N° d’ordre 2010-ISAL-0101 Année 2010 Thèse présentée devant l’Institut National des Sciences Appliquées de Lyon pour obtenir le grade de Docteur École Doctorale : Matériaux de Lyon Spécialité : Matériaux Polymères et Composites par Sébastien LIVI ---------------------- IONIC LIQUIDS MULTIFUNCTIONAL AGENTS OF THE POLYMER MATRICES --------- Soutenue le 02 décembre 2010 devant la Commission d’Examen : JURY DUCHET-RUMEAU Jannick Professeur (INSA Lyon) – Directrice de thèse GALY Jocelyne DR CNRS (INSA Lyon) – Examinateur GANTILLON Barbara Dr (Société TEFAL) – Examinateur GERARD Jean-François Professeur (INSA Lyon) – Co-directeur de thèse PHAM Thi Nhàn Maître de Conférences (Université de Caen) – Examinateur PLUMMER John Christopher Professeur (Ecole Polytechnique Fédérale de Lausanne) – Rapporteur SEGUELA Roland DR CNRS (Université de Lille 1) – Rapporteur
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N° d’ordre 2010-ISAL-0101 Année 2010
Thèse
présentée devant
l’Institut National des Sciences Appliquées de Lyon
pour obtenir
le grade de Docteur
École Doctorale : Matériaux de Lyon
Spécialité : Matériaux Polymères et Composites
par
Sébastien LIVI
----------------------
IONIC LIQUIDS MULTIFUNCTIONAL AGENTS OF THE POLYMER MATRICES
---------
Soutenue le 02 décembre 2010 devant la Commission d’Examen :
JURY
DUCHET-RUMEAU Jannick Professeur (INSA Lyon) – Directrice de thèse GALY Jocelyne DR CNRS (INSA Lyon) – Examinateur GANTILLON Barbara Dr (Société TEFAL) – Examinateur GERARD Jean-François Professeur (INSA Lyon) – Co-directeur de thèse PHAM Thi Nhàn Maître de Conférences (Université de Caen) – Examinateur PLUMMER John Christopher Professeur (Ecole Polytechnique Fédérale de Lausanne) – Rapporteur SEGUELA Roland DR CNRS (Université de Lille 1) – Rapporteur
Ionic Liquids : Multifunctional agents of the polymer matrices
II
INSA de Lyon – Liste des Ecoles Doctorales
III
INSA Direction de la Recherche – Ecoles Doctorales – Quadriennal 2007-2010
SIGLE ECOLE DOCTORALE NOM ET COORDONNEES DU RESPONSABLE
CHIMIE
CHIMIE DE LYON http://sakura.cpe.fr/ED206
M. Jean Marc LANCELIN
Insa : R. GOURDON
M. Jean Marc LANCELIN Université Claude Bernard Lyon 1 Bât CPE 43 bd du 11 novembre 1918 69622 VILLEURBANNE Cedex Tél : 04.72.43 13 95 Fax : [email protected]
E.E.A.
ELECTRONIQUE, ELECTROTECHNIQUE, AUTOMATIQUE http://www.insa-lyon.fr/eea M. Alain NICOLAS
Insa : C. PLOSSU [email protected] Secrétariat : M. LABOUNE AM. 64.43 – Fax : 64.54
M. Alain NICOLAS Ecole Centrale de Lyon Bâtiment H9 36 avenue Guy de Collongue 69134 ECULLY Tél : 04.72.18 60 97 Fax : 04 78 43 37 17 [email protected] Secrétariat : M.C. HAVGOUDOUKIAN
E2M2
EVOLUTION, ECOSYSTEME, MICROBIOLOGIE, MODELISATION http://biomserv.univ-lyon1.fr/E2M2 M. Jean-Pierre FLANDROIS
Insa : H. CHARLES
M. Jean-Pierre FLANDROIS CNRS UMR 5558 Université Claude Bernard Lyon 1 Bât G. Mendel 43 bd du 11 novembre 1918 69622 VILLEURBANNE Cédex Tél : 04.26 23 59 50 Fax 04 26 23 59 49 06 07 53 89 13 [email protected]
EDISS
INTERDISCIPLINAIRE SCIENCES-SANTE Sec : Safia Boudjema M. Didier REVEL Insa : M. LAGARDE
M. Didier REVEL Hôpital Cardiologique de Lyon Bâtiment Central 28 Avenue Doyen Lépine 69500 BRON Tél : 04.72.68 49 09 Fax :04 72 35 49 16 [email protected]
INFOMATHS
INFORMATIQUE ET MATHEMATIQUES http://infomaths.univ-lyon1.fr M. Alain MILLE
Secrétariat : C. DAYEYAN
M. Alain MILLE Université Claude Bernard Lyon 1 LIRIS - INFOMATHS Bâtiment Nautibus 43 bd du 11 novembre 1918 69622 VILLEURBANNE Cedex Tél : 04.72. 44 82 94 Fax 04 72 43 13 10 [email protected] - [email protected]
Matériaux
MATERIAUX DE LYON M. Jean Marc PELLETIER
Secrétariat : C. BERNAVON 83.85
M. Jean Marc PELLETIER INSA de Lyon MATEIS Bâtiment Blaise Pascal 7 avenue Jean Capelle 69621 VILLEURBANNE Cédex Tél : 04.72.43 83 18 Fax 04 72 43 85 28 [email protected]
MEGA
MECANIQUE, ENERGETIQUE, GENIE CIVIL, ACOUSTIQUE M. Jean Louis GUYADER
Secrétariat : M. LABOUNE PM : 71.70 –Fax : 87.12
M. Jean Louis GUYADER INSA de Lyon Laboratoire de Vibrations et Acoustique Bâtiment Antoine de Saint Exupéry 25 bis avenue Jean Capelle 69621 VILLEURBANNE Cedex Tél :04.72.18.71.70 Fax : 04 72 43 72 37
Ionic Liquids : Multifunctional agents of the polymer matrices
IV
Remerciements
V
Remerciements
Ionic Liquids : Multifunctional agents of the polymer matrices
VI
VII
«En science, la phrase la plus excitante que l’on peut entendre,
celle qui annonce des nouvelles découvertes,
ce n’est pas Eureka, mais c’est drôle.»
Isaac Asimov
Ionic Liquids : Multifunctional agents of the polymer matrices
VIII
Résumés
IX
Résumé : Une excellente stabilité thermique, une faible pression de vapeur saturante, une
ininflammabilité, une bonne conductivité ionique ainsi que les différentes combinaisons cations/anions possibles font des liquides ioniques l'objet d'un engouement grandissant de la Recherche. De part ces avantages, les LI se présentent comme une nouvelle voie dans le domaine des polymères, et en particulier dans le milieu des nanocomposites où leur utilisation est essentiellement limitée à la fonction de surfactant des silicates lamellaires. Néanmoins, avant de pouvoir prétendre à un statut d'alternative, il est nécessaire de mettre en évidence les effets bénéfiques de leur utilisation sur les propriétés finales des matériaux polymères.
Dans un premier temps, l’objectif de ce travail a été de synthétiser des liquides ioniques différents par la nature de leur cation et anion mais tous porteurs de longues chaînes alkyles afin de permettre une meilleure compatibilité avec la matrice. Ensuite, les excellentes propriétés intrinsèques des liquides ioniques ont motivé leur utilisation comme agents structurants dans une dispersion aqueuse fluorée. Ainsi, leur rôle d’agents ioniques sur la morphologie, les propriétés physiques, thermiques et mécaniques a été étudié. Dans une seconde partie, les liquides ioniques ont été utilisés comme agents intercalants des silicates lamellaires puis confrontés aux surfactants conventionnels dans le but de préparer des argiles thermiquement stables pour la préparation de nanocomposites thermoplastiques/argiles.
Dans une dernière partie, une faible quantité de ces argiles organiquement modifiées ont été introduites par intercalation à l'état fondu dans deux matrices différentes afin de mettre en évidence les effets de ces nouveaux agents interfaciaux sur les propriétés finales du matériau.
Ionic Liquids : Multifunctional agents of the polymer matrices
X
Ionic Liquids: Multifunctional agents of the polymer matrices Abstract : An excellent thermal stability, a low saturated vapor pressure, a no flammability, a
good ionic conductivity and the different cations / anions combinations possible of ionic liquids are currently the focus of the research. Because of its various benefits, they are as a new alternative in the polymer science, and particularly in the field of the nanocomposites where their use is currently limited to the function of surfactant for the layered silicates. However, before claiming the status of an alternative, it is necessary to highlight the benefits of their use on the final properties of polymer materials.
Initially, the objective of this work was to synthesize different ionic liquids by the nature of cation and anion, but all bearing with long alkyl chains to allow greater compatibility with the matrix. Then, the excellent intrinsic properties of ionic liquids have motivated their use as structuring agents in a fluorinated aqueous dispersion. Thus, their role in ionic agents on the morphology, physical, thermal and mechanical properties was studied. In a second part, ionic liquids have been used as agents intercalating layered silicates and then confronted with conventional surfactants in order to prepare thermally stable clays for the preparation of nanocomposite thermoplastic / clay.
In the last section, a small amount of organically modified clays were introduced by melt intercalation in two different matrices in order to highlight the effects of these new interfacial agents on the final properties of the material.
Chapter I Ionic liquids: State of the art _______________________________________ 27
I.1 Ionic liquids ______________________________________________________________ 29 I.1.1 Origin of ionic liquids _________________________________________________________ 29 I.1.2 Properties of ionic liquids ______________________________________________________ 30 I.1.3 Structure and synthesis of ionic liquids ____________________________________________ 30
I.1.3.1 Effect of cation _____________________________________________________________ 31 I.1.3.2 Effect of anion _____________________________________________________________ 32 I.1.3.3 Synthesis of ionic liquids _____________________________________________________ 33
I.1.4 Applications of ionic liquids ____________________________________________________ 36 I.1.4.1 New alternative to conventional solvents _________________________________________ 36 I.1.4.2 Electrochemistry ____________________________________________________________ 36 I.1.4.3 Homogeneous and heterogeneous catalysis _______________________________________ 36 I.1.4.4 Metal ion capture ___________________________________________________________ 37 I.1.4.5 Chemistry in supercritical medium ______________________________________________ 37 I.1.4.6 Surfactants, plasticizers, and lubricants in polymer science ___________________________ 38
I.1.5 Main limitation of ionic liquids __________________________________________________ 39 I.1.6 Conclusion__________________________________________________________________ 39
I.2 Ionic liquids: A new way in the preparation of polymer/layered silicates nanocomposites ________________________________________________________________________ 40
Conclusions of chapter I ________________________________________________________ 69
References of chapter I _________________________________________________________ 70
Ionic Liquids : Multifunctional agents of the polymer matrices
XII
CHAPTER II POLYMER/IONIC LIQUID INTERACTIONS _____________________________ 75
II.1 New building blocks _____________________________________________________ 77 II.1.1 Introduction _________________________________________________________________ 77 II.1.2 Experimental ________________________________________________________________ 78
II.1.2.1 Materials __________________________________________________________________ 78 II.1.2.2 Processing and characterization of the IL/PTFE films _______________________________ 78 II.1.2.3 Synthesis of ionic liquids _____________________________________________________ 80
II.1.3 Morphology and mechanical performances of polymer/IL blends _______________________ 83 II.1.4 Conclusions _________________________________________________________________ 85
II.2 Nanostructuration of ionic liquids in fluorinated matrix: Influence on the mechanical properties ____________________________________________________________________ 86
II.2.1 Introduction _________________________________________________________________ 86 II.2.2 Results and discussion _________________________________________________________ 87
II.2.2.1 Effect of ionic liquids on the structuration of fluorinated polymer films _________________ 87 II.2.2.2 Effect of ionic liquids on the thermal properties of fluorinated polymer-based blends ______ 90 II.2.2.3 Effect of ionic liquids on the PTFE crystallinity ____________________________________ 91 II.2.2.4 Effect of ionic liquids on the mechanical properties of fluorinated polymer ______________ 93
Conclusions of chapter II ______________________________________________________ 100
References of chapter II _______________________________________________________ 101
Chapter III IONIC LIQUIDS AS NEWS INTERCALATING AGENTS FOR LAYERED SILICATES _ 103
III.1 A comparative study on ionic liquids used as surfactants: Effect on thermal and mechanical properties of high-density polyethylene nanocomposites __________________ 105
III.1.2.1 Materials _________________________________________________________________ 107 III.1.2.2 Synthesis of phosphonium and imidazolium salts _________________________________ 107 III.1.2.3 Organic modification of montmorillonite ________________________________________ 108 III.1.2.4 Processing and characterization of the HDPE/clay nanocomposites ___________________ 110
III.1.3 Results and discussion _____________________________________________________ 111 III.1.3.1 Characterization of modified montmorillonites ___________________________________ 111 III.1.3.2 Thermal stability of modified montmorillonites ___________________________________ 114 III.1.3.3 Structural analysis by WAXD _________________________________________________ 116 III.1.3.4 Surface energies of modified montmorillonites ___________________________________ 118 III.1.3.5 Influence of ionic liquid content _______________________________________________ 118
III.1.4 HDPE/clay nanocomposites _________________________________________________ 120 III.1.4.1 Thermal properties of nanocomposites __________________________________________ 120 III.1.4.2 Mechanical properties of nanocomposites _______________________________________ 121 III.1.4.3 Morphology of nanocomposites _______________________________________________ 122
III.2.3.1 Effect of supercritical carbon dioxide as a exchange solvent on the thermal degradation of the modified MMT ___________________________________________________________________ 127 III.2.3.2 Structural analysis __________________________________________________________ 133 III.2.3.3 Surface energies ___________________________________________________________ 136
Conclusions of chapter III _____________________________________________________ 137
References of chapter III ______________________________________________________ 138
Chapter IV POLYMER/LAYERED SILICATES NANOCOMPOSITES _____________________ 141
IV.1 Synthesis of new surfactants: Effect of the ionic liquids on the thermal stability and the mechanical properties of high density polyethylene nanocomposites _______________ 143
IV.2.3 Results and discussion _____________________________________________________ 164 IV.2.3.1 Characterization of ILs exchanged montmorillonites _______________________________ 164 IV.2.3.2 Effect of interfacial interactions on the material physical properties ___________________ 168
(CF3S03-) dit anions fluorés et les anions conventionnels comme le brome (Br-), le chlore
(Cl-), l'iode (I-) et le chloroaluminate (AlCl4-) pour ne citer que les plus connus. La nature de
l'anion utilisé joue un rôle déterminant sur les propriétés finales des liquides ioniques,
notamment, en ce qui concerne la stabilité thermique des sels. Par exemple, lorsqu'un sel
imidazolium est associé à un anion fluoré (BF4-), sa stabilité thermique est considérablement
améliorée comparée au même LI combiné à un anion bromure (Br-) [2]. Il en est de même sur
la solubilité des liquides ioniques, l'utilisation du 1-méthyl-2-butyl-imidazolium
tétrafluoroborate est soluble dans l'eau alors que le même cation avec l'anion PF6-est non
miscible à l'eau. Dans le domaine des électrolytes et des batteries de piles à combustible, les
anions les plus communément utilisés sont les anions fluorés (BF4-) et (PF6
-). Les différents
anions sont résumés dans le Tableau 1.
Tableau 1 – Les différents anions inorganiques ou organiques les plus souvent rencontrés Inorganic anions Organic anions
F-, Br
-, Cl
-, I
-
BF4-, PF6
-, SbF6
-, AsF6
-
NO3-, ClO4
-
CuCl2-, AuCl4
-, SnCl3
-
CH3CO2-, CH3SO4
-, C6H5SO3
-
CF3CO2-, C(CF3SO2)3
-
N(SO2CF3)2
CF3SO3-
Résumé étendu
Page 5
• Applications des liquides ioniques
- Catalyse
Leur capacité à dissoudre de nombreuses substances comme les catalyseurs ainsi que
leur immiscibilité avec les réactifs et les produits confèrent aux LI un net avantage, très utile
en catalyse homogène et hétérogène. On les retrouve ainsi dans plusieurs réactions: les
réactions de couplage Suzuki-Heck [3], d'oxydation [4], sulfonation [5], isomérisation où les
LI imidazolium et ammonium sont couramment utilisés.
- Elimination des métaux
Les liquides ioniques sont de plus en plus utilisés en remplacement des solvants
organiques traditionnels dans les procédés d'extraction des métaux, en particulier dans le
domaine des déchets nucléaires [6] et de la contamination de l'eau [7]. Par exemple, Les
liquides imidazolium, notamment associés aux anions fluorés PF6-, BF4
- sont utilisés dans
l'extraction des ions sodium, cesium, lithium ou potassium [8].
- Science des polymères
L'utilisation de solvants organiques dans la préparation d'électrolytes de polymères est
fréquente. Néanmoins, des problèmes de volatilité et d’inflammabilité sont générés lorsqu'il
est nécessaire de travailler sur des gammes de température élevées. Ces inconvénients ont
conduit les chercheurs à se tourner vers les liquides ioniques qui contrairement aux solvants
conventionnels possèdent une excellente stabilité thermique, une faible volatilité, une bonne
conductivité et sont ininflammables. Les liquides ioniques imidazolium et pyridinium associés
aux anions PF6-, BF4
-, CF3SO3- et N(CF3SO3)
2- [9-11] ont été largement étudiés. Ainsi, les LI
ont été associés aux polymères électrolytes soit directement par polymérisation à partir du LI
[12] soit par solubilisation du polymère électrolyte dans le LI [13].
Dans l'industrie, les LI sont également utilisés comme plastifiants. Dans ce domaine,
les LI sont de bons substituts des plastifiants traditionnels dans des polymères comme le
PLLA, le PMMA et le PVC [14]. Des études ont également été menées sur l'utilisation des
liquides ioniques à température ambiante (RTIL) et leur grande capacité à réduire le
frottement et l'usure des polymères contre les métaux [15].
Dans le domaine des nanocomposites polymères à base de silicates lamellaires
(montmorillonite), l'utilisation des liquides ioniques ammonium comme agent compatibilisant
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 6
des charges a fait l’objet de nombreux travaux [16]. Toutefois, la faible stabilité thermique
des sels d'ammonium due à l'élimination d'Hoffmann limite grandement leur utilisation lors
de la mise en œuvre à haute température de nanocomposites polymères/argiles. Afin de
contourner cette limitation, l'utilisation de liquides ioniques thermostables basés sur les
cations pyridinium, imidazolium ou phosphonium a été envisagée [17]. Pourtant, l'usage de
ces agents intercalants reste limité en raison du coût très élevé des LI mais également du
manque de choix disponible.
Cette partie bibliographique vise à donner une simple description des LI et de leurs propriétés
attractives. L’utilisation des LI dans le domaine des nanomatériaux reste encore limitée
compte tenu de leur coût. Le grand nombre de combinaisons possibles entre cation, anion et
nature du ligand permet d’envisager par une synthèse à façon une large palette de LI
répondant à un grand nombre d’applications et offrant de nombreuses perspectives.
• Références
[1] C. Chiappe, D. Pieraccini, J. Phys. Org. Chem. (2005); 18:275–297. [2] W.H. Awad, J.W. Gilman, M. Nyden, R.H. Harris, T.E. Sutto, J. Callahan, P.C. Trulove, H.C. DeLong, D.M. Fox, Thermochimica Acta (2004); 409:3. [3] M.J. Earle, S.P. Katdare, World Patent WO 2002030862 (2002). [4] M.J. Earle, S.P. Katdare, World Patent WO 2002030865 (2002). [5] J.F. Brennecke, E.J. Maginn, AIChE J. (2001); 47:2384–2388. [6] R.A. Bartsch, S. Chun, S.V. Dzyuba, Ionic Liquids Industrial Applications for Green Chemistry, American Chemical Society, Washington, DC, (2002); 58–68. [7] S. Chun, S.V. Dzyuba, R.A. Bartsch, Anal. Chem. (2001); 73:3737–3741. [8] H. Luo, S. Dai, P.V. Bonnesen, A.C.I. Buchanan, J.D. Holbrey, N.J. Bridges, R.D. Rogers, Anal.Chem. (2004); 76:3078–3083. [9] Noda, A.; Hayamizu, K.; Watanabe, M. J. Phys. Chem. B (2001); 105:4603. [10] Sakaebe, H.; Matsumoto, H. Electrochem. Commun. (2003); 5:594. [11] Lewandowski, A.; Swiderska, A. Solid State Ionics (2004); 169:21. [12] Ohno, H. Electrochim. Acta (2001); 46:1407. [13] Fuller, J.; Breda, A. C.; Carlin, R. T. J. Electroanal. Chem. (1998); 459:29. [14] M. Rahman and C. S. Brazel, Polym. Degrad. and Stab. (2006); 91:3371–3382. [15] J. Sanes, F. J. Carrión, A. E. Jiménez and M. D. Bermúdez, Wear (2007); 263:658–662. [16] H.L. Tyan, Y.C. Liu, K.H. Wei, Chem Mater. (1999); 11:1942. [17] V. Mittal, European Polymer Journal. (2007); 43:3727–3736.
Résumé étendu
Page 7
Chapitre 2 : Interactions LI/polymère
Dans le domaine des matériaux polymère, les liquides ioniques ont souvent été utilisés
en tant que solvant vert et conducteur dans les gels electrolytes ou en tant que surfactant pour
les charges lamellaires. Mais jusqu’à présent, aucun travail à notre connaissance ne mentionne
l’utilisation des LI en tant qu’agent structurant d’une matrice polymère.
Dans ce deuxième chapitre, nous avons cherché à étudier l’impact du LI, introduit en
tant qu’additif dans la matrice polymère sur la morphologie et les propriétés physiques et
thermomécaniques du polymère. Les effets induits par le LI peuvent être modulés par le large
choix de combinaisons possibles cations/anions. Dans ce travail, nous avons choisi
d’introduire le LI dans une suspension aqueuse fluorée composée de polytetrafluoroéthylène
(PTFE) stabilisée. Le cahier des charges est difficile car le PTFE présente déjà une excellente
stabilité thermique, une résistance élevée aux acides et aux bases et un faible coefficient de
frottement. Quelle sera la plus value apportée par le liquide ionique introduit à un faible taux
(1%) dans la matrice PTFE après filmification ?
• Morphologie des LI dans le PTFE
Dans ce chapitre, différents liquides ioniques ont été synthétisés à partir de cations
pyridinium, imidazolium ou phosphonium et associés soit à des anions halogénés de type
iodure (I-), bromure (Br-) ou hexafluorophosphate (PF6-). Le Tableau 2 présente un récapitulatif
des différents sels synthétisés au cours de cette étude.
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 8
Tableau 2 – Structure chimique des LI synthétisés
Cation Anion Designation
N NH37C18 C18H37
I-
C18C18Im I-
PC18H37
I- Br- PF6
-
C18P I- C18P Br- C18P PF6
-
I-
C18Py I- N
C18H37
- Influence du cation
La microscopie électronique à transmission a révélé des morphologies différentes en
fonction de la nature du cation introduit dans la matrice fluorée. Avec seulement 1% de
liquide ionique, une structuration volumique apparaît dans la matrice. Le liquide ionique
imidazolium (C18C18Im I-) génère deux types de morphologies coexistantes: la première
correspondant à la formation d’agrégats de domaines ioniques tandis que la seconde est
semblable à une morphologie co-continue, similaire à celle du liquide ionique pyridinium
(C18Py I-). A l’opposé, dans le cas du LI phosphonium, une excellente dispersion est observée
avec une structuration à l’échelle du nanomètre. Les différentes structurations des LI dans la
matrice fluorée en fonction de la nature du cation sont représentées sur la Figure 2.
200 nm 200 nm 200 nm
Figure 2 – Influence du cation sur la structuration de la matrice PTFE (imidazolium, pyridinium, phosphonium)
Résumé étendu
Page 9
Malgré la présence des longues chaînes alkyle (18 carbones) comme ligands sur les
différents liquides ioniques qui doivent interagir favorablement avec la matrice hydrophobe
du polytétrafluoroéthylène, la miscibilité des LI dans la matrice polymère est médiocre ce qui
conduit à la création d’une morphologie de phases séparées. De telles morphologies ont déjà
été observées et sont comparables à celles des ionomères dans les mélanges de polymères. En
effet, il est bien connu que le regroupement de paires d'ions dans un milieu de faible constante
diélectrique est responsable de la formation de micro ou de nanostructures qui peuvent être
prédites théoriquement [4, 5]. Le principal paramètre contrôlant la micro séparation de phase
dans un milieu non-polaire sont les interactions dipôle-dipôle entre les paires ce qui induit la
formation d’agrégats ioniques [6, 7]. Dans ce travail, une analogie peut être faite avec la
formation des agrégats de LI, dépendante des interactions entre le polymère et les différentes
combinaisons possibles cations/anions.
- Influence de l’anion
La nature de l’anion a également une influence significative sur la morphologie finale.
La Figure 3 illustre le rôle de l’anion sur la structuration générée dans la matrice PTFE à partir
d’un liquide ionique phosphonium associé soit à un contre anion iodé ou bromé soit à un
fluoré. Les anions bromé et fluoré conduisent à une morphologie grossière composée
d’agrégats. Cette structuration à l’échelle du micron contraste avec la morphologie à l’échelle
nanométrique obtenue avec l’anion iodure.
200 nm200 nm200 nm
Figure 3 – Structuration de l’échelle du micron au nanomètre de mélanges PTFE/LI (1% LI en poids) (C18P I-, C18P Br-, C18P PF6
-)
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 10
• Influence des liquides ioniques sur les propriétés mécaniques des films
PTFE/LI
- Influence du cation
Dans cette partie, les propriétés mécaniques des films structurés en relation avec
leur morphologie ont été également étudiées. Le comportement mécanique est très
dépendant de la nature chimique des liquides ioniques. En effet, l’addition d’1% en poids
de liquides ioniques à base de pyridinium et d’imidazolium associés à l’anion iodure
présentent des performances mécaniques similaires en traction uniaxiale, conformes à
leurs morphologies co-continues identiques. Si des augmentations de module de l’ordre de
38 et 41% sont obtenues respectivement pour les LI pyridinium et imidazolium, une légère
diminution de l’allongement à la rupture de 11% est observée. En revanche, pour le LI
phosphonium qui conduisait à une spectaculaire structuration à l’échelle nanométrique, le
compromis propriétés à rupture/rigidité était fortement amélioré puisque des
augmentations de la rigidité et de la déformation à la rupture sont obtenues avec des
hausses respectives de +160% et +190% comme le montre la Figure 4.
0
2
4
6
8
10
12
14
16
0 100 200 300 400 500 600
Strain at break (%)
Str
es
s (
MP
a)
PTFE
PTFE C18P I-
Figure 4 – Effet du liquide ionique phosphonium (1% en poids) sur les propriétés mécaniques à une vitesse de
déformation de 0.004 s-1 à température ambiante
De meilleures interactions c'est-à-dire une forte cohésion des interfaces due aux
interactions ioniques semblent avoir lieu entre la phase LI phosphonium et la matrice
fluorée ce qui conduit à une augmentation du module PTFE/LI. La phase LI agit comme
un agent de renforcement en formant un réseau de forte cohésion s’apparentant à celui des
réseaux percolants de nanocharges (noir de carbone, silice) mais qui est également capable
d’accomoder des déformations extrêmement importantes (délai de la rupture à haute
déformation).
Résumé étendu
Page 11
- Influence de l’anion
Les propriétés mécaniques peuvent également être modulées par la nature
chimique de l’anion. Ainsi, nous avons observé que le LI phosphonium associé aux anions
bromure et hexafluorophosphate conduit à des augmentations du module de 84% à 115%,
respectivement, en référence au PTFE non chargé. A l’opposé du LI C18P I-, la mauvaise
distribution des agrégats de LI C18P Br- et C18P PF6- dans la matrice fluorée a comme
conséquence une diminution de l’allongement à la rupture comprise entre 22% et 84%
respectivement.
• Influence des vitesses de déformation sur le comportement mécanique
des films polymère
En raison de l’excellente dispersion du LI phosphonium associé au contreanion
iodé dans la matrice fluorée ainsi que du très bon compromis rigidité/plasticité obtenu,
nous avons décidé d’étudier l’effet de la vitesse de déformation sur la morphologie de la
phase LI après déformation ainsi que sur les propriétés mécaniques qui en découlent.
Une des premières observations que nous avons faites est que l’addition du LI
phosphonium dans le PTFE conduit à une cristallisation à très haute vitesse de
déformation (0.2 s-1) avec une augmentation de la cristallinité de 10% en comparaison à
celle de la matrice seule. Pour une même vitesse de déformation, au niveau des propriétés
mécaniques, seule une légère augmentation de la rigidité est obtenue (200 MPa pour le
PTFE/C18P I- vs 170MPa pour le PTFE) qui peut être attribuée à une compétition entre la
réorganisation de la phase LI dans la matrice et la cristallisation sous déformation.
Pour corroborer cette hypothèse, nous avons utilisé de nouveau la microscopie
électronique à transmission (MET) pour révéler les domaines LI dans la matrice sous
Figure 5 – Effet de la vitesse de déformation sur la morphologie du LI phosphonium
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 12
Lorsque la vitesse de déformation augmente, la nanostructure co-continue du LI est
maintenue et orientée dans l’axe de la déformation ce qui signifie que le réarrangement
des domaines ioniques nécessite un temps de relaxation plus important que le temps
caractéristique du processus de déformation. Ce phénomène est très proche de celui
observé par Visser et al [8], qui a proposé un modèle pour les ionomères mettant en
évidence un réarrangement spatial des domaines ioniques au sein de la matrice polymère.
Les auteurs ont également souligné le rôle de la nature des paires ioniques sur la
mécanique et le comportement de déformation.
Pour la première fois, une structuration à l’échelle nanométrique des liquides ioniques
dans un film polymère a été mise en évidence. Nous avons également démontré que les effets
de la nature chimique du LI déterminés par le choix du cation organique : pyridinium,
imidazolium ou phosphonium aussi bien que le choix de l’anion (halogènes ou fluorés)
peuvent affecter la structuration et les propriétés physiques et mécaniques du polymère. En
effet, une combinaison cation/anion adéquate génère une flexibilité sans précédent ainsi
qu’une amélioration significative de la rigidité. Les LI offrent ainsi une nouvelle alternative
pour structurer à l’échelle nanométrique les matériaux polymères.
• Références
[1] M. P. Scott, M. Rahman and C. S. Brazel, Eur Polym J. (2003); 39:1947–1953. [2] F. Avalos, J. C. Ortiz, R. Zitzumbo, M. A. López-Manchado, R. Verdejo and M. Arroyo, App.Clay Sci.
(2009); 43:27–32 [3] Fuller, J.; Breda, A. C.; Carlin, R. T. J. Electrochem. Soc. (1997); 144:L67. [4] A.R. Khokhlov, E.F. Dormidontova, Phys. Uspekhi (2005), 118, 73. [5] I.A. Nyrkova, A.R. Khokhlov, Y.Y. Kramarenko, Polym. Sci. USSR (1990), 32, 852. [6] A. Eisenberg, B. Hird, R.B. Moore, Macromolecules (1990), 23, 4098. [7] I.A. Nyrkova, A.R. Khokhlov, M. Doi, Macromolecules (1993), 26, 3601. [8] S.A. Visser, S.L. Cooper, Polymer (1992), 33, 4705-4710.
Résumé étendu
Page 13
Chapitre 3 : Utilisation des liquides ioniques comme agents intercalants de silicates lamellaires
Depuis les années 80 et depuis les premiers travaux réalisés par l’équipe de Toyota sur
les nanocomposites polyamide-montmorillonite (PA-MMT), le domaine des nanocomposites
à charges lamellaires est en plein essor. En effet, le challenge lié à ces nouveaux matériaux,
vise à améliorer les propriétés finales des matériaux, notamment les propriétés thermiques,
mécaniques et barrière [1] avec un très faible taux de charge inorganique. La clé du succès
réside dans le contrôle de la dispersion des feuillets individuels, décrit comme l’état
d’exfoliation. Mais le manque de compatibilité entre les argiles (hydrophiles) et les
polymères, le plus souvent hydrophobes rend difficile l’obtention de cet état d’exfoliation.
Pour contourner cette difficulté et améliorer la compatibilité entre les argiles et le polymère,
l'utilisation d'espèces organiques nommées agents intercalants ou surfactants, est nécessaire
afin de réduire l'énergie de surface des silicates lamellaires et augmenter les distances
interfoliaires de façon à promouvoir la dissociation des feuillets en vue d’obtenir un état de
dispersion exfolié, plus propice à l'amélioration des propriétés finales des nanocomposites [2,
3]. Jusqu’alors, les sels d’ammonium sont classiquement utilisés.
Toutefois, la faible stabilité thermique des ammoniums quaternaires, qui se dégradent
dès 180°C [4], limite considérablement leur utilisation pour l’élaboration de nanocomposites
polymères/argiles nécessitant des températures de mise en œuvre élevées. Les liquides
ioniques apparaissent alors comme une nouvelle alternative aux ammoniums conventionnels.
L’objectif de ce chapitre a donc été de préparer des argiles organiquement modifiées par des
cations organiques thermostables, en particulier imidazolium et phosphonium connus pour
leur excellente stabilité thermique, et fonctionnalisés par de longues chaînes alkyle afin de
diminuer l’énergie de surface des argiles. En raison d'une offre limitée de liquides ioniques
commerciaux à longues chaînes alkyle (> 14 carbones), nous avons été amenés à synthétiser
des LI à façon.
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 14
• Nomenclature des montmorillonites modifiées
Dans une première partie, afin de démontrer la supériorité thermique des liquides
ioniques sur les sels d’ammonium, une étude comparative sur quatre montmorillonites
modifiées a été décrite. Tout d’abord, nous avons sélectionné deux montmorillonites
commerciales (Nanofil 15 et Nanofil 919) traitées par les agents intercalants diméthyl ditallow
ammonium (MMT-DMDT) et diméthyl benzyltallow ammonium (MMT-DMBT) et nous les
avons comparés aux montmorillonites modifiées par les LI N-octadécyl-N’-
octadécylimidazolium (MMT-I) et octadécyltriphenylphosphonium (MMT-P). La Figure 6
décrit les différents cations organiques qui ont été comparés.
N+
Tallow
CH3
TallowH3C
N+
H3C
H3C Tallow
N
N
C18H37
C18H37
I
P C18H37
I
Versus
Figure 6 – Ammonium vs imidazolium et phosphonium
- Stabilité thermique des MMT modifiées
D’après la littérature [5-7], lorsque l’on modifie la surface des argiles par des cations
organiques, deux types d’interactions interviennent:
- (I) Les interactions de Van der Waals correspondant aux espèces organiques
physiquement adsorbées sur la surface de l'argile.
- (II) Les interactions ioniques correspondant aux espèces réellement intercalées entre
les feuillets de la montmorillonite.
Afin d’identifier et de quantifier le taux d’intercalation des espèces adsorbées et
intercalées, des lavages successifs au méthanol ont été effectués. Ainsi, nous avons démontré
par analyse thermogravimétrique (ATG) que les MMT-I et MMT-P ont des températures de
dégradation correspondant aux espèces physisorbées comprises entre 320°C et 340°C
(évaporation des LI), respectivement comparées aux MMT-DMDT et MMT-DMBT qui se
dégradent à plus basse température 220°C et 270°C.
Concernant les espèces intercalées, les montmorillonites modifiées par les liquides
ioniques imidazolium et phosphonium présentent une meilleure stabilité thermique que les
DMDT P DMBT I
Résumé étendu
Page 15
montmorillonites traitées ammonium puisque les températures de dégradation des nouveaux
surfactants s’étendent sur une plage comprise entre 420-490°C (MMT-I) et 510°C (MMT-P)
contrairement aux ammonium qui commencent à se dégrader à partir de 340-440°C (MMT-
DMDT) et de 300-400°C (MMT-DMBT). La Figure 7 représente le comportement de
dégradation thermique des montmorillonites organiquement modifiées.
Universal V4.2E TA Instruments Figure 8 – Stabilité thermique des MMT-I modifiées par échange standard (a, a’ dérivée de ∆m/m) et sous
ScCO2-eau (b, b’ dérivée de ∆m/m)
Ces résultats ont pu être expliqués par une diminution de la température de fusion des
LI lors de l’exposition au CO2 supercritique en présence d’eau due à la mise en place de
faibles interactions acide-base de Lewis entre la partie basique des LI et la partie acide du
CO2 [12].
En conclusion, nous avons clairement démontré la meilleure stabilité thermique des
silicates lamellaires modifiées par les LI imidazolium et phosphonium comparés aux argiles
traitées par les ammoniums quaternaires conventionnels et l’utilisation du CO2 supercritique
associée à l’eau et aux liquides ioniques permet un échange cationique propre avec une
augmentation significative de la stabilité thermique des argiles modifiées pour des
caractérisations structurales comparables.
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 18
• Références [1] E. Jacquelot, E. Espuche, J.F. Gerard, J. Duchet, P. Mazabraud, J. Polym. Sci. Part B: Polym. Phys. 44 (2) (2006) 431. [2] S.Y. Lee, W.J. Cho, K.J. Kim, J.H. Ahn, M. Lee, J. Colloid Interface Sci. 284 (2) (2005) 667. [3] H. He, J. Duchet, J. Galy, J.F. Gerard, J. Colloid Interface Sci. 295 (2006) 202. [4] W. Xie, Z. Gao, W.P. Pan, D. Hunter, A. Singh, R. Vaia, Chem. Mater. 13 (9) (2001) 2979. [5] L. Le Pluart, J. Duchet, H. Sautereau, J.F. Gérard, J. Adhes. 78 (7) (2002) 645. [6] W. Xie, R. Xie, W.P. Pan, D. Hunter, B. Koene, L.S. Tan, R. Vaia, Chem. Mater. 14 (11) (2002) 4837. [7] W. Xie, Z. Gao, W.P. Pan, D. Hunter, A. Singh, R. Vaia, Thermochim. Acta 367– 368 (2001) 339. [8] S. Boucard, J. Duchet, J.F. Gérard, P. Prele, S. Gonzalez, Macromol. Symp. 194 (1) (2002) 241. [9] C.M. Hansen, A. Beerbower, Kirk-Othmer Encyclopedia of Chemical Technology, second ed., Interscience, New York, 1971. p. 889. [10] S.P. Nalawade, F. Picchioni, L.P.B.M. Jansen, Prog. Polym. Sci. 31 (2006) 19-43. [11] S. Keskin, D. Kayrak-Talay, U. Akman, Ö. Hortaçsu, J. of Sup. Fluids 2007, 43, 150-180. [12] A.M. Scurto, E. Newton, R.R. Weikel, L. Drauker, J.Hallett, C.L. Liotta, W. Leitner, C.A. Eckert, Ind. Eng. Chem. Res. 47 (2008) 493.
I.1.4 Applications of ionic liquids ......................................................................................................... 36 I.1.4.1 New alternative to conventional solvents ................................................................................. 36 I.1.4.2 Electrochemistry ....................................................................................................................... 36 I.1.4.3 Homogeneous and heterogeneous catalysis .............................................................................. 36 I.1.4.4 Metal ion capture ...................................................................................................................... 37 I.1.4.5 Chemistry in supercritical medium ........................................................................................... 37 I.1.4.6 Surfactants, plasticizers, and lubricants in polymer science ..................................................... 38
I.1.5 Main limitation of ionic liquids ..................................................................................................... 39 I.1.6 Conclusion..................................................................................................................................... 39
I.2 Ionic liquids: A new way in the preparation of polymer/layered silicates nanocomposites .................................................................................................................................................. 40
and heterogeneous catalysis and in the polymer science, particularly in polymer gel
electrolytes, lubricants and plasticizers. Despite their many advantages, due to their good
thermal stability, the use of ionic liquids in the field of the nanocomposites is mainly limited
to the role of modifiers agents for layered silicates. We have shown that the influence of ILs
modified clays on the PLS nanocomposites are mixed though. The true potential of ILs based
on the infinite cation/anion combinations is not really exploited. In fact, the reduced choice
and the high cost of the commercial ionic liquids functionalized with different groups (epoxy,
hydroxy, vinyl, and fluorinated chains) are the main causes of this limitation. To achieve
significantly optimized PLS nanocomposites, a lot of studies are needed to find the suitable
association between the specific IL and the matrix.
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 70
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Chapter II: Interactions Polymer/Ionic Liquids
Page 75
Chapter II POLYMER/IONIC LIQUID INTERACTIONS In the field of polymer materials, ionic liquids have often been used as a green solvent
and conductors in the gel electrolyte or as a surfactant to the layered silicates. So far, to our
knowledge, no work mentions the use of IL as structuring agent of a polymer matrix.
In this second chapter, we sought to investigate the impact of the IL, introduced as an additive
in the polymer matrix on the morphology, physical and thermo-mechanical properties of the
polymer. The effects generated by the ionic liquids can be modulated by the wide range of
possible combinations of cations / anions. In this work, we have chosen to introduce the IL in
fluorinated aqueous suspension comprising polytetrafluoroethylene (PTFE) stabilized. The
specification is difficult because the PTFE have excellent thermal stability, high resistance to
acids and bases and a low friction coefficient. What are the contributions of using ionic
liquids introduced at a low rate (1%wt) in the PTFE matrix after film formation?
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Pages
II.1 New building blocks ........................................................................................................... 77 II.1.1 Introduction ................................................................................................................................... 77 II.1.2 Experimental ................................................................................................................................. 78
II.1.2.1 Materials __________________________________________________________________ 78 II.1.2.2 Processing and characterization of the IL/PTFE films _______________________________ 78 II.1.2.3 Synthesis of ionic liquids _____________________________________________________ 80
II.1.2.3.1 Synthesis of phosphonium salt ....................................................................................... 80 II.1.2.3.2 Synthesis of imidazolium salt ........................................................................................ 82 II.1.2.3.3 Synthesis of pyridinium salt ........................................................................................... 82
II.1.3 Morphology and mechanical performances of polymer/IL blends ................................................ 83 II.1.4 Conclusions ................................................................................................................................... 85
II.2 Nanostructuration of ionic liquids in fluorinated matrix: Influence on the mechanical properties ......................................................................................................................................... 86
II.2.1 Introduction ................................................................................................................................... 86 II.2.2 Results and discussion ................................................................................................................... 87
II.2.2.1 Effect of ionic liquids on the structuration of fluorinated polymer films _________________ 87 II.2.2.1.1 Nanostructures in bulk .................................................... Error! Bookmark not defined. II.2.2.1.2 Surface analysis of fluorinated polymer/IL blends ........................................................ 89
II.2.2.2 Effect of ionic liquids on the thermal properties of fluorinated polymer-based blends ______ 90 II.2.2.3 Effect of ionic liquids on the PTFE crystallinity____________________________________ 91
II.2.2.3.1 Influence of organic cation ............................................................................................. 91 II.2.2.3.2 Influence of halide and fluorinated anions associated on phosphonium cation.............. 92
II.2.2.4 Effect of ionic liquids on the mechanical properties of fluorinated polymer ______________ 93 II.2.2.4.1 Dynamical mechanical analysis of fluorinated polymer-IL blends ................................ 93 II.2.2.4.2 Mechanical properties of fluorinated polymer modified using ILs. ..... Error! Bookmark
not defined. II.2.2.4.3 Effect of strain rate on the uniaxial tension behaviour of polymer/IL films .................. 96 II.2.2.4.4 Effect of ionic liquids on the morphology after deformation ......................................... 97
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• Influence of the organic cation
The chemical nature of the organic cation plays a key role on the thermomechanical
properties of polytetrafluoroethylene. At 25°C, the film modified with the imidazolium ion
shows at room temperature the highest value in the elastic moduli E' with an increase of 17%
compared to unfilled film and 47% compared to the film filled with phosphonium ion. Then at
150°C, whatever the organic cation used, similar moduli but always higher than one measured
on PTFE are obtained. At higher temperatures like 250°C, the highest values in moduli are
measured on the PTFE films filled with the phosphonium ionic liquid because of its thermal
stability better than the imidazolium salt one.
• Influence of the conteranion
At room temperature, the addition of phosphonium ionic liquid associated to halide
anions, causes a decrease in modulus of about 20%. On the contrary, for PTFE/C18P PF6-, a
slight increase of modulus is observed. On the other side, as the temperature increases, the
moduli values for the PTFE/IL materials increase slightly compared to the ones of neat PTFE.
This improvement is more significant when the fluorinated anion is used. In fact, the
hexafluorophosphate anion contributes to better thermomechanical behaviour of IL-modified
PTFE. This phenomenom is even more pronounced at higher temperature because of the
better thermal stability of the fluorinated anion.
These changes of storage moduli can be explained by the fact that the ionic liquids
form in the PTFE medium a separated phase which displays a strong cohesion due to the ionic
dipole-dipole interactions. According to the temperature dependence of ionic interactions, the
multiplet aggregates which exist at low temperature could exist in a larger range of
temperature, i.e. to higher temperatures, before reaching the temperature at which ionic forces
become too weak to contribute to the stiffness of the material. This hypothesis could be
similar to the temperature dependence of storage modulus of ionomers with temperature [30].
II.2.2.4.2 High deformation mechanical analysis of fluorinated films
The mechanical properties determined on the fluorinated polymer films with 1 %wt of
ionic liquids are gathered in Table II-5.
Chapter II: Interactions Polymer/Ionic Liquids
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Table II-5 – Effect of ionic liquids on the tensile properties of fluorinated films
(1 wt %) (crosshead speed : 0.004 s-1).
Sample Young’s modulus (MPa)
Strain at break (%)
PTFE 65 180 PTFE/C18P I- 170 522
PTFE/C18P Br- 120 140 PTFE/C18P PF6
- 140 70 PTFE/C18C18Im I- 110 160
PTFE/C18Py I- 90 160
• Influence of the organic cation
The mechanical performance is very dependent on the chemical nature of ionic liquid
used. If 1 wt% of pyridinium and imidazolium ions within the polymer matrix leads to a
similar mechanical behaviour with an increase of modulus of 38% and 41% respectively and a
slight decrease of 11% for the strain at break, the phosphonium ionic liquid introduced in the
fluorinated matrix give an excellent stiffness/failure properties compromise. Indeed, an
increase of 160% of the stiffness and 190% for the strain at break are achieved. Better
interactions seem to take place between the phosphonium ionic liquid functionalized with
long alkyl chains and the fluorinated matrix. In fact, as mentioned before, the confined ionic
liquid phase has a strong cohesion due to the ionic interactions which can contribute to an
increase of the stiffness of the IL-modified PTFE, i.e. the ionic liquid phase acts a reinforcing
agent. As the interfacial interactions between ionic liquid phase and PTFE medium become
better, an efficient stress transfer at the interface could contribute to a higher Young’s
modulus.
• Influence of the conteranion
The mechanical properties can be also tailored from the chemical nature of the anion.
By using the phosphonium ionic liquid, differences are observed for Young’s modulus and
the strain at break as a function of the anion used. For the three anions, a strong increase of
modulus is obtained of 160% with C18P I-, 84% with C18P Br- and 115% with C18P PF6-. On
the other hand, for the strain at break, the phosphonium ionic liquid containing iodide anion
has a plasticizing effect with a large effect (190%) whereas for the C18P Br- and C18P PF6-, the
addition of these salts reduces the fracture behaviour of the fluoride matrix with a decrease of
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 96
22% and 84% respectively. These results are consistent with the morphologies shown
previously. In the case of C18P I-, a very fine structuration is achieved in the fluoride matrix.
Whereas for C18P Br-, the poor distribution of the phosphonium ionic liquid in the polymer
explains the increase in stiffness due to the presence of aggregates and the decrease of the
strain at break. For the PTFE/C18P PF6-films, many well-dispersed ionic aggregates in the
matrix explain the increase in modulus and the higher decrease at the strain at break as
observed in ionomers [36].
II.2.2.4.3 Effect of strain rate on the fluorinated films
• Effect of ionic liquids on the crystallinity and the mechanical properties
Due to the fine dispersion of the phosphonium ionic liquid in the fluorinated film and
the excellent mechanical compromise achieved, the effects of strain rate were investigated on
the Young’s modulus and ultimate properties as well as on the morphology changes after
uniaxial stretching at a given strain. In fact, DSC measurements after deformation at different
strain rates are listed in Table II-6 and the mechanical properties are summarized in Table II-7.
Table II-6 – DSC data on fluorinated strained at different strain rate (5 and 250 mm/min)
Samples Tm (°C) Tc (°C) ∆Hm
(J/g) Xc (%)
PTFE without strain
326 310 32 39
PTFE 0.004 s-1 327 309 32 39 PTFE 0.2 s-1
329 308 40 49
PTFE C18P I-
without strain 328 307 28 34
PTFE C18P I- 0.004 s-1
328 307 42 51
PTFE C18P I- 0.2 s-1
329 308 49 60
Chapter II: Interactions Polymer/Ionic Liquids
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Table II-7 – Effect of the strain rate on tensile properties of the polymer films
Sample Tensile modulus (MPa)
Strain at break (%)
PTFE 0.004 s-1 65 180 PTFE 0.2 s-1 170 450
PTFE/C 18 P I - 0.004 s-1 170 522 PTFE/C 18 P I - 0.2 s-1 200 715
First of all, one can remember that the PTFE films are not oriented due to the
processing method, i.e. drying from a water-based solution followed by heating at 400°C. At
low strain rates, the melting enthalpy of the neat PTFE is unmodified, whereas as the
fluorinated film is strained at high strain rates (0.2 s-1), a crystallization under strain takes
place. Indeed, the high strain rate applied promotes the chain extension which leads to an
increase of crystallinity in the polymer. This increase in crystallinity ratio can be associated to
the increase of Young’s modulus from 65 at 170 MPa.
The addition of phosphonium ionic liquid in the matrix enhances the crystallization
under strain with an increase of 10% compared to the neat polymer film. This increase could
be due to a rearrangement of the ionic liquid phase in the polymer matrix. In fact, the ionic
interactions have a reversible character and the ionic liquid phase based on multiplet
aggregates could be reorganized continuously during strain. Regarding mechanical properties,
this slight increase in the modulus at high strain rate could reflect the effect of competition
between relaxation for the re-organization of ionic liquid phase and deformation.
II.2.2.4.4 Effect of ionic liquids on the morphology after deformation
For a better understanding of the material change during uniaxial tensile test, SAXS
analysis were performed on the PTFE films with and without IL after deformation reached at
different deformation rates as reported by Visser et al. for ionomers [37].
The SAXS images performed on unfilled PTFE and PTFE/C18P I- at different strain
rates, i.e. 0, 0.004, 0.2 s-1 are shown in Figure II-10.
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PTFE Initial State
PTFE/C 18P I--
Initial state 0.004 s-1
0.004 s-1 0.2 s-1
0.2 s-1
PTFE Initial State
PTFE/C 18P I--
Initial state
PTFE Initial State
PTFE/C 18P I--
Initial state 0.004 s-1
0.004 s-1 0.2 s-1
0.2 s-10.004 s-1
0.004 s-1 0.2 s-1
0.2 s-1
Figure II-10 – SAXS images of neat PTFE and PTFE/C18P I- under different strain rates
Without deformation, the isotropic character of the PTFE film due to the processing
method used is observed. As a strain rate (even low) is applied, a configuration with four leaf
clover is observed and in presence of ionic liquid, a different organization is noticed. Indeed,
without strain, the PTFE/C18P I- sample shows an isotropic behavior that is kept at low strain
rates. At a strain rate of 0.2 s-1, this phenomenon disappears in favor of an anisotropic
behavior oriented in six directions. As a consequence, the presence of ionic liquid as a
dispersed nanophase which is based on multiplet clusters could be used to modify the
deformation phenomena of PTFE due to the dynamic character of this phase under strain.
The transmission electron microscopy is used to evidence the morphology of the film.
Figure II-11 shows the different morphologies observed on the strain sample after uniaxial test
carried out at different strain rates.
Chapter II: Interactions Polymer/Ionic Liquids
Page 99
PTFE/C18P I-
Initial state 0.004 s-1 0.2 s-1PTFE/C18P I-
Initial state 0.004 s-1 0.2 s-1
200 nm 200 nm200 nm
Figure II-11 – TEM images of PTFE/C18P I- under different strain rates
After a low strain rate applied, TEM micrographs reveal that the IL nanodomains
which are initially organized as a co-continuous nanostructure (‘spider web’-type) collapse to
form large domains whereas for an higher strain rate, the IL co-continuous nanostructure is
kept and is oriented towards the axis of tension. This means that the relaxation of the IL
nanostructures has a relaxation time larger than the characteristic time of the deformation
process. This phenomenon could be very close to the ones observed by Visser and al [37] who
purposed for ionomers a model invoking ionic aggregate spatial rearrangement within the
polymer matrix. The authors also pointed out the role of the nature of ionic pairs on the
mechanical and deformation behaviour.
II.2.3 Conclusions
For the first time, ionic liquids were used like new building block to achieve a
structuration at nanoscale into a polymer film. We have clearly demonstrated that the effects
of the chemical nature of ionic liquid determined by a choice of the cation: pyridinium,
imidazolium versus phosphonium and the choice of the anion halide versus fluorinated play a
significant role on the structuration and the physical properties of the polymer. A suitable
combination between cation and anion leads to a nanoscale structuration of polymer with an
unprecedented flexibility and a stiffness dramatic improvement. These new building blocks
can be a new alternative in material field for various applications.
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 100
Conclusions of chapter II In this section, we have shown that the introduction of a low rate (1 wt%) of ionic
liquids in the polymer matrix has a behavior similar to that of ionomers. In fact, different
structuring of ILs, dependent on the chemical nature of the cation (pyridinium, imidazolium,
phosphonium) and anion (halide, fluorinated) were highlighted. Thus co-continuous
morphologies are obtained for pyridinium and imidazolium salts while an excellent dispersion
of phosphonium ionic liquid is observed.
Then, the influence of ionic liquids on the mechanical properties has been studied both
in static and dynamic mechanical or significant increases in modulus are observed. The
perfect distribution of the phosphonium IL in polymer film also provides increases in strain at
break. However, at high strain rates, a decrease of the effect of the ionic liquid is observed in
favor of the crystallization under strain.
Chapter II: Interactions Polymer/Ionic Liquids
Page 101
References of chapter II [1] S. Maiez-Tribut, J.P. Pascault, E.R. Soulé, J. Borrajo, R.J.J. Williams, Macromolecules (2007); 40:1268–1273. [2] A. Vermogen, S. Boucard, J. Duchet, K. Masenelli-Varlot, P. Prele, R. Seguela, Macromolecules (2005); 38:9661–9669. [3] J.Lu, F. Yan, J. Texter, Progress in Polymer Science (2009); 34:431–448. [4] W. Xie, Z. Gao, W.P. Pan, D. Hunter, A. Singh, and R. Vaia, Chem. Mater (2001); 13 (9):2979–2990. [5] S. Livi, J. Duchet-Rumeau, T.-N. Pham and J.-F. Gérard, J.colloid Interf Sci (2010); 349:424–433. [6] C.Byrne and T. McNally, Macromolecular Rapid Comm. (2007); 28:780–784. [7] W. H. Awad, J. W. Gilman, M. Nyden, R. H. Harris, T. E. Sutto, J. Callahan, P. C. Trulove, H. C. DeLong and D. M. Fox, Thermochimica Acta (2004); 409:3–11. [8] Q. Guo, J. Liu, L. Chen and K. Wang, Polymer (2008); 49:1737–-1742. [9] X. Yang, F. Yi, Z. Xin and S. Zheng, Polymer (2009); 50:4089–4100. [10] Z. Xu and S. Zheng, Polymer (2007); 48:6134–6144. [11] R. Yokoyama, S. Suzuki, K. Shirai, T. Yamauchi, N. Tsubokawa and M. Tsuchimochi, European Polymer
Journal (2006); 42:3221–3229. [12] L. Priya and J. P. Jog, Journal of Applied Polymer Science (2003); 89:2036–2040. [13] S. Pavlidou and C. D. Papaspyrides, Progress in Polymer Science (2008); 33:1119–1198. [14] S. Sharma and S. Komarneni, Applied Clay Science (2009); 42:553–558. [15] K. Stoeffler, P. G. Lafleur and J. Denault, Polymer Engineering & Science (2008); 48:1449–1466. [16] W. S. Wang, H. S. Chen, Y. W. Wu, T. Y. Tsai and Y. W. Chen-Yang, Polymer (2008); 49:4826–4836. [17] H.-K. Fu, C.-F. Huang, J.-M. Huang and F.-C. Chang, Polymer (2008); 49:1305–1311. [18] L. Li, B. Li, M. A. Hood and C. Y. Li, Polymer (2009); 50:953–965. [19] Z. Spitalsky, D. Tasis, K. Papagelis and C. Galiotis, Progress in Polymer Science (2010); 35:357–401. [20] S. Bose, R. A. Khare and P. Moldenaers, Polymer (2010); 51:975–993. [21] H. Vallette, L. Ferron, G. Coquerel, A.-C. Gaumont and J.-C. Plaquevent, Tetrahedron Letters (2004); 45:1617–1619. [22] A. Safavi and S. Zeinali, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2010); 362:121–126. [23] L. Xu, G. Ou and Y. Yuan, Journal of Organometallic Chemistry (2008); 693:3000–3006. [24] M. Rahman and C. S. Brazel, Polymer Degradation and Stability (2006); 91:3371–3382. [25] K. Park, J. U. Ha and M. Xanthos, Polymer Engineering & Science (2010); 50:1105–1110. [26] A. Vazquez, M. López, G. Kortaberria, L. Martín and I. Mondragon, Applied Clay Science (2008); 41:24–36. [27] H. He, J. Duchet, J. Galy, J.F. Gerard, J.colloid Interf Sci (2006); 295:202. [28] W. Xie, R. Xie, W.-P. Pan, D. Hunter, B. Koene, L.-S. Tan and R. Vaia, Chem Mater (2002); 14:4837–4845. [29] C.G Bazuin, A. Eisenberg, Ind. Eng. Chem. Prod. Res. Dev (1981); 16:41. [30] I. Capek, Adv. Coll. Interface Sci. (2005); 118:73. [31] A.R. Khokhlov, E.F. Dormidontova, Phys. Uspekhi (2005); 118:73. [32] I.A. Nyrkova, A.R. Khokhlov, Y.Y. Kramarenko, Polym. Sci. USSR (1990); 32:852. [33] A. Eisenberg, B. Hird, R.B. Moore, Macromolecules (1990); 23:4098. [34] I.A. Nyrkova, A.R. Khokhlov, M. Doi, Macromolecules (1993); 26:3601. [35] McCrum NG. J Polym Sci (1959);34:355–69. [36] R.F. Storey, D.W. Baugh, Polymer (2000); 41 (9):3205. [37] S.A. Visser, S.L. Cooper, Polymer (1992), 33:4705–4710.
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 102
Chapter III: Ionic Liquids as news intercalating agents for layered silicates
Page 103
Chapter III IONIC LIQUIDS AS NEWS INTERCALATING AGENTS FOR
LAYERED SILICATES Since the 80s and the early work done by Toyota on polyamide-montmorillonite
nanocomposites, the field of layered silicates nanocomposites is booming. In fact, with these
new materials, we seek to improve the thermal, mechanical or barrier properties with a very
low ratio of inorganic filler. The key parameter is the control of the distribution of individual
sheets, described as the state of exfoliation. Nevertheless, the lack of compatibility between
the hydrophilic clay and mostly hydrophobic polymers makes it difficult to obtain this state of
exfoliation. To circumvent this difficulty and improve the compatibility between clay and
polymer, the use of organic species denoted intercalating agents or surfactants, particularly
ammonium salts is necessary to reduce the surface energy and increase interlayer distances of
the layered silicates in order to promote the separation of layers to obtain an exfoliated
dispersion state more conducive to improving the final properties of nanocomposites.
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 104
Pages
III.1 A comparative study on ionic liquids used as surfactants: Effect on thermal and mechanical properties of high-density polyethylene nanocomposites ...................................... 105
III.1.2.1 Materials ................................................................................................................................. 107 III.1.2.2 Synthesis of phosphonium and imidazolium salts .................................................................. 107
III.1.2.2.1 Synthesis of octadecyltriphenylphosphonium salt ...................................................... 107 III.1.2.2.2 Synthesis of N-octadecyl-N'-octadecylimidazolium salt ............................................. 108
III.1.2.3 Organic modification of montmorillonite ............................................................................... 108 III.1.2.4 Processing and characterization of the HDPE/clay nanocomposites ...................................... 110
III.1.3 Results and discussion ............................................................................................................ 111 III.1.3.1 Characterization of modified montmorillonites ...................................................................... 111
III.1.3.1.1 Identification of interactions and effect of the washing .............................................. 111 III.1.3.2 Thermal stability of modified montmorillonites ..................................................................... 114 III.1.3.3 Structural analysis by WAXD ................................................................................................. 116 III.1.3.4 Surface energies of modified montmorillonites ...................................................................... 118 III.1.3.5 Influence of ionic liquid content ............................................................................................. 118
III.1.4 HDPE/clay nanocomposites ................................................................................................... 120 III.1.4.1 Thermal properties of nanocomposites ................................................................................... 120 III.1.4.2 Mechanical properties of nanocomposites .............................................................................. 121 III.1.4.3 Morphology of nanocomposites .............................................................................................. 122
III.2.3.1 Effect of supercritical carbon dioxide as a exchange solvent on the thermal degradation of the modified MMT ..................................................................................................................................... 127
III.2.3.1.1 Thermal stability of imidazolium-modified montmorillonite ..................................... 127 III.2.3.1.2 Thermal stability of phosphonium-modified montmorillonite .................................... 131
Conclusions of chapter III ............................................................................................................ 137
References of chapter III .............................................................................................................. 138
Chapter III: Ionic Liquids as news intercalating agents for layered silicates
Page 105
III.1 A comparative study on ionic liquids used as surfactants: Effect on thermal and mechanical properties of high-density polyethylene nanocomposites
Dialkyl imidazolium and alkyl phosphonium salts were synthesized to be used as new
surfactants for cationic exchange of layered silicates, such as montmorillonite (MMT). The
synthesized phosphonium (PMMT) or imidazolium ion (I-MMT)-modified montmorillonites
display a dramatically improved thermal degradation with respect to commonly used
quaternary ammonium salts. This thermal degradation window can still be shifted toward
higher temperatures after washing of modified clays. Two kinds of organic species can be
identified onto clay: physically adsorbed species versus chemically adsorbed species. To
Evidence the impact of these thermally resistant ionic liquids, the modified montmorillonites
were introduced in a great commodity polymer, i.e., high-density polyethylene. Thermoplastic
nanocomposites with a very low amount of nanofillers were processed in melt by twin screw
extrusion. If the thermal stability of polyethylene is slightly increased with only 2 wt.% of
thermostable made clays, the stiffness–toughness compromise is well improved since a strong
increase in modulus is achieved with both thermostable clays without loss of fracture
properties. But these mechanical performances are mainly obtained with unwashed
thermostable clays because the physically adsorbed organic species onto clay surfaces behave
like a compatibilizer that helps both the dispersion into the PE matrix and improves the
clay/matrix interface quality.
III.1.1 Introduction Although the clays have been recognized for a long time, the attention of academic
and industrial researchers was recently focused on organically modified clays as nanoscale-
reinforcing agents for polymer materials [1–3]. Indeed, the insertion of these lamellar fillers in
polymers can have significant effects not only on the mechanical [4] and barrier performances
[5,6] but also on ablation and flammability resistances [7] due to nanometric dimensions and
high aspect ratios of layered silicates and also due to synergism between polymer and
inorganic nanofillers. Among layered silicates, montmorillonite (MMT) is commonly used
[8]. Nevertheless, to ensure a good compatibility between montmorillonite and polymer
during the processing of nanocomposites, a surface modification of pristine MMT is required.
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 106
By exchanging sodium or calcium cations for organic cations, the surface energy of MMT
decreases and the basal spacing expands [9]. For such a purpose, cationic exchange using
alkylammonium salts is very often used [10,11].
Melt intercalation is by far the most promising method and is industrially preferred for
processing thermoplastic polymer (TP)-based nanocomposites as it can be performed without
use of solvents and by considering conventional tools for processing, i.e., the extrusion
process. However, the process involves working at high temperatures as melting of
conventional TPs requires temperatures above 190°C. Although the commonly used
ammonium salts have been gaining significant success in the processing of polymer/MMT
nanocomposites, a common shortcoming is their low thermal stability. The thermal
degradation of MMT modified by long-chain alkyl quaternary ammonium ions begins from
180°C as shown by TGA studies carried out by Xie et al. [12]. Indeed, the Hoffmann
degradation during processing described in the literature [13,14] could initiate/catalyze
polymer degradation and could affect the physical and mechanical properties of final
materials. To increase the thermal stability of organically modified clays, the use of more
thermally stable compounds, such as ionic liquids based on phosphonium and imidazolium
salts, can offer a new alternative to the ammonium salts [15,16]. These salts may be
considered as ionic liquids because their melting point is below 100°C, and their glass
transition temperature is around 10°C for phosphonium salt and 42°C for imidazolium salt.
Intensive thermal studies on imidazolium and phosphonium salts have shown a better
thermal stability than the alkyl ammonium cations [15,17,18]. Despite many benefits, few
studies using such cations for layered silicate intercalation have been reported in the literature
maybe because of the higher price of these surfactants compared to ammonium salts [19–21].
Moreover, commercially available thermally stable surfactants only incorporate short
aliphatic chains (up to C14).
In this work, efforts have been made to synthesize ionic liquids with long alkyl chains
based on imidazolium (two chains in C18) and on phosphonium salts with three benzyl
groups and only one long aliphatic chain (1 chain in C18). The long alkyl chains cause
expansion of the distance between the layers, and the aromatic groups help to generate a
better intercalation of the clay platelets because aromatic groups can be trapped in the
hexagonal cavities of layers. The thermal stability of the obtained organoclays was compared
with that of ammonium-modified clays. Each ammonium cation shows exactly the same
substituents as the imidazolium cation, either two linear octadecyl chains or an aromatic
group and an aliphatic chain for the phosphonium cation. Then, a high-density polyethylene
Chapter III: Ionic Liquids as news intercalating agents for layered silicates
Page 107
(HDPE) was selected to be mixed with lamellar silicates modified with highly thermally
stable ionic liquids. Finally, the morphology and the thermal and mechanical properties of
nanocomposites processed with the thermally stable ionic liquids were evaluated and
compared with ammonium-modified clay-based nanocomposites [22,23].
III.1.2 Experimental
III.1.2.1 Materials
A sodic montmorillonite, denoted Nanofil 757 (MMT), i.e., an aluminosilicate with
intercalated sodium was chosen as pristine clay and was provided by Süd Chemie (Germany).
The Nanofil 757 has a cation exchange capacity of 95 meq/100 g and is described by the
following formula Na0,65[Al,Fe]4Si8O20(OH)4. Two commercial organically modified
montmorillonites (Nanofil 15 and Nanofil 919) were purchased from Süd Chemie to obtain
reference organophilic clays. Both commercial organoclays are modified with quaternary
ammonium ion carriers of either tallow chains (Nanofil 15) or aromatic groups and tallow
chains (Nanofil 919). Tallow chains have the following composition: 65% C18, 30% C16,
and 5% C14. All chemicals necessary for the synthesis of ionic liquids, i.e.,
triphenylphosphine (95%), imidazole (99.5%), iodooctadecyl (95%), and all the solvents
(toluene, sodium methanoate, pentane and acetonitrile) were supplied from Aldrich and used
as received.
The polyethylene used in this study, called HDPE, is a high-density polyethylene from
Basell, with the trade name Hostalen GF 4750, showing a melt flow index of 0.4.
III.1.2.2 Synthesis of phosphonium and imidazolium salts
III.1.2.2.1 Synthesis of octadecyltriphenylphosphonium salt
In a 100-mL flask was, placed under a positive nitrogen pressure, 1 eq of
triphenylphosphine (5 g) and 1 eq octadecyl iodide (7.3 g). The stirred suspensions were
allowed to react for 24 h at 120°C in toluene (20 mL), and a yellow precipitate was formed.
The reaction mixture was then filtered and washed repeatedly with pentane. Most of the
solvent was removed under vacuum. The synthesis of salts was confirmed by 13C NMR
spectroscopy collected on a Bruker AC 250 (250 MHz) spectrometer. The assignment of 13C
NMR resonance peaks is reported below. 13C NMR (CDCl3): d 14.00 (CH3); 22.67 (CH2Me); 23.2; 29.37–29.66; 30.24; 31.85
(P–CH2); 118.45; 130.43; 133.70; 135.15 (P–C).
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 108
III.1.2.2.2 Synthesis of N-octadecyl-N'-octadecylimidazolium salt
A solution of sodium methoxide was prepared from 1 eq of sodium (0.465 g) using
dry, freshly distilled methyl alcohol (10 mL) in a sealed septum, 100 mL round-bottom, three-
necked flask equipped with a condenser, under nitrogen atmosphere and magnetic stirring.
Imidazole (1 eq, 1.37 g) diluted in acetonitrile (10 mL) was then added into the stirred
mixture of sodium methoxide previously cooled at room temperature. After 15 min, a white
precipitate was formed. The suspension was then concentrated under reduced pressure for 1 h.
The dried white powder was dissolved in acetonitrile, and a solution of octadecyl iodide (1 eq,
7.70 g) diluted in acetonitrile (10 mL) was then added under an inert atmosphere of nitrogen
at room temperature. The mixture was stirred for 1 h and then heated under reflux at 85 C for
about 24 h. A solution of octadecyl iodide (1 eq, 7.70 g) diluted in acetonitrile (10 mL) was
added to the mixture at room temperature. The stirred suspension was heated under reflux at
100 C for about 24 h leaving a brownish viscous oil in each case. After cooling to room
temperature, the solvent was removed by evaporation under vacuum, and the orange-colored
or beige solid was filtered, washed repeatedly with pentane, and dried. Purification of the
resulting imidazolium salts was accomplished by crystallization from ethyl
acetate/acetonitrile: 75/25 mixture. The assignment of 13C NMR resonance peaks is the
evidence of the success of the ionic liquid synthesis. 13C NMR (CDCl3): d 14.10 (2CH3); 22.67 (2CH2Me); 26.23; 28.97; 29.35–29.69;
The montmorillonite (2 g, 1.9 meq) was dispersed in 400 mL of deionized water. The
amount of surfactant added was about 2 CEC, based on the cation exchange capacity (CEC =
95 meq/100 g) of the MMT used [9]. This dispersion was mixed and stirred vigorously at
80°C for 6 h, followed by filtration and continuous washing at 80°C with deionized water
until no iodide ions were detected using an aqueous silver nitrate (AgNO3) solution. The
solvent was removed by evaporation under vacuum. The modified montmorillonite was then
dried for 12 h, at a suitable temperature (not greater than 80°C). The imidazolium,
phosphonium, and the quaternary ammonium ions used for the exchange reactions are
presented in Table III-1. The following abbreviations were used to design the different
Chapter III: Ionic Liquids as news intercalating agents for layered silicates
Page 109
montmorillonites: MMT-Na+ means the pristine montmorillonite. A phosphonium
montmorillonite denoted MMT-P was obtained when octadecyltriphenylphosphonium iodide
was used like surfactant. An imidazolium montmorillonite denoted MMT-I was obtained
when the N-octadecyl-N'-octadecylimidazolium iodide was used as an intercalation agent.
Both commercial montmorillonites modified with ammonium ions are MMT-DMDT for the
montmorillonite carrying a dimethyl ditallow quaternary ammonium as cation and MMT-
DMBT for the montmorillonite modified by a dimethyl (benzylmethyl) tallow quaternary
ammonium.
Table III-1 – Designation of pristine, commercial and synthesized ionic liquid modified montmorillonite
(MMT)
Designation Intercalant Trade name MMT-Na+ - Nanofil 757
MMT-DMDT
Nanofil 15
MMT-DMBT
Nanofil 919
MMT-I
MMT-P
N+
Tallow
CH3
TallowH3C
N+
H3C
H3C Tallow
P C18H37
I
N
N
C18H37
C18H37
I
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 110
III.1.2.4 Processing and characterization of the HDPE/clay nanocomposites
Nanocomposites were obtained by melt intercalation of modified montmorillonite into
a high-density polyethylene (2% by weight) using a twin screw DSM microcompounder. The
mixture was sheared for about 3 min with a 100 rpm speed at 190 C and injected in a 10-cm3
mold at 30 C to obtain dumbbell-shaped specimens. Different nanocomposite samples were
prepared by varying the surface treatment used to modify the montmorillonite and by
considering the nonwashed, exchanged MMT.
Thermogravimetric analyses (TGA) of organically modified clay and composites were
performed on a Q500 thermogravimetric analyzer (TA instruments). The samples were heated
from 30 to 800°C at a rate of 20 K min1 under nitrogen flow.
Surface energy of modified clays was determined with the sessile drop method on a
GBX goniometer. From contact angle measurements taken using water and diiodomethane as
test liquids on pressed modified clay disks, polar, and dispersive components of surface
energy were determined using the Owens–Wendt theory.
Bruker D8 Advance X-ray diffractometer at the H. Longchambon diffractometry
center. A bent quartz monochromator was used to select the Cu Ka1 radiation (k = 0.15406
nm) and run under operating conditions of 45 mA and 33 kV in Bragg–Brentano geometry.
The angle range scanned is 1–102h for the modified clays and 1–302h for the nanocomposite
materials.
Uniaxial tensile measurements were taken using a MTS 2/M electromechanical testing
system at 22 ± 1 C and 50 ± 5% relative humidity. Tensile tests were performed with a speed
of 10 mm min1.
The transmission electron microscopy (TEM) was carried out at the Center of
Microstructures (University of Lyon) on a Philips CM 120 field emission scanning electron
microscope with an accelerating voltage of 80 kV. The samples were cut using an
ultramicrotome equipped with a diamond knife, to obtain 60-nm-thick ultrathin sections.
Then, the sections were set on copper grids.
Chapter III: Ionic Liquids as news intercalating agents for layered silicates
Page 111
III.1.3 Results and discussion
III.1.3.1 Characterization of modified montmorillonites
III.1.3.1.1 Identification of interactions and effect of the washing
According to the literature [9,12,17,24], as a layered clay is modified with an organic
cation, two kinds of interactions between organic cations and inorganic clay can take place:
(i) Van der Waals bonds as the organic species are physically adsorbed on the clay surface.
(ii) Ionic bonds as the species are intercalated in the montmorillonite galleries.
The effect of washing allows identification of the interaction intensity, chemical
versus physical, linking the organic species to the layered silicate surface. After checking the
solubility of ionic liquids in different solvents, methyl alcohol was chosen as solvent for
washing. The effect of the washing is clearly shown on TGA analysis reported in Figure III-1.
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and MMT-I aw (after washing) (heating rate : 20 K.min-1). Three peaks of degradation are observed on the derivative curve of the weight loss
(DTG curve) of imidazolium-modified montmorillonite, before washing (called MMT-I bw).
After two successive washings (called MMT-I aw 2) with methanol, the first degradation peak
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 112
that corresponds to the species physically adsorbed on the montmorillonite surface almost
disappears for the benefit of the second peak. Unlike the first peak, the second increases
significantly. This can be explained by the gradual removal of salt excess on the clay surface
that creates a larger aperture of clay galleries allowing a higher quantity of the salt to be
intercalated between clay layers. Figure III-2 reports DTG curves obtained from the thermal
analysis realized on the phosphonium-treated montmorillonite before (bw) and after washing
(aw).
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Universal V4.2E TA Instruments Figure III-2 – Derivative of TGA curves (DTG) of the MMT-P bw (before
washing) and MMT-P aw (after washing) (heating rate: 20 K.min-1). The first weight loss still corresponds to a partial physisorption at the edges (bearing
polar SiOH groups) or on the external surface of the platelets since the peak decreases after
washing. The fact that it does not completely disappear means that a part of the ionic liquid is
well intercalated but in a peripheral position with respect to the clay gallery as reported by
Davis et al. [25] or is physisorbed from p–SiOH interactions at the edges of the lamellar
silicates. Such portion of surfactant cannot be washed away easily (since it underwent cationic
exchange), but it is not thermally stabilized by the presence of the inorganic silicate platelets
in a confined position. As a consequence, it degrades at the same temperature as the
physisorbed surfactant. On the other hand, the degradation which is evidenced at about 500°C
corresponds to the well-intercalated species between layers.
Chapter III: Ionic Liquids as news intercalating agents for layered silicates
Page 113
The same observation is made on the MMT-DMBT and the MMT-DMDT. A single
washing with methyl alcohol is enough to eliminate the species physically adsorbed. Table
III-2 summarizes the relative mass losses of the physically adsorbed species and the
intercalated species for the modified montmorillonites before and after washing.
Table III-2 – Relative mass loss of physically adsorbed and intercalated species measured by TGA on the modified montmorillonites
On the DTG curves of MMT-DMBT (Figure III-3a), the first peak corresponding to the
degradation of the species physically adsorbed on the surface of clay starts at 220°C, whereas
the phosphonium-modifiedmontmorillonite is still thermally stable at this temperature.
Indeed, in Figure III-3b, the first peak of degradation of the physisorbed species is at 340°C, a
a)
b)
Chapter III: Ionic Liquids as news intercalating agents for layered silicates
Page 115
temperature corresponding to the evaporation one of neat ionic liquid. In this case, a dramatic
difference of 120°C is observed. For the intercalated species in the montmorillonite galleries,
the same trend is observed. Whereas the degradation takes place at about 500°C for MMT-P,
the degradation of intercalated species is extended between 300 and 400°C for MMT-DMBT.
These results show clearly the better intrinsic thermal stability of phosphonium salt compared
with the ammonium salt [12], and the confinement has a similar effect on both salts, i.e., the
temperature shift from adsorbed state to confined state.
Second, the imidazolium montmorillonite and the montmorillonite bearing a dimethyl
ditallow quaternary ammonium as cation was also compared, as both have only two similar
long alkyl chains. Figure III-4 reports the TGA curves and their derivatives.
0 100 200 300 400 500 600 700 800
60
70
80
90
100
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eig
ht
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340°C
440°C
0 100 200 300 400 500 600 700 800
40
50
60
70
80
90
100
% W
eig
ht
Temperature (°C)
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0 100 200 300 400 500 600 700 800
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-0,06
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0,00
Deri
v.W
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ht
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)
Temperature (°C)
MMT-I
320°C
420°C
490°C
Figure III-4 – TGA and DTG curves of the MMT-DMBT (a) and MMT-P
(b) (before washing) (heating rate: 20K.min-1). By considering the thermal analysis carried out on MMT-DMDT and on MMT-I, the
difference is less significant. In Figure III-4a, a shoulder is observed at 270°C, followed by the
degradation of species intercalated at 340 and then at 440°C. In the case of MMT-I (Figure
III-4b), a first clear peak attributed to physically adsorbed species at 320°C is observed,
followed by the degradation of the species intercalated at 420 and 490°C. The increase in the
degradation temperature is much lower between the MMT-I and the MMTDMDT (50°C for
physisorbed species and 80°C for intercalated ones) because both alkyl chains as ligands
display a lower intrinsic thermal stability than benzyl groups. However, keep in mind that the
imidazolium or phosphonium-modified montmorillonites have a much better thermal stability
than the ammonium-treated ones.
a)
b)
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 116
III.1.3.3 Structural analysis by WAXD
The cationic exchange process is clearly detectable by X-ray diffraction as shown in
Figure III-5.
0 2 4 6 8 10
0
500
1000
1500
2000
2500
3000
3500
4000
Inte
nsity (
a.u
)
2θ
MMT-DMDT
d001
= 3.0 nm
0 2 4 6 8 10
0
200
400
600
800
1000
1200
Inte
nsity (
a.u
)
2θ
MMT-Id
001 = 3.7 nm
0 2 4 6 8 10
0
500
1000
1500
2000
2500
3000
3500
4000
Inte
nsity (
a.u
)
2θ
MMT-DMBT
d001
= 1.9 nm
0 2 4 6 8 10
400
450
500
550
600
650
700
750
800
850
Inte
nsity (
a.u
)
2θ
MMT-P
d001
= 4.2 nm
Figure III-5 – X-Ray diffraction spectra of ionic liquid modified MMT before washing:
(a) MMT-DMDT; (b) MMT-I; (c) MMT-DMBT; (d) MMT-P.
Before treatment, the basal spacing of the sodic montmorillonite is 1.2 nm, which
corresponds to the d-spacing of MMT-Na reported in the literature [9]. After organic
treatment in water by the imidazolium and phosphonium salts, the MMT-P displays a (0 0 1)
diffraction peak at 2.12h, corresponding to an interlayer distance of 4.2 nm. This value could
be explained by the swelling of layered silicates due to the steric volume occupied by the
three ring functions and the alkyl chain. For the MMT-I, the diffraction peak situated at 2.42h
is significant at a distance of 3.7 nm, a distance similar to one characteristic of a paraffinic
conformation with trans–trans positions of the alkyl chain. On the other hand, for the
montmorillonite modified with a ditallow quaternary ammonium, i.e., MMT-DMDT, the
lower intercalation distance of 3.0 nm is significant of a paraffinic chain tilted on the clay
surface. For the montmorillonite functionalized with a dimethyl benzyl tallow quaternary
ammonium, MMT-DMBT, the intercalation distance only of 1.9 nm is reduced by nearly half
because the organic chains adopt a pseudo trilayer conformation [26]. For the MMT-P and the
MMT-I, the spectra show intense, thin, and regular diffraction peaks that suggest a long-range
order.
Chapter III: Ionic Liquids as news intercalating agents for layered silicates
Page 117
The effect of washing reported in Figure III-6 was studied by X-ray diffraction on
different montmorillonites.
0 2 4 6 8 10
0
500
1000
1500
2000
2500
3000
3500
4000
Inte
nsity (
a.u
)
2θ
MMT-DMDT awd
001 = 2.4 nm
0 2 4 6 8 10
0
2000
4000
6000
8000
Inte
nsity (
a.u
)
2θ
MMT-I aw
d001
= 2.7 nm
0 2 4 6 8 10
0
500
1000
1500
2000
2500
3000
3500
4000
Inte
nsity
(a.u
)
2θ
MMT-DMBT aw
d001
= 1.9 nm
0 2 4 6 8 10
0
500
1000
1500
2000
2500
3000
Inte
nsity
(a.u
)
2θ
MMT-P aw
d001
= 2.1 nm
Figure III-6 – X-Ray diffraction spectra of ionic liquid modified MMT after washing:
(a) MMT-DMDT; (b) MMT-I; (c) MMT-DMBT; (d) MMT-P.
The washing step leads to a shift of diffraction peaks toward higher angles. The
intercalation distance decreases from 3.0 to 2.4 nm for the MMT-DMDT, from 3.7 to 2.7 nm
for the MMT-I, and from 4.2 to 2.1 nm for the MMT-P after washing with methyl alcohol.
Only the MMT-DMBT does not have any change in gallery height after washing. Washing
with methyl alcohol causes a reorganization of chains that explains the decrease in distances.
But in all cases, washing performed on MMT-P, MMT-I and MMT-DMDT does not induce
any decrease in the intercalation distance below 2.0 nm. This observation is corroborated by
the no change in distance obtained for the MMT-DMBT. As a result, the cationic exchange
leads to a part of organic cations that is really intercalated between the sheets inducing about a
2.0-nm gallery height, while another part of organic cations causes the clay swelling only due
to steric volume of the organic ligands.
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 118
III.1.3.4 Surface energies of modified montmorillonites
The contact angles and surface energy determined by the sessile drop method on
pressed powder are collected in Table III-3.
Table III-3 – Determination of polar and dispersive components of the surface energy on pristine and on exchanged montmorillonites from contact angles with water and diiodomethane (determination on pressed MMT powders) Montmorillonite Θwater (°) ΘCH2I2 (°) γ polar
Both ionic liquids based on phosphonium and imidazolium salts make the
montmorillonite more hydrophobic with a surface energy similar to the surface energy of a
polyolefin [27]. The polar components are very low which evidenced that the hydroxyl groups
are well covered by the organic chains. The steric hindrance of imidazolium and
phosphonium ionic liquids causes an efficient screen of the hydrophilic surface of lamellar
silicates. A stronger hydrophobic character can be obtained with the synthesized ionic liquids
compared to the ammonium cations. Hence, an efficient compatibility must be generated
between ionic liquid-modified nanolayers and polyethylene matrix.
III.1.3.5 Influence of ionic liquid content
The washing effect shows that such a large amount of cationic liquid corresponding to
2 CEC used for cationic exchange is useless for clay treatment. As a result, the ionic liquid
amount can be reduced during cationic exchange. Instead of adding 2 CEC of surfactants,
only 0.5 CEC was added to perform the cationic exchange. The TGA analysis performed
before the washing step on phosphonium-modified montmorillonite with two different
surfactant amounts (2 CEC versus 0.5 CEC) is reported in Figure III-7.
Chapter III: Ionic Liquids as news intercalating agents for layered silicates
Page 119
0 100 200 300 400 500 600 700 800
0
10
20
30
40
50
60
70
80
90
100
% w
eig
ht
Temperature (°C)
MMT-P bw2CEC
MMT-P bw 0,5CEC
Figure III-7 – Effect of the amount introduced of ionic liquid (2 CEC versus 0.5 CEC) used for cationic exchange on thermal stability measured by TGA
before the washing step (heating rate: 20K.min-1)
The similar thermal degradation for both exchange treatments highlights that a lower
amount of surfactant is sufficient to efficiently modify the clay and gives the clay a thermal
stability up to temperatures higher than 400 C. The X-ray diffraction is witnessed that the
amount of ionic liquid can be reduced since the same spectrum is obtained with a lower
surfactant content. The modified clay is characterized by the same intercalation distance as
shown in Figure III-8.
0 2 4 6 8 10
0
200
400
600
800
1000
1200
1400
1600
Inte
nsity (
a.u
)
2θ
MMT-P bw 2 CEC
MMT-P bw0.5 CEC
Figure III-8 – Effect of the amount introduced of ionic liquid (2 CEC versus 0.5 CEC) used for
cationic exchange on intercalation distance measured by WAXD before the washing step
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 120
III.1.4 HDPE/clay nanocomposites
III.1.4.1 Thermal properties of nanocomposites
The thermal behavior of the composites containing 2 wt.% of modified clay was
characterized by thermogravimetric analysis in order to study the effect of imidazolium and
phosphonium-treated montmorillonites on the thermal stability of the nanocomposites. The
thermogravimetric results carried out on polyethylene alone and the polyethylene filled with
modified montmorillonites are shown in Figure III-9.
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igh
t (%
)
0 100 200 300 400 500 600
Temperature (°C)
� PE-(MMT-I bw)� PE-(MMT-P bw)� PE� PE-MMT
Universal V4.2E TA Instruments Figure III-9 – TGA curves of nanocomposites based on polyethylene matrix filled with 2%wt. of sodic montmorillonite (PE-MMT) and of phosphonium (PE-(MMT-P bw)) or imidazolium (PE-(MMT-I bw)) modified montmorillonite (heating rate : 20 K.min-1).
The improvement in thermal stability is not tremendous. The chosen polyethylene is
already thermally very stable up to about 450°C, which corresponds to the temperature from
which starts the modified montmorillonite degradation. Moreover, the samples are analyzed
under inert gas. The addition of only 2 wt.% of the unwashed imidazolium-modified
montmorillonite in the polyethylene matrix does not improve the matrix thermal degradation.
With 2 wt.% of phosphonium-modified montmorillonite, the thermal degradation of the
polyethylene matrix can be only improved by 10°C. By introducing two times more
phosphonium-modified clay, i.e., 5 wt.%, delay in the thermal degradation is also doubled as
shown in Figure III-10.
Chapter III: Ionic Liquids as news intercalating agents for layered silicates
Page 121
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20
40
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We
igh
t (%
)
400 450 500 550 600
Temperature (°C)
� PE-(MMT-P) 5%� PE-(MMT-P) 2%� PE
Universal V4.2E TA Instruments Figure III-10 – TGA curves of nanocomposites based on polyethylene matrix filled with 2 and 5
Mechanical properties of the PE/clays nanocomposites containing 2 wt.% are detailed
in Table III-4.
Table III-4 – Effect of clay washing on tensile properties of the 2 CEC ionic liquid modified montmorillonite-high density polyethylene nanocomposites (2wt%) crosshead speed: 10mm.min-1
*bw: before washing aw : after washing
A strong increase in modulus is obtained when the polyethylene was prepared with
phosphonium or imidazolium-modified montmorillonite: an increase of 40% for the MMT-I
and 50% for the MMT-P. However, this stiffness is reduced when the nanocomposites are
prepared with washed clays or with 0.5 CEC-modified clay. In both cases, only an increase of
20% is observed for both organically modified clays. This decrease in modulus could be
associated with the removal of physically adsorbed species on the edges of the silicate layers
Figure III-12 – Evolution of weight loss as a function of temperature (TGA) and derivative of TGA curves (DTG) of the MMT-I bw (a, a’) and MMT-I CO2 bw (b, b’) (heating rate : 20 K.min-1; nitrogen atmosphere).
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 128
The first weight loss corresponds to a partial physisorption on the edges (bearing polar
SiOH groups) or on the external surface of the platelets since the peak decreases after
washing. The fact that it does not completely disappear after intensive washing is associated
to a part of ionic liquid is well intercalated but in a peripheral position respect to the clay
gallery as reported by Davis et al. [25] or is physisorbed from π-SiOH interactions at the
edges of the lamellar silicates. Such part of surfactant can not be washed away easily (since it
underwent cationic exchange) but it is no thermally stabilized by the presence of the inorganic
silicate platelets in a confined position. As a consequence, it degrades at the same temperature
as the physisorbed surfactant. On the other side, the degradation which is evidenced in a
temperature range from 400 to 500°C corresponds to the well intercalated species. Indeed,
organics inside clay galleries display higher temperatures of degradation. As reported in
previous works [31].
Indeed, on the MMT-I DTG curves, the first peak at 340°C corresponds to physical
adsorption onto clay surface and the second and third peak at about 420°C and 480°C are
related to the imidazolium ionic liquid species really intercalated between clay layers. The
same signature on TGA curve is clear evidence that the cationic exchange of montmorillonite
with imidazolium ionic liquid is possible using only supercritical carbon dioxide as a solvent.
Up to now, only phosphonium ionic liquids with much shorter alkyl were considered in
ScCO2 to modify MMT but working at very high pressures while using a small amount of co-
solvent [32]. The influence of a co-solvent on the modification of lamellar silicates in
supercritical carbon dioxide was also considered. However, there is a significant drawback,
i.e. the formation of sticky powders due to the presence of ionic liquid excess adsorbed on the
surface of montmorillonite having a very high viscosity. As a consequence, the easiest
solution is to reduce the amount of ionic liquid introduced into the autoclave from the use of a
co-solvent as increasing pressure (75 bars) up to reduce the viscosity of ionic liquid required.
According to the literature [33], the solubility of ionic liquid in supercritical CO2
remains extremely low and it is necessary to use organic co-solvents. Wu et al. [34] studied
the effects of organic solvents as acetone or ethanol in ScCO2. Large increase of the solubility
of the ionic liquid in ScCO2 was reported. This phenomenon is explained by strong interaction
of solvent with the ionic liquid due mainly to their high polarity. For this knowledge, we
selected the most polar solvent, i.e. water, that has the huge advantage of being a green
solvent (such as supercritical CO2 and ionic liquids) to design an environmentally sustainable
cationic exchange process. However, compared to organic solvents, it has the disadvantage of
Chapter III: Ionic Liquids as news intercalating agents for layered silicates
Page 129
being CO2-phobic but water could be used as a real co-solvent due to the fact that the
synthesized ionic liquids are soluble in water.
With water as co solvent, the thermal degradation of imidazolium modified
montmorillonite is quite different from one of the modified montmorillonite by conventional
cationic exchanges, i.e. in aqueous solution or ScCO2 medium process (Figure III-13).
Universal V4.2E TA Instruments Figure III-13 – Evolution of weight loss as a function of temperature (TGA) and
derivative of TGA curves (DTG) of the MMT-I bw (a, a’) and MMT-I (CO2 + Water) bw (b, b’) (heating rate : 20 K.min-1; nitrogen atmosphere).
The modified montmorillonite in supercritical carbon dioxide combined with co-
solvent shows both a much earlier degradation of physisorbed species (240°C versus 340°C)
but in the opposite a delayed degradation of intercalated species (540°C versus 420/490°C).
Washed with methyl alcohol that is the more suitable solvent of ionic liquids removes
the physically adsorbed species corresponding to the first degradation peak (Table III-6).
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 130
Table III-6 – Relative mass loss of physically adsorbed and intercalated species determined by TGA (imidazolium modified montmorillonites either in water or ScCO2.
Sample Cationic Exchange process
Physically adsorbed
species (%)
Intercalated species
(%) MMT-I bw Water
31 18
MMT-I aw - 23 MMT-ICO2 bw Supercritical
CO2
27 22 MMT-ICO2 aw - 25
MMT-I(CO2+Water)
bw Supercritical
CO2
(+ 10% water)
47 18
MMT-I(CO2+Water)
aw 9
33
Table III-6 summarizes the relative mass losses of the physically adsorbed species and
the intercalated species for the imidazolium modified montmorillonites either after water
solution or supercritical CO2 medium processes before and after washing with or without co-
solvent. One can observe that before and after washing, the results for imidazolium-modified
montmorillonite (MMT-I) and imidazolium-treated montmorillonite under supercritical CO2
(MMT-I (CO2 + Water)) are similar. In the case of MMT-I (CO2 + Water), when comparing with the
imidazolium modified MMT before washing with a standard cationic exchange process, the
weight percent of physisorbed species is significantly higher (a difference of 16%). We found
almost the same difference (10% instead of 16%) for the intercalated species. Thus, the use of
the supercritical CO2 leads to an increase of the intercalated species ratio up to 33%. This
means that the combined effect of supercritical CO2 and the solubility of imidazolium salts in
water play an important role on the intercalation process.
In order to have a better understanding of the role of the various components,
imidazolium ionic liquids were introduced in the autoclave, heated at 80°C under pressure of
70 bars for 6 hours. The melting temperature of the imidazolium ionic liquid after synthesis
denoted as C18I and after exposure to supercritical CO2, denoted as C18ICO2 are reported in
Table III-7.
Chapter III: Ionic Liquids as news intercalating agents for layered silicates
Page 131
Table III-7 – Effect of exposure to supercritical CO2 on the melting temperature of imidazolium ionic liquid at 80°C
Ionic liquid Exposure under ScCO2
Time (h)
Melting temperature
(°C) C18I 0 71
6 38 C18ICO2 24 33
After 6 hours in supercritical carbon dioxide, a strong decrease of the melting point is
observed. Which remains about the same as the exposure time increases, this effect could be
explained by the presence of ScCO2 which remains soluble in the ionic liquid. In conclusion,
there is a solubility limit of the ionic liquid in the CO2 phase.
According to the literature reported on the influence of supercritical CO2 on ionic
liquids [35, 36], this phenomenon was also observed for example by Kazarian et al. [37] who
reported that the melting point of imidazolium ionic liquids based on a C16 chain and a
fluoride anion was reduced from 25°C after a ScCO2 treatment under a pressure of 70 bars.
Later, another study on the phosphonium and ammonium salts [38] showed in the both cases
an important decrease of 100°C but at higher pressure of exposure (150 bars). Recently,
Scurto et al. [39] concluded that CO2 interacted with the ionic liquid due to the establishment
of weak Lewis acid-Lewis base interactions between basic moieties of the organic salt and the
acidic carbon of CO2.
In conclusion, the decrease of the melting temperature of the ionic liquid after
exposure to the the supercritical CO2 in the presence of water as a co-solvent in which ionic
liquid is soluble allows an optimal cationic exchange and a better intercalation of imidazolium
salt in the clay layer galleries.
III.2.3.1.2 Thermal stability of phosphonium-modified montmorillonite
The same approach was considered for the modification of montmorillonite with
phosphonium ions. The cationic exchange was performed in water under atmospheric
pressure but also in supercritical carbon dioxide. The TGA analysis performed on
phosphonium-modified MMT before washing with methyl alcohol are reported in Figure
III-14.
Ionic Liquids : Multifunctional agents of the polymer matrices
Universal V4.2E TA Instruments Figure III-15 – Effect of ScCO2 combined with water as co-solvent on the thermal degradation of phosphonium-modified montmorillonite by TGA : the derivative of the TGA curves MMT-P (a, a’) bw versus MMT-PCO2+ Water (b, b’) bw (heating rate : 20 K.min-1, nitrogen atmosphere).
With the presence of a co-solvent, a shift of intercalated species is observed (570°C
versus 510°C) whereas the physically adsorbed species are the same degradation behavior
Chapter III: Ionic Liquids as news intercalating agents for layered silicates
Page 133
(320°C versus 330°C). The DSC data realized on phosphonium ionic liquid after synthesis
and after treatment with ScCO2 shows the same behavior in Table III-8.
Table III-8 – Effect of exposure to supercritical CO2 on the melting temperature of phosphonium ionic liquid at 90°C
Ionic liquid Exposure under ScCO2
Time (h)
Melting temperature
(°C) C18P 0 85
6 57 C18PCO2 24 55
The melting temperature of the phosphonium salt is significantly reduced after
treatment with supercritical carbon dioxide (28°C), in the same order of magnitude than for
imidazolium-based ionic liquid. We can expected that, according to the melting temperature
depression, the solubility of the two types of exchanged-montmorillonites ionic liquids in
ScCO2 are similar.
III.2.3.2 Structural analysis
III.2.3.2.1 Imidazolium modified montmorillonite
The effect of the cationic exchange process on the MMT intercalation was studied by X-
ray diffraction and reported in Figure III-16.
0 1 2 3 4 5 6 7 8 9 10
0
2000
4000
6000
8000
10000
12000
14000
16000
18000
20000
22000
24000
26000
28000
30000
32000
34000
Inte
nsity (
u.a
)
2θθθθ
(a)
(b)
(c)
(a)
(b)
(c)
Figure III-16 – Effect of the cationic exchange process on the interlayers distance
measured by X-Ray diffraction spectra of phosphonium ionic liquid modified MMT: (a) MMT-I; (b) MMT-ICO2; (c) MMT-I(CO2 + Water)
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 134
After organophilic treatment by a conventional cationic exchange, the MMT-I displays
a (001) diffraction peak at 2.3°2θ, corresponding to an interlayer distance of 3.7 nm, distance
similar to one characteristic of a paraffinic conformation with trans-trans positions of the alkyl
chain. The CO2 step leads to a shift of diffraction peak towards lower angles with an
interlayer distance of 4.1 nm, corresponding to diffraction peak at 2.1°2θ. The use of water as
a co-solvent leads to reduce the effects of swelling by ionic liquid which is solubilized in
water. The interlayer distance was found to be similar to one of the imidazolium-modified
montmorillonite performed by conventional cationic exchange.
III.2.3.2.2 Phosphonium modified montmorillonite
Figure III-17 shows the X-ray diffraction spectra performed on MMT-P, MMT-PCO2 and
MMT-P(CO2+water).
0 1 2 3 4 5 6 7 8 9 10
0
500
1000
1500
2000
2500
3000
Inte
nsity (
u.a
)
2θθθθ
(a)
(b)
(c)
(a)
(b)
(c)
Figure III-17 – Effect of the cationic exchange process on the interlayers distance
measured by X-Ray diffraction spectra of phosphonium ionic liquid modified MMT: (a) MMT-P; (b) MMT-P CO2; (c) MMT-P(CO2 + Water)
Before treatment, the basal spacing of the sodic montmorillonite is 1.2 nm, which
correspond to the d-spacing of MMT-Na+ reported in literature [7]. After organic treatment in
water by the phosphonium salt, the MMT-P displays a (001) diffraction peak at 2.1°2θ,
corresponding to an interlayer distance of 4.2 nm. This value could be explained by the
swelling of layered silicates due to steric volume of the three ring structure and the alkyl
chain. For the MMT-PCO2, the diffraction peak located at 1.8°2θ, i.e. indicating a d001 of 4.9
nm, a slight increase in the interlayer distance can be explained by the ionic liquid swelling
Chapter III: Ionic Liquids as news intercalating agents for layered silicates
Page 135
under the effect of supercritical CO2. The diffraction spectrum displays also a small peak at
3.1°2θ, corresponding to physically phosphonium salt adsorbed on the surface of
montmorillonite. By using water as co-solvent, the diffraction peak is located at 2.0°2θ, i.e.
indicating a distance of 4.4 nm for MMT-P(CO2 + Water), close to the one of the phosphonium-
treated MMT by conventional cationic exchange. In all the cases, the spectra show intense,
thin, and regular diffraction peaks which suggest a modification on a long range order.
The resulting structure of the ionic liquid-modified montmorillonites, i.e. the d001
distances, as well as the amount of intercalated ionic liquids species can be explained from the
solubility parameters of the various components : ionic liquids, water, and supercritical CO2
the spreading coefficient. In fact, as a conventional route is used, i.e. water solution of ioni
liquid as exchange process medium, the intercalation of imidazolium or phosphonium alkyl-
modified species proceeds from the well-known exchange process described for quaternary
ammonium intercalants [40-41]. As supercritical CO2 is used as medium for intercalation, the
driving force is the spreading of ionic liquid species onto the MMT surface as the surface
polarity of montmorillonite matches the surface tension (or solubility parameter) of the ionic
liquid better than the ScCO2 medium. In the later case, i.e. involving water as co-solvent, as
this one is not soluble in ScCO2, the ionic liquids remain in the water phase leading to a
highly concentrated ionic liquids-water phase, which spreads onto the polar montmorillonite
surface. As a consequence, the process involving water as co-solvent is similar to the
conventional water-solution based protocol.
The solubility of ionic liquids in supercritical CO2 became of a charge interest as such
an understanding requires modelling approaches [42-44] which could be used for practical
purposes [45]. Melting temperature of the phosphonium salt is significantly reduced after
treatment with supercritical carbon dioxide (28°C), in the same order of magnitude than for
imidazolium-based ionic liquid. We can expected that, according to the melting temperature
depression, the solubility of the two types of exchanged-montmorillonites ionic liquids in
ScCO2 are similar.
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 136
III.2.3.3 Surface energies
The contact angles and surface energy determined by the sessile drop method on
pressed powder are collected in Table III-9.
Table III-9 – Determination of polar and dispersive components of the surface energy on pristine and on exchanged montmorillonites from contact angles with water and diiodomethane (determination on pressed MMT powders)
MMT-P ScCO2 co 81.5 ±0.1 45.5 ±0.7 3.6 36.8 40.4 MMT-I ScCO2 co 125.2 ±0.5 41.9 ±0.7 3.6 38.5 42.1
Both ionic liquids based on phosphonium and imidazolium salts make the montmorillonite
more hydrophobic with a surface energy close to the surface energy of a polyolefin [28]. The
polar components are very low which is an evidence that the hydroxyl groups are well
covered by the organic species, especially C18 chains. The steric hindrance of imidazolium
and phosphonium ionic liquids causes an efficient screening of the hydrophilic surface of
lamellar silicates.
For imidazolium and phosphonium-modified montmorillonites under supercritical
carbon dioxide without co solvent, the values are identical. Whereas the montmorillonites
MMT-P ScCO2 co and MMT-P ScCO2 co, i.e. by using water as a co-solvent, the values of
surface energy are slightly higher.
III.2.4 Conclusions In this work, we demonstrated that it is possible to modify lamellar silicates by ionic
liquid phosphonium and imidazolium using solvents as water and supercritical CO2. The
resulting properties are improved thermal stability of intercalated species and better
intercalation between the layers of montmorillonite. This process can be improved and
requires many additional studies on the interactions between different components which are
considered. Nevertheless, this study highlights that several solvents, such as water, ionic
liquids, and supercritical CO2, which are among the most promising components of green
chemistry, could be used relevant surface treatment of layered minerals.
Chapter III: Ionic Liquids as news intercalating agents for layered silicates
Page 137
Conclusions of chapter III
In Chapter III, the surface treatment of layered silicates by intercalating agents like
ammonium, imidazolium and phosphonium has been studied in two ways: the cation
exchange method involving the use of conventional organic solvents and a method
environmentally friendly using supercritical CO2 as solvent.
Firstly, better thermal stability of ILs on the conventional ammonium has been
demonstrated. Indeed, increases of 50°C to 120°C are observed for physisorbed and
intercalated species. In addition, the amount physically adsorbed on the surface of
montmorillonite is compatibilizing agent between the filler and the polymer which results in
thermal and mechanical properties increased.
In a second step, we showed that using supercritical CO2 associated with water as co-
solvent leads to a decrease of melting temperatures of ILs important which leads to a better
intercalation of imidazolium ILs and phosphonium between the clay layers with a sharp
increase of the degradation temperature.
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References of chapter III [1] E.P. Giannelis, Adv. Mater (1996); 8:29. [2] P.C. Le Baron, Z. Wang, T.J. Pinavaia, Appl. Clay Sci. (1999); 15:11. [3] Y.W. Mai, Z.Z. Yu, Polymer Nanocomposites, Woodhead, Cambridge, (2006). [4] L. Le Pluart, J. Duchet, H. Sautereau, Polymer 46 (2005); (26):12267. [5] E. Jacquelot, E. Espuche, J.F. Gerard, J. Duchet, P. Mazabraud, J. Polym. Sci. Part B: Polym. Phys. (2006); 44 (2):431. [6] M.A.Osman,V.Mittal,M.Morbidelli, U.W. Suter, Macromolecules (2003); 36:9851. [7] J.W. Gilman, C.L. Jackson, A.B. Morgan, R. Harris, E. Manias, E.P. Giannelis, Chem. Mater. (2000); 12:1866. [8] S.M. Auerbach, K.A. Carrado, P.K. Dutta, Handbook of Layered Materials, Taylor & Francis, New York, (2004). [9] L. Le Pluart, J. Duchet, H. Sautereau, J.F. Gérard, J. Adhes. (2002); 78 (7):645. [10] S.Y. Lee, W.J. Cho, K.J. Kim, J.H. Ahn, M. Lee, J.colloid Interf Sci. (2005); 284 (2):667. [11] H. He, J. Duchet, J. Galy, J.F. Gerard, J.colloid Interf Sci. (2006); 295:202. [12] W. Xie, Z. Gao, W.P. Pan, D. Hunter, A. Singh, R. Vaia, Chem. Mater. (2001); 13 (9):2979. [13] T.D. Fornes, P.J. Yoon, H. Keskkula, D.R. Paul, Polymer (2001); 42:9929. [14] J.W. Gilman, Appl. Clay Sci. (1999); 15:31. [15] W.H. Awad, J.W. Gilman, M. Nyden, R.H. Harris, T.E. Sutto, J. Callahan, P.C.Trulove, H.C. DeLong, D.M. Fox, Thermochim. Acta (2004); 409:3. [16] J. Zhu, A.B. Morgan, F.J. Lamelas, C.A. Wilkie, Chem. Mater. (2001); 13:3774. [17] W. Xie, R. Xie, W.P. Pan, D. Hunter, B. Koene, L.S. Tan, R. Vaia, Chem. Mater. (2002); 14 (11):4837. [18] H.L. Ngo, K. Le Compte, L. Hargen, A.B. McEven, Thermochim. Acta (2000); 97:357–358. [19] F.A. Bottino, E. Fabbri, I.L. Fragala, G. Malandrino, A. Orestano, F. Pilati, Macromol. Rapid Commun. (2003); 24:1079. [20] J.W. Gilman, W.H. Awad, R.D. Davis, J. Shields, R.H. Harris, C. Davis, Chem. Mater. (2002); 14:3776. [21] V. Mittal, Eur. Polym. J. (2007); 43:3727. [22] C. Lotti, C.S. Isaac, M.C. Branciforti, R. Alves, S. Liberman, R. Bretas, Eur. Polym. J. (2008); 44:1346. [23] S. Filippi, C. Marazzato, P. Magagnigni, A. Famulari, P.V. Arosio, S. Meille, Eur. Polym. J. (2008); 44:987. [24] W. Xie, Z. Gao, W.P. Pan, D. Hunter, A. Singh, R. Vaia, Thermochim. Acta (2001); 339:367–368. [25] R.D. Davis, J.W. Gilman, T.E. Sutto, Clay Clay Miner. (2004); 52 (2):171. [26] F. Bergaya, B.K.G. Theng, G. Lagaly, Handbook of Clay Science, first ed., Elsevier, (2006). [27] C.M. Hansen, A. Beerbower, Kirk-Othmer Encyclopedia of Chemical Technology, second ed., Interscience, New York, (1971). p. 889. [28] S. Boucard, J. Duchet, J.F. Gérard, P. Prele, S. Gonzalez, Macromol. Symp. (2002); 194 (1):241. [29] Chigwada, G., D. Wang, et al, Polym Degrad. and Stab. (2006); 91(4):848–855. [30] S.P. Nalawade, F. Picchioni, L.P.B.M. Jansen, Prog. Polym. Sci. (2006); 31 19–43. [31] S. Livi, J. Duchet-Rumeau, T.-N. Pham and J.-F. Gérard, J.colloid Interf Sci (2010); 349:424–433. [32] E. Naveau, C. Calberg, C. Detrembleur, S. Bourbigot, C. Jérôme, M. Alexandre, Polymer (2009); 50:1438–1446. [33] S. Keskin, D. Kayrak-Talay, U. Akman, Ö. Hortaçsu, J. of Supercritical Fluids (2007); 43:150–180. [34] W.Wu, J.Zhang, B.Han, J.Chen, Z.Liu, T.Jiang, J.He, W.Li, Chem.Comm. (2003); 1412–1413. [35] V. Najdanovic-Visak, A. Serbanovic, J.M.S.S. Esperança, H.J.R. Guedes, L.P.N. Rebelo, M.Nunes da Ponte, Chem.Phys.Chem. (2003); 4:520. [36] M. Roth, J. of Chromatography, A (2009); 1216:1861–1880. [37] S.G. Kazarian, N. Sakellarios, C.M. Gordon, Chem. Comm. (2002); 1314. [38] A.M. Scurto, W. Leitner, Chem. Commun. (2006); 3681. [39] A.M. Scurto, E. Newton, R.R. Weikel, L. Drauker, J.Hallett, C.L. Liotta, W. Leitner, C.A. Eckert, Ind. Eng. Chem. Res. (2008); 47:493. [40] C.B. Hedley, G. Yuan, B.K.G. Theng, Applied Clay Science. (2007); 35:180–188. [41] A. Vasquez, M. Lopez, G. Kortaberria, L. Martin, I.Mondragon, Applied Clay Science. (2008); 41:24–36. [42] X. Ji, H. Adidharma, Fluid Phase Equilibria. In press, Accepted Manuscript. [43] J. Kumelan, A. Perez-Salado Kamps, I. Urukova, D. Turna, J.Chem. Thermodynamics. (2005); 37 (6):595–602.
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Page 139
[44] J.S. Torrecilla, J. Palomar, J. Garcia, E. Rojo, F.Rodriguez, Chemiometrics & Intelligent Laboratory
Chapter IV POLYMER/LAYERED SILICATES NANOCOMPOSITES In this last chapter, we have used these montmorillonites thermally stable during the
preparation of nanocomposites by melt intercalation in two different matrices, high density
polyethylene (HDPE) and polyvinylidene fluoride (PVDF). The influence of ligand, the
variation of chain length, the functionalization of perfluorinated chains, the role of the
imidazolium versus phosphonium cation, and the anion (Br-, I-, PF6-) have been studied on
thermal, physical and mechanical properties as well as on the morphology of the resulting
nanocomposites.
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 142
Pages
IV.1 Synthesis of new surfactants: Effect of the ionic liquids on the thermal stability and the mechanical properties of high density polyethylene nanocomposites ................................ 143
IV.1.2.1 Materials ................................................................................................................................. 144 IV.1.2.2 Processing and characterization of HDPE/clay nanocomposites ............................................ 145 IV.1.2.3 Synthesis of imidazolium and phosphonium salts .................................................................. 146
IV.1.2.3.1 Synthesis of octadecylphosphonium bromide and iodide 1a-1b ................................. 146 IV.1.2.3.2 Synthesis of octadecylphosphonium hexafluorophosphate 1c .................................... 147 IV.1.2.3.3 General procedure for the synthesis of N-alkyl-N’-alkyl imidazolium salts 3a-3c. .... 148
IV.1.2.4 Organic modification of montmorillonite ............................................................................... 149 IV.1.2.4.1 Preparation of phosphonium-MMT ............................................................................ 149 IV.1.2.4.2 Preparation of imidazolium-MMT .............................................................................. 150
IV.1.3 Results and discussion ............................................................................................................ 150 IV.1.3.1 Thermal stability of ionic liquids ............................................................................................ 151 IV.1.3.2 Thermal stability of ionic liquid modified montmorillonites .................................................. 153 IV.1.3.3 Surface Energy of ionic liquid modified montmorillonites ..................................................... 154 IV.1.3.4 PE/modified-montmorillonites nanocomposites ..................................................................... 155
IV.1.3.4.1 Morphology of the nanocomposites ........................................................................... 155 IV.1.3.4.2 Thermal stability of the nanocomposites .................................................................... 157 IV.1.3.4.3 Mechanical properties of the nanocomposites ............................................................ 158
IV.2.2.2.1 Synthesis of ILs with long alkyl chains ...................................................................... 162 IV.2.2.2.2 Synthesis of imidazolium functionalized with perfluorinated chain ........................... 163
IV.2.3.1 Characterization of ILs exchanged montmorillonites ............................................................. 164 IV.2.3.1.1 Thermal stability of ionic liquid-modified montmorillonites ..................................... 164 IV.2.3.1.2 Structural analysis of ionic liquid-modified montmorillonites ................................... 166 IV.2.3.1.3 Surface energy of ionic liquid-treated montmorillonites ............................................ 167
IV.2.3.2 Effect of interfacial interactions on the material physical properties ...................................... 168 IV.2.3.2.1 On the morphology of the PVDF nanocomposites ..................................................... 168 IV.2.3.2.2 Crystallinity of PVDF based nanocomposites ............................................................ 170 IV.2.3.2.3 Mechanical properties of PVDF based nanocomposites ............................................. 174
IV.1 Synthesis of new surfactants: Effect of the ionic liquids on the thermal stability and the mechanical properties of high density polyethylene nanocomposites
Ionic liquids based on alkyltriphenyl phosphonium and dialkyl imidazolium surfactant
salts with long alkyl chains have been synthesized and used as intercalating agents for the
preparation of highly thermally stable organophilic montmorillonites. These new surfactants
behave as conventional organic cations and are easily swollen inducing a high d-spacing.
Thermoplastic nanocomposites with a very low amount of nanofillers have been processed by
melt mixing using a twin screw extruder. The thermal stability of the phosphonium- (MMT-P)
or imidazolium- (MMT-I) modified montmorillonites has been enhanced by up to 100°C
compared with conventional quaternary ammonium cations, making melt mixing of such
modified nanoclays possible with high density polyethylene (HDPE) processed at high
temperature.
IV.1.1 Introduction Recent research dedicated to the introduction of layered silicate-(montmorillonite,
MMT) into polymer matrices demonstrates an increase of thermal stability [1], mechanical
properties [2-4], and reduced flammability [5-7] for numerous types of polymer-clay
nanocomposites. The lamellar and confined structure of inorganic layers in polymer matrix
and the nanoscale dimensions of particles could explain changes in polymer physics, such as
molecular mobility and thermal resistance. This later one is of course also associated to the
thermal stability of the surface modifiers, i.e. interfacial agents. In fact, increasing the thermal
stability of montmorillonite and resulting nanocomposites is a key issue in the developement
of polymer-clay nanocomposites at the industrial scale.
The limited thermal stability of the conventionally used alkylammonium cations [8]
intercalated into layered minerals and the degradation occurring during processing of some
thermoplastic polymers (for example polyolefins) in the presence of nanodispersed MMT
have motivated the development of improved organophilic treatments for layered silicates.
Such new organomodifiers should enable the preparation of polymer/layered silicate
nanocomposites based on polymers that require high melt-processing temperatures [9-10]
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 144
and/or long residence times under high shear as well as for thermoset reactive systems which
are cured at high temperatures.
Ionic liquids (ILs) are organic salts with melting temperature below 100°C. They are
known for their excellent thermal stability, non-flammability, low vapor pressure and high
ionic conductivity. Their properties can be tuned by different associations of cations and
anions. The cation is usually a bulky organic structure with a low symmetry, such as
pyridinium, imidazolium, phosphonium, and ammonium ions. The current research mainly
focused on Room Temperature Ionic Liquids (RTILs) composed of asymmetric N-N
dialkylimidazolium cations bearing short alkyl chains such as butyl-methyl or ethyl-methyl
functionalization. The original part of this work results in the synthesis of ILs functionalized
with long alkyl chains suitable for improving the compatibility and the dispersion of
nanolayered silicates within polyolefin matrix, i.e. hydrophobic medium.
The aim of this study is to synthesize new thermally stable intercalating agents and to
find the most relevant combination of cation and anion to design an ionic liquid efficient to
get organophilic montmorillonites and to improve the physical properties of resulting
nanocomposite. The effect of the chemical nature of cation, imidazolium versus
phosphonium, the influence of the alkyl chain length, i.e. octadecyl versus docosyl, for
imidazolium salts and the effect of the chemical nature of anion, i.e. halide versus fluorinated
for phosphonium salts on the morphology, the thermal, and mechanical properties of high
density polyethylene-based nanocomposites will be investigated.
IV.1.2 Experimental
IV.1.2.1 Materials
A sodic montmorillonite, denoted Nanofil 757 (MMT), i.e. an aluminosilicate with
intercalated sodium was chosen as pristine clay. This one was provided by Süd Chemie Co.
The Nanofil 757 has a cation exchange capacity of 95 meq/100 g and is described by the
following formula Na0,65[Al,Fe]4Si8O20(OH)4. All chemicals necessary to the synthesis of
ionic liquids, i.e. triphenylphosphine (95%), imidazole (99.5%), 1-iodooctadecane (octadecyl
bromide 95%), 1-bromodocosane, (docosyl bromide 96%) and solvents (toluene, methanol,
pentane and acetonitrile) were supplied from Aldrich and used as received.
The polyethylene used in this study, denoted HDPE, is a high-density polyethylene
from Basell, with the trade name Hostalen GF 7260 showing a melt flow index of 0.4.
3c Figure IV-2 – Synthesis of the imidazolium ionic liquids 3a-3c
A solution of sodium methoxide was prepared from sodium (1 equiv.) in dry freshly
distilled methyl alcohol (10 mL) in a sealed septum, 100 mL round-bottomed, threenecked
flask equipped with a condenser, under nitrogen atmosphere and magnetic stirring. Imidazole
(1 equiv.) diluted in acetonitrile (10 mL) was then added into the stirred mixture of sodium
methoxide previously cooled at room temperature. After 15 min, a white precipitate was
formed. The suspension was then concentrated under reduced pressure for 1 h. The dried
white powder was dissolved in acetonitrile and a solution of alkyl halide RX (1 equiv.) diluted
in acetonitrile (10 mL) was then added under an inert atmosphere of nitrogen at room
temperature. The mixture was stirred for 1 h, then heated under reflux at 85 °C for about 24 h.
A solution of alkyl halide R’X (1 equiv.) diluted in acetonitrile (10 mL) was added to the
mixture at room temperature. The stirred suspension was heated under reflux at 100 °C for
about 24 h leaving a brownish viscous oil in each case. After cooling to room temperature, the
solvent was removed by evaporation under vacuum, the beige coloured powder was filtered,
washed repeatedly with pentane and dried. Purification of the resulting imidazolium salts was
accomplished by crystallization from ethyl acetate/acetonitrile: 75/25 mixture. After drying,
alkyl imidazolium salts 3a-3c were fully characterized by spectroscopy. The assignment of 13C NMR spectroscopy resonance peaks is an evidence of the success of the ionic liquid
Figure IV-4 – Effect of the alkyl chain length on the imidazolium ionic liquids: evolution of the weight loss as a function of temperature (TGA, DTG) of the (●) C18C22Im, (■) C22C22Im, (♦) C18C18Im
(heating rate : 20 K.min-1, under nitrogen atmosphere)
The imidazolium cation with C18-C18 alkyl ligands is thermally stable up to 320°C.
Nevertheless, a slight decrease in thermal stability of imidazolium ionic liquid is shown when
the alkyl chain length is increased. With C22-C22 alkyl chains, the weight loss takes as soon as
the thermal analysis starts even if the maximal degradation temperature is slightly higher.
Under oxidative atmosphere, Awad et al [16] also concluded that the thermal stability
decreased as the organic content of the molten salt increased. According to the literature [9],
at high temperature, i.e. 600°C, the imidazole ring is thermally resistant during the thermal
rearrangements of dialkylimidazolium ions which explains that the imidazolium ionic liquids
are not fully degraded.
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The chemical nature of cation, imidazolium versus phosphonium, has no really an effect
on the thermal stability. With the iodide anion, the ionic liquids based on phosphonium cation
display a degradation at the same temperature as one determined for imidazolium cation, i.e.
about 320°C, as reported in Figure IV-5.
The effect of the chemical nature of the anion, halide anions (I-), (Br-) versus fluorinated
anion (PF6-), on the thermal stability of the phosphonium ionic liquids C18P is presented in
Figure IV-5 – Effect of the anion chemical nature associated to phosphonium ionic liquids: evolution of the weight loss as a function of temperature (TGA, DTG) of the
(●) C18P Br-, (■) C18P I-, (♦) C18P PF6-
(heating rate : 20 K.min-1, under nitrogen atmosphere).
The combination of the anion associated with the organic cation has a significant role on
the thermal stability of ionic liquid. Indeed, the use of hexafluorophosphate anion (PF6-)
combined with the phosphonium cation provides an increase of temperature about one
hundred and forty degree celsius compared to the halide salts C18P I- and C18P Br- that are
degraded at the same temperature, about 320°C. This use of fluorinated anion in order to
enhance the thermal stability of the ionic liquids is not new. Awad et al [9] had demonstrated
that the use of anion type PF6- and BF4
- associated to imidazolium salt increased the thermal
stability of one hundred degrees. Regarding the phosphonium ionic liquid, the literature [17,
18] has already reported the lower thermal stability of ionic liquid associated to bromide
anion compared to one obtained in presence of tetrafluoroborate anion.
Figure IV-6 – Effect of the alkyl chain length and the halide anion associated to the imidazolium cation : evolution of the weight loss as a function of temperature (TGA, DTG) of the
Figure IV-7 – Effect of the anion chemical nature associated to phosphonium cation: evolution of the weight loss as a function of temperature (TGA, DTG) of the
(●) MMT-P Br-, (■) MMT-P PF6- (♦) MMT-P I-
(heating rate : 20 K.min-1, nitrogen atmosphere).
Regarding the TGA analysis reported in Figure IV-7 on the phosphonium-modified
montmorillonites with bromide (Br-) and iodide (I-) conteranions, the same degradation is
observed with two main degradation peaks. The first weight loss at 330°C still corresponds to
a partial physisorption on the external surface of the platelets. On the other hand, the
degradation which is evidenced at about 500°C corresponds to the well intercalated species
between clay layers. In the case of MMT-P PF6-, the use of hexafluorophosphate anion causes
a shift measured at 80°C at higher temperatures (410°C instead of 330°C) of the degradation
peak corresponding to physisorbed species.
In conclusion, the montmorillonites modified with the imidazolium and phosphonium
ionic liquids display all both an excellent thermal stability up to nearly 500°C if the
physisorbed species are removed by washing which is a huge advantage for the processing of
polymer/clay nanocomposites at high temperatures.
IV.1.3.3 Surface Energy of ionic liquid modified montmorillonites
To evaluate the interactions able to be generated by the modified montmorillonite
towards the polymer matrix as a function of the alkyl chain length and of the anion nature, the
contact angles and surface energies were determined by the sessile drop method on pressed
powder and the values are collected in (Table IV-1).
Table IV-1 – Determination of polar and dispersive components of the surface energy on pristine and on exchanged montmorillonites from contact angles with water and diiodomethane (determination on pressed MMT powders) Montmorillonite Θwater (°) ΘCH2I2 (°) γ polar
Figure IV-10 – Evolution of weight loss as a function of temperature (TGA, DTG) of the (●) PE/MMT-C18C18Im, (■) PE/MMT-C22C22Im (♦) Neat PE, (○) PE/MMT-C18C22Im
(heating rate : 20 K.min-1, under nitrogen atmosphere).
A very low amount (only 1 wt%) of modified-montmorillonites with imidazolium
ionic liquids is enough to increase the thermal stability of the polyethylene matrix. Indeed, the
thermal decomposition is delayed of 10°C by the addition of imidazolium ionic liquid for the
MMT-C18C18Im and MMT-C18C22Im. A more significant improvement (+ 15°C) is observed
with imidazolium ionic liquid C22C22Im. These results are promising and can be enhanced by
using a larger amount of modified clay for designing PE-nanoclay nanocomposites.
Ionic Liquids : Multifunctional agents of the polymer matrices
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The thermal degradation of nanocomposites processed with phosphonium-treated
Table IV-2 – Tensile properties of the ionic liquid-modified montmorillonites/high density polyethylene nanocomposites at room temperature (10 mm.min-1)
IV.2.2.2.2 Synthesis of imidazolium functionalized with perfluorinated chain
1 equiv. of methylimidazole (3g) and 1 equiv. of 1-Iodo-1H,1H,2H,2H-
perfluorododecane (7.3 g) were placed in a 100 mL flask under a positive nitrogen pressure,
and magnetic stirring for 72 h at 120°C in toluene (40mL). A yellow precipitate was formed.
The reaction mixture was then filtered, washed repeatedly with pentane. Most of the solvent
was removed under vacuum. The synthesis of salts was confirmed by 1H NMR. 1H NMR (CDCN): δ 2.79 (CH2-CF2); 3.79 (CH3N-); 4.45 (CH2CH2N-); 7.35 (CH); 7.42
(CH); 8.60 (N-CH=N).
IV.2.2.3 Organic modification
The montmorillonite (2 g, 1.9 meq) was dispersed in 400 mL of deionised water. The
amount of surfactant added was about 2 CEC, based on the cation exchanged capacity (CEC =
95 meq/100 g) of the MMT used [12]. This dispersion was mixed and stirred vigorously at
80°C for 6 h, followed by filtration and continuous washing at 80°C with deionised water
until no iodide ions were detected using an aqueous silver nitrate (AgNO3) solution. The
solvent was removed by evaporation under vacuum. The modified montmorillonite was then
dried for 12 hours at a temperature below 80°C. The following references were used to denote
the different types of modified montmorillonites: MMT-Na+ for the pristine montmorillonite,
MMT-P for phosphonium montmorillonite as octadecyltriphenylphosphonium iodide was
used as interfacial agent. An imidazolium montmorillonite, denoted MMT-I, was obtained as
N-octadecyl-N’-octadecylimidazolium iodide was used as intercalation agent. MMT-IC12F
denotes the montmorillonite exchanged with imidazolium functionalized with the
perfluorinated chain. The chemical structure of synthesized phosphonium and imidazolium salts
are described in Table IV-3.
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Page 164
Table IV-3 – Pristine and ionic liquid-modified montmorillonites (MMT) Trade name
References
Intercalant
Nanofil 757
MMT-Na+
MMT-I
MMT-P
PC18H37
I
MMT-IC12F
N NC18H37
I
C18H37
N N(CH2)2(CF2)9CF3
I
H3C
IV.2.3 Results and discussion
IV.2.3.1 Characterization of ILs exchanged montmorillonites
IV.2.3.1.1 Thermal stability of ionic liquid-modified montmorillonites
The thermal stability of organically modified montmorillonites was characterized by
Thermogravimetric Analysis (TGA). Figure IV-12 displays the evolution of the weight loss as a
function of temperature performed on the three montmorillonites exchanged either with
imidazolium and phosphonium cations functionalized with long alkyl chains or with
imidazolium cation functionalized with perfluorinated chain (C12). The degradation
temperatures of physically adsorbed and intercalated species are summarized in Table IV-4.
Table IV-4 – Degradation temperatures of the physisorbed and intercalated species for the treated montmorillonites (determined from the maxima of the weight loss by temperature derivative)
Sample Physisorbed temperature
(°C)
Intercalated temperature
(°C) MMT-P 330 510 MMT-I 320 420/480
MMT-IC12F 280 460
As reported in a previous study [11], imidazolium-treated and phosphonium-modified
montmorillonites, MMT-I and MMT-P, have an excellent thermal stability, which can be
useful for processing nanocomposites at high temperatures. Indeed, the organic species,
physically adsorbed on the clay surface are decomposed at around 320-330°C, whereas the
thermal degradation of the species ionically exchanged between the clay layers is delayed in a
range between 420 and 510°C. In the case of MMT-IC12F, the use of a perfluorinated chain
associated with the methylimidazole ring does not induce a better thermal stability. On the
opposite, a decrease of the thermal stability is observed with two degradation peaks at about
280°C for the physisorbed species and 460°C for the intercalated species. This phenomenon
could be attributed to the volatilization of short fluorinated chains.
IV.2.3.1.3 Surface energy of ionic liquid-treated montmorillonites
The contact angles and surface energy determined by the sessile drop method on
pressed clay powder are collected in Table IV-5.
Table IV-5 – Polar and dispersive components of the surface energy on pristine and exchanged montmorillonites from contact angles with water and diiodomethane (determination on pressed MMT powders)
These DSC analyses are in agreement with X-ray diffraction results since the use of
ionic liquids based on phosphonium or imidazolium cations functionalized by a fluorinated
chain as interfacial agents leads to lower crystallinity ratios and higher melting temperatures
associated to the formation of β phase. This result could provide a new perspective on the use
of ionic liquids in the field of membranes as β crystalline form contribute to better dielectrical
and thermal properties. Moreover, additional improvement could be achieved due to the
presence of clay nanoplatelets.
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Page 174
IV.2.3.2.3 Mechanical properties of PVDF based nanocomposites
In addition to the characterization of the morphology and resulting crystallinity of
nanomaterials based on IL-modified silicate nanolayers, the mechanical properties were
analyzed by performing uniaxial tensile tests described in Table IV-8.
Table IV-8 – Tensile properties of the ionic liquid/ poly(vinylidene fluoride) blends and ionic liquid modified montmorillonites- poly(vinylidene fluoride) nanocomposites at room temperature (10 mm.min-1)
Sample Young’s modulus (MPa)
Strain at break (%)
PVDF 940 36 PVDF-P PVDF-I
760 700
30 37
PVDF-MMT 900 30 PVDF/MMT-P 750 32 PVDF/MMT-I 800
80
PVDF/MMT-I C12F 800 250
The values of the moduli and strain at break are not governed by the nature of the
crystalline forms of PVDF, i.e. α or β. In fact, the imidazolium cation has the same
plasticizing effect on the mechanical properties as it is functionalized with alkyl chains that
promotes the α form or with a fluorinated chain that promotes the β form. The phosphonium
cation that generates the β form has the same plasticizing effect and does not improve the
mechanical behavior of PVDF. The mechanical properties are mainly dependent on the
material morphology. The stiffness is less reduced and the failure properties are slightly
enhanced for the nanocomposite showing an intercalated morphology obtained with the ionic
liquids based on long alkyl chains imidazolium cation. The excellent compromise obtained
between stiffness and strength at break is achieved on the nanocomposites containing the
montmorillonite modified with perfluoroalkylimidazolium, i.e MMT-IC12F. As a
consequence, the exfoliated structure is the right morphology to get the best mechanical
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Ionic Liquids : Multifunctional agents of the polymer matrices
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Conclusion générale
Page 179
CONCLUSION GENERALE L'objectif premier de ce travail a été de valoriser et de mettre en avant les effets
bénéfiques et les différentes possibilités proposées par les liquides ioniques dans le domaine
des polymères que ce soit en tant qu'agents renforçants, plastifiants ou encore agents
interfaciaux pour les silicates lamellaires.
Dans un premier temps, nous avons mis en évidence pour la première fois la
structuration des liquides ioniques dans une matrice polymère. Nous avons ainsi démontré que
la nature chimique du cation organique: pyridinium, imidazolium ou phosphonium ainsi que
l'effet de l'anion jouent un rôle essentiel sur les différentes structurations obtenues. En effet,
nous avons observé des morphologies différentes allant de la formation d'agrégats de clusters
ioniques en ce qui concerne l'utilisation du liquide ionique pyridinium (C18Py) à une
excellente dispersion à l'échelle du nanomètre dans le cas du phosphonium (C18P) en passant
par une morphologie co-continue pour l'imidazolium (C18C18Im). Les relations
morphologies/propriétés physiques et mécaniques ont également été établies. Ainsi, nous
avons constaté que le liquide ionique phosphonium le mieux dispersé dans la matrice fluorée
mène à une nette amélioration des propriétés mécaniques du matériau avec une augmentation
du module et de la déformation à la rupture de +160 et +190% respectivement. Les analyses
SAXS et MET ont contribué à une meilleure connaissance de la structuration du matériau
sous sollicitation.
Dans un second temps, l'utilisation des liquides ioniques comme agents intercalants en
remplacement des ammoniums conventionnels a également été discuté. Nous avons ainsi
démontré une meilleure stabilité thermique des argiles modifiées par les liquides ioniques ce
qui permet d'élargir le champ d'utilisation des silicates lamellaires dans le domaine des
nanocomposites thermoplastiques/argiles nécessitant des températures de mise en oeuvre plus
élevées. Nous avons aussi développé un procédé propre de modification des surfaces des
silicates lamellaires basé sur la combinaison CO2 supercritique-eau-liquide ionique avec pour
résultats une amélioration de la stabilité thermique des argiles organiquement modifiées par
les liquides ioniques.
Ionic Liquids : Multifunctional agents of the polymer matrices
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Dans une dernière partie, nous avons montré les effets de ces charges organiquement
modifiées sur les propriétés thermiques et mécaniques de nanocomposites préparés par
intercalation à l'état fondu. Ainsi, nous avons observé que l'influence de la longueur des
chaînes et de l'anion joue un rôle crucial sur la stabilité thermique intrinsèque des liquides
ioniques ainsi que sur les propriétés thermiques et mécaniques des polymères, notamment
dans le cas d'une polyoléfine (PEhd). Ensuite dans une matrice fluorée, le polyfluorure de
vinylidène (PVDF), nous avons démontré que l'utilisation d'un imidazolium fonctionalisé par
une chaîne perfluorée a un effet similaire à celui du cation phosphonium sur la structure
polymorphe du PVDF c'est à dire que leur utilisation en très faible quantité engendre la
formation de la phase β, favorable aux propriétés diélectriques ce qui offre de nouvelles
perspectives dans le domaine de l'énergie et en particulier dans celui des membranes de piles
à combustible. Une exfoliation des feuillets d'argiles dans la matrice polymère ainsi qu'une
augmentation de la déformation à la rupture est également obtenue dans le cas de la
montmorillonite modifiée par l'imidazolium fluoré.
Néanmoins, le coût et l'accessibilité aux liquides ioniques désirés limitent
considérablement leur utilisation dans les polymères. C'est pour ces raisons qu'il est
nécessaire d'intensifier les recherches sur les liquides ioniques afin d'apporter de la
compréhension et de prouver que les avantages des liquides ioniques sont beaucoup plus
importants que leurs inconvénients. Ce travail n'est qu'un aperçu du vrai potentiel des liquides
ioniques en science des polymères. En effet, les différentes combinaisons cations/anions ainsi
que les différentes fonctionnalisations possibles nous laissent penser qu'il est concevable en
théorie de synthétiser une infinité de liquides ioniques à propriétés spécifiques en fonction de
la matrice sélectionnée.
FOLIO ADMINISTRATIF
THESE SOUTENUE DEVANT L'INSTITUT NATIONAL DES SCIENCES APPLIQUEES DE
LYON
NOM : LIVI DATE de SOUTENANCE : … … 2010 Prénoms : Sébastien TITRE :
IONIC LIQUIDS : MULTIFUNCTIONAL AGENTS OF THE POLYMER MATRICES NATURE : Doctorat Numéro d'ordre : 2010-ISAL- Ecole doctorale : Matériaux de Lyon Spécialité : Matériaux Polymères et Composites Cote B.I.U. - Lyon : T 50/210/19 / et bis CLASSE : RESUME : Une excellente stabilité thermique, une faible pression de vapeur saturante, une ininflammabilité, une bonne conductivité ionique ainsi que les différentes combinaisons cations/anions possibles font des liquides ioniques l'objet d'un engouement grandissant de la Recherche. De part ces avantages, les LI se présentent comme une nouvelle voie dans le domaine des polymères, et en particulier dans le milieu des nanocomposites où leur utilisation est essentiellement limitée à la fonction de surfactant des silicates lamellaires. Néanmoins, avant de pouvoir prétendre à un statut d'alternative, il est nécessaire de mettre en évidence les effets bénéfiques de leur utilisation sur les propriétés finales des matériaux polymères. Dans un premier temps, l’objectif de ce travail a été de synthétiser des liquides ioniques différents par la nature de leur cation et anion mais tous porteurs de longues chaînes alkyles afin de permettre une meilleure compatibilité avec la matrice. Ensuite, les excellentes propriétés intrinsèques des liquides ioniques ont motivé leur utilisation comme agents structurants dans une dispersion aqueuse fluorée. Ainsi, leur rôle d’agents ioniques sur la morphologie, les propriétés physiques, thermiques et mécaniques a été étudié. Dans une seconde partie, les liquides ioniques ont été utilisés comme agents intercalants des silicates lamellaires puis confrontés aux surfactants conventionnels dans le but de préparer des argiles thermiquement stables pour la préparation de nanocomposites thermoplastiques/argiles. Dans une dernière partie, une faible quantité de ces argiles organiquement modifiées ont été introduites par intercalation à l'état fondu dans deux matrices différentes afin de mettre en évidence les effets de ces nouveaux agents interfaciaux sur les propriétés finales du matériau. MOTS-CLES : Liquides ioniques ; Nanocomposites ; Silicates lamellaires ; Agents structurants ; CO2 supercritique Laboratoire (s) de recherche : Institut des Matériaux Polymères / Laboratoire des Matériaux Macromoléculaires UMR 5223 INSA de Lyon Directeurs de thèse: Jannick DUCHET-RUMEAU – Jean- François GERARD Président de jury : … Composition du jury : DUCHET-RUMEAU Jannick Professeur (INSA Lyon) – Directrice de thèse GALY Jocelyne DR CNRS (INSA Lyon) – Examinateur GANTILLON Barbara Dr (Société TEFAL) – Examinateur GERARD Jean-François Professeur (INSA Lyon) – Co-directeur de thèse PHAM Thi Nhàn Maître de Conférences (Université de Caen) – Examinateur PLUMMER John Christopher Professeur (Ecole Polytechnique Fédérale de Lausanne) – Rapporteur SEGUELA Roland DR CNRS (Université de Lille 1) – Rapporteur