Poly(siloxane-urethane) crosslinked structures obtained by sol-gel technique

Poly(siloxane-urethane) Crosslinked StructuresObtained by Sol-Gel Technique

MIHAELA ALEXANDRU,1 MARIA CAZACU,1 MARIANA CRISTEA,1 ALEXANDRA NISTOR,1

CRISTIAN GRIGORAS,1 BOGDAN C. SIMIONESCU1,2

1‘‘Petru Poni’’ Institute of Macromolecular Chemistry, Aleea Gr. Ghica Voda 41 A, Iasi 700487, Romania

2Department of Natural and Synthetic Polymers, ‘‘Gh. Asachi’’ Technical University of Iasi, Iasi 700050, Romania

Received 3 December 2010; accepted 26 January 2011

DOI: 10.1002/pola.24602

Published online in Wiley Online Library (wileyonlinelibrary.com).

ABSTRACT: Poly(siloxane-urethane) crosslinked structures were

prepared from isophorone diisocyanate, a,x-bis(hydroxybutyl)-oligodimethylsiloxane and a new hybrid diol containing hydro-

lysable SiAOC2H5 groups besides OH groups. The latest was

synthesized by the acid-catalyzed reaction between 1,3-bis(3-gly-

cidoxypropyl)tetramethyldisiloxane and 3-aminopropyltriethoxy-

silane. The formations of the urethane groups along the

polymer backbone as well as the formation of the silica

domains were first confirmed by the presence of the specific

bands in Fourier transform infrared spectra. The resulted materi-

als were characterized using differential scanning calorimetry,

thermogravimetric analysis and scanning electron microscopy.

The results of the dynamic mechanical analysis (DMA) per-

formed at various frequencies revealed shape memory capabil-

ities for some of the obtained structures. The silica formed

because of the hydrolysis-condensation reactions proved to have

reinforcing effect upon siloxane-urethane structure also evi-

denced by DMA and increasing water vapor sorption capacity as

was measured by dynamic vapor sorption. VC 2011 Wiley Periodi-

cals, Inc. J Polym Sci Part A: Polym Chem 49: 1708–1718, 2011

KEYWORDS: crosslinking; polysiloxanes; silicas

INTRODUCTION Polyurethanes can be considered as one ofthe most versatile polymer materials classes.1 Because of thecapacity to adjust polyurethanes’ molecular structuresaccording to specific property requirements, different typesof polyurethanes can be synthesized and used in a variety ofapplications such as elastomers,2 flexible and rigid foams,3

medical devices,4 adhesives, and coatings.5 Polyurethanesconfer high performance to polymeric materials due to theirtoughness, abrasion resistance, mechanical flexibility, andchemical resistance.6

It is well known that in general, polyurethanes combine rigidhard and flexible soft segments. Depending on the types andcompositions of the two phases as well as the preparationprocedures, the relationships between structure and proper-ties of polyurethanes are extremely diverse and easily con-trolled.7,8 The hard segment is formed from diisocyanatemoieties, whereas the soft segment results from the chainextenders that generally are flexible polyols9 of low molecu-lar weight such as poly(tetramethylene oxide), poly(ethyleneoxide), poly(propylene oxide), poly(monoethylene adipate),polybutadiene and poly(dimethylsiloxane), which contributeto flexibility and elasticity. The hard segment alternatesalong the chain with soft segment and produces distinguish-ing physical and mechanical properties for polyurethane.Because of the differences between chemical structures of

hard and soft segments an incompatibility between thesetwo components often appears that leads to phase separa-tion.10 Different characterization techniques have been usedto study the microstructure of these materials, such as: X-raydiffraction, thermal analysis, birefringence studies, mechani-cal analysis, electron microscopy, light scattering, Fouriertransform infrared (FTIR) spectroscopy, NMR and so on.These techniques have provided important information onthe characteristics of polyurethanes.

Hard segments can link themselves through hydrogen bond-ing and crystallization, making the polyurethane very solidbelow melting temperature. Reversible phase modification ofsoft segment leads to shape memory effect11 and can be con-trolled by molecular weight of soft segment, molar ratiobetween hard and soft segments, polymerization process12,13

usually noticed at a temperature close to Tg.

The unique properties of polyurethanes depend on theirtwo-phases microstructure, where the hard domain acts asreinforcing filler and as a multifunctional crosslinkingsite.14,15 Many researchers investigated polyurethanes asnew materials to improve the permselectivity for gas separa-tion.16–18 Polyurethanes are also considered shape memorypolymers (SMP), and have been extensively researched fromthis point of view since their discovery by Mitsubishi in1988.19,20 Today, shape memory polyurethane can be used

Correspondence to: M. Alexandru (E-mail: [email protected])

Journal of Polymer Science Part A: Polymer Chemistry, Vol. 49, 1708–1718 (2011) VC 2011 Wiley Periodicals, Inc.

1708 WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

as medical device or biological microelectromechanical sys-tems, smart fabrics, deformable handle, and others. Most ofresearches had the physically crosslinking of segmented poly-urethanes as main goal. This process is used to memorizethe original shape and to retain the dimensional stabilityduring deformation while the entangled or crystallizedmolecular chains of soft segments are used as a reversiblephase.21,22 The relaxation process of polyurethane chains canbe minimized by introducing ionic groups23/mesogen units24

into the hard segments, by the crosslinking of hard andsoft segments,25 or through the control of soft segmentarrangement.26

A combination between polyurethanes and polysiloxanes,both well known as versatile-multipurpose materials, hasattracted special attention due to the advantages: a goodthermal stability and high flexibility at low temperaturesgiven by the siloxane part and better mechanical strengthand abrasion characteristics contributed by polyurethane.The incorporation of PDMS increases the difference betweenthe solubility parameters of the hard and soft segments.27

It have been studied the influence of the silica in situ formedthrough sol-gel process using tetraethoxysilane (TEOS) onthe shape memory effect and mechanical properties of poly-urethane. The authors found an important relationshipbetween this type of properties and TEOS content. A goodshape fixation and a shape recovery of more than 80% werereported for polyurethane-silica samples.28

Xu et al.29 provided a new direction in obtaining shape mem-ory systems based on polyurethane with mechanical proper-ties and shape memory easy to adjust. The SiAOASi bondsformed through hydrolysis and condensation of SiAOC2H5

have an important reinforcement effect in restricting the mo-bility of chains, the fact proved by the Tg values. The samplesincorporating a high amount of diol-containing hydrolysablegroups have lower shape fixing and faster shape recoveryspeed. The excellent shape memory behaviors of theobtained materials recommend them for applications inbiomechanics.

Following the same direction as above, but using differentdiols, in this article, we prepared a series of poly(siloxane-urethane) cross-linked structures by silica domains, whichmay not only act as the net points for dimension stabilitybut also act as inorganic fillers for reinforcement. The poly(siloxane-urethane) crosslinked structures have been charac-terized from the aspects of surface morphology and thermaland dynamic mechanical properties.

EXPERIMENTAL

Materials1,3-Bis(3-glycidoxypropyl)tetramethyldisiloxane (GPTMD) (pu-rity > 95%, b.p. ¼ 184 �C, d204 ¼ 0.996); 3-aminopropyltrie-thoxysilane (APTES) (purity > 99%, b.p. ¼ 213–216 �C, d204 ¼0.949); dimethylformamide (DMF) (purity > 99.8%, b.p. ¼152–154 �C, d204 ¼ 0.950, n20D ¼ 1.430) freshly distilled beforeuse; a,x-1,3-bis(hydroxybutyl)tetramethyldisiloxane, HB0, (pu-rity � 97%, b.p. ¼ 148 �C, d204 ¼ 0.93); isophorone diisocya-

nate (IPDI) obtained from Fluka (purity � 97%, b.p. ¼ 153 �C,d204 ¼ 1.061); octamethylcyclotetrasiloxane, D4, (purity > 99%,b.p. ¼ 175 �C, d204 ¼ 0.955, n20D ¼ 1.396) were obtained fromFluka. Purolite CT-175, a styrene-divinylbenzene ion exchangerwith -SO3H groups (4.1 mequiv g�1) was dehydrated by azeo-trope distillation with toluene and vacuumation at 110 �C/10mmHg. Dibuthyltin dilaurate (DBTDL), d204 ¼ 1.055) wasreceived from Merck-Schuchardt.

EquipmentsFTIR spectra were recorded on a Bruker Vertex 70 FTIRspectrometer. The analyses were performed in transmissionmode, in the 400–4100 cm�1 range, at room temperaturewith 2 cm�1 resolution and accumulation of 32 scans. Theground samples were incorporated in dry KBr and processedas pellets.

Thermogravimetric measurements (TG) were performed inthe 0–750 �C temperature range at a heating rate of 10 �Cmin�1 in air using a Q-1500D System.

The thermal transitions of the hybrid materials were deter-mined with a Pyris Diamond differential scanning calorime-try (DSC), power compensated differential calorimeter typefrom Perkin Elmer. The scans were performed on a tempera-ture range of �150 � 100 �C at a heating rate of 20 �Cmin�1. Helium gas was purged through the cells at 20 mLmin�1 to assure an inert atmosphere and good thermal con-ductivity. Before measurements, the DSC instrument was cali-brated for temperature and energy scale using n-hexane andpure water as recommended standards for LN2 range of DSCanalysis. The glass transition temperature was calculated asa midpoint of the heat capacity of the sample from secondheating scan.

The dynamic mechanical analysis (DMA) tests were con-ducted on a PerkinElmer Diamond apparatus, in tensionmode, under nitrogen atmosphere, from –150 �C to 250 �C,at 1 Hz and 2 �C min�1. The films had the dimensions of10 � 10 � 0.3 mm3. Multifrequency scans (0.5, 1, 2, 5, and10 Hz) were made in the same temperature range, with thesame heating rate.

Morphology of the films in fracture was investigated byScanning Electron Microscope type Quanta 200 operating at20kV with backscattering electrons in low vacuum mode.Before analysis, the films were covered with a thin layer ofgold by sputtering (EMITECH K550X).

Water-vapor sorption capacity of the film samples was meas-ured by using the fully automated gravimetric analyzer IGA-sorp supplied by Hiden Analytical, Warrington (UK). Anultrasensitive microbalance measures the weight change asthe humidity is modified in the sample chamber at a con-stant regulated temperature. The measurement system iscontrolled by a user-friendly software package.

Synthesisa,x-Bis(hydroxybutyl)oligodimethylsiloxanea,x-Bis(hydroxybutyl)oligodimethylsiloxane (HB) (Mn ¼ 740)was prepared by acid equilibration of the HB0 with D4. The

ARTICLE

POLY(SILOXANE-URETHANE) CROSSLINKED STRUCTURES, ALEXANDRU ET AL. 1709

cation-exchanger, Purolite CT-175 was used as catalyst (2.5wt % reported to the reaction mixture). The equilibrationwas performed at 70 �C, 4 h after which, the catalyst wasseparated by filtration. The reaction mixture was devolatil-ized by heating at 150 �C/5 mmHg. The obtained oligomerhas a viscous oil aspect. The molecular weight of theoligomer was estimated by 1H NMR spectra based on the ra-tio between the intensity of the peaks assigned to protonsfrom dimethyl (at 0.08 ppm) and those from butyl (0.54–3.63 ppm) units (Fig. 1).

Hybrid Diol1,3-Bis(3-glycidoxypropyl)tetramethyldisiloxane (3.6261 g,0.01 mol) was introduced into a 100 mL three-neck flaskequipped with a magnetic stirrer, a drop funnel, and a N2

inlet. 3-Aminopropyltriethoxysilane (3.5858 g, 0.02 mol) wasdropped slowly into the flask, and then 25 mL of DMF and0.1 g DBTDL were added. The reaction mixture was vigo-rously stirred and refluxed at 75 �C for 4 h. Finally, a viscoustransparent fluid resulted (7.17 g).

Poly(siloxane-urethane) Crosslinked StructuresInto a three-neck flask equipped with magnetic stirrer and anitrogen inlet, HB (1.77 g, 0.0024 mol), IPDI (0.67g, 0.0030

mol), DMF (8.15 mL), and DBTDL (0.0084 g) were introduced.After stirring under N2 at 80 �C for 4 h, the temperature wasdecreased to 70 �C. An appropriate amount of hybrid diol GDsolution (1.73 mL, 0.0014 mol) was added, in such mannerthat the molar ratio IPDI/(HBþGD) � 1. The reaction pro-ceeded at 70 �C for another 4 h. The resulted viscous mixturewas poured into a large amount of ice-cooled ether. Afterwashing for three times with ice-cooled ether, the resin wasredissolved in DMF to produce a 30 wt % solution. H2O and1.0 M HCl were added in the molar ratio H2O:HCl:OAC2H5 ¼2:0.018:1, and the solution was stirred at room temperaturefor 2 h. The resulted opaque yellow sol was then poured intoa Teflon mold, and kept at 60 �C for 24 h, and 120 �C foranother 24 h in a vacuum oven. The resulted product was la-beled Pu1. The other two samples Pu2 and Pu3 were obtainedusing the corresponding ratios presented in Table 1. While thesiloxane content maintains almost constant, the crosslinkingdegree increases due to the increasing ratio GD/HB.

RESULTS AND DISCUSSION

Two siloxane diols were prepared to be used in the obtainingof siloxane-urethane crosslinked structures. a,x-Bis(hydroxy-

FIGURE 1 1H NMR spectrum of the a,x-bis(hydroxybutyl)oligodimethylsiloxane.

TABLE 1 Feed Amounts of the Reactants used to Obtain Poly(siloxane-urethane) Crosslinked Structures

Samples

Reactants

Aspect of the Product

HB

(mol)

GD

(mol)

IPDI

(mol)

DBTDL

(g)

HCl

(mL)

H2O

(mL)

Pu1 0.0024 0.0006 0.0030 0.0084 1.09 0.12 Yellow transparent flexible film

Pu2 0.0020 0.0011 0.0030 Yellow transparent flexible film

Pu3 0.0017 0.0013 0.0030 Brittle yellow transparent film

JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA


butyl)oligodimethylsiloxane (HB) was prepared by equilibriumpolymerization of D4 with 1,3-bis(hydroxybutyl)tetramethyldi-siloxane in presence of a cation exchanger as a catalyst(Scheme 1). The molecular weight number of the diol esti-mated by 1H NMR was 740.

For the second diol, 1,3-bis(3-glycidoxypropyl)tetramethyldi-siloxane was reacted with 3-aminopropyltriethoxysilane inmolar ratio 1:2 in presence of DBTDL as a catalyst whenepoxy cycle was opened with the formation of OH group andcoupling to the aminopropyl group, according to Scheme 2.Thus, a siloxane diol having also hydrolysable trialkoxysilanegroups was obtained.

The formation of the presumed siloxane diol was verified byFTIR spectroscopy (Fig. 2). The disappearance of the bandscharacteristic to the epoxy groups in the reagent, at 3052,1159, and 910 cm�1 confirms the reaction occurrence.Another arguments, which confirm that the reactionoccurred is the disappearance of the band from 1590 cm�1

assigned to primary amine ANH2 (Fig. 2, APTES), theappearance of the band from 1672 cm�1 assigned to second-ary amine ¼¼NH, and the band from 1192 cm�1 assigned to¼¼C¼¼N-bond from secondary amine (Fig. 2, GD). The siloxanespecific bands are also visible in the diol spectrum: 2935cm�1 (CAH aliphatic), 1254 cm�1 (SiACH3), 1089 cm�1

(SiAOASi), 797 cm�1 (SiACH3).

A three steps procedure was applied to prepare the siloxane-urethane crosslinked structures. The first step consists in thereaction between IPDI and a,x-bis(hydroxybutyl)oligodi-

methylsiloxane (HB). Then, newly synthesized diol GD wasadded when an extension of the chain formed in the firststep occurs in the presence of DBTDL (Scheme 3). The thirdstep consists in adding H2O and HCl, when the hydrolysisand condensation of the BSiAOC2H5 groups occur that leadto the forming of the silica network concomitantly with thecuring of the polymer (Scheme 4).

The BSiAOASiB domains formed can act as network nodesto provide dimensional stability but also act as reinforce-ment ‘‘inorganic fillers.’’ The presence of flexible siloxane seg-ments gives reversibility to network deformation with poten-tial application as shape memory material.

FTIR Spectroscopy. Figure 3 shows the FTIR spectra of thepoly(siloxane-urethane) crosslinked structures obtained byusing different HB/GD molar ratios in the spectral range4100–400 cm�1. The absence of the band assigned to isocya-nate group at 2258 cm�1,30,31 in FTIR spectra of the cross-linked structures indicates the formation of the urethanechain (Fig. 3). The FTIR spectra of the obtained structurespresent the bands typical for polyurethane: ANH (free andbonded) around 3364 cm�1, ACH2 at 2796–2958 cm�1, andC¼¼O in urethane group at 1701 cm�1. The hydrolysis of theSiAOAC2H5 groups and their condensation is sustained bythe disappearance of the band at 1100 cm�1 assigned toSiAOAC bonds from GD. The band around 1197 cm�1 in thespectra of the sample Pu2 corresponds to SiAOASi stretch-ing of crosslinked silica structures resulted by self-condensa-tion of SiAOH groups of hydrolyzed ethoxy groups.32

SCHEME 1 The reaction scheme to obtain the a,x-bis(hydroxybutyl)oligodimethylsiloxane.

SCHEME 2 The reaction scheme to obtain the hybrid diol GD.

ARTICLE


Formation of the silica network is confirmed also by theband from 474 cm�1.33 The band at about 3364 cm�1 andthe shoulder at 953 cm�1 are associated with SiAOH groupsattached to the silica networks formed in the described con-ditions. In the FTIR spectrum of the poly(siloxane-urethane),crosslinked structures (Fig. 3, Pu2) are also visible the bandscharacteristic for the dimethylsiloxane sequences at 1259and 800 cm�1. The SiAOASi band visible in spectrum of HBat 1032–1092 cm�1 is now overlapped with silica SiAOASiband as well as with CAOAC band.

Thermogravimetric Measurements. Results of thermogra-vimetric analysis (TGA) of the samples in the 0–750 �C tem-perature range, in air, are shown in Figure 4. The slightweight loss below 250 �C can be assigned to the evaporationof the remnant water and organic solvents. However, it canbe observed that, in this stage, the curves become more ab-rupt from the sample Pu1 to Pu3, in this order increasingthe amount of GD used in the reaction. As a result, it canpresume that the condensation of the residual groups withethanol evolving occurs in this stage. This order is main-tained until almost the end of the main decomposition stageoccurring in the range 250–400 �C. This behavior could sup-port assumption that the weak point is GD moiety containingsiloxane-aliphatic but also NH and CAOAC groups. At T >

400 �C, an inversion occurs in this order as a result ofincreasing the residue amount at higher amount of used GDdiol that is the main generator of silica.34 Thus, the samplewith the higher crosslinking degree, Pu3, yields a largeramount of residue as compared with Pu1 sample.

According to literature,35,36 the weight loss below 250 �Ccould be also assigned to the breaking of urethane bond con-sidered as the thermally weakest link yielding to a transientchar in thermo-oxidative condition. In the temperature range250–350 �C the further combustion of organic moieties29,30

and the thermo-oxidative degradation of the transient char37

occur which yield high temperature residues at 750 �C.

Differential Scanning Calorimetry. The DSC scans of poly(siloxane-urethane) crosslinked structures are shown inFigure 5. It can be noticed that Tg values at low temperaturerange specific to polysiloxane moieties are not considerablyaffected by the presence of the urethane bonds or crosslinks, all samples showing almost the same value, around of�124 �C. We can conclude that different crosslinking degreesof networks have not a significant influence on amorphousphase. Also, it does not identify on the DSC curves any melt-ing process around �40 �C assignable to poly(siloxane)chains, proving the high degree of crosslinking.

Instead, it was recorded as a melting process in the tempera-ture range of 20–80 �C, which depends on the crosslinkingdegree. The siloxane-urethane segments form crystallinity,which melts in this temperature range. Melting temperaturesfor these segments increase with GD content and decreasewith HB one containing long dimethylsiloxane chain in softphase. A small content of crystallizable dimethylsiloxanechains permits formation of crystals, which can be an

SCHEME 3 The reaction scheme for the obtaining of the polyurethane structures.

FIGURE 2 FTIR spectra of the reactants and hydrolizable

diol GD.



explanation for the light sharpening of melting endotherms.But crystallinity developed in crosslinked structuresdecreases with increasing of GD amount and decreasing ofHB amount. The crosslinking degree has an opposite effecton the values of melting enthalpies.

Dynamic Mechanical Analysis. The particularity of thesynthesized structures is represented by the fact that thepermanent network is realized through three means: thechemical crosslinkings, the polyurethane networks resultedat low temperatures due to the association of hydrogenbonding and the existence of siloxane crystalline domains.The reversible thermal-phase transition originates from the

glass transition of the amorphous phase of the polymer, themelting of the crystalline domains, and the dissociation ofhydrogen bondings of polyurethane at higher temperature.

Figure 6 presents the variation of the storage modulus (E0),loss modulus (E00), and loss factor (tan d) with temperaturefor the samples that could be studied by DMA, Pu1 and Pu2.On a DMA curve, the drops in E0 and the peaks of E00 and tand reports on the physical transition of polymers. The E00 peakappears always at a lower temperature than tan d peak.Some relevant dynamic mechanical characteristics of thesamples are included in Table 2. In the negative temperaturedomain, the gradual decrease of E0 and the small peaks onE00 and tan d reflects the glass transition of amorphous sil-oxane chain segments. Because of the higher crosslinkingdegree in Pu2 compared to Pu1, the storage modulus ishigher in the glassy range, and the glass transition of thesoft domains estimated as the onset of E0 drop starts athigher temperature (Table 2). The large transition peakspanning between 40 �C and 125 �C consists of a shoulderaround 65 �C and a peak at 90 �C for Pu1 and at 97 �C forPu2.

SCHEME 4 The reaction scheme for the preparation of the crosslinked structures.

FIGURE 3 FTIR spectra for the reactants and poly(siloxane-ure-

thane) crosslinked structures.

FIGURE 4 The TGA curves of poly(siloxane-urethane) cross-

linked structures.

ARTICLE


These peaks may correspond to the phenomena emphasizedin DSC by the peaks developed in the 20–90 �C temperaturerange but that could be better detected by DMA.

The multifrequency experiment (0.5, 1, 2, 5, and 10 Hz) wasperformed to establish the origin of the two peaks [Fig.7(a,b)]. For both polymers, the first peak decreases withincreasing frequency, whereas the second increases withincreasing frequency. When the tan d peak is exclusively theresult of a relaxation, an increase of E0-at the expense of E00-takes place with increasing frequency.38–41 Considering thattan d is the ratio E00/E0, this fact is reflected as a decrease oftan d peak. It is not the case for the second peak whose am-plitude increases with increasing frequency. Therefore, atleast one more phenomenon happens in the same time withglass transition that increases the mobility of polymer chains.It can be the melting of the crystalline phase of siloxanedomains. Moreover, it is well known that the cleavage ofinterurethane hydrogen bonds starts above 90 �C.42 Even ifit was expected that the polymer Pu2 has more importantrigidity than Pu1, the phenomena that overlap the glass tran-sition guarantee for an important decrease of E0 during thetransition. In addition, the cohesion of the network isnot destroyed, and during the transition, the E0 modulus isalways higher than the E00 modulus.

Because of the coexistence of the siloxane and urethanesequences, it is expected that such structures to show shapememory.

SMP are polymeric materials that after deformation areable to go back to the initial shape under the action of anexternal stimulus.8,43–48 More often, this stimulus is the tem-perature. Generally speaking, the procedure includes the fol-lowing steps:

• application of a deformation at high temperature, usually atemperature corresponding to the rubbery plateau regionof the polymer DMA curve (predeformation);

• cooling to a lower temperature situated in the glassyregion, where the polymer is fixed (storage);

• heating again beyond the glass transition temperature,where a SMP recovers the original shape (recovery).

A reversible phase transition is used to switch between thetemporary shape and the permanent shape. It could be a

FIGURE 5 The DSC scan plots for the poly(siloxane-urethane) crosslinked structures.

FIGURE 6 The dependence of storage modulus (E0) and loss

modulus (E00) on temperature for samples Pu1 and Pu2 at 1 Hz

frequency.



glass transition and/or a melting and the corresponding tem-perature is known as the shape memory temperature (Ts).Almost all the polymers have a transition. However, a partic-ular structure is needed to obtain shape-memory proper-ties.8,45–47 First, the structure should include a permanentnetwork obtained through physical crosslinking or chemicalcrosslinking. This network impedes the plastic flow of chainsand saves the permanent shape. Then, the structure containsa reversible phase whose transition helps switching between

the temporary shape and the permanent shape. As has beenstated by many authors, a DMA experiment can suggestwhether a material fulfills some preliminary demands tofunction as SMP.11,49,50 These requirements are emphasizedby many authors and are excellently summarized in ref. [8].A high glassy storage modulus (Eg0) renders good shape fixityto the polymer. An appropriate value of rubbery state stor-age modulus (Er0) will ensure both large deformations in therubbery state and high elastic recovery at high temperatures.

TABLE 2 The Main Parameters of DMA Curves

E 0 � 10�6

(Pa) (�140 �C)E 0 � 10�6

(Pa) (20 �C), (Eg0)

Tg (�C)

E 0 � 10�6 (Pa)

(125 �C), (Er0) Eg

0/Er00E 0

onset E 00peak

tan dpeak

I II

Pu1 920 290 40 46 65 90 0.43 675

Pu2 1500 570 48.5 55 68 97 2.1 272

FIGURE 7 The dependence of tan d on temperature for samples: (a) Pu1 and (b) Pu2.

FIGURE 8 SEM micrographs of the broken surfaces deposed on Al supports and coated with Au.

ARTICLE


A high ratio of Eg0/Er0, that is, as at least two orders of magni-tude, makes possible in the same time, the easy deformationat temperatures higher than Ts and the high resistance to de-formation at temperatures lower than Ts. Usually, Eg0 and Er0

are taken 25 �C below and above Tg. The ratio Eg0/Er0 (Table2) and the stability of the prepared networks recommendthese two polymers to be tested for shape memory capabil-ities. It can be noticed some differences between samplesand after relaxation, EPu20 > EPu10 (5.5 � 105 Pa, Pu1; 2.6 �106 Pa, Pu2), due to the network hardening.

Scanning Electron Microscopy. Scanning electron micros-copy (SEM) micrographs of the cross section of the preparedfilm structures are displayed in Figure 8. In the case of Pu1sample, the general view of the fractured surface revealed ahomogeneous structure characteristic to the crosslinkedpolymers. As the GD amount increases, the texture changes,thus, in Pu2 sample, the texture becomes cylindrical.

The EDX analyses, performed to identify the nature of theatoms present in the samples at a depth of 100–1000 nmfrom the surface, proved the presence of all expected ele-ments (C, O, Si, and N) (Fig. 9). It can be noticed that Si per-centage detected by EDX on the fracture surface increaseswith increasing the crosslinking degree in the whole mass ofthe sample.

Vapor Sorption Capacity. Water-vapors uptaking capacityfor the samples at 25 �C in the 0–90% relative humidity(RH) range was investigated by using the IGAsorp equip-ment. The vapors pressure was increased in 10% humiditysteps, every having a pre-established equilibrium timebetween 40 and 50 min. At each step, the weight gained was

measured by electromagnetic compensation between tareand sample when equilibrium was reached. An anticondensa-tion system was available for vapor pressure very close tosaturation. The cycle was ended by decreasing the vaporpressure in steps to obtain also the desorption isotherms.The drying of the samples before sorption measurementswas carried out at 25 �C in flowing nitrogen (250 mL

FIGURE 9 EDX analysis of poly(siloxane-urethane) crosslinked structures. [Color figure can be viewed in the online issue, which is

available at wileyonlinelibrary.com.]

FIGURE 10 Rapid water-vapors sorption isotherms for cross-

linked structures.



min�1) until the weight of the sample was in equilibrium atRH < 1%. The sorption–desorption isotherms registered inthese conditions are presented in Figure 10.

It can be seen as a clear influence of the diols ratio on thesurface properties of the resulted structures. Increasing thefeed amount of GD will lead to rising in silica domains withits porosity and SiAOH groups on the surface, which willfavorize the water sorption. This presumption is supportedby the isotherms presented in Figure 10, and the mainparameters evaluated on their basis and summarized inTable 3.

GAB and BET kinetic models applied to the obtained datagave the values presented in Table 3. These models are veryoften used for modeling of the sorption isotherms and arebased on following (1) BET and (2) GAB equations:

W ¼ Wm � C � RH1� RHð Þ � 1� RHþ C � RHð Þ (1)

W ¼ Wm � C � K � RH1� K � RHð Þ � 1� K � RHþ C � K � RHð Þ (2)

where: W, weight of sorbed water; Wm, weight of waterforming a monolayer; C, sorption constant; RH, relativehumidity; and K, additional constant for the GAB equation.51

BET model describes the sorption isotherms up to a RH of40%, depending on the type of sorption isotherm and on thetype of material. This method is limited mainly to isothermsof type II but can also describe the isotherms of types I, III,and IV.52 GAB model covers a larger range of humidity condi-tions, up to 90% and is also used for finding out the mono-layer sorption and surface area values. This model describesvery well the water sorption in multilayers.51,53,54 The aver-age pore size for all the samples was calculated consideringthat pores have a cylindrical form.

As can be seen, the values for surface area increase with GDcontent. The presence of hydroxyl groups on the surface ofSiO2 network obtained after the hydrolysis reactions ofethoxy groups from GD leads to an increase of water sorptioncapacity. For the sample Pu3, GAB equation did not describewell the shape of the moisture sorption isotherm. Due to theincrease in the sample porosity, the rate of desorption pro-cess is smaller than of sorption one (the presence of hystere-sis in the shape of isotherm) because of capillary phenom-enon. The loop of the hysteresis depends on the average pore

size. According to the values presented in Table 3 and as canbe seen in the Figure 10, there is a good correlation betweenthe loop of the hysteresis and the value of the average poresize. The loop is bigger (the hysteresis is more pronounced)for the sample in which the average pore size has 1.28 nmvalue. By increasing the crosslinking degree, the obtainedhybrid materials have a higher water sorption capacity (theincrease in hydrophilicity), this property improving their bio-compatibility, which is considered a major drawback of poly-urethane shape memory materials.

CONCLUSIONS

A three-step procedure was used to prepare crosslinkedsiloxane-urethane structures. In the first step, isophoronewas reacted with a,x-bis(hydroxybutyl)oligodimethylsiloxane(HB) followed in the second step by the extension of thisprecursor with a new hybrid siloxane diol containing addi-tional hydrolysable groups. In the third step, acid-catalyzedhydrolysis of the ethoxy groups occurred resulting the cross-linked siloxane-urethane structures that differ by the feed ra-tio between the two diols. The formation of the expectedstructures was verified by FTIR. The effects of the usedhydrolysable groups’ amounts on some properties of theresulted structures were investigated by DSC, TGA, DMA,SEM, and DVS. The results revealed that increasing theamount of such groups leads to the improvement of the me-chanical properties and the water-vapor sorption capacity.While Tg value corresponding to the siloxane phase remainsunmodified, a melting isotherm own to these structuresdevelops in 20–80 �C temperature range. Based on the tran-sitions from this temperature range, better detected by DMAat various frequencies, the shape memory capability of twoof the prepared structures was revealed. The increasing ofthe hydrolysable groups content that are silica generatorsalso induces slight modifications in the morphology of theresulted structures.

This research was financially supported by European RegionalDevelopment Fund, Sectoral Operational Programme ‘‘Increaseof Economic Competitiveness,’’ Priority Axis 2 (SOP IEC-A2-O2.1.2-2009-2, ID 570, COD SMIS-CSNR: 12473, Contract 129/2010-POLISILMET). M. Cristea acknowledges the financialsupport of European Social Fund–‘‘Cristofor I. Simionescu’’Postdoctoral Fellowship Programme-(ID POSDRU/89/1.5/S/55216), Sectorial Operational Programme Human ResourcesDevelopment 2007-2013.

TABLE 3 The Main Parameters from Water Sorption–Desorption Isotherms

Samples

Weight

(%)

The Average

Pore Size (nm)

BET Analysis GAB Analysis

Monolayer

(g g�1)

Area

(m2 g�1)

Monolayer

(g g�1)

Area

(m2 g�1)

Pu1 5.6476 1.28 0.0250 88.077 0.0290 101.853

Pu2 8.7123 1.55 0.0319 112.285 0.0367 129.068

Pu3 21.1041 2.12 0.0564 198.255 – –

ARTICLE


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Poly(siloxane-urethane) crosslinked structures obtained by sol-gel technique

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