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European Polymer Journal 49 (2013) 3889–3896
Contents lists available at ScienceDirect
European Polymer Journal
journal homepage: www.elsevier .com/locate /europol j
Macromolecular Nanotechnology
Synthesis and properties of near IR induced
self-healablepolyurethane/graphene nanocomposites
0014-3057/$ - see front matter � 2013 Elsevier Ltd. All rights
reserved.http://dx.doi.org/10.1016/j.eurpolymj.2013.10.009
⇑ Corresponding author. Address: Rm 7205, 7th Eng. Bldg. (Chem.
Eng.Bldg.), Republic of Korea. Tel.: +82 (0)51 510 2406.
E-mail address: [email protected] (B.K. Kim).
Jin Tae Kim a, Byung Kyu Kim a,⇑, Eun Young Kim a, Sun Hong Kwon
b, Han Mo Jeong ca Department of Polymer Science and Engineering,
Pusan National University, Busan 609-737, Republic of Koreab
Department of Naval Architecture and Ocean Engineering, Pusan
National University, Busan 609-735, Republic of Koreac Department
of Chemistry, University of Ulsan, Ulsan 680-749, Republic of
Korea
a r t i c l e i n f o a b s t r a c t
RN
AN
OTE
CHN
OLO
GY
Article history:Received 26 August 2013Received in revised form
2 October 2013Accepted 9 October 2013Available online 19 October
2013
Keywords:Self-healable polyurethane
nanocompositesGrapheneInfrared light absorptionNanofiller
OLE
CULA
A series of self-healable polyurethane (SHPU)/modified graphene
(MG) nanocompositeswere synthesized from poly(tetramethylene
glycol) (PTMG) and 4,40-methylene diphenyldiisocyanate (MDI) with
minute amounts (0–1 wt%) of MG which was chemically
modifiedgraphene oxide (GO) with phenyl isocyanate and reduced in
the presence of phenylhydr-azine.
MG gave dual effects of reinforcing filler and light absorption
medium. That is, SHPU/MGnanocomposites showed significantly
enhanced Young’s modulus and near infrared (NIR)absorption along
with increased glass transition temperature (Tg). However, break
strengthand break strain decreased at high GO contents (MG075,
MG100) implying that MG dis-turbs chain orientations.
The self-healing behavior of nanocomposites was done by
intermolecular diffusion ofpolymer chains which was accelerated by
thermal energy generated by NIR absorptions.The self-healing effect
was most pronounced with 0.75 wt% MG (MG075) where the
elasticstrain energy was even greater than the fresh material up to
over 200% strain. Further addi-tion of MG (MG100) induced more
light absorption, but physically disturbed the interchaindiffusion
to reduce the self-healing efficiency.
� 2013 Elsevier Ltd. All rights reserved.
CRO
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1. Introduction
Polyurethanes (PUs) are a most versatile engineeringmaterial
which is synthesized by a simple polyadditionreaction of polyol,
isocyanate and chain extender. Theyfind a variety of industrial
applications including coatings,adhesives, sealants, elastomers
(often abbreviated byCASE), primer, sports goods, medical devices,
textile finishaside from the various foam products [1–3].
Graphene, atomically thin two-dimensional sheets ofcarbon, has
emerged as the subject of enormous interestbecause of its
exceptional micromechanical and electrontransport properties.
Graphene has a high basal plane elastic
modulus, E � 1 TPa; ultimate strength, rultimate � 130 GPa;and
room temperature charge-carrier mobility, l �10,000 cm2/V s [4].
Moreover, the perfect sp2 carbon-network structures of the graphene
materials ensure themto have excellent thermal conductivity and IR
response[5–8]. With regard to the IR induced self-healing
ofpolymer, fast heat transfer as well as high IR absorptionis of
prime importance since heat dissipated by IR at thesurface should
quickly transfer into the center to actuatethe self-healing. In
this regard, graphene is preferred overthe carbon nanotube as well
as carbon black [9–10].
It is essential to highly exfoliate the graphite into layersto
significantly enhance the mechanical and electricalproperties of
polymers at extremely small loading[11–14]. Oxidative exfoliation
of natural graphite by acidtreatment has been a most efficient
method. However,the damage to graphene’s sp2 carbon network
would
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severely affect the properties of graphene, such as mechan-ical
[15], electrical [11], thermal [8], and optical
absorptionproperties [6]. This can be solved by reducing
grapheneoxide (GO) which can restore sp2 carbon network [5,11].
And owing to their hydrophilic nature, GO sheets canonly be
dispersed in aqueous media that are incompatiblewith most organic
polymers. Exfoliation behavior ofgraphite oxide can be altered by
changing the surfaceproperties of GO sheets by way of chemical
functionaliza-tion. The isocyanate treatment reduces the
hydrophiliccharacter of GO sheets by forming amide and
carbamateester bonds to the carboxyl and hydroxyl groups of
graph-ite oxide, respectively. As a result, such
isocyanate-derivatized graphite oxides no longer exfoliate in
waterbut readily form stable dispersions in polar aproticsolvents
such as N,N-dimethylformamide (DMF). Thesedispersions of
isocyanate-derivatized graphite oxide allowGO sheets to be
intimately mixed with many organicpolymers, facilitating synthesis
of graphene–polymercomposites [11]. Also, the isocyanate treated GO
canimprove miscibility with polymer, especially PU which
issynthesized from the same isocyanate groups. A numberof studies
regarding the polymer/graphene nanocompos-ites have been done, but
most of studies focused onthe improvement of mechanical property,
electricalconductivity or gas barrier property [11–14,16–18].
Polymers with the ability to repair themselves after sus-taining
damage could extend the lifetimes of materialsused in many
applications [19,20]. When fractured, poly-mer can regain the
physical properties of the original poly-mer [21] either
autonomically [22], or in response to anexternal stimulus such as
heat [23] or pressure [24]. Heal-able polymeric systems may for
example contain encapsu-lated monomers and polymerization catalysts
[22,25], orlatent functionalities which are able to participate in
ther-mally-reversible, covalent bond-forming reactions [23,26].It
has also been shown that non-covalent interactions, spe-cifically
hydrogen bonds [8] may be used to effect healingwithin a
supramolecular polymer blend (albeit in the pres-ence of a
plasticising solvent) [24]. If the commodity PU isendowed with the
ability to repair itself, vast applicationswill be opened up as
functional polymer [27–29].
To the best knowledge of the present authors, the nearIR (NIR)
absorption of graphene has not been reported forself-healing of
polymers in open literature. We synthesizedPU/graphene
nanocomposites to introduce the light in-duced self-healing by
graphene as well as to reinforce thePU. To enhance the miscibility,
graphene was chemicallymodified by phenyl isocyanate (pi-GO), and
subsequentlyreduced in phenyl hydrazine to restore sp2 carbon
net-work. Mechanical, thermal, optical properties as well asthe
self-healing ability of the PU/modified graphene (MG)nanocomposites
were measured.
2. Experimental
2.1. Raw materials
Polytetramethylene ether glycols (PTMG, Mn = 650g/mol) and 1,
3-butandiol (1,3-BD, Aldrich) were driedand degassed at 80 �C under
vacuum for 3 h before use.
4,40-Methylene diphenyl diisocyanate (MDI, BASF),
phen-ylhydrazine (Aldrich), phenyl isocyanate (Kanto Chem.)were
used as received. Graphite powder (Conductinggrade, 325 mesh) was
purchased from Alfa Aesar.
2.2. Preparation of GO
Hummers’ method was used to prepare the GO [16,30–32]. The
desired amounts of graphite (10 g) and NaNO3(7.5 g) were first put
in a three neck flask and mixed thor-oughly. Subsequently an
appropriate amount of H2SO4(621 g) was added and stirred in an ice
water bath to ab-sorb the heat of mixing. KMnO4 (45 g) was then
slowlyadded during the next 1 h, followed by cooling for the next2
h. The mixture was allowed to stand for 5 days at 20 �Cwith mild
stirring. The liquid obtained was added to one li-ter of 5 wt%
H2SO4 aqueous solution for 1 h with stirring.Then, 30 g of 30 wt%
H2O2 aqueous solution was addedand stirred for the next 2 h.
Finally, the mixture was puri-fied following the cyclic procedure
which is well describedin our earlier work.
2.3. Preparation of pi-GO
GO (675 mg) was loaded into a 500-ml round bottomflask equipped
with a stirrer under nitrogen atmosphere.Anhydrous DMF (67.5 ml)
was added to create an inhomo-geneous suspension. Phenyl isocyanate
(3.22 g) was thenadded and stirred for 7 days (Scheme 1). Then, the
reactionmixture was poured into methylene chloride (675 ml)
tocoagulate the product. The product was filtered, washedwith
additional methylene chloride (675 ml), and dried un-der vacuum
[33].
2.4. Synthesis of pi-GO/polyurethane nanocomposites
The overall reaction scheme for the synthesis of thecomposite is
shown in Scheme 2. Molar excess of PTMGand 1,3-BD were reacted with
MDI at 60 �C for 1 h in a500-ml four-necked flask with a mechanical
stirrer, ther-mometer, condenser and a nitrogen injection tube.
Thisreaction yields OH-terminated PU with theoretical molec-ular
weight of 15,000, that were determined by the indexof [OH]/[NCO]
> 1. Then, the various compositions ofpi-GO dispersed in DMF
(1–2 mg/ml) (Table 1) were fedinto a bial which was suspended in
water of sonicationbath operating in 40 kHz frequency. Reduction of
the dis-persed material was carried out with phenyl
hydrazine(2.06–4.12 g) at 60 �C for 24 h with stirring [34]. The
nano-composite solution was then added dropwise to room-temperature
methanol (600–1000 ml) with stirring [35].Coagulated product was
isolated by filtration and washedwith methanol (400–800 ml).
Composite was redissolvedin DMF (70 g) with stirring and
sonication, followed bycasting and drying on a Teflon plate.
Thickness of the driedfilm was approximately 0.4 mm.
2.5. Characterizations
Fourier transform infrared (FT-IR) spectroscopy
(MattsonSatellite) was used to confirm the formations of OH
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Scheme 1. Modification of graphene oxide (GO) by phenyl
isocyanate.
Scheme 2. Overall reaction scheme to prepare PU/MG
nanocomposite.
Table 1Recipe for the preparation of PU/MG nanocomposites.
Series Polyurethane (g) Pi-GO (wt%)
PTMG650 MDI 1,3-BD MDI
MG000 15.52 5.48 2.38 6.62 0MG050 0.5MG075 0.75MG100 1
Fig. 1. FT-IR spectra of the OH-terminated polyurethanes.
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terminated PU and GO. Standard X-ray PhotoelectronSpectroscopy
(XPS) measurements were done using aVG-Scientific ESCALAB 250
spectrometer with an AL KaX-ray source. Thermal properties of the
cast films weredetermined using a Differential Scanning
Calorimetry(DSC, Q100). The samples were first heated to 100 �C
toerase their thermal history and then cooled to below�50 �C at 20
�C/min under nitrogen. Glass transitiontemperature (Tg) was
measured during the second heatingcycle at 10 �C/min. Morphology of
the film was examinedusing a scanning electron microscopy (Zeiss
FE-SEMSUPRA25). Sample was cryogenically fractured beforeviewing.
Near infrared (NIR) light absorption spectra wereobtained by
FT–UV–VIS–IR Spectrometer (VERTEX 80).Tensile properties of the
cast films were measured at roomtemperature with a Universal
Testing Machine (UTM, LloydLRX) at a crosshead speed of 500 mm/min.
Microtensiletest specimens were prepared according to ASTM D
1822.
For quantitative healing test, nanocomposite film wasscratched
to a depth of 50–60% or cut using a razor bladein frozen state to
avoid any accidental deformation of the
film. And care was taken to ensure that heating and healingof
the scratched or cut film occur mainly by the NIRabsorption;
experiment was done in darkroom at roomtemperature and effects of
convection and radiation byheated lamp were possibly minimized by
vertically placingthe horizontal film below lamp at a distance of
ca 20 cmwhile the lamp was continuously cooled by a fan. In caseof
cut, as soon as the film was cut, the ruptured edges werebrought
into close contact with but with no overlap in fro-zen state. Then,
the film was exposed to NIR (HANA 0E17)for a certain time. The
power density delivered to the sam-ple was 20 mW/cm2 as measured
using a light intensitymeter. The temperature was measured by
infrared ther-mometer. The healed specimens were subjected to
tensiletest as above. Healing efficiency is defined using the
mod-ulus of toughness [19,36] as:
Healing efficiencyð%Þ ¼ toughnesshealedtoughnessoriginal
� 100 ð1Þ
Modulus of toughness is a measure of the strain energyrequired
to break the material and corresponds to the areaunder the
stress–strain curve [36,37].
3. Results and discussion
3.1. Characterization of GO, pi-GO, PU
Fig. 1 shows that the characteristic absorption peak ofNCO at
2,270 cm�1 completely disappeared upon complet-ing the OH
terminated polyurethane forming reactions.
-
Fig. 2. C1s survey XPS scans of pristine (a) graphite and (b)
GO.
Table 2Atomic concentrations in graphitic derivatives from
XPS.
Graphite (%) GO (%) Pi-GO (%)
C1s 94.39 62.38 68.69N1s – – 2.77O1s 5.16 35.12 28.54Sp2 0.44
2.51 –
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The formations of GO and pi-GO were confirmed byXPS. The C1s
deconvolution spectrum of pristine graphite(a) shows a single peak
at 284.61 eV, while that of GO (b)shows three different peaks at
284.7, 286.9, and 289.4 eV,corresponding to the C@C/CAC bonds in
the aromaticrings, C (epoxy and alkoxy) groups, and C@O groups,
Fig. 3. SEM images of the cryogenically fracture surfaces
respectively (Fig. 2) [16,32]. The atomic compositions ofthe
three graphitic materials are given in Table 2. After oxi-dation,
the oxygen content in GO increased from up to 35%suggesting the
formation of CAO bonds. In addition to car-bon and oxygen, sulfur
was also detected on the surface ofGO, which may be traced to their
ions being physicallytrapped in the closed pores or covalently
esterified withsurface hydroxyls [38]. Upon treating the GO with
phenylisocyanate, carbamate and amide groups appeared.
3.2. Morphology and properties of PU/MG nanocomposites
3.2.1. SEMFig. 3 shows the SEM morphology obtained from the
cryogenically fracture surface of the PU/MG nanocomposites.
of films for (a) MG050, (b) MG075 and (c) MG100.
-
Fig. 4. DSC curves of PU/MG nanocomposite films.
Fig. 5. Stress–strain behavior of PU/MG nanocomposite films (25
�C).
Table 3Thermal and mechanical properties of PU/MG nanocomposite
films.
Series Tg (�C) E (MPa) rb (MPa) eb (%)
MG000 �5.48 17.78 6.78 607.16MG050 �3.04 18.69 7.10 774.77MG075
�4.45 40.85 3.96 596.01MG100 �3.49 42.56 3.73 472.17
Fig. 6. NIR absorbance of PU/MG nanocomposites films.
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It is seen that primary MG particles are well dispersed
inpolymer matrix at low contents (MG050, MG075). How-ever,
particles are agglomerated at high content (MG100).
3.2.2. DSC and UTMFigs. 4 and 5 show thermal and mechanical
properties
of PU/MG nancomposites while the detailed data areshown in Table
3. The Tg of PU is increased with the addi-tion of MG due to the
restricted motion of PU chains in the
presence of MG. The initial modulus monotonically in-creases
with the addition and increasing amount of MGwhile the break
strength and elongation at break increasedwith MG050 but decreased
with MG075 and MG100. Thissuggests that the nanoparticles disturb
the orientationsof polymer chain at high elongations at high
loadings [39].
3.2.3. NIR measurementsThe NIR absorbance increases with
addition of graphene
[5]. Fig. 6 shows the NIR absorbance of the MG serieswhere the
NIR absorbance of PU (MG000) is increased over10 times with MG050
due to existence of graphene whichhave many sp2 carbons and it is
further increased withMG075 and MG100 due to the increased content
of MGviz. sp2 carbon. It is mentioned that unfortunately theNIR
absorbance for the MG075 and MG100 was over theinstrument.
3.3. Tensile and healing tests
First, we assume that self-healing behavior of
PU/MGnanocomposites will be done by intermolecular diffusionof
polymer chains which is proportional to time and tem-perature. So,
this intermolecular diffusion can be acceler-ated by thermal energy
which is generated by NIRabsorptions.
Fig. 7 is photographs showing that the PU/MG nano-composite film
bended by 180� largely recovers the defor-mation upon exposure to
the NIR, or the cryogenically cutfilm is healed or rebonded by NIR
irradiation.
Fig. 8 and Table 4 shows the temperature and healingefficiency
changes of the MG000 film with the irradiationtime. Temperature of
the film increases from 25 �C to asteady value of about 30 �C in
about 1 h. The steady stateis established as the rate of heat
generation by NIR absorp-tion becomes equal to the rate of heat
loss by the interfaceheat transfer. As the heat generation within
the film in-creases heat loss at the surface increases according
to
q ¼ hðTs � T1Þ ð2Þ
where q is the heat flux, h the heat transfer coefficient, Tsthe
surface temperature, and T1 the ambient temperature.
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Fig. 7. Photographs showing NIR induced healing of PU/MG
nanocomposite films.
Fig. 8. Healing efficiency and temperature rises of MG000 vs.
NIRirradiation time.
Table 4Healing efficiency and temperature rises of MG000 at
various NIRirradiation time.
Irradiationtime (h)
Sampletemperature (�C)
Healingefficiency (%)
MG000 0 25 00.5 29 4.371 30 7.161.5 30 13.692 30 17.023 30
39.63
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As the irradiation time increases, Ts should increase to givean
increased q until it becomes equal to the rate of heatgeneration by
irradiation, and the time is ca. 0.5 h in the
present experiment. On the other hand, the healing effi-ciency
increases slowly up to about 2 h beyond which it in-creases
rapidly. The time lag between the temperature andhealing efficiency
rises is an indication of slow intermolec-ular diffusion of polymer
chains.
If MG000 which absorbs little amount of NIR is exposedto NIR
excessively, it can be healed enough due to contin-uous NIR energy
though its amounts are small. Maybe anypolymers which can absorbs
NIR even though its amountsare very small can be healed enough on
excessive NIRexposure. So, we determined NIR irradiation time for 2
hto clearly observe self-healability of MG series.
The stress–strain behaviors of the nano-composite weremeasured
for the irradiation time of 2 h (Fig. 9 and Table 4).Even at low
strain, the elastic strain energy (area under thes–s curve) of the
healed film (MG(I2)) is significantly re-duced for MG000 while it
is mostly recovered with theaddition of MG. Surprisingly with
MG075, the elastic strainenergy even increased over the fresh film
upon NIR irradi-ation, which needs further investigation to clarify
themechanism. MG absorbs NIR and augments the film tem-perature in
proportion to its content but the thermal equi-librium is
established at ca 30–33 �C regardless of itscontent (Table 5),
while the interchain diffusion becomesless plausible at higher
content so that an optimumcontent seems to exist, which is MG075
for the presentwork. The healing efficiency which is in the order
ofMG050 < MG100 < MG000 < MG075 again assures this.
4. Conclusions
PU/graphene nanocomposite were prepared to rein-force and
augment the NIR absorption of polyurethane(MG000), graphite was
oxidized and chemically modifiedwith phenyl isocyanate and reduced
in the presence ofphenylhydrazine to form modified graphene (MG).
Withthe addition of MG, glass transition temperature and
initialmodulus of the PU increased while break strength
andelongation at break showed an increase at low (MG050)and
decrease at high content (MG075, MG100) implying
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Fig. 9. Stress–strain curves of PU/MG nanocomposite films before
and after NIR irradiation for 2 h: (a) MG000, (b) MG050, (c) MG075,
and (d) MG100.
Table 5Healing efficiency and temperature rises of MG series
with NIR irradiationfor 2 h.
Series Irradiationtime (h)
Sampletemperature (�C)
Healingefficiency (%)
MG000 2 30 17.02MG050 30–33 13.42MG075 30–33 39.12MG100 30–33
15.86
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that the GO could disturb chain orientations at high load-ings
and high elongations.
MG significantly enhanced the NIR absorbance leadingto a
self-healable PU/MG nano-composite, where the effectwas most
pronounced with MG075 which showed elasticstrain energy greater
than the fresh material up to ca250% strain. However, at high
content MG disturbs inter-chain diffusion to give a decrease of
self-healing efficiencyas compared with the appropriate amount of
loading. It isexpected that the self-healable polyurethane would
ex-pand its vast applications as a functional polymer.
Acknowledgement
This study was supported by the National ResearchFoundation of
Korea through GCRC-SOP(No. 2011-0030013).
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Synthesis and properties of near IR induced self-healable
polyurethane/graphene nanocomposites1 Introduction2 Experimental2.1
Raw materials2.2 Preparation of GO2.3 Preparation of pi-GO2.4
Synthesis of pi-GO/polyurethane nanocomposites2.5
Characterizations
3 Results and discussion3.1 Characterization of GO, pi-GO, PU3.2
Morphology and properties of PU/MG nanocomposites3.2.1 SEM3.2.2 DSC
and UTM3.2.3 NIR measurements
3.3 Tensile and healing tests
4 ConclusionsAcknowledgementReferences