European Polymer Journal 49 (2013) 3889–3896

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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: bkkim@pnu.edu (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

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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

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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.

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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

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.

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

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