Nanocomposites based on polyurethanes and carbon nanoparticles: preparation, properties and...

Journal ofMaterials Chemistry A

FEATURE ARTICLE

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Nanocomposites b

EcUnrDitPS

cmacn

Institute of Problems of Chemical Physics

Chernogolovka, 142432, Russia. E-mail: ba

Tel: +7 4965224476

Cite this: DOI: 10.1039/c3ta10204a

Received 15th January 2013Accepted 13th February 2013

DOI: 10.1039/c3ta10204a

www.rsc.org/MaterialsA

This journal is ª The Royal Society of

ased on polyurethanes and carbonnanoparticles: preparation, properties and application

Elmira Badamshina,* Yakov Estrin and Margarita Gafurova

The review reports on methods for the formation of polyurethane nanocomposites containing fullerene

C60, carbon nanotubes, graphene, graphene-like structures, and nanodiamonds which show advanced

performance as compared with the parent polymers. Properties and possible fields of application of the

nanocomposites are described. Special attention is paid to research of relationships between structural

changes in a polymer matrix under the influence of nanomodifier and composite properties.

1 Introduction

Polymer compositematerials play an important role in all kinds ofhuman activity and the eld of application of such materialsbecomes wider from year to year.However, conventionalmethodsfor preparing materials, which would meet state-of-the-art strictperformance requirements with the use of high concentrations ofconventionalllingmodiers, exhausted theirpotential.1Nowit isclear that the solution to the problem of the design of advancedpolymer composites is provided by a lower degree oflling, whichis possible when using new type llers, namely, nanoparticles, i.e.particles of �1 and �100 nm in size.2 Such materials are, inparticular, new forms of carbon discovered in the last decades,which show fundamentally unusual properties caused by theirunique structure and small size of particles.

lmira Badamshina studiedhemistry at the Mendeleevniversity of Chemical Tech-ology of Russia (Moscow). Sheeceived her PhD in 1980 andoctor of Chemistry in 2011. Shes a Deputy Director of the Insti-ute of Problems of Chemicalhysics of Russian Academy ofcience. Her current researcheld is polymer chemistry,overing aspects of reactionechanism, synthesis and char-cterization of polymeromposite materials, includinganocomposites.

of RAS, 1, Academician Semenov Ave,

[email protected]; Fax: +7 4965221770;

Chemistry 2013

Among awide variety of known carbonnanostructures suchasfullerenes, carbonnanotubes (CNT), graphene andgraphene-likecarbon structures (GNS), nanodiamonds (ND), astralens, ‘nano-cones’, ‘nano-onions’ and others, the rst four structures haveattracted the greatest attention of researchers. Lots of publica-tions arededicated to (i) thedevelopmentofpreparationmethodsfor these modiers; (ii) the analysis of their properties andmethods of introduction into polymers; (iii) the relationshipsbetween the components in the nal composites and the reasonsfor changing the structure and, accordingly, the properties ofpolymermatrices as a result ofmodication; and (iv) the possibleareas of application of these new materials. All known types ofpolymers including polyurethanes (PU) are being investigated.

Most commercially used PU are multi-block copolymers.Macromolecules of such copolymers are formed of alternatingrigid polyurethane or polyurethane-urea segments and sooligoether or oligoester units. As a result of thermodynamicincompatibility of the segments of different nature, there isphase separation in PU. The phase consisting of rigid segments

Yakov Estrin studied chemistryat the Kazan Institute of Chem-ical Technology of Russia(Kazan), and then he studied atthe post-graduate level at theInstitute of Problems of Chem-ical Physics of Russian Academyof Science. He received his PhDin 1969 and Doctor of Chemistryat the same Institute in 1992where he works as a HeadResearcher. His current researcheld is polymer chemistry,

covering aspects of kinetics and reaction mechanism, polymeranalysis, including liquid chromatography, synthesis and charac-terization of polymer nanocomposites.

J. Mater. Chem. A

http://dx.doi.org/10.1039/c3ta10204a

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Journal of Materials Chemistry A Feature Article

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performs as nodes of the physical network and so segmentsprovide high-elastic properties to these materials. In theabsence of chemical cross-linking these materials are solubleand show properties of thermoplastic elastomers. However,sometimes a network of covalent bonds (chemical network) isformed in addition to a physical network. The covalent bondnetwork provides insolubility and infusibility to the materials.Such PU do not necessarily consist of segmented macromole-cules and, consequently, there is no phase separation.

Due to a variety of properties of PU differing in nature andlength of segments, the degree of their crystallinity, molecularmass (MM), the presence and density of the chemical network orthe lackof it, PUarewidelyusedas lacquerandprotectivecoatings,adhesives, sealing components, insulators, gaskets of clothes andfootwear, implants, etc. Recently developed PU based materialscan be used for saving information and recording individualprograms, which, as expected, will nd their application.3

When considering publications related to PU compositeswith carbon nanoparticles (CNP), it should be noted thatthough PU are important and widely used polymers, thenumber of publications on the analysis of possibility of essen-tial improvement of their properties using CNP is unreasonablylow to our opinion, compared with publications on modica-tion of many other polymers. There are almost no reviews onthis topic. For example, there is no information on PU in therecently published detailed review dedicated to the applicationof fullerenes in macromolecular chemistry,4 and minimalattention is paid to PU–CNP systems in the reviews on polymercomposites containing CNP. In particular, PU–CNT compositesare cited only in 2 references of 221 in the review,5 in which theproblem of the effect of pre-treatment of CNT on compositeproperties was discussed. There are 299 references in thereview6 and 223 references in the review7 on polymer compos-ites involving graphene and graphene-like materials, and only 2and 8, respectively, refer to PU composites.

This review shows the results of the search for methods offormation of PU composites involving fullerene C60, CNT, GNSand ND for preparing advanced functional materials. It includesthe data on properties and possible elds of application andreports on possible mechanisms of the effect of CNP on apolymer matrix, which provides better performance

Margarita Gafurova studiedchemistry at the St PetersburgState Institute of Technology. Shereceived her PhD at the Instituteof Problems of Chemical Physicsof Russian Academy of Science in1988. At present, she works as asenior researcher at the sameInstitute. Her research interestsare focused on the synthesis andcharacterization of polymercomposite materials, includingnanocomposites.

J. Mater. Chem. A

characteristics of PU materials. There is also brief informationon the history of the discovery, preparation methods andproperties of CNP in corresponding sections of the review.

An important problem should be noted, which arises whennanoparticle–polymer composites including PU–CNP systemsare formed. This problem consists of uniform distribution ofnanoparticles in a polymer matrix. In the absence of uniformdistribution, the potential capability of such composites cannotbe realized in full. The problem is a result of the high surfaceenergy of nanoparticles and, as a consequence, the tendency toform stable aggregates hardly susceptible to disintegration inwater and liquid organic media including polymers.

Polymer composites containing CNP can be prepared eitherby covalent bonding of nanoparticles and their functionalizedderivatives with a polymer matrix or mechanical introduction ofCNP into a polymer when no chemical bonds are formedbetween the matrix and nanoparticles. The use of functional-ized CNP is preferable since two tasks at the least can be solvedsimultaneously:

- the task of compatibility of nanoparticles and a modiedmaterial and uniform distribution of CNT is solved easier; moreefficient modication is attained as a result of the formation ofstrong bonds between CNT and a matrix.

- the presence of chemically active functional groups on thenanoparticle surface allows them to be used as building blocksin organic synthesis, particularly, in the formation of PUnanocomposites.

2 Fullerene containing polyurethanecomposites

Fullerene molecules are empty closed polyhedrons. Thefullerene cage is formed by pentagons and hexagons composedof only carbon atoms. The number of carbon atoms varies fromtens to hundreds depending on fullerene synthesis conditions.Fullerenes, which follow the rule of isolated pentagons, each ofwhich is surrounded by ve hexagons, aremost stable. FullereneC60 found in the products of graphite vapor condensationformed under laser pulse in a helium atmosphere8 (the NobelPrize in chemistry 1996) follows this rule. C60 is the most acces-sible compound of the fullerene family. Themajority ofmethodsfor preparation and investigation of properties and possibleelds of application have been developed for fullerene C60. Thepossibility of ‘placing’ various functional groups on the surfaceof the cage of theC60molecules is determined by its high enoughreactivity caused by unusual structure of the molecules.9,10

Fullerene containing PU can be prepared using a classicalscheme of urethane formation. Covalent bonding of a fullerenecage with a polymer matrix can be realized only if surfacefunctional groups of the fullerene (rst of all hydroxyl ones) areable to react with isocyanate groups (–NCO) of a co-reagent.†

† Apparently, it is possible to prepare poly(isocyanate) derivatives of fullerene typeC60(NCO)n or C60(RNCO)n. However, the synthesis of such compounds is complexenough, and their use as polyfunctional reagents requires maintaining specialconditions for storage and reactions due to the high sensitivity of isocyanategroups to moisture.

This journal is ª The Royal Society of Chemistry 2013


Fig. 2 Scheme of the parent compounds for cross-linked PU (adapted fromref. 29).

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Two types of hydroxyl containing fullerene derivatives areknown: (a) fullerenols C60(OH)n, in which oxygen atoms aredirectly bound to carbon atoms of the cage and n can be up to27,11 and (b) C60(ROH)m, in which OH-groups are bound withthe fullerene cage through alkyl, aryl, alkyl- or arylamine andother linkers. There are quite enough methods for preparationof hydroxyl containing fullerene derivatives. In particular, thesynthesis of fullerenols comprising 16 OH-groups by hydrolysisof polynitroadducts,12,13 polycyclosulfonated derivatives of C60

(ref. 14) and polybromosubstituted fullerenes15 yielded prod-ucts containing 10–12 hydroxyl groups.

Fullerenols bearing a great number of hydroxyl groups areprepared, particularly, by the reaction of borane derivatives offullerene with a H2O2 + NaOH mixture using various cata-lysts.11,16,17 Methods for preparation of hydroxylated fullerenesand for synthesis of water soluble fullerenols are generalized inref. 18 and 19, respectively. Preparation of hydroxyl containingfullerene derivatives of the second type is described, for example,ref. 20–25. In particular, some publications20,23–25 report on thesyntheses of, correspondingly, mono-(hydroxyalkyl)-fullerene –

C60(CH3)5(CH2)3OH, bis-(hydroxyaryl)-methane-fullerene – C60-C(PhOH)2, an adduct bearing three OH-groups, and poly-(hydroxyalkyl)-fullerene – (Alk)nC60(CH2CH2OH)n (PHAF), wheren is 5 on average. Generally, fullerene derivatives of the secondtype involve a lower number of hydroxyl groups than fullerenols.

However, someof the hydroxyl fullerene derivatives cannot beused for the formation of PU composites because of complicatedsynthetic procedures, instability of some composites or poorcompatibility with components used in syntheses of PU.

It is reported26 that star-shaped and cross-linked PU weresynthesized using the reaction of fullerenol prepared by thedescribed procedure14 with macrodiisocyanate based on poly-tetramethylene glycol (PPMG) and 4,40-diphenylmethane diiso-cyanate (MDI). Star-shaped polymers were prepared in a largeexcess of macrodiisocyanate. Non-reacted isocyanate groupswere deactivated by 1-dodecanol aer the completion of reac-tion. The polymers appear as soluble viscous liquids. At aNCO : OH < 10 ratio, cross-linked polymers with fullerenecrosslinks were obtained. In thermal stability and physical–chemical parameters, new cross-linked polyurethanes essen-tially exceed model PU based on the macrodiisocyanate andconventional cross-linking agent, namely, 1,1,1-tris-(hydroxy-methyl)-ethane. For example, the value of penetration

Fig. 1 Scheme of synthesis of PU bearing C60 pendants (adapted from ref. 23).


threshold, according to the data of thermomechanical analysis,was 190 �C for cross-linked PU containing fullerene, 130 �C forconventional cross-linked PU, and 120 �C for linear (non-cross-linked) PU. The value of breaking elongation (up to 580%) forcross-linked PU containing fullerene is 9 times higher than thatfor model cross-linked PU and 16 times higher than that forlinear PU.26,27 The system comprising cross-linked PU contain-ing fullerene with an applied layer of conducting polyanilineretains conductivity under extension up to 500% elongation.28

The authors also prepared star-shaped PU via the reaction of thepre-polymers with fullerene, which showed higher thermalstability as compared with linear PU elastomers and with cross-linked 1,1,1-tris-(hydroxymethyl)-ethane.

The data reported in the publications16,27,28 are evidence ofgreat advantages of using fullerenols instead of conventionallinkers in syntheses of PU.

Linear PU with fullerene side spacers were synthesized usingthe reaction of equimolar quantities of hexamethylene diiso-cyanate (HMDI) and bis(4-hydroxyphenyl)-methanofullerene inthe presence of a catalyst, namely, 1,4-diazabicyclo-[2,2,2-octane] (DABCO), see Fig. 1.23

The authors conclude from the data of UV-Vis spectroscopyand cyclic voltammetry that the electron structure of C60

remains unchanged in the prepared composite material andthis allows unusual properties of the material to be predicted.

Cross-linked PU lms, which show large ultrafast nonreso-nant optical third-order nonlinearity at telecommunicationwavelengths, were formed via the reaction of a trifunctionalhydroxyl derivative of fullerene with multi-functional isocyanatebased on trimethylolpropane (1,1,1-tris-(hydroxymethyl)-propane, TMP) and n-xylylene diisocyanate24,29 as shown in thescheme of Fig. 2.

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The reaction of urethane formation can be used to gramacromolecules bearing corresponding terminal functionalgroups to the C60 molecule. For example, an amphiphiliclinear polymer with a fullerene core and two polyethyleneoxide arms attached to it via the urethane fragment wasprepared from fullerenol, diisocyanate and polyethyleneoxide. This polymer forms spherical aggregates in suchsolvents as water, tetrahydrofuran (THF) and dimethylforma-mide (DMF). The aggregates involve 540–1020 individualmacromolecules, as was shown by gel permeation chroma-tography, transmission electron microscopy (TEM) and lightscattering.30

The authors of ref. 31 and 32 for the rst time used twoapproaches to the synthesis of fullerene containing PU: (i) theuse of prepared PHAF25 as a cross-linker similarly to thatreported in ref. 26 and (ii) the addition of native C60 to thereaction mixture.

In the rst case, cross-linked PU elastomers were preparedvia the reaction of polypropylene glycol (PPG), 2,4-toluylenediisocyanate (TDI) and PHAF at an equimolar NCO : OH ratio.It was shown that the concentration of network nodes inprepared PU almost coincides with the concentration ofintroduced PHAF, while the use of TMP as a cross-linkerunder similar conditions provides a linking efficiency equalto �30%. The authors suppose that hydroxyl groups of thePHAF molecule are incapable of intramolecular association,which is a main factor for inefficient intramolecular cycliza-tion when using conventional aliphatic polyols as cross-linkers.

In the second case, fullerene was added as a toluene solu-tion (the solvent was then removed) to the reaction mixture ofPPG, TDI and 4,40-diamine-3,30-dichlorodiphenylmethane as across-linker. The concentration of C60 was varied from 1.2 �10�7 (0.009 wt%) to 4.8 � 10�7 mol cm�3 (0.036 wt%), whilethe concentration of chemical nodes was independent of theconcentration of fullerene and was �10�4 mol cm�3. Never-theless, it is seen from Table 1 that a small addition offullerene provides essentially higher performance parametersof the material, namely, strength, Young's modulus andbreakdown elongation of the elastomer. It should be notedthat depending on the concentration of fullerene all para-meters pass through maxima. The observed phenomenon ismost probably due to that at low concentrations (a different

Table 1 Dependence of physical–mechanical parameters of PU based on PPG,TDI and 4,40-diamino-3,30-dichlorodiphenylmethane on the concentration of C60

(based on ref. 31)

No. ofsample

[C60] � 107,mol cm�3

Tensilestrength, MPa

Young'smodulus,MPa

Breakdownelongation, %

1 0 26.6 32.8 4802 1.2 32.4 46.5 5803 2.0 35.1 57.2 5904 3.0 32.9 55.5 3405 4.8 30.1 43.4 320

J. Mater. Chem. A

value for each polymer) fullerene uniformly dispersed in apolymer seemingly appears as complexes of individual mole-cules or small clusters with macromolecular chain fragments.With increasing concentration of C60, its molecules and/orsmall clusters aggregate to large clusters weakly soluble in thepolymer. The clusters are less efficient as modiers. Possibly,the size of the formed aggregates is higher than the nanosizedones. As a result, there are changes in the mechanism of theinuence of fullerene on structure and, hence, properties ofthe polymer.

One of the important elds of application of fullerene andother CNP as additives to polymers is the task of higher thermalstability of materials including PU, and lower content of toxi-cants formed as a result of thermal decomposition (combus-tion). The methods for investigating the mechanism andkinetics of decomposition are described in detail and para-meters determining thermal stability of PU and methods forstability increase are considered.33 Of particular interest amongthe latter ones is the use of functionalized derivatives of C60

which can form covalent bonds with the polymer matrix. Thederivative can be used in the absence of such bonds as well.Along with various organoelement and inorganic modiers,carbon nanotubes and thermally expanded graphite are alsoused.

An interesting example of the use of fullerene in the designof an unusual polymeric material is described.34 Using TEM,SEM and SAXS, the authors showed that the particles of poly-urethane amide dendrimer of the 3rd generation attached to thefulleropyridine molecule form aggregates in the THF–watermixture. The aggregates appear as ribbons with ordered layersof the C60 derivative located between ribbons. The authorsdetermined the length and wideness/thickness ratio for theobserved assemblies. It is suggested that their formation is dueto the ability of urethane and amide groups to form hydrogenbonds between dendrons (Fig. 3).

There are publications reporting on the effect of minoramounts of the fullerene-containing products, for example,mixtures of fullerenes C60 and C70, fullerene-containing carbonblack and others on the properties of polymers. Thus, theauthors of ref. 35 observed a signicant increase in tensilestrength of PU bearing terminal trimethoxysilyl groups, aertheir cross-linking upon addition of 0.01–0.5 wt% of fullerenecarbon black containing fullerene (up to 2.5 wt%). Breakdownelongation also somewhat increases but Young's modulusdecreases considerably at the same time. Insertion of fullereneC60 or the mixture of fullerenes C60 and C70 (0.01–0.06 wt%)enhanced Young's modulus of epoxy resin and very slightlychanged its compression strength and breakdown elonga-tion.36 However, the mixture of C60 and C70 (0.02–0.08 wt%)increased Young's modulus for polyamide-12 almost by 1.5times and its compression strength more than by 1.5 times.Pure C60 is less effective to these parameters. Nevertheless,breakdown elongation increases by 25–100% in both cases. Itis interesting that all mechanical parameters of polyamide-12signicantly increase at the insertion of 0.1–0.5 wt% ofgraphite, channel black or fullerene black washed fromfullerene.



Fig. 3 Scheme of the dendrimeric derivative of C60 (adapted from ref. 34).


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3 Polyurethane–carbon nanotubecomposites

Stretched hollow cylindrical structures, namely, CNT werefound additionally to fullerenes in products of carbon vaporcondensation formed as a result of electric-arc evaporation ofgraphite.37 CNT are specied by a high aspect ratio, i.e. lengthcan be hundreds and thousands times longer than diameter (dis from <0.7 nm to dozens of nanometres). Now many methodsfor preparation of CNT have been developed additionally to theelectric-arc one.38–40 These nanoparticles are specied by varioustypes differing in the number of layers (single-wall carbonnanotubes (SWCNT) are graphene sheet rolled to cylinders, andmulti-wall carbon nanotubes (MWCNT) are such cylinders(walls) inserted into one another), geometry, and chirality,which is determined by the angle of orientation of the graphenelattice relative to the tube axis. The orientation affects electricproperties of CNT, which either possess metallic conduction orsemiconducting properties.

A combination of such properties as ultra-small size and highaspect ratio, high specic surface, record high values of strengthand Young'smodulus, thermal and chemical stability combinedwith capability to chemical conversion, high electrical conduc-tivity ofmetallic and emission ability of semiconductingCNT is areason for which nanotubes are investigated in both academicand applied research. Academic research of CNT is associatedwith that CNT simultaneously show properties of molecules andsolid and can be considered as a state of matter intermediatebetween molecular and condensed ones. It can be assumed thatsuch exotic matter can show unexpected features of behaviorunder different conditions including polymeric media.

One of the elds of search for commercial use of CNT is theanalysis of possibility of their use in the development of poly-mers, particularly, polyurethane composites with improvedperformance properties.

3.1 The problems of formation of the nanocomposites andthe ways of their solution

Polymer composites can be formed by introducing both nativeCNT and their functionalized derivatives into polymers. The


possibility of functionalization of CNT is provided by their highenough reactivity. It is suggested by many researchers that highreactivity of CNT is due to distortions of a graphene lattice(defects) generated in the synthesis of CNT or as a result ofexternal strain inuences. The surface defects accumulateexcess surface energy and, hence, show high reactivity ascompared with a regular graphene structure.

Though there are enough reactive centers, the use of nativenanotubes when forming polymer composites is difficultbecause of the tendency to aggregation due to high surfaceenergy related to distortion of the cylindrical surface on the onehand, and a high aspect ratio providing cooperative van derWaals interaction between nanotubes on the other hand. Theaggregates generally formed as bundles or bars consisting oftens and hundreds of individual nanotubes are with difficultydispersed to smaller species. This hinders their solubilizationin dispersion media, impedes compatibility with polymermatrices, and results in a lower value of the active surface ofnanoparticles, accessible to forming interphase polymer–CNTinteractions.

Functionalization of the CNT surface provides easier deag-gregation and, hence, higher ability of CNT to solubilize since itprovides lower surface energy, disturbs a cooperative characterof van der Waals interaction between nanotubes and provideshigher accessibility of the CNT surface to interaction withreagents and dispersion medium. Experiments with nanotubesinvolve most oen sonication (ultrasonic machining, USM) ofthe reaction mixture, which intensies deaggregation processesof nanotubes and gives rise to defects on their surface. This, inturn, provides higher chemical reactivity of the tube. CNTbundles deaggregate essentially easier in organic media andwater in the presence of surfactants (SF).

The chemistry of CNT has been studied well. It has beenshown that CNT react with various compounds from almost allclasses of chemical compounds, both organic and inorganic, toform adducts bearing various functional groups.41–46

PU composites were prepared using non-functionalized(native) CNT,47–50 oxidized51–55 and aminated CNT,56–58 and CNTcontaining ester48 and other functional groups.59–61

Many methods for CNT oxidation are known.62–68 Themethod used most commonly is treatment of CNT with a

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HNO3 + H2SO4 mixture. In some cases HCl is added to thereaction mixture.62–64 As a result, COOH-groups are graed toCNT. Hydroxyl, carbonyl and ether groups are formed atessentially lower concentrations. Oxidation is the most impor-tant type of functionalization of CNT since oxygen containingsurface groups can perform as linkers for bonding other variousfunctional groups and fragments, namely, biologically active,uorescent, oligomeric and polymeric including polyurethaneones.

3.2 Preparation methods and properties of thenanocomposites

A noticeable increase in strength and Young's modulus of acomposite as compared with the parent polymer can beobtained by insertion of CNT into PU. Some other propertiescan improve as well. However, positive results to a great extentdepend on the method of insertion of CNT into a polymer. Bynow, many methods for the formation of polymer compositeswith CNT have been described (see, for example, the reviews 42and 69–72). The method for preparation of nanocompositesdepends on the structure of PU, kind of CNT, the presence andtype of surface functional groups and purpose of end products.

The primary prerequisites for the preparation ofmost efficientcompositematerials are uniformdistributionofCNT ina polymermatrix and optimal interphase interaction, which could provideefficient mechanical stress transfer from the polymer matrix tonanotubes. An inhomogeneous distribution results in highbrittleness of the composite that manifests itself in the destruc-tion of individual CNT at relatively low loads with respect to thewhole sample.Obviously, this ispossibleprovidedonly that strongbinding between CNT and amatrix exists. This phenomenon wasobserved, for example, for the system MWCNT–urethane–dia-crylate oligomer EBECRYL 4858.73 The composite was formed as alm of �200 mm thickness by photopolymerization on a glassysupport. The lm was subjected to extension, and cut into thinlayers parallel to the surface with a diamond blade. According tothe TEM data, nanotubes in a polymer matrix had multipleruptures under mechanical force. The calculated normalizedlength of the destroyed nanotubes, l/DNT, was from 5 to 20 (l –length, DNT – outer diameter of nanotubes).

One can develop the most efficient composites if the func-tion of CNT distribution in a polymer is known and the factorsaffecting the function are optimized. For example, the effect ofelectric elds with 10 Hz and 1 kHz frequencies on the distri-bution function and degree of orientation of SWCNT (0.03 vol%) upon photocopolymerization of the dimethyl acrylate–urethane oligomer with hexane diol dimethylacrylate wasinvestigated using Raman spectroscopy, optical microscopy andconductivity measurements.74,75 It was found that a higher-frequency eld provides better mutual orientation of nanotubesin a copolymer matrix. The percolation threshold in the elddirection is attained even at SWCNT concentrations lower than0.03 vol%, and low-frequency conductivity of the compositeincreases by �8 orders of magnitude as compared with unlledpolymer. The presence of nanotubes at such concentrations

J. Mater. Chem. A

very weakly affects the conductivity of copolymer at zero-eldpolymerization.

It is reasonable to consider two principally different methodsfor the formation of polymer nanocomposites with participa-tion of CNT: (i) CNT are added to the prepared polymer and (ii)the urethane formation reaction is performed in the presence ofCNT – the in situ method.

In the rst method it is possible either to use solvents to bothdissolve polymers and disperse CNT (generally to prepare forthin lms), or mix nanoparticles with a polymer melt.

Obviously, the rst method is applicable to linear polymersand cannot be used to synthesize cross-linked PU nano-composites, which can be prepared using the in situ methodonly. When using this method it is possible to mix liquidcomponents of the prospective composite with CNT withoutusing any solvent or the components dissolved in a volatilesolvent are mixed with nanoparticle dispersion and the solventis removed either under vacuum upon reaction mass stirring orwhen the composite is cured as a thin lm.

3.2.1 Synthesis from prepared polymers. Each particularsystem used to prepare composites requires an individualapproach to the choice of the most efficient synthetic proce-dure. If the polymer is soluble in volatile solvents, it is reason-able to add CNT to solution of the prepared polymer since itresults in the most uniform distribution of nanoparticles in apolymer matrix and, consequently, the best characteristics ofthe composite. A disadvantage of this method is the possibilityof reaggregation of a nanomodier upon slow evaporation ofthe solvent. Economic and ecological restrictions sometimesprevent the use of this method. Nevertheless, themethod can beused in laboratory experiments. In industry it can be used toprepare thin lms or bers and foam plastics since the solventcannot be removed from bulky blocks. Solutionmethods are theonly usable ones when polymers are either infusible or ther-mally unstable at temperatures higher than the plastic statetransition temperature.

In most cases bulky products can be fabricated from nano-composites by mixing llers with polymer melts. For thispurpose, it is necessary to use high-power and efficient mixingmachines able to process high-viscous mixtures. The method iseconomy and environmentally friendly, and can be efficientenough for polymers, which do not tend to thermal decompo-sition in melts. However, PU based composites are very rarelyprepared bymixing inmelt. For example, it was found that high-strength composite bers can efficiently be prepared by theelectrospinning method when PU (and polystyrene as well) aremixed with SWCNT in melt.48 Young's modulus for lled PUbers was observed to be 27 times higher as compared withnon-lled PU bers. The strength of membranes based on PUbers containing 1 wt% of SWCNT is 46% and 104% higher fornative SWCNT and SWCNT functionalized by ester groups,respectively. Young's modulus of the membranes from thecomposite bers increases by 215 and 250%, respectively, ascompared with non-lled bers. TEM measurements show thatsmall-size SWCNT bars are oriented along the ber axis.

Generally, PU–CNT composites are prepared by mixingdispersions of CNT with solutions of PU followed by solvent



Fig. 4 Scheme of grafting of segmented PU macromolecules to MWCNT(adapted from ref. 81).


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removal by evaporation. The components of so-preparedcomposites can be bound both covalently and non-covalently.

It is reported76 that native and hydroxylated SWCNT (0.1–2.6wt%) were added to the solution of the prepared polymer usingUSM followed by solvent evaporation from thin lm. Thenanocomposite was formed using a block-copolymer based on aPTMG polyurethane thermoplastic elastomer with a highcontent of hard segments of non-aromatic nature, which provedto be benecial for dispersing nanotubes. Young's modulus andtensile strength of the composite inclusive functionalized tubesincrease by a factor of�1.5 with respect to the characteristics ofunlled PU and conductivity increases approximately by twoorders of magnitude aer the percolation threshold has beenattained.52 However, in the case of native SWCNT, conductivitycan increase by 8 orders of magnitude, and the differences inphysical–mechanical characteristics of composites with SWCNTof both types are considered by the authors to be statisticallyincidental.

CNT modied by graing both low-molecular-weightcompounds and macromolecules can be used to prepare poly-mer nanocomposites.42,77 Highest dispersancy of CNT in amatrix is attained if a polymer well-compatible or identical tothe polymer of a matrix is graed. It is reported60,61 that SWCNTfunctionalized by alkoxysilane groups were treated by poly-urethaneurea block-copolymers based on polydimethylsiloxanebearing either [tris-(ethoxy)silyl]propylurea or phenyl-tetrae-thoxydisiloxane terminal groups, TDI and glycerol, which wasused as a chain extender. In the rst case, the block-copolymercovalently bound to MWCNT. In the second case, the formationof non-covalent complexes due to –p–p interactions was onlyobserved. However, in both cases strength increases by as muchas 3–4 times with growing content of nanotubes to 5 wt% in thecomposite as compared with the parent block-copolymer. Highdispersancy of graedMWCNT in a block-copolymer matrix wasnoted.

Graing of a polymer similar to the polymer of the matrix toCNT was reported.78 Macromolecules of segmented PU preparedfrom tetramethylene glycol, TDI and 2,2-bis(hydroxymethyl)propionic acid as a chain extender were graed to MWCNT pre-functionalized with chloroanhydride groups. These PU con-tained side carboxyl groups in the backbone. The side carboxylgroups reacted with chloroanhydride groups of MWCNT to formanhydride groups covalently bound with the nanotube surface.Upon addition of MWCNT (from 1 to 10 wt%) well dispersed inDMF to the solution of the same PU, Young's modulus increasesby 70%, strength of thematerial increases by a factor of three andglass-transition temperature increases by 15 �C, while break-down elongation decreases by as much as >1.5 times.

A PU composite with tensile strength 270% and Young'smodulus70%higher thantheparametersof theparentpolymerwasprepared by mixing the solution of PU oligomer bearing terminalNCO-groups and dispersion of aminated MWCNT (4 wt%).57

Composite lms including those with increased electricalconductivity are formed generally using the method of castingfrom solution.49,79,80 For example, the conductivity of compositelms prepared by casting from the mixture of water dispersionof PU andMWCNT (sodium dodecyl sulfate as a SF) was 3 orders


of magnitude higher than that of parent PU even at a CNTconcentration of 0.5 wt%. With increase in the concentration ofnanotubes to 7 wt%, lm conductivity remains almostunchanged being 2–4 orders of magnitude lower than that ofconducting or semiconducting tubes.80 Nevertheless, thermalconductivity of lms at a CNT concentration of 7 wt% increasesno more than 8.6%.

The effect of structure of rigid segments of PU moleculesgraed to MWCNT on the electrical conductivity of compositelms was investigated.81 The rigid segments were formed from1,4-butanediol (BD) and either aliphatic diisocyanate, hexa-methylene diisocyanate (HMDI), or aromatic MDI. The exiblesegments were formed from polyethylene glycol (PEG), Fig. 4.

PUmatrices were prepared fromsimilar components inDMF.Composite lms were cast from the mixture of dispersion offunctionalized CNT with PU dissolved in DMF. It was found thatthe structure of the rigid segments affects dispersancy ofMWCNT in PU: compatibility of MWCNT modied by PU basedonHMDIwith the similar polymer is higher than that ofMWCNTmodied by PU based on MDI with the MDI based polymer. Theauthors suppose that forMDI, the ability of the rigid segments tocrystallize due to hydrogen bonds formation is lower than forHMDI because of steric hindrances produced by bulky phenylgroups. The authors also suggest that forMDI, it results inpoorercompatibility of MWCNT with a matrix and provides theformation of larger MWCNT aggregates in a matrix. A result ofsuch aggregation is a lower percolation threshold (0.52 wt% ascompared with 0.69 wt% for HMDI composites) and a higherconductivity of composites at a saturation concentration of 2.5wt% of MWCNT for MDI derivatives as compared with HMDIderivatives (�10�2 S cm�1 against �10�4 S cm�1).

The effect of nature and content of rigid segments in PUbased on polycaprolactone diol (PCLD) and polycarbonate(exible segments), HMDI or MDI with BD (rigid segments) onthe electrical conductivity and mechanical properties of nano-composites was studied in a wide range of concentration ofcarboxylated MWCNT (from 0.1 to 20 wt%).82 The compositeswere prepared in a THF : DMF mixture of 1 : 1 ratio. It has beenshown that the percolation threshold corresponds to a volumefraction (f) of MWCNT of �0.02, and at f � 0.15, conductivityattains the value of �1 S cm�1 close to that of pure MWCNT,while conductivity of native PU is close to 10�9 S cm�1. Young's

J. Mater. Chem. A


Fig. 5 Scheme of in situ encapsulation of MWCNTwith a polymer layer (adaptedfrom ref. 84).


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modulus and strength of composites increase by as much asseveral times with the concentration of MWCNT. For PU with aminimal content of the rigid segments these parameters grow toa greater extent. However, the values of breakdown elongationdecrease by almost an order of magnitude. It is noted thatchanges in mechanical parameters with growing f arepronounced more drastically for PU based on HMDI rather thanonMDI. The authors assume that the strengthening role of CNTis due to that their length essentially exceeds the size ofdomains formed by both exible and rigid segments. Therefore,CNT bind with several nodes of a physical network simulta-neously to make it more rigid, and the character and strength ofthese bonds depend on both the fraction of rigid segments inPU and their nature.

Water-soluble SWCNT functionalized by PEG or poly-(aminobenzene sulfonic acid) (PABSA) and THF-soluble CNTfunctionalized by octadecylamine (ODA) were used to preparecompounds with segmented thermoplastic PU (TPU).58 Thispolymer was used as water emulsion with particles of <3 mm size(Waterborne Polyurethane Dispersion –Hydrosize�U2-01). Theproperties of prepared composites sharply differ from eachother. Water-soluble nanotubes promote crystallization of ex-ible segments, while THF-soluble SWCNT promote crystalliza-tion of rigid segments. The authors suppose the selective effectof nanotubes on so and rigid segments depends on the surfacechemistry of CNT and structure of segments. Composite lmscast from THF solution tended to retain strength and elasticityat higher levels of lling. The rst type lms lost strength andembrittled at CNT concentrations higher than �10 vol%. Theauthors assume that the loss of elasticity was due to that water-soluble SWCNT preclude exible segments straightening underloading. Being concentrated in rigid segments, the second typeSWCNT do not affect mechanical properties of composites.Such controlled reinforcement allows one to form compositeswith a Young's modulus up to 250 MPa and a plateau of highelasticity of 8 MPa for water-soluble composites at a content ofnanotubes equal to �12.5 vol%. Maximal strength (�60 MPa)and maximal values of breakdown elongation (to 750%) wereattained for both ODA–SWCNT and PABSA–SWCNT. In bothcases these values corresponded to a concentration of SWCNTof #1 vol% and with increasing concentration rapidly drop forwater-soluble composites. The experimental values of physical–mechanical parameters of composites were essentially higherthan those for unlled PU.

Mechanical properties of composites can be governed by theintroduction of controlled quantities of CNT into thermoplasticPU.83 Young's modulus of composites En grows from 0.4 up to2.2 GPa, stress at 3% deformation, s3¼3%, grows from 10 up to50 MPa when the CNT concentration changes from 0 to 40 wt%at almost unchanged tensile strength sb, but breakdown elon-gation 3b and impact strength T decrease dramatically from 555down to 3% and from 177 down to 1 MJ m�3, respectively. As aresult, composites with low contents of CNT are similar toelastomers in their properties, and at high concentrations ofCNT they behave similarly to thermoplastics. Material inclusiveof �15 wt% of CNT behaves as a rigid thermoplastic showinghigh Young's modulus and impact toughness (1.5 GPa and

J. Mater. Chem. A

50 MJ m�3, respectively). The stress at 3% deformation is36 MPa, the tensile strength is 55 MPa, and the breakdownelongation is 100%. The authors discuss the mechanism ofpolymer strengthening, which is considered to a great extent tobe due to binding of exible segments of thermoplastics bytheir adsorption on the nanotubes surface. Such binding resultsin changes in the ratio between exible and rigid segments infavor of the latter.

Prima facie, the mechanisms of the effect of CNT on prop-erties of TPU based composites proposed by the authors inworks of ref. 58, 82 and 83 contradict one another. However,these contradictions are most probably seeming since allproposed mechanisms can be realized, and the contribution ofeach mechanism depends on particular conditions, namely,matrix nature and structural features of CNT (aspect ratio,degree of dispersion, the presence and chemical nature ofgraed groups, etc.), and their concentration in a composite.

3.2.2 In situ synthesis. Additionally to the use of preparedpolymers, most of the work reports on the in situ preparation ofPU–CNT nanocomposites. Many researchers prefer a solvent-free method when nanotubes are dispersed in one of theprecursors used in the synthesis of desired polymers. Thisallows one to avoid the problem of limited survivability of areaction mixture. Most commonly used CNT bear varioussurface functional groups – from small COOH and OH to bulkypolymer structures prepared by multi-step syntheses. Thefunctional groups provide the formation of strong covalentbonds between CNT and a polymer matrix in the syntheses ofcomposites.

Multi-step functionalization is described, for example, in ref.84. OH-groups primarily attached to the MWCNT surface wereetheried by the reaction with poly(acryloyl chloride). Excesschloroanhydride groups were etheried by ethylene glycol. Theso-prepared polyol graed to CNT (concentration of MWCNTwas �10 wt%) was used to form PU by a step-by-step interactionwith MDI and BD in DMF (Fig. 5).

Composite strength at 0.1 wt% MWCNT increases by 60% ascompared with parent PU, and Young's modulus increases by�6%.

It is reported85 that PPG and TDI based macrodiisocyanateand PCLD were graed step-by-step to hydroxylated SWCNT.Then modied nanotubes were dispersed in PCLD, andsynthesis of PU was performed via the reaction with MDI andBD. At 0.7 wt% of a nanomodier the prepared composites



Table 2 Physical–mechanical characteristics of the PU composites89

Samplenumber

Concentrationof CNT, wt%

Young'smodulus,MPa

Tensilestrength,MPa

Breakdownelongation, %

1 0 3.65 � 0.18 1.71 � 0.07 675 � 562 0.2 6.24 � 0.37 4.20 � 0.15 1337 � 93 0.5 6.60 � 0.23 7.79 � 0.13 1223 � 714 1.0 7.00 � 0.05 10.18 � 0.16 1172 � 27


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show enhanced thermal stability. Their strength increases by�40%, and Young's modulus increases by 300% as comparedwith parent PU, without noticeable drop of breakdown elonga-tions. As compared with the composites described above, thoseprepared by adding native SWCNT to PU show high strengthand lower Young's modulus, and their thermal stability isnoticeably lower than that of non-modied PU.

A similar investigation was performed using hydroxylatedMWCNT, PPG and TDI based macrodiisocyanate as the rstoligomer, and the second one was trifunctional PPG (MM was6000).86 Strength and Young's modulus for the composite con-taining 1 wt% of graed MWCNT increase by �30% ascompared with parent PU and viscosity of the uncured mixtureincreases as compared with that of parent polyol to a less extentthan at similar addition of native or hydroxylated MWCNT.

It should be noted that though the data reported in publi-cations 85 and 86 were obtained under similar conditions andalmost by the same research team, they partially contradict eachother possibly because of the use of different type nanotubes(SWCNT and MWCNT) and polymers differing in composition.This is additional evidence of that each system requires anindividual approach for optimum results.

The preparation of PU–carboxylated CNT composites isdescribed.52,87 A segmented polyurethane block-copolymerbased on macrodiisocyanate (PCLD + MDI) and TMP involving36% of rigid segments was prepared in the presence ofcarboxylated MWCNT (Fig. 6). As a result of the reactionof excess hydroxyl groups of TMP with carboxylic groups ofMWCNT, covalent ester bonds of a polymer matrix with thenanotube surface are formed at 100 �C that results in theformation of a cross-linked insoluble polymer.

The introduction of carboxylated MWCNT into block-copolymers (1–5 wt%) provides essentially higher Young'smodulus and strength and thermal stability of the polymers. Itis noted that the introduction of non-modied MWCNT at asimilar concentration is accompanied by the sharp decrease instrength and breakdown elongation.

Composites of MWCNT and hyperbranched PU based onPCLD, MDI and castor oil were also prepared by in situ poly-merization. The concentration of rigid segments in thecomposites was different. The composites were well soluble inorganic solvents (up to 40 wt%).88 The composites show a well-pronounced memory effect and better mechanical properties ascompared with unlled hyperbranched PU.

The PU–MWCNT composites showing better characteristicsas compared with the parent polymer were synthesized using acombination of in situ polymerization and lm casting from

Fig. 6 Scheme of the PU–carboxylated CNT composites (adapted from ref. 87).


solution.89 A pre-polymer based on poly(ethyleneglycol adipate)and isophorone diisocyanate (IPDI) prepared in methyl ethylketone (MEK) was mixed with MWCNT dispersed in DMF. Thena polyurethaneurea polymer was produced via the reaction withisophorone diamine. TEM and SEMmeasurements showed thatnanotubes are homogeneously enough distributed in a polymermatrix and weakly aggregated. Physical–mechanical character-istics of the composite lms are listed in Table 2.

Young's modulus for the composites increases by 70% evenat 0.2 wt% of MWCNT and weakly grows to 1 wt% of SWCNT,while tensile strength grows monotonically from 1.7 to 10.2MPa in the same concentration range. The values of breakdownelongation almost double at 0.2 wt% of nanotubes and slightlydecrease with further increase of their concentration. Thetemperature of the beginning of thermal decompositionincreases from 350 up to 362 �C.

The introduction of 0.1 to 0.5 wt% of alkoxysilylaminatedMWCNT shortened to an aspect ratio of 15–100 into thepolyurethaneurea composition based on PTMG, MDI and ami-noethyl-aminopropyl-triethoxysilane or aminopropyl-tri-ethoxy-silane in DMF allows transparent lms to be prepared in situ.The degree of swelling of the lms increases in water and dropsin DMF with increasing concentration of nanotubes. At themaximal percentage of CNT, material strength increases by 60%and Young's modulus increases by as much as >3 times at smallchanges in the values of breakdown elongation.59

Cross-linked products were prepared as lms by UV photo-polymerization of MWCNT suspended with hyperbranchedpolyurethane resin.90 Covalent bonds between nanotubes and amatrix were found during polymerization. The presence of 0.1wt% of nanotubes in a cured composite provides essentiallyhigher Young's modulus, tensile strength and hardness of theso-prepared lms as compared with those prepared from non-modied resin.

The authors of ref. 91 analyzed the effect of additives to 0.2 wt% of pre-carboxylated MWCNT (40–60 nm in diameter andseveralmmlong) on soundabsorptionproperties of polyurethanefoam based on oligoether and MDI (in the presence of SF)synthesized by the method of free foaming using water as afoaming agent. The electronmicroscopy data show that CNT arehomogeneously dispersed in the polymer of open porous struc-ture. The prepared foams are specied by essentially higherstrength and Young's modulus with the increase in concentra-tion of MWCNT at a relatively small increase in density. Thecomposite lms prepared by hot pressing show a 1.3-foldincrease in acoustic activity (integral acoustic absorption coef-cient) with the CNT concentration increase up to 1 wt%.

J. Mater. Chem. A


Fig. 8 Scheme of cross-linked PU synthesis (adapted from ref. 94).


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Thermally cured tough PU–MWCNT composites weresynthesized by mixing nanotubes with IPDI. Then poly-etherpolyol was added to the suspension and the resultingmixture was poured in Teon molds for preparing samples.92

The addition of 0.1 wt% of MWCNT to PU provides a sharpincrease in breakdown elongation from 80 up to 300%. Young'smodulus increases by �100 and �500% at 0.1 and 1 wt% ofMWCNT, respectively. At similar concentrations tensilestrength increases by 400% as compared with parent PU. Theformation of covalent bonds between nanotubes and a polymermatrix is noted, which provides uniform distribution ofMWCNT in the matrix and their efficient effect on mechanicalcharacteristics of composites.

If in the previous works, PU composites containing from 0.1wt% up to several or even dozens of percent of CNT aredescribed, ref. 93–95 apparently for the rst time report onsignicant changes observed in the main mechanical charac-teristics of the cross-linked poly(urethaneurea) elastomer uponthe formation of nanocomposites with an ultrasmall concen-tration of SWCNT at the level of <0.01 wt%. It was found that thevalues of tensile strength and Young's modulus attain maximalvalues (by factors of 1.5 and 2.5, respectively, than those for theparent polymer) even at a nanotube concentration of 0.002 wt%.With the further increase in concentration, the parameterssharply decrease and at 0.018 wt% of SWCNT tensile strengthlowers as compared with the parent polymer (see Fig. 7),breakdown elongations being about 800%.

In reports94 and 95 the non-trivial experimental data aregeneralized. The cross-linked polyurethaneurea elastomer wassynthesized on the base of macrodiisocyanateurethane, madefrom PTMG and TDI, in which isocyanate groups were blockedby 3-caprolactam (CL). Therefore, the mixture of oligomer with2,6-toluylene diamine as a curing agent can be stored at roomtemperature for a practically unlimited long time. When heat-ing the mixture, diamine forms polyurea chains displacing CLfrom its adduct with terminal NCO groups of the oligomer.Cross-linking is realized due to the formation of biuret groupsin the reaction of excess of NCO groups with urea groups of thebackbone. CL is also displaced at this stage. Free CL polymer-izes to polyamide-6, which forms an individual phase as

Fig. 7 The dependences of Young's modulus and tensile strength of cross-linkedPU on SWCNT concentrations (based on ref. 94).

J. Mater. Chem. A

particles of submicron size as a result of incompatibility withthe PU matrix under curing conditions, as shown in Fig. 8.SWCNT were introduced into the initial oligomer mixture aerUSM of their suspension in a volatile solvent. Then the solventwas vacuum-evaporated upon stirring, the mixture was pouredin plate- or dumb-bell-shaped Teonmolds and cured at 140 �C.

The observed results94 cannot be interpreted in terms ofreinforcement effect of a continuous network formed uponintroducing CNT into a polymer since their concentration isorders of magnitude lower than percolation threshold generallyobserved. The authors assume that the changes in physical–mechanical parameters of the composite are associated with thechanges in structure of a major part of bulk of the polymermatrix under the effect of active surface of CNT. In particular,the effect consists of the essential increase in glass-transitiontemperature for the composite (from ��55 to �45 �C) and theappearance of the second glass-transition temperature at�0 �C.

Such a phenomenon is associated with transferring theorientation effect of matrix layers adjacent to the surface onadjacent matrix domains. The orientation effect transferred‘from point to point’ over dozens and hundreds of nanometerswas predicted theoretically96,97 and observed in experiments97–99

with different polymers and nanosized llers.The extreme character of the dependence of composite

characteristics on the concentration of CNT is interpreted bythe authors of ref. 94 by a sharp increase in the degree ofaggregation of nanotubes aer attaining the threshold ofcompatibility with a matrix of individual CNT or their weaklyaggregated assemblies, and the formation of large aggregateswith smaller active surface, which can perform as structuraldefects in materials. It is shown100 that the observed extremecharacter of the changes in Young's modulus of the compositeat a concentration of CNT of up to �0.015 vol% can bedescribed in terms of the gradient model of an interphase layer.

It should be noted that polymer composites consisting ofCNT in concentrations from tens percent to several percents(>1 O 10 kg per ton) shall nd wide application neither atpresent nor in the nearest future since the scale of production ofCNT are incommensurable with the scale of production ofpolymers, and CNT are high expensive. Nevertheless,




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composites comprising <0.01 wt% (<100 g per ton) of CNT canbe quite accessible, particularly, in cost. These composites havethe obvious advantages: (i) material density remains almostunchanged as a result of lling, therefore, weight of productsdoes not increase – moreover, it can be lowered due to higherstrength; (ii) the problem of contribution of cost of expensiveCNP to cost of nal products shall be solved; and (iii) theproblem of scaling up production of nanocomposites atminimal changes in conventional technologies for productionof PU products shall easily be solved.

Obviously, research in this concentration range is promisingfor both better understanding the nature of interactions in thepolymer matrix–nanoller system and a wide range ofcommercial application.

Interesting results were obtained when studying the prop-erties of a series of hybrid nanostructures synthesized from CNTand hyperbranched PU.101,102 In situ functionalization (one-pot)of carboxylated MWCNT by hyperbranched polyurethaneureassynthesized via the reaction of TDI with diethanolamine isdescribed in ref. 101. It has been shown that a network formsfrom individual nanotubes uniformly covered by a �15 nmthick polymer layer. The percentage of the polymer in theproduct attains 83.5 wt%. The composite is well-soluble in polarsolvents such as DMF, dimethylacetamide, N-methyl-pyrrolidone, and dimethylsulfoxide. The prepared compositesare described in detail in ref. 102. The authors showed that bothintra- and intermolecular hydrogen bonds form due to thepresence of a large quantity of proton donor and protonacceptor groups in the surface layer of the ‘core–shell’ structure.The intramolecular hydrogen bonds provide the formation of‘curved’ conformations of nanoparticles in solution, whichstraighten up at high shear rates and form a network of inter-molecular hydrogen bonds. This is displayed in a sharp increasein viscosity (almost 2 orders) at high shear rates at roomtemperature. Nevertheless, viscosity of solutions drops twice at80 �C and then weakly changes with the shear rate growth. Theauthors attribute such behavior to that though orientationalstraightening of particle is realized at high shear rates, thesystem of hydrogen bonds cannot be formed at elevatedtemperatures and nanoparticles are free in solution. Features ofbehavior of hyperbranched PU–MWCNT solutions were studieddepending on a carbon/polymer component ratio in acomposite.

4 Polyurethane nanocomposites consistingof graphene and graphene-likenanostructures

Search for new nanocarbon modiers to design polymercomposites with improved operation parameters is in progress.At present special attention of researchers is attracted to gra-phene and graphene-like nanostructures. Now there are nounied terms to denote such carbon forms (akes consisting offew graphene layers). Dening graphene as a 2D single-layercarbon sheet of one atom thickness consisting of condensed six-membered rings, it seems to be incorrect to dene structuresconsisting of several (n) layers as ‘n-layer graphene’, and proper


graphene as ‘single-layer graphene’. However, such terms as‘single-layer graphene’, ‘two- and three-layer graphene’, ‘few-layer graphene’, etc. are found in publications. We tried to avoidsuch denitions using the term ‘graphene-like nanostructures’to denote the above carbon nanoforms including graphene.Nevertheless, in some cases we used the authors' terms. Therelative novelty of these nanocarbon objects requires at least abrief preliminary acquaintance with them before presenting themain material of this section.

4.1 Preparation methods and properties of graphene andgraphene-like nanostructures and problems of theirapplications in polymer composites

As mentioned above, graphene is a 2D single-layer sheet of oneatom thickness and is a basic element of the crystal lattice ofgraphite. Graphene layers in the graphite lattice consist ofregular hexagons with 0.142 nm side. Carbon atoms are locatedin vertices of hexagons. The layers are located at �0.34 nmdistances parallel to each other and their double-sided specicsurface is 2630 m2 g�1.103

Interest to graphene was evoked by the pioneering studies byK. S. Novoselov and A. K. Geim et al. on preparation andinvestigation of graphene in ‘free’ state104–110 (the Nobel Prize in2010). As for other carbon nanoparticles, graphene is investi-gated in both academic and applied research work. Interest tographene is evoked particularly by that it combines theproperties of nanoparticles and quantum characteristics ofmacromaterials. Such a combination allows the features of two-dimensional crystal structures to be revealed.103 Commercialinterest to graphene is specied by its high mechanical rigidity,thermal and chemical stability, thermo- and electro-conductivity, good electromechanical characteristics, nonlinearoptical and some other unique properties.103,110–113 Hundreds ofpapers published are aimed at the search for possibilities ofusing graphene in micro- and nanoelectronics, while it showsproperties valuable for other applications including the devel-opment of polymer composites.

Among known methods for preparation of gra-phene,103,104,114–117 the most promising in respect to scaleproduction is the chemical liquid method using colloidaldispersions of precursors comprising graphene layers.115 Pene-tration of solvent molecules into the interlayer space of aprecursor lowers the energy of van der Waals interaction andincreases the distances between the layers to separate them upto exfoliation. This method is advantageous to allow relativelyeasy chemical modication of the GNS surface, i.e. variousfunctional groups are covalently graed depending on thepossible eld of application. Moreover, colloidal dispersions ofgraphene particles are necessary for such technological stagesas mixing with other composite components, casting of lms onsurfaces and others.

Graphite and its derivatives, particularly, graphite oxide (GO)and intercalates of various chemical compounds – products ofmolecules or atoms penetration between graphene layers – aregenerally used as precursors.103,118 Then precursors are con-verted to a set of particles involving structures composed of

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several graphene layers additionally to graphene. Polydispersityof GNS in the number of carbon layers can preclude fromachieving the expectable results when trying to use them inapplied research since the properties of nanoparticles aredependent on structural parameters, especially on the numberof layers. Of importance are also such characteristics as particlesize, boundary structures, type and quantity of defects andfunctional groups formed in layers during synthesis.103,112,119

These parameters are dependent on the preparation procedure,reaction conditions and nature of a precursor. An originalmethod for preparation of GNS was proposed.120 The methodconsists in that graphite is milled in a planetary-type mill inliquid CO2. In contrast to particles prepared from GO of dozensand hundreds of micrometers in size, few-layer particles (fromdozens to hundreds of nanometers in size) have no defects andany functional groups inside the layers and are fringed bycarboxyl groups.

Preparation of few-layer graphene nanoplatelets (<2 nmthickness) by reduction of GO in aqueous solution of hydrazinewas described.114 The prepared material shows a high value ofspecic surface (466 m2 g�1). In contrast to GO, its electro-conductivity is close to that of graphite and the percolationthreshold is almost an order of magnitude lower than that ofgraphite.

To avoid the effect of functional groups inevitably containedin GNS when using chemical methods of synthesis, someauthors prepared ‘few-layer graphene’ using prolonged USM ofgraphite akes either in solvent121 or one of the reagents used inthe synthesis of PU122 followed by centrifugation of the mixture.Concentration GNS in the prepared suspension can reach�4 mg mL�1.

With the increase in the number of layers (sometimes up totwo only), characteristics of the nanomaterial generally becomeworse. Electrophysical characteristics are most sensitive to thenumber of layers. Therefore, of principal importance is the useof graphene itself in the design of nanoelectronics. For otherpurposes, particularly, when forming polymer composites,experiments showed no need in strict requirements to thenumber of layers. In these cases few-layer GNS were found to beefficient.118,123,124 In the works on the preparation and the anal-ysis of properties of PU–GNS composites considered below,mixtures of particles of exfoliated graphene precursors con-sisting of few-layer GNS and graphene itself were used. At such avalue of specic surface, which is inversely proportional to thenumber of layers, this is also important in terms of applicationof graphene as a polymer modier.

Two approaches are proposed to solve the problem of poly-dispersity of GNS:

- USM of suspensions or solutions of parent compounds inorganic solvents or water including additives of SF, polymersand biomolecules as stabilizers of colloidal systems and

- chemical functionalization of precursors.119,125–129

4.2 Methods for the formation of the nanocomposites

When forming polymer composites by covalent bonding of GNSwith a matrix, functional groups able to react with functional

J. Mater. Chem. A

groups of a polymer should be on the GNS surface. Chemicalbehavior of GNS is discussed, in particular, in the review.115 Anideal graphene particle can be considered as a giant macro-molecule, whose reactivity is dened by an extended poly-aromatic p-system with terminal coordinatively unsaturatedcarbon atoms. The latter are generally bound with OH- orCOOH-groups slightly differing in properties from similargroups in phenols and aromatic carboxylic acids, respectively.115

The formation of functional groups on the basal surface ofgraphene provides the generation of defects in the aromaticconjugated structure and growth of reactivity of a domainadjacent to carbon atoms which formed such bonds. Then the‘chain reaction’ initiated in the primary attack point is realized.The reacted surface domains deform since they cannot remainin the plane, and being strained, the domains show high reac-tivity.115 Due to high polarizability of the p-system graphene canreact with organic and inorganic reagents in reactions of bothelectrophilic and nucleophilic addition. It was found that insome reactions, particularly, in those with diazonium salts,graphene is more reactive than two-layer (by an order ofmagnitude) and multi-layer GNS.130

Among the reactions of functionalized derivatives of gra-phene with organic compounds, of special interest is the reac-tion with isocyanate which provides the preparation of apolyurethane nanocomposite.131–133 For example, GO surface-functionalized by hydroxyl, epoxy groups, and terminal carboxylgroups easily reacts with isocyanates, including those bearingother functional groups, to form urethane from basal hydroxyland amide from terminal carboxyl groups,131 as shown in Fig. 9.The work reports on a comparative analysis of activity of phenyl,tert-butyl, cyclohexyl, hexyl, 4-cyanophenyl, 4-acetylphenyl and4-azidosufonylphenyl isocyanate in reactions with GO. 4-Ace-tylphenyl isocyanate was shown to be the most active. Itattaches to GO almost three times faster than the least activetert-butyl isocyanate. It is noted that such treatment allowed thepreparation of GO derivatives (iGO) which showed lowerhydrophilicity and were able to fully exfoliate in polar organicsolvents. The presence of azidosulfonyl, nitryl or ketone groupsin iGO allows further modication of the GO nanoparticlessurface to be performed. PU–GNS and PU–CNT composites areprepared using similar methods described in Section 3.However, an individual approach is desirable for each system inselecting a modier and a method of its introduction into acomposite.

4.2.1 Synthesis from prepared polymers. As mentioned inSection 3, nanocomposites are formed using meltable and/orsoluble pre-prepared polymers only. For polyurethanes theseare TPU or waterborne polyurethanes (WBPU). TPU basednanocomposites shall be considered rst as most widely usedpolyurethane materials.

Physical–mechanical parameters and thermal stability ofTPU based composites (TPU Irogran PS455-203) containingfrom 0.1 to 1 wt% of one of three types of GNS, namely: GOmodied by isocyanate (iG), sulfonated GO (sG) and GO reducedby hydrazine hydrate (hhG), were analyzed.134 The compositeswere prepared bymixing of sonicated GNS dispersions with TPUdissolved in DMF. Then the mixture was cast in molds and the



Fig. 9 Scheme of GO surface functionalization via treatment of isocyanates (adapted from ref. 131).


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prepared lms were dried. The highest strength was demon-strated by the samples with sG. Their strength and Young'smodulus were higher than those of the unlledmatrix by 75 and120%, respectively. This effect is interpreted by higher dis-persancy and uniform distribution of sG in a polymer matrix ascompared with other GNS. The rate of thermal decompositionof the samples containing iG was found to be higher than thoseof the samples with other type llers. The TPU–sG compositeshowing the maximal extinction coefficient at 500–1000 nm isrecommended for use in infrared-triggered actuators. It wasshown that such a nanocomposite lm (1 wt% of iG) 30 � 5 �0.05 mm in size pre-stretched by 100% is able to restore theinitial size liing the load of 21.6 g at a height of 3.1 cm underIR irradiation as a result of heating due to energy absorption bysG. The ability to absorb energy (up to 40 J g�1) and restore sizeremains unchanged aer irradiation during 11 test cycles. Theunmodied polymer is transparent and, hence, does notrespond to irradiation.

It is reported135 that TPU was modied using ‘graphitenanoplatelets’ prepared from thermally treated graphite inter-calated by sulfuric acid. Even at amaximal concentration of GNS(3.9 vol%), breakdown elongation for the composite remainshigh enough (to 600%) at essentially increased dynamicmodulus of elasticity, thermal stability and ignition resistance.

The conducting PU–GNS composite was prepared by mixingTPU solution prepared from PCLD (2000 g mol�1), 4,40-methy-lene-bis-(cyclohexyl isocyanate) and BD136 with thermallyreduced GO (TRGO) dispersed in MEK. The composites wereprepared as lms. The concentration of ller was varied from0.5 to 7 wt%. The general formula determined for TRGO parti-cles from the data of elemental analysis was C10H0.51O0.5. It wasshown that TRGO particles are well dispersed in a polymermatrix and, hence, no chemical modication of their surface isrequired. At a 1 wt% content of TRGO conductivity of thecomposite increases by 10 times and at a content of 2 wt%conductivity increases by 7 orders of magnitude as comparedwith parent PU. The value of Young's modulus noticeablyincreases from �460 to �650 MPa. However, tensile strengthand breakdown elongation measured at room temperature


remarkably decrease. A polymer matrix is reinforced attemperatures higher than the melting point for so segments,Tms z 41–42 �C. It was found from the DSC data that nano-modier precludes crystallization of polycaprolactonesegments, i.e. DHms essentially decreases with the increase inconcentration of TRGO. Therefore, the reinforcing effect ofllers lowers to a certain extent because of soening induced bythe lowering of degree of crystallinity in the Tg–Tms range.

Successful preparation of high-modulus PU–GNS elasto-meric composites was reported.135,137,138 When introducing GOinto TPU, chemical bonds formed between rigid TPU segmentsand the nanoparticle surface provide efficient strain transferunder loading, and GO precludes the formation of crystallites inthe hard phase of the polymer.137 When introducing 4.4 wt% ofGO, Young's modulus for the composite increases by �10 timesand hardness increases more than by 4 times. As a result,polymer resistivity to scratching sharply increases and it can berecommended as materials for coatings.

Interest of researchers to WBPU used as coatings andadhesives is caused by their manufacturability, namely, lowviscosity at highMM of the polymers, and strict requirements toenvironmental safety of production, which provide for usingsolvent-free technology. Nevertheless, the introduction of GNSinto WBPU-composites in quantities sufficient for forming acontinuous conducting network seems to be promising for useof WBPU in nanoelectronics, particularly, as materials forshields protecting from electromagnetic radiation and anti-static coatings.

Ref. 139 reports on preparation of WBPU–TRGO compositesby mixing water suspensions of the modier and PU synthe-sized from polybutylene adipate, IPDI, dimethylolpropionicacid and BD. It was shown that for lms of the preparednanocomposites strength and breakdown elongation increaseby 35 and 15%, respectively, if the GNS concentration increasesup to 1 wt%. These parameters are observed to decrease ascompared with unmodied PU with the further increase ofTRGO concentration, and Young's modulus decrease as well.Electrical conductivity of the composites increases by 5–6 ordersof magnitude at GNS percentage >2 wt%.

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Properties of WBPU (Hydrosize U2-01) based graphene con-taining composites are described in ref. 121. The authors'efforts were focused on forming the composite which wouldcombine hardness and strength of TPU with elasticity, partic-ularly, with high impact strength. For that, composites wereprepared which contained exfoliated few-layer GNS to concen-tration of 90 wt%, by mixing GNS suspensions and polymerdissolved in DMF and THF. Exfoliated graphene was preparedby prolonged USM of graphite dispersion in DMF followed bycentrifugation to separate graphite particles. Aer mixing thesolutions of components, composite lms were formed as aresult of solution evaporation and vacuum drying. The lmsshowed essentially higher strength even upon introducing 0.5wt% of GNS. Tensile strength grew from 25 MPa for TPU to 35MPa for the composite. With the further increase in concen-tration of GNS, an intricate dependence of strength on theconcentration was observed, namely, the drop to concentrationof 30 wt%, the growth to�55 wt%, the decrease with the furtherconcentration increase. The optimal nanocomposite contained�55 wt% of GNS. Being a thermoplastic, it showed Young'smodulus (E) and tensile strength (sr) close to those of poly-carbonate but essentially higher breakdown elongation (3r) andductile fracture energy (T) (see Table 3). It was also found thatGNS particle size determined by the centrifuge bowl rotationrate upon product fractioning noticeably affects the value ofYoung's modulus for composites. Young's modulus decreaseswith the particle size decreasing, while the values of strengthand breakdown elongation slightly increase.

It is reported140 that the WBPU–GNS composite was preparedby a solution method using concentrated water dispersion ofGNS prepared by hydrothermal reduction of GO in the presenceof polyvinylpyrrolidone. Parent PU was synthesized from PTMG,IPDI, dimethylpropionic acid and 1,2-ethylenediamine as waterdispersion. The composites were prepared by mixing PU andGNS dispersions. Conductivity of the composite containing 4 wt% of GNS was found to be close to those of other compositesprepared by the solution or the in situ method,139,141 and thepercolation threshold was �0.2 instead of 1 wt% in the lastworks. The authors found that with the decrease of initialconcentration of GNS suspension, used for preparation of thecomposite, from 4 to 2 wt%, conductivity of the preparedcomposite increased by 40 times, though the content of thenanoller was equal (4 wt%) in both cases. It is noted thatthermal stability of the composite is slightly higher thanthermal stability of parent WBPU.

It is known that PU of such type are quickly destroyed underUV radiation. As a result, extrication of free GNS particles fromthe WBPU based composite is quite probable. It can impede the

Table 3 Mechanical properties of WBPU–GNS composites and polycarbonate121

MaterialContent ofGNS, wt% E, GPa sr, MPa 3r, % T, MJ m�3

WBPU–GNS 45 0.75 24 40 7WBPU–GNS 55 1.5 37 15 4Polycarbonate — 1.3 45 4 1

J. Mater. Chem. A

use of the new material. The effect of UV radiation at sunlightwaves on WBPU composites containing 2 wt% of GO wasstudied.142 Rapid photodestruction of the composite underradiation is accompanied by oxidation and loss of sample mass,and changes in surface morphology. Prior to irradiation thesurface layer contains a limited number of nanoparticles. Incourse of 15–137 day irradiation the increase in density isobserved, the GO particles in the near-surface layer grow in sizeand their shape changes. The particles protrude from thesurface. The authors consider this phenomenon to be due toagglomeration of ller particles with the increase in theirconcentration as a result of gradual removal of a polymer fromthe surface layer of the sample. The obtained results can beuseful for the analysis of potential risks when using PU–GNScomposites when under illumination.

4.2.2 In situ synthesis. A PU-composite was synthesized bysuccessive treatment of GO using toluylene diisocyanate andamphiphilic oligoether-ester based on PEG and decane-dicarboxylic acid. The composite showed amphiphilic propertiesand formed stable enough dispersions in water and DMF.132

The composites prepared from WBPU based on oligoesterPCLD and oligoether PTMG via in situ polymerization141 werecompared with similar composites based on prepared WBPU.139

It is noted that in contrast to the previous work,137 a strongerinteraction of a polymer matrix with the ller surface provides afourfold increase in Young's modulus as compared with that ofthe parent polymer. It is also noted that the introduction of GNSprovides crystallization of the so segments of PU andprecludes crystallization of the rigid ones. GNS were found toaccelerate thermal destruction of WBPU.

Attention should be paid to the statement of the authors ofref. 136, 139, and 141 that the methods for preparation ofcomposites proposed by them do not require pre-functionali-zation of GNS. Nevertheless, they call these materials func-tionalized. Pro forma, there is no contradiction in thisstatement. Hydroxyl, carboxyl, and epoxy functional groups ableto further conversion are partially present on the GNS surfacewhen GO is thermally reduced. However, to avoid misunder-standing of terms it seems reasonable to use the term ‘func-tionalized’ nanoparticles as applied to those which werepurposely treated to gra functional groups.

The composition of rigid blocks of PU reported ref. 139 and141 was identical. It was shown by TEM that the GNS particlesare uniformly distributed in the polymer matrix to produceefficient electroconducting channels allowing compositeconductivity to be increased by 5–6 orders of magnitude ascompared with original WBPU at concentrations of GNS equalto 2–3 wt%. It should be noted that crystallinity of the rigidsegments of the composites was observed to be noticeably lowerthan that in parent polymers. However, crystallinity of the sosegments in composites prepared by the in situ method wasobserved to be higher than that in parent PU.141 The authors ofref. 139 consider that the fourfold growth of Young's modulusof the composite as compared with an unlled polymer is due tothe higher crystallinity of the so segments and to the strongerpolymer–ller interaction. This is well pronounced especially ascompared with the composite prepared by mixing a polymer



Table 4 Physical–mechanical parameters for the composites taken from ref. 143

Sample N [GNS], wt% sr, MPa 3r, % Conductivity, S cm�1

1 0 10.8 � 0.8 195 � 19 1.13 � 10�11

2 0.1 12.5 � 1.1 291 � 24 1.02 � 10�10

3 0.5 16.4 � 0.9 386 � 32 6.24 � 10�10

4 1.0 23.2 � 1.6 448 � 28 3.06 � 10�9

5 2.0 36.3 � 2.0 535 � 35 2.15 � 10�8


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with GNS, whose Young's modulus is lower than that of originalPU.139

The in situ method for preparation of TPU–GNS via thereaction of PTMG with MDI143 in the presence of GO reduced byhydrazine114 was found to be efficient. When the concentrationof GNS changes from 0.1 to 2 wt%, the values of tensile strengthand breakdown elongation monotonically increase to attain thevalues of 3.5 and 2.7 times, respectively, higher than those forthe unmodied polymer, and conductivity grows by 3 orders ofmagnitude (Table 4).

The preparation of composites based on GNS and PU ther-moplastics on the base of BD and HMDI or IPDI by the in situmethod is described in ref. 122. To avoid any chemical modi-cation, GNS as a mixture of few-layer graphene particles andgraphene nanoribbons were prepared by long time USM ofgraphene akes followed by centrifugation similarly to thatdescribed in ref. 121 directly in one of the reagents. Thismethod allows one to prepare dispersions in which theconcentration of GNS is 1–4 mg mL�1 (the yield of GNS was2–8% of parent graphite).

The properties of so-prepared TPU essentially change in thepresence of GNS. They change differently for HMDI–BD basedPU (PU-A) and IPDI–BD based PU (PU-B). For PU-A, glass-tran-sition temperature lowers from �50 down to �30 �C, and forPU-B, it grows from �90 up to �110 �C even at a concentrationof GNS of �0.015 wt%.

With the further increase in concentration to 0.06 wt%, theglass-transition temperature for composites remains almostunchanged. Rheological behavior of melts of these compositesis also different. The behavior of PU-A is characteristic of non-Newtonian plastics. A strong dependence of viscosity on shearrate is observed. PU-B show almost Newtonian behavior at highconcentrations of GNS (0.03–0.1 wt%). For both TPU, GNSmodifying results in the decrease of both Young's modulus andviscosity of melts, more than ten times for PU-A and by 3.5orders of magnitude for PU-B.

Unfortunately, neither molecular-weight distribution norintrinsic viscosity of the prepared PU was measured. Therefore,it is not clear whether GNS affect the character of melts owing,or MM of the polymers decreases in the presence of GNS. Thelatter assumption is supported by both orders of magnitudedecrease of viscosity, which is typical for the dependence of meltviscosity on MM of polymers, and the results of the work on asignicant increase in viscosity of melts of composites obtainedby the introduction of GNS into prepared PU (for example, thereview8).

4.2.3 Comparison of the methods. Themost interesting arethe publications in which the researchers compare


nanocomposites prepared by different methods to selectoptimal ways of PU modication.

The authors of ref. 138 found that in TPU-functionalizedGNS nanocomposites prepared by in situ polymerization, poly-mer adhesion to GNS is essentially higher than that of thecomposite made by mixing the prepared polymer with the ller.The introduction of GNS into prepared TPU provides theincrease in Young's modulus and electrical conductivity, andstrength and breakdown elongation simultaneously decrease.

Polyurethane composites based on TPU (ester polyol Dal-torez P765 + MDI + BD) were prepared using GO and GO func-tionalized through the reaction with monoisocyanates, TRG orgraphite powder.133 Three methods for preparation of compos-ites were compared: (i) mixing with prepared TPU in melt or (ii)mixing in solution (DMF) and (iii) in situ polymerization insolution. The best results were obtained when mixing modierswith the prepared polymer in solution. Depending on the aspectratio (Af), Young's modulus for the composites with 1.7 vol% ofller is 6–10 times higher than that for parent TPU. Ifcomposites are prepared by the in situ method or in melt, theincrease in Young's modulus is �3 times lower. The surfaceconductivity increases by 9 orders of magnitude at the sameTRG concentration when using the solution and the in situmethods, while mixing of the components in the melt leads tomuch more modest results, namely, resistance decreases by 5–6orders of magnitude. Permeability of lms with respect tonitrogen decreases by more than 10 times for compositessynthesized from prepared TPU in solution. Provided equalconcentration of GNS, the in situ method was found to be �5times less active and mixing in melt was found to be two timesless active. In all parameters graphite is less effective than GNSeven at a tenfold higher concentration.

The authors of ref. 144 made a comparative analysis ofproperties of nanocomposites based on TRG and TPU or WBPU,formed from both prepared polymers and using the in situmethod.136,138,139,141 It has been shown that the increase inYoung's modulus, i.e. the reinforcement effect of nanoparticlesand the growth of electrical conductivity, ismore intensive whenthe in situmethod is used, which provides a stronger interactionof composite components as compared with the method ofmixing modier suspension with polymer solution. Thisconclusion is in contradiction with that reported in the publi-cation.133 The latter method was reported to be much more effi-cient as comparedwith the in situmethod. This fact is additionalevidence of necessity of an individual approach to the selectionof an optimal method for the formation of nanocomposites.

A recent publication145 describes the in situ preparation ofPU–GNS composites. Graphene nanoplatelets were rst ballmilled in alcoholic solution of PTMG (MM ¼ 1000) and BD, andthen alcohol was removed, and TDI and catalyst were added.The PTMG : BD : TDI ¼ 1 : 2 : 3.15 molar ratio was used,(NCO) : (OH) ¼ 1.05. It was shown that the use of ball millingallows one to prepare conducting composites with a lowerpercolation threshold (2 wt% of llers) and better mechanicalproperties in contrast to simple stirring (percolation thresholdis 6%). It was also shown that graphene nanoplatelets werediminished signicantly in size (from dozens of microns to

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several microns) in the process of ball milling, their thicknesswas also diminished considerably. Similar composites wereprepared using carbon black by both ball milling and stirring.However, the composites with carbon black showed the worstresults in comparison with PU–GNS composites prepared bystirring. The dependences of strength and breakdown elonga-tions for PU–GNS composites prepared by ball milling on theconcentration of ller pass through maxima at about 1 wt% ofllers and then weakly decrease with increasing concentrationof the ller. At the same time the characteristics of the restcomposites fall fast and monotonically.

5 Nanodiamond containingnanocomposites

NDanddiamondblend (DB) are of certain interest asmodiers ofproperties of polymers including PU. DB is a primary product ofdetonation decomposition of high-power explosive mixturesshowing negative oxygen balance, in non-oxidized medium. DBconsists of diamond nanoclusters with a carbon shell of intricatecomposition, which involves metal compound admixtures.Chemical purication ofDBprovides preparation ofND – orderedcarbon nanostructures with a diamond-like crystal lattice.146,147 Aprimary ND particle consists of a diamond core, whose diameterwas estimated to be from#3 nm to 8 nm (themost possible valueis 4 nm (ref. 148)), covered by a �1 nm thick carbon shell ofdisturbeddiamondstructure. It isnoted thatDB isa less expensiveand more available product than ND, but it is not a less efficientnanomodier for polymers.149 One of the most important taskswhich is successfully being solved now is to lower the tendency ofthe latter to aggregation to provide the preparation of high-qualitypolymer composites using nanodiamond modiers.150–152 Thedevelopment of methods for ND surface functionalization aimedat the design ofmore efficient modifying nanomaterials is also inprogress.153–158 The commercial production of these products hasbeen organized in Russia and their cost is relatively low.147–149 Atthe present timeNDandDBare, perhaps, themost accessible andcheap forms of CNP. Unfortunately, there are only a few publica-tions reporting on the preparation of ND or DB containingcomposites based on polymer matrices of different nature, whichshow improved elastic-strength, tribological and dielectric char-acteristics, increased chemical stability, and lowered combusti-bility and inammability.153,159–164

In particular, the introduction of 0.05–0.15 wt% of ND andDB into polyurethane foams provided higher adsorptioncapacity of foams and technological effectiveness of systems,165

and when 0.2 wt% of DB is introduced into polyester urethane,strength and elasticity of the material increases by 3 and 1.5times, respectively.166 Hopeful results of the above mentionedexperiments together with accessibility of DB and ND can giveimpetus to further research aimed at their efficient use inpolymers, particularly, polyurethane nanocomposites whichwould show essentially improved performance.

6 Conclusions

The experimental data on the formation of polyurethanenanocomposites containing fullerene C60, carbon nanotubes,

J. Mater. Chem. A

graphene nanostructures, and nanodiamonds obtained fromexperiments aimed at the development of new-generationpolyurethane materials are evidence of the potential use ofcarbon nanoparticles as modiers for PU. PU composites havebeen prepared whose properties such as strength, Young'smodulus, electrophysical properties, gas barrier and othersnoticeably and even cardinally exceed characteristics ofunmodied parent polymers.

For example, the TPU–GNS composite was formed121 inwhich strength and hardness of the original polymer arecombined with the properties of elastomer. The compositewhich showed high gas barrier characteristics and high enoughelectrical conductivity and strength was also formed.133 Anessential growth of conductivity (up to 8 orders of magnitude)was observed for the composites containing CNT andGNS.55,76,135,136,139–144 Some of such composites were proposed tobe used in nanoelectronics.135,141,167 A fullerene containingpolyurethane material was developed which showed prominentnonlinear optical properties. The material was recommendedfor use in telecommunication technology.28

Shape-memory PU–CNP composites were recommended forefficient electroactive switches,168–170 and as infrared-triggeredactuators.134 Nanocomposite PU bers were prepared, whichwere reinforced by CNT introduced into the polymer.48,171

The recent review on polymer composites containing CNT172

reports on trends and prospects for using nanoparticlesincluding CNT to stabilize and increase durability of polymers,particularly, PU.

Most of the publications report on that essential improve-ment of properties of PU is observed at relatively low concen-trations of llers, from 0.1 wt% to a few percent, and in thesome publications – even at ultralow concentrations of CNT(from 0.001 to 0.01 wt%).31,32,93,94 Such hopeful results at suchlow concentrations of modiers are of special interest since PU–CNP composites of this type show several advantages which canbe commercialized.

Though noticeable success is achieved in research of PU–CNP composites, there are a lot of problems to be solved. Itshould be noted that the problems are common for all polymercomposites. In particular, this is a tendency of CNT to aggre-gation which precludes uniform distribution of CNT in a poly-mer matrix. However, homogeneous distribution of CNT in apolymer matrix is a necessary condition for preparing materialswith optimal properties. Therefore, the development of newmethods and techniques and modication of known methodsand techniques for breakdown of CNP aggregates are still anurgent task.

It is known that macroscopic properties of polymer nano-composites are determined by interphase contacts of dispersionmedium (polymer matrix) and a dispersed phase (nano-particles). The contact strength is determined by either energyof van der Waals interactions or additionally (or mainly) thepresence of covalent bonds between a matrix and the surface ofnanoparticles. Energy of these interactions should be esti-mated. The effect of energy on composition, structure andorientation of macromolecules in interface areas and the areasize, and strain transfer from polymer to nanoparticles under




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loading should be studied. Because of experimental difficultiesin the study of the interphase interactions and mechanisms ofcomposite breaking, the development of theoretical models,which would allow the pilot study to be optimized and newproperties of materials to be predicted, is of great importance.

In general it is clear that the improvement of properties ofnanocomposites as compared with parent polymers is associ-ated with certain structural changes in a polymer matrix underthe effect of a high developed surface of nanoparticles.However, mechanisms of both covalent and non-covalentinteraction of CNP with polymers and possible reasons ofstructural rearrangements in macromolecules as a result ofthese interactions are far from being elucidated. Therefore, thestudy of details of formation of structures in composites,particularly, the changes in phase composition, degree of crys-tallinity, parameters of ne structure of a polymer componentin the presence of nanoparticles and other various factorsaffecting properties of materials is of current importance. Thesefactors can be methods and conditions of the formation ofnanocomposites, chemical nature and size of chain fragmentslocated between urethane groups, the ability of TPU macro-molecules to segregation of rigid chain fragments, etc.

Each type of CNP discussed in the present review has bothadvantages and disadvantages. Properties and modicationability of CNT and GNS are compared with respect to PU7,121,173

and some other polymers.174 The author of ref. 171 supposesGNS to be more preferable as modiers to change all propertiesof the polyurethane matrix. The authors of ref. 7 and 121 are notso categorical in their statements. Nevertheless, in some casesthey prefer using GNS. It is stated172 that though the degree ofimprovement of characteristics of PU and other polymers ach-ieved using CNT or GNS is compatible, GNS are preferred whenCNT are not efficient, for example, to improve gas barrierproperties of materials. Obviously, when selecting a modier oreven determining expediency of its commercial use, an impor-tant role will be played by economic evaluation and outlooks.

The analysis of publications cited in the present reviewallows the following conclusions to be drawn:

(1) The use of all types of CNP in the described polyurethanesystems under certain conditions provides either quantitativejump or qualitative changes in some polymer properties.

(2) The comparative analysis of efficiency of nanomodiersat similar concentrations reported in some publications ishardly correct because for each system and for each modierthere is, apparently, its own optimal concentration of modier,and efficiency of modier should be compared at its optimalconcentration in a composite.

(3) The data on the behavior of PU systems and other poly-mers at an ultralow content of CNP added are scanty so far.Nevertheless, the available data allow one to nd new unusualpatterns of relationship and new unusual phenomena.

(4) Potential of carbon nanomodiers for improving perfor-mance of PU and other polymer materials has not been realizedin full until now.

Prospects of commercialization of PU–CNP nano-composites depend on the intended scope of their application.If we are talking about small-tonnage products (sensors,


actuators and other small parts in complex devices), specialproblems should not arise, provided, of course, that thecharacteristics of new materials and related devices aresuperior in comparison with existing devices. In the case oflarge-tonnage production (constructional materials, machineelements, protective and decorative coatings, etc.), the cost ofCNP and the scale of their production come to the fore.Apparently, from this point of view the most promising areGNS and ND (or DB), since the ways of preparation allowproduction installations to be scaled. Nevertheless, it isobvious that the search for effective modication of polymersby ultralow concentrations of CNP may be useful if electro-conductivity of the composites is not needed.

List of abbreviations

BD
1,4-Butanediol CL 3-Caprolactam DABCO 1,4-Diazabicyclo[2,2,2-octane] DB Diamond blend DMF Dimethylformamide CNP Carbon nanoparticles CNT Carbon nanotubes GNS Graphene nanostructures GO Graphite oxide HMDI Hexamethylene diisocyanate IPDI Isophorone diisocyanate MDI 4,40-Diphenylmethane diisocyanate MWCNT Multi-wall carbon nanotubes MEK Methyl ethyl ketone MM Molecular mass ND Nanodiamonds ODA Octadecylamine PABSA Poly(aminobenzene sulfonic acid) PCLD Polycaprolactone diol PEG Polyethylene glycol PPG Polypropylene glycol PHAF [Poly(hydroxyalkyl)]fullerene PTMG Polytetramethylene glycol PU Polyurethanes SF Surfactant SWCNT Single-wall carbon nanotubes TEM Transmission electron microscopy TDI 2,4-Toluylene diisocyanate THF Tetrahydrofuran TMP 1,1,1-Tris-(hydroxymethyl)-propane TPU Segmented polyurethane
thermoelastoplast
TRGO Thermally reduced graphite oxide USM Ultrasonic machining WBPU Waterborne polyurethanes
Acknowledgements

The work was supported by Russian Foundation for BasicResearch (grant 12-03-00919-a).

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Notes and references

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Nanocomposites based on polyurethanes and carbon nanoparticles: preparation, properties and...

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