Materials Science and Engineering A399(2005)368–376Nanocomposite materials based on polyurethane intercalated intomontmorillonite clayAhmed Rehab∗,Nehal SalahuddinChemistry Department,Faculty of Science,University of Tanta,31527Tanta,EgyptReceived in revised form31March2005;accepted7April2005AbstractPolyurethane organoclay nanocomposites have been synthesized via in situ polymerization method.The organoclay has been prepared by intercalation of diethanolamine or triethanolamine into montmorillonite clay(MMT)through ion exchange process.The syntheses of polyurethane–organoclay hybrids were carried out by swelling the organoclay into different kinds of diols followed by addition of diisocyanate. The nanocomposites with dispersed structure of MMT was obtained as evidence by scanning electron microscope and X-ray diffraction (XRD).The results shows broaden with low intense and shift of the peak characteristic to d001spacing to smaller2θand the MMT is dispersed homogeneously in the polymer matrix.Also,the TGA showed that the nanocomposites have higher decomposition temperature in comparison with the pristine polyurethane.Keywords:Polyurethane nanocomposites;Nanocomposites;Polyurethane organoclay;Intercalated polymers;Layered silicate;Polymer–clay nanocomposite1.IntroductionPolymer composites were widely used in electronic and information products,consumer commodities and the con-struction industry.In these polymer composites,inorganic materials were used to reinforce polymers with the idea of taking advantage of the high heat durability and the high mechanical strength of inorganic and the ease of processing polymers.Clays have been extensively used in the polymers industry either as reinforcing agent to improve the physico-mechanical properties of thefinal polymer or as afiller to reduce the amount of polymer used in the shaped structures, i.e.,to act as a diluent for the polymer,thereby lowering the economic high cost of the polymer systems.The effi-ciency of the clay to modify the properties of the polymer is primarily determined by the degree of its dispersion in the polymer matrix,which in turn depends on the clay’s parti-cle size.However,the hydrophilic nature of the clay surfaces impedes their homogeneous dispersion in the organic poly-∗Corresponding author.Tel.:+2040350804;fax:+2040350804.E-mail addresses:rahmed@.eg,rehab220956@(A.Rehab).mer phase.The interfacial incompatibility between inorganic and organic polymers existed owing to the difference in the nature of their individual intermolecular interaction forces and often caused failures in these inorganic–organic compos-ites.One approach to alleviate the interfacial and the tenacity problem in these polymer composites is to chemically bond the inorganic and polymers through the sol–gel method[1]. The composite materials prepared by sol–gel method suf-fered the drawback of large shrinkage during the removal of the solvent.The other approach is to uniformly disperse the inorganic in the polymer matrix in the nanometer scale to form inorganic–polymer nanocomposites[2].Nanocomposites are a class of composites in which the reinforcing phase dimensions are in the order of nanome-ters[3].Layered materials are potentially well suited for the design of hybrid nanocomposites,because their lamel-lar elements have high in-plane strength,stiffness and a high aspect ratio[4].The smectic clays(e.g.,montmorillonite) and related layered silicates are the materials of choice for polymer nanocomposite design for two principal reasons:first,they exhibit a very rich intercalation chemistry,which allows them to be chemically modified and made compatible with organic polymers for dispersal on a nanometer lengthA.Rehab,N.Salahuddin/Materials Science and Engineering A399(2005)368–376369scale.Second,they occur ubiquitously in nature and can be obtained in mineralogically pure form at low cost.The layered clay–polymer nanocomposites can be prepared by replacing the hydrophilic Na+and Ca+exchange cations of the native clay with more hydrophobic onium ion to form a polymer-clay hybrid through two ways.Thefirst is the inter-calation of a monomer into the clay interlayer and subsequent heat treatment for polymerization[5].The second is the direct intercalation of a preformed polymer into the layered clay[6].Since the development of Nylon-6–clay nanocomposite by Toyota researchers[7],extensive studies on polymer–clay nanocomposites have been investigated in order to obtain new organic–inorganic nanocomposites with enhanced properties.The use of clay or organically modified clay as precursors for preparation of nanocomposites has been studied into various types of polymer systems including polyamide6[8],polyoligo(oxyethylene)methacrylate[9], epoxy[10],polyimide[11],polyester[12],polypropylene [13],polyacrylamide[14],polypyrrol[15],polystyrene[16], poly(p-phenylene vinylene)[17],polyethylene oxide[18], polycaprolactone[19]and polymethyl methacrylate[20].Polyurethane(PU)elastomers are a family of segmented polymers with soft segments derived from polyols and hard segments from isocyanates and chain extenders[21].PU elastomers represent one of the most attractive elastomers because they have the advantages,such as the best abra-sion resistance,outstanding oil resistance and excellent low-temperatureflexibility.They also exhibit the widest variety of hardness and elastic moduli that justfill in the gap between plastics and rubbers.Thefirst example of elastomeric polyurethane–clay nanocomposites with greatly improved performance proper-ties compared to the pristine polymer was reported by Wang and Pinnavaia[22].Preparation,characterization and proper-ties of polyurethane–clay nanocomposites have been reported by a various researchers[23–33].However,there also exist some disadvantages concerning with thermal stability and barrier properties.To overcome the disadvantages,the present work will discuss our initial efforts to synthesis different structure of PU–MMT nanocom-posites.Since the physical properties of the resulting mate-rials are derived from their structures and study the effect of organoclay percentage on the resulting nanocomposites (Scheme1).2.Experimental2.1.MaterialsMontmorillonite(Na-MMT)minerals were supplied by ECC America Inc.,under the trade name Mineral Colloid-BP asfine particles with an average particle size of75m and cation exchange capacity(CEC)of90m equiv./100g and interlayer spacing of9.6˚A.Diethanolamine from Riedel-De Haen AG Seelze-Hannover;triethanolamine from GDR Co.Germany;1,3-butylene glycol,diethylene glycol and triethylene glycol from Aldrich;tolylene-l,4-diisocyanate (TDI)from Fluka were used as supplied.Dimethylformamide (DMF)from Adwic(Egypt)was used after distillation and drying over molekularSieb.2.2.Preparation of materials2.2.1.Preparation of modified clay I a,bThe MMT(10g)was swelled in600ml of distilled water followed by addition of20g diethanolamine dropwise with stirring.The suspension was stirred for24h at room tem-perature followed by addition of dilute HCl(1:1)to obtain slightly acidic medium(pH∼5.5–6)then the stirring was continued for24h at room temperature.The suspension was allowed to stand for a few hours,filtered off using sintered glass(G4),washed many times with distilled water,then dried at∼35◦C under vacuum to yield10.85g of MMT-diethanolamine intercalate product.The product was retreat-ment with diethanolamine by swelling in mixture of300ml DMF and300ml water followed by addition of20g of diethanolamine and the procedure was repeated as previously to give11.05g of MMT-diethanolamine intercalate I a.The intercalation of MMT(10g)with triethanolamine (20g)was carried out by the same procedure described in synthesis of I a to give11.5g of I b.The structural properties were measured directly by infrared(IR),Fig.1;thermo-gravimetric analysis(TGA),Fig.2;calcinations,elemental microanalysis,swelling data and X-ray diffraction(XRD), Fig.3.2.2.2.Preparation polyurethane–MMT compositesThe materials were prepared by swelling the modified clay in the diol followed by addition of the diisocyanate as in the following procedure.0.52g of I a swelled in4.66g(50mmol) of1,3-butylene glycol for∼5h with stirring followed by addition8.87g(50mmol)of tolylene-2,4-diisocyanate with stirring at room temperature(∼20◦C).The polymerization was started after a few minutes(the viscosity of the mix-ture was increased)and completed very fast.After about2h, the formed solid product was suspended in DMF then pre-cipitated in distilled water.The white powder product was filtered off and washed several times with water then dried under reduced pressure at−35◦C to give13.2g(94%yield) of(II a).The other samples(II b–h)were prepared by the same procedure using different amount of modified clay(I a)with different diols and another modified clay(I b)with the same diisocyanate as illustrated in Table1.2.2.3.Preparation of linear polymersThe linear polyurethanes were prepared by polycondensa-tion technique using a mixture of diol and diisocyanate as in the following procedure:2.83g(30mmol)of1,3-butylene glycol was cooled in ice bath then added3.6ml(4.35g, 30mmol)of2,4-tolylene diisocyanate with stirring.The mix-ture was stirred for a few minutes then the temperature370 A.Rehab,N.Salahuddin/Materials Science and Engineering A399(2005)368–376Fig.1.Infrared spectra of the organoclay I a,b and PU–organoclay nanocom-posites II a–h and linear polyurethanes III a–d.increased gradually to the room temperature(∼20◦C)for about2h.The formed solid product was dissolved in DMF then precipitated in distilled water.The white powder was filtered off,washed several times with water then dried under reduced pressure at∼35◦C to give5.3g(74%yield)of prod-uct III a.The other samples(III b–d)were prepared by the same procedure using different diols and the same diisocyanate as illustrated in Table2.2.2.4.Analytical proceduresInfrared spectra were carried out on a Perkin-Elmer1430 Ratio-recording infrared spectrophotometer using the potas-sium bromide disc technique in the wavenumber range of 4000–400cm−1.Thermogravimetric analysis was obtained by using a TGA 50Shimadzu(thermal gravimetric analyzer).The heating rate was10◦C/min in all cases in the temperature range ∼30–800◦C in nitrogen atmosphere.Calcination measurements:A definite weight of the sam-ple was introduced into a porcelain crucible and dried in an electric oven at120◦C overnight,then introduced into an ignition oven and the temperature was increased to1000◦C and adjusted at this temperature for15h.The loading of each sample expressed as the weight loss by ignition per100g of the dry sample.The data of all prepared samples are listed in Table4.X-ray diffraction measurements were carried out using a Phillips powder diffractometer equipped with a Ni-filtered Cu K␣radiation(λ=1.5418˚A)at scanning rate0.005◦s−1, diverget slit0.3◦.Measurements were made for the dried product to examine the interlayer activity in the composite as prepared.Morphology of the composite was examined by a Joel JXA-840scanning electron microscopy(SEM)equipped with an energy dispersive X-ray detector to examine the mor-phology and particle size of MMT in the polymer–MMT composites.Specimen was deposited on double-sided scotch tape and examined at their fracture surface.3.Results and discussionTo disperse MMT nanolayers in a polyurethane matrix, it was necessary tofirst replace the hydrophilic inor-ganic exchange cations of the native mineral with more organophilic diethanolamine or triethanolamine.The ion exchange was carried out between sodium cation in MMT and ammonium groups in diethanolamine or triethanolamine.The presence of these group in the galleries of MMT renderstheScheme1.Synthesis of intercalated polyurethanes.A.Rehab,N.Salahuddin /Materials Science and Engineering A 399(2005)368–376371MMT organophilic and promote the absorption of diol into the interlayer of MMT and improve the particle–matrix inter-actions,since diethanolamine and triethanolamine contains functional group which react with diisocyanate.Polyurethane nanocomposites were prepared by solvation of the organoclay with the diol.It was found that the modified clay was swelled easily in the diols at room temperature.This solvation was followed by adding the diisocyanate.The synthesis of new organic–inorganic nanocomposite materials was achieved by the intercalation of polyurethane onto functionalized montmorillonite clay through in situ polycondensation polymerization technique.Thenanocom-Fig.2.TGA thermogram of PU–organoclay nanocomposites (a)II a–d with different ratios of organoclay I a and linear polyurethanes III a ,(b)II e–h with different diols,(c)TGA thermogram of linear polyurethanes III a–d .372 A.Rehab,N.Salahuddin/Materials Science and Engineering A399(2005)368–376Table1Polymerization data of intercalated samples II a–hRun Modified clay b Diol Diisocyanate a Productg wt%c Type d g mmol g mmol g Yield(%) II a0.523.831,3-Bu-G 4.66508.875013.293.9II b 1.177.981,3-Bu-G 4.62508.875011.377.1II c 1.6510.911,3-Bu-G 4.63508.875014.092.4II d 2.8017.211,3-Bu-G 4.60508.875014.092.5II e0.5943.749TEG7.12478.184710.6567.21II f0.5913.703DEAm 5.80559.575510.7967.61II g0.5953.717DEG 5.84559.575511.8574.03II h0.2563.6411,3-Bu-G 2.4325 4.35255.5178.37a Diisocyanate is2,4-tolylene diisocyanate.b Modified clay in all cases is I a except in the sample II h the modified clay is I b.c wt%=(weight of dry modified clay/total weight of all components introduce the polymerization process).d1,3-Bu-G,butane diol;DEAm,diethanolamine;DEG,diethylene glycol;TEG,triethylene glycol.Table2Polymerization data of linear polymer samples III a–dRun Diol Diisocyanate b Product Type a g mmol g mmol g Yield(%) III a1,3-Bu-G2.83304.35305.31173.97III b DEG3.25304.35301.23316.22III c TEG4.556304.35305.08357.07III d DEAm4.4404.35305.36661.33a1,3-Bu-G,butane diol;DEAm,diethanolamine;DEG,diethylene glycol; TEG,triethylene glycol.b Diisocyanate is2,4-tolylene diisocyanate.posites were synthesized through the intercalation of diols into organoclay I a,b interlayers followed by addition of TDI to produce the intercalated polyurethanes II a–h.The yields of the products were ranging from67%to94%,as shown in Table1.It was found that the1,3-butylene glycol gives high yields than the other diols in the polymerization into organoclay.The different ratios of organoclay used during the polymerization do not appear as an important factor to affect the yield percent of the product.It was found that the percentages of yield in the nanocomposites is higher than the yield percentage of linear polyurethane(Table2),which may be attributed to catalytic effect for the clay.The struc-tural composition and properties of the product materials was determined by several analytical techniques.The data in Table3illustrate that a high intercalation yields for I a,b occurred.Also,the swelling data indicated that the organoclay I a,b account for higher swelling in the organic solvents and lower swelling in water.While,the affinity to water still present due to the presence of OH group(I b>I a). Moreover,the swelling behavior increased in the polar and aprotic solvents than the non-polar solvent(the swelling followed the order DMF>1,4-dioxane>water>acetone> benzene.The IR spectra of all the prepared samples were illustrated in Fig.1and Table3.The spectra of modified clay I a,b shows that the reported NH stretch band near3425cm−1and NH bend band near1630cm−1are shifted quite substantially to regions associated with+NH3vibration which facilitate the ion exchange with MMT.A characteristic band at464cm−1 for Si–O and at3626cm−1for OH group are shown.This free OH band at3626cm−1in organoclay was disappeared in nanocomposite indicating the strong interaction are occur-ring between OH group in organoclay and the isocyanate forming the isocyanate paring the+NH3band near1630cm−1in organoclay with nanocomposite,it is clear that this band is shifted to higher wavelength near1710cm−1 indicating that an interaction occur between organoclay and the polymer.The spectra of polyurethane III a shows the absorbance appeared at1724cm−1that was assigned to hydrogen-bonded urethane carbonyl(C O),1413cm−1to a secondary urethane amide(C–NH).The spectra of the syn-thesized PU–modified clay shows II a,peaks at1712was caused by the stretching of urethane carbonyl group(C O) and the2927and2864cm−1were due to the asymmetric and symmetric C–H stretching vibration.The3317cm−1peak resulted from the N–H group in hydrogen bonding;the main features of various bond vibration and hydrogen bonding of these PU–modified clay nanocomposites remained the same as that of neat PU.These results deduce that there were no major chemical structural changes in PU,owing to the pres-ence of organoclay.Table3Characterization of modified montmorillonite clayRun no.Microanalysis Loading Swelling(%)IR(ν,cm−1)C%H%N%wt%mmol/100g a Acetone H2O Bz DMF Dioxane Free OH CH aliphatic N+Si0I a 3.2 1.9 1.413.6111.437142301689534342930,285215201046,523,464I b 6.3 2.9 1.3517104.61332031932818133582934,289914891045,523,465a Number of mmol of nitrogen per100g of clay,from calcination;(loss of weight/molecular weight)×1000.A.Rehab,N.Salahuddin/Materials Science and Engineering A399(2005)368–376373Fig.3.XRD pattern of the organoclay I a and PU–organoclay nanocomposites(a)II a–d,(b)II a and II e–g and(c)II h.Table4X-ray diffraction and thermal analysis data for the intercalated samplesSample X-ray data a Calcination TGA data b2θd-Spacing Polymer(%)Clay(%)Weight loss(%)infirst stage Weight loss(%)in second stage Residue I a 6.214.2611.788.316.5–83.5I b 6.3513.9415.684.420.3–79.7II a 5.914.9896.963.0455.141.53.4II b 6.014.7397.22.859.533.86.7II c 6.014.7389.5410.4658.432.510.1II d 5.715.5090.689.3257.428.114.5II e Shoulder>49––50.840.98.3II f 4.519.6496.43.654.442.82.8II g 4.519.6498.11.962.234.43.4II h 4.022.09––51.541.71.8a2θ(◦);d-spacing(˚A).b First stage≈30–350◦C,second stage350–800◦C and residue at800◦C.374 A.Rehab,N.Salahuddin/Materials Science and Engineering A399(2005)368–376Thermal analysis of polyurethane and intercalated mate-rials were determined by both calcination and TGA data listed in Table4and Fig.2a–c.The data and thefigure shows the weight loss encountered during heating the PU–modified clay materials were ranging from85%to98%as determined by both TGA and calcination.The associated weight loss evi-dent in TGA curves is nearly compatible with the calcination measurements.The TGA curves for all samples indicate that there are two stages of decomposition.Thefirst stage is the major and sharp,which involve the thermal decomposition of the intercalated polymers,specially the polymers present on the surfaces of the layers of the clay.The decomposition tem-perature in this stage was started at≈200◦C and take place to≈350◦C,which corresponds the weight loss ranging from 54%to62%.In this stage,there is no clear difference between the samples.Also,it was found that the composites degrade slightly faster than the pure polymer.This may be attributed to the degradation of the small molecules between the interlay-ers.The second stage is broad,in which the weight loss rang-ing from32%to42%in the temperature range≈300–700◦C. In this stage,the composites displayed higher thermal resis-tance than pure polymers.This stage was attributed to further decomposition of the rest intercalated polymers,specially the polymers present in the interlayers of the clay or some salts in the interlayer of the clay or interval the clay mineral loses OH groups and the crystallographic structure collapsed [9].The crystal structure of MMT consists of two-dimensional layers formed by fusing two silica tetrahedral sheets to an edge-shared octahedral sheet of aluminum hydroxide.Stack-Fig.4.(a)Scanning electron micrograph of PU–organoclay nanocomposites II d,(b)Elemental mapping for Si of PU–organoclay nanocomposites II d.A.Rehab,N.Salahuddin/Materials Science and Engineering A399(2005)368–376375ing of layers of clay particles are held by weak dipolar or van der Waals forces[34].XRD is powerful technique to observe the extent of silicate dispersion,ordered or disordered struc-ture in the polyurethane nanocomposites.Fig.3a and c show typical XRD for the organoclay I a,b.The001reflection has sharp intense peak at2Θ=6.2,6.35for I a and I b,respec-tively.The d001spacing was calculated and listed in Table4 from peak positions using Bragg’s law d=λ/2sinθ.It is clear that the d-spacing for Na-MMT(9.6˚A)increased to(14.26, 13.94˚A)since the small inorganic Na+cation is exchanged by onium group in ethanolamine and diethanol amine through an ion exchange process.Fig.3a–c presents three series of XRD corresponding to polyurethane clay nanocomposites: (a)with different ratios of organoclay,(b)with different types of diols and(c)with different types of organoclay.In polyurethane clay nanocomposites II a–d,the position of the peak corresponding to intercalated organoclay show some change to smaller2Θ=6.0–5.7as in Table4and Fig.3a.It is note worthy that the sharp peak obtained in organoclay I a due to a more narrow distribution of the interlamellar spac-ing become broad and have small intensity.The intensity decrease(i.e.,broadness increase)with decreasing the per-centage of organoclay.This suggests that the stacking of the silicate layers become disordered.In one earlier work[35], the same results obtained for polymethyl methacrylate–MMT composites.On the contrary,it is interesting tofind that at constant ratio of organoclay,the peak characteristic to001 plane in II f,g is shifted to higher d-spacing=19.64˚A.This confirmed that the polyurethane is intercalated between the layers.However in II e,XRD is featureless of ordered struc-ture and there is no apparent peak of the clay that can be detected as in Table4and Fig.3b.It is clear that the type of diol affect on the structure of the resulting nanocompos-ites.This diol is used to swell the organoclay before addition toluene diisocyanate Fig.3c.In II h,the peak characteristic to001is shifted to smaller2Θ=4and the intensity of the peak is small.The broadness of the peak may suggest that clay show some mixture of intercalated and exfoliated struc-ture.However,the exfoliated structure of the silicate layers is not judged from only this diffractograms.These results con-firm that modified MMT with different chemical structure, different percentage of clay lead to various degree of the dispersion in the polymer matrix.These results similar to the one described using another structures in PU nanocomposites [36].SEM examination of the fracture surface of the compression-molded samples did not reveal the inorganic domains at the maximum possible magnification.Fig.4a shows a micrograph of the fracture surface at9000magnifica-tions.It is observed that there is no mineral domains could be seen.The search for any aggregation was aided by an energy dispersion X-ray probe.An image for element mapping for Si was shown in Fig.4b.The uniformity of the white dots representative of Si,indicates that the mineral domain are submicron and are homogenously dispersed in the polymer matrix.4.ConclusionA series of polyurethane organoclay nanocomposites were synthesized by in situ polymerization using different kinds of diols and toluene diisocyanate in the presence of montmoril-lonite clay modified with diethanolamine or triethanolamine. 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