Waterborne trifunctionalsilane-terminated polyurethane nanocomposite with silane-modified clay

Waterborne Trifunctionalsilane-Terminated PolyurethaneNanocomposite With Silane-Modified Clay

SANKARAIAH SUBRAMANI, JUN-YOUNG LEE, SUNG-WOOK CHOI, JUNG HYUN KIM

Department of Chemical Engineering and Biotechnology, Yonsei University, 134 Shinchon-Dong, Sudaemoon-Ku,Seoul-120-749, Republic of Korea

Received 28 March 2007; revised 4 July 2007; accepted 9 July 2007DOI: 10.1002/polb.21285Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: Trifunctional organosilane-modified clay was synthesized and used to pre-pare waterborne trifunctionalsilane-terminated polyurethane (WSPU)/clay nanocom-posite dispersions in this study. Qualitative evidence of the presence of chemicallyattached silane molecules on clay were confirmed by Fourier transform infrared spec-troscopy. The grafted amount and the grafting yield were determined by thermogravi-metric analysis and the obtained results were in good agreement with the cationexchange capacity of pristine clay. X-ray diffraction and transmission electron micros-copy examinations indicated that the clay platelets are mostly intercalated or par-tially exfoliated in the SPU matrix with a d-spacing of �2.50 nm. Clay does not influ-ence the location and peak broadness of the glass transition temperature of soft seg-ment as well as hard segment domains in the WSPU/clay films. WSPU/claydispersion with higher clay content exhibits a marginal increase in the average parti-cle size, but silane modified clay has a pronounced effect compared with Cloisite 20A-based nanocomposites. In addition, the incorporation of organophilic clay can alsoenhance the thermal resistance and tensile properties of WSPUs dramaticallythrough the reinforcing effect. The improvement in water and xylene resistance ofthe silane modified clay nanocomposites proved that trifunctional organosilane can beused as effective modifiers for clays. Storage stability results confirmed that the pre-pared nanocomposite dispersions were stable. This method provides an efficient wayto incorporate silane modified clay in SPU matrix. VVC 2007 Wiley Periodicals, Inc. J Polym

Sci Part B: Polym Phys 45: 2747–2761, 2007

Keywords: clay; modification; nanocomposites; polysiloxanes; polyurethane; water-borne

INTRODUCTION

Polyurethanes (PU) are functional polymerswhose properties can be tailor-made by simplyadjusting the compositions to meet the highlydiversified demands of modern technology, foruses such as coatings, adhesives, reaction injec-tion molding, fibers, foams, rubbers, thermoplas-

tic elastomers, and composites.1 Conventionalpolyurethane products, such as coatings andadhesives, contain a significant amount of or-ganic solvents and highly reactive isocyanatemonomers. As waterborne polymer systems areenvironment-friendly, the solvent-based PUshave been gradually replaced by waterborne PUsin the recent years and widely used in coatingsand adhesives industries. Solvent and water-based PUs are modified to make it as advancedmaterials either by varying PU microstructuresor by dispersing inorganic fillers, especiallythrough incorporation of nano-sized layered

Correspondence to: J. H. Kim (E-mail: [email protected])

Journal of Polymer Science: Part B: Polymer Physics, Vol. 45, 2747–2761 (2007)VVC 2007 Wiley Periodicals, Inc.

2747

silicates within the PU continuous matrix. MMT,a layered silicate with lamellar shape, hasattracted intensive research interest recently, forthe preparation of PU/clay nanocomposites. Thisis because the lamellar platelets of MMT displayhigh in-plane strength, stiffness, and aspect ra-tio.2 Depending upon the organization of the sili-cate layers in a polymer matrix, two types ofmorphology can be achieved in the nanocompo-sites: Intercalated or exfoliated. In general, thereare various methods that can be used to preparepolymer/montmorillonite; exfoliation–adsorption,in situ intercalative polymerization, melts inter-calation, and template synthesis.3–9

After the development of the Nylon/MMTnanocomposite,10 a large number of new polymer/clay nanocomposites based have been investi-gated.11–16 There are many papers in the litera-ture about solvent-based PU/clay nanocompo-sites. These research papers have described theeffect of incorporation of nanolayers of mineralclay on the thermal stability,17–19 mechanicalstrength,20–22 morphology and elasticity23 prop-erties of these nanocomposites. A few reportsabout waterborne PU/clay nanocomposites havebeen found in the literature, which were pre-pared by a prepolymer mixing process.24–27 Longchain organoammonium compounds are widelyused in the modification of pristine clay and thereare relatively few reports on the modification ofclay by organosilanes.28,29 In our previous report,we have studied in detail about the synthesis ofN-(2-aminoethyl)-3-aminopropyl)trimethoxysilane-modified clay and its effect on the novel silylated(PU-Acrylic hybrid/clay) nanocomposites.30 Toour best knowledge, there has been no informa-tion presented about the study of aminoalkyltri-functionalsilane modified clay and water-borneisocyanate free silylated polyurethane dispersion(WSPU) and its nanocomposites.

The main objectives of this work were to syn-thesize trifunctionalsilane modified clay and thestudy of modified clay by XRD, FT-IR, and TGA,and to study the morphology, physical properties,glass transition, and mechanical properties of theWSPU/clay nanocomposites by various analyses.

EXPERIMENTAL

Materials

Poly(tetramethylene glycol) (PTMG, Molecularweight ¼ 2000 g/mol, OH functionality ¼ 2.0,Dongsung Chemical, Korea) and 1,4-butane diol

(BD, Yakuri Pure Chemicals Co., Japan) weredried in a vacuum at 80 8C for 12 h before use.Dimethylol propionic acid (DMPA, Aldrich, USA)was dried at 508C for 24 h in a vacuum oven. N-methyl-2-pyrrolidone (NMP, Lancaster, UK) andacetone were stored in well-dried molecularsieves. Isophorone diisocyanate (IPDI, Aldrich,USA), triethylamine (TEA, Duksan Pharmaceu-tical Co., Korea) and [3-(phenylamino)propyl]tri-methoxysilane (PAPTMS, Aldrich, USA) wereused as received. A natural sodium montmoril-lonite clay, Cloisite Na1 (Southern Clay Product,USA), with a cation exchange capacity (CEC) of92.6 mequiv/100 g was dried at 80 8C for 24 hunder vacuum conditions. An organophilic clay,Cloisite 20A, specific gravity of 1.77 (g/cm3),(Southern Clay Product, USA) was dried at80 8C for 24 h under vacuum conditions. Thecations of natural montmorillonite in this orga-nophilic clay were replaced by dimethyl anddihydrogenated tallow quaternary ammoniumions. The weight loss at ignition of this clay is38% and the modifier concentration is 95mequiv/100 g. The organic surface modifier, (3-aminopropyl)trimethoxysilane (APTMS, Aldrich,USA) was used as received. The catalyst, di-butyl tin dilaurate, (DBTDL, Aldrich, USA) wasused as received. Aqueous dispersions were pre-pared using Milli-Q water of 18 MO.

Preparation of Inorganic-Organophilic Clay

The inorganic clay (MMT) was modified withaminosilane using a cation-exchange techniqueto increase the interaction between the hydro-phobic polymer and the hydrophilic clay, and toincrease the interlayer spacing of the clay. Theequation for calculating the intercalating agentneeded for a cation-exchange reaction is

92:6=100 3 10 ðg clayÞ 3 1:2

¼ X=Mw of intercalating agent3 13 1000 ð1Þ

where X represents the amount of intercalatingagent used, 92.6/100 is the cationic exchangecapacity (CEC) of 92.6 mequiv per 100 g ofMMT clay, and 1.2 (>1) indicates the excessamount of intercalating agent used. The interca-lant, APTMS (0.012 mole) and screened naturalclay (10 g) were gradually added to 1000 mLof 0.01 N HCl solution for better clay disper-sion. Then the dispersion was heated to 60 8Cand continuously stirred for 12 h to obtaincompletely ion-exchanged hydrophobic inorganic-

2748 SUBRAMANI ET AL.

Journal of Polymer Science: Part B: Polymer PhysicsDOI 10.1002/polb

organophilic clay. Finally, the treated clay waswashed with demonized water several times toremove residual chloride or cations. To ensurethe complete removal of chloride ions, the fil-trate was titrated with 0.1 N AgNO3 until nofurther AgCl precipitated. The filter cake wasthen placed in a vacuum oven at 80 8C for 24 h.The dried cake was ground and screened with a325-mesh sieve to obtain the inorganic-organo-philic clay.

Synthesis of Water-Borne SPU/ClayNanocomposites

A 250-mL rounded, four-necked separable flaskwith a mechanical stirrer, nitrogen inlet, ther-mometer and condenser, was charged withPTMG, and DMPA was dissolved in NMP, BD,and modified clay. The reaction was performedin a silicone oil bath maintained at a constanttemperature under a nitrogen atmosphere, andthe mixture was agitated for 12 h at 80 8C forthe exfoliation of clay by PTMG. After thoroughmixing, IPDI and 0.03% DBTDL catalyst wereadded to the reaction mixture and the reactionwas allowed to obtain NCO-terminated prepoly-mer for 2.5 h. Then, a calculated amount ofPAPTMS was slowly added and the reaction con-tinued for a further 1.5 h. After cooling to 60 8C,acetone (10 g) was added to reduce the viscosityof the SPU/clay hybrid and then TEA was addedand the reaction was continued for another30 min. Finally, water was added with vigorousstirring to obtain uniform dispersion, and ace-tone was removed under a low vacuum at 50 8C.The solid content of the resulting product wasadjusted to 30%. The prepared nanocomposites

were cast on a silicone trough and the waterwas allowed to evaporate at room temperature.The remaining moisture was removed under avacuum at 60 8C for 24 h. The nanocompositescrosslink by the formation of silyl ether linkagesupon removal of water via a combination of hy-drolysis and condensation of trimethoxysilylgroups. No additional additives or catalystsneed to be used for crosslinking to occur. Thecompositions for the preparation of SPU/claynanocomposites are given in Table 1.

Polymer Recovery

Five milliliter of toluene was added to 0.5 g ofthe synthesized SPU/clay nanocomposites whilestirring for 6 h at 60 8C. A clear SPU/clay solu-tion was obtained after filtration. The clear SPUsolution was then gradually stirred into 10 mLof 1% LiCl solution that was prepared by usingequal volume ratio (v/v) of toluene and DMF.The finished mixture was placed at room tem-perature for 48 h to perform the reverse ion-exchange reaction. After the ion exchange, thesolution was centrifuged at 5000 rpm for10 min. The supernatant liquid after centrifugewas distilled under reduced pressure to removethe solvent; thus, the SPU polymer wasobtained by reversed ion exchange on the sili-cate layer.

Characterization

Wide Angle X-ray diffraction experiments wereperformed directly on the film samples using anX-ray diffractometer (Rigaku D/MAX – 2500H)at 40 kV and 100 mA with a Cu Ka radiation

Table 1. Composition of SPU/Clay Nanocomposites (weight in Grams)

Ingredient C0 C1 C3

Sample

C5 A1 A3 A5

PTMG-2000 12.50 12.50 12.50 12.50 12.50 12.50 12.50DMPA 1.41 1.41 1.41 1.41 1.41 1.41 1.41BD 0.51 0.51 0.51 0.51 0.51 0.51 0.51NMP 6.00 6.00 6.00 6.00 6.00 6.00 6.00IPDI 7.00 7.00 7.00 7.00 7.00 7.00 7.00PAPTMS 4.06 4.06 4.06 4.06 4.06 4.06 4.06TEA 1.07 1.07 1.07 1.07 1.07 1.07 1.07Clay (%)Cloisite 20A 0.00 1.00 3.00 5.00 – – –APTMS-Clay – – – – 1.00 3.00 5.00

Acetone 10 g was used to reduce the viscosity of the prepolymer.

TRIFUNCTIONAL ORGANOSILANE-MODIFIED CLAY 2749


source (k ¼ 1.5404 A), at a scan speed of 48 / minand in the range of 1.5–308. Nanocompositesamples were measured as films, with thick-nesses of 0.1 and 0.3 mm. FTIR analyses wereperformed using a Bruker Tensor 27 FTIR ana-lyzer (Bruker Optics, Germany) in the transmis-sion mode in the range 400–4000 cm�1 at roomtemperature with a resolution of 2 cm�1 andaccumulation of 32 scans. The powder samplewas mixed with KBr and pressure packed toobtain pellets. The 1H NMR spectrum of theSPU polymer in DMSO-d6recovered from thenanocomposite was recorded at 25 8C with aFourier transform Bruker 400 MHz spectrome-ter. Samples (25 mg) were dissolved in DMSO-d6 (1 mL).

The samples for the transmission electron mi-croscopy (TEM, JEOL JEM3010, Japan) studywere prepared by placing the SPU/clay nanocom-posite films into epoxy capsules. The capsuleswere cured at 70 8C for 24 h in a vacuum oven.Then, the cured epoxies containing SPU/claynanocomposites were microtomed into 50-nmthick slices in a cryogenic ultra-microtome sys-tem. Subsequently, a 3-nm thick layer of carbonwas deposited on these slices, and the slices placeon 200-mesh copper nets for TEM observation.

The particle size of the WSPU/clay nanocom-posite dispersion was measured using a BI- par-ticle sizer ZPA (Brookhaven Inst. Co.). Dynamicmechanical properties were determined using adynamic mechanical analyzer (Seiko Exstar6000, DMA/SS6100) with a tensile mode. Thegeometry of the specimens was 10 mm (length)3 7 mm (width) 3 0.1 mm (thickness). The sam-ples were cooled to �100 8C, equilibrated for3 min, and then heated to 150 8C at a constantheating rate of 5 8C/min and a frequency of1 Hz, under a nitrogen atmosphere.

Tensile properties of the dispersion-cast filmswere measured using a universal tensilemachine (INSTRON, UK) at a crosshead speedof 0.1 m/min. Sample specimens were preparedby cutting the films with a die of dimension of10 mm width and 40 mm length, the grip dis-tance was set at 20 mm. The thicknesses of thefilms were between 0.1 and 0.3 mm. For eachfilm, three specimens were tested and the aver-age value was reported. Fractographs wereobserved with an FE-SEM (JEOL JSM6500FField Emission SEM). The fracture surfaces ofthe tensile specimens (�0.5 mm thick) werecoated with a layer of gold or platinum beforeSEM characterization. A thermo-gravimetric

analysis (TGA, TA Instruments, Q50, USA) wascarried out and the sample weights were 3–10 mg. Experimental runs were performed from30 to 800 8C at a heating rate of 10 8C/min in anitrogen atmosphere, with a gas flow rate of30 mL/min.

The gel content was calculated as follows: Asample of �0.1 g (W1) was wrapped in 300-meshstainless steel mesh of a known mass (W2) andexposed to 100 mL of xylene at 100 8C for 24 h.The stainless steel mesh was then removed andthe mass was measured after vacuum drying at80 8C for 24 h (W3). The degree of crosslinkingwas measured in terms of the percentage of gelcontent, using the equation

Gel contentð%Þ ¼ ðW3=W1Þ 3 100 ð2Þ

The water and xylene resistance values of thefilms were tested as follows: preweighed dryslabs (5 mm 3 5 mm in size) were immersed inDI water and xylene, respectively, at 25 8C. Af-ter immersion, the samples were blotted withlaboratory tissue and weighed. The swelling(solvent uptake) was expressed as the weightpercentage of water in the swollen sample

Swellingð%Þ ¼ ðWS �WDÞ=WD 3 100 ð3Þ

where, WD is the weight of the dry sample andWS is the weight of the swollen sample.

RESULTS AND DISCUSSIONS

Silylation of Natural Clay With Organo-FunctionalTrialkoxysilane

It is well known that pristine clay is hydrophilicand can form a stable dispersion in water, inwhich clay is dispersed as isolated sheets orsmall domains consisting of a few sheets.31 Aschematic illustration of silylation of pristineclay is shown in Scheme 1. There are two pur-poses for modifying pristine clay by organo-func-tional trialkoxysilane. The first one is to modifythe surface of clay with silane through the con-densation reaction of the OH group of clay withthe alkoxy group of silane; another is to replacesodium ions for a better exfoliation/dispersion ofmodified clay in SPU prepolymer. This methodalso promotes the oligomerization of silane dur-ing surface modification, since the condensationreaction can be catalyzed by the presence ofwater. Changes in morphology of the clay were



monitored using XRD. The XRD diagrams ofpristine clay as well as modified clay are pre-sented in Figure 1. The d-spacing of the pristineclay was about 1.17 nm, which agrees well withthe literature reports.32 The d-spacing of themodified clay and Cloisite 20Awere �1.64 nm (2h¼ 5.408) and �2.42 nm (2h ¼ 3.608), respectively.The d-spacing difference between the pristineclay and modified clay was about 0.5 nm, and itwas estimated that there may be a single molecu-lar layer between each clay layer. The increase inthe basal spacings suggests that polycondensatesare formed in the interlaminar space. This isbecause the silane preferentially replaces the so-dium ions on the surface of the clay platelets withammonium ions from the silane and are graftedat the edge of the clay platelets because of exis-tence of a hydroxyl group at the edge of the clayplatelets by the hydrolysis-condensation reac-tion.33 Modification with APTMS did not affectthe microstructure much because it is a shortchain oligomer. It is believed that there is a possi-bility of formation oligomers or siloxane linkagesby hydrolysis-condensation (crosslinking) of freesilanol [��Si(OH)3] groups. These may able topenetrate the external part of the interlaminarspace and push the clay sheets apart.29 It is clearthat the combined reaction of ion replacement ofthe surface of the clay platelets and grafting at

the edge of the clay platelets by a short chain,and preferentially the same modifier molecule,controls or limits the interlayer spacing of theclay sheets. However, even with such minimalmodification, the resulting SPU nanocompositesexhibit much better morphology.

TG analysis was used to examine the stabilityof organophilic groups on nanoclay. TGA curvesof the pristine clay, APTMS-clay and Cloisite 20Aare shown in Figure 2. The onset temperature ofthe degradation, as measured in a TGA at aramp rate of 20 8C/min, is about 270 8C for Cloi-site 20A and the second step of the degradation,which starts at 330 8C, may be attributed to therelease of olefin and amine.34 The unmodifiedclay shows negligible decomposition up to 800 8C.The initial drop in mass at �150 8C of theunmodified clay suggests that there may be the

Scheme 1. Schematic illustration of silylation ofpristine clay.

Figure 1. XRD patterns of (a) Pristine clay, (b)APTMS-modified clay, and (c) Cloisite 20A.

Figure 2. TGA curves of (a) Pristine clay, (b)APTMS-modified clay, and (c) Cloisite 20A.



loss of intragallery water upon exposure of clayto elevated temperature. In the region of 600 8C,a mass loss has been previously assigned to theloss of structural hydroxyl water from MMT.APTMS modified clay exhibits a higher onsettemperature, about 300 8C, and the seconddecomposition step start above 400 8C. Thedecompositions are probably due to the loss ofstructural hydroxyl water and organosilyl groupsof modified clay, respectively. The organosilylgroup is released very slowly between 400 and500 8C and more quickly between 500 and700 8C. Similar results have been reported inthat the organosilyl groups grafted to the sili-cates are highly thermal stable and that decom-position does not begin below 400 8C.35

The grafted amount was determined usingthe following equation based on the weight loss,W200–600, between 200 and 600 8C, correspondingto silane degradation.29

Grafted amount ðmequiv=gÞ¼ ð103 W200�600Þ=ð100�W200�600ÞM ð4Þ

where M (g/mol) is the molecular weight of thegrafted silane molecules. The grafting yield,which corresponds to the percentage of silanemolecules effectively involved in the couplingreaction, was calculated as follows:

Grafting yield ð%Þ¼ grafted amount 3 100=½silane� ð5Þ

where [silane] (mequiv/g) is the initial silane con-centration. The weight loss between 200 and600 8C, grafted amount and grafting yield arelisted in Table 2. The weight loss between 200and 600 8C was 14.45% and the grafted yield was

78.3%.The grafted amount of silane determinedby TGA analysis was 0.94 mequiv/g, which wasin good agreement with the cation exchangecapacity (CEC, 92.6 mequiv/100 g) of the pristineclay. These results show that the sodium ions ofthe pristine clay are completely replaced by qua-ternary ammonium ions of the APTMS modifier.It is also believed that the trifunctional APTMSmodifier can form oligomers attached to the clayedges by two or three Si��OMe groups. It isexplained that the formation of chemical bondsbetween the individual clay platelets gives rise toirreversibly attached clay stacks.29 FTIR is acommon tool to characterize the organic treat-ment of clay. The FTIR spectra of the pristineclay, APTMS-clay, and Cloisite 20A clay) in theregion 4000–400 cm�1 are shown Figure 3. Thebroad OH peaks of the pristine clay at 3425 and1635 cm�1 can be attributed to the hydratedwater in the intercalated clay region36 and theintense peak at 3628 cm�1 is assigned tohydroxyl group present in clay layers. The bandsat 1115, 1035, and 915 cm�1 can be collectivelyattributed to Si��O stretching vibrations.37 Thespectrum of silane-modified clay displays almostthe same pattern as that of pristine clay, exceptfor some new bands of ��CH stretching of alky-lammonium at 2937 and 1473 cm�1 (��CH2 bend-ing), which indicate the grafting of organicgroups on the clay mineral surface.

Synthesis and Physical Properties of SPU/ClayNanocomposite Dispersions

SPU/clay nanocomposites (Cloisite 20A-based andAPTMS-Clay-based) dispersions were synthe-sized by adding 1, 3, and 5% of organoclay. Therecipes are given in Table 1. Pure SPU was alsoprepared and studied to compare the results with

Table 2. Properties of Pristine Clay, Modified Clay, and Cloisite 20A

Clay

Property

OrganicModifier

Cation Exchange Capacity(mequiv/100 g clay) 2h8

d001

(nm)Weight

Loss (%)aGrafted Amount

(mequiv/g)bGraftedYield (%)c

Cloisite Na1 None 92.6 7.2 1.17 – – –APTMS-clay APTMS 92.6 5.4 1.64 14.45 0.94 78.3Cloisite 20A 2M2HTd 95 3.6 2.42 – – –

a Weight loss between 200 and 600 8C.b Determined using eq 4.c Determined using eq 5.d Dimethyl, dihydrogenated tallow.



the SPU/clay nanocomposites. The synthetic pro-cess for the preparation of SPUs at a ratio of 1.4NCO/OH and their dispersions are outlined inScheme 2. Since SPU prepolymers were highly

sensitive to moisture and more viscous than thenanocomposites, these were diluted with acetonebefore dispersing the prepolymers in water toavoid precrosslinking.26 The DMPA content waskept at 0.021mole in these experiments. The car-boxylic groups of the DMPA were neutralized byequimolar amounts of TEA and the pH of the dis-persions was basic. The solid content of all the dis-persions were checked and adjusted to 30%. Allthese experiments were carried out with care in anitrogen atmosphere to avoid premature cross-linking or gelation. The WSPU/clay dispersionsshowed no precipitates or unstable aggregates.

The representative FTIR spectra of WSPU/clay nanocomposite A3 and PAPTMS areshown in Figure 4. Strong absorptions at 1700cm�1 ( free C¼¼O stretching of urethane and car-boxylic groups), 1656cm�1 (ion interacted C¼¼Ostretching peaks), 2900 cm�1 (CH2 stretchingvibration of PTMG), 1100 cm�1 (C��O��Cstretching vibration of PTMG, Si��O��C stretch-ing and Si��O��Si asymmetric stretching vibra-tion of amino silane), 3250–3300 cm�1 (N��H

Figure 3. FT-IR spectra of (a) Pristine clay, (b)APTMS-modified clay, and (c) Cloisite 20A.

Scheme 2. Synthetic scheme of WSPU/clay nanocomposite.



stretching), 1530–1560 cm�1 (N��H bending),1210–1240 cm�1 (the stretching vibration of theC¼¼O group of urea combined with the N��Hgroup) and about 800 cm�1 (Si��C stretchingand Si��O��C deformation) proved the forma-tion of WSPU/clay nanocomposite (A3) withPAPTMS.26 A representative 1H-NMR spectrumof the recovered SPU prepolymer from nanocom-posite (C1) is shown in Figure 5. In Figure 5,1H-NMR signals for the recovered SPU prepoly-mer were assigned as follows: d ¼ 7.35 for thephenyl group of PAPTMS; d ¼ 1.9–2.2 for the��CH2 groups attached to PAPTMS; d ¼ 4.0 forthe methylene group of DMPA; d ¼ 1.1 for the

methyl group of DMPA; d ¼ 1.5 and 3.4 for themethylene groups of PTMG; d ¼ 1.0 (��CH3), 2.7(��CH2), and 3.9 (��CH) for the various protonsof IPDI. Apart from above groups, there existtwo kinds of ��NH groups in the polymer thatconfirms the formation SPU prepolymer: one isin the urethane unit and the other is in theurea unit. The 1H peak at 6.95 ppm is assignedto the ��NH group of the urea unit. The down-field 7.1 ppm is assigned to the ��NH group inthe urethane unit since the ��NH group isattached to the carboxylic group.

In aqueous dispersions, the organoclay‘swells’ (i.e. its layers are separated by hydra-tion) which provides for good dispersions in theWSPU/clay nanocomposite. Generally, the parti-cle size of the WSPU/clay nanocompositedepends upon many factors, such as the type ofisocyanate, polyols, hydrophilicity, degree ofneutralization, viscosity of the prepolymer, claycontent, and crosslinking density. The particlesize of the WSPU/clay nanocomposite disper-sions (30 wt % solid contents) are presented inTable 3. In these two series, the particle size ofthe dispersions was between 40 and 60 nm. Thismarginal difference could be due to the presenceof the same molar level of the neutralizedDMPA in all dispersions and the dispersion ofNCO-free WSPU prepolymer in water. Anotherreason for the marginal increase of average par-ticle diameter with increasing clay content couldbe the hydrophobic nature of organosilyl groups

Figure 4. FT-IR spectra of PAPTMS and A3.

Figure 5. 1H NMR spectrum of the recovered SPU from the nanocomposite (C1).



of the modified clay. This increase also couldpossibly result from the ionic interactionsbetween WSPU and clay. The WSPU dispersionis electrostatically stabilized by ��COO� ions ofneutralized DMPA. Therefore, a stable electricaldouble layer is formed around each WSPU parti-cle. The cations of exfoliated clay platelets inter-act with ��COO� ions and interfere with thedouble layer of pristine WSPU dispersion. In theself-emulsification of ionomers, particle sizeincreases with a decrease in hydrophilicity,which is governed by the ionic content of PUprepolymers.27 In these experiments, the con-tent of organoclay is another variable in bothsets of dispersions, and the variations showedthat clay has a marginal effect on the particlesize. This result shows that the clay particleswere well-dispersed in the SPU polymer matrixwithout any agglomeration probably by thehydrophobic nature of both the modified claysurface and the matrix. Since the isocyanate(NCO) groups were completely end-capped withPAPTMS, there were no side reactions and pre-cipitation of the dispersion material was notobserved.26 The WSPUs and their nanocompo-sites showed excellent stability during the pe-riod of the ageing test.

Morphology of SPU/Clay Nanocomposite Films

The XRD is the most powerful technique formonitoring the formation and structure of inter-calated or exfoliated organoclays. XRD patternsof the nanocomposite films are shown in Figure 6(A,B). It can be seen from the XRD pictures thatthere is no distinguishable peak in the SPUnanocomposite films of A1, A3, A5, and the nano-composite film of C1 in the range of 2h ¼ 2–308.

This data shows that the d-spacing increaseswhen the SPU is combined with the clay, prob-ably because the SPU penetrates into the galleryspace and forces the galleries apart. There ismore chance for SPU chains to intercalate orexfoliate into the gallery with APTMS-silicateand with less silicate in the nanocomposite.38

The wetting ability between SPU and modifiedclay is improved after modification of clay byAPTMS, because the surface OH groups at theedges of the layers have been lost, and thisavailed to better intercalation or exfoliation proc-esses. In addition, the interlayer attraction isreduced because of the decrease of cationexchange capacity value. On the other hand,weak peaks were noticed in the XRD patterns ofC3 and C5, when the organophilic clay contentwas 3.0 and 5.0%, respectively. The peaks (2h ¼4.88) may have occurred because the content of

Table 3. Tensile Properties and Particle Size ofSPU/Clay Nanocomposites

Sample

Property

TensileStrength (MPa)

Elongation(%)

ParticleSize (nm)

C0 10.9 110 42.7C1 15.8 212 43.2C3 13.3 105 45.7C5 12.9 180 49.5A1 17.9 191 43.1A3 15.2 303 53.9A5 13.7 175 54.3

Figure 6. A: XRD patterns of (a) C0 (b) C1 (c) C3and (d) C5 SPU/clay nanocomposite films. B: XRDpatterns of (a) C0 (b) A1 (c) A3 and (d) A5 SPU/claynanocomposite films.



organoclay increases in the nanocomposites. Thisindicates that the silicate layers of organoclaysdispersed in the SPU matrix lose their structuralregistry because of intercalation. This result alsosuggested that the layered silicates in the SPU/clay nanocomposites (C3 and C5) might bemostly intercalated in the SPU matrix.26,39 It hasbeen reported that the complete and effectiveentry of the PU molecules into the organic modi-fied silicate layers that caused a thorough exfoli-ation of the silicate layers in the SPU matrix,could not be achieved at a higher content of orga-noclay.23 For many solvent-based PU/clay nano-

composites with an absence of ions in PU moi-eties, the electrostatic forces between the clayplatelets would have a tendency to squeeze thePU polymer chains out and subsequently resultin an intercalated structure.39,40 For a certainWPU/clay nanocomposite, the aforementionedsqueeze effect was overcome by the ionic attrac-tions between anionic WPUs and cationic clayplatelets,27 resulting in an exfoliated silicate sys-tem. The pure WSPU and WSPU/clay nanocom-posites exhibited a broad diffraction halo at 2h ¼198. This diffraction halo is associated with theamorphous phase of WSPU.

Figure 7. TEM images of (a) C1 and (b) C5 SPU/clay nanocomposite films and(c) A1 and (d) A5 SPU/clay nanocomposite films.



In addition to XRD, TEM provides additionalinformation that will aid in the interpretation ofthe XRD results. Typical TEM images of SPU/clay nanocomposites of samples C1, C5, A1, andA5 are shown in Figure 7 (a–d), and these can beobserved with the presence of one or several claylayers in the SPU matrix, which indicates thedispersion of clay layers in the SPU matrix andthe formation of nanocomposites. TEM micro-graphs also prove that most clay layers are dis-persed homogeneously into the polymer matrix.The dark lines in the micrographs represent theclay layers, and the spaces between the darklines are interlayer spaces. Some single silicatelayers and ordered intercalated or exfoliatedassembled layers of clay are well dispersed in theSPU matrix. At higher magnification, Figure 7shows that the organoclay is mostly intercalatedor partially exfoliated in the SPU matrix withlayer distances of 2–4 nm in 1% clay loadednanocomposites (C1 and A1). The 5% clay loadednanocomposites are mostly intercalated (C5 andA5). The dark lines are the cross-sections of sin-gle or possibly multiple-silicate platelets. Thisresult confirms that the layered silicates aremostly intercalated or partially exfoliated in theSPU nanocomposites, and there is much less sep-

aration between individual silicate layers. Onthe basis of the aforementioned TEM and XRDresults, the SPU/clay dispersions samples wereapparently nanocomposites with mostly interca-lated or partially exfoliated structures of organo-clay.26,30,39 Both the decrease of interlayer attrac-tion and the improvement of wetting ability pro-mote the intercalation of SPU into interlayers. Itis worth to mention that the study of APTMS-clay in pure SPU/clay nanocomposite will providemore basic information on the dispersion mecha-nisms of clay in the polymer matrix instead of itsstudy in water-borne SPU/clay nanocomposite.

Tensile Properties of SPU/ClayNanocomposite Films

Mechanical properties were measured and com-pared to obtain a supportive evidence of theenhanced intercalation of clay in both series ofnanocomposites. As shown in Table 3, silylationof pristine clays imparts a change of the tensilestrength to nanocomposite materials. It wasclear from the test results that both the tensilestrength and the elongations at break increasedas the organoclay content increased in both se-ries of nanocomposites, implying that the

Figure 8. SEM images of fractured surface of (a) C0, (b) C1, (c) C5, (d) A1 and (e)A5 SPU/clay nanocomposite films.



greater dissociation of clay silicate layers andtheir homogeneous distribution in the polymermatrix was well established by surface modifica-tion of the clay. Among the two series of thenanocomposites, APTMS-based SPU nanocompo-site films exhibited higher tensile propertiescompared to the commercial Cloisite 20A-basedSPUs. Swelled clay can be regarded as a cross-linking center of the SPU polymer via anAPTMS-modifier, consequently promoting notonly the tensile strength but also the elongationof the nanocomposites. The tensile strength oforganoclay-reinforced nanocomposites is about20–65% higher than that of the pure SPU (C0).The elongation at break of the nanocompositesis one to two times higher than that of the pureSPU. The improved tensile strength of the nano-composites indicates that there is a strong inter-facial interaction between the silicate surfaceand the nearby silylated polymer chains.30,41

Fracture Morphology of SPU/ClayNanocomposite Films by SEM

The tensile fracture surfaces of SPU and itsnanocomposites are shown in Figure 8. It can beseen from Figure 8 that the fracture surface ofthe pure SPU polymer is smooth because ofquick or brittle failure. However, on addition ofclay particles, the crack surface becomes rough.The roughness increases as clay content in-creases in the matrix. The fracture roughnessindicates that the resistance to propagation ofthe crack is strong and the crack does notpropagated as easily as it does in pure SPU.30

The fracture surface roughness indicates thatcrack propagation is large and increased the tor-turous path of propagating crack.42 This effectresults in higher stress to failure and causedthe improved strength of the nanocomposites.The fracture surface of SPU with 1% APTMS-clay is smoother with large cracks than pure

Figure 9. (a,b): Storage modulus and tan d of theSPU/clay nanocomposite films.

Figure 10. (a,b): TGA thermograms of the SPU/claynanocomposite films.



SPU. At 5% APTMS-clay, the fracture surfacebecomes rougher with large cracks, unlike pureSPU. This indicates that particles are well dis-persed in the SPU polymer matrix and do notpeel off from material as a crack propagates.This also indicates that the bonding between thematrix and clay particle is strong.

Dynamic Mechanical Property of SPU/ClayNanocomposite Films

Figure 9 presents the storage moduli (E0) andtan d of the nanocomposite films. The storagemodulus, E0, and tan d of APTMS-modified clayand commercial clay nanocomposites are com-pared with that of the pure SPU. The nanocom-posites have higher E0 values than those ofpure SPU over the whole temperature rangeand E0 of nanocomposites increase with anincrease in the content of organoclay in both se-ries of nanocomposites [Fig. 9(a)]. Adding thesame content of clay (1 wt %), E0 of A1 washigher than that of C1 and the values of bothA1 and C1 were also higher than those of pureSPUE. This may have been due to the increasein the polarity of APTMS-modified clay, whichenhanced its compatibility with the SPU. Theimprovement in the storage modulus withsmall clay loading may have resulted from thestrong interaction between the organoclay andthe polymer matrix.26

Figure 9(b) shows two damping (transition)peaks for the SPU/clay nanocomposites. Thepeaks are ascribed to the glass transition tem-perature of the soft and hard-segments of thenanocomposites. The dominant peaks around90–110 8C represent the glass transition of thehard-segment portion of the SPU and the weak

broad peaks around �60 to �80 8C are theglass transition temperature of the soft-seg-ment portion of the SPU. It was reported inour earlier paper that the glass transition tem-perature of both hard and soft-segment por-tions of the SPU had a marginal effect by theaddition of or increase in a small part by wt %of clay.30 The change in the glass temperatureis due the interaction between SPU and clay. Itwas also found that there is not much differ-ence in the glass temperature of the two typesof the nanocomposites as seen in figure. How-ever, by adding clay into SPU, there is littleincrease in the glass transition temperature inboth cases. Further a detailed study is re-quired to find out the effect of the two clays inthese nanocomposites.

Thermal property of SPU/ClayNanocomposite Films

The introduction of inorganic components intoorganic materials can improve their thermal sta-bility because these species possess good ther-mal stability.30,43 The TGA curves of the pureSPU and the SPU/clay nanocomposites areshown in Figure 10 and residual weight percen-tages are listed in Table 4. The onset decomposi-tion temperature (TOD) of the nanocompositesand the pure SPU has some difference, but it isnot remarkable. However, the maximum decom-position temperature (Tmax) and the residualweight percentage of the nanocomposites werehigher than pure SPU. It is clear from the resid-ual weight percentages that APTMS-clay nano-composites have more residual weight afterdecomposition. The increase in thermal stabilitycould also be attributed to the high thermal

Table 4. Water and Solvent Swelling and Gel Content Percentages of SPU/ClayNanocomposites

Sample

Property

Residual Weightat 6008C by TGA

Water Swell (%)After 3 Days

Xylene Swell(%) in 24 h

GelContent (%)

C0 3.3 8.9 132.8 62.5C1 6.1 7.7 131.0 65.0C3 7.0 6.5 125.4 68.7C5 7.9 5.8 120.1 68.9A1 6.9 5.6 128.6 69.6A3 8.5 4.1 114.3 75.0A5 10.9 3.2 100.0 76.3



stability of clay and the interaction between theclay particles and the polymer matrix. Also, thesiloxane (��Si��O��Si��) network is formedfrom the methoxysilane hydrolysis-condensationreaction. In addition, thermal resistanceincreases with an increase in clay content.Therefore, the introduction of inorganic compo-nents into organic materials can enhance theirthermal resistance, as the dispersed silicatelayers hinder the permeability of volatile degra-dation products out of the material.44,45

Swelling of SPU/Clay Nanocomposite Films inWater and Xylene and Gel Content

Water and xylene resistance and gel content ofthe nanocomposite films were tested and arelisted in Table 4. The swelling percentages offilms containing 1, 3, and 5% clay were lowerthan those of pure SPU film containing 0% clay,and decreased with an increase in clay content.The water and xylene resistance and gel contentof the APTMS clay-based nanocomposite filmswere remarkably superior to the commercialclay-based nanocomposite films. The presence ofdispersed impermeable silicate layers in theSPU matrix reduced water/solvent swells andincreased water/solvent resistance.24,30 Improve-ments in water and xylene resistance of nano-composites could be due to the intercalation oforganoclay and the hydrophobicity of the treatedclay. It is well-known that polysiloxanes havegood water repellency and solvent resistanceproperties. Water and xylene resistance are im-portant properties for many applications in coat-ings and films, especially for the water-basedpolymer systems.

CONCLUSIONS

Silane-modified organoclay and water-borneSPU/clay nanocomposites by incorporating sil-ane-modified clay were prepared with a prepoly-mer mixing process. The grafted amount of silanedetermined by thermogravimetric analysis wasin good agreement with cation exchange capacityof pristine clay. This confirmed the ion-exchangereaction with aminosilanes and possibility of thegrafting on the clay edges by silylation. X-ray dif-fraction and transmission electron microscopyexaminations indicated that the clay plateletswere mostly intercalated or partially exfoliatedin the SPU matrix. Clay does not influence thelocation and peak broadness of the glass transi-

tion temperature of the soft segment as well ashard segment domains in the WSPU/clay films.

The tensile and thermal properties of silanemodified nanocomposites were superior thanthose of commercial clay incorporated nanocom-posites.

The silane modified clay nanocompositesshowed higher water and xylene resistance com-pared with pure SPU and commercial clay-basednanocomposites. The most significant feature ofthe technique is that a very small amount of sil-ane modifier is required to facilitate the welldispersion of the clay, which leads to betterphysical, mechanical, and thermal properties ofthe resulting nanocomposites. This study effec-tively describes a way to prepare one-packWSPU/clay nanocomposites that have remark-able storage stability.

This work is financially supported by the Ministryof Education and Human Resources Development(MOE), the Ministry of Commerce, Industry andEnergy (MOCIE), and the Ministry of Labor (MOLAB)through the fostering project of the Lab of Excellency.We also gratefully acknowledge the research fundingby the Korea Institute of Industrial Technology Evalu-ation and Planning (Project Number 10016568).

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