Waterborne Polyurethane Nanocomposites Having Shape Memory Effects

SOO KYUNG LEE,1 SUNG HO YOON,1 ILDOO CHUNG,1 ANDREAS HARTWIG,2 BYUNG KYU KIM1

1Department of Polymer Science and Engineering, Pusan National University, Busan 609-735, Korea

2Fraunhofer-Institute for Manufacturing Technology and Applied Materials Research, Adhesive Bonding Technology and

Surfaces, Wiener Strasse 12, Bremen D-28359, Germany

Received 29 July 2010; accepted 31 October 2010

DOI: 10.1002/pola.24473

Published online 2 December 2010 in Wiley Online Library (wileyonlinelibrary.com).

ABSTRACT: UV curable waterborne polyurethane/silica nano-

composites were designed and synthesized with functionalized

silicas, where the functionalization was made with allyl isocya-

nate. The incorporated silica particles gave triple effects of

multifunctional chemical cross-links, reinforcing fillers, and

stress relaxation retarders. Consequently, functionalized silica

incorporated into the polymer chains showed significantly

improved mechanical and thermal properties than the simple

addition of unmodified silica. Notably, over 99% shape fixity

and shape recovery with minimum cyclic hysteresis were

obtained for the repeated cycles at 1% loading of the modified

silica. VC 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym

Chem 49: 634–641, 2011

KEYWORDS: composite; dispersion; polyurethane; shape mem-

ory; silica modification; UV cure

INTRODUCTION Waterborne polyurethanes (WPUs) are ver-satile environmentally friendly materials that are increas-ingly being used in coatings and adhesives for wood andautomobiles, as well as for numerous flexible substrates,such as textiles, leather, paper, and rubber.1–3 Generally,WPUs have inferior drying rate and a slower development ofadhesion compared to the conventional solvent borne PUs.In addition, their applications have been limited by the typeof raw material and the manufacturing process. However,problems related to their properties and processing canlargely be resolved by proper molecular designs and hybrid-ization with other materials.4–7

Much attention has recently been paid to improve the ther-mal and mechanical properties of organic polymers by incor-porating nano-size inorganic particles.8–13 Among many addi-tives, nanostructured silica is a most commonly used onedue to its low price, chemical stability, and favorable mor-phologies.14 However, miscibility enhancement has to be con-sidered first since simple addition of inorganic particles toorganic polymer often aggravates properties due to the poordispersion and low interactions between particle surface andpolymer.

Surface modification of the inorganic particles by chemicalreaction with organic materials is an effective way toimprove miscibility and interphase interaction with polymermatrices.15 However, few has been evolved for the net-worked polymers grafted onto the modified silica with nowork devoted to polyurethane which needs thermal stability

to extend the application temperature as well as mechanicalproperties.

UV curing is a well-known technology which has found alarge variety of industrial applications because of its uniqueadvantages such as energy saving, high production speed,high product durability and high scratch resistance.16–20

However, for UV cure of acrylate monomers, unmodifiedsilica particles rather disturb the curing reactions betweenthe vinyl groups and lower the conversion.21 In this regard,chemical modification of inorganic materials is regardedbefore they are incorporated into the organic materials. Liuet al.22 functionalized silica particles with epoxy compoundsand Shirai et al.23 with isocyanate groups to graft linearpoly(methyl methacrylate) onto the silica particle leadingtypically to the so called core-shell structure.

On the other hand, shape memory polymers (SMP) whichare composed of reversible and fixed phases are smart mate-rials that respond to stimuli such as temperature.24–26 SMPshave traditionally been used as temperature sensors andactuators. However, recently applications are opened up formedical applications including sutures, actuators, catheters,and stents27–30 though their applications are often limited bythe inferior mechanical properties. So, polymer compositeswith reinforcing fillers including nanoclay, silica, as well asglass fibers have been used.31–34 However, only a limitedsuccess was reported. In fact, the shape recovery is nega-tively influenced by the orientation of the rigid fillers.35

Organoclay modified with reactive sites exhibited poor shape

Correspondence to: B. Kyu Kim (E-mail: bkkim@pnu.edu)

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 49, 634–641 (2011) VC 2010 Wiley Periodicals, Inc.

634 WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

fixity and recovery due to the hindered crystallization of thesoft segments.

In this work, nano-size silica particles were chemically modi-fied with allyl isocyanates to synthesis WPU/silica nanocom-posites before the composites were cast and UV cured. Thechemically modified silica particles are for the first timebeing incorporated into the WPU by covalent bond. In thisway the particles are expected to be multifunctional cross-links and relaxation retarders as well as reinforcing filler,and become the fixed phases of SMP. Shape memory func-tionalization of WPU could be utilized for environmentallyfriendly coatings for automobile, shoes, and textiles wheresmall dent or scratch can be fixed by simply applying heat.Shape memory WPU may be used as adhesive where auto-matic de-bonding at elevated temperature is desired. Thedispersion size, thermal, mechanical, dynamic mechanicaland surface properties and shape memory behaviors of thecast films have been studied and compared with those ofunmodified silica as well as unfilled WPU.

EXPERIMENTAL

Raw MaterialsPolytetramethylene ether glycols (PTMG, Mn ¼ 250 g/mol,Aldrich) was dried and degassed at 80 �C under vacuum for2 h before use. Dimethylol butanoic acid (DMBA, Aldrich)was dried at 50 �C for 48 h in a vacuum oven. Isophoronediisocyanate (IPDI, Aldrich), dibutyltin dilaurate (DBTDL,Aldrich), hydroxyethyl acrylate (HEA, Aldrich) and allyl iso-cyanate (Aldrich) were used as received. Triethylamine (TEA,Fluka) was dried over 4 Å molecular sieves before use.Nano-size hydrophilic fumed silica (Aerosil 200) withaverage primary particle size of 12 nm was obtained fromEvonik-Degussa. This material forms aggregates with a dia-meter in the range of 100 nm and agglomerates with asize of some microns. Due to its high specific surface of200 m2/g it forms a large interphase with the polymer.

Modification of Silica ParticlesSilica particles dispersed in DMF were mixed and reactedwith allyl isocyanate for 24 h at room temperature to obtainthe vinyl terminated silica (Scheme 1). Then the sample wasfiltered and washed with acetone.

Preparation of Polyurethane-Silica NanocompositesThe overall reaction scheme to synthesis the composite isshown in Scheme 2. Beside the DMF used for particle modifi-cation no organic solvent was used for the synthesis. Thefirst step was to build up the isocyanate-terminated potentialionic or carboxylic acid segments from IPDI and DMBA.Then, these ionic segments were subsequently reacted withPTMG and IPDI to obtain NCO-terminated prepolymers.These prepolymers were end-capped with HEA at 50 �C for

SCHEME 1 Reaction scheme to modify the silica with allyl

isocyanate.

SCHEME 2 Overall reaction scheme to prepare WPU

nanocomposites.

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WATERBORNE POLYURETHANE NANOCOMPOSITES, LEE ET AL. 635

4 h. The completion of end capping reaction was confirmedby FTIR measurements (Fig. 1). Then silica particles modi-fied with allyl isocyanate were fed to the prepolymers. Theformulations used to prepare the WPUs are given in Table 1.

A 500 mL round-bottomed, four-necked separable flask witha mechanical stirrer, thermometer, condenser with a dryingtube, and a pipette outlet was used for the reaction. Thereactions were carried out in a constant temperature oilbath. The urethane-forming reactions were carried out at80 �C in the presence of an organometallic catalyst (0.03%DBTDL based on the total solid) until the theoretical NCOvalues (determined by dibutyl amine back titration36 andFTIR measurements) were obtained. When the urethane-forming reaction was completed, the reaction mixture (pre-polymer þ silica) was cooled to 50 �C, and the carboxylicacid groups were neutralized by an equimolar amount TEAduring the next 2 h.

An aqueous dispersion of PU with a solid content of 30%was obtained by adding water to the mixture using a tubingpump. 4 wt % of photo initiator with respect to the solidpolymer was added to the aqueous dispersion which wascast into film and dried under UV. Reactions between themodified silicas and HEA capped polyurethanes occur at thisstep.

CHARACTERIZATIONS

ATR–FTIR Spectroscopy and 1H NMRAn ATR–FTIR (Bruker IFS 66) and a 600 MHz Solid-StateNMR Spectrometer (UnityINOVA600) were used to confirmthe chemical modification of silica particles with vinyl isocya-nates. The resolution of the ATR–FTIR was 2 cm�1.

Powder samples for infrared and 1H NMR measurementswere prepared after evaporating the solvent used for modifi-cation of silica particles.

Dispersion SizeThe number average diameter of the WPU was measured bya light scattering method (Autosizer, Malvern IIC), using aHe-Ne laser with wavelength 633 nm. The sample was first

diluted in deionized water to 0.5%, followed by built-inultrasonic wave treatment for 10 min to homogenize thedispersion.

ViscosityThe dispersion viscosities were measured with a Brookfieldviscometer using an LV.2 spindle.

Contact AngleContact angles of the dispersion cast films with deionizedwater drop were measured with a conventional contact anglegoniometer (G-1, Erma). Tests are made at room tempera-ture and at least five runs are made to report the average.

Water Swell and Gel ContentSwelling was measured by immersing the films in water atroom temperature. The percentage of swelling for a particu-lar film was determined by measuring its weight increase asa function of time,

% swell ¼ ðW �WoÞWo

� 100 (1)

where W0 is weight of dried film and W is weight afterwater absorption.

The degree of network formation was determined by meas-uring the gel content. For this the dried film was immersedin toluene, a solvent for linear polyurethane for 168 h (7days) at room temperature and the gel content was calcu-lated by the following equation;

% gel ¼ W

Wo� 100 (2)

where Wo is the weight of initial dried film and W is theweight of dried film after immersing in solvent.

Tensile PropertiesTensile properties were measured with a universal testingmachine (Tinius Olsen 1000) at a crosshead speed of 500mm/min using specimens prepared according to ASTM D-1822. Tests were performed at room temperature and atleast five samples were measured for each kind of material.

FIGURE 1 FTIR spectra of the polyurethane prepolymer before

(a) and after (b) HEA capping. [Color figure can be viewed in

the online issue, which is available at wileyonlinelibrary.com.]

TABLE 1 Composition of the Waterborne

Polyurethane/Silica Nanocomposites

Series

PTMG

250 (mol)

IPDI

(mol)

DMBA

(mol)

HEA

(mol)

Silica

(wt%)

Modified

Silica (wt%)

U0 4.91 6.52 0.61 2 – –

U0.5 0.5 –

M0.5 4.91 6.52 0.61 2 0.5

M1 1

M2 2

M5 5

Total solid ¼ 30 g.

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Dynamic Mechanical PropertiesDynamic mechanical properties were performed with adynamic mechanical thermal analyzer (Rheometric Scientific,DMTA MK-IV). The experiments were carried out at 10 Hz,4 �C/min, and 3% strain over a temperature range from�40 to 100 �C. The sample size was A single cantileverbending was used to deform the sample having dimension of4 � 25 � 0.5 mm (width � length � thickness).

Shape Memory PropertiesThe shape memory properties were characterized using anUTM (Tinius Olsen 1000) attached with a heating chamber.The sample was first heated to the glass transition tempera-ture (Tg) þ20 �C and loaded to a maximum strain (em) of100%, followed by cooling and unloading at Tg �20 �C.Upon unloading, part of the strain (em � eu) is instantane-ously recovered, leaving an unload strain (eu). Then, thesample was reheated to the loading temperature of Tgþ20 �C to recover the strain, leaving a substantial amount ofpermanent strain (ep). These three steps complete onethermomechanical cycle. Shape fixity and shape recovery aredefined as:

% shape fixity ¼ euem

� 100 (3)

% shape recovery ¼ erem

� 100; (4)

where er ¼ eu � ep is the recovered strain.

Thermogravimetric AnalysisTGA analysis has been done using a TGA Q50 (TA instru-ments). 8–10 mg of sample was put in an alumina crucibleand heated at 5 �C/min under N2 atmosphere, where theflow rate of N2 was 60–40 mL/min.

RESULTS AND DISCUSSION

ATR–FTIR Spectroscopy and 1H NMRThe IR spectra of NCO-terminated PU prepolymer and HEAare given in Figure 1, which shows that the absorption peakat about 2270 cm�1, corresponding to the stretch vibration ofthe NCO group, has completely disappeared upon cappingwith HEA. ATR–FTIR (Fig. 2) and 1H NMR spectra (Fig. 3)provide evidence of reaction between allyl isocyanate and theSi-OH group of silica particles. The IR-spectra of modified silicashow the characteristic peaks of the NH group (3335 cm�1),C¼¼C group (1633 cm�1) and CNH group (1579 cm�1) whichare formed by the urethane forming reactions. The 1H NMRspectrum also confirms the reaction. The signal at 5.96 ppm isassigned to the protons of CH and those at 5.28 and 3.99 ppmare assigned to the protons of CH2, indicating the successfulincorporation of allyl isocyanate onto the silica surfaces.37

Dispersion Size and Solution ViscosityThe viscosity of prepolymer solution and dispersion size(Table 2) increase in the increasing order of

FIGURE 2 ATR-FTIR spectra of unmodified silica particles (a)

and after modification with allyl isocyanate (b). [Color figure

can be viewed in the online issue, which is available at

wileyonlinelibrary.com.]

FIGURE 3 1H NMR spectrum of unmodified silica particles (a) and

after modification with allyl isocyante (b). [Color figure can be

viewed in the online issue, which is available at wileyonlinelibrary.

com.]

TABLE 2 Particle Sizes and Solution Viscosities of the

Dispersions, Contact Angles and Gel Contents of the Cast Film

Series

Particle

Size (nm)

Viscosity

(cP)

Contact

Angle (�)Gel Contents

(%)

U0 147.8 442.6 60 70.3

U0.5 184.9 691.1 69.1 67.1

M0.5 188.1 726.4 66.4 86.9

M1 195.4 794.3 70.6 88.4

M2 245.3 1157.6 81.3 80.7

M5 346.8 1831.2 97.8 77.5

Standard deviations of particle size, viscosity and contact angle were

2.5, 3.0, and 3.5%, respectively.

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WATERBORNE POLYURETHANE NANOCOMPOSITES, LEE ET AL. 637

U0 < U0:5 < M0:5 < M1 < M2 < M5 (5)

The prepolymer viscosity and dispersion size increase withthe addition and increasing amount of silica and the effect ismore pronounced with the modified silica (M). The solutionviscosity directly contributes to the breakup of the dispersedphase during phase inversion with water addition. Assumingstress continuity at interfaces, the smaller dispersed phaseviscosity leads to greater dispersed phase deformationaccording to

_cd ¼ gcgd

_cc (6)

where g and _c are the viscosity and the rate of shear, andthe subscripts c and d designate continuous and dispersedphases, respectively.38 As the content of silica increases, thesolid content of the prepolymer increases and gives anincrease in viscosity and dispersion size as well.39 Theapplied fumed silica forms typically aggregates with a size inthe range of 100 nm which cannot be broken into smallerparticles. As this material is hydrophobic, it should be sur-rounded by the polymer by energetic reasons. With increas-ing amount of fumed silica it becomes more likely that morethan one aggregate is in one polymer particle and that thesize of the aggregates increases by the interaction with resid-ual smaller particles. This leads to the increased size of thepolymer particle with increasing amount of silica.

At the same content of silica (0.5%), modified silica showshigher solution viscosity and larger dispersion size than theunmodified one. The larger particle size of modified silicashows that the hydrophobic particles are embedded to ahigher content in the polymer compared to the hydrophilicnon-modified particles due to the minimization of interfacialenergy. Fumed silica is a typical compound to increase theviscosity of compounds which is caused by particle-particleinteractions. These interactions are higher if the particles arein a medium with low interaction with the particles surface.This means that hydrophobic particles increase the viscosityof aqueous dispersions much stronger than hydrophilic par-ticles do and that what is measured is mainly determined bythe viscosity of the continuous aqueous phase of the polymerdispersion.

Contact AngleContact angles of the UV-cured nano-composites increasewith the addition and increasing amount of silica due to theinclusion of hydrophobic particle (Table 2) leading to alower surface energy. Surprisingly it is also seen that modi-fied silica gives almost the same contact angle with theunmodified one. It is well-known that roughness of the sur-face as well as the surface energy influences the contactangle. The similar contact angles for samples with unmodi-fied and modified fumed silica implies that surface rough-ness is changed on the nono- or microscopic scale by theparticles and influences also the contact angel. Change ofsurface roughness is due to the relatively large size of theagglomerates typical for fumed silica.

Water Swelling and Gel ContentIt is seen that the equilibrium and rate of water swellincrease with addition of silica (Fig. 4), which indicates thatfree path of water molecules is increased in the presence ofsilica particle. The equilibrium and rate of water swellingare by far the greatest with 1% modified silica (M1). Thisimplies that the free path increases up to 1% silica beyondwhich it decreases. The decrease is related to the poor dis-persion of silica particles which on the other hand is relatedto the auto-inhibition reactions which take place betweenthe growing acrylate chain and allyl groups bound at theparticles surface (see later).

Gel fraction of the virgin resin (U0, 70.3%) decreases withthe addition of unmodified silica particles (U0.5, 67.1%)which indicates that the unmodified particles disturbthe cure reactions of the vinyl groups during UV curing(Table 2). As expected, gel content increases with the addi-tion of modified silica particle showing a maximum of 88.4%at 1% modified silica particle (M1). The increase over theunmodified particles is due to the cross-linking reactions viathe silica particles.

Mechanical PropertiesWith the addition of unmodified silica (U0.5), the ultimatestrength and yield strength increase over the unfilled one(U0) due to reinforcing effect of silica particle (Fig. 5). Thechain extension by vinyl groups of silica surface (M0.5) fur-ther increases modulus, strength, and strain energy (areaunder the curve). Our earlier work40 also showed that rein-forcing effect is most pronounced when the nano-size silicais incorporated into the polymer chains by covalent bonding.Maximum reinforcement is obtained with 1% (M1) modifiedsilica, beyond which the effect is decreased implying that theamount of silica particles is in excess.

FIGURE 4 Water swelling of the cast film versus immersion

time. [Color figure can be viewed in the online issue, which is

available at wileyonlinelibrary.com.]

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Dynamic Mechanical PropertiesDynamic mechanical properties of PU nanocomposites (Fig.6) show that all the samples show relatively narrow glass-rubber transition region and large difference between theglass state and the rubbery state moduli. A large differencein modulus at glass and rubbery states are essential forshape memory applications. The narrow transition is due tothe intensive phase mixing between the soft and the hardsegments, which was achieved by incorporating low-molecu-lar-weight polyols. The low molecular weight of polyol(PTMG250) gives many urethane linkages which induceintensive phase mixing between soft and hard segments viahydrogen bonding. The glass transition temperature (Tg) interms of tan d peak position nearly does not increase by thesimple addition of silicas (U0.5) compared with unfilled one.On the other hand, Tg increased over 20 �C with modifiedsilica (Series M). This implies that silica particles in PUchains retard segmental motions much more than thosephysically embedded in the polymer matrix. The increase issmall with over 1% silica.

With the simple addition of silicas, rubbery state modulus isslightly increased compared with unfilled one. However, therubbery plateau is well defined and increased with the modi-fied silica showing a maximum at 1% (M1). This confirmsthat the vinyl groups of silica surface provide reaction siteswith PU to form network structure with silica being thecross-link center. However, with further addition of silicarubbery modulus rather decreases indicating that silicaparticles do not take part in the cross-linking of the polyur-ethane domains. This implies that the auto-inhibition reac-tion of the allyl groups dominates at high modified silicacontent. That is, addition of allyl groups of particle to theacrylate termini of PU chains is followed by allyl stabilizationwith other particles. This auto-inhibition prevails at high par-ticle contents leading to the decreased cross-links betweenacrylate terminated polymer chains as well as those usingsilica particles.

Shape Memory PropertiesCyclic loading and unloading behavior of the PU nanocompo-sites are shown in Figure 7. Unfilled PU (U0) shows poorshape fixity and shape recovery. The two properties arerespectively related to the rapid chain relaxation and lowrubbery modulus of this material. Moreover, with increasingnumber of cycles (N), shape fixity and recovery decrease dueto the hysteresis in thermo–mechanical cycles. With the sim-ple addition of silica particles to the PU (U0.5), shape fixityand shape recovery increase over the unfilled one due to theincreased relaxation time and reduced polymer mobility.When the relaxation of oriented polymer chains is slow,more polymer chains remain stretched during loading andsubsequent cooling and induce large recovery during thereheating step.24

Shape memory properties, especially shape recovery andhysteresis to thermomechanical cyclic loading are signifi-cantly improved with modified silica extension (Series M)due to the formation of networks by the reactions between

FIGURE 5 Stress-strain behavior of the WPU nanocomposites.

[Color figure can be viewed in the online issue, which is avail-

able at wileyonlinelibrary.com.]

FIGURE 6 Storage modulus (a) and tan d (b) of WPU nanocom-

posite versus temperature. [Color figure can be viewed in the

online issue, which is available at wileyonlinelibrary.com.]

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WATERBORNE POLYURETHANE NANOCOMPOSITES, LEE ET AL. 639

vinyl groups of PU and modified silica. Network structuregives high rubbery modulus with the magnitude dependingon the network density and minimizes cyclic hysteresis sincechain breakages and slippages are minimized with net-worked structure. Notably, M1 gives over 99% shape fixityand recovery up to the 4th cycles. Low thermomechanicalcyclic hysteresis is a highly desirable property for shape

memory materials since such materials are often subjectedto repeated loading and unloading during their applications.With further addition of silica (>1 wt %), shape memory per-formance became worse implying that those silica particleswhich are not incorporated in chain would disturb chainmotions and impede elastic properties of the materials.41 Inother word chain termination by the autoinhibition reactionof the radical allyl polymerization leads to a decreasingdegree of cross-linking as discussed earlier. If autoinhibitionwould not take place, the cross-linking density wouldincrease leading to increasing brittleness or too high modu-lus which are also in conflict with good shape memoryproperties.

Thermal PropertiesThe thermal stability of polyurethane nanocomposites hasbeen increased a little with the addition of silicas throughoutthe temperature range tested (Fig. 8). It is well-known thatthermal stability of organic phase can be enhanced by theaddition of inorganic phase which possess good thermalstability. The thermal stability is enhanced more with modi-fied silicas (Series M) than unmodified one. Notably, M1show the highest stability, indicative of fine dispersion ofsilica particles. This again confirms incorporation of the par-ticles into the polymer network at low amount. At higherconcentrations thermal stability rather decreases due to thedecreased cross-linking density by the allyl autoinhibitionreactions.

Above 500 �C, it can be verified that the char yield and thecontents of inorganic materials are the same. This mightmean that organic phase is entirely decomposed into gasesduring the decomposition of PU nanocomposite.

CONCLUSIONS

Waterborne polyurethane/silica nanocomposites showed thatthe effects of silica particles are much pronounced when the

FIGURE 7 Thermomechanical cyclic behavior of U0 (a), U0.5

(b) and M1 (c) for the first four cycles.

FIGURE 8 TG curves of WPU nanocomposites versus tempera-

ture. [Color figure can be viewed in the online issue, which is

available at wileyonlinelibrary.com.]

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particles are incorporated into the polymer chains via cova-lent reactions.

Modified silica augmented the viscosity of prepolymer solu-tion and accordingly the particle size of dispersion over theunmodified one due to the improved particle distribution.Nevertheless the fumed silica essentially forms aggregatesand agglomerates with a typical size of 100 nm which is inthe range of the dispersion size. Consequently, roughnessand hence the contact angle of the dried polymer film withnanoparticle is different from the virgin one.

With the addition of silica, swelling, initial and rubbery mod-uli, strengths, and thermal stability of the film were increased,and the effects especially shape recovery and hysteresis tocyclic loading are significantly improved with modified silicadue to the formation of networks by the silica particles. It hasbeen shown that silica particles incorporated into the polymerchains function as multifunctional chemical cross-linkers, ori-entation relaxation retarder and reinforcing fillers. Neverthe-less, as the silica content increases over 1 wt %, all the aboveproperties showed a decrease due probably to the auto-inhibi-tion of the allyl compounds, that is, termination by auto-inhibi-tion seems dominant, yet to be determined.

The research is in the program of PNU-IFAM JRC and NCRC,both supported by the National Research Foundation of Korea,and organized at PNU.

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WATERBORNE POLYURETHANE NANOCOMPOSITES, LEE ET AL. 641