Research Article

Received: 22 December 2009, Revised: 29 January 2010, Accepted: 1 February 2010, Published online in Wiley Online Library: 12 May 2010

(wileyonlinelibrary.com) DOI: 10.1002/pat.1693

Effect of polymerically-modified clay structureon the morphology and properties ofUV-cured polyurethane acrylate/claynanocomposites

Dan Chena, Yuanhua Xua, Yangling Zangb and Shengpei Sua*

Polyurethane acrylate (PUA)/clay nanocomposites

Polym. Adv

were prepared by UV-curing from a series of styrene-basedpolymerically-modified clays and PUA resin. Effect of the chemical structure of the polymeric surfactants on themorphology and tensile properties of nanocomposites has been explored. X-ray diffraction (XRD) and transmissionelectron microscopy (TEM) experimental results indicated that surfactants having hydroxyl or amino groups showbetter dispersion and some of the clay platelets were fully exfoliated. However, the composites formed from pristineclay and other polymerically-modified clays without hydroxyl or amino groups typically contained both tactoids andintercalated structure. The mechanical properties of PUA composites were greatly improved where the organoclaysdispersed well. Thermogravimetric analysis (TGA) and differential scanning calorimeter (DSC) were carried out toexamine the thermal properties of the composites. The results showed that the loading of polymerically-modifiedclays do not effect the thermal stability, but increased the Tgs of PUA/clay composites. Copyright� 2010 JohnWiley &Sons, Ltd.

Keywords: polyurethane acrylate; polymerically-modified clay; composite; UV-curable

* Correspondence to: S. Su, Key Lab of Resource Fine-processing and AdvancedMaterials, College of Chemistry and Chemical Engineering, Hunan NormalUniversity, Changsha, Hunan 410081, China.E-mail: sushengpei@gmail.com

a D. Chen, Y. Xu, S. Su

Key Lab of Resource Fine-processing and Advanced Materials, College of

Chemistry and Chemical Engineering, Hunan Normal University, Changsha,

Hunan 410081, China

b Y. Zang

Department of Fine Chemicals, Hunan Research Institute of Chemical

Industry, Changsha, Hunan 410007, China

Contract/grant sponsor: Xiaoxiang Scholar Foundation, Hunan Normal Uni-

versity; contract/grant number: Chem 050613.

Contract/grant sponsor: Hunan Provincial Nature Science Foundation; con-

tract/grant number: 07JJ5073. 1

INTRODUCTION

Photoinduced polymerization is a well-documented techniquefor the rapid and precise formation of crosslinked polymers fromliquid monomers and has numerous uses in paint, ink, andcoating industries.[1–3] A significant body of work has beenundertaken on the free radical polymerization of acrylate-basedresins, due to their high reactivity and broad ranges of backbonechemistries. Within the acrylate family, urethane acrylates haveunique properties such as high flexibility, toughness, andchemical resistivity, which are widely used in the coatingindustry.[4–6] However, depending on requirements, it is desirableto formulate polymer film systems with higher glass transitiontemperatures, good barrier properties, low shrinkage, andenhanced mechanical properties.[4,5,7]

During the last three decades, polymer layered silicatenanocomposites have attracted much attention from materialscientists. This interest is mainly driven by the promise of greatlyenhanced properties such as tensile strength, flammabilityresistance, and barrier properties, etc.[8,9]

So it is not a surprise that UV-cured clay nanocomposites,which take advantages of both UV-curing and nanoclaytechnologies, have attracted so much attention in recent yearssince Zahouily et al.[10] first reported their preparation ofUV-cured polymer clay nanocomposites. However, the enhance-ment of clay nanocomposite properties is dependent on the size,the dispersion, and structure of clays. Because there is no extrashear in the curing process, nanodispersion of clays in theprepolymeric matrix is especially important for obtainingexfoliated UV-cured clay nanocomposites.

. Technol. 2011, 22 1919–1924 Copyright � 2

Most recent research has focused on clay functionalizationwith reactive monomers, initiator, or both monomer and initiatorto realize the exfoliation of the clay platelets through the in situreactions taking place between the layers.[11–18] Functionalgroups, such as acrylate, methacrylate, and benzophenones, havebeen used to expand the clay layers. Composites withpredominantly intercalated and only partially exfoliated struc-tures have been obtained in these cases. To overcome the defectsof presently available modified clays, the application ofpolymerically-modified clays has been investigated.[9,19–22] Thegreatly expanded gallery and tunable properties of polymeri-cally-modified clays are expected to offer a platform for theformation of UV-cured composites.

010 John Wiley & Sons, Ltd.

919

D. CHEN ET AL.

1920

The objective of this study is to explore the effect of thepolymerically-modified clay structure on the morphology andproperties of UV-cured polyurethane acrylate (PUA)/clay nano-composites. Various techniques including X-ray diffraction (XRD),transmission electron microscopy (TEM), thermogravimetricanalysis (TGA), differential scanning calorimeter (DSC), andtensile testing were used to examine the morphologies, thermalproperties, and tensile behavior of the composites.

EXPERIMENTAL

Materials

PUA purchased from BASF Co. (Ludwigshafen, German) was usedas telechelic oligomer. The reactive diluent, tripropyleneglycoldiacrylate (TPGDA), was provided by UCB Co. (Shanghai, China).4-methyl diphenyl ketone and triethanolamine were purchasedfrom Huihong Chemical Reagent Co., Ltd. (Changsha, China).Pristine sodium montmorillonite (Na-MMT) was provided byZhejiang Fenghong Clay Chemicals Co., Ltd. (Hangzhou, China;cationic exchange capacity (CEC)¼ 90meq/100 g). Acrylamide(AM), styrene (St), acrylacid (AA), vinylbenzylchloride (VBC),methyl methacrylate (MMA), 30% trimethylamine aqueoussolution, and N,N0-azobis (isobutyronitrile) (AIBN) were allpurchased from Shanghai Shanpu Chemical Reagent (Shanghai,China) and used without further purification. Tetrahydrofuran(THF) was purchased from Yueyang Chemical (Yueyang, China)and distilled before using.

Preparation of polymerically-modified clays

In general, the polymerically-modified clays were prepared bycation exchange of the sodium ions in the Na-MMT with thecorresponding polymeric quaternary ammonium salts. Thepolymeric quaternary ammonium salts were prepared by thereaction of trimethylamine with the corresponding polymers,which were synthesized by free-radical copolymerization of St,VBC, and a third monomer. The scheme for preparingpolymerically-modified clays was reported in our previouswork.[21]

The polymerically-modified clays were selected to explore theeffect of the polymeric surfactants structures on the dispersion ofclay particles in PUA matrices. The only difference among thisseries of polymeric surfactants is that, in the preparation of thesepolymeric surfactants, the third monomer is MMA, acrylic acid,and AM. Their corresponding modified clays were denoted asPSMMA-clay, PSAA-clay, and PSAM-clay. The organic content ofthese polymeric-modified clays is 56% for PSMMA-clay, 49% forPSAA-clay, and 55% for PSAM-clay, as calculated by TGA data.

Solvent-swelling experiment

The solvent-swelling properties of the modified clays werecharacterized using the method described in literature.[23] Theamount for PSAM-clay, PSAA-clay, and PSMMA-clay is 0.68, 0.58,and 0.67 g, respectively, all with the same amount of inorganicclay (0.3 g). 0.3 g of pristine clay was processed in the same wayand used for comparison.

Preparation of PUA/clays UV-cured composites

40 g of the reactive diluent TPGDA and polymeric-modified clays(the loading of clay is 3wt%) were added to 40 g of PUA, and this

wileyonlinelibrary.com/journal/pat Copyright � 2010 John Wiley

mixture was stirred for 6 hr to disperse the clay in the resin. Then2.4 g of 4-methyl diphenyl ketone and 2.4 g of triethanolaminewere added to the mixture. To avoid premature polymerizationcaused by the light, the samples were wrapped by aluminum foil.After homogenization using high speed emulsifier at 208C for 1 hrand by sonication for 4 hr, the sample was degassed undervacuum to remove the entrapped air. Sample of pure PUA wasprocessed in the same way and used for comparison.The resins were injected into a 2-mm thick organic glass gasket

(length 150�width 50 mm), sandwiched between two glassslides and PET films to allow easy removal of the samples.Samples were exposed to the UV radiation from a high pressuremercury lamp (output power of 600W/cm2). The incident lightintensity at the sample position was measured by radiometry tobe 400W/cm2. A shutter was used to allow an accurate control ofthe exposure time of the sample by the light. Samples wereexposed to the UV lamp for a total time of 2min (lmin on eachside) at ambient temperature.

Instrumental

TGA was conducted from 30 to 8008C and 208C/min scan rate ona Ntezsch STA409PC instrument (Bavaria, Germany) undernitrogen atmosphere. All TGA results are averages of a minimumof three determinations; temperatures are reproducible to �38C,while the error bars on the non-volatile material are �3%. DSCwas performed on a Ntezsch DSC200F3 instrument (Bavaria,Germany) from 20 to 1008C under nitrogen atmosphere and108C/min scan rate. Specimens (8–12mg) were cut as powder toavoid effect of the heat transfer as far as possible and thespecimen were encapsulated in the aluminum pans. Twoconsecutive heating and cooling runs were employed using108C/min heating and cooling rates respectively in both runs. Thepowder samples for XRD were obtained by grinding and sievingto <270 mesh and then mounted in the Philips sample holder.XRD patterns were collected from 1 to 10o using a step size of0.028 on a Bruker-D8 instrument (Karlsruhe, Germany) usingmonochromatic CuKa radiation. TEM was performed on aJEOL-1230 instrument (Tokyo, Japan) under an acceleratingvoltage of 100 kV. The samples were ultra-microtomed with adiamond knife at room temperature to give thin sections whichwere approximately 70 nm. The sections were transferred fromthe knife edge to 600 hexagonal mesh Cu grids. Tensile testingwas carried out with a Kexin WDW3020 electronic universaltesting machine (Changchun, China) according to GB1040-92standard.

RESULTS AND DISCUSSIONS

Swelling capacity of polymerically-modified clays

In general, the stability of dispersed clays is related to the swellingcapacity of the clays in the solvent, and the morphology of curedcomposites is certainly related to the dispersion state of clays inthe prepolymers.[24] In the present experiment, it is important toform a stable suspension of clays because there is no extra-shearin UV-exposure period. The swelling capacities of clays used inthis experiment were tested in the prepolymers. The X-raypatterns for the clays before and after swelling are shown in Figs 1and 2, respectively. The d-spacing of pristine clay has increasedfrom 1.2 to 1.6 nm after being swollen in prepolymers as shown inFigs 1(A) and 2(A). There is no obvious peak in the XRD patterns of

& Sons, Ltd. Polym. Adv. Technol. 2011, 22 1919–1924

Figure 1. XRD patterns obtained from clays (before swelling): (A)—

pristine clay; (B)—PSAA-clay; (C)—PSMMA-clay; and (D)—PSAM-clay.

Figure 3. Solvent swelling ability of different clays.

EFFECT OF POLYMERICALLY-MODIFIED CLAY STRUCTURE

polymerically-modified clays after being swollen in prepolymersas shown in Fig. 2(B), (C), and (D). However, it is true that thesedata can be taken as an indication that the d-spacings ofpolymerically-modified clays were all enhanced to more than8.8 nm as calculated by l¼ 2d sinu where 2u¼ 18. The datashown in Fig. 3 suggest that the swelling capacity oforganic-modified clays is greatly increased compared to pristineclay. Moreover, this increase is strongly dependent on themodifier used. The swelling capacity of PSAA-clay in theprepolymers increased almost three times that of pure clay,which was the best among three kinds of polymerically-modifiedclays. This can be attributed to the formation of hydrogenbonding between the modifiers and prepolymers due to thepolymerically-modified clays having hydroxyl (PSAA-clay), amidegroups (PSAM-clay), and carbonyl groups (PSMMA-clay).

Effect of polymerically-modified clay structure on themorphologies of formed nanocomposites

The structures of the polymeric surfactants used for themodification of clays in this experiment are expected to havean effect on the morphology of the UV-cured PUA nanocompo-

Figure 2. XRD patterns obtained from clays (after swelling): (A)—pris-

tine clay; (B)—PSAA-clay; (C)—PSMMA-clay; and (D)—PSAM-clay.

Polym. Adv. Technol. 2011, 22 1919–1924 Copyright � 2010 John Wil

sites. Figure 4 shows XRD patterns of PUA/clay compositesprepared from pristine clay and polymerically-modified clays.One can see no obvious diffraction peak. These results suggestthat the clay platelets are probably exfoliated or completelydisordered. These results alone cannot confirm this premise asthe observed absence of scattering could be due to geometryeffects or lack of sensitivity at the low level of clay loading.To confirm the structures of PUA/clay and PUA/polymerically-

modified clay nanocomposites, TEM experiments were per-formed. TEM images of polymerically-modified clays andcomposites are shown in Fig. 5. The composites formed frompristine clay contain many aggregates of clay as shown inFig. 5(A). Figure 5(B1), (B2), and (B3) show the morphologies ofPSMMA-modified clay, and the low and high magnificationimages of the nanocomposites formed from the PSMMA-modified clays. It can be seen that most of the particles in thecomposites are much smaller than that observed in pristineclay-based composites, and ‘tactoids’ are still present. Acomparison between the high magnification images of PSMMA-modified clays and its PUA composites, shown in Fig. 5(B1) and

Figure 4. XRD patterns obtained from: (A)—pure PUA; (B)—PUA/PSAA-clay-3%; (C)—PUA/PSMMA-clay; (D)—PUA/PSAM-clay; and (E)—PUA/

clay-3%.

ey & Sons, Ltd. wileyonlinelibrary.com/journal/pat

1921

Figure 5. TEM images obtained from: (A)—PUA/pristine clay; (B)—PUA/PSMMA-clay; (C)—PUA/PSAA-clay; and (D)—PUA/PSAM-clay (2—low

magnification, 3—high magnification, and 1—the corresponding clays).

D. CHEN ET AL.

1922

(B3), indicates that the clay galleries are further expandedbecause of the intercalation of polymers. Figure 5(C) and (D) showthe structures of PUA nanocomposites formed using the PSAAand PSAM-modified clays. The clays in the composites areextensively dispersed as indicated in the low magnificationimages. The high magnification images shown in Fig. 5(C3) and5(D3) reveal that these composites have an exfoliated structurewith some stacked layers. In this experiment, the content of clayare the same, the significant difference among them lies in thechemical structures of polymeric surfactants used in themodification of clays. There are hydroxyl and amide groups in

wileyonlinelibrary.com/journal/pat Copyright � 2010 John Wiley

the PSAA and PSAM surfactants which could form hydrogenbonds with the prepolymers; the hydrogen bonding may be thereason for the difference in morphologies of these claycomposites.

Thermal analysis of PUA/clay nanocomposites

The thermal stability of PUA and its nanocomposites has beenevaluated using TGA. The TGA traces obtained from pure PUA andPUA nanocoposites are shown in Fig. 6. It shows that the smallclay loading in PUA/clay nanocomposites does not affect the

& Sons, Ltd. Polym. Adv. Technol. 2011, 22 1919–1924

Figure 6. TGA traces obtained from pure PUA and its nanocomposites.

Figure 7. DSC traces obtained from: (A)—PUA; (B)—PUA/pristine clay;(C)—PUA/PSMMA-clay; (D)—PUA/PSAA-clay; and (E)—PUA/PSAM-clay.

Figure 8. DSC traces obtained from: (A)—PUA; (B)—PUA/PSAM-

clay-1%; (C)—PUA/PSAM-clay-3%; and (D)—PUA/PSAM-clay-5%.

Figure 9. Tensile strength of PUA and its composites.

Figure 10. Elongation at break of PUA and its composites.

EFFECT OF POLYMERICALLY-MODIFIED CLAY STRUCTURE

1

thermal stability of PUA. The experimental results indicate thatmorphological structures do not affect the thermal properties ofnanocomposites at clay loading of 3%, and this is in agreementwith what is usually seen for clay nanocomposites.[9]

The Tgs of the various UV-curing composites were evaluatedby DSC. Figures 7 and 8 show DSC traces obtained from UV-curedPUA and PUA/clay nanocomposites. The nanocomposite withclays have higher Tg values than the pure PUA because of theconfinement of the polymer chains between the clay layers,which limited segmental motion of the polymer chain.[25]

Figure 8 showed an increase of Tg with the increasing clayloading. On the other hand, Fig. 7 showed that nanocompositePUA/PSAA-clay and PUA/PSAM-clay had higher Tg than PUA/PMMA-clay and PUA/pristine clay. In the swelling capacityexperiment, polymerically-modified clays have also indicatedthat there were stronger interactive attractions between the claysand prepolymers in comparison to the pristine clay, especiallyPSAA-clay and PSAM-clay. This strong interactive attraction mayalso contribute to the observed increase in Tgs of compositesformed from different clays.[26,27]

Polym. Adv. Technol. 2011, 22 1919–1924 Copyright � 2010 John Wil

Mechanical properties

Tensile properties of nanocomposites formed from polymer-ically-modified clays are shown in Figs Fig. 99 and Fig. 1010. The

ey & Sons, Ltd. wileyonlinelibrary.com/journal/pat

923

D. CHEN ET AL.

1924

experimental results indicate that the nanocomposites formedfrom the clays having hydroxyl or amide groups (PSAA andPSAM-modified clay) have significantly higher tensile strengththan the ones formed from pristine clay or clays having MMAgroups. TEM experimental results show that nanocompositesbased on PSMMA consist of large aggregates, while nanocom-posites based on PSAA and PSAM display better dispersion andexfoliated structures. Furthermore, DSC and swelling capacitytests have shown that PSAA and PSAM-modified clays havestronger interactive attractions with PUA matrix, which wasattributed to hydrogen bonding resulting in a higher compat-ibility. The above two aspects have contributed to the highperformance of nanocomposites formed from PSAA andPSAM-modified clays.

CONCLUSION

A comprehensive study on the morphologies and properties ofPUA/clay UV-cured nanocomposites prepared from differentpolymerically-modified clays has been performed. Experimentalresults indicated that morphologies and tensile properties of thecomposites are dependent on the chemical structure of modifiersused in the preparation of clays. Polymerically-modified clayswith hydroxyl and amide groups are promising additives for highperformance UV-cured PUA/clay nanocomposites.

Acknowledgements

We thank Zhejiang Fenghong Clay Chemicals Co. Ltd. for clayproducts, Dr. Chao Zisheng (College of Chemistry and ChemicalEngineering, Hunan University) for obtaining the digital XRD.

wileyonlinelibrary.com/journal/pat Copyright � 2010 John Wiley

REFERENCES

[1] W. D. Cook, Polymer 1992, 33, 2152.[2] C. Decker, Prog. Polym. Sci. 1996, 21(4), 593.[3] K. M. Dean, S. A. Bateman, R. Simons, Polymer 2007, 48, 2231.[4] E. Andrzejewska, Prog. Polym. Sci. 2001, 26(4), 605.[5] H. J. Assumption, L. J. Mathias, Polymer 2003, 44(18), 5131.[6] I. Sideridou, V. Tserki, G. Papanastasiou, Biomaterials 2002, 23(8),

1819.[7] J. Nie, L. A. Lindin, J. F. Rabek, Acta Polymer. 1998, 49, 145.[8] S. S. Ray, M. Okamoto, Prog. Polym. Sci. 2003, 28, 1539.[9] J. G. Zhang, E. Manias, C. A. Wilkie, J. Nanosci. Nanotechnol. 2008, 8,

1597.[10] K. Zahouily, S. Benfarhi, T. Bendaikha, J. Baron, In Proc Rad Tech

Europe Conf, 2001; 583.[11] M. K. Dean, A. S. Bateman, R. Simons, Polymer 2007, 48, 2231.[12] Y. Y. Wang, T. E. Hsieh, Chem. Mater. 2005, 17, 3331.[13] F. M. Uhl, S. P. Davuluri, S. C. Wong, D. C. Webster, Polymer 2004, 45,

6175.[14] L. Kellera, C. Deckera, K. Zahouilya, J. M. Le-Meins, Polymer 2004, 45,

7437.[15] S. Benfarhi, C. Decker, L. Keller, K. Zahouily, Eur. Polym. J. 2004, 40, 493.[16] B. Paczkowska, S. Stzelec, B. Jederzejewska, L.A. Linden, J. Pacz-

kowski, Appl. Clay. Sci. 2004, 25, 221.[17] S. C. Lv, W. Zhou, S. Li, W. F. Shi, Euro. Polym. J. 2008, 44, 1613.[18] Y. L. Zang, W. J. Xu, G. P. Liu, D. Y. Qiu, S. P. Su, J. Appl. Polym. Sci. 2009,

111, 813.[19] P. Liu, Appl. Clay. Sci. 2007, 38, 64.[20] S. P. Su, D. D. Jiang, C. A. Wilkie, Polym. Degrad. Stab. 2004, 83, 333.[21] R. Y. Chen, F. B. Peng, S. P. Su, J. Appl. Polym Sci. . 2008, 108, 2712.[22] D. Chen, Y. L. Zang, S. P. Su, J. Appl. Polym Sci. . 2010, 116, 1278.[23] J. P. Zhang, R. F. Liu, A. Li, A. Q. Wang, Polym. Mater. Sci. Eng. 2005, 21,

169.[24] X. Kornmann, H. Lindberg, L. A. Berglund, Polymer 2001, 42, 1303.[25] M. Ogawa, K. Kuroda, Bull. Chem. Soc. Jpn. 1997, 70, 2593.[26] O. P. Valmikanathan, O. Ostroverkhova, I. S. Mulla, K. Vijayamohanan,

S. V. Atre, Polymer 2008, 49, 3413.[27] K. M. Lee, C. D. Han, Polymer 2003, 44, 4573.

& Sons, Ltd. Polym. Adv. Technol. 2011, 22 1919–1924