Effects of TEOS Contents on Swelling Behaviors and Mechanical Properties of Thermosensitive Hybrid Gels

Transcription

1 Effects of TEOS Contents on Swelling Behaviors and Mechanical Properties of Thermosensitive Hybrid Gels Wei-Jen Huang, Wen-Fu Lee Department of Chemical Engineering, Tatung University, Taipei, Taiwan A series of organic-inorganic hybrid thermosensitive gels with three different structures and different contents of tetraethoxysilane (TEOS) were prepared from N-isopropylacrylamide (NIPAAm), and N,N 0 -methylenebis-acrylamide (NMBA) and TEOS [N-IPN]; NIPAAm, 3- (trimethoxysilyl) propyl methacrylate (TMSPMA) as coupling agent and TEOS [NT-IPN]; and NIPAAm, TMSPMA and TEOS [NT-semi-IPN] by emulsion polymerization and sol-gel reaction in this study. The effect of TEOS content on the swelling behavior, mechanical properties, and morphologies of the present gels was investigated. Results showed that the properties of the gels would be affected by the gel networks such as IPN or semi-ipn, existence of TMSPMA as the bridge chain between networks, and content of TEOS. The NT-semi- IPN gel had higher swelling ratio because poly (NIPAAm) moiety in the semi-ipn gels was not restricted by NMBA network. However, the IPN gels such as N-IPN and NT-IPN had good mechanical properties and lower swelling ratio, but had bad thermosensitivity due to the addition of coupling agent, TMSPMA, into the gel system that resulted in denser link between organic and inorganic components. Increasing TEOS content would also reduce the thermosensitivity of the hybrid gels. The morphology showed that IPN gels had partial aggregation (siloxane domain) and showed some denser phases. POLYM. COMPOS., 31: , ª 2009 Society of Plastics Engineers INTRODUCTION Hydrogel is a crosslinked polymer that is not soluble in water. Hydrogels are sensitive to environmental stimuli and have response to these stimuli such as temperature, ph, solvent composition, light, ion strength [1 7]. Because of the nature of stimuli-sensitive hydrogels, they can be applied as separation membranes, biosensors, artificial muscles, chemical valves and drug delivery devices [8]. Poly(N-isopropylacrylamide) [poly(nipaam)] hydrogels are well known to show thermally reversible swellingshrinking behavior around 328C in aqueous solutions with Correspondence to: Wen-Fu Lee; wflee@ttu.edu.tw Contract grant sponsor: National Science Council of the Republic of China; contract grant number: NSC E DOI /pc Published online in Wiley InterScience ( VC 2009 Society of Plastics Engineers respect to a lower critical solution temperature (LCST). Many studies have been focused on the field of controlled drug delivery, [9] regulation of the activity of enzymes, [10, 11] and thermo-controlled chromatography [12]. Organic-inorganic hybrids are attractive materials with heat resistance, mechanical and electrical properties, and radiation resistance [13]. It is expected that the hybridization of poly(nipaam) and inorganic compound improves the mechanical strength of poly(nipaam) gels [14]. For some purposes of superabsorbent, inorganic component such as Vermiculite, mineral powder, or TiO 2 were introduced into organic component to discuss the swelling properties or photocatalytic degradability [15 17]. Some hybrid hydrogels containing silyl group were prepared in our previous studies [18 21]. For example, two series of NIPAAm-siloxane copolymeric gels were prepared from NIPAAm and silane monomer such as 3- acrylamido propyl trimethoxysilane (APTMOS) or 3-(trimethoxysilyl) propyl methacrylate (TMSPMA).Their drug release behavior was investigated and the results showed that the release ratios of caffeine in the copolymeric gels would be decreased with an increase of the content of silane monomer and the compactness of the gel network [21]. In addition, a series of hybrid gels with IPN or semi-ipn structures were prepared from NIPAAm, tetraethoxysilane (TEOS, 5 mol%), and TMSPMA (5 mol%). The effect of coupling agent, TMSPMA, and different gel structures of the present hydrogels on swelling behaviors, mechanical properties, and morphologies was investigated in our previous report [22]. In this research, we prepared a series of hybrid gels with IPN or semi-ipn structures. The effect of TMSPMA as coupling agent and different gel structures and different TEOS contents on swelling behaviors, mechanical properties, and morphologies for the NIPAAm and TEOS was investigated. EXPERIMENTAL Materials N-isopropylacrylamide (NIPAAm) (TCI, Tokyo, Japan) was recrystallized in n-hexane before use. 3-(Trimethoxysilyl) POLYMER COMPOSITES -2010

2 TABLE 1. Feed compositions and yields and equilibrium swelling ratios (at 258C for the hybrid gels (in mol%). Sample NIPAAm TMSPMA NMBA TEOS Yield (%) SR (g/g) N-IPN-TEOS N-IPN-TEOS N-IPN-TEOS N-IPN-TEOS N-IPN-TEOS N-IPN-TEOS NT-SIPN-TEOS NT-SIPN-TEOS NT-SIPN-TEOS NT-SIPN-TEOS NT-SIPN-TEOS NT-SIPN-TEOS NT-IPN-TEOS NT-IPN-TEOS NT-IPN-TEOS NT-IPN-TEOS NT-IPN-TEOS NT-IPN-TEOS propyl methacrylate (TMSPMA) as coupling agent obtained from ACROS (Geel, Belgium) and N,N,N 0,N 0 -tetramethylethylene diamine (TEMED) as an accelerator obtained from Fluka Chemical (Buchs, Switzerland) were used as received. N,N 0 -methylenebisacrylamide (NMBA) as a crosslinking agent and ammonium persulfate (APS) as an initiator were purchased from Wako Pure Chemical (Osaka, Japan). TEOS was purchased from Showa Chemical (Tokyo, Japan). Sodium lauryl sulfate (SLS) was purchased from Nippon Shiyaku Kogyo (Osaka, Japan) Preparation of Organic-Inorganic Hybrid Thermosensitive Gels Three series of hybrid gels prepared were mainly composed of NIPAAm (95 mol%)/tmspma (5 mol%) or NIPAAm (100 mol%), adding NMBA (3 mol%) as crosslinker for organic component, and adding TEOS (0 50 mol%) as crosslinker and modifier for inorganic component. The feed compositions are listed in Table 1. Typically, NIPAAm, TMSPMA, TEOS and 0.2 mol% SLS as emulsifier were dissolved in 10 ml of deionized water, and then carried out the pre-emulsification by super-sonication (TP690, ELMA) until the solution reached to homogeneous phase. Then, 3 mol% NMBA and 0.5 mol% APS based on total monomer concentration, as crosslinker and initiator, respectively, were added into above solution. Finally, 0.5 mol% TEMED was added as an accelerator. The mixture was immediately injected into the space between two glass plates. The gelation was carried out at 258C for 2 days. After the gelation was completed, the gel membrane was cut into disks, and immersed in 1 M HCl(aq) for 2 days to proceed the sol-gel reaction. Then, the gels were immersed in excess deionized water to remove unreacted monomers and HCl. After above process, the swollen gels were dried by freeze-drying. Measurement of Swelling Ratio The preweighed dried gels (W d ) were immersed in deionized water at fixed temperature until swelling equilibrium was attained. The gel was removed from the water bath, tapped with delicate task wipers to remove excess surface water, and weighed as the wet weight of the gel (W w ). The swelling ratio (Q) was calculated from the Eq. 1: Q ¼ðW w W d Þ=W d ð1þ Physical Properties Measurement The mechanical strengths of these gels were measured by uniaxial compression experiment with universal tester (LLOYD, LRX; J. J. Lloyd, Poole, UK). The shear modulus was calculated from Eq. 2: [20,23] s ¼ F=A ¼ Gðk k 2 Þ ð2þ where s is compression stress, F is the compression load, A is the cross-sectional area of swollen gels, and k is the compression strain (L/L 0 ) (L 0 is the initial thickness of wet gel, and L is the thickness of wet gel after compression). At low strain, a plot of shear stress versus 2(k 2 k 22 ) would yield a straight line whose slope is shear modulus (G). The effective crosslink density (q) can then be calculated from shear modulus and polymer volume fraction (m 2 ) as follows: q ¼ G=m 2 1=3 RT ð3þ where R is the gas constant and T is the absolute temperature. Morphologies of Hybrid Gels Samples were equilibrated in deionized water for 1 day, and then the swollen gels were frozen-dried for 2 days. The gels were immersed in liquid nitrogen and fractured. The fractured specimens were examined for morphological details by using scanning electron micrograph (SEM) (Joel JSM5600, Tokyo, Japan) with an acceleration voltage of 15.0 kv. The specimens were coated a gold metal layer to provide proper surface conduction. RESULTS AND DISCUSSION Preparation of Organic-Inorganic Hybrid Gels With Different Contents of TEOS To investigate the effect of different gel structures and silane coupling agent TMSPMA on the properties of 888 POLYMER COMPOSITES DOI /pc

3 SCHEME 1. The structures of hybrid gels. thermosensitive hybrid gels, a series of NIPAAm hybrid gels with different structures were prepared, that is, N- IPN, N-semi-IPN, NT-IPN, and NT-semi-IPN, and investigated in previous study [22]. The structures of the designed hybrid gels are shown in Scheme 1. NT-IPN gel is an interpenetrating polymer network composed of organic network that was formed by NMBA and inorganic siloxane network that was formed by sol-gel process of TEOS. These two networks were interconnected by covalent bonding through sol-gel process of coupling agent, vinyl silane monomer TMSPMA; NT-semi-IPN gel is a semi-interpenetrating polymer network composed of linear poly(nipaam) and inorganic siloxane network, the linear polymer and siloxane network were interconnected through coupling agent, TMSPMA. N-IPN gel is an interpenetrating polymer network composed of organic poly(- NIPAAm) network and inorganic siloxane network, and there existed no covalent bonding between two networks. From a structural point of view, types of network and existence of interconnection between networks would affect the properties of the gels. Based on this reason, the effect of existence of silane coupling agent and types of interpenetration on the properties of these three types of gels are investigated. To further investigate the effect of gel structures on swelling behaviors and mechanical properties, a series of DOI /pc POLYMER COMPOSITES

4 SCHEME 2. Schematic diagram of structural difference of IPN and semi-ipn gels. [Color figure can be viewed in the online issue, which is available at organic-inorganic hybrid gels with different TEOS contents were prepared in this research. For hybrid gels which mainly composed of organic network and inorganic network with or without covalent bonding via introduction of coupling agent, their situations of inorganic network and formation of gel structures could be modified by modulation of TEOS content. The feed compositions are listed in Table 1. TEOS and TMSPMA would form siloxane networks as sol-gel process was carried out under acid catalysis. In normal situation, gels would be more condensed as the content of inorganic component increased. But in this system, gel networks became loosen and collapsed due to phase separation when the TEOS content were too high to form homogeneous phase. Thus, the swelling behaviors and mechanical properties were not only affected by hydrophobicity of inorganic composition but also affected by the discontinuous gel structural situation caused by phase separation under high TEOS addition. The gel conformations with different TEOS contents were represented in Scheme 2. As the TEOS content raised from 0 to 5 mol%, growing siloxane network made the gel structure more compacted. When TEOS content was higher than 5 mol%, the phase separation occurred FIG. 1. Effect of content of TEOS on equilibrium swelling ratios of hybrid gels at 258C. 890 POLYMER COMPOSITES DOI /pc

5 FIG. 2. Effect of temperature on swelling behaviors of (a) N-IPN hybrid gels, (b) NT-SIPN hybrid gels, and (c) NT-IPN hybrid gels with different TEOS contents. and reduced the compactness of gels and resulted in loosen and collapsed gel structures. The results in Table 1 also indicated that the yields of hybrid gels decreased with increasing TEOS content. The reason was supposed to be resulted from the collapse of gel structures causing by phase separation under too high TEOS content. Among these gels, because the organic crosslinking agent (NMBA) was not added in the feed of semi-ipn gels and resulted in lower yields due to relatively loosen structures. Effect of Temperature on Swelling Behavior of Hybrid Gels With Different TEOS Contents The effect of TEOS content on equilibrium swelling ratios of hybrid gels at 258C shown in Figure 1 indicated that the swelling ratios of the hybrid gels with different gel structures had the tendency in the order NT-SIPN [ N-IPN [ NT-IPN. The results and the reason were the same as mentioned in previous study [22]. The swelling ratios of hybrid gels decrease with increasing TEOS content for each kind of hybrid gels. This is because the addition of TEOS increased the hydrophobicity and the gel structure became more compact via increasing siloxane component. Figure 2a c showed the effect of temperature on swelling behaviors of the hybrid gels with different TEOS content. The swelling ratios of all hybrid gels decreased as the temperature increased. The swelling ratios of N- IPN gels with different TEOS contents at different temperatures are shown in Figure 2a. For N-IPN gels with TEOS content less than 5 mol%, differences were DOI /pc POLYMER COMPOSITES

6 FIG. 3. Effect of TEOS content on swelling ratios for (a) N-IPN hybrid gels, (b) NT-SIPN hybrid gels, and (c) NT-IPN hybrid gels at different temperatures. observed between swelling ratios of hybrid gels that swelled at low temperature and deswelled at high temperature. There were more drastic changes of swelling ratios around C, that is, the gels had more apparent critical gel transition temperature (CGTT). For N-IPN gels with TEOS content higher than 10 mol%, the swelling ratios decrease with increasing temperature, but the decreasing degree was not as drastic as the N-IPN gels with lower TEOS content. This is because the addition of TEOS would decrease the thermosensitivity of the gels and the siloxane networks of TEOS would not be affected by temperature. Hence, the shrinkage degrees of the gels with higher TEOS content were less than the gels with lower TEOS content at temperature higher than 328C, and caused the swelling ratios of the gels with lower TEOS content were lower than the gels with higher TEOS content in high temperature environment. The effect of temperature on swelling ratios of NT- SIPN gels with different TEOS content shown in Figure 2b indicated that swelling ratios of hybrid gels decreased with increasing temperature and increasing TEOS content. The decreasing degree of swelling ratios of the gels with TEOS content less than 5 mol% was more apparent than the gels with TEOS content higher than 10 mol% as the temperature increased. Thus, the gels with lower TEOS content have better thermosensitivity. At high temperature, the swelling ratios of NT-SIPN0 and NT-SIPN1 gels were not decreased to lower than those of the gels with higher TEOS content as the N-IPN series behaved (Fig. 2a). That is, swelling ratios for N-IPN gels with lower TEOS contents would lower than N-IPN gels with higher TEOS contents, but swelling ratios for NT-SIPN gels with lower TEOS contents would higher than NT- SIPN gels with higher TEOS contents. This is because crosslinking network of organic component were not formed in NT-SIPN gel networks, and linear polymeric NIPAAm component would not be restricted by crosslinking bonding. Thus, semi-ipn gels with looser structure 892 POLYMER COMPOSITES DOI /pc

7 resulted in higher water content even though the gels were hydrophobic at high temperature environment. The swelling behaviors of NT-IPN gels with different TEOS content at various temperatures were similar to N-IPN gels and the results were shown in Figure 2c. In summary, comparisons of thermosensitivity of gels with low TEOS content were further described as follow: (1) Thermosensitivities of N-IPN gels were better than those of NT-IPN gels. This is because coupling agent, TMSPMA, in NT-IPN gels would increase the hydrophobicity and decrease the thermosensitivity, poly(nipaam) networks of N-IPN gels would not be restricted by inorganic siloxane network via covalent bonding providing by TMSPMA and resulted in better thermosensitivity; (2) Thermosensitivities of N-IPN gels were better than those of NT-SIPN gels. Looser semi-ipn structure caused the gels have relatively high swelling ratios than other gels even at high temperature and resulted in less difference of swelling ratios between temperatures at higher and lower than CGTT. Moreover, the linear poly(nipaam) component of NT-SIPN were restricted by inorganic siloxane networks via covalent bonding providing by TMSPMA even if the linear poly(nipaam) component was relatively free in networks and resulted in less thermosensitivity than N-IPN gels; (3) Thermosensitivities of NT-IPN gels were similar to NT-SIPN gels from the results of Figure 2a and c. The factors that affect the thermosensitivity in organicinorganic hybrid gel system were described as follows: (1) Freedom of thermosensitive component. In this study, organic part of poly(nipaam) was the thermosensitive component. The less the restriction of thermosensitive component, the more the apparent of response to temperature change and resulted in better thermosensitivity. (2) Hydrophilicity of gels. The hydrophobic component which was independent to formation and separation of bonding between thermosensitive component and water molecules would reduce the thermosensitivity of hybrid gels. In this study, the coupling agent, TMSPMA, and TEOS were the hydrophobic component; (3) Compactness of gel structure (IPN or semi-ipn). In this study, loosen structure of semi- IPN increased the water content and counterbalanced the thermosensitivity performance of poly(nipaam). FIG. 4. Mechanical properties of the hybrid gels with different contents of TEOS: (a) modulus and (b) crosslinking density. Effect of TEOS Content on Swelling Ratios of Hybrid Gels at Fixed Temperature The information of the relationships of swelling ratios, temperatures, and TEOS content in Figure 2 was converted and the results are shown in Figure 3. In Figure 2, the main concept was focused on the effect of temperature on swelling ratio of the hybrid gels. The main concept in Figure 3 was focused on the effects of TEOS content on swelling ratios of the hybrid gels at a fixed temperature. Effect of TEOS content on swelling ratios for N-IPN, NT-SIPN, NT-IPN hybrid gels at fixed temperature were respectively shown in Figure 3a c. For each kind of the hybrid gels, there existed similar behaviors. At temperature lower than 308C (T\ CGTT), the swelling ratios of hybrid gels decreased with increasing TEOS content due to the hydrophobicity of TEOS. At temperature higher than 358C (T[ CGTT), the swelling curves existed a reversed behavior to TEOS content. Swelling ratios of hybrid gels decreased as TEOS content increased from 0 mol% to 5 mol%. When TEOS content is higher than 5 mol%, swelling ratios increased rather than decreased as the hybrid gels behaved at lower temperatures. Further increase of TEOS content also caused the swelling ratio decreased, but the decrease ranges were not so drastic. The main reason for this phenomenon should be resulted from the phase separation of hybrid gels caused by high TEOS content; the resulted loosen gel structure made the gel imbibe more water. Therefore the hybrid gels with higher TEOS content DOI /pc POLYMER COMPOSITES

8 FIG. 5. The microphotographs of the cross-sectional morphology of the hybrid gels. might have higher swelling ratios than those gels with lower TEOS content at temperature higher than CGTT. At temperatures lower than CGTT, poly(nipaam) component was hydrophilic and swelled. The effect of hydrophilicity of poly(nipaam) component was more significant than the loosen structure resulted from phase separation causing by TEOS addition. Thus, swelling ratios were almost simply affected by hydrophobicity of TEOS at temperature lower than CGTT. At temperatures higher than CGTT, poly(nipaam) component converted to hydrophobic phase, and effect of loosen structure caused by phase separation became more obvious. Among these gels, the swelling behaviors of NT-SIPN gels were slightly different from N-IPN and NT-IPN gels. The swelling ratios of N-IPN (Fig. 3a) and NT-IPN (Fig. 3c) gels with TEOS content less than 5 mol% were all lower than those gels with TEOS content higher than 10 mol% at the temperature higher than 308C. This is because the organic components poly(nipaam) in NT- SIPN0 and NT-SIPN1 gels were not restricted by crosslinking networks and resulted in higher swelling ratios than those gels with TEOS content higher than 10 mol% even at the temperature higher than CGTT. That is, water absorption for NT-SIPN0 and NT-SIPN1 gels were much 894 POLYMER COMPOSITES DOI /pc

9 FIG. 5. (Continued) more than the water absorption by loosen structures of gels with high TEOS content. Mechanical Properties The compressive moduli G and effective crosslinking density q x for the present gels were calculated from Eqs. 2 and 3. The effects of TEOS content on mechanical strength (compressive modulus) and crosslinking density of organic-inorganic hybrid gels were shown in Figure 4a and b, respectively. The results in Figure 4 indicate that NT-IPN has highest gel strength and crosslinking density and the maximum modulus and maximum crosslinking density of hybrid gels were obtained at TEOS ¼ 5 mol%. Theoretically, introducing inorganic component to prepare organic-inorganic hybrid gels would improve the mechanical properties of hybrid materials, but the mechanical strength and crosslinking density were suddenly reduced when TEOS content higher than 5 mol% for the present hybrid gels in this study. It was supposed that the addition of higher content of TEOS caused the phase separation and resulting in a loosen structure of the gel. Hence, the gel strength and crosslinking density diminished. The effect of loosen structure caused by phase separation was also reflected on the swelling behaviors and microscopic DOI /pc POLYMER COMPOSITES

10 morphologies of the hybrid gels in this research. Although the phase separation were exited as TEOS content higher than 10 mol%, the improvement of mechanical properties by adding inorganic component would still be reflected on whole performance and resulted in slight increase of mechanical properties as the TEOS content increased. Morphologies of the Hybrid Gels The microphotographs of the cross-sectional morphology of the hybrid gels with different TEOS contents are shown in Figure 5. Figures showed that the pores inside the hybrid gels would become smaller and denser when the TEOS content increased. These figures also showed that the pores for the hybrid gels with higher TEOS content become interconnect. For gels with TEOS content less than 5 mol%, there existed larger and independent pores that shown in Figures 5-(1a e), 5-(2a e), and 5- (3a e). But, for gels with TEOS content higher than 10 mol% in Figures 5-(1f j), 5-(2f j), and 5-(3f j), the morphologies of gel structure became loosen and the porous structures became more interconnected. Check the micrographs of gels with 10 mol% TEOS with magnitude of 5,000 in Figures 5-(1e), 5-(2e), and 5-(3e), the gel morphologies of the gels with IPN structures had more compact than those of the gels with semi-ipn structures. While TEOS content raised up to 30 mol% or 50 mol%, the gel structures became more fractured and the pores inside the gels became connected that shown in Figures 5-(1h, j), 5-(2h, j), and 5-(3h, j). The factors affecting the gel structures and pore formations were mainly caused by phase separation between organic and inorganic parts. Swelling behaviors and mechanical properties were dependent on gel structures and reflected on gels morphologies. Gels with loosen structures such as shown in Figures 5-(1d j), 5-(2d j), and 5-(3d j), might increase water absorption but reduce the gel strength. CONCLUSIONS A series of hybrid gels with different network type and different TEOS content were prepared by emulsion polymerization and sol-gel process, and the effect of TEOS content on swelling ratios, temperature-dependence, mechanical properties, and morphologies of the hybrid gels were investigated. Results showed that the swelling ratios and thermosensitivities of the hybrid gels decreased with an increase of TEOS content. As the content of TEOS increased, the phase separation occurred that resulted in loosen gel structure and consequently showed a weaker modulus and lower crosslinking density. SEM morphologies also showed that interconnect porous structures of hybrid gels were formed by phase separation caused by increased TEOS content. REFERENCES 1. J. Ricka and T. Tanaka, Macromolecules, 17, 2916 (1984). 2. Y. Hirokawa and T. Tanaka, J. Chem. Phys., 81, 6379 (1984). 3. J. Grignon and A. M. Scallan, J. Appl. Polym. Sci., 25, 2829 (1980). 4. A. S. Hoffman, J. Control Release, 6, 297 (1987). 5. T. Tanaka, D. Fillmore, S. Sun, I. Nishio, G. Swislow, and A. Shah, Phys. Rev. Lett., 45, 1636 (1980). 6. J. Hrouz, M. Ilvasky, K. Ulbrich, and J. Kopecek, Eur. Polym. J., 17, 361 (1981). 7. S. Katayama, Y. Hirokawa, and T. Tanaka, Macromolecules, 17, 2641 (1984). 8. N.A. Peppas, J. Bioact. Compat. Polym., 6, 241 (1991). 9. Y.H. Bae, T. Okano, R. Hsu, and S.W. Kim, Makromol. Chem. Rapid Commun., 8, 481 (1987). 10. T. Okano, Y. H. Bae, and S.W. Kim, J. Control Release, 11, 255 (1990). 11. A.S. Hoffman, A. Afrassiabi, and L.C. Dong, J. Control Release, 4, 213 (1986). 12. A.S. Hoffman, J. Control Release, 6, 297 (1987). 13. T. Ogata, T. Nonaka, and S. Kurihara, J. Membr. Sci., 103, 159 (1995). 14. H. Schmidt, J. Non-Crystal Solids, 73, 681 (1985). 15. Q. Tang, J. Lin, J. Wu, Y. Xu, C. Zhang, J. Appl. Polym. Sci., 104, 735 (2007). 16. J. Wu, Y. Wei, J. Lin, S. Lin, Polymer, 44, 6513 (2003). 17. Q. Tang, J. Lin, Z. Wu, J. Wu, M. Huang, Y. Yang, Eur. Polym. J., 43, 2214 (2007). 18. W.F. Lee and W.Y. Yuan, J. Appl. Polym. Sci., 84, 2523 (2002). 19. W.F. Lee and W.J. Lin, J. Polym. Res., 9, 23 (2002). 20. W.F. Lee and W.J. Lin, J. Polym. Res., 10, 31 (2003). 21. W.F. Lee and W.J. Huang, Mater. Sci. Forum, (2003). 22. W.J. Huang and W.F. Lee, J. Appl. Polym. Sci., 111, 2025 (2009). 23. L.R.G. Treloar, The Physics of Rubber Elasticity, Clarendon Press, Oxford (1975). 896 POLYMER COMPOSITES DOI /pc