s-PVA Hydrogels Prepared by ... - Springer Link

Fibers and Polymers 2012, Vol.13, No.8, 955-962

DOI 10.1007/s12221-012-0955-5

Investigation of a-PVA/s-PVA Hydrogels Prepared by Freezing-thawing Method Mei Huang, Dongdan Cai, Yanhua Liu, Jun Sun, Jianjun Wang, Chuanxiang Qin, Lixing Dai*, and Yamaura Kazuo1 College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China 1 Faculty of Textile Science and Technology, Shinshu University, Ueda-city, Nagano-prefecture 386, Japan (Received January 10, 2012; Revised March 12, 2012; Accepted March 20, 2012) Abstract: The hydrogels of atactic poly (vinyl alcohol) (a-PVA) and syndiotactic poly (vinyl alcohol) (s-PVA) with different blend ratios were prepared by freezing-thawing processes. The effect of s-PVA on gelation behavior of the blend was investigated in terms of gelation temperature (Tgel) and hydrogel melting temperature (Tgm). And swelling behavior, crystallization, thermal properties, morphology of the blend hydrogels were also studied. With the increase of s-PVA, Tgel of the blend solution and Tgm of the blend hydrogels increase. Both crystallinity and crystallite dimensions based on the XRD profiles are nearly monotonically increasing functions of s-PVA content. FTIR results indicate the number of hydrogen bonds raises with s-PVA increasing. DSC results demonstrate s-PVA favors improvement of hydrogels thermal stability. According to SEM images of hydrogels, the increase of cross-linking caused by s-PVA in the blend hydrogels results in denser structure, which in turn leads to increased gel fraction (G) and Hardness. 50/50 (a-PVA/s-PVA) blend hydrogel has a denser structure with EWC of 73.6 %, hardness of 22.8 HA and Tm of 236.15 oC. The result indicates blending a-PVA and s-PVA is a useful method to form the hydrogel having good thermal stability and relative high degree of swelling. Keywords: Syndiotactic poly (vinyl alcohol), Atactic poly (vinyl alcohol), Freezing-thawing, Blend hydrogel

of this PVA hydrogel to be too weak to use as a joint material [22]. The desire for PVA hydrogel properties is different in different application. The poor thermal properties of PVA hydrogel also limit its application. As far as s-PVA is concerned, it is easier to form hydrogels from its aqueous solutions than a-PVA [23-25]. As expected, the mechanical and thermal properties of s-PVA hydrogels are superior to those of a-PVA. But water content of s-PVA hydrogels is much lower than that of a-PVA [26-31]. With these considerations, we tried to prepare a-PVA/s-PVA blend hydrogels in this paper. There have been some papers about the a-PVA/s-PVA series. Tanigami investigated miscibility in the crystal phase for blends between two types of PVAs with different syndiotacticities, and found the blends of sPVA and a-PVA can produce cocrystallized phases, the results indicate blending of s-PVA with a-PVA will be a useful technique to produce PVA materials with higher melting temperatures from the cheaper a-PVA material [32,33]. Matsuzawa studied the effects of annealing temperature on the crystallinity, solubility, degree of swelling and the hygroscopicity of the blends of a-PVA and s-PVA, they found the crystallinity of the blends increased with increasing annealing temperature, and in the 200 oC annealed blends containing s-PVA about 50 %, the insoluble fraction of which in boiling water was larger than the fraction of s-PVA in each blend [34]. Lyoo investigated the influence of blend ratios of a-PVA/s-PVA on the rheological properties of sPVA/a-PVA/water solutions, they found yield stress was higher for s-PVA/a-PVA blend solution with larger s-PVA content, this suggested that more domains with internal order were present at blend solution having larger s-PVA

Introduction Hydrogels, polymer networks which can swell to an equilibrium but do not dissolve in water [1], have been considered as a promising biomedical material for their similar physical properties to human tissues and the eximious tissue compatibility [2]. In recent years, increasing attention has been paid to the hydrogels application, especially that of PVA hydrogels. PVA hydrogels have the advantages of nontoxicity, good biocompatibility [3-5], and excellent mechanical properties, as well as their super water-absorbing capability which make them popular to be used as biomedical materials [6,7], such as wound dressing [1,8,9], articular cartilage replacement [10], cell immobilization [11], and tissue engineering [12]. According to the location of the hydroxyl group on the chain, PVA is classified into three types: atactic PVA (aPVA), syndiotactic PVA (s-PVA) and isotactic PVA (i-PVA). Generally, PVA hydrogels, as we known, are made from aPVA. The properties of a specific hydrogel are extremely important in selecting which materials are suitable for a desired application. However, a-PVA hydrogels have many advantages, a critical barrier to their use is the low strength [13-16]. There have been lots of attempts to improve hydrogel mechanical strength, such as chemical cross-linking or freezing-thawing [17-19]. As early as 1975, Peppas and coworkers had succeeded in producing strong PVA hydrogel by the freezing-thawing method and using it as an artificial cartilage [20,21]. But Ushio thought the mechanical strength *Corresponding author: [email protected] 955

956

Fibers and Polymers 2012, Vol.13, No.8

content [35]. However, there has not yet been any information on a-PVA/s-PVA blend hydrogels till now. In this study, the aqueous solutions of the blends of aPVA/s-PVA were made into hydrogels through the freezingthawing method. The structure-property relationships of these hydrogels were revealed by investigating their gel behavior, swelling behavior, morphology, crystallization and thermal properties.

Experimental Materials a-PVA was purchased from Sinopharm Chemical Reagent Co., Ltd., which had a degree of polymerization of 1750, and a degree of hydrolysis of 98 %. s-PVA was obtained by saponication of poly (vinyl trifluoroacetates) prepared by radical polymerization of vinyl triuoroacetate at 60 oC and some data of the polymer are shown in Table 1. Preparation of PVA Hydrogels 5 wt. % a-PVA/s-PVA aqueous solutions with different ratios were prepared by dissolving the PVAs in deionized water in a sealed tube at 120 oC for 4 h and were kept for 1 h to ensure homogenization. The prepared aqueous solutions were put into molds, and cooled to room temperature, and then subjected to one to seven cycles of freezing for 10 h at 30 oC and thawing for 2 h at room temperature to form hydrogels. The a-PVA/s-PVA (DP: 1100) hydrogels exposed to three freezing-thawing cycles were used for measurements of gel fraction, equilibrium degree of swelling, equilibrium water content, hardness and morphology, while the same hydrogels dried at room temperature for 48 h and further dried at 50 oC in vacuum oven for 48 h were used for XRD, FTIR and DSC measurements. Measurements Gelation Temperature and Hydrogel Melting Temperature Gelation temperature (Tgel) and hydrogel melting temperature (Tgm) were measured by the test tube tilting method [36-38]. A series of a-PVA/s-PVA solutions with different weight ratios were prepared in sealed test tube. The sealed test tubes containing solution were cooled from 100 oC to 0 oC in water bath, and then to below 0 oC in a water, ice and salt Table 1. Main parameters of s-PVA used Degree of DiadDegree of b b saponification syndiotacticity polymerizationa (%) (%) 1 800 57.2 99.8 2 1100 57.1 99.8 3 4380 57.3 99.8 a Based on intrinsic viscosity of the acetylated sample and bbased on 1 H-NMR spectrum. Samples

Mei Huang et al.

bath. The temperature at which the solution from mobile becomes immobile was regarded as Tgel. The sealed test tubes containing hydrogels were warmed from 0 oC to 100 oC, the temperature at which the hydrogels from immobile to mobile was regarded as Tgm. Gel Fraction The a-PVA/s-PVA hydrogels were immersed in distilled water at room temperature for 15 days to eliminate the unreacted species, and then dried to a constant weight in vacuum at 50 oC. The weight ratio of the dried hydrogels before and after immersion can be thought as an index of the degree of crosslinking or gel fraction [39]. So gel fraction G of hydrogels was calculated according to the following equation: W G ( % ) = -------d × 100 Wi

(1)

Where Wi and Wd are the weights of the dried hydrogels before and after immersion, respectively. Equilibrium Degree of Swelling and Equilibrium Water Content The a-PVA/s-PVA hydrogels were immersed in distilled water at room temperature until the weights of the samples were constant. Before weighing the samples, the surface water was removed with filter paper. Then the samples were dried in vacuum at 50 oC to reach constant weight. The equilibrium degree of swelling (EDS) and equilibrium water content (EWC) were calculated as follows: Ws – W d EDS( % ) = ----------------- × 100 Wd

(2)

Ws – Wd EWC ( % ) = ----------------- × 100 Ws

(3)

where Ws is the weight of the swollen sample at equilibrium state and Wd is the final dried sample weight after immersion. Hardness The hardness measurement was carried out using type A shore durometer on the a-PVA/s-PVA hydrogels. The data were recorded using average value of five samples for each hydrogel. Crystallization Behavior Measurement Wide-angle X-ray powder diffraction profiles of the dried hydrogels were obtained at room temperature with a diffractometer using Ni-filtered Cu Kα radiation (λ=1.5405 Å) at 40 kV and 40 mA and scans at 0.026 deg(2θ)/s in the 2θ range from 10 to 60 o. Apparent crystalline dimensions along the [101] lattice direction was evaluated from the width at half-height of the peak centered at around 2θ=20 o by using the Scherrer formula: kλ t = -------------βcosθ

(4)

where t is crystalline dimension, k is Scherrer geometric or

a-PVA/s-PVA Hydrogels Prepared by Freezing-thawing Method


957

shape factor, which is taken to be 0.89 as a constant, λ is the wavelength of the X-rays, β is the width at half-height of the peak, and θ is the Bragg angle. The crystallinity was determined from the XRD patterns of the samples by applying the formula: Ac - × 100 C ( % ) = --------------Aa + Ac

(5)

where Ac is the area of the crystalline zone, and Aa is the area of the amorphous zone. Fourier Transform Infrared Spectroscopy (FTIR) To characterize the presence of specific chemical groups in the a-PVA/s-PVA hydrogels, the FTIR spectra of PVA hydrogel films were recorded in the range between 4000 and 800 cm-1 during 32 scans. Thermal Behavior Measurement The thermal property of dried hydrogels was measured by differential scanning calorimetry (Q200, TA Co., USA), 510 mg of dried hydrogels was placed in an aluminum pan and heated at a scanning rate of 10 oC/min from 40 to 260 oC in a nitrogen atmosphere. Morphology The a-PVA/s-PVA hydrogels were investigated using scanning electron microscopy (S-4700, Hitachi Co., Tokyo, Japan). Because it was difficult to observe the hydrogel structure by SEM owing to the presence of water in the native state of hydrogels, therefore, the hydrogel samples were freeze-dried for 48 h to prevent contraction of the hydrogels before taking SEM photographs.

Results and Discussions Gelation Behavior The effects of the content of s-PVA and its DP on the Tgel of a-PVA/s-PVA blend solution and Tgm of a-PVA/s-PVA blend hydrogels are presented in Figure 1. Whatever s-PVA molecular weight, the blend solution with higher s-PVA content forms gels at higher temperature. Moreover, with the increase of s-PVA molecular weight, Tgel of the blend increases. Although the DP 4380 is higher four times than the DP 800, the difference in Tgel of their blends is invariably about 6 oC, which suggests s-PVA molecular weight has limited effect on Tgel. Intermolecular hydrogen bonding increases in the blend with the s-PVA content or its DP increasing, leading to more insoluble crystallites and physical crosslinking points in the blend solutions at a relatively high temperature, which gives answers for why gelation takes place at higher temperature for the blend solutions containing higher s-PVA content. For the similar reason, the Tgm of aPVA/s-PVA blend hydrogels rises distinctly with s-PVA content increasing. The effect of s-PVA molecular weight on Tgm is similar to that on Tgel as mentioned above for the similar reason. Another important parameter affecting the properties of

Figure 1. Effects of the content and DP of s-PVA on (a) Tgel of aPVA/s-PVA blend solution, and (b) Tgm of a-PVA/s-PVA blend hydrogels (exposed to one cycle of freezing for 24 h at 0 oC).

hydrogels is the number of freezing-thawing cycles. The relations between Tgm of a-PVA/s-PVA (70/30) blend hydrogels and freezing-thawing cycles are shown in Figure 2. With an increase in the number of freezing-thawing cycles, Tgm of sample increases, but this tendency is not so evident after 3 freezing-thawing cycles, especially in the samples where sPVA DP is 4380 and its Tgm is nearly independent of the number of freezing-thawing cycles (~96 oC). Therefore, in the following experiments, the number of freezing-thawing cycles of all the hydrogels is set as 3. The higher s-PVA DP, Tgm of the blend hydrogels is larger at the same freezingthawing cycle. The intermolecular hydrogen bonding would be stronger when the DP of the s-PVA is higher [40], so the internal crosslinked structure becomes more stable. Crystallization Behavior The X-ray diffraction profiles of different samples are

958


Figure 2. Effect of freezing-thawing cycles on Tgm of a-PVA/sPVA (70/30) blend hydrogels (exposed to repeated cycles of freezing for 10 h at −30 oC and thawing for 2 h at room temperature).

Figure 3. X-ray powder diffraction profiles for dried a-PVA/sPVA blend hydrogels with different ratios. The inset is the calculated crystallinity and crystallite dimensions (along the [101] lattice direction) for dried blend hydrogels as a function of s-PVA content. DP of s-PVA: 1100.

shown in Figure 3. The diffraction profiles exhibit a strong peak in the 2θ range 19-20 o corresponding to the [101] reflection of PVA, which demonstrates that there are some crystals presented in the hydrogel sample. As shown in the inset of Figure 3, both crystallinity and crystallite dimensions (along the [101] lattice direction) are nearly monotonically increasing function of s-PVA content. The crystallinity changes from 39.4 % (a-PVA/s-PVA: 100/0) to 46.9 % (aPVA/s-PVA: 0/100), and the crystallite dimension changes from 31 to 39 Å correspondingly. The more s-PVA content means the more intermolecular hydrogen bonding, which promotes the production of more small crystal nucleus. On

Mei Huang et al.

Figure 4. FTIR spectra for dried a-PVA/s-PVA blend hydrogels with different ratios.

the other hand, the increase of s-PVA content promotes the increase of the crystallite dimension, while s-PVA has higher crystallite dimension than a-PVA as reported by Cho [41]. Typical FTIR spectra for a-PVA/s-PVA hydrogel samples are shown in Figure 4, which clearly indicates the major characteristic absorption peaks of PVA. The C-H broad alkyl stretching band can be observed at 2850-3000 cm-1. The absorption peaks at approximately 1431 cm-1 may be assigned to CH2 bending vibration. The strong hydroxyl bands for the -OH stretching vibration are observed at 3200-3600 cm-1. The absorption peak of -OH of a-PVA/s-PVA hydrogel sample shifts to a lower wave number with increasing sPVA, as Figure 4 shows, it from 3350 cm-1 (a-PVA/s-PVA: 100/0) shifts to 3279 cm-1 (a-PVA/s-PVA: 0/100). The shifts of -OH peak position are caused by the hydrogen bonds increase [42], which indicates the number of hydrogen bonds raises with s-PVA increasing. An important absorption peak at 1140 cm-1 related to C-O-C stretching is observed which verifies the crystalline structure of a-PVA/s-PVA hydrogel sample [43-45]. The FTIR results have endorsed the former findings based on XRD spectrums of a-PVA/s-PVA hydrogels. A series of DSC traces were obtained for the dried blend hydrogels with different a-PVA/s-PVA (DP: 1100) ratios, giving evidence of Tm of the dried blend hydrogels in the range 210-250 oC as shown in Figure 5. There is only onepeak endotherm in every DSC curve though the blend ratios of a-PVA and s-PVA are different, which indicates the cocrystallization happened between a-PVA and s-PVA by virtue of their isomorph [32]. The position of the endotherm gradually shifts to higher temperatures and the endotherm peak becomes sharper with s-PVA content increasing. The endothermic peak of pure a-PVA hydrogel is broad and shallow owing to low degree of crystal perfection. With the s-PVA content increasing, the endothermic peak progressively


becomes narrower as shown in Figure 5, which suggests sPVA favors a fairly perfect crystallization. The melting temperatures, heat of fusion calculated from the whole melting endotherm of the DSC curves are plotted against the content of s-PVA as shown in the inset of Figure 5. The increment in Tm and ∆H with addition of s-PVA indicates that s-PVA facilitates crystallization and improves thermal stability. A dependency of gel fraction G (%) of blend hydrogels on the s-PVA content is shown in Figure 6. After repeated freezing-thawing process, G (%) rises with s-PVA content increasing. Such as G (%) is 78.3 % for pure a-PVA hydrogel, it goes up to 85.2 % for blend hydrogel containing 30 % s-PVA, and it is 89.79 % when s-PVA is 50 %, which


959

means that s-PVA does favor to the hydrogels stability. The increase in s-PVA content leads to more hydrogen bonding between PVA chains, which in turn results in more crystallites. Morphology Analysis The SEM micrographs of the blend hydrogels with aPVA/s-PVA ratio of 100/0, 50/50 and 0/100 are shown in Figure 7. As we seen, hydrogels are porous network structure which forms in the freezing-thawing process by producing ice crystals [46,47], and the ice crystals mainly generate in the space between crosslinking points where PVA molecules are less. The morphology of a-PVA hydrogel shown in Figure 7(a) presents an irregular porous structure, along with some fibers, that of a-PVA /s-PVA hydrogel shown in Figure 7(b) has a uniform and clear smaller porous structure like nets, and that of s-PVA hydrogel shown in Figure 7(c) only remains smallest pores. The morphological difference between pure s-PVA and a-PVA hydrogels reflects the distinction in cross-linking based on degree of syndiotacticity. When s-

Figure 5. DSC thermograms for a-PVA/s-PVA dried blend hydrogels with different ratios, the inset is the Tm and ∆H of dried blend hydrogels as a function of s-PVA content. DP of s-PVA: 1100.

Figure 6. Gel fraction (G %) of a-PVA/s-PVA blend hydrogels with different ratios, DP of s-PVA: 1100.

Figure 7. SEM micrographs of a-PVA/s-PVA hydrogels: (a) aPVA/s-PVA (100/0), (b) a-PVA/s-PVA (50/50), and (c) a-PVA/sPVA (0/100), DP of s-PVA: 1100.

960


Mei Huang et al.

Figure 8. Schematic representation of (a) pure a-PVA hydrogel, (b) blend a-PVA/s-PVA hydrogel, and (c) pure s-PVA hydrogel molecular structure after the freezing-thawing process.

Figure 9. Hardness of a-PVA/s-PVA blend hydrogels with different ratios, DP of s-PVA: 1100.

PVA is added into a-PVA, the number of intermolecular hydrogen bonding increases corresponding to an increase in degree of physical cross-linking between PVAs (as illustrated in Figure 8), leading to some increase in the number of pores and decrease in their size, both of which contribute to a relatively denser structure. The increase in hardness of the blend hydrogels with s-PVA increasing as shown in Figure 9 can also be explained by the above morphology analysis. The swelling of hydrogels can reflect cross-linking and it can also be used as a measure of the hydrophilicity of the polymer network, indirectly reflecting morphological structure. As shown in Figure 10, both of EDS and EWC curves exhibit decreasing trends with s-PVA content increasing. EDS decreases dramatically from 592.31 % for pure a-PVA hydrogel to 178.13 % for pure s-PVA hydrogel, while EWC from about 86 % to 64 %, reflecting the increase in degree of crosslinking and dense morphology structure with the increase of s-PVA content in a-PVA/s-PVA hydrogels. The hydrophilicity and mechanical properties are extremely important in selecting which hydrogel are suitable for

Figure 10. (a) EDS and (b) EWC of a-PVA/s-PVA blend hydrogels with different ratios, DP of s-PVA: 1100.

biomaterials, as described in material selection and design criteria of PVA hydrogels for use as synthetic cartilage replacements [48]. But different properties are desired in different application. In this study, the result of 70/30 (aPVA/s-PVA) blend hydrogels with EWC of 80.25 %, hardness of 16.1 HA, Tm of 232.68 oC, the 50/50 (a-PVA/s-PVA) blend hydrogels with EWC of 73.6 %, hardness of 22.8 HA, Tm of 236.15 oC and the 30/70 (a-PVA/s-PVA) sample with EWC of 68.81 %, hardness of 30.3 HA, Tm of 237.98 oC confirms that the blend hydrogels have improved swelling property comparing with pure s-PVA hydrogel, and enhanced thermal stability, mechanical strength contrasting with pure a-PVA hydrogel. All analyses above are taken into consideration, we think the 50/50 (a-PVA/s-PVA) hydrogel is better than others concerning the imbalance between the thermal, mechanical properties and swelling properties. The result indicates blending a-PVA and s-PVA is a useful method to form the hydrogel having good thermal stability and relative


high degree of swelling.

Conclusion In this study, the blend hydrogels of a-PVA/s-PVA with different ratios were prepared through the freezing-thawing methods. FTIR results indicate the number of hydrogen bonds raises with s-PVA increasing, and the variation in the number of hydrogen bonding results in difference in gel behavior, swelling behavior, morphology, crystallization and thermal properties of a-PVA/s-PVA hydrogels as studied above. The increment in Tgel, Tgm, G, hardness, crystallinity, crystallite size, Tm and ∆H are observed with increasing the s-PVA content. The result of 50/50 (a-PVA/s-PVA) blend hydrogels with EWC of 73.6 %, hardness of 22.8 HA, Tm of 236.15 oC confirms that the blend hydrogels have better swelling property than pure s-PVA hydrogel and higher thermal stability than pure a-PVA hydrogel which is atributed to the dense structure arising from an increase in hydrogen bonds. The result reveals blending a-PVA and sPVA is a useful method to form the hydrogel having good thermal stability and relative high degree of swelling.

Acknowledgements This work was funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.

References 1. Y. A. Han, E. M. Lee, and B. C. Ji, Fiber. Polym., 9, 393 (2008). 2. T. J. Smith, J. E. Kennedy, and C. L. Higginbotham, J. Mater. Sci., 45, 2884 (2010). 3. F. Xu, Y. Li, X. Wang, J. Wei, and A. Yang, J. Mater. Sci., 39, 5669 (2004). 4. J. Zhang, S. Cheng, and R. Zhuo, Colloid Polym Sci., 281, 580 (2003). 5. J. H. Park, M. R. Karim, I. K. Kim, I. W. Cheong, J. W. Kim, D. G. Bae, J. W. Cho, and J. H. Yeum, Colloid. Polym. Sci., 288, 115 (2010). 6. C. M. Hassan and N. A. Peppas, Adv. Polym. Sci., 153, 37 (2000). 7. D. Zhao, G. Liao, G. Gao, and F. Liu, Macromolecules, 39, 1160 (2006). 8. X. Yang, K. Yang, S. Wu, X. Chen, F. Yu, J. Li, M. Ma, and Z. Zhu, Radiat. Phys. Chem, 79, 606 (2010). 9. M. Kokabi, M. Sirousazar, and M. Z. Hassan, Eur. Polym. J., 43, 773 (2007). 10. R. Ma, D. Xiong, F. Miao, J. Zhang, and Y. Peng, Mater. Sci. Eng., C, 29, 1979 (2009). 11. J. Wang, W. Hou, and Y. Qian, Biotechnol. Tech., 9, 203 (1995). 12. C. T. Lee, P. H. Kung, and Y. D. Lee, Carbohyd. Polym.,


961

61, 348 (2005). 13. J. A. Stammen, S. Williams, D. N. Ku, and R. E. Guldberg, Biomaterials, 22, 799 (2001). 14. M. Qi, Y. Gu, N. Sakata, D. Kim, Y. Shirouzu, C. Yamamoto, A. Hiura, S. Sumi, and K. Inoue, Biomaterials, 25, 5885 (2004). 15. Z. Gu, J. Xiao, and X. Zhang, Bio-Med. Mater. Eng., 8, 75 (1998). 16. X. Tong, J. Zheng, Y. Lu, Z. Zhang, and H. Cheng, Mater. Lett., 61, 1704 (2007). 17. M. J. D. Nugent and C. L. Higginbotham, J. Mater. Sci., 41, 2393 (2006). 18. Y. Mori, H. Tokura, and M. Yoshikawa, J. Mater. Sci., 32, 491 (1997). 19. Y. Pan, D. Xiong, and X. Chen, J. Mater. Sci., 42, 5129 (2007). 20. N. A. Peppas, Makromol Chem, 176, 3433 (1975). 21. N. A. Peppas, Biomat Med Dev Art Org, 7, 421 (1979). 22. K. Ushio, M. Oka, S. Hyon, T. Hayami, S. Yura, K. Matsumura, J. Toguchida, and T. Nakamura, J. Biomed. Mater. Res., 68B, 59 (2004). 23. J. H. Choi, S. W. Ko, B. C. Kim, J. Blackwell, and W. S. Lyoo, Macromolecules, 34, 2964 (2001). 24. J. H. Choi, W. S. Lyoo, and S. W. Ko, Macromol. Chem. Phys., 200, 1421 (1999). 25. Y. Nagara, T. Nakano, Y. Okamoto, Y. Gotoh, and M. Nagura, Polymer, 42, 9679 (2001). 26. K. Yamaura and S. Matsuzawa, J. Appl. Polym. Sci., 29, 4009 (1984). 27. K. Yamaura and S. Matsuzawa, J. Appl. Polym. Sci., 30, 3225 (1985). 28. K. Yamaura and S. Matsuzawa, J. Appl. Polym. Sci., 31, 2139 (1986). 29. K. Yamaura, T. Tanigami, and S. Matsuzawa, J. Appl. Polym. Sci., 32, 4343 (1986). 30. K. Yamaura, T. Masuzawa, T. Tanigami, and S. Matsuzawa, J. Appl. Polym. Sci., 35, 593 (1988). 31. K. Yamaura, A. Hayakawa, T. Tanigami, and S. Matsuzawa, J. Appl. Polym. Sci., 35, 1621 (1988). 32. T. Tanigami, Y. Shirai, K. Yamaura, and S. Matsuzawa, Polymer, 35, 1970 (1994). 33. T. Tanigami, H. Hanatani, K. Yamaura, and S. Matsuzawa, Eur. Polym. J., 35, 1165 (1999). 34. S. Matsuzawa, K. Yamaura, M. Nagura, and T. Fukuta, J. Appl. Polym. Sci., 35, 1661 (1988). 35. W. S. Lyoo, J. H. Yeum, O. W. Kwon, D. S. Shin, S. S. Han, B. C. Kim, H. Y. Jeon, and S. K. Noh, J. Appl. Polym. Sci., 102, 3934 (2006). 36. L. Dai, K. Ukai, S. M. Shaheen, and K. Yamaura, Polym. Int., 51, 715 (2002). 37. L. Dai, S. Hijioka, T. Tanaka, and K. Yamaura, J. Polym. Eng., 28, 485 (2008). 38. Y. M. Chung, K. L. Simmons, A. Gutowska, and B. Jeong, Biomacromolecules, 3, 511 (2002).

962


39. M. Kokabi, M. Sirousazar, and Z. M. Hassan, Eur. Polym. J., 43, 773 (2007). 40. S. S. Kim, I. S. Seo, J. H. Yeum, B. C. Ji, J. H. Kim, J. W. Kwak, W. S. Yoon, S. K. Noh, and W. S. Lyoo, J. Appl. Polym. Sci., 92, 1426 (2004). 41. J. D. Cho, W. S. Lyoo, S. N. Chvalun, and J. Blackwell, Macromolecules, 32, 6236 (1999). 42. K. Yao, J. Cai, M. Liu, Y. Yu, H. Xiong, S. Tang, and S. Ding, Carbohydr. Polym., 86, 1784 (2011). 43. H. S. Mansur and A. A. P. Mansur, Mater. Res. Soc. Symp. Proc., 873, K1.9.1 (2005).

Mei Huang et al.

44. H. S. Mansur, R. L. Orefice, and A. A. P. Mansur, Polymer, 45, 7193 (2004). 45. H. S. Mansur, C. M. Sadahira, A. N. Souza, and A. A. P. Mansur, Mater. Sci. Eng., C, 28, 539 (2008). 46. R. Ricciardi, F. Auriemma, C. D. Rosa, and F. Laupretre, Macromolecules, 37, 1921 (2004). 47. N. A. Peppas and S. R. Stauffer, J. Controlled Release, 16, 305 (1991). 48. J. C. Bray and E. W. Merrill, J. Biomed. Mater. Res., 7, 431 (1973).