THE STRUCTURE AND PROPERTIES OF CHITOSAN/PPLYETHYLENE GLYCOL/SILICA TERNARY HYBRID ORGANIC-NORGANIC

Chinese Joumal of Polymer Science Vol. 26, No.5, (2008), 621-630Chinese Journal ofPolymer Science⑥2008 World ScientificTHE STRUCTURE AND PROPERTIES OF CHITOSANPOLYETHYLENEGLYCOL/SILICA TERNARY HYBRID ORGANIC-INORGANIC FILMSRui Song* , Rui Xue', Ling-hao He", Ying Liu* and Qiao-ling Xiao'#Henan Provincial Key Laboratory of Surfuce & Interface Science, Zhengzhou University of Light Indusry,Zhengzhou 450002, ChinaCollege of Chemistry and Chemical Engineering, Graduate University of Chinese Academry of Sciences,Beiing 100049, ChinaAbstract The termary hybrid flms consisting of chiosan (CS), polyethylene glycol (PEG) and nano-sized silica which wassurface-modified by amino groups (RNSA) were prepared. The structures of the blend membranes were characterized byattenuation total reflection-infrared spectroscopy (ATR-IR), X-ray diffraction (XRD), optical microscopy (OM) anddifferential scanning calorimetry (DSC). The results showed that the addition of silica affected not only the distribution andcrytallinity of PEG on the sample surface. but also the phase coarseness and the crystalline structure of chitosan in the blendsystem. Moreover, PEG changed the crystalline structure of chitosan. Upon annealing (at 100C for 1 h), the blends wouldshow the altered crystalline structure of chitosan, the reinforced phase coarseness, as well as the decreased miscibility andinteraction between chitosan and PEG.Keywords: Chitosan; Poly(ethylene glycol); Silica; Annealing.INTRODUCTIONChitosan is a polycation derived from chitin, a natural polysaccharide usually obtained from carapaces of themarine crustaceans such as crabs and shrimps'". Chitosan has amino groups and hydroxyl groups on itsbackbone, which on one hand makes chitosan itself hydrophilic and on the other hand brings chitosan a .polycationic property. Several studies have indicated that chitosan has some important advantagesl-9I, such asbiodegradation, antibiosis, biocompatibility, non-toxicity, good film-forming characters and excellent chemical-resistant properties which allow its use in many areas such as in clinicsl2), drug delivery systems'3, solidpolyelectrolytes'*, surfactants'sl, membranes for ultra-filtration, reverse osmosis and evaporation' 0.Owing to its functional properties, chitosan has been widely investigated for the last two decades onindustrial and biomedical applications, which is expected to be useful in the development of composite materialssuch as blends or hybrids with other polymers and inorganic nanoparticles. Specially, blends with otherpolymers such as polycaprolactonelol, polyvinylpyrolidone) (PVP)", DNAI!12, nylonl'3l, polyethylene glycol(PEG)4-I7I, starch!'8, pullulan!'!, poly(vinyl alcohol) (PVA)I9 221 et al. have shown favorable properties, whichare recognized as materials supporting chitosan to obtain material with sufficient mechanical strength of preparedfilms and fibers. Some researches about the compatibility and interaction between the components have beenreported. Among all the alternatives, poly(ethylene glycol) (PEG) is of particular interest owing to its usefulproperties such as low toxicity, immunogenicity, biocompatibility and biodegradability. Alexeev et al. foundsignificant improvement in mechanical properties of chitosan blending with PEGIS, l6l. Kolhe concluded that'This work was prially subsidized by Henan Innovation Project for中国煤化工ens (HAIPURT")program.“Corresponding author: Rui Song (宋锐), E-mail: rsong@ gucas.ac.cn.MYCHCNMHGReceived June 20, 2007; Revised August 23, 2007; Acepted September 1，2w/622R. Song etal.blending was a more efficient way to improve ductility of chitosan. For blend consisting of PEG and chitosanwhich was supposed to be the parially miscible blend, the improvement of blend in compatibility and propertiesis dependent on 'well-dispersion', crystallinity of PEG as well as phase coarsening and intermolecularinteractions4. However, the influences on crystal properties of each other have not well been understood. Inaddition, hybriding with inorganic nanoparticles such as silial2-21 or carbon nanotube (CNT)281 was usuallyconsidered to be an effective method to improve the properties of materials by interaction with components.However, the influence of inorganic nanoparticles on blends had been investigated scarcely. Since the propertiesof films were settled by factors such as crystalline structures of ingredients, morphologies of films and molecularinteractions, the investigation about the influences of inorganic nanoparicles on these properties will be highlynecessary.The objective of this investigation was to explore the effects of silica on the chitosan/PEG blend flms,including dispersion and crystal properties of PEG as well as phase separation and coarsening. Besides themiscibility and interaction between chitosan and PEG, the influence of silica on the process of annealing wasalso investigated in order to understand the interaction involved.EXPERIMENTALMaterialsChitosan with a degree of deacetylation of 93% was received from Shanghai Reagent Company, China. PEG(Mw = 10000) was received from Tianjin Chemical Reagent Company, China. Silica with surface modified byamino groups and with diameter of ca. 20 nm (RNSA hereinafter), was supplied by Prof. Z.J. Zhang in HenanUniversity, China.Preparation of Hybrid MembranesChitosan was dissolved in 2 vol% aqueous acetic acid solution at a concentration of 2 wt%. Appropriate amountof PEG and RNSA were added to the solution, and the mixture was thoroughly stired until there was no airbubble in the solution. Films were obtained by casting solution on a glass dish and allowing the solvent toevaporate at 40°C. To check the annealing effect, all samples were treated at 100°C under vacuum for I h, andthen cooled down to room temperature in the vacuum.CharacterizationsAttenuation total reflection-infrared spectra (ATR-IR Bruker-TENSOR 27) were used to probe surface characterof films. For all the measurements, the spectra were obtained with 32 scans at a resolution of 2 cm ! interval. Anoptical microscope (OM, Nikon, E600POL) was used to acquire the morphology and phase coarsening of thesamples. TA Instruments Q100 was used to perform differential scanning calorimetry (DSC) analysis. About5 mg of samples were sealed in aluminum pans. After samples were cooled to 0°C and maintained at thetemperature for about 1 min, DSC curves of all samples were collected during heating from 0°C to 100°C at aspeed of l0 K/min. Prior to DSC runs, the temperature and heat transition of the instrument were calibrated withindium and zinc standards. A continuous nitrogen flow (20 mL /min) was maintained during the DSC scan. AnX-ray diffractometer (XRD, Bruker, D8) was used to inspect the diffractograms of the blend membranes at 25°C.The X-ray source was Ni-itered CuKa radiation (30 kV and 30 mA). The dry membranes were mounted onaluminum frames and scanned at a speed of 0.5 (°9/min from 5° to 40 (20).RESULTS AND DISCUSSIONSTable 1 shows the different compositions investigated, here all the compositions are expressed as weight ratios,and the corresponding appearances of all samples are shown in Fig. I. Pure chitosan flm was transparent, andblending with RNSA and PEG would decrease the transparence of films since RNSA as opaque inorganicparticles could weaken the transparence of films. PEG as an organic polymer also could imDact the transparenceof films by interaction with chitosan, ie, initial incorporation中国煤化工:nce a lte; upondouble the PEG loading, the films became non-transparefHC N M H G' the appearanceTemnary Hybrid Films of ChitosanPolyethylene GlycoUSilica623Table 1. Compositions of the prepared hybrid filmsCPROCS/PEGRNSACPR1CS/PEG/RNSAT CPR2CS/EG/RNSACPR0-11/0/0CPRI-11/0.2/0CPR2-11/0.4/0CPR0-21/0/0.05CPR1-21/0.2/0.05CPR2-21/0.4/0.05CPR0-31/0/0.1CPR1-31/0.2/0.1CPR2-31/0.4/0.1CPR0-41/0/0.2CPR1-41/0.2/0.2CPR2-410.4/0.2CPR0-3 CPR0-4CPR1- CPR1-2 CPR1-3 CPR1-4Fig. 1 The appearance of all samples50 um. 50μm .CPR1-1CPR1-3 .CPR1-4 .Soum50um_(PR2-2中国煤化工”Fig. 2 Optical microscope results 0.MYHCNMHG624R. Song et al.could be viewed as the competition between two processes. (1) Chitosan could interact with PEG throughhydrogen bonding. (2) Phase separation due to the limited compatibility between chitosan and PEG could beobserved for a wide range of compositions.Optical Microscope (OM) ObservationsThe optical microscope (OM) observations are shown in Fig. 2. For all samples, no trace of RNSA particles wasfound, which implied the homogeneous distribution and no aggregation of RNSA in the flms.Homogeneous phase morphology was generally observed in group CPRO and samples CPRI-1, CPR1-2;however, coarsening of morphology was seen in samples CPR1-3, CPR1-4 (Fig. 2). That means phase coarseningwas reinforced by adding adequate RNSA in the samples. According to the point of Kolhe, coarsening ofmorphology implied that the occurrence of phase separation'. So in this case RNSA particles would promotethis process.More complex structures were observed in group CPR2 (Fig. 2): compared with pure chitosan films(CPR0-1) which showed no obvious crystal morphology, blend membrane with PEG indicated a branch-likecrystal in morphology, and this morphology had been mentioned previously about chitosan crystal29]. Theappearance of branch-like crystal suggested that the addition of PEG could destroy the original crystallinestructure of chitosan. This point will be discussed further in the following XRD results. In addition, RNSAdisplayed no obvious impact on phase coarsening in group CPR2 that was correlated to the special structure offilms in the above discussion.50um.50 pum50um_50 umCPR0-1CPR0-2CPR0-3CPR0-450m_50um_CPRI-1CPR1-2CPR1-3CPR1-4d 50 um_. 50um50um_CPR2-1CPR2-2_CPR2- 3CPR2-4中国煤化工Fig. 3 Optical microscope resuts of the |.MYHCNMHGTemary Hybrid Films of ChitosanPolyethylene GlycoUSilica625To check the annealing effect, all the samples were annealed at 100°C for 1 h (Fig. 3). In contrast to CPR0,CPRI-1 and CPR1-2 - - no significant change in morphology was detected - - the phase coarsening of samplesCPR2, CPR1-3 and CPR1-4 was enhanced due to the annealing process. Therefore, phase separation was inducedby annealing process, and this observation was coincident with the report by Kolhell4.ATR-IR CharacterizationIt seemed that the addition of RNSA showed no effect on pure chitosan (Fig. 4), but had a profound effect onchitosan/PEG blends as probed with the ATR-IR technique which can give the near-surface infrared absorptionof samples (Figs. 5, 6). In Fig. 5, the characteristic absorption of PEG became stronger with increasing of RNSA.The absorption band of chitosan at 1251 cm ! was covered by double peaks of PEG at 1241 cm' and 1280 cmi +!.Moreover, the absorption peaks at 1466, 1371, 1360, 1113, 962 and 842 cm' " attributed to the absorption of PEGappeared and became more obvious with the adding of RNSA. The enhanced characteristic absorption of PEGsuggested a tendency that RNSA would promote PEG's accumulation on the surface of samples. However, thereseems no difference upon adding RNSA for group CPR2 (Fig. 6), as could be also corroborated from the aboveOM resuts (Fig. 2).PR0-1，:AA1800 1600 1400 12001000Wavenumber (cm~)Fig.4 ATR-Infrared spectra for surface of flms for CPRO samplesIn each pair, top: annealed; bottom: non-annealed1466 cm~'1131 cm:;rM1360 cm:!281 cm~'M962 cm~1241 cm\; 842 cmCSCPRl-I~CPR2-1(CPR1-2/CPR2-CPRI-3PR1-4PEG180016001400120018001600800Wavenumber (cm~')Wavenumber (cm )Fig. 5 ATR-Infrared spectra for surface of films for中国煤化工ce of fims .CPRI samples:YHCNMHG626R. Song et al.Upon annealing, RNSA still failed to affect the infrared absorption of pure chitosan fIm (Fig. 5), whileother samples displayed the increased absorption peaks assigned to PEG, suggesting an accumulation of PEGcomponent on sample surface (Figs. 7, 8). On one hand, this observation was related to the annealing temperature(100°C) that was well above the melting temperature of PEG. At this temperature, PEG melted and migrated, andthen it would recrystallize during the subsequent cooling step. Therefore, for samples that contained more PEG,the influence of annealing on fims would become more intense. On the other hand, the structure change ofchitosan, upon annealing, also played an important part in the accumulation of PEG on film surfaces. This aspectwill be discussed futher in the analysis of XRD measurements.|csCPR2-1CPRI-)CPR2-2CPRI-3-CPRI-3CPR2-3 .人CPRI-4.CPR2-4w MwN人人PEC1800160014001200100800100000Wavenumber (cm~)Wavenunber (cm)Fig. 7 ATR-Infrared spectra for surface of CPRIFig.8 ATR-Infrared spectra for surface of CPR2samples before and after annealingsamples after and before annealingIn each pair, top: annealed; bottom: non-annealedX-ray Diffraction AnalysisSo far, at least seven polymorphs have been proposed for chitosanl2. 30-321, including "tendon-chitosan",“annealed",“I-2", "L-2",“form I", “form II", and“noncrstalline". Ogawa et al. have proposed three forms:noncrysalline, hydrated crystalline and anhydrous erysallinels0l. The X-ray diffraction spectra of hybrid films areshown in Fig. 9. The peaks around 2θ= 11.70, 14.2°9, 17°, 18.6° and 23° should be assigned to chitosan: thepeak around 20 = 11.7° corresponding to the hydrated crystalline structure'sol; while the peaks around 20 = 14.2°and 23° corresponded to the anhydrous crystalline structure and the amorphous structure, respectively30l.Meanwhile, the reflections for PEG were at 2θ= 19.5°, 24°, 26.7° and 33.39. As shown in Fig. 9(a), RNSAexhibited no influence on the crystalline structures of chitosan and PEG. The diffraction of PEG tended to coverthe reflection of chitosan with increasing PEG, and the addition of PEG impacted the crystal of chitosan: purechitosan and chitosan hybriding with RNSA tend to crystallize in the form of hydrated crystaline (20= 11.7°)and amorphous structure (20 = 23°). Once blending with less amount of PEG, the main crystalline structures werein hydrated crystalline (20 = 11.79) and anhydrous structure (2θ = 14.2*). With more content of PEG, thhydrated crystalline (20 = 11.7) would prevail in chitosan structure. As indicated, the gradual disappearance ofamorphous一indicative diffraction (2θ = 23°) for samples CPRI and CPR2 suggested PEG would promote thecrystallization of chitosan.The variation in the crystalline state of chitosan could be ascribed to the double effects of PEG: (1) based onthe above -discussed role of PEG in the blend, it could weaken the interaction between chitosan and water andthus allow chitosan to crystallize in the form of the anhydrous structure (see the XRD result of CPR1); (2) on theother side, PEG is a water-soluble molecule, more PEG would中国煤化工me and thus makethe hydrated crystalline structure be formed as revealed in thention in the crystalMYHCNMHGstructure could correspond to the crystal morphology in OM oblTemary Hybrid Films of Chiosan/Polyethylene GilcolSilica627CPR2-3CPR2-1CPR1-3CPRI-3whn共CPRI-1CPR1-ICPR0-3CPR0-1CPRO-I104182263034101418222630320(°)20()Fig. 9 X-ray spectra for samples (a) annealed and (b) without annealingAfter annealing process, both the hydrated crytalline (20 = 11.7) and anhydrous crysalline (20 = 14.29)were weaken and the amorphous structure (20 = 23°) was reinforced for pure chitosan and chitosan afterblending with RNSA (Fig. 9b). As for the chitosan blending with PEG, the sharp peaks were replaced by thediffusing peaks around 23° (20), which represented the amorphous structure of chitosan. This observationsuggested that the structure of chitosan blending with PEG would readily be changed upon annealing. Besides, asincreasing the amount of RNSA and PEG, some obvious peaks of PEG at 33.30 (20) and even sharp peak ofchitosan at 15.5° (20) appeared, implying the improved crystalline properties. More importantly, the peak at 20 =15.5° on CPR2-3 had some shift comparing with the anhydrous crystalline structure of chitosan at 14.2° (20)before annealing. According to the Bragg equation, the distance between the plans in each direction becamesmaller due to the increased 20 value, so it was noted that the structure of chitosan after annealing becametighter, and this aspect was consistent with the report of other literatures!9O 321. The process of structure changeof chitosan during dehydration is schematically shown in Fig. 10. In crystal structure of hydrated chitosan, watermolecules entered into the formation of hydrogen bond. After dehydration, molecular chains of chitosan beganto shift and shrink the structure of chitosan due to abruption of hydrogen bondl33I. Then the ATR-IR resultsbecame understandable: the shrinkage of chitosan structure would make the meling PEG migrate and thenDehydration(a) Crystal structure of hydraled chitosan(b) Crystal structure of anhydraed chitosanFig. 10 Plausible solid-state transformation from the hydr中国煤化工n(b)of tendon chitosan (Shaded polymer chains denote up-poinHydrogen bonds before and after transformations are also ::TYHCNMHG628R. Song et al.re-crystallize on the film surface. In addition, the enhanced phase coarsening upon annealing as indicated inthe OM photographs (Fig. 3) could be explained by the altered structure of chitosan as well as migrated andre-crystallized PEG.One more point should be emphasized was that the crystallinity of PEG had been increased by increasing thecontent of RNSA in samples CPRI since the peak of PEG at 33.3° (20) became more obvious. The change ofcrystallinity for PEG in other samples was not easy to be observed by XRD measurements due to severeoverlapping of the diffraction peaks in the diffraction patterm.Thermal Behavior AnalysisThe depression of melting point (Tm) of a crystalline polymer blended with other polymers provides informationabout their miscibility and interactionl54. Both chitosan and PEG are crystalline polymers; pure PEG melts atca. 60*C, while chitosan undergoes thermal degradation at ca. 270°C prior to melting. Thus, the meling point ofPEG was monitored to understand the miscibility and the interaction between the two polymers. This view wassupported by the Nishi-W ang equation' 5 that was used to obtain the interaction parameter (x12):11-R V。工Tm AHzaVw212(1-42尸where subscripts 2 and I refer to PEG and chitosan, respectively, T° and Tm2 are the equilibrium meltingtemperature (K) of PEG in blend and that of pure PEG, respectively; R is the universal gas constant, OHzu is theheat of fusion permole of 100% crystalline PEG, V2u and Vu are molar volumes of the repeating units of thepolymers and 4 represents volume fractions of PEG in the blend. According to this equation, the interactionparameter (X12) which represents miscibility and interaction is dependent on the depression of melting point (Tm)of a crystalline polymer after blending with other fixed content polymer. The more negative X12 derived from themore decreased melting point, suggests better misciblility and stronger interaction between both of polymers.The result from DSC showed a decrease in the melting temperature (Tm) of PEG for all samples compared withthe pure PEG, suggesting that chitosan and PEG are miscible (Table 2).Table 2. Melting properties of PEG in all samples'CPRI-L CPR1-2 CPR1-3 CPR1-4_ CPR2-1_ CPR2-2 CPR2-3 CPR2-4Tm(°C)47.41 .52.1952.2452.7952.1752.9251.8453.3253.4555.4855.2755.5455.42AH (J/g)24.3523.9738.2040.2043.3141.1821.7523.2544.2443.8444.7245.68'In each group of data, up: non- annealed; down: annealedFor CPR1 samples, the meling peak of PEG displayed a transition from weak to strong with addition ofRNSA, suggesting the increasing of melting enthalpy (which correlates with crystallinity) (Fig. l1a). However,after CPRI-3, the data of endotherm melting enthalpies (OH) noted in Table 2, will not increase again. Thisobservation would be ascribed to the fact that RNSA could act as the nucleating agent in the crystallization ofPEG: a ltte content of RNSA would not induce PEG to crystallize until its loading beyond some value: once thecrytallization is initiated, the crystallinity will be almost completed due to the strong crystalline tendency ofPEG. Therefore, the crytallinity will reach a plateau after initial increasing. This view could be also used tounderstand the invariant 0H pre- or post-adding RNSAor annealing process (see Table 2, sample CPR1-3,CPRI-4 and CPR2).中国煤化工MYHCNMHGTemary Hybrid Films ofChitosan/Polyethylene GlyoUSilica629-0.2pCPR1-ICPRI-I-0.5-0.4CPR1-2CPR1-2/CPR1-3-0.6CPR1-4-0.8--1.5--1.2-2.01.4L104505560654050 556Temperature ('C)Temperature (C)Fig. 11 The melting curves of PEG in the CPR1 samples with (a) non annealed and (b) annealedThe invariant Tm indicated RNSA had no effect on miscibility and interaction of blends. However, afterannealing, the increased TmS compared with non-annealed samples were found in all samples, suggesting adecrease in miscibility and interaction between chitosan and PEG led by annealing (Fig. 11b, Table 2)CONCLUSIONSThe interaction of components in the chitosan/PEG/silica temary hybrid films had been investigated. There is astrong molecular interaction between chitosan and PEG, so that the original crystal structures of chitosan andPEG have been changed. The addition of RNSA would serve as a nucleating agent for PEG and compel PEG toaccumulate on the surface of films. Annealing would decrease the miscibility and interaction between chitosanand PEG, enhance the phase coarsening, as well as change the crystalline structure of chitosan blended withRNSA and PEG.ACKNOWLEDGMENT We are grateful t0 Prof. Z.J. Zhang of the Henan University for supplying the silica nanoparticlesused in this research.REFERENCES1Shahidi, F., Arachchi, J.K.V. and Yeon, YJ, Trends Food Sci. Technol, 1999, 10: 37Badawy, M.E.1. Rabea, EL and Rogge, T.M.. Biomacromolecules, 2000, 5: 5893 Sashiwa, H., Yajima, H. and Aiba, S.,. Biomacromolecules, 2003, 4: 12444 Wan, Y., Creber, KA.M. and Peppley, B.. Polymer, 2003, 44: 10575 Ngimhuang, J. Furukawa, J. and Satoh, T.. Polymer, 2004, 45: 8376 Chanachai, A.. Jiraratananon. R.. and Uttapap, D.. J. Membr. Sci, 2000, 16: 2717 Rhim, J.W.,. Hong, S.I. and Park, H.M.. J. Agric. Food Chem, 2006, 54: 58148 Wan, Y.. Wu, H. and Yu, A.X. Biomacromolecules, 2006, 7: 13629 Wang, X.Y Du, Y.M. and Yang, J.H.. Polymer, 2006, 47: 673810 Sarasam, A.R. and Krishnaswamy, R.K., Biomacromolecules, 2006, 7: 113111 Devi, A.D., Smitha, B. and Sridhar, s.. J. Membr. Sci. 2006, 280: 4512 Strand, S.P.. Danielsen, s. and Christensen, B.E., Biomacromolecules, 2005, 6: 335713 Lin, S.J, Hsiao, W.C. and Jee, S.H.. Biomaterials, 2006, 27: 5079中国煤化工14 Kolhe, P. and Kannan, R.M., Biomacromolecules, 2003. 4: 17315 Alexeev, VL, Kelberg, E.A. and Bronnikov, s.v, Polym. Sci,, SetjYHCNMHG630R. Song et al.16 Alexeev, VL.. Kelberg, E.A. and Evmenenko, G.A.. Polym. Eng. Sci, 2000, 40: 121117 Zhang, M.,. Li, X.H. and Gong, Y.D, Biomaterials, 2002. 23: 264118 Lazaridou, A. and Biliaderias, C. Carbohydr. Polym, 2002. 48: 17619 Kubo, s. and Kadla, J.F. Biomacromolecules, 2003, 4: 56120 Jin, L. and Bai, R.B., Langmuir, 2002, 18: 97651 Koyanoa, T. Koshizakib, N. and Umeharab, H., Polymer, 2000, 41: 446122 Zhen, H.P., Nie, J. and Sun, J.F.. Acta Polymerica Sinica(in Chinese), 2007, (3): 23023 Oki, A., Qiu, X.D., Alawode, O. and Foley, B.. Mater. Lett, 2006, 60: 275124 Fuentes, s.. Retuer, PJ. and Uilla, A.. Biomacromolecules, 2000, 1: 23925 Uragami, T,. Katayama, T. and Miyata, T. Biomacromolecules, 2004, 5: 156726 Rutakormpituk. M.. Carbohydr. Polym., 2006, 63: 22927 Martnez, Y, Retuert, J. and Mehrdad, Y.P.. Polymer, 2004, 45: 325728 Wang, S.F., Shen, L. and Zhang. W .D.. Biomacromolecules, 2005, 6: 306729 Mo, X.M. Wang. P. and Zhou, G.E.,. Chem. J. Chinese Universitiesin Chinese), 1998, 19: 98930 Ogawa, K.. Hirano, s. and Miyanishi, T, Macromolecules, 1984, 17: 97331 Saito, H.. Ryoko, T. and Ogawa, K., Macromolecules, 1987, 20: 242432 Clark, G.L. and Smith, A.F., J. Phys. Chem., 1936, 40: 86333 Okuyama, K., Noguchi, K. and Miyazawa, T.. Macromolecules, 1997, 30: 584934 Sarasam, A. and Madially. s.v., Biomaterials, 2005, 26: 550035 Nishi, T. and Wang, TT, Macromolecules, 1975, 8: 909中国煤化工MYHCNMHG