ORIGINAL PAPERCrystal and Thermal Properties of PLLA/PDLA Blends Synthesized by Direct Melt PolycondensationDakai Chen •Jing Li •Jie RenPublished online:29May 2011ÓSpringer Science+Business Media,LLC 2011Abstract We herein report the effects of the component ratio and method of blending on the synthesis of stereo-complex poly(lactic acid)(SC-PLA)based on poly(L -lactic acid)(PLLA)and poly(D -lactic acid)(PDLA)prepolymers.PLLA and PDLA were prepared by direct melt polycon-densation of lactic acid (DMP).Combined with the dual catalyst system,PLA prepolymers with M w more than 20,000were prepared by DMP.PLLA was mixed by powder blending or melt blended with PDLA.It is revealed that melt-point and spherulite growth rate of SC-PLA is strongly dependent on the perfection of SC structure.The melt point of PLA can be increased by nearly 50°C because of the particular strong intermolecular interaction between PLLA and PDLA chains.Solid-state polycon-densation (SSP)is an efficient method to increase the molecular weight of SC-PLA,but it can have a negative effect on the regularity of linear chains of SC-PLA.Thermogravimetry analyzer (TGA)results show that SC structure cannot cause the delay reaction on the thermal degradation of PLA.Keywords SC-PLA ÁSpherulite growth rate ÁThermal degradationIntroductionThe heat distortion temperature (HDT)of common PLA products prepared by injection molding is only about 58°C,and this low heat-resistance of PLA has consider-ably limited the application area of PLA products.For example,as packaging products,it is not suitable for heat resistant containers.Such as hot lunch box,soup bowl,cup and other food utensils,it cannot be used as microwave containers.Even when used for non-heat-resistant pipe,building material,board,stationery,box and other mate-rials,it may deform when stored and transported in the summertime.To sum up,common PLA is not suitably be used as engineering plastics for household,electrical and electronic components and automotive products,etc.Thus,the modification of the heat resistance of PLA is very important to broaden the application area of PLA products.Although PLA is a crystalline polymer,there is almost no crystallization during molding.The slow crystallization rate causes the low heat-resistance of PLA.There are some factors that affect the crystallization rate of PLA,including the structure of PLA,molding condition and content of nucleating agent [1,2].Structural factors include molecular symmetry,molecular chain flexibility,molecular weight and the structure of branched-chain.Molding conditions include the cooling rate,the stretch ratio [3–7].For now,the general methods to improve the heat resistance of PLA are blending technology [8–10],cross-linking technology [11–13]and nanocomposite technology [14–16].When the interaction of polymer chains with different tacticity prevails over that of the polymers with same tacticity,it can cause a stereoselective interaction in polymer chains.The most typical stereocomplex structure is formed between stereoisomers of poly(Methyl Methac-rylate)(PMMA)[17].There are many optically activeD.Chen ÁJ.Li ÁJ.Ren (&)Institute of Nano-and Bio-Polymeric Materials,School of Material Science and Engineering,Tongji University,200092Shanghai,Chinae-mail:renjie6598@J.RenKey Laboratory of Advanced Civil Engineering Materials,Ministry of Education,Tongji University,200092Shanghai,ChinaJ Polym Environ (2011)19:574–581DOI 10.1007/s10924-011-0301-9polymers can form the stereocomplex structure,such as stereoisomers of poly(alpha-methyl-alpha-ethyl-beta-pro-piolactone),poly(c-benzyl glutamate),poly(c-methyl glu-tamate),poly(tert-butylene oxide),poly(tert-butylthiirane), poly(1-methylbenzyl methacrylate),poly(menthyl methac-rylate)and poly(a-methyl-a-ethyl-b-propiolactone)[18]. Ikada et al.first reported the formation of stereocomplex of PLLA and PDLA in1987[19].For the special space structure of PLLA and PDLA,the levorotatory molecular chain and dextrorotary molecular chain will form the supplementary structure in the arrangement process and form the much tighter space structure.This tighter stack structure can cause the intermolecular van der Waals force play greater role in some properties such as thermal property,crystal property and mechanical property.Ouchi et e the branched PLA to synthesize the stereocom-plex poly(lactic acid)and the result show the branched stereocomplex poly(lactic acid)has the higher melting point,tensile strength and young modulus[20].All of them have been added in text according to the reviewer’s com-ment.Japanese scientists have done a lot of work in the research of the SC structure.Tsuji et al.wrote a series of papers about the preparation process and formation mechanism of SC structure[21,22].Fukushima et al.also did a series of researches in SC structure,and found some new preparation methods of SC structure[23,24].In this study,SC-PLA are prepared by powder-blending or melt-blending based on PLLA and PDLA synthesized by DMP and the mixing ratio of prepolymers,regularity of linear PLA chains and thermal properties of SC-PLA are discussed.ExperimentalMaterialsL-lactic acid(88%L-lactic acid in water,99%optically pure)was supplied by PURAC,Holland.D-lactic acid(90% L-lactic acid in water,98%optically pure)was supplied by Suzhou Chi-Yun Co.,Ltd.,China.Stannous dichloride dihydrate(SnCl2Á2H2O),p-toluenesulfonic acid monohy-drate(TSA)and chloroform(99%)were obtained from Shanghai Guoyao Co.,Ltd.,China.1,1,1,3,3,3-Hexafluo-roisopropanol(HFIP)was received from Shanghai Da-Rui Chemical Co.,Ltd.,China.All chemicals were used without further purification.Synthesis of PrepolymersLactic acid was purified by heating at90and100°C for dehydration for1h under mechanical stirring,respectively. Then the water present in the system was removed with the inner pressure reduced slowly to30Torr over2h.After the inner pressure had reached30Torr,the heating and dehydration were continued for2h at120°C and contin-ued for further4h at140°C to obtain a transparent viscous melt of oligo(L-lactic acid)(OLLA)or oligo(D-lactic acid) (ODLA).Subsequently,the purified products were mixed with the binary catalysts SnCl2Á2H2O(0.4wt%)and TSA, and the number of moles of TSA was equal to that of SnCl2Á2H2O.Each mixture was then heated from140to 165°C at the rate of10°C/h,then stirred under a reduced pressure of10Torr for8h to carry out DMP.Theflask was purged with nitrogen.The rotation speed was approximately100rpm.The PLLA and PDLA obtained by this DMP had a medium molecular weight.The binary catalysts could effectively catalyze the DMP by preventing the discoloration of the polymer.Synthesis of Stereoblock Poly(lactic acid)(sb-PLA)and SC-PLAPLLA and PDLA prepared above were mixed in a prede-termined weight ratio,charged in a two-neckedflask,and heated to180°C under the protection of nitrogen.When the prepolymers melted completely,PLLA and PDLA were fully mixed by mechanical agitation with the blending time of1h.After cooling,the product of blending wasfinally pulverized with an electric mill,and dried in vacuum at room temperature for2h.The pulverized melt-blend was subsequently charged into a test tube and subjected to SSP with the predetermined conditions.The heating tempera-ture was raised by10°C increments from140to160°C every10h at0.5Torr.The produced polymer was ana-lyzed without purification.CharacterizationThe average molecular weight and molecular weight dis-tribution were determined using gel permeation chroma-tography(GPC;Waters150C USA).Polymer was dissolved in CDCl3containing5vol%HFIP at a concen-tration of1*2g/L.The molecular weight was calculated from the elution volume of PMMA standards with narrow molecular weight distribution.Differential scanning calo-rimetry(DSC)was performed with a TA Q100thermal analyzer under a nitrogenflow of20mL/min at a heating rate of20°C/min.Wide angle X-ray diffraction(WAXS) powder patterns were recorded on a Rigaku D/max using nickel-filtered Cu K a radiation with a wavelength of 0.1542nm in a2h range of5*40°at a scan rate of2°/min operated at40kV and130mA.1H-NMR and13C-NMR spectra were recorded using a Bruker500MHz high-resolution spectrophotometer(In CDCl3containing5% HFIP and0.03%tetramethylsilane).A polarized opticalmicroscope(LEICA DMLP)equipped with a Linkam THMS600hot stage was used to investigate the spherulitic morphology and growth of PLLA and PDLAfilms and PLLA/PDLAfilm.The samples werefirst heated to200 and250°C for PLLA,PDLAfilms and PLLA/PDLAfilm, respectively,at100°C/min,and held at these tempera-tures for3min to destroy thermal history,then cooled at 100°C/min to an arbitrary crystallization temperature(T c) in the range of100*200°C,and then held at the T c. Annealing lasts for given time periods.TGA was per-formed on a Q100thermogravimetric analyzer(Tainstsh, USA)at a heating rate of20°C/min,and examined under flowing air(80mL/min)over a temperature range from ambient to700°C.Results and DiscussionMelting Point,Molecular Weight and Crystalline Structure of SC-PLAFor PLLA prepolymer(M1)and PDLA prepolymer(M2), Fig.1shows that there are clear melting endothermic peaks in the vicinity of160°C in the heating process of scanning calorimetry.There are different degrees of increase in the melt-point of the blending of M1and M2is different after they were blended with different methods.The PLLA powder and PDLA powder is blended under the melting point of PLLA and PDLA prepolymers,which is called powder-blending.The PLLA powder and PDLA powder is blended above the melting point of PLLA and PDLA prepolymers,which is called melt-blending.The melting endothermic peak of powder-blending of M1/M2=50:50 (P1)is in the vicinity of175°C,which is only15°C higher than that of M1and M2.This phenomenon indicates that there are only a small amount of P1forms the SC structure.Apparently,simple powder blending cannot make the PLLA and PDLA chains mixed fully.The molecular chains of M1and M2should be coupled together to form a more perfect SC structure.In molten state of melt-blended of M1/M2=50:50(P2),molecular chains of M1and M2get the maximum freedom of movement, therefore they can be mixed fully together to form a more perfect SC pared to the products of powder blending,melt blending is a more effective method to the preparation of SC-PLA and the heat resistance of PLA can be improved effectively.For the melt-blending products of M1/M2=90:10(B1),there are three melting endothermic peaks in the heating thermal scan curve.The peaks in the vicinity of160and170°C may be attributed to the for-mation of crystals of PLLA and PDLA,respectively. Because of the uneven distribution of molecular weight of samples,the molecular chains infirst melting area can promote the movement of molecular chains in other areas and enhanced the formation of another crystalline region resulting in the emergence of the second melting peak.The melting peak of SC-PLA is still in the vicinity of210°C. But the melting enthalpy of SC-PLA is much lower than that of PLA homopolymer,indicating only a small amount of PLA forms the SC structure.For the melt-blending products of M1/M2=75:25(B2),there are two different crystalline areas observed.The two melting endothermic peak in the vicinity of160and210°C are the melting peak of PLA homopolymer and SC-PLA,respectively.The melting enthalpy at210°C is much greater than that at 160°C,showing that the SC structure proportion of B2is larger and contains more perfect stereocomplex crystals. The sample B3is melt-blending products of PLLA and PDLA prepolymers with the ratio of50/50.When the content of PDLA approaches to50%,for the melt-blending products of M1/M2=50:50(B3),there is only one clear melting endothermic peak in the vicinity of210°C,indi-cating that the structure of B3is mainly SC structure.The sample P2is products of B3after the reaction of solid-state polycondensation.For the SSP products of B3,the melting point is aslo increased by nearly50°C.DSC results show that when the proportion of PDLA(XD)=0.5,a more perfect SC structure can be achieved.When the XD decreases continuously,the homopolymer crystal zone will appear,and the proportion of SC structure begins to reduce. When the XD approaches0,only a small amount of SC structure is formed and homopolymer crystallization dominates.The detailed results of DSC and GPC are pre-sented in Table1.Figure2shows that the characteristic diffraction peaks of M1and M2are in the2h=15°,16°,18.5°and22.5°, and all of them are attributed to the crystals of PLLA and PDLA homopolymer.In SC structure,the van derWaals Fig.1DSC curves of M1,M2,B1,B2,B3,P1and P2force causes a special energy interactions that leads to theformation of the accumulation of b crystals,which result in the high melting point SC-PLA.The crystals of SC-PLA is b crystalline form,and the characteristic peaks are in the 2h =12°,21°and 24°.The DSC results show that the structure of B3and P2is mainly SC structure,which is entirely consistent with the XRD results.The diffraction peaks of P1is still the characteristic peak of a crystalline,indicating powder blending is not a favorable method to the forming of SC structure.Regularity of Linear PLA ChainsFor analyzing the sequential structure of the SSP products of blending products,the 1H-NMR of M1,M2,B1,B2,B3,P1and P2were measured.The NMR spectra of the CH 3and CH groups of these PLA molecules are shown in Fig.3a and b.Somewhat broader peaks appear on theresonance lines of B3in Fig.3a and B3in Fig.3b with the increase of the proportion of PDLA,indicating that movement of PLA chains are confined in local area.As shown in Fig.4,these two broad peaks may be attributed to chains connecting vertical-linked regions for the perfect stereocomplex PLLA/PDLA mixtures.This double-strand vertical-linked structure causes the melting peaks at the vicinity of 210°C in the heating thermal scan curve.However,the not obvious broad peaks of curve B1and curve B2in Fig.3a and b may be attributed to chains connecting cross-linked regions that are probably com-posed of stereocomplex microcrystallite.These stereo-complex microcrystallite cause the melting peaks in the vicinity of 160and 170°C in the heating thermal scan curve [25,26].As seen from curve B1and curve B2in Fig.3a and b,it appears that the chains in the stereocom-plex microcrystallite show no obvious NMR spectrum because of their entirely frozen mobility.The growth rate of stereocomplex microcrystallite functioning as cross-links is extremely low.Sharp lines identical with those of the homopolymers M1and M2,are observed even when the proportion of PDLA is up to 25%,suggesting the existence of freely movable PLA chains around the ste-reocomplex area.As shown on curves B1and B2in Fig.3a and b,the spectra of the homopolymer solution remain unchanged.Curve P1and P2in Fig.3a and b are the 1H NMR spectra of expanded regions of the methyl and methine proton signals.The spectra of powder-blending of SSP products P1(M w =50kDa)has sharp lines identical with those of the homopolymers M1and M2without stereo defects [27,28].This method cannot make the PLLA chains and PDLA chains fitted together.To a certain extent,the PDLA plays the role of nucleating agent to perfect the stereocomplex microcrystallite just as shown in curve P1in Fig.1.The peak of P2in Fig.3a and b are not as broad as that of B3.In SSP,the chain elongation withoutTable 1Typical results of the DMP,powder-blending,melt-blending,SSP and the summary of the thermodynamic data of the blends;the heat of fusion (D H m )of hc crystals becomes smaller in the blends with higher proportional PDLA Code PLLA %PDLA %Polymer recovery M w a M w /M n a T m b °C D H m b J/g M110008320,000 1.4156c 50c M201009121,000 1.3158c50c B190109527,000 1.5160,170c /210d 43c /15d B275259131,000 1.5160c /212d 5c /59d B350509139,966 1.5210c 60c P150509850,647 1.9172c 79d P2505089135,1862.2208d70da Determined by GPC relative toPMMA standardsb Measured by DCS (heating rate;20°C/min)c For hc crystals dFor sccrystalsFig.2WAXS profiles for M1,M2,B1,B2,B3,P1and P2crystalization can affect the perfect stereocomplex structure indicating that the SSP products P2comprise both block chains of PLLA and PDLA just as presented in Fig.4.The 13C-NMR spectra of the methine and carbonyl region of M1,M2,B1,B2,B3,P1and P2are shown in Fig.3c and d.The signal assignment of the theoretical stereosequence distributions has been done in earlier reports [29–31].In the methine region the resonance lines due to signal A (iii,iis,sii and sis tetrads)are found at69.1ppm and that due to signal B (isi tetrads)are found at 69.2ppm.The resonance lines of signal C (iss/ssi tetrads)which could also be formed by the transesterification are observed at 69.4ppm when the transesterification occurs during polymerization.In the spectra of the methine region new lines are observed as a combination of tetrads con-taining an ss segment.As shown in Fig.3c,signals B and C are present in curves B3and P2,as compared to those observed in curve M1,M2,B1,B2and P1.Signals D at 169.54ppm is assigned to the isotactic (i,mm)sequence of the carbonyl carbon atom of successive LLA units.Signal E in the region between 169.31and 169.39ppm are ten-tatively assigned to the heterotactic (h,rm)sequence.The signal F at 169.20ppm is assigned to the syndiotactic (s,rr)sequence that is not observed for PLLA and PDLA homopolymers,while it is clearly observed for racemic PLA.Therefore,the appearance of the large peak D and the small peaks E and F and the absence of consecutive r units (e.g.,rr,rrr,…)are indicative of the blocky nature of the polycondensation products.As shown in Fig.3d,signal E and F are presented in curve B3and P2as compared to those observed in curve M1,M2,B1,B2and P1.However,in the methine region of the NMR spectra in Fig.3c an increase in intensities of signal B and C is seen as com-pared to those observed in polylactide acid.Similarly,inFig.3NMR spectra of M1,M2,B1,B2,B3,P1and P2:a Expanded methyl proton signals from the 1H-NMR spectra,b Expanded methine proton signals from the 1H-NMR spectra,c Expanded methine resonances from the 13C-NMR spectra andd Expanded carbonyl signals from the 13C-NMRspectraFig.4Synthesis of sb-PLA by melt/solid-state polycondensationthe carbonyl carbon region of the NMR spectra in Fig.3d an increase in intensity of signal E and F is also observed.There are small changes in the signal B,C,E and F between the melt-blends and the final SSP products.These small changes in the enantiomeric phenomenon suggest that little unit racemization has taken place during the melt-blending and SSP.The intramolecular ester exchange reaction and its reverse reaction will affect the PLA molecular sequence in the SSP process.The racemization reactions are most likely due to a dynamic equilibrium of ester interchange reactions occurring between the polymer chains.There are two ways in the ester interchange reactions.One is acyl-oxygen cleavage,which does not involve the chiral carbon.The other is alkyl-oxygen cleavage,in which the covalent bond between oxygen and the chiral carbon breaks and subse-quently reforms;this results in an inversion of the config-uration.The NMR results indicate that as the XD increases,the probability of alkyl-oxygen cleavage increases,and which results in the formation of the inverted configuration.And the melt-blending promotes greater inversion than SSP.In the process of melt-blending,the D -lactic acid units seems to be incorporated into the backbone in a purely random manner,whereas in the SSP,the LLA and DLA are embedded into the polymer chain as small blocks and separated from each other as shown in Fig.4.Fig.5The polarized optical micrographs of the isothermal crystallization of PLLA (a –c )at a crystallization temperature of 120°C,PDLA (d –f )at a crystallization temperature of 120°C,sb-PLA (g –i )at a crystallization temperature of 120°C,sb-PLA (j –l )at a crystallization temperature of 160°C and sb-PLA (m –o )at a crystallization temperature of 180°CSpherulite Morphology and GrowthThe crystalline morphology and the spherulitic growth process of M1,M2and P2were investigated by POM.The isothermal crystallization temperature of M1and M2is 120°C and the isothermal crystallization temperatures of P2are 120,160and 180°C.The T c of 180°C is higher than the T m values of PLLA and PDLA (158and 162°C,respectively).As shown in Fig.5,the size of the spherulite of P2is much smaller than that of M1and M2,indicating that the high density and rapid growth of crystals disturb the normal growth of the radius of spherulitie.The average spherulitic radius is then plotted against the isothermal crystallization time as shown in Fig.6.The spherulitic radius linearly increases with the isothermal crystallization time,and the spherulitic growth rate (G)can be evaluated from the slope of theses lines.These results indicate that both the G and the spherulitic morphology are affected by the SC structure of sb-PLA.Thermal Properties of sb-PLA and SC-PLAPLA is one material that is very sensitive to high temper-ature.There is obvious thermal degradation when the temperature is over 200°C.The thermal degradation of PLLA is a very complex process,involving intermolecular and intramolecular transesterification,racemization and the reaction between free radicals and non-radicals derived from the fracture of the molecular chains.Thermal degra-dation is generally dependent on the residual metal oxides,molecular weight of polymer,final structure of polymer chains and thermal degradation conditions.The thermal degradation process of PLLA,PDLA and sb-PLA were analyzed by TGA at the heating rate of 20°C/min withresults shown in Fig.7.From Fig.7,it can be seen that the thermal degradation of PLLA,PDLA and sb-PLA is started in a relatively narrow temperature range (200*280°C)with weight loss.The difference of quality loss temperature of PLLA,PDLA and sb-PLA is not obvious and their quality loss temperature range is basically same.This is mainly because the content of metal Sn and lactide residues of polymers are basically the same,resulting in the similar thermal degradation mechanism.The thermal degradation of sb-PLA cannot be improved by SC structure.The ther-mal degradation behavior of sb-PLA is the process that the rupture of chemical bond of main chain and forming molecular fractionlet and it is mainly caused by the inter-molecular and intramolecular transesterification and the depolymerizing reaction affected by metal Sn.ConclusionIn this article,a stereocomplexed PLA is created by simply melt mixing of PLLA with PDLA.The generation of SC structure of the sb-PLA and SC-PLA improves the melting point,spherulite growth rate and shorter induction period as compared to PLLA and PDLA homopolymers.The SC structure has little effect on the thermal degradation of sb-PLA.The investigation indicates that the chain elon-gation reaction produced by SSP can reduce the regular unit sequence of SC-PLA.Acknowledgments This work is supported by the National High Technology Research and Development Program of China (No.2006AA02Z248),the Program of Shanghai Subject Chief Scientist (No.07XD14029)and the fund of Shanghai International co-opera-tion of Science and Technology (No.075207046).Fig.6Dependence of the spherulite radius on the isothermal crystallization time for PLA homopolymer chains of M1and M2at 120°C and sb-PLA chains of P2at 120,160and 180°CFig.7TG curves and expanded signals from the 150to 250°C interval TG curves of M1,M2,P1and P2References1.Lim LT,Auras R,Rubino M(2008)Prog Polym Sci33(8):820–8522.Sobkowicz MJ,Dorgan JR,Gneshin KW,Herring AM,McKinnon JT(2008)J Polym Environ16(2):131–1403.Slager J,Domb AJ(2003)Adv Drug Deliver Rev55(4):549–5834.Narladkar A,Balnois E,Grohens Y(2006)Macromol Symp241:34–445.Tsuji H(2005)Macromol Biosci5(7):569–5976.Na B,Tian NN,Lv RH,Zou SF,Xu WF,Fu Q(2010)Macro-molecules43(2):1156–11587.Harris AM,Lee EC(2008)J Appl Polym Sci107(4):2246–22558.Suryanegara L,Nakagaito AN,Yano H(2009)Compos SciTechnol69(7–8):1187–11929.Tsuji H,Sawada M,Bouapao L(2009)Acs Appl Mater Inter1(8):1719–173010.Pan P,Liang Z,Zhu B,Dong T,Inoue Y(2009)Macromolecules42(9):3374–338011.Muroya T,Yamamoto K,Aoyagi T(2009)Polym Degrad Stabil94(3):285–29012.Quynh TM,Mitomo H,Nagasawa N,Wada Y,Yoshii F,TamadaM(2007)Eur Polym J43(5):1779–178513.Karikari AS,Edwards WF,Mecham JB,Long TE(2005)Bio-macromolecules6(5):2866–287414.Zhou WY,Duan B,Wang M,Cheung WL(2009)J Appl PolymSci113(6):4100–411515.Zhao YY,Qiu ZB,Yang WT(2009)Compos Sci Technol69(5):627–63216.Hu X,An HN,Li ZM,Geng Y,Li LB,Yang CL(2009)Mac-romolecules42(8):3215–321817.Deuring H,Vanekenstein GORA,Challa G,Mason JP,Hogenesch TE(1995)Macromolecules28(6):1952–195818.Fukushima K,Kimura Y(2006)Polym Int55(6):626–64219.Ikada Y,Jamshidi K,Tsuji H,Hyon SH(1987)Macromolecules20(4):904–90620.Ouchi T,Ichimura S,Ohya Y(2006)Polymer47(1):429–43421.Tsuji H,Horii F,Hyon SH,Ikada Y(1991)Macromolecules24(10):2719–272422.Tsuji H,Tezuka Y(2004)Biomacromolecules5(4):1181–118623.Fukushima K,Furuhashi Y,Sogo K,Miura S,Kimura Y(2005)Macromol Biosci5(1):21–2924.Fukushima K,Hirata M,Kimura Y(2007)Macromolecules40(9):3049–305525.Fukushima K,Kimura Y(2008)Polym Sci Pol Chem46(11):3714–372226.Thakur KAM,Kean RT,Hall ES,Kolstad JJ,Munson EJ(1998)Macromolecules31(5):1487–149427.Thakur KAM,Kean RT,Hall ES,Kolstad JJ,Lindgren TA,Doscotch MA,Siepmann JI,Munson EJ(1997)Macromolecules 30(8):2422–242828.Shyamroy S,Garnaik B,Sivaram S(2005)J Polym Sci Pol Chem43(10):2164–217729.Tsuji H,Horii F,Nakagawa M,Ikada Y,Odani H,Kitamaru R(1992)Macromolecules25(16):4114–411830.Zell MT,Padden BE,Paterick AJ,Thakur KAM,Kean RT,Hillmyer MA,Munson EJ(2002)Macromolecules35(20): 7700–770731.Kasperczyk JE(1995)Macromolecules28(11):3937–3939。
