Investigations On The Morphology And Melt Crystallization Of Poly(L-lactide)-Poly(Ethylene Glycol) Diblock Copolymers

Ring opening polymerization of L-lactide was realized in the presence of monomethoxy poly(ethylene glycol),using zinc lactate as catalyst.The resulting PLLA-PEG diblock copolymers were characterized by using ~1H-NMR,SEC,WAXD DSC,POM and TEM.All the copolymers were semicrystalline,one or two melting peaks being detected depending on the composition.The phenomenon of double melting of PLLA block has been discussed.PLLA-PEG diblock copolymers present composition-dependent thermal properties due to the mutual influence on crystallization of both blocks.The presence of PEG disfavors the crystallization of PLLA,and vice versa.The equilibrium melting temperature(T_m~0) of PLLA blocks decreases with decreasing PLLA block length and is much lower than that of PLLA homopolymer.The Avrami exponent n,nucleation constant K_g and free energy of foldingσ_e were obtained for a copolymer with almost equivalent PLLA and PEG block lengths by using Avrami equation and Lauritzen-Hoffmann theory.Both K_g andσ_e are higher than those of the PLLA homopolymer,which suggests that nucleation is more difficult in the case of the copolymers.The lower spherulite growth rate(G) of the diblock copolymer compared with PLLA homopolymers indicate that the chain folding ability decreases due to the attachment of PEG block.In addition,we adopted the extending technique largely used in biology to fabricate films with a bilayer structure to further investigate the crystallization mechanism of diblock copolymers without entanglemant in the artificial double layer systems.