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, and DSC. All the copolymers were semicrystalline, one or two melting peaks being detected depending on the composition. Equilibrium melting temperature (Tm0) of PLLA blocks was determined for three copolymers with different EO/LA molar ratios. Tm0 decreased with decreasing PLLA block length. A copolymer with equivalent PLLA and PEG block lengths was selected for melt crystallization studies and the resulting data were analyzed with Avrami equation. The obtained Avrami exponent is equal to 2.6 ± 0.2 in the crystallization temperature range from 80 to 100°C. In addition, the spherulite growth rate of PLLA-PEG was analyzed by using Lauritzen-Hoffmann theory in comparison with PLLA homopolymers. The nucleation constant was found to be 2.39 × 105 K2 and the free energy of folding equal to 53.8 erg/cm2 in the range of 70–94°C, both higher than those of PLLA homopolymers, while the spherulite growth rate of the diblock copolymer was lower. POLYM. ENG. SCI., 2008. © 2007 Society of Plastics Engineers