Thermal properties of the multi-stereo block poly(lactic acid)s with various block lengths

Multi-sb-PLAs with molecular weights of approximately 60,000 (MSB-60) and 130,000 (MSB-130) with various block lengths were synthesized. These have a structure in which di-block copolymers consisting of L- and D-blocks with the same length are connected with 1,12-dodecanediol (DMG), an initiator and hexamethylene diisocyanate (HMDI), a chain extender alternately. DSC analyses revealed that Tg increases with the increase in the block length and the multi-sb-PLA with a lower molecular weight has higher Tg at a constant block length. These are attributable to the higher concentration of DMG and HMDI in the multi-sb-PLA. Tm increases with the increase in the block length and the molecular weight of multi-sb-PLA and those of MSB-60 and MSB-130 get close to a constant value at higher block length. The variation of Tm seems to be attributable to change in the lateral dimension of lamella rather than that of the thickness. Tc in the heating process from the amorphous solid decreases and that in the cooling process from the melt increases with the increase in the block length, indicating that the multi-sb-PLA with a long block has a wider temperature range of the crystallization.