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 T g increases with the increase in the block length and the multi-sb-PLA with a lower molecular weight has higher T g at a constant block length. These are attributable to the higher concentration of DMG and HMDI in the multi-sb-PLA. T m 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 T m seems to be attributable to change in the lateral dimension of lamella rather than that of the thickness. T c 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.