Effect of the block length and the molecular weight on the isothermal crystallization behavior of multi-stereoblock poly(lactic-acid)s

Multi-stereoblock PLAs (multi-sb-PLA)s having the controlled block sequences between PLLA and PDLA were synthesized by the chain-extension of tri-stereoblock PLAs. These multi-sb-PLAs only crystallize into the stereocomplex without forming any traces of homo-chiral crystal. The effects of the block length and the total molecular weight on the isothermal crystallization behavior and the higher order structure of the multi-sb-PLAs were investigated. Observation of isothermal crystallization behavior revealed that the multi-sb-PLA with longer blocks and higher molecular weights started the crystallization at a lower temperature within a shorter period of time in a wider temperature range. The crystallization rate increases with increasing block length and the multi-sb-PLA with a longer blocks achieves higher degree of crystal. The multi-sb-PLA with a higher molecular weight forms more complete structure of the spherulite than that with a low molecular weight. Annealing of the amorphous multi-sb-PLA produced sc with two different lamellar thicknesses. One which melts at a higher temperature is a main sc phase with a lamellar thickness merely determined by the block length and the molecular weight regardless of the thermal history. The chains in the amorphous region only crystallized upon annealing and gradually grew with increasing annealing temperature giving some addition to the lamellar thickness of the samples detected by SAXS.