Critical Size and Formation Mechanism of Secondary Nuclei in Melt-Crystallized Polylactide Stereocomplex Crystals

Abstract

Revealing the molecular mechanism and determining the length scale of the rate-limiting ordering process have been a challenge in the field of polymer crystallization. Previously, we proposed a theory to determine the size of critical nuclei in single-component crystals from the variation of nucleation kinetics with different dilution ratios. Here, we further extend this theory to polylactide (PLA) stereocomplex crystals (SCs) consisting of poly(l-lactide) and poly(d-lactide). By diluting only l-lactide units or both l-lactide and d-lactide units via random copolymerization, the number of l-lactide units and that of two types of units in a critical secondary nucleus were obtained. The results show that a critical secondary nucleus of an SC bulk crystal consists of 22–27 l-lactide units and the same number of d-lactide units when crystallized from melt at temperatures ranging from 140 to 170 °C. These repeating units form two poly(l-lactide) stems and two poly(d-lactide) stems. The four stems in a secondary nucleus of SC are chosen from approximately three to four chains. According to the number of chains, at most one-third of PLA chains has once adjacent re-entry folding in the secondary nuclei of SC crystallized from melt. These results provide rich information on the ordering process during the melt crystallization of SC crystals and are beneficial for understanding the crystallization mechanism.