The impact of template effect at different temperatures on homocrystallization-to-stereocomplexation transformation in Poly(L-lactide)/Poly(d-lactide) racemic blends

Abstract

Stereocomplexation, formed between enantiomeric poly(L-lactide) (PLLA) and poly(d-lactide) (PDLA), is 50 °C higher than the melting point of PLLA (or PDLA) homocrystallites (HCs). In a certain temperature range, stereocomplexation can induce homocrystallization-to-stereocomplexation (HC-to-SC) transformation through template effect. In this work, a series of temperature programs designed through differential scanning calorimetry (DSC) were used to study the impact of template effect at different temperatures (TH-S) on HC-to-SC transformation in PLLA/PDLA racemic blends. It was found that HCs can be efficiently converted to stereocomplex crystallites (SCs) through template effect when TH-S was 210 °C. When TH-S was 220 °C, SCs with large lamellar crystal thickness were obtained, which had a higher melting point, about 10 °C higher than the conventional SCs. At a TH-S of 230 °C, the melt-crystallization peaks of HCs and SCs were clearly observed during the DSC cooling process because PLLA/PDLA helical pairs were still retained in the PLLA/PDLA melt. The results demonstrated that the residual PLLA/PDLA helical pairs can accelerate remarkably the crystallization rate of SCs and generated new SCs due to template effect, and the new SCs significantly accelerated the crystallization rate of HCs resulted from the heterogeneous nucleation effect. Non-isothermal crystallization behavior of PLLA/PDLA racemic blends at different cooling rates was also studied. The results indicate that the lower cooling rate promotes SCs formation more strongly than the promotion effect on HCs.