Isothermal and non-isothermal crystallization behavior of poly(l-lactic acid): Effects of stereocomplex as nucleating agent

The effects of incorporated poly(d-lactic acid) (PDLA) as poly(lactic acid) (PLA) stereocomplex crystallites on the isothermal and non-isothermal crystallization behavior of poly(l-lactic acid) (PLLA) from the melt were investigated for a wide PDLA contents from 0.1 to 10 wt%. In isothermal crystallization from the melt, the radius growth rate of PLLA spherulites (crystallization temperature (Tc)≥125 °C), the induction period for PLLA spherulite formation (ti) (Tc≥125 °C), the growth mechanism of PLLA crystallites (90 °C≤Tc≤150 °C), and the mechanical properties of the PLLA films were not affected by the incorporation of PDLA or the presence of stereocomplex crystallites as a nucleating agent. In contrast, the presence of stereocomplex crystallites significantly increased the number of PLLA spherulites per unit area or volume. In isothermal crystallization from the melt, at PDLA content of 10 wt%, the starting, half, and ending times for overall PLLA crystallization (tc(S), tc(1/2), and tc(E), respectively) were much shorter than those at PDLA content of 0 wt%, due to the increased number of PLLA spherulites. Reversely, at PDLA content of 0.1 wt%, the tc(S), tc(1/2), and tc(E) were longer than or similar to those at PDLA content of 0 wt%, probably due to the long ti and the decreased number of spherulites. This seems to have been caused by free PDLA chains, which did not form stereocomplex crystallites. On the other hand, at PDLA contents of 0.3–3 wt%, the tc(S), tc(1/2), and tc(E) were shorter than or similar to those at PDLA content of 0 wt% for the Tc range below 95 °C and above 125 °C, whereas this inclination was reversed for the Tc range of 100–120 °C. In the non-isothermal crystallization of as-cast or amorphous-made PLLA films during cooling from the melt, the addition of PDLA above 1 wt% was effective to accelerate overall PLLA crystallization. The X-ray diffractometry could trace the formation of stereocomplex crystallites in the melt-quenched PLLA films at PDLA contents above 1 wt%. This study revealed that the addition of small amounts of PDLA is effective to accelerate overall PLLA crystallization when the PDLA content and crystallization conditions are scrupulously selected.