Stereocomplex crystallization of high-molecular-weight poly(l-lactic acid)/poly(d-lactic acid) racemic blends promoted by a selective nucleator

- A novel stereocomplex-type nucleator of PLLA/PDLA blend is found.
- The nucleator can selectively promote stereocomplex crystallization of HMW PLLA/PDLA blend.
- Nucleation mechanism of nucleator-modified PLLA/PDLA blend is proposed.

Stereocomplex (sc) crystallization of poly(l-lactic acid)/poly(d-lactic acid) (PLLA/PDLA) is highly effective to improve the heat resistance of poly(lactic acid) (PLA). However, sc crystallization is much less predominant than homocrystallization (hc) in the high-molecular-weight (HMW) PLLA/PDLA racemic blend. Herein, we report a simple and straightforward method to promote the sc crystallization and suppress the hc crystallization for HMW PLLA/PDLA blends by using a novel sc-type nucleator, i.e., zinc phenylphosphonate (PPZn). Effects of PPZn nucleator on nonisothermal, isothermal crystallization kinetics, crystal modification, polymorphic crystalline transition, spherulite morphology, and dynamic mechanical properties of PLLA/PDLA blends were investigated. PPZn nucleator exerts dual effects on the crystallization of PLLA/PDLA blends. It not only accelerates the crystallization rate but also selectively promotes the formation of sc crystallites for HMW PLLA/PDLA blends under all the crystallization conditions investigated including nonisothermal melt, cold crystallizations, and isothermal crystallizations in a wide temperature range (80-160 °C). In the isothermal crystallization at 140 °C, the crystallization half-time of PLLA/PDLA blend decreases from 15.2 to 0.28 min with the incorporation of 1 wt% PPZn, accompanied by the increase of nucleation density and decrease of spherulitic size. Due to the enhanced sc crystallization, storage modulus of HMW PLLA/PDLA blend at high temperatures (170-200 °C) increases significantly after the addition of PPZn. The nucleator-promoted sc crystallization of HMW PLLA/PDLA blend is attributed to the improved PLLA/PDLA intermolecular interactions and lattice matching between nucleator and sc crystallites.