Role of carbon nanotube grafted poly(l-lactide)-block-poly(d-lactide) in the crystallization of poly(l-lactic acid)/poly(d-lactic acid) blends: Suppressed homocrystallization and enhanced stereocomplex crystallization

Abstract

Self-nucleation is considered to be the ideal case for the nucleation of polymer crystallization; however, the nucleation effect of stereocomplex (sc) crystallites for sc crystallization of poly(l-lactic acid)/poly(d-lactic acid) (PLLA/PDLA) blend is hard to be evaluated, due to that sc crystallites form through intermolecular packing of PLLA and PDLA molecular chains and sc crystallite is a good nucleating agent for the homocrystallization (hc) crystallization of PLLA or PDLA. In the current work, we proposed a facile strategy to tailor the sc crystallization behavior of PLLA/PDLA blend through in-situ preformed sc crystallites. Herein, Multi-walled carbon nanotube (CNT) grafted PLLA-block-PDLA copolymer (CNT-L-D) was synthesized and incorporated into an equimolar high-molecular-weight PLLA and PDLA blend to study its effect on the sc crystallites formation. The results manifested that, sc crystallization is highly enhanced by the addition of CNT-L-D, and sc crystallites can be formed exclusively in the blend with 10wt% CNT-L-D during isothermal and nonisothermal melt crystallization. With the addition of CNT-L-D into the PLLA/PDLA blend, the sc crystallites formed preferentially between grafted PLLA-b-PDLA induced by CNT work as self-nucleation sites for sc crystallization. The nucleation densities are increased, and the competition between hc and sc crystallization which is the key factor influencing the exclusive formation of sc crystallites is tailored simultaneously. With significantly improved nucleation density and reduced sc spherulites size, sc spherulites cannot be entered by any other growing hc crystalline lamellae and the surface of sc spherulites cannot be covered with hc crystallites, leading to exclusive formation of sc crystallites.