Preferential formation of the stereocomplex crystals of poly(L‐lactide) and poly(D‐lactide) blend by epoxidized soybean oil under nonisothermal crystallization

Abstract

Formation of stereocomplex crystals (SCs) in enantiomeric poly(L‐lactide) (PLLA) and poly(D‐lactide) (PDLA) blends is an effective way to improve the heat resistance and mechanical performance of polylactide (PLA) products. However, obtaining PLAs with high stereocomplex content is not straightforward as the homocrystal and SCs occur simultaneously and compete with one another. In this work, various amounts of epoxidized soybean oil (SO) or SO at 3%–20% by weight were added to equal amounts of the stereoisomers (PLLA/PDLA; 50/50). From differential scanning calorimetry, it was found that epoxidized SO increased the crystallization temperature, facilitated the SC crystallization, and suppressed the homecrystal formation, whereas SO did not affect the crystallization behavior for the racemic blends. Moreover, epoxidized SO reduced the glass transition temperature, cold crystallization temperature, and melting temperature of the racemic blends. We examined how epoxidized SO affected the crystallization of PLLA/PDLA at cooling rates from 2 to 20°C/min. We discovered that epoxidized SO still encouraged SC crystallization. The activation energy, calculated from Takhor and Kissinger equations, indicated that adding 5 wt% epoxidized SO effectively promoted the crystallization and facilitated the SC formation of the PLLA/PDLA blend.