The difference of equilibrium melting point between poly(l-lactic acid) and poly(l-lactic acid)/poly(d-lactic acid) blends: cases with three molecular weights: Difference of equilibrium melting point between PLLA and PLLA/PDLA blends

In order to explore the origin of the higher melting point of poly(lactic acid) (PLA) stereocomplex crystal (SC) than that of homo-crystal (HC), the equilibrium melting point (T-m(0)) differential between SC and HC was determined using the Hoffman-Weeks method. The results showed that, for PLA samples with M-n around 16, 20 and 65 kg mol(-1), the T-m(0) differential between SC and HC is around 36, 42 and 55 degrees C, respectively. Thus, the higher melting point of SC compared to HC does not stem from Tm0 differential only. For PLA samples with lower M-n, the supercooling differential between poly(l-lactic acid) (PLLA)/poly(d-lactic acid) (PDLA) blends and PLLA is smaller than that with higher M-n, which means chain diffusion behavior is crucial for SC formation in PLLA/PDLA blends. The fact that the SC adopts the intermolecular parallel arrangement rather than the adjacent chain folding is verified by the greater slope of the melting point of SC versus crystallization temperature fitting curve when M-n is relative higher. (c) 2018 Society of Chemical Industry