Effect of molar mass on the α′/α-transition in poly (l-lactic acid)

Abstract

The effect of molar mass on the reorganization behavior of α′-crystals of poly (l-lactic acid) (PLLA) has been investigated by fast scanning chip calorimetry. The α′-crystals of PLLA were formed by isothermal crystallization of the relaxed melt at 90 °C or lower temperatures, and were then heated at different rates between 2 and 200 K s−1. Slow heating permits for all PLLA samples of different molar mass between 2 and 576 kDa reorganization of α′- into α-crystals. It was found that the α′/α-transition is suppressed if heating occurs faster than 10 K s−1 in case of the sample with a molar mass of 576 kDa, while the critical heating rate to suppress the formation of α-crystals is increased to 30 and 100 K/s in PLLA with molar masses of around 100 kDa and 2 kDa, respectively. The faster kinetics of the α′/α-transition in case of shorter macromolecules may be explained by faster melting of smaller α′-crystals, faster growth of α-crystals from the non-isotropic melt containing remnants/self-seed from molten α′-crystals, and/or a higher number of such α′-crystal remnants/self-seed.