Effect of molar mass on the alpha'/alpha-transition in poly (L-lactic acid)

The effect of molar mass on the reorganization behavior of alpha'-crystals of poly (L-lactic acid) (PLLA) has been investigated by fast scanning chip calorimetry. The alpha'-crystals of PLLA were formed by isothermal crystallization of the relaxed melt at 90 degrees 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 alpha' - into alpha-crystals. It was found that the alpha'/alpha-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 alpha-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 alpha'/alpha-transition in case of shorter macromolecules may be explained by faster melting of smaller alpha'-crystals, faster growth of alpha-crystals from the non-isotropic melt containing remnants/self-seed from molten alpha'-crystals, and/or a higher number of such alpha'-crystal remnants/self-seed. (C) 2017 Elsevier Ltd. All rights reserved.