Temperature-dependent polymorphic crystallization of poly(L-lactide)s on the basis of optical purity and microstructure

A series of poly(L-lactide) stereocopolymers with various optical purities (100%-88%) were synthesized by ring-opening copolymerization of the enantiomeric feedstock mixtures of L-lactide and D-lactide at 140 degrees C for 16 h under reduced pressure. The obtained relationship between microstructure and optical composition of poly(Llactide) (PLLA) copolymers confirmed that the stannous octoate-catalyzed lactide polymerization proceeded by nonstereoselectivity without racemization and transesterification reactions. As a result, the small amounts of D-lactide units inserted randomly into the predominant Llactide units chains following Bernoullian pairaddition chain-end control mechanism, which regulated the stereoregularity of PLLA and tailored its physical properties. Furthermore, the effect of optical purity (OP) on temperature-dependent polymorphic crystallization behavior of PLLA was investigated. The critical temperatures (T-alpha/alpha') for the formation of alpha and alpha' crystal modifications were obtained from WAXD and DSC, respectively. It was found that the T-alpha/alpha' declined by about 10 degrees C from 120 degrees C to 90 degrees C for every 2 -3% decrease in OP within the range of 100-92%, and for the PLLA samples with OP <= 90%, a form crystal was mainly generated at the crystallization temperature as low as 80 degrees C. Finally, the crystal growth kinetics was analyzed by using Lauritzen-Hoffman theory to obtain the transition temperature (TII-III) from regime II to regime III for each PLLA. The correlation between the critical temperature (T-alpha/alpha') and the transition temperature (TII-III) for PLLAs with different optical purities was also discussed. (C) 2017 Elsevier Ltd. All rights reserved.