Effects of poly(hexylene succinate) amount on the crystallization and molecular mobility of poly(lactic acid) copolymers

Poly(lactic acid) (PLA), poly(hexylene succinate) (PHSu) and new block PLA-b-PHSu copolymers with mass ratios of 90/10 and 95/5 were synthesized. PHSu was chosen, as a proper agent to tune the crystallizability and to introduce biodegradability on PLA. Emphasis was given on crystallization behaviour, which is assessed by DSC, XRD and PLM. Also, to reveal any direct effects of the copolymer structure and composition on molecular mobility, TMDSC and BDS measurements were performed. FTIR was used as an accompanying technique. It was found that the slow crystallization rate of PLA becomes faster due to the presence of PHSu in the copolymers, while by XRD and DSC was proved that the degree of crystallinity decreases. Regarding molecular mobility of PLA at the presence of PHSu, the calorimetric (DSC) as well as the dynamic (alpha relaxation, BDS) glass transition, exhibit strong acceleration, weak enhancement in strength and suppressed cooperativity. These effects correlate well with the progressive decrease of molar mass in copolymers by increasing PHSu content. Local dynamics of PLA is not affected by PHSu, as expected.