Effect of molecular weight on the cold-crystallization of biodegradable poly(ethylene succinate)

Abstract

A series of poly(ethylene succinate) (PESu) samples with varying molecular weight were synthesized by the melt polycondensation method. The effect of the molecular weight on the cold-crystallization and the subsequent melting behaviour of melt-quenched PESu was studied by means of standard DSC and Step Scan DSC. Recrystallization and multiple melting followed the cold-crystallization of the samples. In general a melting–recrystallization–remelting scheme was supposed. The phenomenon is more pronounced for the low molecular weight samples. Higher degree of crystallinity is achieved during heating of low molecular weight samples. Cold-crystallization kinetics was also studied using microscopic and macroscopic models. The modified Avrami and the Ozawa methods seem to describe well the experimental data in contrast to the Tobin model. The spherulite growth rates during isothermal crystallization from the melt were studied using polarizing light microscopy (PLM). They were found to decrease with increasing molecular weight. A regime transition was observed at about 70 °C and the K g I I I / K g I I ratio was close to 1.8. Furthermore, the nucleation constant K g of the Lauritzen–Hoffmann equation was estimated using either isothermal PLM measurements or non-isothermal DSC cold-crystallization data and the assumption that the growth rate is inversely proportional to the half crystallization time. No clear trend of K g with the polymer molecular weight was observed. Finally, the effective activation energy at different relative degrees of crystallinity was estimated using the differential isoconversional method of Friedman. Based on these values, both Lauritzen–Hoffman parameters ( U * and K g) were evaluated using the overall rates of non-isothermal cold-crystallization according to the method of Vyazovkin and Sbirrazzuoli. A clear increase of U * with the molecular weight was observed meaning that the effect of diffusion is more pronounced in polymers having higher average molecular weight