Physical changes and sorption/desorption behaviour of amorphous and semi-crystalline PLLA exposed to water, methanol and ethanol

•Penetrant-induced cold-crystallisation of PLLA, kinetics.•Penetrant-induced restructuration of glassy structure of PLLA.•Sorption and desorption kinetics of water, methanol and ethanol in PLLA.•Remarkably slow desorption of alcohols due to crystal caging of penetrant molecules.

The purpose of this study was to study the effects of water, methanol and ethanol on the structure of fully amorphous and semi-crystalline poly(l-lactic acid) (PLLA), which is important for applications in which the material is in contact with body fluids (water), and also in order to tailor properties by adjusting the crystallinity and glass transition temperature using penetrants. Amorphous and semi-crystalline PLLA tubes were exposed to the afore-mentioned liquids at 37 °C and the mass crystallinity, fictive temperature and the rigid amorphous fraction were assessed by DSC and WAXS. The diffusivity and solubility of the penetrant were assessed by gravimetric sorption and desorption experiments. Water has a plasticizing effect on the glassy structure, which enhances the equilibration of the glass as revealed by a lowering of the fictive temperature of the subsequently dried samples, but the plasticization was not sufficient to induce cold-crystallisation. The moderate effect of water at 37 °C was further demonstrated by the almost constant water diffusivity. Both methanol and ethanol induced cold-crystallisation at 37 °C, primarily forming crystals of the α′-form, which in the case of methanol led to a marked increase in the fictive temperature and the formation of a rigid amorphous fraction in the subsequent dried samples. The crystal growth was restricted according to an Avrami analysis of the crystallisation kinetics data. The complexity of the sorption and desorption kinetics of methanol and ethanol was caused by the progressing cold-crystallisation on sorption, which resulted in several phenomena. Very notable was the extremely high concentration-dependence of the penetrant diffusivity as revealed on desorption, which could not be explained by plasticization alone. Instead it is believed that the formed crystals caged a significant fraction of the penetrant molecules.

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