Polylactide compositions. The influence of ageing on the structure, calorimetric and viscoelastic properties of PLA/calcium sulfate composites

High-performance composites prepared by melt-blending polylactide (PLA, l/d isomer ratio of 96/4) with various amounts of ß-anhydrite II (AII), the dehydrated form of calcium sulfate hemihydrate obtained by a specific thermal treatment at 500 °C, have been aged to study the evolution of their physical and mechanical properties with time. The effect of 1-year ageing under ambient conditions (below Tg of PLA) for selected composites, i.e., filled with 20 and 40 wt% AII, was determined and compared to unfilled PLA with the same processing and ageing history. Samples with an initial amorphous PLA matrix, obtained by fast quenching from the melt, were characterized before and during ageing. The changes in physical parameters have been studied using dynamic mechanical thermal analysis (DMTA), differential scanning calorimetry (DSC), X-ray diffraction (XRD) and density measurements. Surprisingly, for all the samples, an increase of the storage modulus (E') was recorded, as a result of ageing. This improvement was ascribed to the reorganization of the PLA structure induced by ageing. The structural reorganization was also reflected by a slight increase of PLA density and changes in thermal behaviour. The X-ray investigations showed unchanged crystallographic structure of AII both during blending with molten PLA and in the composite systems after ageing. The surprising stability of the thermo-mechanical properties of PLA and PLA/AII composites is in agreement with the results of size exclusion chromatography analysis (SEC) which did not show significant changes of PLA molecular weights brought out by ageing.