Composites of a PLA with SBA-15 mesoporous silica: Polymorphism and properties after isothermal cold crystallization

Abstract

Composites based on an L-rich poly(lactic acid), PLA, and different contents in mesoporous SBA-15 silica have been prepared by extrusion in order to evaluate the influence of incorporation of mesoporous particles on the structural PLA features and on their ultimate mechanical performance. For that, the samples have been isothermally cold crystallized for relatively long times at different temperatures, ranging from 65 to 120 °C. The results from DSC, X-ray diffraction and Raman spectroscopy show that the pure α′ modification is obtained at the lower annealing temperatures, while the α form is in majority above 85 °C. The temperature range at which α′ and α polymorphs were developed is somewhat atypical, being associated with the particular D stereoisomer content of the PLA used. Furthermore, several phase transitions are observed depending on the crystallinity reached and the polymorphs developed during the isothermal crystallization from the glass: an additional cold crystallization, the α′/α transformation and the subsequent melting processes. These transitions have been evaluated thoroughly by DSC measurements and real-time variable temperature experiments with synchrotron radiation. It was also found that rigidity of these materials is influenced by both the presence of SBA-15 particles and by the amount and perfection of crystallites. Thus, the relative reinforcement effect of SBA-15 particles is importantly reduced by differences in long spacing PLA values for the materials crystallized between 85 and 120 °C, mainly in the composites with the lowest SBA-15 content. Those are the temperatures where the α polymorph is the major existing crystalline structure.