Influence of the Filler Nature on the Crystalline Structure of Polylactide-Based Nanocomposites: New Insights into the Nucleating Effect

This work deals with the influence of the nature of a filler on the thermally induced crystalline structure of polylactide (PLA)-based nanocomposites. Particularly two types of silicated clay both having a platelet-like shape, i.e., talc and kaolin, were melt compounded with PLA, and the crystalline structure involved during isothermal crystallization was followed in situ by means of both wide- and small-angle X-ray scattering techniques. Results indicate that even if the two types of clay are chemically similar, i.e., they both exhibit silicate surfaces, only talc plays the role of nucleating agent on the PLA crystallization. Moreover, structural analyses reveal that for PLA nanocomposites filled with talc a transcrystallization process occurs and that, in addition to the perpendicular growth of the PLA crystals toward the clay surface, a well-ordered nanometer scaled structure can be formed depending on the crystallization temperature. Particularly, for the first time, an epitaxial growth process has been evidenced, and crystallographic orientation relationships between the PLA crystals and the talc platelets have been determined. In opposition, the same behavior was not encountered for the kaolin-based nanocomposites. From a fundamental point of view, this study shows that the existence of favorable interactions between the polymer and the filler is not the only parameter involving the nucleating effect process. Indeed, the existence of crystallographic relationships between the filler and the polymer crystalline structure is also a necessary condition to observe a nucleating effect. Finally, this work also highlights that incorporating a well-chosen filler into a polymer matrix can be an efficient route to induce nanostructured materials.