The effect of nano-sized filler particles on the crystalline-amorphous interphase and thermal properties in polyester nanocomposites

Poly(epsilon-caprolactone) (PCL) nanocomposites containing carbon nanotubes, layered silicates, sepiolite or polyhedral oligomeric silsesquioxane (POSS) were characterized by advanced thermal analysis. Adding carbon nanotubes or layered silicates increases the excess heat capacity in quasi-isothermal crystallization experiments by MTDSC, pointing to an increased fraction of reversibly melting and crystallizing material. In situ AFM during isothermal crystallization and SAXS during subsequent heating reveal an altered PCL semi-crystalline morphology. The nucleating ability of the carbon nanotubes induces a striking transcrystalline morphology, promoting the growth of secondary PCL crystals with an increased fold surface energy and a reduced melting temperature. The altered crystalline-amorphous interphase at the crystal fold surface houses the material that reversibly crystallizes and melts under quasi-isothermal crystallization conditions. A fraction of this interphase is not able to crystallize but converts into glassy, rigid amorphous material upon cooling prior to reaching the PCL main glass transition temperature. Low aspect ratio sepiolite and POSS nanofillers barely nucleate the PCL crystallization and therefore hardly affect the PCL semi-crystalline morphology and thermal behavior.