Isothermal crystallization and spherulite morphology of poly(ethylene terephthalate)/Na⁺-MMT nanocomposites prepared through solid-state mechanochemical method

Differential scanning calorimetry (DSC) and polarized optical microscopy were used to comparatively investigate the multiple melting behavior and isothermal crystallization kinetic of pure PET and PET/Na⁺-MMT nanocomposites prepared through solid-state mechanochemical method and conventional method. The results showed that both pure PET and Na⁺-MMT-filled PET system isothermally treated at different crystallization temperature exhibited three melting endotherms, including peak I, II and III in the DSC heating process. The increases in both crystallization temperature and Na⁺-MMT loading and the mechanochemical pan-milling prove to be beneficial to the formation of crystals related to endotherm II in the primary crystallization process. The mechanochemical pan-milling effect and the increase in the Na⁺-MMT loading also decrease the size of the formed spherulite. The investigation of isothermal crystallization kinetics indicated that the calculated Avrami exponent n of the milled sample (PETSN_x) shows the higher value than that of both pure PET and the conventional unmilled sample (PETCN_x) and also increases with increasing the Na⁺-MMT content. In addition, the calculated crystallization activation energy of PETSN_x is found to be lower than that of pure PET as well as PETCN_x.