Isothermal Crystallization, Melting Behavior and Mechanical Properties of Polyethylene Terephthalate (PET) and Reorganized PET (RPET)

Analysis of the isothermal crystallization kinetics of polyethylene terephthalate (PET) and reorganized PET was carried out using differential scanning calorimetry (DSC). Reorganization of PET process results in material with widely different organizations, structures, and morphologies of their polymer chains and different material properties. In thermal study of PET and RPET, the Avrami equation explained the primary stage of the isothermal crystallization of both PET and RPET. In the crystallization process of PET and RPET, the secondary crystallization occurred simultaneously with the primary crystallization and the two processes obeyed different Avrami time dependences. Lauritzen–Hoffman’s (L-H) secondary nucleation theory and the Lauritzen Z-test explained the microscopic description of the crystallization kinetics. The L-H analysis revealed that both PET and RPET crystallization followed regime I to II transitions at temperatures between 194 and 240°C. The folding surface free energies of RPET, determined from the Lauritzen–Hoffman equation, were σe = 61.2 erg/cm2 and 64.9 erg/cm2; and the works of chain folding were q = 7.38 Kcal/mol and 7.83 Kcal/mol, for regime I and regime II, respectively, whereas for PET they were σe = 50.2 erg/cm2 and 70.4 erg/cm2; and the works of chain folding were q = 6.06 Kcal/mol and 8.49 Kcal/mol, for regime I and regime II, respectively. Polarized optical microscopy observations showed that different morphologies were developed in RPET than PET when crystallized from the melt. The mechanical properties and morphology of PET and RPET were studied using a universal tensile machine and atomic force microscopy, respectively. The morphological study, the mechanical properties and the thermal behavior of RPET confirmed that the RPET had an enhanced crystallization rate and mechanical parameters.