Isothermal and nonisothermal crystallization kinetics of 2-methyl-1,3-propanediol substituted poly(ethylene terephthalate)

Differential Scanning Calorimetry (DSC) and a modified Light Depolarizing Microscopy (LDM) technique, capable of producing controlled cooling rates up to 5,000°C/min, was used to evaluated the isothermal and nonisothermal crystallization kinetics of poly(ethylene terephthalate) (PET) copolymers containing 2-methyl-1,3- propanediol (MPDiol) as a comonomer unit. The results of these experiments indicate that both the isothermal and nonisothermal crystallization kinetics are reduced by the addition of this comonomer. In this case it appears that the more flexible glycol group does not increase crystallization rates by promoting chain folding during crystallization, as has been suggested for some other glycol modified PET copolyesters. The melting behavior was also examined where it was found that the inclusion of these comonomers served to decrease the observed melting temperature. The isothermal melting behavior was examined using a Hoffman-Weeks approach, showing very good linearity for all copolymers tested and predicted an equilibrium melting temperature of 280.0°C for PET homopolymer. The remaining copolymers showed a marked decrease in Tm0 with increasing copolymer composition. The results of the analysis of the melting behavior support the claim that these comonomers are excluded from the polymer crystal during growth. Nonisothermal crystallization experiments showed similar results, both at moderate cooling rates examined using DSC and at high cooling rates using the modified LDM technique. The experimental data were examined both as a function of cooling rate and temperature where in both cases it was shown that the incorporation of MPDiol results in the reduction of the rate of crystallization. Likewise, the cooling rate required to quench the materials to the glassy state (QCR) was estimated using the LDM technique where it was shown that an increase in MPDiol content reduces QCR. From this experiment, Continuous Cooling Transform (CCT) diagrams were proposed for each of the three materials.