Crystallization conditions effect on molecular weight of solid-state polymerized poly(ethylene terephthalate)

Abstract

Number-average molecular weight (\( \overline{M}_{n} \)) variation of polyethylene terephthalate with respect to crystallization temperature and time, and solid-state polymerization (SSP) time were studied using response surface experimental design method. All experiments were conducted in a fluidized bed reactor. \( \overline{M}_{n} \) values were calculated by Mark–Houwink equation upon determining intrinsic viscosity (IV) of samples. Two suitable models were proposed for \( \overline{M}_{n} \) and IV, based on the regression coefficient. It was observed that \( \overline{M}_{n} \) increases with decrease in crystallization temperature and increase in crystallization time and SSP time. It was shown that SSP time is the most important parameter based on statistical calculations. Crystallization time, crystallization temperature and SSP time were determined 60 min, 160 °C and 8 h, respectively, in order to achieve maximum \( \overline{M}_{n} \). Density measurements were applied to study the overall crystallinity of samples. Based on density results it was revealed that percent of crystallinity is not the only factor that affects the \( \overline{M}_{n} \) of polymer. Differential scanning calorimeter was used to analyze thermal properties of the samples. All samples showed two melting peaks. It was observed that the lower melting temperature peak is related to the isothermal crystallization process temperature. Polarized light microscopy was used to study spherulitic structures of polymer films after crystallization process. It was shown that the sample with smallest spherulite size had the maximum \( \overline{M}_{n} \) equal to 26,000 g/mol.