Chemical kinetics and diffusion in poly(butylene terephthalate) solid-state polycondensation: Experiments and theory

Postpolymerization was obtained in solid PBT sheets, by annealing in a dry nitrogen stream at 214°C. After different reaction times, the samples were cut into thin slices. The M_w profile within the sample was obtained through intrinsic viscosity measurements in a phenol–TCE mixture. At each location inside the sample, the [η] vs. time curve typically shows a maximum, which is rather broad at the midplane and much sharper at the external surface. The phenomenon was mathematically described by accounting for both diffusion and chemical reactions within the slab; five simultaneous chemical reactions have been considered. The observed behavior is found to be essentially due to two competing processes, i.e., the diffusion of the low molecular weight species generated during the polycondensation and the thermal degradation reaction. The model predictions are compared with the experimental data showing a satisfactory agreement.