Influence of cross-link density on the non-isothermal crystallization kinetics of polybutylene terephthalate

Kinetics of crystallization of the samples of cross -linked polybutylene terephthalate with different cross-link densities was studied experimentally using conventional differential scanning calorimetry (cooling rates 5 to 40 K & sdot;min-1) and fast scanning calorimetry (cooling rates 0.5 to 5000 K & sdot;s-1). The samples were obtained by copolycondensation of terephthaloyl chloride, 1,4-butanediol, and glycerol. The cross -linking degree was characterized using 1H NMR spectroscopy. An increase of the number of glycerol cross -links in polymer leads to a decrease in the crystallization and melting temperatures and suppression of crystallization process. The critical rate of crystallization decreases monotonically with increasing cross -linking degree from 2700 K & sdot;s-1 for linear polybutylene terephthalate to 110 K & sdot;s-1 for the sample with 270 mol & sdot;m-3 cross-link density. At the cross-link densities higher than 270 mol & sdot;m-3, the polymer does not crystallize even at 5 K & sdot;min-1 cooling rate. The observed dependence of the cold crystallization enthalpy on the previous cooling rate suggests that the homogeneous nuclei formation process also slows down with an increase in the cross -linking degree.