Modes of polymer crystallization kinetics examined using fast scanning calorimetry

The crystallization mode of polymers is determined by the modes of nucleation and crystal growth, and the understanding is of great practical importance. The basic research should be based on isothermal crystallization, in which the rates of nucleation and crystal growth are constant. Recent advances in fast scanning calorimetry (FSC) enabled rapid temperature jumps required to examine isothermal crystallization kinetics near the glass transition temperature T_g, which is of interest in basic science, and in the high supercooling range, which corresponds to the processing conditions having practical importance. We examined the modes of isothermal polymer crystallization kinetics on the basis of the Kolmogorov–Johnson–Mehl–Avrami model using FSC. For many polymers, the temperature dependence of the crystallization rate exhibits a double peak. We investigated poly(butylene terephthalate), PBT, one of these polymers, which does not change its crystal form, over a wide temperature range.

As indicated by the Avrami exponent n, the two peaks arise from different nucleation modes. Of these, the high-T_c peak is due to the growth of spherulites initiated by the nucleation from foreign heterogeneities (n≅3), and is strongly influenced by a nucleating agent, NA, the effect of which can be quantitatively evaluated using a model suggested by Avrami 80 years ago. Regarding the low-T_c peak, we focused on the microscopic observations using AFM for the samples prepared by FSC in the T_c range with the change in the Avrami exponent from 3 to 4, and confirmed the continued size reduction of the spherulites becoming sub μm-scale due to rapid increase in nucleation from homogenous melt at lower T_c. We think this finally leads to the formation of nm-scale nodules near T_g. The Avrami exponent once risen to 4 rapidly drops to less than 2 at further lower T_c. Based on the growth inhibitory effect of rigid amorphous fractions proposed by Schawe, we applied Temperature-Modulated FSC to differentiate amorphous fractions, and confirmed that the remaining amorphous fractions (~ 70 %) are all rigid near T_g.

As a future challenge, it will be important to confirm whether a similar mechanism occurs in other polymers that show double peaks and the role of crystal polymorphism.