Crystal Domains and Crystallization Kinetics of Poly(butylene terephthalate)

The relationship between the changes in the crystallization kinetics and the crystal domains of poly(butylene terephthalate) was examined under isothermal conditions. In the target range of the crystallization temperature Tc across the double peak of the crystallization rate, the Avrami exponent n characterizing the nucleation and growth kinetics of the crystal domains exhibited a continuous change in the range 3 ≤ n ≤ 4. This was in correspondence to the change in the nucleation mode for spherical domains, such as spherulites and nodules, between nucleation from foreign heterogeneities and that from the homogeneous melt. We identified the morphology of the crystal domains responsible for this change, that is, whether spherulites or 10 nm scale granular nodules are involved in the change in the crystallization kinetics. Isothermally crystallized samples were prepared by applying a temperature jump using chip-sensor-based fast-scanning calorimetry. Optical microscopy and atomic force microscopy revealed a continuous change in the size of spherulites around a 1 μm scale across the Tc range of the crossover change because of the higher nuclei density at lower Tc. This behavior confirms that the Avrami exponent n = 4 is attained by the kinetics of crystal domains of submicrometer scale spherulites initiated by nucleation from the homogeneous melt in the low-temperature peak; spherulites of this scale were confirmed by the addition of talc as a nucleating agent in the high-temperature peak. As a limiting case of the continuous change, the extremely high nuclei density at low Tc and the corresponding reduction in the size of spherulites resulted in the formation of 10 nm scale granular nodules, which did not coalesce because of the rigid amorphous fraction formed around them near the glass transition temperature.