Dual types of spherulites in poly(octamethylene terephthalate) confined in thin-film growth

Spherulite morphology and growth kinetics of poly(octamethylene terephthalate) (POT), cast on single-side glass or confined between two slides in thin-film forms, were characterized using polarized versus nonpolarized optical microscopy, scanning electron microscopy (SEM), and wide-angle X-ray (WAXD) analysis. POT can simultaneously display solely one type of spherulite or dual types of spherulites (double-ring-banded and ringless ones), depending on T c or T max imposed. Fractions of these two types depend on T c when quenched from a fixed T max = 160 degrees C. At lower T c's, POT exhibits higher crystallization rates leading to higher fractions of ringless spherulites; at higher T c's, POT exhibits lower crystallization rates leading to ring-banded spherulites. At intermediate to high T c's where the growth kinetics of POT could be monitored, the ring-band type dominates and the fraction of ringless spherulites is insignificantly small. Both ringless and ring-banded spherulites can be seen in regime III ( T c = 70-110 degrees C), with fractions of ringless type of spherulites decreasing with temperature. Thus, growth kinetics for POT was mainly focused on the regime of ring-banded spherulites. In regime III, the ring-band pattern is more orderly concentric with smaller inter-ring spacing (1-2 mum) for lower T c's but intermediately larger spacing (3-5 mum) for higher T c's. The orderly lamellar orientation in the ring-bands in contrast with the inter-ring valley region is discussed. In regime II (115 degrees C and above), the ring-band pattern is first distorted to highly zigzag irregularity at higher T c's and then eventually disappears at extremely high T c, with the lamellar crystals eventually turning dendritic with no rings. Apparently, the types of spherulites in polymers are more influenced by the growth rates as determined by T c and slightly less by T max, but not by the substrate surface nucleation.