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 °C. At lower 7_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 °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 μm) for lower T_c's but intermediately larger spacing (3-5 μm) 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 °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.