Morphology and growth rate of spherulite of cyclic poly(ε-caprolactone) having a triazole group at the closing point

To clarify the effect of triazole (TR) group at the closing point within the cyclic polymer chain on the crystal growth, we studied morphology and the growth rate of spherulite G of cyclic poly(ε-caprolactone) with a TR group at the closing point (C-TR-PCL) as a function of the degree of supercooling ΔT using polarizing optical microscopy. We prepared C-TR-PCLs synthesized by the ring-closing reaction from their linear precursors with M_w = 30000 and 50000, and cyclic PCL polymerized by ring expansion reaction (C-PCL) having no TR group with M_w = 26000, and linear PCL with/without a TR group at a chain end (L-TR-PCL-OH and L-PCL) with M_w = 26000 and 36000, where M_w was determined by GPC using linear polystyrene standards. We found that C-TR-PCL and L-TR-PCL-OH show banded spherulites at small ΔT and band-less normal spherulites with maltese cross at large ΔT. Whereas, C-PCL and L-PCL only showed band-less normal spherulites at any ΔT. This speculates that steric hindrance of TR group near the surface of crystals will become significant at small ΔT. G obeyed the equation, G = G₀exp(−B/TΔT), for all the samples. B of C-PCL and L-PCL is constant irrespective of TΔT, however, that of C-TR-PCL and L-TR-PCL-OH showed two values depending on TΔT. At the region of lower B, i.e., small ΔT, the banded spherulites could be observed. The change of B corresponds to the morphological change of the spherulites. Since the size of the crystalline region closely related to lamellar thickness becomes large at small ΔT, TR groups excluded from the crystals become to assemble near the surface of the crystals. This is the reason why B becomes low at small ΔT. In C-TR-PCL with higher M_w and L-TR-PCL-OH, the decrease of B with decreasing ΔT becomes small compared to that of C-TR-PCL with lower M_w due to relatively small effect of TR group.