Molecular weight dependence of the growth rate of spherulite of cyclic poly(ε-caprolactone) polymerized by ring expansion reaction

Abstract

To elucidate the molecular weight (Mw) dependence of the growth rate of spherulite (G) of cyclic polymers polymerized by the ring expansion reaction, we studied the G of cyclic poly(ε-caprolactone) (C-PCL) growing from the melt as a function of the crystallization temperature (Tc) and degree of supercooling (ΔT) using polarizing optical microscopy. We prepared C-PCLs comprising an exclusively repeating unit, excluding other structural units with Mw = 17000, 33000, 41000, 74000 and 85000 and linear PCLs (L-PCL) with Mw = 16000, 35000 and 60000, where Mw was determined by GPC using linear polystyrene standards.G obeyed the equation, G = G0exp(−B/TΔT), for all the samples. The activation energies of secondary nucleation (B) of C-PCL were almost identical except for the C-PCL with the lowest Mw. Since B is proportional to the end surface free energy of the secondary nucleus, it implies that the regularity of the folding surface of C-PCL with high Mw is independent of Mw.Compared to C-PCL and L-PCL having almost the same Mw, G of L-PCL is higher than that of C-PCL at the same TΔT. This suggests that G of C-PCL and L-PCL is not solely controlled by the entanglement because the entanglement of C-PCL is lower than that of L-PCL. We found the following associations: G0∝Mw−1.2 for C-PCL and G0∝Mw−0.67 for L-PCL. The difference in the power of the G0 dependence on Mw reflects the difficulty of performing sliding diffusion in the secondary nucleus. This strongly supports the hypothesis that the absorption behavior of C-PCL chains in the secondary nucleus on the growth front is significantly different from that of L-PCL.