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

To elucidate the molecular weight (M-w) dependence of the growth rate of spherulite (G) of cyclic polymers polymerized by the ring expansion reaction, we studied the G of cyclic poly(epsilon-caprolactone) (C-PCL) growing from the melt as a function of the crystallization temperature (T-c) and degree of supercooling (Delta T) using polarizing optical microscopy. We prepared C-PCLs comprising an exclusively repeating unit, excluding other structural units with M-w = 17000, 33000, 41000, 74000 and 85000 and linear PCLs (L-PCL) with M-w = 16000, 35000 and 60000, where M-w was determined by GPC using linear polystyrene standards. G obeyed the equation, G = G(0)exp(-B/T Delta T), 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 M-w. 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 M-w is independent of M-w. Compared to C-PCL and L-PCL having almost the same M-w, G of L-PCL is higher than that of C-PCL at the same T Delta T. 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: G(0)alpha M-w(-1.2) for C-PCL and G(0)alpha M-w(-0.67) for L-PCL. The difference in the power of the G(0) dependence on M-w 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.