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(ε-caprolactone) (C-PCL) growing from the melt as a function of the crystallization temperature (T_c) and degree of supercooling (ΔT) 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 = 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 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 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∝M_w ^−1.2 for C-PCL and G₀∝M_w ^−0.67 for L-PCL. The difference in the power of the G₀ 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.