Isotope Effect on the Melt–Isothermal Crystallization of Polyoxymethylene D/H Random Copolymers and D/H Blend Samples

Abstract

We have successfully synthesized a series of polyoxymethylene (POM) random copolymers between the deuterated (D) and hydrogeneous (H) monomeric units by a cationic polymerization reaction. The randomness of D and H units in the copolymers was characterized by the quantitative analysis of 13C NMR and Fourier transform infrared spectral data. The equilibrium melting point T°m was estimated on the basis of Gibbs–Thomson equation using the experimental data of DSC melting points plotted against the crystallite thickness evaluated from the small-angle X-ray scattering data. The T°m of pure D-POM (208.5 °C) is higher than that of pure H-POM (homopolymer, 190.0 °C). The T°m changes systematically with the D content in the copolymer. The blend samples between D-POM and H-POM (homopolymer) show the similar D content dependence, but the T°m is as a whole higher than that of the copolymer. Another type of blend samples consisting of D-POM and H-POM containing small amount of ethylene oxide (Duracon) shows the similar but slightly different D content dependence of T°m, compared with homopolymer case, since Duracon’s melting point is about 5 °C lower than the latter. The kinetics of melt–isothermal crystallization behavior of these copolymers have been investigated using the time-dependent DSC data collected at the various isothermal crystallization temperatures, from which the crystallization rate constant (k) and growth dimension (n) were estimated on the basis of Avrami’s plot, where the induction time of nucleation was corrected. In parallel, the tangential line of Avrami’s curve at the crystallinity 0.5 was analyzed, from which the crystallization rate was estimated. These two methods of analysis were found to give almost the same results. By comparing the thus-obtained parameters among all the samples, several important results were obtained: (i) the POM-D shows the lowest crystallization rate compared with the POM-H samples; (ii) the crystallization rate decreased gradually with the increment of the D content in both the cases of D/H random copolymers and D/H blends when compared at the same ΔTc or the degree of supercooling; (iii) the existence of regimes I and II has been detected, the boundary temperature of which was found to change systematically depending on the D content. The crystallization rates were compared also between the two types of D/H blend samples, which showed remarkably different behaviors as a whole. In this way the isotopic effect on the thermal and crystallization behavior of D/H POM copolymers and blends has been revealed for the first time.