Melting process and the equilibrium melting temperature of polychlorotrifluoroethylene

Abstract

A new method of estimating the equilibrium melting temperature, Tm, of a polymer isdescribed, and applied to polychlorotrifluoroethylene (PCTFE). Experimentally de-termined values of the so-called observed melting point,!F^(obs), are plotted as a function of the isothermal crystallization temperature, Tx. When freed of secondary effects, such as recrystallization, the data fit a straight line of positive slope on a Ti (obs) versus Tx plot, Txbeing the abscissa. This line is then extrapolated to its intersection with the line T ^ (obs) = Tx. The temperature at this intersection is Tm. This intersection is at 224 °C for PCTFE, and Tm is quoted as 224 ± 1 °C. (The highest melting point actually attained for a specimen was 218.2 °C.) The value of Tm estimated using the extrapolation procedure is compared with that estimated using the customary method of slow stepwise warming. A theoretical justification is given for making the type of plot mentioned above. The most important assumption used in the theory is that one of the dimensions of the growing crystal retains a value rather close to that of the appropriate growth nucleus during an iso-thermal crystallization, the other two dimensions being large in comparison. Combination of this with the fact that the relevant dimension of the growth nucleus will vary as the re-ciprocal of the degree of supercooling leads to the prediction of melting points that increase linearly with crystallization temperature. The assumption that one of the dimensions of