Melting process and the equilibrium melting temperature of polychlorotrifluoroethylene

Abstract

A new method of estimating the equilibrium melting temperature, T m, of a polymer is described, and applied to polychlorotrifluoroethylene (PCTFE). Experimentally determined values of the so-called observed melting point, T'm(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 T'm (obs) versus Tx plot, Tx being the abscissa. This line is then extrapolated to its intersection with the line T'm (obs)=Tx. The temperature at this intersection is T m. 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 T m 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 isothermal 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 reciprocal 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 the crystal retains a value fairly close to that of a growth nucleus can readily be justified on the basis of polymer crystal growth with chain folds. Its justification in the case of the customary bundlelike mode of crystallization is less clear. It is demonstrated experimentally that even the largest detectible crystals in PCTFE are onIy about 70 percent thicker than a primary nucleus, when secondary effects are minimized. The application of the theory to systems other than PCTFE is discussed briefly, and some preliminary measurements on polyethylene mentioned. Some points relating to the shape of the melting curves of highly crystalline polymers are also brought out.