Lamellar Crystallization and Melting of Polyoxymethylene

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Abstract
Polyoxymethylene has been crystallized isothermally from the melt and from solution, and has been analyzed for the lamellar fold period as related to the melting behavior. The melt‐crystallized samples gave intense low‐angle x‐ray and DTA peaks, and in addition produced weak x‐ray reflections and shoulders on the DTA melting peaks. The strong peaks yielded Tm 0(s)=210±10°C, and two independent methods of calculating δ e (s) place its value at 150 erg/cm2 with a maximum variation of ±40 erg/cm2. A method of plotting the data is proposed which allows a determination of δ from the experimental data of about 14 erg/cm2. The weak peaks, although not as accurate as the strong ones, yielded Tm 0(w)=182°±2°C and δ e (w)=37±10 erg/cm2. Dried single‐crystal mats gave a single low‐angle x‐ray reflection which when plotted with the appropriate melting point caused the data to lie with the weak bulk data. A second, higher temperature endotherm appears upon slow melting of the single crystals. It has not yet been fully explained, but it appears to be related to an abrupt increase in fold period.

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