Transitions and Relaxations in Polytetrafluoroethylene

Abstract

The modulus and internal friction of polytetrafluoroethylene were measured with longitudinal waves at a frequency of 12 Mc between 248° and 548°K and the fluorine magnetic resonance was studied between 77° and 375°K. The samples covered a wide range of crystallinities and included a specimen which had not been sintered (as polymerized material which had not been heated above the melting temperature). The results resolve discrepancies which existed in the literature and introduce new information about the relaxations and first‐order transitions in polytetrafluoroethylene. In the ultrasonic work it is shown that the ``19°C'' and ``30°C'' diffuse first‐order crystalline transitions can be studied independently of the crystalline relaxation which occurs at 418°K at 12 Mc. The ``19°C'' transition is not observed but the ``30°C'' transition causes an appreciable decrease in the modulus. X‐ray data show that this accompanies a decrease in the rotational order of the lattice. This transition is found to occur over a wider temperature range in unsintered polymer (possibly because of a distribution of the lengths of the molecular segments in the ordered regions). An NMR absorption line from the crystalline regions is resolved and its narrowing above 280°K is attributed to rotational motions associated with the first‐order transitions. Comparison with published data shows that the narrowing occurs over a wider temperature range in unsintered polymer. Consideration of a distribution of relaxation times suggests that the narrowing above 190°K of an NMR absorption line from the amorphous regions results from the molecular motion which gives rise to a relaxation observed at 263°K in the ultrasonic measurements. Another amorphous relaxation is observed at 470°K in the ultrasonic measurements and the activation parameters are obtained. Examination of these parameters for the two relaxations suggests that the higher‐temperature relaxation should be assigned to the larger molecular segments. For all the relaxations, the parameters follow a relation which obtains for activated processes in inorganic solids.