Structural study of a series of ethylene–tetrafluoroethylene copolymers with various ethylene contents, Part 2: Phase transition behavior investigated by temperature dependent measurements of X-ray fiber diagrams

Phase transitional behavior has been investigated for a series of ethylene (E) and tetrafluoroethylene (TFE) two-component copolymers on the basis of the temperature dependent measurement of X-ray fiber diagrams taken for the uniaxially-oriented samples. The usage of such uniaxially-oriented samples has allowed us to clarify the structural changes more definitely than before. So far, the crystalline transition from the low-temperature phase to the high-temperature phase had been assumed to occur continuously, but the detailed analysis of the X-ray reflections at higher scattering angles revealed for the first time that the transition is of the thermodynamically-discontinuous first-order type between the monoclinic and the pseudohexagonal phases. This phase transition temperature shifted toward lower temperature side as the TFE content became higher. The analysis of the X-ray reflection profile allowed us to imagine that low-temperature shift of the transition point is considered to occur due to the easier thermal motion of the planar-zigzag chains around the chain axis because of looser packing of chains containing higher population of bulky CF2CF2 groups. From these experimental data a phase diagram has been built up to show the phase transition behaviors of a series of E/TFE copolymers in a systematic manner. The characteristic features of the structural disordering including trans–gauche conformational exchange have been also discussed on the basis of the data of polarized infrared and Raman spectra combined with the X-ray diffuse scatterings.