STRUCTURAL VARIATIONS IN RANDOM COPOLYMERS OF TETRAFLUOROETHYLENE WITH KIND AND CONTENT OF COMONOMER UNITS

Random copolymers of tetrafluoroethylene with minor amounts of fluorin ated comonomers (hexafluoropropene, perfluoromethylvinylether and perf luoropropylvinylether) of different compositions are characterized mai nly by X-ray diffraction but also by differential scanning calorimetry and density measurements in an attempt to establish the possible incl usion of the respective side groups into the crystallites. Structural characterizations are conducted at room temperature (where all the con sidered copolymer samples are in the less ordered crystalline form I) as well as at -40 degrees C (where all the considered copolymer sample s are in the more ordered crystalline form II). Increases of the later al dimension of the unit cell and decreases of the size and amount of crystalline domains, with comonomeric unit content, are observed which are similar for different comonomer units (of largely different bulki ness) as well as for the two crystalline forms. The reported experimen tal results suggest a substantial exclusion from the crystallites not only of the bulkier comonomeric units (from perfluoropropylvinylether) , but also, at least for high comonomer compositions, of the smaller c omonomeric units (from perfluoromethylvinylether and hexafluoropropene ). (C) 1998 Elsevier Science Ltd. All rights reserved.