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Copolymers of 1,1‐difluoroethene with tetrafluoroethene, chlorotrifluoroethene, and bromotrifluoroethene have been prepared by emulsion polymerization, and their crystallinity has been investigated by means of x‐ray diffraction. The results are discussed in relation to copolymer composition and structure.

Tetrafluoroethene (TFE)–chlorotrifluoroethene (CTFE) and TFE–bromotrifluoroethene (BTFE) copolymers have been synthesized by solution copolymerization over the entire range of comonomer composition. Crystallinity data are reported and first‐ and second‐order transitions have been investigated by DSC. Glass transition temperatures of TFE‐CTFE copolymers vary in a nonlinear fashion in the range defined by the homopolymers conforming best to the Johnston equation; the behavior in the TFE‐BTFE system is more linear. Whereas TFE‐BTFE copolymers show a steep decrease of melting temperature at higher BTFE content, due to the amorphous character of the polymers, more regular behavior was found for TFE‐CTFE copolymers. Enthalpies of fusion are also reported. The results are discussed in relation to copolymer composition and structure and are compared with data on tetrafluoroethene–hexafluoropropene (FEP) fluorocarbon resins.

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