The crystal structure of the alternating copolymer of hexafluoroisobutylene and vinylidene fluoride

The crystal structure of the alternating copolymer of hexafluoroisobutylene and vinylidene fluoride, P(HFIB/VF₂), was determined using x-ray diffraction techniques. Oriented specimens of the copolymer were produced by drawing melt-pressed film at 300°C. The unit cell of P(HFIB/VF₂) is orthorhombic with dimensions at a = 1.064, b = 1.837, and c = 0.783 nm. The chain conformation is a 2₁ helix with an approximate trans-trans-gauche-gauche backbone dihedral-angle sequence. Severe intrachain steric crowding exists; as a consequence, the chain is quite rigid and some of the backbone bond angles are highly extended. The unit cell contains four such chains packed according to the symmetry of space group Iba2. Neighboring chains in a crystal are mechanically interlocked; in combination with the rigidity of the chains, this is the basis for the superior mechanical properties of the copolymer compared to PTFE. A structure containing up to 20-30% of head-to-head residues was found to be statistically indistinguishable from a completely head-to-tail structure.