Empirical Potential Structure Refinement of semi-crystalline polymer systems: polytetrafluoroethylene and polychlorotrifluoroethylene

Empirical potential structure refinement (EPSR) simulations are performed on total neutron scattering data from powder samples of polytetrafluoroethylene (PTFE) and polychlorotrifluoroethylene (PCTFE), both at 300K. For PTFE a number of Bragg peaks are visible in the scattering data and these are found to be consistent with a lattice spacing $a(=b)=5.69(1)$\AA\ with a dihedral angle along the (helical) chain of 166$^\circ$ which gives a repeat distance along the chain (c-axis) of $\sim$19.6\AA. The positions of the Bragg peaks are well reproduced by this model, although there is a mismatch in the amplitudes of some of the higher order reflections between simulation and data. For PCTFE there is only one visible Bragg peak (100) which is well reproduced by a hexagonal lattice of atactic parallel polymers with a spacing of $a(=b)=6.37(1)$\AA. In this case the absence of distinct reflections along the polymer c-axis makes characterisation of the internal dihedral angle difficult, but a model with nearly straight \textit{trans} (zig-zag) structure gives best agreement with the data. For PCTFE little change in structure could be discerned when the material was heated to 550K, apart from a slight increase in lattice spacing. In both cases there is substantial diffuse scattering between the Bragg peaks, and this is correctly replicated by the EPSR simulations.