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

Abstract

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.