Preferred orientation of crystallites in uniaxially deformed polytetrafluoroethylene

X-ray methods have been used to determine the preferred orientation of crystallites in PTFE sheets stretched varying amounts in the trigonal phase at room temperature. Spherical harmonic analysis of the x-ray data was carried out to obtain inverse pole figures. At high elongations results show preferential alignment of the c axis (chain direction) with the extension axis. At lower elongations there is also a tendency for (h0h̄l) poles to be aligned with the extension axis. The results at low extension are interpreted as being caused by slip occurring on {1120} planes along the tangent to the helix formed by the PTFE molecule. Experiments carried out in the hexagonal phase above 30°C, and in the triclinic phase below 19°C support these conclusions.