Lattice dynamics of polytetrafluoroethylene

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Abstract
The normal modes of vibration propagating along the unique c axis of stretch‐oriented polytetrafluoroethylene at −11, 25, and 40°C have been studied by means of coherent neutron inelastic scattering methods. No significant effects associated with the 19 and 30°C phase transitions have been definitely observed. The results, together with related information obtained from Raman scattering and infrared absorption measurements, have been analyzed in terms of various models of the forces both within and between the polymer chains. The effects of interchain forces on the shapes of the dispersion curves and on the selection rules for radiation scattering and absorption are discussed. It has not been possible to establish a satifactory model for describing all the available data, mainly because of the mathematical complexity involved in treating the dynamical behavior of the coupled chains. The calculations nevertheless indicate that this type of model is capable of providing at least qualitative agreement with experiment. The elastic constant C 33 may be estimated from the limiting slope of the longitudinal acoustic branch: we obtain C 33 =1.50± 0.10 (10 12 dyn / cm 2 ) , a constant for all three temperatures within experimental error.