Far infrared measurements on the 19°C phase transition in polytetrafluoroethylene

Abstract

The far infrared spectra of crystalline polytetrafluoroethylene has been obtained at temperatures above and below the 19°C phase transition. These measurements have shown, for the first time, the appearance of four lattice modes below 19°C at 42, 46, 55, and 84 cm−1. Upon cooling the lines become more intense and shift to higher frequencies indicating an increasing intermolecular interaction between the two molecules per unit cell. The appearance of these lattice modes, along with previously reported Raman and x‐ray data, supports the idea that the unit cell of polytetrafluoroethylene below 19°C belongs to the monoclinic crystal system with space group P21 containing two molecules per unit cell.