The Crystal Structure of Polytetrafluoroethylene, Forms I and IV

Polytetrafluoroethylene (PTFE) is unusual among polymers in that it has four crystal phases, designated I–IV, with two of the phases (I, IV) incorporating substantial angular motion (librations) of linear segments within a precisely defined lateral lattice array. Data not included in the authors' earlier publications are presented to interpret two unsolved aspects of the crystal structure: 1) the narrow (11K) range of Form IV (1 atm) and 2) the possibility of another transition in Form I. The existence of Form IV is attributed to an “energy well” from space group P31. The molecular behavior in Form I is attributed to (L–R) molecular defects (changes in chirality) that increase rapidly in number and speed of translation along the axis with increasing temperature.