Raman spectroscopic evidence for conformational deformation in the high pressure phase of polytetrafluoroethylene

An investigation of polytetrafluoroethylene (PTFE), nC20F42, and a random copolymer of tetrafluoroethylene and hexafluoropropylene (TFE–HFP) under pressure has been carried out using Raman spectroscopy in conjunction with a diamond anvil cell. Both nC20F42 and the TFE–HFP copolymer were found to undergo a phase transition in the 7–9 kbar range similar to PTFE as evidenced by the observation of the 625 cm−1 Raman band characteristic of phase III. With increasing pressure in the 10–52 kbar range, a continual variation in intensity of the bands at 285 and 395 cm−1 was observed and found to differ for the three fluorocarbon materials studied. By correlating a change in the I285/I395 ratio with a change in sample crystallinity (as determined by x- ray), it has been determined that an increase in transplanar content with pressure is responsible for the observed band intensity changes in the high pressure phase of PTFE.