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

Molecular symmetry change of perfluoro-n-alkanes in 'Phase I' monitored by infrared spectroscopy

Phase diagram of polytetrafluoroethylene (PTFE) comprises four regions. Phases II and IV are characterized by twisted perfluoroalkyl (Rf) chains having different twisting rate of 13/6 and 15/7, respectively, while Phase III is characterized by a planer trans-zigzag molecular skeleton like a normal alkyl chain. These are confirmed by X-ray and electron diffraction and have already been established. Unlike these, Phase I is left an unresolved matter. This phase is complicated indeed and is not symbolized by a single molecular structure. At an ambient pressure, Phase I is the temperature region above 30 ºC (303 K), and the helical molecular structure is supposed to be gradually untwisted with an elevating temperature. This untwisting image is roughly suggested by the diffraction, neutron scattering, and thermal expansion techniques, but the conventional approaches have all experimental limitations because the untwisting accompanies disorder (or defect) in the twist along the chain. To explore the transition between two different helical structures of the Rf chain having disordered structures, vibrational spectroscopic techniques are expected to be an alternative approach. For infrared spectroscopy, for example, the twisting rate of the molecule is simply recognized as a degree of molecular symmetry. Here, we show that the band progression peaks of the CF2 symmetric stretching vibration mode are quite sensitive and useful for pursuing the molecular symmetry change in Phase I for both peak intensity and position using perfluoro-n-alkanes having different chain length covering both even and odd number of the CF2 groups.

Time-Resolved Vibrational Spectroscopy of Polytetrafluoroethylene Under Laser-Shock Compression

Shock-wave-induced high pressure and nanosecond time-resolved Raman spectroscopic experiments were performed to examine the dynamic response of polytetrafluoroethylene (PTFE) in confinement geometry targets. Time-resolved Raman spectroscopy was used to observe the pressure-induced molecular and chemical changes on nanosecond time scale. Raman spectra were measured as a function of shock pressure in the 1.2-2.4 GPa range. Furthermore, the symmetric stretching mode at 729 cm^-1 of CF₂ was compared to corresponding static high-pressure measurements carried out in a diamond anvil cell, to see if any general trend can be established. The symmetric stretching mode of CF₂ at 729 cm^-1 is the most intense Raman transition in PTFE and is very sensitive to change in pressure. Therefore, it can also be utilized as a pressure gauge for large amplitude shock wave compression experiments. A maximum blueshift of 12 cm^-1 for the 729 cm^-1 vibrational mode has been observed for the present experimental pressure range. A comparative study on the similarities and differences from the earlier work has been done in detail. One-dimensional radiation hydrodynamic simulations were performed to validate our shock compression results and are in very good agreement.

High-Pressure-High Temperature (HP-HT) Stability of Polytetrafluoroethylene: Raman Spectroscopic Study Up to 10 GPa and 600 ℃

The increasing demand for use of polymers at extreme conditions makes important the exploration of their behavior in a wide pressure and temperature range, which remains unknown for polytetrafluoroethylene (PTFE), one of the most common materials. An in situ Raman spectroscopic study of PTFE shows that it is stable within the range of 2-6 GPa at 500 ℃ and up to 12 GPa at 400 ℃. At T > 500 ℃ and P > 3.5 GPa, the graphitization of PTFE is observed, but judging from the preservation of liquid run products, PTFE can be used as a material for sample container up to 600 ℃ at this pressure. The obtained data allow the suggestion that the triple point between liquid, solid, and decomposed (carbonized) PTFE is located between 3 and 4 GPa at about 550 ℃, by analogy with the behavior of polycyclic aromatic hydrocarbons.

High-pressure Raman spectroscopy and X-ray diffraction studies of a terpolymer of tetrafluoroethylene-hexafluoropropylene-vinylidene fluoride: THV 500

High-pressure Raman spectroscopy and X-ray diffraction of THV 500, a terpolymer of tetrafluoroethylene-hexafluoropropylene-vinylidene fluoride, were performed using diamond anvil cells (DAC). Changes in the interatomic spacing as well as shifts of several of the vibrational bands as a function of pressure were measured up to approximately 10 GPa. The changes in interatomic spacing and shifts of the vibrational bands are compared to those of polytetrafluoroethylene, showing the effects of copolymerization and reduced crystallinity. The high-pressure behavior of polymers is a relatively unexplored field but is becoming increasingly important due to applications where polymers experience extreme conditions.