Transitions in glasses at low temperatures

Abstract

Length-temperature measurements from −180°C to the glass transition temperature have been made on a poly(vinyl alkyl ether) series, poly(ethylene terephthalate), and a tetrafluoroethylene-trifluoropropylene copolymer series. Consistent with low frequency dynamic measurements, the poly(vinyl alkyl ethers) exhibit a glass-glass transition‡ Tgg [1] between Tg and Tg −70°C with evidence of another at about −160 to −180°C. With increasing side-chain length, the expansion coefficient below Tg- and the magnitude of the g-g transition increase, and the value of the Simha-Boyer free-volume product decreases, suggesting retention of excess free volume by the flexible pendant group. This behavior is analogous to that observed for poly(alkyl methacrylates) except that the latter system exhibited two g-g transitions, the lower of which coincided with Tgg [1] observed here. Poly(ethylene terephthalate) samples, both amorphous and crystalline, exhibit two T > Tg transitions at about −85 and −40°C. Dynamic results resolve only one relaxation in this temperature range. The glass temperatures of the tetrafluoroethylene-trifluoropropylene copolymer series, as determined by both dilatometry and differential scanning calorimetry, extrapolate to a Tg, for 100% amorphous PTFE of 11-16°C. Two g-g transitions, present in each homo-polymer, persist in the copolymers, indicating that only a small number of molecular units are involved. The magnitude of these relaxations, however, as measured by the change in expansion coefficient, is dependent on copolymer composition.