David B. Curliss

Isochoric and isobaric glass formation : Similarities and differences

Pressure-volume-temperature (PVT) studies were performed on a glass-forming polymer, poly(carbonate) (PC), under both isobaric and isochoric (constant volume) conditions. An isochoric glass transition was observed and the formation points were found to be consistent with those obtained isobarically. Although the isobaric and isochoric responses were, as expected, the same in the rubbery state, the glassy state values were found to be different and dependent upon the glass formation history. The isobaric data exhibited larger changes in going from the rubber to the glass, hence a stronger glass transition, than did the isochoric data. Inserting the experimental values for the thermal expansion coefficient α and isothermal compressibility β, into appropriate thermodynamic relations, measures of the strength of each transition are defined. Strength estimates based on literature values of α and β are compared to the experimental measures of the isochoric and isobaric transitions. In addition, both the isobaric and isochoric PVT results were analyzed in terms of the Fox and Flory free volume theory which assumes that the glass transition is an iso-free volume state. While the isobaric results were consistent with the Fox and Flory theory, the isochoric results were not consistent with the idea of an iso-free volume glass transition.

Effects of Thermal History and Jet Fuel Absorption on the Properties of APC-2

The response of APC-2 (AS4/PEEK) processed with different thermal histories simulating different fabrication techniques was investigated. Three thermal histories were evaluated: (1) slow cooled (SC) from the melt state at 2°C/min; (2) rapidly quenched (Q) from the melt state by cold press transfer; and (3) quenched and annealed (Q/A) at 160°C. The SC APC-2 was found to have a fully developed semicrystalline matrix and the highest strength and modulus for flexural and shear tests, while the Q APC- 2 was found to have an essentially amorphous matrix and the greatest fracture toughness. In uniaxial creep tests of [±45°]2 S coupons both exposed and unexposed to JP-4, the unexposed Q specimens had the highest creep rates at room temperature and 100°C. The exposed SC specimens had the highest creep rates at 100°C because of the plasticization effect of the absorbed JP-4.