A suitable specimen was selected to compare the glass transition measurement by several thermal analysis techniques. The specimen, an epoxy composite, was analyzed by Thermal mechanical analysis (TMA) using three different methods, including flexure, expansion, and stress relief, as a function of temperature. A dynamic mechanical analyzer (DMA) was then performed at three frequencies. A portion of the specimen was then selected for differential scanning calorimeter (DSC) analysis. Special care was taken to ensure that the specimen pre-treatment was identical and that thermal lag errors were corrected for, so that a direct glass transition comparison could be made between analytical techniques. The best agreement in Tg values between the various measurement methods was obtained between: DSC heat flow midpoint; TMA onset of expansion, flexure or stress relief; and the onset of the storage modulus for DMA data taken at 0.1 Hz. These were all within one degree of 122°C. The best correlations with the DSC and TMA data to be obtained from a 1-Hz DMA run was that of the onset of the loss modulus peak (121.7°C). The methods employed are described, and the results discussed.

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A commercially available liquid crystal polymer system having a very weak glass transition is investigated in order to illustrate the special problems of measurement and interpretation inherent in high-sensitivity Tg measurement. Differential scanning calorimetry, thermomechanical analysis, and dynamic mechanical analysis were employed over the temperature region from ambient to 175°C. A transition between 75 and 125°C with a midpoint around 100°C was detected by most techniques. The use of the maximum in the derivative of the measured signal as the primary assignment of the glass transition temperature is recommended. The results are discussed with reference to other literature on this polymer system.