Determination of Glass Transition Temperature by DTA or DSC

The heat capacity (Cp) of amorphous, materials in the solid state is lower than that in the liquid state. On this account, a deflection of the baseline can be observed at the glass transition temperature (Tj,). Occasionally, a small endothermic peak based on enthalpy relaxation (see Section 4.1.6) can be observed. Based on the Cp change, can be determined experimentally using DTA or DSC. The Tg value depends markedly on the thermal history of the sample and the experimental conditions. The measurement should be performed with the same standardization procedure as in ASTM D 3418-82 and JIS K 7121-1987. 

Before the measurement, maintain the sample at a temperature of 23 2 C and a relative humidity of 50 5% for more than 24 h (or under other desired conditions) for conditioning. 

Weigh about 10 mg of the sample with a precision of 0.1 mg. If the sample contains a large amount of additive, 5-10 mg of pure sample (e.g, polymer) is sufficient. Since milligram quantities of sample are used, it is essential to ensure that the samples are homogeneous and representative. 

DTA sample vessel. Heat the sample from ambient up to 30 C above the final temperature for a amorphous polymer or 30 C above the final temperature for a crystalline polymer. After maintaining this temperature for 10 min, cool to 50 °C below the glass transition temperature. 

After maintaining the apparatus at 50 °C below the glass transition temperature, heat the sample at a rate of 20 °C min up to 30 C above the end of the transition temperature. The sensitivity of the apparatus should be adjusted, resulting in more than a 10% deflection of full-scale of the recorder before and after the glass transition temperature. (Computer-equipped apparatus can carry out this process automatically). 

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