Glass transition temperatures of polymer materials, measured by thermomechanical analysis

Abstract

The linear expansions of two materials have been measured, a double-base propellant and a carboxyl-terminated polybutadiene. The glass transition temperature, T~ and expansion coefficients below and above T, have been calculated. The influence of the heating and cooling rates and sample thickness has been investigated. The resull~s show that the value of Tg is dependent on the rates of heating and cooling but not on the sample thickness. Extrapolating to zero rate gives the same Tu for both heating and cooling. The expansion coefficients are not influenced by the rates of heating and cooling or by the sample thickness.In measurements of the glass transition temperatures T~ of polymers it has been found that the value of T~, is dependent on the rate of heating. Strella [1 ] has made an investigation of Tg for two polymers by DTA. He found an increase of Te with increasing heating rate. Further, Boyer [2] mentions in his review the influence of the heating rate on the value of the glass transition temperature. The dependence of T~, on the heating rate has also been investigated by Wolpert et al. [3] with DTA measurement, and by Prud'homme et ah [4] when measuring heat capacity with DSC. Nielsen [5] states that the glass transition is generally measured in experiments which correspond to a time scale of seconds or minutes. If the time scale in the experiments is shortened, the apparent T~ is raised, and if the time scale is lengthened to hours or days, Tx is lowered. Shen and Eisenberg [6] have come to the same conclusion in their review.The reason for this investigation was that in measurements of the expansion coefficients and Tg of polymer materials the value of Tg varied with the rate. In order to have a reliable value of Te it was necessary to establish the influence of different rates of heating and cooling with this particular apparatus. At the same time it was of great interest to know the influence of the sample thickness on the results. Experimental MaterialsThe investigation was made on two materials. One is a double-base propellant, that is the propellant is mainly made of nitrocellulose and nitroglycerine. The nitroglycerine functions as a plasticizer for the polymer nitrocellulose.

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