The Application of Dynamic Mechanical Methods to T g Determination in Polymers: An Overview

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Abstract
Many factors are involved with establishing Tg values from Dynamic Mechanical Methods. These can be grouped as: (a) instrumental factors (b) test frequency (c) material characteristics (d) choice of Tg criterion Instrumental factors may include temperature calibration, thermal gradients, sample size, clamping effects, and sample geometry. Increasing test frequency causes a shift of Tg to higher temperatures. Material characteristics include such factors as the degree of crystallinity in crystalline polymers, the degree of crosslinking in thermosets, the specific thermal and mechanical history of the material, and possible moisture effects. The Tg criterion selected relates to the subjective choice of viscoelastic function used to establish Tg. For example, the peak value of E" will give a different result than the peak tanδ. In this paper we will review these various aspects of determining Tg, citing examples in each category which illustrate problems in assigning appropriate Tg values.