The thermal expansion of solids is a consequence of anharmonicity of thermal oscillations of the substance particles. The thermal expansion of polymers has a number of peculiarities connected with various physical transitions occurring in the polymer as temperature is increased. To estimate experimentally the temperature coefficient of volumetric expansion, the temperature dependence of the specific volume of the polymer is determined. Schematically, this dependence is depicted in Figure 13.

Figure 13: Schematic representation of the dependence of specific volume V on temperature T (dilatometric curve) (rate of heating q ₁ > q ₂ > q ₃ > q ₄).

This dependence as a broken line is typical of many polymers near the glass transition temperature, T _g. At temperatures below the glass transition temperature this dependence is flatter than in the range of temperatures above it. Hence if T < T _g, the temperature coefficient of volumetric expansion (which represents a tangent of dilatometric dependence) is smaller than when T > T _g. In the first case, the temperature coefficient of volumetric expansion is designated as ? _G, and in the second one - ? _L. In this connection, the specific volume of the polymeric substance may be calculated by equations

(III.1)

(III.2)

where V _g is the specific volume of the polymer at the glass transition temperature; T is temperature.

The dilatometric dependence shown in Figure 13 is rather simplified. In fact, we are...