For a number of liquids it has been shown previously that an unambiguous and physically reasonable free volume can be evaluated from internal pressure, P¡ = (dE/dV)T, measurements. The free volume is given as t>f = Vb = RT/{P + P¡), the b representing a quasilattice occupied volume. This approach to free volume has now been extended to solutions for which Pi of both the pure components and the mixtures are available. It is found experimentally that the occupied volume of the mixture, bm, is the mole fraction average of the occupied volume of the components. The excess entropy and excess enthalpy of mixing are shown experimentally to be given by -SE = R[xi In vn + X2 In ut2 -In urm] and EP = PimVm -xiPiiVi -X2P12V2 for all nonelectrolyte systems tested. Since internal pressure measurements for mixtures are rare, an equivalent approach is to compute P¡m from the Pi values for the pure components and the measured excess volumes, V6, through the relation Pim = {RT/(xiVn + 2 2 + V^)] -P.Excellent agreement is found between calculated and measured excess properties. As always with such solution calculations, the results are very sensitive to input data and hence more complicated methods than those proposed here would appear to be unwarranted.

It has been well established in the literature that the internal pressure, Pi = (∂E/∂V)T, of a polymer in the glassy state is about half the value expected from the behavior of the polymer just above the glass temperature, Tg. Consideration of this behavior in terms of a recent analysis of factors affecting internal pressures leads to the conclusion that the expression for the total energy of a glass must include a volume‐dependent stored energy term, a term not present above Tg. This stored energy could be associated with actual bond and segment deformations in the glassy state. Brittleness and solvent cracking behavior of glasses will be strongly dependent on this stored elastic energy which can be modified by altering the molding conditions under which the glass is formed.

The free volume, vf, of liquids is defined in many ways. Comparison of solid and liquid behavior indicates that the definition for free volume in terms of the internal pressure of the liquid (aE/dV) T , is physically reasonable. Application of the definition of free volume, vf = f i T / ( a E / a V )~, to polymethylenes, coupled with surface energy values, leads to an evaluation of both polymer segmental volume, Vs, and free volume per segment, (vf) s, as a function of temperature. These equilibrium thermodynamic measurements of VS and (vf ) s lead to an energy of activation for viscous flow in good agreement with viscosity studies. Information of this type could be of great use in considering many current problems in polymer flow such as the effect of pressure on viscosity.--