Semi-empirical equation of state for polymers and simple liquids in non-critical and critical regions

Abstract

The semi-empirical equation of state for the polymers polystyrene (PS), polyisobutylene (PIB) and polydimethyl siloxane (PDMS) and the simple liquids benzene, n-heptane, carbon tetrachloride and argon at a negligible external pressure has been determined using the experimental data of thermal pressure coefficient γ v , molar volume V and thermal expansion coefficient α p . The equation of state obtained in this work is expressed by P = ( X β o V )c 1 exp(α −1 oI o)T − c o exp(a −1 oI o) , where X = (V c − V) V and I o = ∫ V V c X β o V dV , V c is the critical volume and α 0, β 0, a 0, c1 and c 0 are constants determined experimentally. We have obtained good agreement between the values of α p and γ v calculated and observed, within 1–2% over a wide temperature range. The semi-empirical equation of state in the critical region is expressed by V c − V∝( T c − T) 0.33, P − P c∝( V c − V) 5.0, β −1 T ∝( T c − T) 1.33 and α −1 p∝( T c − T) 0.67 for β 0 = 2.0, which agrees semi-quantitatively with the experimental critical indices.