STRUCTURAL EFFECTS ON THE VAPORIZATION OF HIGH-MOLECULAR-WEIGHT ESTERS

To understand the effect of molecular weight and branching on the heats of vaporization (ΔHv) and their flow behavior ΔHv and viscosity (η) were measured at different temperatures in the high molecular weight ester series: linear flexible di-n-alkyl sebacates and compact branched triglycerides with molecular weight ranging from 300 to 900. ΔHv° values (ΔHv corrected to 298 K) have been obtained with experimental ΔHv and also computed according to the group additivity method; a smaller -CH2- group value of 3.8 kJ mol-1 compared to the normal value of 5.0 kJ mol-1 is found to give good agreement with the experimental data (within 2-5% error). Both ester series have the same ΔHv° irrespective of their molecular features, namely, shape, flexibility, and polarity, suggesting the coiling of the molecules during vaporization. The segmental motion of these ester series during their flow and its dependence on their molecular features unlike ΔHv° are demonstrated by the correlation of the enthalpy of activation for viscous flow (ΔH‡) and the ratio ΔEv/ΔH‡ = n (ΔEv is the energy of vaporization) with molecular weight. © 1990 American Chemical Society.