Pressure dependence of polymer chain conformational parameters

Abstract

Configurational statistics of chain molecules relates the average spatial configuration and the other configuration-dependent properties of a chain to the relative free energy AF of rotational isomers for each rotatable skeletal bond. The statistical weights uij = exp[ Mij/(RT)] are assigned to the pairs of conformations about adjoining bonds and are arranged as elements of statistical weight matrix’). The comparison of calculated and experimental values of configuration-dependent properties is the usual way for the assignment of statistical weights uij . Alternatively, uij can be determined from the conformational energy calculations for the compounds. Customarily, only the dependence of uij on temperature is assumed in conformational studies. For low molecular weight compounds there are many examples of the striking environmental (especially solvent) influence on conformational equilibria2). In configurational statistics this effect can be accounted for by introducing the solventdependent statistical weights uij . For example, the solvent effect on mean chain dimensions is simulated by a variation of some uij values’). It is common belief that the environmental effect on torsional potentials about single bonds and on the associated conformational free energies AF is important in polar systems only. However, there are also indications of the importance of the environmental effect on the conformational equilibria for nonpolar e.g. hydrocarbon molecule^^^^). One of the possible reasons for it is the modification of torsional potential in the condensed phase due to the different volumes of trans and gauche conformers’). In other words, the statistical weights for hydrocarbon chain molecules in the condensed phase are affected by changes of internal or external pressure or by changes of sample volume. In this paper the pressure variation of statistical weights of the polymethylene chain is estimated as a representative case of non-polar polymers.