(251d) A Comparison of Direct and Indirect Methods for Calculation of Polymer Solubility Parameters from Molecular Simulations

The cohesive energy density of polymeric materials has found widespread application in areas ranging from miscibility prediction to estimation of other properties such as Youngs and bulk moduli and surface tension (1). Experimentally, while the cohesive energy of a small molecule liquid can usually be deduced directly from heat of vaporization data, the corresponding quantity for polymers must be obtained by indirect methods (e.g. from swelling measurements of lightly crosslinked polymer interacting with liquids of different cohesive energies). In principle therefore, molecular simulations, which probe intermolecular interactions directly, offer a useful alternative method for consistently determining cohesive energy. In practice the quality of predictions from molecular simulations depends on a number of factors including the quality of the force field used, the ability to effectively sample a representative set of the diverse conformations normally accesible to polymers, and on the actual method used to deduce the cohesive energy.

In view of the fundamental importance of the cohesive energy especially in the area of the thermodynamics of polymer miscibility, we have conducted a series of investigations in which the cohesive energy density and solubility parameter of a variety of common polymers are obtained from computer simulations using both direct and indirect methods. In the first part of the study, the accuracy of the force field used is assessed by performing calculations of cohesion parameters of related small molecules. Next, a series of bulk polymers are constructed and analyzed to determine how well the conformations generate sample the conformation space available to the polymer chains. This is followed by calculation of the cohesive energy as obtained by direct nonbonded energy analysis of the conformations generated, and by a series of indirect studies in which interaction of the polymer surface with a variety of liquids is studied in a manner which has previously been demonstrated to be successful in determining the cohesive energy density of the rigid engineering polymer Ultem (2).

References

(1) see, for example, van Krevelen, D.W. and Hoftyzer, P.J., ?Properties of Polymers, Their Estimation and Correlation with Chemical Structure?, 2nd Edition, Elsevier, New York (1976).

(2) Eichinger, B.E., Rigby, D. and Stein, J., Polymer 43, 599 (2002).