Local structure and phase behavior of dense polymer-particle mixtures: improved theory and comparison with simulation

The polymer reference interaction site model (PRISM) has been extensively applied to study the equilibrium behavior of polymer nanocomposites (PNC). The theory predicts three typical phases or states of aggregation: depletion clustering, steric stabilization via discrete adsorbed layers, and polymer-mediated bridging or networking, depending on polymer-particle attractive interaction, size ratio, composition, chain length and total packing fraction. While the existence of such microstructures appears to qualitatively agree with simulations and experiments, the accuracy of the predicted pair correlation functions and phase behavior has not been thoroughly examined. By performing systematic simulations, we find the PRISM pair correlation functions, selected thermodynamic properties and phase behaviors based on the commonly used Percus-Yevick and Hypernetted Chain closures sometimes incur significant errors, especially near bridging and depletion spinodal boundaries. A variety of new closure approximations are explored and we find that over a wide range of parameter space the modified Verlet approximation provides a major improvement in accuracy of structural and thermodynamic behavior for both PNCs and the corresponding atomic or colloidal mixtures.