9730976 Lipson This is a renewal grant funded jointly by the Materials Theory Program in the Division of Materials Research and the Theoretical and Computational Chemistry Program in the Chemistry Division. The theoretical research is targeted at understanding how the microscopic nature of fluids and their mixtures is correlated with their macroscopic behavior. Systems of interest include both simple and complex molecules. In addition, a unique opportunity afforded by this work is the ability to compare the results for a lattice model with those for a continuum model using the same theoretical approach. The behavior of complex fluids has been of high interest in recent years, and this has been stimulated by the increasingly sophisticated kinds of measurements becoming accessible. In addition, the ability to simulate mixtures of dense fluids has expanded dramatically within the last decade. Thus, more data are beginning to appear whcih are capable of testing statistical mechanical theories, particularly for complex liquid mixtures. The research conducted here involves an integral equation technique known as Born-Green-Yvon (BGY) theory. Using the BGY formalism theoretical descriptions of lattice and continuum systems have been derived, and comparisons between the results using the two have been initiated. The lattice theory has resulted in simple closed-form expressions for thermodynamic quantities of interest. The advantages of lattice theory include its accessibility to non-theorists, and the ability to test it using lattice simulation results on relatively complex fluids and mixtures, which are more plentiful than continuum simulation data. The continuum theory is capable of tackling more subtle issues involving the interplay between local structure and bulk properties. However, continuum solutions involve numerical methods and simulation data on mixtures are not yet plentiful. The development of analogous lattice and continuum theories yields the p ossibility of determining what kinds of equilibrium properties are expected to be sensitive to the imposition of a lattice constraint. This research will focus on developing an understanding of polymer solutions and blends, building on the demonstrated ability of the lattice BGY theory to describe pure fluids, simple alkane mixtures and polyethylene solutions. This work will involve analysis of data, including equation of state information and (new to these efforts) small angle neutron scattering results, in order to obtain the characteristic microscopic parameters. Having determined what minimum data set is required to characterize a system, the goal is then to predict less accessible properties, such as the pressure dependence of the coexistence curve. Such information is important in deciding on processing conditions, for example. The BGY theory is also capable of probing the effects of structural differences (for example polyolefin blends) and energetic differences (such as ocur in strongly interaacting mixtures) on miscibility in an effort to understand at a more sophisticated level the balance between these two. %%% This is a renewal grant funded jointly by the Materials Theory Program in the Division of Materials Research and the Theoretical and Computational Chemistry Program in the Chemistry Division. The theoretical research is targeted at understanding how the microscopic nature of fluids and their mixtures is correlated with their macroscopic behavior. Systems of interest include both simple and complex molecules. In addition, a unique opportunity afforded by this work is the ability to compare the results for a lattice model with those for a continuum model using the same theoretical approach. Research will focus on polymer solutions and blends. Besides providing fundamental insight on these materials, the results will be of importance in the processing of these materials. ***
