A LIQUID-STATE THEORY OF DENSE STAR POLYMER FLUIDS

A liquid-state theory is presented for the conformational properties a nd intermolecular structure of concentrated solutions and bulk melts o f purely repulsive star-branched polymers. The theory generalizes earl ier work on linear chains and is based on a solvation potential, expan sion of the single-chain free energy to second order in monomer intera ctions, and interaction-site integral equation theory. Information abo ut the chemical structure of polymers, in particular intra- and interm olecular interaction potentials with a finite excluded volume per mono mer, may be included straightforwardly. The theory does not assume inc ompressibility in the melt and suffers no loss of accuracy in the shor t-chain, small-arm-number limit. Also presented here is a study of con formational properties, including radius of gyration, mean arm end-to- end distance, monomer density profile, and local persistence length, f or concentrated solutions and melts of a hard-sphere, branched-pearl-n ecklace model of star polymers with a moderate (4-12) number of arms. Stretching of the star arms near the branch point is found, and a conc omitant swelling of the star with respect to the fully ideal state is predicted, even at meltlike densities and even in the few-arm limit. M easures of the swelling are compared with experimental measurements, t he Daoud-Cotton ''blob'' model, and the ''exclusion zone'' model of Bo othroyd and Ball. The physical origin of the stretching is also discus sed in detail.