Osmotic second virial cross coefficients for star and linear polystyrenes

Experimental osmotic second virial cross coefficients are reported for line ar and 8-arm star polystyrenes in three solvents: toluene, cyclohexane, and methylcyclohexane. The osmotic second virial cross coefficient for 8-arm s tar and linear polystyrene is always positive and within the osmotic second virial coefficients measured for the single polymers. The positive cross c oefficient indicates net repulsion between the two different polymers in di lute solution. The extent of repulsion is greatest in toluene and least in cyclohexane. To relate the macroscopic second virial coefficient to microsc opic interactions, the potential of mean force between linear and 6-arm sta r polymers was computed by molecular simulation. The interaction between no nbonded polymer segments is given by a square-well potential. Well width wa s set equal to one half of the segment diameter. Different solvent conditio ns were investigated by using different well depths. Potentials of mean for ce were then used to compute the osmotic second virial cross coefficients. (C) 2000 American Institute of Physics. [S0021-9606(00)51231-8].