THERMODYNAMIC PROPERTIES OF DILUTE AND SEMIDILUTE SOLUTIONS OF REGULAR STAR POLYMERS

The osmotic modulus of solutions of polybutadiene stars with f = 32, 64, and 128 arms have been measured from the dilute to the semidilute regime in the good solvent cyclohexane by static light scattering. Some complementary results on low molecular weight stars have been obtained using SANS. Values of the dimensionless virial ratio g = A(3)/A(2)(2)M approximate to 0.62 for these many-arm stars accord well with the hard-sphere value(5/8). The concentration dependence of the osmotic modulus near the polymer coil overlap concentration, A(2)Mc approximate to 1, is characterized by a steep increase. This behavior agrees with arguments by Witten, Pincus, and Cates which indicate a steplike rise in the osmotic modulus where the step height scales as f(3/2). In the semidilute regime the osmotic modulus becomes identical with that of a homogeneous linear polymer solution. A linear polybutadiene sample was also considered for comparison with the star polybutadiene measurements.