THERMODYNAMIC PROPERTIES OF DILUTE AND SEMIDILUTE SOLUTIONS OF REGULAR STAR POLYMERS

The osmotic modulus of solutions of polybutadiene stars with f = 32, 6 4, and 128 arms have been measured from the dilute to the semidilute r egime in the good solvent cyclohexane by static light scattering. Some complementary results on low molecular weight stars have been obtaine d 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 h ard-sphere value(5/8). The concentration dependence of the osmotic mod ulus near the polymer coil overlap concentration, A(2)Mc approximate t o 1, is characterized by a steep increase. This behavior agrees with a rguments by Witten, Pincus, and Cates which indicate a steplike rise i n the osmotic modulus where the step height scales as f(3/2). In the s emidilute 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 measuremen ts.