Dynamic Viscoelasticity and Concentration Dependence of Micelle−Gel Transition of Styrene and N-tert-Butylacrylamide Diblock Copolymer Solutions

We present dynamic viscoelastic studies of styrene and N-tert-butylacrylamide block copolymers (S(278)NtBAM(517) and S(93)NtBAM(252)) in 1-octanol as a function of copolymer concentration at 15, 25, and 35 degrees C. Dilute solutions of these diblock copolymers in 1-octanol, a selective solvent for N-tert-butylacrylamide units, yield spherical micelles as evidenced by transmission electron microscopy (TEM). Dynamic light scattering (DLS) is used to determine hydrodynamic radius of the micelles as a function of temperature (15, 25, and 35 degrees C). Dilute solutions of these block copolymers behave as viscoelastic fluids in the low frequency range. At higher concentrations, these copolymer solutions form glass-clear gels. Rheological measurements show that these block copolymer solutions exhibit power-law behavior at the gel point (G'(omega) approximately G''(omega) approximately omega(n)). In this paper, we discuss the formation and properties of critical gel states of S(m)NtBAM(n) in 1-octanol based on (1) concentration dependence of micelle-gel transition, (2) determination of critical state by rheology, and (3) temperature dependence of critical gel concentration and material parameters.