Dynamic Mechanical Properties of Concentrated Polystyrene Solutions at 40 MHz

Publisher Website
Google Scholar
Abstract
Measurements of dynamic shear impedance at 40 MHz and room temperature are reported for two polystyrene‐solvent systems as a function of concentration. These measurements were made by an ultrasonic reflectance technique. The polymers had sharp molecular‐weight distributions, M̄w/M̄n∼1.08, and molecular weights of 2.39×105 and 2.67×105. The concentrations ranged from about 1% to 20% and 3% to 30% in di‐n‐butyl phthalate, DBP (a near theta‐solvent) and in toluene (a good solvent) for the two polymers, respectively. Results are reported in terms of the in‐phase, G′, and out‐of‐phase, G″—ωηs, components of the dynamic shear modulus of the polymer and in terms of the reduced dynamic viscosity (η′—ηs)/(η—ηs), where η′ is the dynamic viscosity, ηs is the solvent viscosity, and η is the steady‐flow solution viscosity. Results on the polystyrene—toluene system indicate that the system undergoes a change from Zimm‐like to Rouse‐like behavior with increasing concentration and, hence, with reduced frequency. No evidence of a reduced high‐frequency limiting viscosity is observed and the value for the lowest concentration falls on a line extrapolated from earlier data of Lamb and Matheson. Results on the polystyrene DBP system indicate that the slope of logG′ vs log volume fraction of polymer increases at about 5% concentration, the predicted concentration for the onset of entanglement coupling. As concentration increases, a reduced high‐frequency limiting viscosity is present up to the entanglement concentration, but beyond this point the reduced dynamic viscosity decreases and indicates a trend toward Rouse‐like behavior at 20% concentration. Within experimental uncertainty neither solvent exhibits viscoelastic behavior.