Broadband ultrasonic spectrometry of polystyrene-cyclohexane critical mixtures

Abstract

Mutual diffusion coefficients, shear viscosities, and broadband ultrasonic attenuation spectra in the frequency range 100 kHz to 300 MHz have been measured for solutions of polystyrene in cyclohexane at two degrees of polymerization N and various temperatures near the critical. The exponent y(η) in the power law representation of the critical part in the viscosity deviates substantially from the universal value y(η) = 0.0435: y(η) = 0.028 (N = 288) and y(η) = 0.014 (N = 6242). Also, the adiabatic coupling constant g and the amplitudes ξ(0) and Γ(0) in the power laws of the correlation length and the relaxation rate of fluctuations, respectively, depend on N. This is especially obvious with the relaxation rates, for which Γ(0) = 5.8×10(9) at N = 288 and Γ(0) = 6.1×10(7) with the larger polymer results. A noteworthy feature is the difference between the relaxation rates from the diffusion coefficients and shear viscosities on the one hand and from the ultrasonic spectra on the other. Near the critical temperatures the latter Γ values deviate from power law behavior, indicating a coupling between the concentration fluctuations and structural isomerizations of the polymers.

Chain Dynamics of Ethylene Oxide Oligomer Melts. An Ultrasonic Spectroscopy Study

“…Recently, however, it was found that the critical fluctuations of the polystyrene−cyclohexane binary system reveal clear indications for their coupling to the polymer conformational variations. 37 In a series of papers, 38−41 ultrasonic attenuation spectra of poly(ethylene glycol) oligomers, measured in the frequency range between 5 MHz and 2.5 GHz revealed two Debye-type relaxation terms with discrete relaxation times. These terms were discussed to be due to a three-state model of chain conformational transitions, namely equilibria between trans− trans and trans−gauche, trans−gauche and gauche−gauche, as well as gauche−gauche and trans−trans rotational isomers.…”

“…Yet the spectra were too small-band to allow for reliable conclusions about the underlying modes of the local segmental motions . Other investigations were focused on the critical demixing behavior of polymer–polymer and polymer–low-weight solvent , mixtures. The contributions from noncritical processes in the ultrasonic spectra of such mixtures were largely masked by those from the critical fluctuations.…”

“…The contributions from noncritical processes in the ultrasonic spectra of such mixtures were largely masked by those from the critical fluctuations. Recently, however, it was found that the critical fluctuations of the polystyrene–cyclohexane binary system reveal clear indications for their coupling to the polymer conformational variations …”