Motion associated with a single rouse segment versus the alpha relaxation. 2

Abstract

The dynamics in polystyrene melt and concentrated solution as probed by depolarized photon-correlation spectroscopy has been shown to reflect the motion associated with a single Rouse segment. In the concentrated solution case (entanglement-free), the analysis using the frictional factor K (= zeta < b(2)>/kT pi(2)m(2)) extracted from the viscosity data in terms of the Rouse theory and aided by the Monte Carlo simulation based on the Langevin equation of the Rouse model confirms the conclusion in a precise manner. In the melt case (entangled), the Rouse-segmental motion as observed by depolarized photon-correlation spectroscopy is compared with the alpha relaxation and the highest Rouse-Mooney normal mode extracted from analyzing the creep compliance J(t) of sample A reported in the companion paper. Another well-justified way of defining the structural (alpha-) relaxation time is shown basically to be physically equivalent to the one used previously. On the basis of the analysis, an optimum choice tau(s) = 18 <tau >(G) (<tau >(G) being the average glassy-relaxation time) is made, reflecting both the temperature dependence of WG and the effect on the bulk mechanical property by the glassy-relaxation process. In terms of thus defined tau s, two traditional ways of defining the alpha-relaxation time are compared and evaluated. It is shown that as the temperature approaches the calorimetric T-g, two modes of temperature dependence are followed by the dynamic quantities concerning this study: One includes the time constant of the highest Rouse-Mooney normal mode, tau(v); the temperature dependence of the viscosity corrected for the changes in density and temperature, eta/rho T; and the average correlation time obtained by depolarized photoncorrelation spectroscopy, <tau(c)>. The other, being steeper, is followed by the alpha-relaxation time tau(s) derived from the glassy-relaxation process and the temperature dependence of the recoverable compliance J(r)(t) as obtained by Plazek. The comparison of the dynamic quantities clearly differentiates the motion associated with a single Rouse segment as characterized by tau(v) or <tau(c)> from the alpha-relaxation as characterized by tau(s); due to the lack of clear definition of these two types of motion in the past and the proximity of one to the other in the time scale-actually the two crossing over each other-as the temperature is approaching T-g, the two modes could be easily confused. Below similar to 110 degrees C, the rate of <tau(c)> changing with temperature lags behind that of tau(v) is explained as due to the loss of effective ergodicity taking place in the system.