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  • Title: Motion associated with a single rouse segment versus the alpha relaxation. 2.
    Author: Lin YH.
    Journal: J Phys Chem B; 2005 Sep 22; 109(37):17670-8. PubMed ID: 16853261.
    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 (=zetab2/kTpi2m2) 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 Jt 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 tauS = 18tauG (tauG being the average glassy-relaxation time) is made, reflecting both the temperature dependence of tauG and the effect on the bulk mechanical property by the glassy-relaxation process. In terms of thus defined tauS, two traditional ways of defining the alpha-relaxation time are compared and evaluated. It is shown that as the temperature approaches the calorimetric Tg, two modes of temperaturedependence are followed by the dynamic quantities concerning this study: One includes the time constant of the highest Rouse-Mooney normal mode, tauv; the temperature dependence of the viscosity corrected for the changes in density and temperature, eta/rhoT; and the average correlation time obtained by depolarized photon-correlation spectroscopy, tauc. The other, being steeper, is followed by the alpha-relaxation time tauS derived from the glassy-relaxation process and the temperature dependence of the recoverable compliance Jr(t) as obtained by Plazek. The comparison of the dynamic quantities clearly differentiates the motion associated with a single Rouse segment as characterized by tauv or tauc from the alpha-relaxation as characterized by tauS; 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 Tg, the two modes could be easily confused. Below approximately 110 degrees C, the rate of tauc changing with temperature lags behind that of tauv is explained as due to the loss of effective ergodicity taking place in the system.
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