Whole range of chain dynamics in entangled polystyrene melts revealed from creep compliance: thermorheological complexity between glassy-relaxation region and rubber-to-fluid region. 1.
The rubber(like)-to-fluid region of the creep compliance J(t) results reported by Plazek of two nearly monodisperse polystyrene melts in the entanglement region have been quantitatively analyzed in terms of the extended reptation theory (ERT), giving the frictional factor K (= zetab2kTpi2m2) in quantitative agreement with the values obtained previously from analyzing the relaxation modulus Gt line shapes as well as calculated from the viscosity and diffusion data-a quantity shown independent of molecular weight as expected from the theory. Using the successful description of Jt in terms of ERT in the rubber(like)-to-fluid region as the reference frame in time, the glassy-relaxation process microG(t) that occurs in the small-compliance short-time region of J(t) can be studied in perspective. As shown from the analysis in terms of a stretched exponential form for microG(t) incorporated into ERT, the temperature dependence of the energetic interactions-derived microG(t) process being stronger in a simple manner than that of the entropy-derived ERT processes accounts fully for the uneven thermorheological complexity occurring in J(t) as initially observed by Plazek. When the results of analysis being displayed in the G(t) form, the relative roles of the energetic interactions-derived dynamic process and the entropy-derived ones in polystyrene are clearly revealed. It is shown that at the calorimetric glass transition temperature (Tg) the contribution from energetic interactions among segments to G(t) at the time scale of the highest Rouse-Mooney normal mode greatly exceeds that derived from entropy, indicating vitrification at the Rouse-segmental level. At the same time the Rouse-Mooney normal modes provide an internal yardstick for estimating the characteristic length scale of a polymer at Tg, giving approximately 3 nm for polystyrene. On the basis of the obtained results, the basic mechanism for the thermorheological complexity occurring in polystyrene is analyzed. It is shown that this basic mechanism should be also responsible for the breakdown of the Stoke-Einstein equation in relating the translational diffusion constant and viscosity as observed in glass-forming liquids, such as OTP and TNB, in approaching Tg from above.
Lin
Y-H
YH
Department of Applied Chemistry, National Chiao Tung University, Hsinchu, Taiwan. yhlin@mail.nctu.edu.tw