Browsing by Author "Juang, JH"

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Chain dynamics of concentrated polystyrene solutions studied by depolarized photon-correlation and viscosity measurements
(1999-05-08)
Lai, CS
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Juang, JH
;
Lin, YH
;
應用化學系
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Department of Applied Chemistry
Concentrated solutions of nearly monodisperse polystyrene samples in cyclohexane in the theta condition and in the entanglement-free region have been studied by means of the depolarized photon-correlation spectroscopy and viscosity measurements. It is shown that the viscoelastic behavior of the studied systems is described by the Rouse theory and that in agreement with the theoretical analysis the main polymer dynamic process probed by the depolarized Rayleigh scattering is basically the reorientational motion associated with a Rouse segment of the polymer chain, whose relaxation is independent of the scattering angle and the molecular weight. In addition to the main dynamic process, the tail region of a very fast process associated with the sub-Rouse-segmental motions can be observed, whose existence is expected from the theoretical analysis. (C) 1999 American Institute of Physics. [S0021-9606(99)51817-5].
Onset of entanglement
(1999-01-12)
Lin, YH
;
Juang, JH
;
應用化學系
;
Department of Applied Chemistry
The onset of entanglement was determined from studying the viscoelastic spectra of the blends consisting of two nearly monodisperse polystyrene polymers: component one having a molecular weight slightly less than the entanglement molecular weight M-e (=4 rho RT/5G(N)), whereas that of component two being greater than M-e (such a blend is referred to as the blend solution). It is shown that because of the absence of the hydrodynamic interaction the viscoelastic spectrum of the blend solution can be described by applying the Rouse theory to both components in the entanglement-free region. As entanglements among the chains of component two occur, when its weight fraction, W-2, increases above a critical point, the viscoelastic response of component one remains described by the Rouse theory. With the Rouse viscoelastic response of component one as the internal reference, the onset of chain entanglement among the chains of component two can be determined by monitoring the deviation of the viscoelastic response of component two from being described by the Rouse theory as a function of increasing mil. According to the study, entanglement starts to occur in the close neighborhood of M-e' (the entanglement molecular weight of the blend solution and M-e' = MeW2-1) for the polymer blend solution or equivalently M-e for the monodisperse polymer melt well below M-c' or M-c (M-c' = McW2-1, where M-c is the critical molecular weight of the zero shear viscosity of the polymer melt).