For an entangled solution of linear polystyrene (PS 545k; M = 545k) in dibutyl phthalate (DBP), the stress relaxation after constant-rate uniaxial elongation was examined with an extensional viscosity fixture mounted on ARES (TA Instruments). The PS concentration, c = 52 wt%, was chosen in a way that the entanglement density M/Me of the solution coincided with that of PS 290k melt (M = 290k). After the elongation at the Rouse-based Weissenberg number Wi(R) ~ 3 up to the Hencky strain of 3, the short time stress relaxation of the solution was accelerated by a factor of ~4, which was less significant compared to the acceleration (by a factor of ~ 10) noted for PS melt under a similar condition. This result, being in harmony with the mild acceleration found for a 52 wt% blend of PS 545k with an oligomeric styrene (M = 1k), suggested that the segmental friction is reduced when the chain is highly stretched and oriented but this reduction weakens on an increase of the concentration of un-stretchable solvent molecules. This change of the stretch/orientation reduction of the friction with the solvent concentration appears to be consistent with the monotonic thinning of the steady-state elongational viscosity seen for melts and the lack of monotonic thinning observed for the semidilute solutions. Results for less concentrated solutions will be also presented on site.
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85th Annual Meeting of The Society of Rheology, 2013