Extensional viscosities η e(ε•) of polystyrenes (PS) with high molecular weights in benzyl butyl phthalate (BBP; a good solvent for PS) and dioctyl phthalate (DOP; a θ solvent for PS) were studied in the dilute region for viscosity as a function of the extensional rate with an opposing jets extensional rheometer. The η e(ε•) data obtained with use of different orifice geometries agree with each other within experimental errors. The extensional viscosity at low ε• limit, η 0e is about three times larger than the corresponding zero - shear viscosity in accordance with Trouton's rule. In high ε• region, the extensional - thickening was observed for all the solutions. The critical extensional rate ε• * at which the η e(ε•) deviates from η 0e is confirmed to be related to the longest relaxation time of the solutions.
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