Photon-correlation velocimetry of polystyrene solutions in extensional flow fields

A cross slot device has been used to produce elongational flow fields in dilute solutions of narrow fractions of high molecular weight ($\text{M}\text{?}{}_{\mathrm{w}}$ between 2.8 × 10⁶ and 2 × 10⁷) atactic polystyrene. Strainrates obtainable were sufficient to extend individual molecules. Photon-correlation microvelocimetry has been used to measure velocity profiles in a channel of the cross slot as a function of distance from the centre of the cross, polymer concentration and flow rate. It has been found that in solutions of concentration about or greater than $\text{c1}$ and at flow rates sufficient to extend the polymer molecules a minimum in velocity is observed at the centre of the channel. This minimum sets in at flow rates around that at which extension of the polymer molecules is first observed through birefringence observations, but the depth of the minimum decreases rapidly with decreasing polymer concentration below $\text{c1}$. The velocity profile evolves into the usual parabolic profile away from the region of elongational flow. At higher strain rates a birefringence in the solution near the channel walls was also observed and preliminary measurements indicate that two symmetrical minima or shoulders in the velocity profile may be associated with this observation.