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MPR: Cross-slot geometry

Extensional flow

The extensional flow of polymers can have a very strong influence on their flow properties in the molten state. However, it is usually more difficult to characterise properties for extensional flow than for simple shear. As such, in many characterisations, extensional flow is simply overlooked.

We have developed a simple apparatus to generate pure extensional flows in a controlled environment using the Multi-Pass Rheometer. It is capable of delivering precise flows through a known geometry. This special test section has been designed to generate a stagnation point using cross-slot flow. The pistons move towards and away from each other to force the polymer through a cross-slot geometry. Nitrogen pressure is applied to the side arms in order to force the polymer back into the system so that multiple experiments can be carried out on the same sample. The flow through the cross slot is controlled by pistons capable of delivering flowrates between 1 and 1,000 mm3/s. In the cross slot apparatus, this corresponds with maximum extension rates between 0.2 and 200 s-1.

Animation showing the operation of the MPR cross-slot.

Schematic diagram of the MPR cross-slot geometry

The resulting polymer flows can be characterised using pressure transducers inserted into the barrels of the rheometer and by observing the resulting optical birefringence patterns.

 Picture of the optical module containing cross-slot inserts mounted on MPR4

Photograph of MPR4 set up for a cross-slot flow experiment 

 The cross-slot flow birefringence pattern obtained from a sample of polydisiperse polystyrene

The flow birefringence pattern obtained from polydisperse polystyrene at a maximum extension rate of 4s-1

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