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MPR: Optical Birefringence Measurements

One of the advantages of the MultiPass Rheometer is that optical birefringence measurements can be conducted at the same time as a sample is sheared. This technique utilises a specially designed optical test module. Metal inserts are slotted into the test module to form the slit through which the sample will flow. Stress-free quartz windows are then inserted at right angles, allowing the flow to be observed.


 Diagram of the MPR optical module and its inserts Example MPR optical module inserts 
Diagram showing the different components of the optical test module Photograph of a pair of optical module inserts

Optical Birefringence generates a 'contour map' of stresses observed by taking advantage of the optical anisotropy of polymers under stress. To obtain an optical birefringence pattern, the flowing sample is illuminated with monochromatic light. A polariser and analyser, placed at 90° to each other before and after the sample, ensure that a "dark field" is obtained when the sample is unstressed (i.e. at rest).

 Digram of the optical apparatus used for birefingence studies with the MPR
 Diagram apparatus used to obtain stress birefringence patterns

The images are recorded by the camera and saved directly to a computer. The observed birefringence patterns can then be modelled using numerical simulation software such as Polyflow or flowSolve.


Optical apparatus used on MPR4   Birefringence pattern obtained from HDPE
Photograph of the optical apparatus set up on MPR4 The optical brirefringence pattern obtained from high density polyethylene flowing in the steady state through a contraction-expansion slit at 155°C. The flow direction is from bottom to top at an apparent wall shear rate of 12 s-1. The slit dimensions are: length = 1.5mm, width = 1.4mm depth = 10mm. The slit width contracts from 10mm to 1.4mm. Picture courtesy of Lino Scelsi.


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