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.
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| 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).
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| 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.
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| 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. |