Image icon  Birefringence in a film of polyethylene

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Description: The colours in the image are the result of birefringence and relate to the residual stress in the film. The colour is generally uniform (with some contrast where an additional thickness of film exists or where wrinkling has resulted in a different apparent thickness). This is indicative of both a uniform film thickness and of the uniformity of the drawing process used to make the film. In the region of the tear, however, the colours indicate strain-induced orientation: because the material is above the glass transition temperature of PE (-30 °C) chain sliding occurs readily under and applied stress and the molecules become aligned along the tensile axis. The strain is permanent and does not relax when the stress is removed. However, if the material were annealed at ~100 °C, it would relax towards its original configuration, and uniform colour would again be observed.
Keywords: alignment • birefringence • extrusion • film • polyethylene (PE) • polymer • polythene
Categories: Science approaches > Microstructure
Materials > Polymers & elastomers > Plastics > Thermoplastics > Polyethylene (PE)
Processes > Thermal treatments > Annealing
Testing, analysis & experimentation > Metallography
Scale > Micro
Created by: J A Curran, Department of Materials Science and Metallurgy, University of Cambridge
DoITPoMS, University of Cambridge
Published by: DoITPoMS, University of Cambridge
License: This resource is released under the Creative Commons Attribution Non-Commercial Share Alike license (2.0 UK: England & Wales).
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Date created: 03 October 2002
Date added: 21 August 2009
Resource ID: 1606