Image icon  Birefringence in a film of polypropylene

 
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Description: The colours in the image are the result of birefringence and relate to the residual stress in the film, following the biaxial stretching process. The uniformity of the colour (with contrast only where an additional thickness of film exists or where wrinkling has resulted in a different apparent thickness) is indicative of both a uniform film thickness and of the uniformity of the drawing process used to make the film. Note that unlike polyethylene, the film has not been permanently strained in the region of the tear. This is because the monomer is bulkier and the material is relatively close to its glass transition temperature (~8 °C for PP, ~-30 °C for PE). Chain alignment and subsequent sliding is therefore much more sluggish and does not occur readily under an applied stress.
Keywords: alignment • birefringence • extrusion • film • polymer • polypropylene (PP)
Categories: Science approaches > Microstructure
Materials > Polymers & elastomers > Plastics > Thermoplastics > Polypropylene (PP)
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
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Date created: 03 October 2002
Date added: 21 August 2009
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Resource ID: 1605