Image icon  Strain induced birefringence in a thermoformed PET cup

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Description: The base of the cup is the least strained part of the sheet. Nevertheless, residual strain, increasing with radius, is evident when the specimen is viewed between crossed polars. The polymer chains are more highly aligned where the strain is greatest and this leads to greater birefringence (rotation of polarised light). Hence a circumferential pattern of colours is observed. If heated above the glass-transition temperature of PET (70-80 °C), the cup will tend to retract towards its unstrained form of a sheet. The dark 'Maltese cross', spreading out horizontally and vertically from the centre, indicates the extinction directions or the orientations of the two polarising films, where the intensity of transmitted light is lowest. Note: most clear plastic cups are made from polystyrene (PS).
Keywords: alignment • birefringence • cup • drawing • polyester • polyethylene terephthalate (PET) • polymer • strain • thermoforming
Categories: Science approaches > Microstructure
Materials > Polymers & elastomers > Plastics > Thermoplastics > Polyethylene (PE)
Testing, analysis & experimentation > Metallography
Scale > Micro
Created by: DoITPoMS, University of Cambridge
J A Curran, Department of Materials Science and Metallurgy, University of Cambridge
Published by: DoITPoMS, University of Cambridge
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Date created: 03 October 2002
Date added: 21 August 2009
Resource ID: 1601