Dissemination of IT for the Promotion of Materials Science (DoITPoMS)

DoITPoMS Micrograph Library Full Record for Micrograph 568

Full Record for Micrograph 568

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Micrograph no
Brief description
Strain induced birefringence in a thermoformed PET cup
alignment, birefringence Link to MATTER Glossary entry for birefringence, cup, drawing Link to MATTER Glossary entry for drawing, polyester, polyethylene terephthalate (PET), polymer Link to MATTER Glossary entry for polymer, strain Link to MATTER Glossary entry for strain, thermoforming
Polyethylene terephthalate (PET)
Not specified
Standard codes
The cup is formed by thermoforming; pressing a plug into a heated sheet of PET
The principle use of poly(ethylene terephtalate) is in the packaging of soft drinks. The resulting bottles are shatterproof, impermeable to the carbon dioxide (PET is amorphous), and can withstand pressure.
Sample preparation
Cross-polarised light microscopy
Length bar
8 mm
Further information
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 deg 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).
J A Curran
Department of Materials Science and Metallurgy, University of Cambridge
Licence for re-use
Attribution-Non-Commercial-Share Alike 2.0 UK: England & Wales