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

DoITPoMS Micrograph Library Full Record for Micrograph 573

Full Record for Micrograph 573

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Micrograph no
Brief description
Birefringence in a film of polyethylene
alignment, birefringence Link to MATTER Glossary entry for birefringence, extrusion Link to MATTER Glossary entry for extrusion, film, polyethylene (PE), polymer Link to MATTER Glossary entry for polymer, polythene
Low density polyethylene (LDPE)
Not specified
Standard codes
This film is made by biaxially stretching heated material during the extrusion process.
Polyethylene is the most common polymer. Low density polyethylene (LDPE) is widely used as a film for packaging
Sample preparation
The packet has been torn open, to demonstrate residual chain alignement around the tear
Cross-polarised light microscopy
Length bar
14 mm
Further information
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 deg 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 deg C, it would relax towards its original configuration, and uniform colour would again be observed.
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