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

DoITPoMS Micrograph Library Full Record for Micrograph 572

Full Record for Micrograph 572

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Micrograph no
Brief description
Birefringence in a film of polypropylene
alignment, birefringence Link to MATTER Glossary entry for birefringence, extrusion Link to MATTER Glossary entry for extrusion, film, polymer Link to MATTER Glossary entry for polymer, polypropylene (PP)
Polypropylene (PP)
Not specified
Standard codes
This film is made by biaxially stretching heated material during the extrusion process.
Polypropylene has good resistance to chemicals and to fatigue. It is widely used in food packaging in the form of rigid tubs, lids and screw-tops as well as in flexible (but inextensible) films.
Sample preparation
The packet has been torn open, to demonstrate its inextensibility
Cross-polarised light microscopy
Length bar
10 mm
Further information
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 deg C for PP, ~-30 deg C for PE). Chain alignment and subsequent sliding is therefore much more sluggish and does not occur readily under an applied stress.
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