Polymer stiffness V temperature question

[opticsman]

Hi . Not my normal area as my name implies. I would like to know about temperature effects on polymers ( or plastics) , particularly commonly available polymers . I understand that a lot of such polymers become less stiff with temperature but is the reverse possible ( in a common polymer?) . In other words can some become more rigid at higher temperature? Many thanks in advance. Opticsman

 

[patprimmer]

It is possible if the heat causes cross links to form.

Regards

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[Demon3]

Normally a polymer will soften due to heating. These effects are well documented and you can find graphs for example on the net of how they will behave. An amorphous polymer will soften so much that it will not hold a load as it gets close to it's glass transition temperature (Tg) whereas a semi-crystalline polymer retains load bearing capability above Tg and all the way up to near the melting point (Tm). Addition of fillers, especially anisotropic ones like glass fiber substantially improve the high temperature load bearing capability of polymers.

As Pat mentioned, about the only way to increase modulus at high temperature is if you actually change the polymer chemically by heating, i.e. induce cross-links. For example an uncured epoxy (like glue) will harden a lot on heating due to the cure (cross-linking). Other ways are possible but not likely to be very useful in practice.

 

[Demon3]

You can find the modulus versus temperature for amorphous and semi-crystalline polymers here

http://www.azom.com/details.asp?ArticleID=83#_Amorphous_Polymers

 

[opticsman]

Hi Thanks demon and pat Very helpful. The plot of E versus temperature decreases with T on the curve i saw for the two types of polymer ( your link) . I guess what I am asking is can the plot of E fall against T for a polymer. Ideally as a reversible process?

 

[patprimmer]

Normally, only if you get cross linking. Cross linking is not normally reversible.

The examples are two pot adhesives, polyester resins used in the FRP industry, and thermoplastics going hard and brittle due to thermal ageing.

Another possibility is if a semi crystalline thermoplastic is moulded under conditions that produce amorphous mouldings, the E vs T will increase if the parts are heat treated to increase crystallinity. That is heated to softening point and cooled slowly.

Regards

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[HDS]

This behavior is typical of most materials including metals and ceramics. I don't know of a material that gets stiffer with temperture.