As “working range” or “useful working range” in dielectric and/or mechanical loss measurements is characterized the range between two transitions namely α- and β- ones in polymers. Many uses of polymers require considerable rigidity without brittleness. This behavior is often evidenced in a temperature range below the- transition but above subsidiary processes such as the β- transition. Depending on the special use of a polymer the size of this range should be small or large. Which are the factors affecting the size and the shape of this part of loss curve and how this can be controlled in a desirable way?
In amorphous polymers the alpha transition belongs to the glass transition, while the beta transition to some local motion (it can be side group motion as in polyacrylates and methacrylates, or short segment motion in polymers not having rotating side groups, e.g. polyolefins). Amorphous polymers exhibiting significant beta relaxation usually have good impact strength (e.g PMMA or polycarbonate). Below the beta transition these motions are frozen, therefore they cannot absorb impact energy, the material becomes brittle. In semicrystalline polymers the "safe" range (leathery behavior) is between the crystalline melting point and the glass transition of the amorphous phase. There are several excellent books on the relaxation transitions of various polymers. Here I mention only two old ones: Mc Crum, Read, Williams: Dielectric and Anelastic Effects in Polymers and Hedvig. Dielectric Spectroscopy of Polymers.
In amorphous polymers the alpha transition belongs to the glass transition, while the beta transition to some local motion (it can be side group motion as in polyacrylates and methacrylates, or short segment motion in polymers not having rotating side groups, e.g. polyolefins). Amorphous polymers exhibiting significant beta relaxation usually have good impact strength (e.g PMMA or polycarbonate). Below the beta transition these motions are frozen, therefore they cannot absorb impact energy, the material becomes brittle. In semicrystalline polymers the "safe" range (leathery behavior) is between the crystalline melting point and the glass transition of the amorphous phase. There are several excellent books on the relaxation transitions of various polymers. Here I mention only two old ones: Mc Crum, Read, Williams: Dielectric and Anelastic Effects in Polymers and Hedvig. Dielectric Spectroscopy of Polymers.
HI Dr Gyorgy Banhegyi, thank you for your response. Actually there is a lot of work about the transitions themselves. What I am looking for, putting this question, is to activate people concentrating their interest on the “useful working range” and the responsible mechanisms. I would extremely appreciate having your ideas-opinions about it.
I agree with Gyorgy that for amorphous and semi-crystalline polymers, the practical alpha and beta is different.
Generally it is not easy to change or control this range. When adding plastizers, we can shift Tg (alpha relaxation) to lower temperature, but doesn't shift beta relaxation, or just lower the beta relaxation intensity ( antiplastizers can suppress the loss peak of beta).
When we add some nanoparticles with strong interaction with polymers, usually both alpha and beta relaxation shift to high temperature and alpha relaxation is more likely to shift more. That means we can make a bigger range by adding nanoparticles. See papers: http://pubs.acs.org/doi/abs/10.1021/mz200253x
Another way could be used is polymer blending. Professor J Runt in Penn State University in the US has many papers about using polymer blend to control the beta relaxation.
It can be controlled by post curing the polymer in higher temperature
Dear all,
for Dr. Alisa Buchman. I think you mean annealing instead of curing, since most conveniently is to talk about these transitions in the case of thermoplastics. Regards
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