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Could be there material that begins melting/softening when it's temperature is lowered? I would say no, but I've seen enough physics to know that not always life is so easy.

Moreover I think I've heard something about it, but can't remember a thing.

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    $\begingroup$ Nothing melts when the temperature is lowered, but a solid material with a negative coefficient of thermal expansion might theoretically get softer $\endgroup$
    – Jim
    May 27, 2014 at 19:53
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    $\begingroup$ @Jim would a negative CTE imply it softens? Or just that it expands as it cools? Maybe the two are correlated, I've never really thought about it... $\endgroup$
    – tpg2114
    May 27, 2014 at 19:57
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    $\begingroup$ @tpg2114 I wouldn't say it implies that it softens, but I can't imagine anything that gets softer as it contracts. The only time I can imagine something getting softer is also when it expands (allows more cushioning of each molecule/atom), so I said negative CTE $\endgroup$
    – Jim
    May 27, 2014 at 20:04
  • $\begingroup$ @Jim I have the same intuition but since expanding in my head is usually accompanied by more degrees of freedom due to increased energy, I'm having trouble separating the two concepts... I'd be very interested to see a question and answer about the relationship between stiffness and CTE $\endgroup$
    – tpg2114
    May 27, 2014 at 20:15
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    $\begingroup$ The crust going soft relates to absorption of moisture (evaporating from inside the bread) followed by drying out (as the insides have cooled down and stop supplying moisture). I think that changes the material - it's "crust plus bread" not just "crust" any more. $\endgroup$
    – Floris
    May 27, 2014 at 20:33

3 Answers 3

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Materials that exhibit entropic elasticity will theoretically exhibit this effect. For example, elastomers that at a temperature far greater than the glass transition temperature.

In entropic elasticity, stiffness arises not from stretching mechanical bonds, but by decreasing the entropy of the polymer chains in the material. A derivation of this phenomenon is beyond the scope of this question, but here's the punchline:

The initial shear modulus of the material is proportional to the absolute temperature. This fits the bill for a material that "softens when it gets colder."

Here is a link to the Wikipedia page for rubber elasticity. This phenomenon is described in more detail here. http://en.wikipedia.org/wiki/Rubber_elasticity

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You may wish to google "retrograde melting" - some allows/mixtures do melt with cooling. See, e.g., http://xraysweb.lbl.gov/uxas/Publicatons/Papers/pdfs/hudelson%20adv%20mat.pdf

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There's a nitinol wire that stiffens when warm and softens when cool. It's been used in various patented heat engine applications. see this reference http://www.imagesco.com/articles/nitinol/09.html

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