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Well my book says that on heating- amorphous solids crystalline at some temperature. As far as I know , amorphous solids have a very disordered structure compared to crystalline solids and on further heating the level of disorder should increase. So why do they become more ordered or crystalline at high temperatures or am I missing something fundamental in here?

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What they say in your book is right. Take the case of polyethylene terephthalate (PET) which is a kind of polyester used in water bottles for example. It's an amorphous polymer (remember the bottles are transparent). When it is heated, the strained chains have the possibility to move and reorder themselves and the polymer becomes partially crystalline. This fact can readily be proved experimentally by heating a piece of PET (over its glass transition temperature, around $74 ^0\mathrm{C}$), then quenching the sample. It becomes white and not transparent which means it's partially crystalline. We can monitor this transformation using a specific instrument called differential scanning calorimeter, where we can notice the formation of an exothermic peak corresponding to crystallization.

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    $\begingroup$ But why does this happen? Isn't it violating thermodynamic principles? $\endgroup$ – Karan Singh Apr 3 '15 at 19:33
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    $\begingroup$ Of course not. You can imagine the polymer chains entangled and frozen in an amorphous state. The role of heating is to liberate these entangled chains, so some parts of chains are arranged in regular order, these regions are called crystalline regions. In between these ordered regions chains are randomly oriented. I agree this is weird. But this property is specific of long chains polymers $\endgroup$ – Yomen Atassi Apr 3 '15 at 19:43

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