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Why do thin plastic sheets contract when heated, contradictory to the behavior of most other materials ?

What are the things going on at the molecular level ?

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  • $\begingroup$ what material are you considering? $\endgroup$ – permeakra Feb 8 '16 at 7:03
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When plastic sheets are produced, they are rapidly cooled to keep the polymer chains oriented in a way that makes the sheets nice and flat. This is a relatively high-strain orientation since it is associated with the energy level of the molecules at the casting temperature.

Once the plastic is heated above its glass transition temperature, the polymer chains are no longer locked in that high strain orientation. They relax to a low energy orientation- curled and bending in a way that shrinks the bulk material.

As for the precise mechanism, I'm not exactly sure. I would guess that the shrunken conformation is entropically favorable because there are more arbitrary bends. This would decrease the Gibbs free energy, making it a more stable shape.

Alternatively, it could be that hydrogen bonding between chain elements makes the folded shape more enthalpically favorable.

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    $\begingroup$ Just for fun I heated up some yoghurt containers ... they all flattened out to the flat sheet of plastic they had been before they were heat-pressure deformed into cups. $\endgroup$ – Gyro Gearloose Feb 11 '16 at 21:28
  • $\begingroup$ It's the entropy for all polymers. Surface energy makes up a small additional term. $\endgroup$ – Karl Jun 28 '16 at 22:02
  • $\begingroup$ It's not glass transition, but melting of nanocystallites, which crosslink the sheet, stabilising it. The Tg of Polyethylene is below room temperature, and your saran wrap would be very brittle if it had to be used below its Tg. $\endgroup$ – Karl Jun 28 '16 at 22:04

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