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 ?
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.
Sheets are usually made by film blowing, or some other technique which involves uniaxial or biaxial stretching followed by rapid cooling.
The cooling (and, usually, crystallisation) of the material is usually so fast that the polymer chains have no chance to relax from their elongated state.
In the film or sheet, the chains are now frozen in in their oriented, stretched state, which strongly increases the toughness of the film. If you heat it again, so the microcrystallites melt, the film relaxes into its next best entropically favoured form, which is a wrinkly ball.
Different plastic types may behave differently, like thermoplastic and thermoset plastic. Thermoplastic sheet is made by melting the plastic. It further classifies into amorphous and semi crystalline type. Amorphous material doesn't have a sharp melting point. Semi crystalline material has a sharp melting point. Thermoset material like phenolic is not melted to form. For thermoplastic amorphous material, like ABS, acrylic, PVC, polycarbonate, etc, I think it depends on how you heat it up. If you heat it up from one side, the heat distribution is not even. One side is hotter and one side is cooler. One side then has a greater thermal expansion than another. The difference in thermal expansion creates an internal stress. When the temperature gets higher, the plastic goes softer. Until the plastic structure is weaken to a point that it cannot hold the stress, the plastic sheet bends. Another scanerio is the internal stress already built up during manufacture. It highly depends on the manufacture process, the material use and the thickness. When you heat it up (evenly), the material is anyway soften. When the plastic sheet is so soft that it cannot hold the internal stress. It bends. For thermoplastic semi crystalline material, like Polyethylene, It itself is so soft having a low flexural modulus. It does not have much intenal stress otherwise it will contract itself.. When you heat it up closed to the melting point, it turns to a liquid state. The surface tension makes a tendency for it to turn to a sphere. Hence it contracts. For thermoplastic sheet like phenolic, it behaves very different from thermoplastic when heated. It may bend a little but more often it burns.