The effect that heat has on polymers depends in part on what type of polymer it is. A thermoplastic polymer will be a crystalline solid below a certain temperature, then undergo a glass transition when it softens, then melts, and finally burns. A thermoset, or cross-linked, polymer will remain solid until it begins to char and burn.
Glass Transition Temperature
Solid polymers are usually semicrystalline. The long chains arrange themselves so portions are structured in crystals and others are loose. At a low temperature the loose, or amorphous, regions are glassy and brittle. As the heat increases, the polymer passes through the glass transition temperature and the chains in the amorphous regions are free to move, making the polymer more flexible. It is important that polymers in finished products remain above or below the glass transition over the expected temperature range of use so that the properties do not change. Additives can change the glass transition temperature.
With more heat, a thermoplastic polymer will reach the melting temperature. The melting only affects the crystalline regions. The polymer chains become unstructured. With all of the polymer chains, amorphous and moving, the polymer behaves like a liquid and can be recycled into new shapes. However, if the polymer is a thermoset, there are chemical bonds, or cross links, between the chains that keep them from moving apart. A thermoset polymer does not melt. A completely amorphous polymer does not melt but will become thinner and flow more easily with increased heat.
Above a certain temperature the chemical bonds in either the thermoplastic liquid or the thermoset solid will begin to break and small molecules will be emitted. Some polymers will burn with flames, others will emit black smoke, and others will only char. After time the polymer will be reduced to ash.
Critical Temperatures Depend on Polymer Chemistry
The glass transition, melting and degradation temperatures of a polymer depend on the type of polymer. All polymers are long chains of carbon atoms, but different types contain other chemical groups within the chain, which alters the expected behaviour. A polymer that contains molecular chains of various lengths will react more gradually and over a larger temperature range than a polymer with molecular chains of similar lengths. A polymer with short chains branching from the main chain will also behave differently than a linear polymer.