The defining property of a thermoplastic polymer is that it can be heated from a solid state to a viscous liquid state and then cooled back down to solid, and that this heating and cooling cycle can be applied multiple times without degrading the polymer:

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Thermoplastic polymers can be repeatedly melted and solidified by heating and cooling, which allows them to be reshaped multiple times without significant degradation of their properties. This reversibility is a key characteristic distinguishing them from thermosetting polymers, which cannot be melted and reshaped after their initial curing process.

The Defining Property of a Thermoplastic Polymer Is That It