Polymers contain different morphologies on the molecular level which give rise to their macro properties. Long range disorder in the polymer chain is representative of amorphous solids, and the chain segments are considered amorphous. Long range polymer order is similar to crystalline material, and chain segments are considered crystalline.
The thermal characteristics of polymers are fundamentally different from those of most solid materials. Solid materials typically have one melting point, the T, above which the material loses internal molecular ordering and becomes a liquid. Polymers have both a melting temperature T and a glass transition temperature T. Above the T, the polymer chains lose their molecular ordering and exhibit reptation, or mobility. Below the T, but still above the T, the polymer chains lose some of their long-range mobility and can form either crystalline or amorphous regions. In this temperature range, as the temperature decreases, amorphous regions can transition into crystalline regions, causing the bulk material to become more crystalline over all. Below the T, molecular motion is stopped and the polymer chains are essentially frozen in place. In this temperature range, amorphous regions can no longer transition into crystalline regions, and the polymer as a whole has reached its maximum crystallinity.
Hoffman nucleation theory addresses the amorphous to crystalline polymer transition, and this transition can only occur in the temperature range between the T and T. The transition from an amorphous to a crystalline single polymer chain is related to the random thermal energy required to align and fold sections of the chain to form ordered regions titled lamellae, which are a subset of even bigger structures called spherulites. The crystallization of polymers can be brought about by several different methods, and is a complex topic in itself.