Melting is the transition between a crystalline solid and a liquid. The melting point of a small molecule is very well-defined at a given pressure. Water, for instance, melts at 0oC when P = 1 atm.
Polymers, on the other hand, do not have a single well-defined melting point. When a polymer "melts" it slowly becomes "leathery," then "tacky," and then liquid over a fairly broad temperature range42. The crystalline portion of the polymer is a nonequilibrium distribution of a large number of crystallites of different sizes and in different environments. They all do not melt at exactly the same temperature.
David Whisnant (Wofford College). Partial support for this work was provided by the National Science Foundation's Division of Undergraduate Education through grants DUE #9950809 and DUE #9950296. Additional support was provided by the Camille and Henry Dreyfus Foundation.