What Rules the Melting Point of Semi-Crystalline Polymers?

The melting point Tm of semi-crystalline polyethylene terephthalate is correlated with the thickness lc/nm of the crystalline lamellae by the unique relation 1/Tm = 1/594.5 + 0.001759/lc. This correlation is independent of the temperature of crystallization, the molecular weight or the annealing. This demonstrates that the melting point is ruled only by the characteristics of the crystalline parts. The molar heat of fusion is 18.4 kJmol−1 per crystallized segment and the corresponding entropy change 30.5 JKmol−1. Both are independent of lc. In the amorphous parts the two segments adjacent to the crystalline lamella have their entropy reduced by 15 J/Kmol as a consequence of the limitations in their possibilities of orientation. However this is a constant effect, independent of the thickness of the amorphous zones, possible entanglements or other characteristics of the amorphous parts.