What Rules the Melting Point of Semi‐Crystalline Polymers?

Abstract

The melting point Tm of semi-crystalline polyethylene terephthalate is correlated with the thickness Ic/nm of the crystalline lamellae by the unique relation l /T, = 1/594.5 + 0.001759/Ic. 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-I per crystallyzed segment and the corresponding entropy change 30.5 JKmol'l. 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. THE PROBLEMWhen polymers crystallize, they do it in a much more complicated way than low molecular crystals. In particular thermal treatment and annealing can modify the melting temperature T , .Moreover DSC measurements have shown that in some cases the melting at a given temperature is followed by a recrystallization and a second melting at higher temperatures.The origin of this complexity lies in the fact that polymer molecules only very seldom crystallize in their full length, as do for instance the low molecular alkanes. The most encountered type of crystallization, found for instance in polyethylene, polyacrylonitrile, polypropylene, polyoxymethylene, polyvinylalcohol, polyethylene terephthalate etc. is that where the polymer chains adopt a folded configuration. The chain regularly re-enters crystal zones. Such behaviour leads to the formation of a lamellar structure first considered by Keller [l], where crystalline lamellae are formed by neighbouring sequences of the chains that have acquired rigidity and are separated from each other by amorphous zones containing the chain-folds, the chain-ends and the interlamellar links (consisting of noncrystallized parts of a given chain, the other parts of which belong to two different lamellae) (FIG. 1).Important characteristics of such lamellar type of crystallization are the long spacing L separating the basis of two successive lamellae, that can be found by small-angle X-ray diffraction, and the fraction of crystallinity xc which can be derived among others from the density.The average thickness Ic of the crystalline layers and the average thickness la of the amorphous layer are then given byIn general Ic and la are of the order of a few nm. The lateral dimensions of the lamellae are much larger and reach in general several micrometers. Both Ic and la can be modified by annealing. Moreover it has been observed that, at a given crystallization temperature, Ic often exhibits a slight spontaneous increase after the initial formation of the lamellae.From a thermodynamic point of view the first fundamental question is that of the influence of Ic and la on the melting temperature...

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“…Also, Δ H ° m and σ e are the melting enthalpy of an ideal polymer crystal per unit volume which is equal to 171 j/cm 3 , 67 and the surface free energy per unit area, correspondingly. Huyskens and coworkers rewritten Equation for semi‐crystalline PET as Equation 68 . According to the obtained results of this equation (Table 5), the thickness of crystal decreased with increasing the content of chain extender. …”

“…Huyskens and coworkers rewritten Equation (2 for semi-crystalline PET as Equation ((3. 68 According to the obtained results of this equation (Table 5), the thickness of crystal decreased with increasing the content of chain extender.…”

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