Cross-linked polymers equilibrium melting temperature

The premise that the crystalline-liquid transformation in polymers possesses all the characteristics of a first-order phase transition can be subjected to further testing. Predictions can be made with respect to the influence of added species, either low molecular weight or polymeric, the incorporation of comonomers, cross-linking and chain orientation, on the equilibrium melting temperature and the crystallinity level. The analysis of such systems, following phase equilibrium theory, will be given in the following chapters. It will be found that these apparently diverse subjects can be treated from a unified point of view. 

If the network structure is such that the crystallization of the cross-linked units is not restricted, AH and A5 can be taken to be independent of the fraction of units cross-linked. Under these conditions, A5 is identified with the entropy of fusion of the pure non-cross-linked polymer, and the ratio of A5 to AH is identified with the equilibrium melting temperature of the pure polymer. If, however, steric requirements are such that cross-linked units are excluded from the crystalline regions, an alteration will occur in these quantities. The presence of cross-linked units in the molten phase and not in the crystalline phase results in an increase in A 5 (when compared with the non-cross-linked polymer) of an amount Rp per mole of chain units. The melting temperature must accordingly be depressed for this reason, as long as AH is unaffected by the presence of cross-links. 

Semicrystalline polymers show a more complicated DMA picture. It was indicated in Fig. 6.4 that the glass transition results only in a partial softening because of the high level of cross-linking due to the crystals. Only above the melting temperature is the fully liquid state reached. A typical torsion pendulum result, obtained with an instrument similar to that in the sketch of Fig. 6.18, is shown in Fig. 6.24 for linear polyethylenes of different crystallinity. Three regions of maxima in G and tan S can be found below the equilibrium melting temperature of 414.6 K. Customarily these relaxations are called a, p, and y. A detailed interpretation of such DMA data has been... 

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