Polyethylene nonequilibrium melting

Polyethylene data are shown in Fig. 2.23. At the equilibrium melting temperature of 416.4 K, the heat of fusion and entropy of fusion are indicated as a step increase. The free enthalpy shows only a change in slopes, characteristic of a first-order transition. Actual measurements are available to 600 K. The further data are extrapolated. This summary allows a close connection between quantitative DSC measurement and the derivation of thermodynamic data for the limiting phases, as well as a connection to the molecular motion. In Chaps. 5 to 7 it will be shown that this information is basic to undertake the final quantitative step, the analysis of nonequilibrium states as are common in polymeric systems. 

Figure7.11 illustrates the irreversible behavior ofthe same TCB and polyethylene, but under nonequilibrium conditions. The DSC data were collected after quenching to 354 and 388 K for crystallization. Comparing with Fig. 7.10 shows reasonable agreement for TCB. For polyethylene, crystallization at the higher temperature, 378 K, raises the eutectic and melting temperatures relative to crystalUzation at 354 K. The coohng traces, in contrast, show even lower crystallization temperatures for the polymer and at higher concentration, the TCB phase diagram is also affected. It is of particular interest that both the eutectic temperatures and the eutectic concentrations...

An experimental phase diagram of polyethylene dissolved in 1,2,4,5-tetrachlorobenzene (TCB) is shown in Figure 16b (132). Both polymer and low molar mass solvent have similar equilibrium melting temperatures. On the left-hand side of the phase diagram, the liquidus line follows equation 47, the right-hand side does not follow equation 46. Again, this indicates the usual nonequilibrium state of polymer crystals. Besides too low melting temperatures, low... 

Finally, the bottom left curves show that above about 260 K, melting of small, metastable crystals causes abnormal, nonlinear deviations in the heat capacity versus crystallinity plots. The measured data are indicated by the heavy lines in the figure. The thin lines indicate how continued additivity would look. The points for amorphous polyethylene at the left of the figure represent the melt and agree with the extrapolation of the measured heat capacities from the melt. All heat capacity contributions above the thin lines must thus be assigned to nonequilibrium melting, as was discussed in Sect. 4.7.1. 

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