Free energies of fusion, and derived thermodynamic properties, are reported for a series of pure long chain n-alkanes, containing from 19 to 42 carbon atoms, in the temperature range --35OC+ melting point. Orthorhombic, monoclinic (for which the sub-cell is orthorhombic), triclinic, and hexagonal phases are investigated and the properties of the corresponding polyethylene structures extrapolated using the Flory-Vrij equation. The orthorhombic form (extrapolated m.p., 141~6°C) is always stable under normal conditions but the triclinic modification is only 3-4 J g-l less stable and, once formed, will remain indefinitely at all temperatures until these are such that there is considerable lattice mobility.PHASE BEHAVIOUR OF n-ALKANES AND POLYETHYLENE together with (l), the solid heat capacity c * , ~ at rounded temperatures. The first three of these quantities are reproduced with negligible loss of accuracy over the whole temperature range with the exception of the region (ca. 10°C) immediately below the transition point (fig. 2), where AH and A S are experimentally less than calculated owing to pre-melting effects. The effects virtually compensate for AG. TABLE 1.-FREE ENERGY AGN OF FUSION OF VARIOUS n-ALKANES A 2 x 104 A3 X 106 J g-1 deg.-Z J g-1 deg.-3 At J 0-1 deg.-1 structure N A0 J g-1 triclinic 22 33.74 -*7398 -9.12 3-62 24 38-12 -*7290 -9.54 3.74 19 22.04 -*7411 -6.26 7-00 orthorhombic 23 33-58 -a7205 -8.19 4.51 42
Results are presented of parallel calorimetric and dilatometric experiments on several meltcrystallized polyethylenes, with various thermal histories, over the range -35" +liquidus. Enthalpic and weight fraction degrees of crystallinity are in good agreement up to 6O"C, where direct comparison with n-alkane data is possible. Hence the interfacial surface enthalpy of the crystallites is small and there is evidence to show such behaviour continues to the melting point. The derived heat of fusion of perfectly crystalline polyethylene exhibits considerable curvature at low temperatures ; at the melting point it is 307 J g-l, some 5 % above the highest previous estimate. The temperature dependence of the free energy of fusion indicates an extended chain melting point of 141 f 1°C.
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