Heat capacities of paraffins and polyethylene

Abstract

The heat capacities of the homologous series of paraffins from n-propane to polyethylene in the solid state are analyzed by use of the advanced thermal analysis system (ATHAS) developed for linear macromolecules. While the skeletal heat capacity contributions linked to the intramolecular vibrations follow a simple functional relationship with the number of chain atoms ({Theta}{sub 1} = 519 {minus} 3,508/n{sup 2}), one detects for the heat capacities linked to the intermolecular vibrations a clear odd/even fluctuation ({Theta}{sub 3}{sup odd} = 158 {minus} 116.7/{radical}n; {Theta}{sub 3}{sup even} = 158 {minus} 103.6/{radical}n). All prior measured data are included in the analysis in addition to new data on n-hexatriacontane (C{sub 36}H{sub 74}), n-tetratetracontane (C{sub 44}H{sub 90}), and n-pentacontane (C{sub 50}H{sub 102}). Similarly, the heat capacities in the liquid state can be generated within a precision of {plus minus} 1.7% (rms error), using empirical contributions from the CH{sub 2} and CH{sub 3} groups (C{sub p}{sup CH{sub 2}} = 17.33 + 0.04551T; C{sub p}{sup CH{sub 3}} = 30.41 + 0.01479T). With heat capacities of the solid and liquid known, the transitions and thermal functions H,S, and G can be derived and extrapolated to paraffins not measured. For all homologues a large increase in heat capacitymore » beyond that expected from the contributions of vibrations is observed 50-100 K below the fusion temperature. This increase in heat capacity is shown to be in line with the gradual increase in conformational disorder proven by polyethylene crystal simulation using supercomputers.« less