CONFORMATIONAL PROPERTIES OF LIQUID POLYETHYLENE

The conformational entropy Sc of linear polyethylene liquid is calculated at Tm = 414 K for the three-state rotational model taking into account the “pentane interference”. The volume contribution Sv is estimated using a volume-dependent (dp/dT)v which is derived via the Tait equation for liquid compressibility. The values are (cal deg-1 mol-1 CH2) Sc -1.295 and Sv =1.057, and the sum is in excellent agreement with the experimental fusion entropy for extended-chain crystals: ΔSm = 2.362. Previous studies using isolated or unperturbed chain values for the rotational energy have over-estimated Sc and based on a volume-independent (dp/dT)v have under-estimated Sv. From a consideration of the pressure dependence of Tm up to 2000 bars, it is found that both Sc and f, the fraction of flexed bonds, remain constant at Tm while ΔSm and Sv decrease. The “conformational volume” Vc associated with the rotational isomerization is estimated to be Vc = 4.31 cm3/mol of flexed bonds. At Tm, Vc constitutes 30% of ΔVm. For statistical mechanical calculations of the conformational properties, the simple rotational partition function due to Temperley is shown to be an adequate approximation to a more complex function derived later. © 1979, American Chemical Society. All rights reserved.