Detailed atomistic molecular-dynamics simulation of the orthorhombic phaseof crystalline polyethylene and alkane crystals

Results are presented for the structural and thermodynamic properties of th e orthorhombic phase of crystalline polyethylene (PE) from detailed atomist ic molecular-dynamics (MD) simulations in the NPT statistical ensemble. Two different PE crystal systems have been investigated: (a) paraffins consist ing of an odd number of carbon atoms such as C23H48 and (b) infinite length PE monocrystals, at temperatures ranging from T=150 to 298 K. The results support the experimentally verified chain herringbone arrangement in the cr ystal for both systems. The infinite-chain systems, however, are characteri zed systematically by a higher density and a more regular structure, partic ularly at low temperatures, where all chains are found in their all-trans c onfiguration. At temperatures close to 273 K, the finite-chain length cryst als start presenting gauche defects whose number increases rapidly upon fur ther increasing the temperature. No such gauche defects are observed for th e infinite-chain length crystals at the range of temperatures (T < 300 K) i nvestigated. In a second step, the atomistic, crystalline PE configurations accumulated during the NPT MD runs are analyzed within the quasi-harmonic approximation to obtain their infrared (IR) vibrational spectra. IR spectra of the infinite-chain length crystals obtained at different temperatures s how a shift of the peaks of the CH2 symmetric and asymmetric stretching ban ds to lower wave numbers with increasing temperature. No shift is observed for the vibrational wave numbers of the other bands. The wave number shift of the CH2 symmetric and asymmetric stretching bands is found to be related linearly to temperature with a slope equal to -0.033 cm(-1)/K for both ban ds, in approximate agreement with experiment. (C) 2001 American Institute o f Physics.