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

Results are presented for the structural and thermodynamic properties of the orthorhombic phase of crystalline polyethylene (PE) from detailed atomistic molecular-dynamics (MD) simulations in the NPT statistical ensemble. Two different PE crystal systems have been investigated: (a) paraffins consisting 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 crystal for both systems. The infinite-chain systems, however, are characterized systematically by a higher density and a more regular structure, particularly at low temperatures, where all chains are found in their all-trans configuration. At temperatures close to 273 K, the finite-chain length crystals start presenting gauche defects whose number increases rapidly upon further increasing the temperature. No such gauche defects are observed for the infinite-chain length crystals at the range of temperatures (T < 300 K) investigated. 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 show a shift of the peaks of the CH2 symmetric and asymmetric stretching bands 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 bands, in approximate agreement with experiment. (C) 2001 American Institute of Physics.