Methyl branches in hydrocarbon crystals: Calculation of relaxation parameters

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Abstract
A site model has been used in conjuction with potential-energy calculations to examine the role of isolated molecules with methyl branches in the mechanical relaxations of a linear hydrocarbon host crystal. The results indicate that there are two possible relaxation modes and that the one involving molecular rotation is energetically favorable over the one involving rotation and translation. For some modes of deformation, the calculated relaxation strengths are comparable to the experimentally measured ones. Furthermore, the barriers determined in this work yield calculated curves of the logarithmic decrement as a function of temperature which are comparable to the experimental ones. The relaxation is much weaker for unbranched chains in the planar zigzag conformation.