Experimental test of the validity of the use of the n-alkanes as model compounds for polyethylene

n-Alkanes are widely used as model compounds for the dynamics of the crysta lline region of polyethylene; this use relies on the assumption of the tran sferability of the forcefield of the n-alkanes, a stringent test of this as sumption is a comparison of the dispersion curves of the n-alkanes with tho se of polyethylene; the only direct measurements of the dispersion curves o f polyethylene are for the nu(5) acoustic branch, the longitudinal acoustic mode (LAM), an in-plane skeletal mode of perdeuterated polyethylene; in th e present work we have observed (by inelastic neutron scattering spectrosco py) and assigned (by the use of density functional theory calculations) the LAM modes of perdeuterated n-hexadecane in order to provide the first expe rimental test of the validity of the assumption.