Quasi-elastic neutron scattering and NMR investigation of the molecular dynamics in bitumen compounds

Abstract

The preliminary results of the combined NMR and neutron scattering investigations on bitumen compounds are reported. The goal of the study is to bring information about the molecular dynamics in various phases of these compounds and on this basis to test and/or improve the microscopic models for them. The quasielastic peaks in the neutron scattering spectra give direct information about diffusion motions via the linewidth dependence on the transfer momentum k/sup 2/. On this basis we calculate an overall diffusion constant D/sub n/ = 1.26*10/sup -7/ cm/sup 2//s, at T = 293K. The temperature dependence of the NMR spin-lattice relaxation time is well described by two V-type relaxation mechanisms that can be associated with two simultaneous and independent reorientational molecular motions, characterized by the correlation times /spl tau//sub c/(1) = 20/spl middot/10/sup -9/ s and /spl tau//sub c/(2) = 0.44/spl middot/10/sup -9/ s (T = 293K). Combining the NMR output /spl tau//sub c/(1) and /spl tau//sub c/(2) and the neutron scattering output (D/sub n/), it was possible to determine the upper limits for the radii of the molecular fragments involved in these motions: a/sub 1/ = 10/spl Aring/ and a/sub 2/ = 2/spl Aring/. The first value is in good agreement with the neutron incoherent elastic structure factor data which predicts a molecular scattering "globule" of radius d /spl cong/ 9/spl Aring/.