NMR Self-Diffusion and Transverse Relaxation Time in Bitumen: The Effect of Aging

Abstract

In this investigation the dynamics of two types of bitumens with different penetration grade were tested by using dynamic shear rheometry (DSR) and Nuclear Magnetic Resonance (NMR) at unaged conditions, and upon both short- and long-term artificial aging. The gel-sol transition temperature $T_{gel\to sol}^{{}^{*}}$ was found to increase with increasing the time of aging treatment. Arrhenius parameters of the viscosity were found, unexpectedly, to be correlated with those of simple liquids, suggesting that the two kinds of systems, although chemically and physically quite different, share the same basic process at the molecular level. The molecular dynamics has been then investigated by NMR Pulsed Field Gradient Stimulated-Echo (PFGSE) and relaxometry (Carr-Purcell-Meiboom-Gill, CPMG, spin-echo pulse sequence) to capture the effect of aging upon dynamics variables such as self-diffusion coefficients D and transverse relaxation times T₂ . The translational diffusion at T> $T_{gel\to sol}^{{}^{*}}$ of the light molecular components of both types of bitumens was characterized by broad distributions of D which were found independent of the experimental time scale up to 0.2 s. Similarly, T₂ data could be described as a continuous unimodal distributions of relaxation times determined both at T< $T_{gel\to sol}^{{}^{*}}$ and T> $T_{gel\to sol}^{{}^{*}}$ .

Keywords: Gamma distribution; NMR relaxometry; NMR self-diffusion; aged bitumen; dynamic shear rheometry.