A revised relationship between molecular weight and reduced angular frequency in δ-method applied to unmodified petroleum bitumens

The δ-method has been developed to estimate molecular weight distributions of bitumens from phase-angle master curves. The method uses a correlation between the cross-over frequency(ωC) and the molecular weight (MW) that was developed from average MW measured by vapour pressure osmometry. This study proposes a revised correlation whose parameters have been calculated from Gel Permeation Chromatography (GPC), which can be conducted at a low level of dilution preserving the interactions in the bitumen bulk. Furthermore, data from the GPC allow the optimisation of the parameters considering the entire MW distribution (MWD), thus improving the predictive capability of the method. Three unmodified petroleum bitumens at different levels of ageing and three rheological models were used in the optimisation. Two groups of parameters were identified; one for hard (e.g. Pen 35-50) and another one for soft-medium bitumens (e.g. Pen 50–70 or Pen 70-100). The apparent-MWDs (A-MWDs) calculated with the new parameters show improved correlation with the respective distributions and average molecular weight from GPC.