Correlation of the Viscosity and the Molecular Weight of Silicone Oils with the T₂ NMR Relaxation Times

Nuclear magnetic resonance (NMR) was used to determine the T₂ NMR relaxation times of silicone oils. T₂ characterizes the molecular mobility of fluids. The T₂ times, furthermore, correlate with both the zero-shear-rate viscosity and the molecular weight of the corresponding silicone oil. Thus NMR provides the possibility of determining simultaneously the zero-shear-rate viscosity and the molecular weight of silicone oils by means of T₂ experiments, A comparison of rheological NMR experiments and experiments for the determination of the molecular weight show that there are correlations between the zero-shear-rate viscosity, the T₂ relaxation time and the molecular weight of silicone oils. The temperature was varied from -20 to 70 °C, the molecular weight from 650 to 250 000. A critical molecular weight of approximately 20 000 can be derived from the dependence of q o and M , for the silicone oils studied (Fig. 3). This fact is less pronounced in a T₂-M_w plot (Fig. 2 ). For a rough estimate of the molecular weight the simple potential fit in Fig. 2 can be used. Due to the existence of the critical molecular weight, however, a piecewise fitting of the low and high molecular parts of the curve is recommendable. The T₂ experiment should be performed at temperatures sufficiently below 0 °C to increase the accuracy of the measurement. Summarizing the findings presented, it can be concluded that both the molecular weight and the viscosity of silicone oils could be determined - after calibration - with the help of a less criticaland fast Tand fast T2 experiment. This possibility could be quite helpful for process and quality control for the production and handling of silicone oils. experiment. This possibility could be quite helpful for process and quality control for the production and handling of silicone oils.