Viscosity measurement of silicone oils based on diffusion rates in polydiphenylacetylene films

Polydiphenylacetylene derivatives with different alkyl side chains (methyl, ethyl, and n-propyl groups) via a silylene linkage (PDPA-C1, PDPA-C2, PDPA-C3) were prepared. The polymer films were examined as a solid-state sensor for measuring the viscosity of silicone oils based on the diffusion rate in the films. When the films were brought into contact with silicone oils, the fluorescence (FL) emission increased with time to reach an FL enhancing equilibrium. PDPA-C1 showed faster and more significant FL enhancement because of the larger free volume and higher degree of side phenyl stacking. The relationship between the time to reach FL enhancing equilibrium (t(eq)) and silicone oil viscosity (eta) was well expressed by a modified Forster-Hoffmann equation: log t(eq) = C + x log eta, where x is the probe-dependent constant. The slope x values, which indicate viscosity sensitivity, were 0.37 for PDPA-C1, 0.35 for PDPA-C2, and 0.36 for PDPA-C3 in thin films and 1.15 for PDPA-C1 in a thick film, indicating high viscosity sensitivity depending on film thickness. The relationship between probe FL intensity and fluid viscosity, typically expressed by the Forster-Hoffmann equation, was never established in this system. The viscosity determination method using the modified Forster-Hoffmann equation proved very useful for identifying unknown viscosities of commercial silicone oils.