Linear and nonlinear shear studies reveal supramolecular responses in supercooled monohydroxy alcohols with faint dielectric signatures

Abstract

Monohydroxy alcohols (MAs) with methyl and hydroxyl side groups attached to the same carbon atom in the alkyl backbone can display very weak structural and supramolecular dielectric relaxation processes when probed in the regime of small electrical fields. This can render their separation and assignment difficult in the pure liquids. When mixing with bromoalkanes, a faint Debye-like process can be resolved dielectrically for 4-methyl-4-heptanol. To achieve a separation of structural and supramolecular processes in pure 4-methyl-4-heptanol and 3-methyl-3-heptanol, mechanical experiments are carried out in the linear-response regime as well as using medium-angle oscillatory shear amplitudes. It is demonstrated that first-order and third-order nonlinear mechanical effects allow for a clear identification of supramolecular viscoelastic modes even for alcohols in which they leave only a weak signature in the linear-response shear modulus. Additionally, the nonlinear rheological behavior of 2-ethyl-1-hexanol is studied, revealing that its linearly detected terminal mode does not coincide with that revealed beyond the linear-response regime. This finding contrasts with those for the other MAs studied in this work.