Nonlinear variation of optical absorption and rheological behavior with concentration in dispersed poly(vinyl pyrrolidone) of small molecules in water

Abstract

Interaction of poly(vinyl pyrrolidone) PVP with water molecules is of interest for possible drug applications and other chemical reactions. In this purview, aqueous solutions in 0.1–10 g/dl PVP were studied in terms of the optical absorption spectrum and rheology in this work. The rheology determines a polymer nature of the solutions in the selective concentrations (C). A strong absorption band, which occurs in an asymmetric shape at wavelength maximum λ max = 247 nm ( π → π ⁎ electronic transition) in 10 g/dl PVP, shifts nonlinearly to a λ max value as small as 222 nm upon decreasing the C-value. The λ max follows a parabolic path as a function of increasing the C-value in this range. The band narrows down from an average 55 nm bandwidth Δ λ in 10 g/dl PVP to a value 15 nm in the 0.1 g/dl PVP in predominant intramolecular interactions in selective conformers in dilute solutions. An opposite effect of red-shift and band broadening occurs in solute–solvent interactions. The viscosity thus increased hardly from a value 10 × 10 − 4 Pa s in the 0.1 g/dl PVP to as much as 37 × 10 − 4 Pa s (i.e., as large a factor ∼ 4) in the 10 g/dl PVP at a shear rate 100 s − 1 . A weak concentration-induced viscosification of dispersed PVP molecules in water is important for the medicinal values.