Humidity affects the viscoelastic properties of supramolecular living polymers

Abstract

We investigate the linear and nonlinear viscoelastic response of a supramolecular gelator formed by association of a bisurea monomer (EHUT) via hydrogen bonds, in a nonpolar organic solvent (dodecane). Past experimental studies reported contradictory results concerning the concentration dependence and the magnitude of the terminal relaxation time. The discrepancies among data were attributed to the presence of water and, in particular, its chain stopper effects on the EHUT supramolecular assembly. Here, we present new measurements with this system under conditions of controlled humidity, achieved by means of a simple setup developed for this purpose. We resolve the discrepancy and demonstrate that humidity (both during sample storage and measurement) can substantially affect the linear viscoelastic response (frequency spectra). The magnitude of the terminal relaxation time, τt, is significantly reduced (by as much as 1 order of magnitude) and its concentration dependence follows τt ∼ c0.77 ± 0.06. This scaling differs from the theoretical prediction with nearly double exponent (1.25). We tentatively attribute this difference to the polydispersity in the supramolecular chain length, which results mainly from the EHUT self-assembly association mechanism, and propose that constraint release effects of smaller chains should be considered as they may weaken the concentration dependence of the terminal relaxation. On the other hand, the magnitude of the plateau modulus G0 is affected by humidity to a lesser degree (not exceeding 50%), whereas its concentration dependence is in accordance with predictions, G0 ∼ c2 ± 0.21. Finally, the nonlinear viscoelastic properties are also affected, again experiencing weakening in the presence of humidity.