Relaxation Dynamics at Different Time Scales in Electrostatic Complexes: Time-Salt Superposition

Abstract

In this Letter we show that in the rheology of electrostatically assembled soft materials, salt concentration plays a similar role as temperature for polymer melts, and as strain rate for soft solids. We rescale linear and nonlinear rheological data of a set of model electrostatic complexes at different salt concentrations to access a range of time scales that is otherwise inaccessible. This provides new insights into the relaxation mechanisms of electrostatic complexes, which we rationalize in terms of a microscopic mechanism underlying salt-enhanced activated processes.

Polyelectrolyte assisted charge titration spectrometry: Applications to latex and oxide nanoparticles

“…"# --value is also in good agreement with the stoichiometry found in ITC on the same compounds. We have found that stoichiometric PSS/PDADMAC mixtures without added salt form a viscous coacervate phase that sedimented rapidly [63,64]. Observed between glass slides, the coacervate appears as 5 µm droplets (Fig.…”

Linear viscoelasticity of complex coacervates

“…In numerous examples, the elastic behavior dominates (i.e., the storage modulus (G') dominates) [8,61,137,138,171,175,[177][178][179][180], while in other coacervates the viscous or liquid-like behavior (i.e., the loss modulus (G") dominates) [95,103,171]. Meanwhile, there are other systems where a crossover is observed between the two regimes, with G" dominating at low frequencies, and G' dominating at higher frequencies [61,71,72,79,111,130,162,171]. Considering these observations in the context of the expected characteristic responses for polymeric materials suggests that the differences in the observed material properties are the result of experimental limitations and thermodynamic factors that limit experimental access to the full range of frequency behavior, rather than an inherent characteristic of the material itself.…”

“…Spruijt et al proposed a model for these rheological observations based on "sticky" Rouse (or Zimm) polymer dynamics, where ionic bonds between polyelectrolyte chains act as "sticky" points that enhance the effective friction of polymer chains and decrease their mobility, thus slowing down stress relaxation [79,111]. This model provides a platform for developing and testing our understanding of molecular-level interactions within the material, and reasonable agreement was obtained between the model and the data.…”

“…Spruijt et al proposed a way of overcoming the experimental limitations of a single sample through the use of a time-salt superposition [79,111]. Superposition has long been used as a tool in rheology where an experimental variable such as temperature or strain rate is used as a means for accelerating activated processes.…”

“…This type of mechanical characterization can give insight into the molecular-level interactions present in the material, and is critical to enable effective processing of these materials. Generally, coacervates have been reported to have a Newtonian, or a shear-rate independent viscosity [71,111,171,172], as well as a shear thinning response [2,87,111,124,[171][172][173][174][175], depending on the shear rate. The occurrence of shear thinning is suggestive of a structural change in the material, and research has suggested that the tendency for shear thinning in coacervate-based materials may correlate with the strength of the underlying electrostatic interactions [2,174].…”