The rheology of polymer latices containing 35 kg/mole polyethylene oxide chains with C16 terminal hydrophobes is related quantitatively to the composition. Correcting the solution concentration and the particle volume fraction to account for adsorption of polymer permits a clear distinction between the contributions the solution and those from the particles. The relaxation spectrum then clearly direct interactions between the particles that increase the low shear viscosity and produce a power law spectrum that controls the modulus at low frequencies.
We report the rheology of the micellar solutions formed from narrow-molecular-weight poly(ethylene oxide) chains fully endcapped with C16 or C18 alkanes and correlate the viscoelasticity, characterized by the high-frequency modulus G∞' and a single relaxation time λ, with the measured characteristics of the micellar solutions. Scaling G∞' by p3/2kT/τRH3 (p = aggregation number, RH = hydrodynamic radius, 1/τ = attractive virial coefficient, k = Boltzmann's constant, and T = temperature) and plotting against the hydrodynamic volume fraction of micelles collapses the two sets of data. The relaxation time scales as the diffusion time, μRc3/kT, for a free hydrophobe escaping from a micellar core of radius Rc times a Boltzmann factor accounting for an association energy that increases linearly with hydrophobe length. The low shear viscosity follows as ηo = λG∞'.
We report the first clear observations of an entropy-driven phase transition between a dilute micellar "gas" and a disordered but highly associated micellar "liquid" realized with aqueous solutions of poly(ethylene oxide) chains fully end-capped with C16 and C18 hydrophobes. Dynamic light scattering and capillary viscometry determine the radii and aggregation numbers of the micelles and, together with the coexisting concentrations, permit estimates of the strength of the entropic attraction through the adhesive hard-sphere model. The behavior is qualitatively consistent with expectations for the entropy gain from the exchange of end blocks between cores of flowerlike micelles consisting of associative triblock copolymers.
The viscoelasticity of latex dispersions containing triblock associative polymers exhibits multiple modes of relaxation. Here we confirm that the behavior at high frequency is imparted by the associated solution and characterized by a high frequency modulus and relaxation time comparable to the neat micellar solution at the same concentration. At low frequencies, diffusional modes of the particles generate a power law spectrum of relaxation times. Here the time scales and the volume fraction dependence of the contribution reflect the slower dynamics of particles incorporated into a percolating network via weak attractions between the adsorbed polymer layers. The viscoelasticity of the dispersions is correlated by superimposing the two modes.
A protein's stability may range from nonexistent, as in the case of intrinsically disordered proteins, to very high, as indicated by a protein's resistance to degradation, even under relatively harsh conditions. The stability of this latter group is usually under kinetic control because of a high activation energy for unfolding that virtually traps the protein in a specific conformation, thereby conferring resistance to proteolytic degradation and misfolding aggregation. The usual outcome of kinetic stability is a longer protein half-life. Thus, the protective role of protein kinetic stability is often appreciated, but relatively little is known about the extent of biological roles related to this property. In this Perspective, we will discuss several known or putative biological roles of protein kinetic stability, including protection from stressors to avoid aggregation or premature degradation, achieving long-term phenotypic change, and regulating cellular processes by controlling the trigger and timing of molecular motion. The picture that emerges from this analysis is that protein kinetic stability is involved in a myriad of known and yet to be discovered biological functions via its ability to confer degradation resistance and control the timing, extent, and permanency of molecular motion.
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