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41237-G7
Probing the Nanoscale Structure of Colloid-Semiflexible Polymer Suspensions
Introduction. The multicomponent interactions and structure in colloid-polymer solutions underlie the phase behavior, viscoelasticity, stability, and vitrification of these systems. In turn, the unique properties of colloid-polymer solutions and other filled systems play significant roles in many industrial applications. Suspensions of particles in polymer solutions occur in a remarkable range of technologies and products, from paints and coatings to consumer and personal care products.
The behavior of colloid-polymer solutions depends on the nature of the colloid-colloid, colloid-polymer and polymer-polymer interactions and correlations. For instance, it has long been recognized that dilute solutions of non-adsorbing polymer induce attractive depletion interactions between colloidal particles. Even these fairly simple systems exhibit rich phase behavior, including regions of coexistence between fluid-fluid and fluid-crystal phases. However, as the polymer concentration exceeds the entanglement transition, the polymer size becomes greater than that of the dispersed particles, or the particle concentration becomes high, the structural correlations, amenable to treatment by mean-field descriptions in the simplest cases, become more complex. Theoretical approaches using refined models of the polymer-particle direct correlations illustrate the complex nature of the colloid-polymer and polymer-polymer interactions. For instance, the modified polymer reduced interaction site model based on the Percus-Yevick closure approximation (PRISM-mPY) takes into account local entropic repulsions between the particle and polymers and the loss of conformational entropy of polymer segments close to the particles.
Results. In this work, the dynamics of colloidal particles dispersed in semiflexible polymer solutions of F-actin with different average filament lengths were investigated under conditions where the attractive interactions between filaments and particles were reduced. Two distinct regimes of the particle dynamics were found as a function of filament length, in good agreement with the expected transition from the dilute to the tightly entangled regime. Particle dynamics were then compared to theories of dilute and entangled rod-like and semiflexible polymers using the generalized Stokes-Einstein equation. In the dilute regime, the particle dynamics were in good agreement with the theoretical models of rod-like and semiflexible polymer solutions. However, in the tightly entangled regime, the anomalous particle dynamics reflect the formation of a local depletion layer surrounding the embedded particles. Using the viscoselastic shell model of Levine and Lubensky, we characterized the depletion layer thickness. The size of the depletion layer scales with the empirical non-locality length obtained from PRISM theory. Importantly, the non-locality length describes both the filament length and particle size dependence of the depletion layer thickness in entangled F-actin solutions.
A no-cost extension enabled us to carry out additional work with industrial sponsors. We studied the behavior of depletion-induced gels in vesicle-polymer mixtures. The vesicle dispersion was prepared from a commercial-grade dichain cationic surfactant through a standard milling process. To induce a depletion attraction between vesicles, we added a cationic polymer. Vesicles (ö = 0.05~0.3) were systematically mixed with polymer (Cp = 0.01~2.0wt%). As density gradients developed, an interface formed between a turbid vesicle-rich phase and a polymer-rich phase up to Cp = 0.2wt%. Increasing the polymer concentration further formed a gel, which subsequently collapsed. Height profiles of the gels were characterized by a slow initial rising for a finite delay time, a rapid collapse, and a slow final compaction to an equilibrium height. However, we observed a remarkably different polymer concentration dependence on the initial collapse rate. Unlike other colloidal gels, we found that the initial rising velocity increased with increasing polymer concentration. This surprising behavior could be accounted for by an increase in the permeability for solvent backflow, which is directly related to the characteristic pore area of the gel, obtained using confocal microscopy.
Significance. Our experiments demonstrated that microrheology is well suited to quantify the nanometer scale structure of polymer solutions, such as depletion near solid surfaces. Our work has also demonstrated a significant advantage of high-frequency microrheological experiments, such as diffusing wave spectroscopy; since the polymer network is expected to undergo a relatively small amount of structural rearrangement on the time scales of the probe particle motion, microrheology effectively becomes a “hydrodynamic scattering” experiment that probes polymer structure over a region comparable to the particle size. Finally, these experiments provided an important experimental validation of recent statistical mechanical theory of polymer-colloid solution nanostructure under conditions when strong polymer-particle and particle-particle correlations occur. This represents an initial step towards a first-principles understanding of the microscopic interactions (both entropic and enthalpic) that will lead to better control of the phase behavior, structure, stability and rheology of these systems. The work was recently designated “of special interest” in a recent survey of high frequency rheology and microrheology. We have extended this work to study the collapse of vesicle dispersions in the presence of non-adsorbing polymer depletant, which has important industrial applications related to product stability and shelf life.
