Published: April 05, 2011 r2011 American Chemical Society 5211 dx.doi.org/10.1021/la200022j | Langmuir 2011, 27, 5211–5221 ARTICLE pubs.acs.org/Langmuir Particle Restabilization in Silica/PEG/Ethanol Suspensions: How Strongly do Polymers Need To Adsorb To Stabilize Against Aggregation? So Youn Kim and Charles F. Zukoski* Department of Chemical and Biomolecular Engineering, University of Illinois at Urbana—Champaign, Urbana, Illinois 61801, United States I. INTRODUCTION Polymers are commonly used to control interactions in colloidal suspensions. Nonadsorbing polymer produces a depletion attraction the strength of which scales on the polymer solution osmotic pressure and the range of which are controlled by the polymer radius of gyration. Increasing nonadsorbing polymer con- centration leads to aggregation, gelation, liquidliquid, and crystal- lization phase separations. Adsorbing polymers are used as steric stabilizing aids where the approach of two particles restricts adsorbed polymer configurations resulting in an entropic repulsion. Steric stabilization is commonly used to stop aggregation at high ionic strengths. Clearly, to confer these properties, the polymer must adsorb with sufficient strength that it is not displaced by Brownian encounters. Here we address the question of the magnitude of the strength of attraction between polymer segments and the particle surface that will confer stability. 13 Polymer-induced interactions, of course, occur in addition to other forces acting on the particles such as van der Waals attrac- tions and electrostatic repulsions. In combination, these forces determine particle stability and the state of colloidal aggregation. Thus, interpreting the state of colloidal aggregation requires understanding how polymer concentration alters all interactions controlling particle stability. An example of a situation where the state of aggregation must be controlled as polymer concentration is increased comes in creating polymer nanocomposites by mixing polymer and particles in a low molecular weight solvent and subsequently removing the solvent. These initial conditions are similar to those that result in depletion forces. The aggregation and gelation that can result from depletion attractions are often undesirable. 1,4 While many studies have inves- tigated the effects of polymers on the state of aggregation at relatively low polymer concentrations, 58 fewer have investigated particle stability in concentrated polymer solutions as the melt concentration is approached. Well-understood and tested models for depletion attractions predict monotonic growth in the strength of depletion attractions with increasing polymer concentrations. 3,7,9 Thus, if particles aggregate at low polymer concentration, the paths leading to stable particles following solvent removal are not obvious. Never- theless, such paths exist. Here, we explore how stable polymer nanocomposite melts can be prepared with the solvent removal method in a system that is aggregated at low polymer concentration but stable at high polymer concentration. The system investigated involves 44 nm diameter St€ober silica particles suspended in ethanol and 400 molecular weight polyethylene glycol (PEG 400). We characterize the thermodynamic and structural properties of nanoparticles suspended in low molecular weight solvent as the polymer concentration increases toward that of the polymer melt. Our measurements suggest that particles are stabilized by electrostatic repulsion in pure ethanol. 10 As polymer concentration is increased, the particle charge decreases and particles Received: January 3, 2011 Revised: March 16, 2011 ABSTRACT: We study the effects of increasing the concentration of a low molecular weight polyethylene glycol on the stability of 44 nm diameter silica nanoparticles suspended in ethanol. Polymer concentration, c p , is increased from zero to that characterizing the polymer melt. Particle stability is accessed through measurement of the particle second-virial coefficient, B h2 , performed by light scattering and ultrasmall angle X-ray scattering (USAXS). The results show that at low polymer concentration, c p < 3 wt %, Bh2 values are positive, indicating repulsive interactions between particles. Bh2 decreases at intermediate concentrations (3 wt % < c p < 50 wt %), and particles aggregates are formed. At high concentrations (50 wt % < c p ) B h2 increases and stabilizes at a value expected for hard spheres with a diameter near 44 nm, indicating the particles are thermodynamically stable. At intermediate polymer concentrations, rates of aggregation are determined by measuring time-dependent changes in the suspension turbidity, revealing that aggregation is slowed by the necessity of the particles diffusing over a repulsive barrier in the pair potential. The magnitude of the barrier passes through a minimum at c p ≈ 12 wt % where it has a value of ∼12kT. These results are understood in terms of a reduction of electrostatic repulsion and van der Waals attractions with increasing c p . Depletion attractions are found to play a minor role in particle stability. A model is presented suggesting displacement of weakly adsorbed polymer leads to slow aggregation at intermediate concentration, and we conclude that a general model of depletion restabilization may involve increased strength of polymer adsorption with increasing polymer concentration.