(198d) Microstructure of State Behavior of Polymer Suspension

When colloids are suspended in a solvent and polymer is added, particles experience a net attraction. This depletion attraction leads to liquid/liquid phase separation or gelation depending on the polymer to particles size ratios and the polymer concentration. Most studies of depletion forces have focused on polymer-colloid mixtures where the polymer does not adsorb to the particle surface and have investigated the dilute polymer solution behavior. Our goal is to explore depletion forces as a function of polymer concentration and strength of attraction between particles and polymer segments in concentrated polymer solutions.

This work is important in developing an understanding of the state of dispersion of particles in polymer nanocomposites. Application of these materials depends on the state of aggregation of particles in the polymer melt which is related with polymer induced particle interactions. Our studies will investigate the role of polymer adsorption in mitigating depletion forces in polymer solutions and in the melt state. Our studies will enable a deeper understanding of how to tune particle-polymer interactions to ensure desired properties in nanocomposites.

In previous work, we confirmed that in polyethylene oxide, PEO, melts polymer adsorb to silica particles such that suspensions are stable and experience essentially hard sphere repulsions. Here we look at the role of adsorption on the polymer induced particle interactions in a tertiary system consisting of particles, polymer and low molecular weight solvent. In these studies, 44 nm diameter particles are suspended in ethanol and PEO is added. From Small Angle X-ray Scattering (SAXS), we verified that at a low polymer concentration, the particles aggregate. As the polymer concentration increases, the particles restabilize depending on the polymer to particle size ratios and interaction between polymer and particle. Our studies confirm that the second virial coefficient moves from positive to negative and back to positive as the polymer concentration increases. Depletion restabilization is a poorly understood phenomena of significance in both the production of nanocompistes and in the formulation of many suspensions.

Here, we focus on strength of attraction between particles and polymer segments in concentrated polymer solutions by investigating structure factors at high polymer concentrations which is believed where particles are stable.

Depletion forces change with polymer molecular weight and the strength of the polymer segment interactions with the particles surface. The polymer segment interactions can be modified by the substitution of polymer such as PTHF, which is known adsorb less strongly to the particles surface. Our goal in these studies is to demonstrate the impact of changes in segment /surface interaction strength on particle stability and suspension microstructure.

Characterizing this specific system provides invaluable information about particle packing and particle interaction, ultimately, this work will lead to greater insights in how to control particle aggregation in polymer nanocomposites.