No abstract

Instabilities often arise in the sedimentation of colloidal particles, even when using the density gradient technique. These instabilities occur at volume fractions as low as 10−3 or 10−4, causing mixing of particles throughout the suspension, rather than the smooth sedimentation of particles. Here, we show that the mixing is due to a classical Rayleigh−Bénard instability. The sedimentation process is modeled in an approximate manner, and experiments are done to test whether the approximate model gives maximum stable volume fractions in the correct range. Sedimentation is a well-studied problem, and yet, to our knowledge, this is the first time that the well-known instability has been described in terms of a Rayleigh−Bénard instability.

The results confirm the grafting of PPA on HCN and TBACN, quantify the extent of PPA grafting, and identify various grafting modes (mono-, bi-, and tridentate). All of these aspects are found to be dependent on the layered materials' exfoliation state, suspension processing conditions, and solvent composition. The results are rationalized in terms of a plausible mechanism of the grafting process.

This work explores the factors that control the dispersion of exfoliated montmorillonite (MMT) in poly(vinyl alcohol) (PVOH) during solution blending and solvent evaporation. Nanocomposite films were prepared by solution blending of aqueous PVOH solutions with dilute suspensions of fully exfoliated MMT platelets (as confirmed by AFM). Dynamic light scattering (DLS) indicates that addition of MMT suspensions to PVOH solutions results in undesired particle aggregation and thus poor MMT dispersion in cast films (as evidenced by transmission electron microscopic images and gas permeation measurements). We believe that PVOH bridging induces MMT platelet aggregation. To counteract bridging aggregation, we explore the novel idea of pretreating the MMT surface with a small amount of compatible polymer prior to solution blending with PVOH. We hypothesize that "pretreating" the MMT platelet surfaces with adsorbed polymer in dilute suspensions will protect the platelets from bridging aggregation during solution blending and solvent evaporation. MMT/PVOH composite films have been prepared using low-molecular-weight PVOH as the pretreatment polymer; and low-, medium-, and high-molecular-weight PVOH as the matrix polymer. A PEO-PPO-PEO triblock copolymer (F108 from the Pluronics V R family) was also evaluated as the pretreatment polymer. DLS shows that pretreated MMT platelets are less susceptible to aggregation during blending with PVOH solutions. Results compare the crystalline structure, thermal properties, dynamic mechanical properties, gas permeability, and dissolution behavior of MMT/PVOH films incorporating untreated versus pretreated MMT.

This paper examines the relationship between rheology and the qualitative appearance of dried, mica-based paint coatings used in the aerospace industry. The goal is to identify key rheological characteristics indicative of poor coating visual appearance, providing a screening tool to identify unsatisfactory paint formulations. Four mica paints were studied, having coating visual appearances ranging from very poor to very good. Strain sweeps indicated that the poor-quality paints have a smaller % strain midpoint in the linear visco-elastic range; while the good-quality paints have a lower G’/G” cross-over point in frequency sweeps. Thixotropy experiments utilizing single and multiple-loop hysteresis cycles plotting shear stress as a function of shear rate showed that the base mica paints with good appearance had nearly constant, reversible profiles in the forward and the backward directions; while the mica paints with poor appearance were irreversible with a noticeable gradual change in shear stress as more loops are run. The difference in area between the forward and the reverse curves was determined, leading to a quantifiable criterion that can differentiate good paints from poor paints with significance testing. This work would establish the first rheology model using hysteresis loops to predict the visual properties of mica-based paints.