ABSTRACT
The way that polymer brushes respond to shear flow has important implications in various applications, including antifouling, corrosion protection, and stimuli-responsive materials. However, there is still much to learn about the behaviours and mechanisms that govern these responses. To address this gap in knowledge, our study uses in situ X-ray reflectivity to investigate how poly(styrene sulfonate) (PSS) brushes stretch and change in different environments, such as isopropanol (a poor solvent), water (a good solvent), and aqueous solutions containing various cations (Cs+, Ba2+, La3+, and Y3+). We have designed a custom apparatus that exposes the PSS brushes to both tangential shear forces from the primary flow and upward drag forces from a secondary flow. Our experimental findings clearly show that shear forces have a significant impact on how the chains in PSS brushes are arranged. At low shear rates, the tangential shear force causes the chains to tilt, leading to brush contraction. In contrast, higher shear rates generate an upward shear force that stretches and expands the chains. By analysing electron density profiles obtained from X-ray reflectivity, we gain valuable insights into how the PSS brushes respond structurally, especially the role of the diffuse layer in this dynamic behaviour. Our results highlight the importance of the initial chain configuration, which is influenced by the solvent and cations present, in shaping how polymer brushes respond to shear flow. The strength of the salt bridge network also plays a crucial role in determining how easily the brushes can stretch, with stronger networks offering more resistance to stretching. Ultimately, our study aims to enhance our understanding of polymer physics at interfaces, with a particular focus on practical applications involving polymer brushes.
ABSTRACT
Polyelectrolyte brushes consist of charged polymer chains attached on one end to a surface at high densities. They are relevant for many practical applications ranging from biosensors to drug delivery to colloidal stability. Their structure and functionality can be dramatically influenced by multivalent counterions in the solution environment. In this work, the surface forces apparatus (SFA) and atomic force microscopy (AFM) were used to investigate the effects of three alkaline earth divalent cations, Mg2+, Ca2+, and Ba2+, on the structures of polystyrenesulfonate (PSS) brushes tethered to mica and silicon oxide surfaces. While all these ions caused significant shrinkage of the height of the PSS brushes, strong ion-specific effects were observed. Mg2+ and Ca2+ caused homogeneous shrinkage; Ba2+ led to pinned-micelle like inhomogeneous structures. Isothermal titration calorimetry (ITC) demonstrated that this ion specificity was mainly caused by the difference in binding energy between sulfonate groups and the divalent cations. Considering the abundance of divalent cations in industrial processes, natural environments, and biological systems, the understanding of strong ion-specific effects of divalent counterions is of great importance for theoretical studies and various applications involving polyelectrolytes.
ABSTRACT
Much research has focused on the discovery and description of long-ranged forces between hydrophobic solids immersed in water. Here we show that the force between high contact-angle solids in concentrated salt solution (1 M KCl) agrees very well with van der Waals forces calculated from Lifshitz theory for separations greater than 5 nm. The hydrophobic solids are octadecyltrichlorosilane-coated glass, with an advancing contact angle of 108°. Thus, in 1 M salt solution, it is unnecessary to invoke the presence of a hydrophobic force at separations greater than 5 nm. Through measurement in salt solution, we avoid the necessity of accounting for large electrostatic forces that frequently occur in pure water and may obscure resolution of other forces.
