Hydration in Weak Polyelectrolyte Brushes

Abstract

Weak polyelectrolytes (PEs) are complex because intertwined connections between conformation and charge are regulated by the local dielectric environment. While end-tethered PE chainsso-called PE "brushes"are archetypal systems for comprehending structure−property relationships, it is revealed that the reference state nominally referred to as "dry" is, in fact, a situation in which the chains are hydrated by water vapor in the ambient. Using charge-negative PE homopolymer brushes based on methacrylic acid and copolymer brushes that incorporate methacrylic acid and 2hydroxyethylmethacrylate, we determine self-consistently the water content of PE films using neutron reflectometry under different hydration conditions. Modeling multiple data sets, we obtain dry polymer mass density and layer thickness, independent of adsorbed water, and PE brush profiles into different pH solutions. We show that hydration of the chains distorts, here by as much as 30%, the quantification of these important physical parameters benchmarked to films in ambient conditions.

Swelling Behavior and Nanomechanical Properties of (Peptide-Modified) Poly(2-hydroxyethyl methacrylate) and Poly(poly(ethylene glycol) methacrylate) Brushes

“…These numbers appear to be slightly larger compared to those reported in Table 1, but this is expected based on how the two different data analysis packages describe roughness. 18 The polymer density profiles of the swollen brushes that are shown in Figure 6 reveal that these thin films consist of a polymerrich layer (φ = 0.85 -0.90) close to the substrate and a second layer that is swollen to a much higher degree (φ = 0.50) at the brush -D 2 O interface. This is an interesting observation as it indicates that these hydrophilic polymer brush films near the polymer -substrate interface only swell very little and are composed mostly of polymer.…”

“…The overall swelling ratio and the polymer volume fractions that can be derived from the neutron reflectivity experiments are in good agreement with the ellipsometry results. The roughnesses of the brush films as estimated 18 from neutron reflectivity are relatively small and range from 4.0 to 6.1 nm for the "dry" samples and from 7.9 -13.2 nm for the D 2 O swollen films. These numbers appear to be slightly larger compared to those reported in Table 1, but this is expected based on how the two different data analysis packages describe roughness.…”

Monomer volume fraction profiles in pH responsive planar polyelectrolyte brushes

“…Various modeling techniques such as scaling theory, analytical theory, molecular dynamics simulations, and numerical self‐consistent field theory (SCFT) have been used to study this coupling, leading to different predictions about height of the brushes as functions of experimental variables such as grafting density, molecular weight, salt concentration, and pH of the buffer solution. Similarly, a large body of experimental work has been focused on the response of polyelectrolyte brushes to the aforementioned variables.…”

“…In contrast to these developments on the modeling side, heuristic forms for monomer volume fraction profiles have been used to interpret experimental data such as neutron reflectivity profiles for pH responsive polyelectrolyte brushes. Furthermore, an indirect measure of the monomer volume fraction profile such as height of the brush, which is defined as the second moment of the profile and is more accessible in experiments, has been used when comparing experimental results with modeling predictions.…”

“…The numerical SCFT overcomes limitations of the analytical theory and inclusion of additional effects resulting from dipolar interactions provides a more accurate and relatively quicker prediction of the brush parameters when it is complemented with the analytical theory. Also, monomer volume fraction profiles extracted from neutron reflectivity experiments for poly(2‐(dimethylamino)ethyl methacrylate) (PDMAEMA) and polyacidic poly(methacrylic acid) (PMAA) brushes are used to demonstrate that a stepwise use of the analytical theory followed by the more refined numerical SCFT scheme can lead to relatively quick and accurate estimation of the brush parameters.…”