The behavior of poly(n-butyl acrylate) (PnBA) spread at the air-water interface has been studied for a full range of surface coverages and several molecular weights. At low and intermediate surface coverages, the surface pressure-area isotherm behavior of the polymer is found to follow the expected scaling laws. In the dilute regime the pressure is an increasing function of surface coverage and a decreasing function of molecular weight. In the semidilute regime the surface pressure becomes independent of molecular weight, and a Flory exponent for the twodimensional radius of gyration is found to be ν = 0.57 ( 0.02. Beginning in the high coverage concentrated regime, at a surface pressure of around 15 mN/m, and through the full coverage regime (where the water in the subphase is fully covered and not exposed to air), X-ray reflectivity (XR) measurements show the formation of a continuous waterfree monolayer (i.e., one monomer thick) film of the polymer. At surface concentrations above the transition point to the full coverage regime (alternatively called the "collapsed" regime hereafter for the reason that will become apparent below), Brewster angle microscopy (BAM) shows that the excess polymer material does not distribute uniformly in the polymer film layer but instead leads to formation of micrometer-scale isolated globular domains of roughly uniform size. Further, it was observed that the number of such domains increases as the surface polymer concentration is increased, whereas the size of the globular domains is largely unaffected by the concentration variation. X-ray grazing incidence diffraction (GID) indicates that these domains are regions of bulklike (amorphous) polymer. These and other observations, including the invariant nature of the monolayer throughout the compression (confirmed by XR), the plateau nature of surface pressure-area isotherm throughout the collapsed regime, and the reversible nature of the domain formation (evidenced by BAM), suggest that the globular domains formed at high surface concentrations of PnBA are in a type of coexistence with the uniform monolayer. A simple thermodynamic model considering the entropic penalty of confining the polymer chains to monolayer, the translational entropy of the domains, and the surface energy of the interface is made in order to understand the behavior of the polymer as it becomes excluded from the monolayer. This argument suggests that the excess polymer should form a single large domain in order to minimize the large surface energy at the water-polymer interface. The presence of many small domains suggests the domains are kinetically trapped in a local, rather than global, equilibrium.

The height of weakly basic polyelectrolyte brushes in the osmotic brush regime is studied as a function of the grafting density using a numerical self-consistent field theory derived from the (semi)grand canonical partition function. The theory is shown to properly account for the local nature of the charge equilibrium and to capture the basic behaviors of polyelectrolyte brushes. On one hand, we find, in agreement with recent experiments, that the scaling of brush height with grafting density can be qualitatively different at intermediate chain lengths than that predicted by basic scaling arguments. This difference is attributed to the relative strength of electrostatic type interactions compared to finite segment size packing constraints. On the other hand, the trend of decreasing brush height with increasing grafting density predicted by the classic scaling analysis is recovered for large molecular weight polymers immersed in a solution of very weak ionic strength.

We investigated, by experiment and theory, the lateral structure of a weak polyelectrolyte brush at various added salt concentrations and chain grafting densities. Model poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) brushes with grafting density gradients were developed for this study by using a novel "Langmuir-Blodgett-deposition-under-compression" (LB\C) method. Fluid AFM images of these brushes indicate that the lateral structure of the brush system is sensitive to both added salt concentration and grafting density. Under low salt conditions, 0-20 mM NaCl, the brush structure shows strong microscopic lateral heterogeneities at high grafting densities; both the width and height of the heterogeneities increase with increasing grafting density but are independent of the salt concentration. As the bulk salt concentration is increased to an intermediate regime, 60-100 mM NaCl, these heterogeneities become smaller in size and number, coexisting with smooth homogeneous regions. At high enough concentrations, 300-500 mM NaCl, the entire surface becomes homogeneous. A simple free energy-based Flory-type argument is presented which explains the essential features of the thermodynamic behavior of the brush system. In the zero-salt limit, relatively few monomers are charged, and the hydrophobicity of the backbone chain drives the collapse/aggregation of the chains. At high salt concentrations, the brush chains become sufficiently charged to overcome the hydrophobic nature of the monomers and stabilize the homogeneous state. However, at intermediate salt concentrations, it is found that the osmotic pressure of the counterions surrounding the charged polymer moieties can be decreased by collapsing the chain structure while simultaneously decreasing the number of charges along the backbone and releasing small ions into the bulk solution. This effect, which we term "osmotic instability", serves to destabilize the homogeneous brush configuration.

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We present a theoretical study of the phase behavior of mixed brushes composed of charged and noncharged polymers that are mutually incompatible. We derive the self-consistent-field (SCF) equations from the canonical ensemble for a system of polyelectrolyte and uncharged polymers with added salt. Within the saddle point approximation, the modified Edwards Hamiltonian results in expressions for the chemical potential and species density fields in terms of the Green's function propagator and the nonlinear Poisson−Boltzmann equation for the electrostatic potential. These SCF equations were fully numerically analyzed to achieve results that are exact within the assumption of lateral mean field. The two-dimensional phase behavior of the mixed brushes (assuming that the brushes are laterally mobile) was examined using the conventional free energy of mixing analysis. The predictions on the effects of such control variables as the charge content of the polyelectrolyte species, ionic strength of the medium, and the surface grafting density of the chains on the conformation properties and phase behavior of the mixed brushes are presented. The results suggest that an increase in the effective charge on the polyelectrolyte (through variation of either charge content or ionic strength) favors the mixed state, while increasing the grafting density favors phase separation.