AmericanChemicalSociety.com

This project aims to study the structure and phase behavior of novel mixed polymer brush systems, namely, the mixed brushes composed of polyelectrolyte and neutral polymer chains. This novel mixed brush system can offer previously unavailable opportunities (i) to produce mesoscopic surface patterns of various length scales by long-range-frustrated lateral phase separation between the two chain types and (ii) to create functional interfaces with surface properties switchable between charged and non-charged states. During the past two years, our efforts have focused on developing basic experimental and theoretical techniques for studying this problem. Notable progress has been made on both experimental and theoretical domains.

With a goal to develop new and better experimental model systems and techniques for studies of polyelectrolyte brushes, we designed and developed atom transfer radical polymerization procedures for producing model poly(2-(dimethylaminoethyl)methacrylate)-b-poly(n-butyl acrylate) (PDMAEMA-PnBA) diblock copolymer materials, and studied the nanostructure and thermodynamic behavior of these polyelectrolyte-containing amphiphilic block copolymers at the air-water interface; this diblock copolymer comprises a positively-chargeable PDMAEMA block, and a water-insoluble PnBA block which anchors the accompanying PDMAEMA segment to the air-water interface. This work proved that the PDMAEMA-PnBA block copolymer forms a molecularly flat monolayer at the air-water interface that can thus be used as a convenient model system for thorough investigation of the PDMAEMA polyelectrolyte brushes [Macromolecules 2008, 41(23), 8960]. We also demonstrated that the PDMAEMA-PnBA monolayer at the air-water interface can be reliably transferred onto a graphite substrate for AFM imaging under aqueous conditions. Using these procedures, we performed an investigation of how the thermodynamic stability of the weak polyelectrolyte PDMAEMA brush is influenced by variations of the grafting density and the ionic strength of the medium. We observed experimental evidence of lateral nanoscale heterogeneities in weak polyelectrolyte brushes which have been only theoretically predicted before; the PDMAEMA brush system becomes laterally heterogeneous at zero or low concentrations of added NaCl, and this heterogeneity disappears with increasing NaCl concentration.

More recently, extending this initial investigation, the work has been directed toward exploring in greater detail the exact mechanisms by which added NaCl influences the thermodynamic stability of the PDMAEMA brush at various grafting densities. We conducted more detailed AFM measurements on the PDMAEMA brushes over wider ranges of NaCl concentration and polymer grafting density. Analysis of these experimental results based on a simple theoretical model suggested an interesting possibility that at low concentrations of added NaCl, the PDMAEMA brush chains become laterally aggregated primarily, not because of the hydrophobicity of the monomers, but instead because of the collapse-induced charge neutralization of the ionizable groups which reduces the osmotic stress caused by the accumulation of counterions in the vicinity of the polyelectrolyte segments. This mechanism termed as “osmotic instability” does not require the polymer to be hydrophobic in character in order to induce a tendency for the brush chains to collapse or aggregate [Langmuir 2009, accepted]. An important implication of this discovery is that contrary to conventional wisdom, electrostatic charges on polymers can make the polymers less soluble in water under certain circumstances. For the present, this explanation remains a speculation, and further investigation is in progress.

On the theory side, we have earlier shown the derivation of a Green’s function-based self-consistent field (SCF) formalism from the canonical partition function for a polyelectrolyte brush system. Predictions of this SCF model support the proposed idea of two-dimensional lateral phase separation in a mixture of polyelectrolyte and neutral polymer chains grafted to a surface [Macromolecules 2006, 39(22), 7757]. Last year, we further extended this original analysis into an investigation of the phase behavior of charged and non-charged mixed brushes grafted onto curved interfaces, and found that the effect of curving the surface is to favor a mixed state [Macromolecules 2008, 41(7), 2735]. Recently, we successfully generalized this formalism to capture the local, reversible nature of electrical charges on weak polyelectrolytes, and showed that this model predicts a correct description of an experimentally observed behavior of weak polyelectrolyte brushes which evades scaling theory explanation [Journal of Physical Chemistry B 2009, 113(32), 11076].

Building upon the above-stated accomplishments, we are currently performing combined experimental and theoretical studies of the structure and phase behavior of a mixed polymer brush system composed of PDMAEMA and poly(ethylene oxide) chains. In recent experiments, we discovered that in this mixed brush system, the chemical dissimilarity between the two species results in the formation of microscopic phase-separated structures (results to be published in a future publication; also see our TOC graphic entry) for reasons that are not understood. Similar long-range-hindered phase separation phenomena have recently gained increasing attention as a mechanism responsible for the formation of microscopic patterns observed in various 2D mixed systems, such as biological membranes, assemblies of mixed peptide amphiphiles, and bilayer electronic systems. A study of the mixed brushes will allow rigorous verification and expansion of our understanding of this widely-observed phenomenon, generally termed as “frustrated phase separation”.