Strong Polyelectrolyte Brushes via Alternating Copolymers of Styrene and Maleimides: Synthesis, Properties, and Stability

Polyelectrolyte brushes are of great interest for various applications from smart actuators to fouling resistant surfaces. Strong stretching due to proximity and Coulombic repulsion of the neighboring chains is a key to their function, which creates tension along the polymer chains and leads to degrafting of polymer brushes from the surfaces. A promising approach to lower the tension along the backbone is to reduce the number of charged units. Herein, we report a series of strong polyelectrolyte brushes with precisely controlled architectures, based on the alternating copolymers of styrene and N-substituted maleimides. Alternating copolymerization is employed to ensure the homogeneous distribution of charged units and dilute the charge density. Tailor-made maleimide monomers bearing hydrocarbon spacers with cationic groups are copolymerized with styrene via surface-initiated Activators ReGenerated by Electron Transfer Atom Transfer Radical Polymerization (SI-ARGET ATRP). Swelling behavior of the brushes is investigated as a function of molecular weight and pH. In solution atomic force microscopy (AFM) analyses and water contact angle measurements revealed that it is possible to obtain PEBs with good hydration properties, by combining hydrophobic and ionic units in an alternating sequence. Results also proved that the spacers play an important role in swelling behavior and wettability of brushes. Stability of the brushes is evaluated through incubation tests, and AFM is used to follow the changes in thickness and surface morphology. Brushes retained more than 80% of their original height after 3 weeks in buffer solutions. This study provides a novel approach to design PEBs with enhanced stability and adjustable macromolecular architecture

Visualization of the pH Response through Autofluorescent Poly(styrene-<i>alt</i>-<i>N</i>‑maleimide) Polyelectrolyte Brushes

Polyelectrolyte brushes can undergo reversible conformational transitions in response to changes in environmental pH and ionic strength. Therefore, they offer great potential for the design of “smart” surfaces and surface-based sensing devices. Herein, we report weak acidic polyelectrolyte brushes with pH-dependent autofluorescence based on alternating copolymers of styrene and tailor-made N-maleimides, which exhibit “clusteroluminescence” due to the through-space conjugation of π-chromophoric subfluorophores. Swelling behavior of the polyelectrolyte brushes was evaluated as a function of pH via in-solution atomic force microscopy (AFM) analyses. The correlation between the pH-induced conformational transitions and autofluorescence was confirmed with confocal laser scanning microscopy (CLSM) and two-photon laser scanning microscopy. Poly(styrene-alt-N-maleimide)-based well-defined, stable polyelectrolyte brushes, generating optical signals from conformational changes without conventional fluorophores, may enable the design of sensors and optoelectronic devices