(192e) Responsiveness of Multi-Responsive Weak Polyelectrolyte Brush Grafted Nanoparticles with Varying Brush Characteristics

Polymer brushes have been used in a variety of fields, including catalysis, stimuli responsive surfaces, lubrication, and photovoltaic devices. These polymer brushes are capable of achieving a variety of conformations depending on brush characteristics such as degree of polymerization, grafting density, and monomer composition. Brush grafted nanoparticles consist of polymer chains densely grafted to or from the curved surface of nanoparticles. Polymer brushes consisting of stimuli responsive polymers add an additional tuning parameter to control the brush conformation.

Theoretical scaling theories have been extensively studied for a variety of polymer brush systems, from neutral polymers to charged polymers, from planar surfaces to cylindrical or spherical surfaces, block copolymer micelles and star polymers. Experimentally equivalent studies of polymer brush-grafted nanoparticles are relatively scarce, in part due to difficulties in synthesizing well-controlled structures.

Previous experiments in our group showed that weak cationic polyelectrolyte brush grafted nanoparticles of poly(2-(dimethyl amino)ethyl methacrylate) (PDMAEMA) grafted from a silica core showed pH-dependent swelling and a weak salt induced de-swelling, leading to pH-dependent adsorption (Langmuir 2014, 30, 4056-4065). In the current work, we investigate the responsiveness of PDMAEMA-based brush-grafted nanoparticles with varying grafting density, degree of polymerization, and chain composition. We also explore the effect of temperature and photo-isomerization of photo-responsive azobenzene-containing co-monomers, where cis- and trans- states differ in their hydrophobicity.

Brush-grafted nanoparticles were synthesized by surface-initiated atom transfer radical polymerization from initiator-functionalized silica nanoparticles. Dynamic light scattering was used to measure pH-induced and temperature-induced changes in hydrodynamic radius. White light and longwave UV light were used to reversibly photo-isomerize azobenzene co-monomers to shift copolymer hydrophobicity. The effect of photoisomerization on brush swelling was measured by dynamic light scattering.

Dynamic light scattering results showed a monotonic decrease in grafted brush thickness with increasing pH, as expected for decreasing degree of PDMAEMA ionization, for all variations of grafting density, degree of polymerization, and brush composition. Grafting densities were varied by a factor of ten. Over this range, grafting density did not influence the scaling of brush thickness with respect to degree of ionization. Existing scaling theories showed good agreement with the experimental results. The relative responsiveness of PDMAEMA brushes were found to collapse on a master curve for all combinations of brush characteristics, including those with copolymerized brushes of PDMAEMA and azobenzene. Increasing temperature was found to cause brush de-swelling for moderately charged brushes. Photo-isomerization of azobenzene units had a minor, but measurable, effect on brush swelling. Individually, degree of polymerization and grafting density dictate the brush thickness, but the general pH-responsive scaling behavior of normalized brush thickness was found to be independent of brush characteristics: normalized brush thicknesses for all samples examined collapsed onto a single master curve. Thus, while brush conformations are dependent on brush characteristics such as grafting density or degree of polymerization, the responsive behavior of these materials is not. Temperature changes and photo-isomerization of the azobenzene units were found to also be potentially useful tuning parameters in manipulating brush heights.

Magnetic brush grafted nanoparticles have been synthesized by similar methods as their silica counterparts. These nanoparticles are to be cross-linked into responsive fibrils under the influence of a magnetic field to then be used as soft material actuators. The responsiveness of these fibrils will then be compared to the responsiveness of their individual nanoparticle constituents as part of a hierarchical design.