pH-responsive Films Research Articles

Single-composition three-dimensionally morphing hydrogels

We describe an approach to create pH responsive films that can transform from a flat state to a pre-described deformation. Our approach to fabricate these films is based on the use of a monomeric mixture of acrylic acid and a diacrylate crosslinker. Upon UV initiated free-radical polymerization the different monomers have different reactivities, which enables polymerization induced diffusion during spatially modulated UV intensity gradients. By means of this process the crosslink density within the film can be tuned spatially both in-plane and over the film thickness. The crosslink density determines the degree of swelling upon actuation of the formed hydrogel by a pH change. We demonstrate this effect by pH-switchable systems which are either substrate-attached exhibiting surface topologies upon actuation or freestanding that can undergo geometrical changes in the form of bending, curling or morphing into even more complex shapes.

PH-Responsive Films of Electrostatically Adsorbed Arborescent Copolymers

The adsorption of charged dendrigraft (arborescent) copolymers of different generations (G1, G2) and side chain molecular weights (Mn ≈ 5000 or 30,000) on silica surfaces in water, was monitored by the quartz crystal microbalance dissipation (QCM-D) technique. The topology of the adsorbed copolymers on mica was also investigated by AFM measurements. The PS-P2VP [polystyrene-graft-poly(2-vinylpyridine)] copolymers readily interact with a silica or mica surface and form a thin layer in acidic water (pH 2) due to the positively charged P2VP shell branches. The adsorbed arborescent PS-P2VP films expanded and collapsed reversibly in water upon cycling between low and high pH values, respectively. As the generation number increased, the density of copolymer molecules adsorbed onto the surface decreased due to stronger intermolecular electrostatic repulsions. The adsorption density also decreased significantly for copolymers with longer P2VP chains due to their more expanded conformation on the surface.

Synthesis, Multilayer Film Assembly, and Capsule Formation of Macromolecularly Engineered Acrylic Acid and Styrene Sulfonate Block Copolymers

We report the use of copolymers synthesized with specific block ratios of weakly and strongly charged groups for the preparation of stable, pH-responsive multilayers. In this study, we utilized reversible addition-fragmentation chain transfer (RAFT) polymerization in the synthesis of novel pH-sensitive copolymers comprising block domains of acrylic acid (AA) and styrene sulfonate (SS) groups. The PAA x- b-SS y copolymers, containing 37%, 55%, and 73% of AA groups by mass (denoted as PAA 37- b-SS 63, PAA 55- b-SS 45, and PAA 73- b-SS 27, respectively), were utilized to perform stepwise multilayer assembly in alternation with poly(allylamine hydrochloride), PAH. The ratio of AA to SS groups, and the effect of the pH of both anionic and cationic adsorption solutions, on multilayer properties, were investigated using ellipsometry and atomic force microscopy. The presence of SS moieties in the PAA x- b-SS y copolymers, regardless of the precise composition, lead to films with a relatively consistent thickness. Exposure of these multilayers to acidic conditions postassembly revealed that these multilayers do not exhibit the characteristic large swelling that occurs with PAA/PAH films. The film stability was attributed to the presence of strongly charged SS groups. PAA x- b-SS y/PAH films were also formed on particle substrates under various adsorption conditions. Microelectrophoresis measurements revealed that the surface charge and isoelectric point of these core-shell particles are dependent on assembly pH and the proportion of AA groups in PAA x- b-SS y. These core-shell particles can be used as precursors to hollow capsules that incorporate weak polyelectrolyte functionality. The role of AA groups in determining the growth profile of these capsules was also examined. The multilayer films prepared may find applications in areas where pH-responsive films are required but large film swelling is unfavorable.

Preparation of Ultrathin Self-Standing Polyelectrolyte Multilayer Membranes at Physiological Conditions Using pH-Responsive Film Segments as Sacrificial Layers

A new system to obtain ultrathin self-standing polyelectrolyte multilayer membranes at physiological conditions is introduced. On the surface of a substrate, a hybrid film structure composed of two compartments, (1) a pH-responsive film segment formed via hydrogen bonds and (2) a polyelectrolyte multilayer film on top of 1, was assembled. The pH-responsive polymer multilayer segments disintegrate at a neutral pH and release self-standing polyelectrolyte multilayer films. The obtained self-supporting polyelectrolyte multilayer membranes had thicknesses of 55 to several hundred nanometers and areas of a few square centimeters, approximately. The preparation method introduced here avoids harsh release conditions and thus broadens the choice of materials that can be incorporated into the self-standing film.