Mark A. Hempenius

Poly(ferrocenylsilane) Gels and Hydrogels with Redox-Controlled Actuation.

This concise paper discusses poly(ferrocenylsilane) (PFS)-based organometallic gels and hydrogels as a novel class of redox-responsive materials. First, the use of silicon-bridged spirocyclic [1]ferrocenophane crosslinkers for creating PFS networks swellable in organic solvents is described. Optical properties of PFS composite colloidal crystal films, composed of monodisperse silica spheres embedded in such PFS networks, are shown to be influenced by solvent swelling or redox chemistry, allowing the use of these composites in photonic crystal full-color displays. The synthesis of networks composed of water-soluble PFS polyanions or polycations is discussed. In response to redox stimuli, a polyanionic PFS-hydrogel exerted a pressure, which could render it useful as an actuator.

Surface Nano- and Microstructuring with Organometallic Polymers

This paper gives an overview of the use of poly(ferrocenylsilane)s in the surface patterningof silicon substrates. Due to the presence of iron and silicon in their main chain, poly(ferrocenylsilane)sshow a very high resistance to reactive ion etching, allowing one to transfer polymer patternsdirectly onto the substrate. Methods for introducing etch-resistant polymer patterns on substratesurfaces include soft lithography approaches such as microcontact printing, directed dewetting, andcapillary force lithography. Next to top-down methods, self-assembly strategies are discussed. Phaseseparation in thin films of asymmetric organic–organometallic block copolymers leads to theformation of nanoperiodic organometallic patterns. The use of such thin films as nanolithographictemplates is demonstrated. Surface patterning can also be realized using electrostatic self-assemblyof organometallic polyions. Layer-by-layer deposition of poly(ferrocenylsilane) polyanions and polycationson chemically patterned substrates allows one to guide the growth of multilayer thin films and toproduce patterned organometallic coatings.

Redox-Active Cross-Linkable Poly(ionic liquid)s

The synthesis of a new class of cross-linkable redox-responsive poly(ferrocenylsilane)-based poly(ionic liquid)s (PFS-PILs) is reported. PFS-PILs self-cross-link at low concentrations into nanogels or form macroscopic hydrogel networks at higher concentrations. PFS-PILs proved to be efficient dispersants in the microemulsion polymerization of methyl methacrylate, producing stable PFS-poly(methyl methacrylate) latex suspensions.

Redox active gels: synthesis, structures and applications

Recent years have brought significant advances and developments in the area of redox-responsive polymers. This review outlines major developments that focused on the incorporation of redox responsive components that are covalently attached as pending or crosslinking groups along a polymer backbone. Among the major contestants, ferrocenes, conjugated polymers, tetrathiafulvalene, transition metal ions and disulfide bonds represent major contributions in this field. The unique technological feature of these responsive gels capable of redox stimuli controlled and reversible changes is the ability to tune their shapes, color, solubility, or display sol-to-gel transitions.

Probing the Collapse Dynamics of Poly(N‐isopropylacrylamide) Brushes by AFM: Effects of Co‐nonsolvency and Grafting Densities

Collapse of poly(N-isopropylacrylamide) (PNIPAM) brushes in the mixed solvent system (water/methanol 50% v/v) is studied by in-situ atomic-force microscopy (AFM). PNIPAM brushes with three different grafting densities and similar chain lengths are synthesized via surface-initiated atom-transfer radical polymerization. By changing the solvent from water to a water/methanol (50% v/v) mixture, the polymer brushes switch from a swollen to collapsed state. AFM force measurements using a silica colloidal probe attached to the tip are employed to obtain the Youngs moduli of the polymer brushes in different solvation states. The collapse dynamics of the brush is followed by monitoring the pull-off force (adherence) in situ. The modulus of the swollen high-density polymer brush is four times lower than that of the same brush in the collapsed state. It is shown that in the case of the high-density polymer brush with a thickness (t(in water) ) of 900 nm, the collapse takes place in a time scale of ~25 s, whereas the collapse occurs faster for the medium-density brush (t(in water) = 630 nm) and much more rapidly for the low-density brush (t(in water) = 80 nm). This difference in the response kinetics is primarily ascribed to the time needed for solvent exchange in the polymer brushes.

Breathing Pores on Command: Redox‐Responsive Spongy Membranes from Poly(ferrocenylsilane)s

Redox-responsive porous membranes can be readily formed by electrostatic complexation between redox active poly(ferrocenylsilane) PFS-based poly(ionic liquid)s and organic acids. Redox-induced changes on this membrane demonstrated reversible switching between more open and more closed porous structures. By taking advantage of the structure changes in the oxidized and reduced states, the porous membrane exhibits reversible permeability control and shows great potential in gated filtration, catalysis, and controlled release.

Preparation of a Rapidly Forming Poly(ferrocenylsilane)-Poly(ethylene glycol)-based Hydrogel by a Thiol–Michael Addition Click Reaction

The synthesis of a rapidly forming redox responsive poly(ferrocenylsilane)-poly(ethylene glycol) (PFS-PEG)-based hydrogel is described, achieved by a thiol-Michael addition click reaction. PFS bearing acrylate side groups (PFS-acryl) was synthesized by side group modification of poly(ferrocenyl(3-iodopropyl)methylsilane) (PFS-I) and characterized by (1) H NMR, (13) C NMR, and FT-IR spectroscopy. The equilibrium swelling ratio, morphology, rheology, and redox responsive properties of the PFS-PEG-based hydrogel are reported.

Electrografting of Stimuli-Responsive, Redox Active Organometallic Polymers to Gold from Ionic Liquids

Robust, dense, redox active organometallic poly(ferrocenylsilane) (PFS) grafted films were formed within 5 min by cathodic reduction of Au substrates, immersed in a solution of imidazolium-functionalized PFS chains in the ionic liquid 1-ethyl-3-methylimidazolium ethyl sulfate. The electrografted polymer films were employed as an electrochemical sensor, exhibiting high sensitivity, stability, and reproducibility.

Poly(N-isopropylacrylamide)–poly(ferrocenylsilane) dual-responsive hydrogels: synthesis, characterization and antimicrobial applications

Novel hydrogels composed of thermo-responsive poly(N-isopropylacrylamide) (PNIPAM) and redox-responsive poly(ferrocenylsilane) (PFS) macromolecules were formed by photopolymerization. PFS chains bearing acrylate side groups were copolymerized with NIPAM and N,N′-methylenebisacrylamide in tetrahydrofuran in a predetermined ratio under ultraviolet light-emitting diode (UV-LED) irradiation at a wavelength of 365 nm, in the presence of a photoinitiator. Crosslinking occurred smoothly, providing homogeneous hydrogels. The equilibrium swelling ratio, rheology and morphology of these hybrid PNIPAM–PFS-based hydrogels were investigated. The thermo-responsive properties of the hydrogels were studied as a function of the PFS oxidation state. In situ fabrication of silver nanoparticles inside the hydrogel network via reduction of silver nitrate by the PFS chains led to hydrogel composites. These composites showed strong antimicrobial activity while maintaining a high biocompatibility with cells

Probing the thermal collapse of poly(N-isopropylacrylamide) grafts by quantitative in situ ellipsometry

We demonstrate the potential of in situ spectroscopic ellipsometry for the investigation of the chain segment density profile and layer thickness during the temperature-induced, reversible collapse-expansion transition of poly(N-isopropylacrylamide) (PNIPAM) grafted layers. Here, we study PNIPAM films with variable grafting densities in aqueous systems, which were produced by atom-transfer radical polymerization (ATRP). In our attempt to obtain a realistic quantitative description of the thickness of our swollen PNIPAM layers, various models were implemented to fit the ellipsometric data. As expected, we found that the swelling ratio is strongly dependent on the grafting density. From the ellipsometry results, the density and thickness variation accompanying the collapse transition across the lower critical solution temperature (LCST) was characterized. The collapse can be adequately explained by considering the PNIPAM film to consist of two layers: (i) a dense layer near the surface and (ii) a more diluted layer on the side of the film exposed to the solvent. Analysis of the optical response reveals a gradient density profile within these layers.