Kaihuan Zhang

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.

Highly Swellable, Dual-Responsive Hydrogels Based on PNIPAM and Redox Active Poly(ferrocenylsilane) Poly(ionic liquid)s: Synthesis, Structure, and Properties

Highly swellable, dual-responsive hydrogels, consisting of thermo-responsive poly(N-isopropylacrylamide) (PNIPAM) and redox-responsive poly(ferrocenylsilane) (PFS) based poly(ionic liquid)s (PILs) are formed by photo-polymerization. PFS chains bearing cross-linkable vinylimidazolium (VIm) side groups are copolymerized with NIPAM in aqueous solutions under ultraviolet light (λ = 365 nm) in the presence of a photoinitiator. The PFS-PILs serve as a macro-cross-linker and also provide redox responsiveness. The swelling ratio, morphology, and lower critical solution temperature (LCST) of the hydrogels are studied as a function of the PNIPAM/PFS ratio. The value of the LCST is dependent on the choice of the counterion of the PIL and the PNIPAM/PFS ratio. The hydrogel is employed as a reducing environment for the in situ fabrication of gold nanoparticles (AuNPs), forming AuNP-hydrogel composites. The localized surface plasmon resonance peak of the as-synthesized Au nanoparticles inside the hydrogel could be tuned by altering the temperature.

Organometallic polymers for electrode decoration in sensing applications

Macromolecules containing metals combine the processing advantages of polymers with the functionality offered by the metal centers. This review outlines the progress and recent developments in the area of electrochemical chemo/biosensors that are based on organometallic polymers. We focus on materials in which the metal centers provide function, allowing these materials to be used in electrochemical sensing applications based on various transduction mechanisms. Examples of chemo/biosensors featuring organometallic polymers that possess Fe, Os, Co and Ru are discussed.

Continuous fabrication of multi-stimuli responsive graphene oxide composite hydrogel fibres by microfluidics

Microfluidics appeared in the 1990s as a promising technology and has received considerable attention in developing stimuli-responsive hydrogel fibres in microscale for tissue engineering and actuation devices. In this work, thermo- and electro-responsive graphene oxide/poly(N-isopropylacrylamide)/sodium alginate (GO/PNIPAM/SA) hydrogel fibres were prepared via microfluidics and off-chip free radical polymerization. The composite hydrogel fibres were characterised using FTIR, SEM, and DSC. The thermo-triggered volume-phase transition and electrically triggered bending behaviours were also investigated. The results show that the hydrogel fibres have porous internal structures and the pore size becomes smaller with the increase of GO content due to the hydrogen bonding between the amide groups of PNIPAM chains and oxygen-containing groups on the GO nanosheets. Besides this, the incorporation of increased GO content enlarges the swelling ratio of the hydrogel fibre. The hydrogel fibres also exhibit bending behaviour under the non-contact direct current electric field.

Hydrogels with a Memory: Dual-Responsive, Organometallic Poly(ionic liquid)s with Hysteretic Volume-Phase Transition

We report on the synthesis and structure–property relations of a novel, dual-responsive organometallic poly(ionic liquid) (PIL), consisting of a poly(ferrocenylsilane) backbone of alternating redox-active, silane-bridged ferrocene units and tetraalkylphosphonium sulfonate moieties in the side groups. This PIL is redox responsive due to the presence of ferrocene in the backbone and also exhibits a lower critical solution temperature (LCST)-type thermal responsive behavior. The LCST phase transition originates from the interaction between water molecules and the ionic substituents and shows a concentration-dependent, tunable transition temperature in aqueous solution. The PIL’s LCST-type transition temperature can also be influenced by varying the redox state of ferrocene in the polymer main chain. As the polymer can be readily cross-linked and is easily converted into hydrogels, it represents a new dual-responsive materials platform. Interestingly, the as-formed hydrogels display an unusual, strongly hysteretic volume-phase transition indicating useful thermal memory properties. By employing the dispersing abilities of this cationic PIL, CNT-hydrogel composites were successfully prepared. These hybrid conductive composite hydrogels showed bi-stable states and tunable resistance in heating–cooling cycles.

Metal nanoparticle loading of gel-brush grafted polymer fibers in membranes for catalysis

We report on the preparation, characterization, and catalytic activity of microporous membranes featuring palladium (Pd) nanoparticles (NPs). The membranes consisted of polycaprolactone (PCL) microfibers featuring gel-brush layers of poly(hydroxyethyl methacrylate) (PHEMA). Pd nanoparticle loading was achieved by in situ reduction of Pd2+, coordinated to carboxylate groups in the brush, in aqueous Pd(NO3)2 electrolytes by using NaBH4. Gel-brushes were obtained via surface-initiated atom transfer radical polymerization (ATRP) polymerization. The membrane mats prior to functionalization were fabricated by electrospinning of PCL solutions. The PCL included mixtures of Br terminated PCL chains with a non-functional polymer. The electrospun fibers thus featured Br at their surface, which functioned as initiators, and allowed us to polymerize polymer gel-brushes at the fiber surface. The formation of Pd nanoparticles was evidenced by SEM and TEM. The membranes obtained had a large specific surface area and high porosity, which enabled high concentrations of metal nanoparticle loadings. The structure and morphology of the membranes were characterized by FTIR, SEM, TGA, and static contact angle measurements. The membranes obtained showed pronounced catalytic activity due to the presence of Pd NPs. As a proof-of-principle experiment we performed the catalytic reduction of 4-nitrophenol to 4-aminophenol in continuous flow-through catalysis.

Metallopolymers as responsive materials: shifting equilibrium by tuning structure

This thesis describes the synthesis and characterization of a series of poly(ionic liquids) (PILs), or polyelectrolytes, from poly(ferrocenylsilane)s (PFSs) and their applications in responsive materials. PFSs are a fascinating class of metallopolymers, with a backbone consisting of alternating ferrocene and silane units. Redox-active ferrocene units provide unique redox-responsive properties, and the presence of silane groups offers great opportunities for post-polymerization modification. However, synthetic access to soluble, well-characterized metallopolymers with controllable properties, functionalities, and processability in aqueous media has proved to be a significant challenge. In this thesis, poly(ferrocenyl(3-iodopropyl)methylsilane) (PFS-I) was readily synthesized by transition metal-catalyzed ring-opening polymerization and employed as suitable precursor for post-polymerization modification because of the reactivity of its haloalkyl moieties, i.e. via the Menschutkin reaction or Strecker sulfite alkylation. Aiming at diversified functionalities that give rise to enhanced responsiveness to stimuli or even to multiple stimuli, a variety of substituents has been explored in this thesis. By tuning polymer structures and compositions, understanding the dynamic phase behavior, using different cross-linking chemistries, and applying external fields or constraints under appropriate processing conditions, a variety of applications are explored in this Thesis, including artificial muscles from electrospun hydrogel microfibers, porous membranes and micro-particles with breathing pores, an active plasmonic system from AuNP-hydrogel composites, carbon nanotube-hydrogel composites with bi-stable states and tunable resistance, an electrically switched smart window device, and symmetric redox flow batteries.