Materials Science and Technology of Polymers at the University of Twente

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. By tuning polymer structures and compositions, 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

In Chapter 1, a short introduction to the topics, the motivation and an overview of the research are presented.

In Chapter 2, polymer materials, especially stimuli-responsive organometallic polymer materials, are briefly reviewed from the perspective of their equilibrium/non-equilibrium and dynamic phase behavior, and a wide variety of well-defined polymer structures and architectures from different processing technologies are discussed.

In the following main chapters, poly(ferrocenyl(3-iodopropyl)methylsilane) (PFS-I) is employed as a suitable precursor for post-polymerization modification because of the, 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, including cross-linkable methacrylamide for hydrogels and microfibers (Chapter 3), vinylimidazolium for the fabrication of porous materials (Chapter 4 and 5) or employed as a macro-crosslinker (Chapter 6), a particular ion pair containing quaternary phosphonium and alkylsulfonate moieties with a specific hydrophobic-hydrophilic balance (Chapter 7 and 8), viologen species as second redox center (Chapter 9), and reactive primary amine for interfacial polycondensation (Outlook). These post-polymerization modified substituents enable the introduction of controllable properties and complex functionalities in aqueous media targeting a broad range of applications by understanding the dynamic phase behavior and using different cross-linking chemistries and polymer processing technologies.

In Chapter 3, a cross-linkable methacrylamide-bearing polycation (PFS-DMAPMA) was synthesized by allowing PFS-I to react with N-[3-(dimethylamino)propyl] methacryl-amide at room temperature for 24 h. Redox responsive bulky hydrogels were formed instantaneously via a one-step thiol-Michael addition reaction between PFS-DMAPMA and poly(ethylene glycol) (PEG) cross-linker under ambient aqueous conditions. This relatively tough hydrogel displayed a reversible change in color and size (volume) upon oxidation and reduction under physiological salt conditions, even under high stress. By employing the electrospinning technique, hydrogel fibers with a submicrometer diameter were produced in an aligned organization, which makes it suitable for artificial muscles in further biomimetic applications.

In Chapter 4, a cationic PFS-based polyelectrolyte bearing vinylimidazolium bis(tri-fluoromethylsulfonyl)imide side groups (PFS-VImTf2N) and a typical organic acid poly(acrylic acid) (PAA), were employed to prepare a new type of redox responsive porous membrane via electrostatic complexation between these two polyelectrolytes. The aqueous NH3 solution as a weak base initiated pore formation by deprotonating carboxylic acid (COOH) groups of the PAA chains to form carboxylate groups (COO), thus activating interchain ionic bonding and structural rearrangements between PAA and surrounding cationic PFS-VImTf2N. The PFS-VImTf2N/PAA complex membrane demonstrated redox responsive behavior, which had a strong influence on its porous structure. By taking advantage of the structure changes in the oxidized and reduced states, the porous membrane exhibited reversible permeability control and showed great potential in gated filtration and catalysis. Additionally, the fabrication of porous microspheres was also demonstrated in Chapter 4 by using a simple co-flow capillary device. Monodisperse droplets were obtained from an oil-in-oil emulsion and further solidified during solvent extraction. Pore formation was then induced via an ammonia treatment, yielding porous particles with a diameter of 290 µm, approximately. These porous, redox-responsive switchable structures could provide further opportunities for controlled release as well as for drug delivery.

In Chapter 5, the porous structure and redox behavior introduced in Chapter 4 were further investigated by small-angle X-ray scattering (SAXS) using synchrotron radiation combined with electrochemical impedance spectroscopy. In order to gain more insight into structure variations during electrochemical treatment, the scattering signal of the porous membrane was detected directly from the films at the electrode surface, using a custom-built in-situ SAXS electrochemical cell. All experiments confirmed the morphology changing between more “open” and more “closed” cells with approximately 30% variation in the value of the equivalent radius (or correlation length), depending on the redox state of ferrocene in the polymer main chain. This property may be exploited in applications such as reference-electrode-free impedance sensing, redox-controlled gating, or molecular separations.

In Chapter 6, we reported on the synthesis and characterization of highly swollen, dual-responsive hydrogels formed by photo-polymerization of N-isopropylacrylamide (NIPAM) and fully water soluble PFS-vinylimidazolium chloride (PFS-VImCl). Compared with the earlier reported dual-responsive network made from PNIPAM and PFS bearing acrylate side groups, the hybrid structure described here possessed significantly improved swellability in water due to the presence of imidazolium groups attached to the PFS chain. Dual-responsive PNIPAM/PFS-VIm hydrogels were also employed in this chapter as a reducing agent for the in-situ formation of gold nanoparticles by utilizing the redox properties provided by the PFS segments. Optical properties of the nanoparticles formed within the hydrogel were tuned by the stimuli-induced volume-phase transition of the hybrid materials. Therefore, the as-synthesized AuNP-hydrogel composites could be used as an active plasmonic system with reversible changes in their optical response.

In Chapter 7, alkylphosphonium side groups were introduced to PFS by a simple quaternization reaction between PFS-I and tri-n-butylphosphine (PBu3) in a mixture of THF and DMSO, forming a novel organometallic poly(ionic liquid) (PIL), PFS-PBu3. The iodide counterions were exchanged with alkyl sulfonate counterions by dialysis in the corresponding aqueous salt solution. Different from neutral thermo-responsive polymers, e.g., PNIPAM, which exhibit a constant cloud point temperature, the phase transition temperature of PFS-PBu3 charge-compensated with alkyl sulfonate changes over a wide range depending on polymer concentration and the type and concentration of additional salts. The obtained polymers were also used as a dispersant for carbon nanotubes and as building blocks for hybrid hydrogels, by using the cross-linking chemistry described in Chapter 3. In addition to the redox-switchable thermo-responsive volume-phase behavior, surprisingly, the reduced PFS-DMAPMA/PBu3 hydrogel exhibited a strongly hysteretic volume-phase transition (around 20 °C) and showed long-lived bi-stable states at room temperature. This design strategy is expected to open new opportunities in the field of functional soft materials.

In Chapter 8, we demonstrated for the first time a one-step synthetic approach to thermo-responsive PILs, [PFS−SO3][nBu4P], via the Strecker reaction, by choosing tetraalkylphosphonium sulfite as an effective and versatile nucleophile. Notably, this simple approach directly introduced sulfonate side groups with phosphonium counterions to PFS-I without any further dialysis. The introduced pendant ion pairs were selected to possess a suitable hydrophobicity, to impart the PFS chains with a lower critical solution temperature (LCST)-type phase transition upon mixing with water, and also with an “isothermal” phase transition induced by a redox trigger, towards further electrical switchability. This organometallic polymer holds great promise as a new glazing material allowing dual modulation. A simple, smart window device was fabricated and could be switched between a transparent and non-transparent state for 100 thermal and 100 redox cycles without any change in its optical characteristics.

In Chapter 9, PFS-Viologen was synthesized by allowing PFS-I to react with N-Ethyl-4,4’-bipyridinium iodide. By introducing a second redox center, a viologen species, to the side groups of PFS, a symmetric polymeric redox flow battery was proposed based on this new polymer. This new redox-active polymer, PFS-Viologen, had a good solubility in both water and ionic liquid, exhibited electrochemically reversible disproportionation reactions and thus could serve as both anolyte and catholyte redox materials in a symmetric flow cell. This chapter is expected to inspire a new strategy for designing aqueous or ionic liquid flow batteries.