Materials Science and Technology of Polymers

Functional macromolecules and smart polymer networks for ion separation,
reduction and delivery

Thesis cover Bram Zoetebier: Functional macromolecules and smart polymer networks for ion separation, reduction and delivery

In this thesis we tackle the synthesis, characterization and function of novel organometallic polyanions, polycations, and their corresponding hydrogels and explore the use of these redox-responsive water-soluble or water-swellable materials in applications where either their charge or their redox activity leads to new opportunities, such as in metal nanoparticle fabrication and transfection. In addition, crown ether-functionalized aromatic polymers and organometallic polymers will be explored for ion separation and – sensing.

After a brief introduction and outline of the thesis, a broad introduction to the field of stimulus responsive polymers is given in Chapter 2. This literature overview is focused on redox responsive hydrogels and their application in the fabrication of metal nanoparticles.

Stimulus-responsive gels attract much attention due to their wide applicability in areas including sensing, controlled release, actuators and artificial muscles. In Chapter 3 we describe the synthesis of polyferrocenylsilane (PFS) polyion hydrogels, where crosslinks were formed between strained alkynes and azide groups in a copper-free Huisgen cycloaddition reaction. The anionic organometallic hydrogels display a reversible collapse and reswelling upon chemical oxidation and reduction as they are switched between a zwitterionic and a polyanionic state.

Following the development of covalently crosslinked, redox responsive organometallic hydrogels we explored their ability to reduce transition metal salts to the corresponding metal nanoparticles (NPs) and observed that the hydrogels provided a facile platform for fabricating such nanoparticles. Salts of metal cations with selected redox potential were conveniently converted to unaggregated metal NPs without employing external reducing agents. The production of metal NPs often requires the use of organic ligands, salts, surfactants or other capping agents, which may significantly reduce their (catalytic) activity. Nanoparticle fabrication inside a polymer network circumvents these issues and suppresses excessive growth of the particles.

In addition to polyanions, PFS polycations are also explored in this project. Chapter 4 shows the potential that PFS polycations have to be used in selective gene delivery applications. The positive charge of the cationic PFS condenses DNA into polyplex NPs in which DNA is surrounded by PFS chains. The polyplex particles have a high positive charge at the surface which results in a high gene delivery efficiency compared to conventional polymers used in cell transfection experiments such as polyethyleneimine (PEI).

PFS derivatives are suitable for sensing applications due to their excellent redox properties. Different functionalities are easily introduced by side-group modification. It has been shown that thin and dense redox-active films can be immobilized on electrode surfaces. Chapter 5 combines these redox-active films with crown ethers, allowing specific cation recognition and sensing. Preliminary results, achieved with crown ether functionalized PFS films, show a response to µM cation concentrations.

Next to PFS, various crown ether containing poly(arylene ether ketone)s (PAEKs) were also synthesized and described in Chapter 6. PAEKs are the most commonly known high performance materials used for ion exchange and fuel cell membranes. By incorporating crown ether units in the main chain of these aromatic polymers, monovalent ion-selective membranes and ionic polymer metal composite actuators were prepared. The suitability of these systems for potassium and lithium ion separation was demonstrated.

In the outlook, Chapter 7, PFS polyanionic hydrogels were investigated further. The intriguing and substantial swelling and collapse upon reduction and oxidation were combined with the self-healing capabilities of β-cyclodextrin/adamantyl crosslinks. The resulting supramolecular hydrogel is a system responsive to multiple stimuli, including redox stimuli and the addition of competing guest molecules. By employing 8-arm instead of 4-arm crosslinking polymers, more mechanically robust hydrogels are expected to be formed.