Date of Degree

9-2018

Document Type

Dissertation

Degree Name

Ph.D.

Program

Chemistry

Advisor

Nan-Loh Yang

Advisor

Shi Jin

Committee Members

David Locke

Subject Categories

Materials Chemistry | Polymer Chemistry

Keywords

Nitrocatecholic Polymer, Iron Oxide Nanomagnets, Boronic Acid, Binding Enhancement Factor

Abstract

Magneto-responsive polymer-iron oxide materials have been fabricated by coating various polymers on iron oxide nanoparticles (NPs). The stability of magneto-responsive polymer-iron oxide materials relies on the selection of appropriate coating anchors, such as catechols and silanes. This dissertation describes two robust polymer coating systems on iron oxide NPs. One system focuses on thermo-responsive nitrated catechol-N-isopropylacrylamide polymers with binding enhancement factors (EnF) of ~40 and ~20 towards iron oxide NPs and a boronic acid compared to the polymers without the nitro group. The second system involves coating of poly(vinyl alcohol) (PVA) on iron oxide NPs via a silane anchor and fabrication of stable PVA magneto-ferrogels.

Catecholic polymers play key roles as versatile adhesives and coatings with universal binding affinity in interface systems toward inorganic and organic materials, but their reversible binding and tendency to be oxidized led to undesired aggregation of iron oxide NPs and loss of small functional molecules derived from boronic acids. Compared to catechol, nitrocatechol displays improved binding affinity and anti-oxidation ability. Nitrocatecholic random copolymers carrying multiple multidentates can potentially show improved binding affinity toward the surface of materials. However, the retardation and inhibition effects of nitrocatechol monomers pose a challenge to syntheses of structurally well-defined nitrocatecholic polymers via free radical polymerization.

We developed a strategy to synthesize nitrocatecholic polymers without retardation and inhibition during the synthetic process. First, the monomer, dopamine methacrylate (DMA), was protected with tert-butyldimethylsilyl groups. Then, the nitrocatecholic homopolymer and copolymer, polynitrodopamine and poly(nitrodopamine-co-N-isopropylacrylamide), i.e., P(NDMA-co-NIPAM) were obtained via thermal free radical polymerization and nitration of the catechol ring using acetyl nitrate, followed by deprotection.

Rigorous enhancement factors (EnF) toward Fe3O4 NPs (diameter: ~15 nm), rendered by the nitro group, were determined via competitive binding investigation between polymers with only the nitro group difference from the same parental polymer. The competitive binding to Fe3O4 NPs between catecholic and nitrocatecholic copolymers, poly(dopamine-co-N-isopropylacrylamide), i.e., P(DMA-co-NIPAM), and P(NDMA-co-NIPAM), was performed in neutral methanol under sonication in a nitrogen atmosphere. The interaction of polymer-Fe3O4 through C-O-Fe bonds was established using FT-IR. With the copolymer containing only 5% of nitrocatechol units, remarkable EnF, ~40, were obtained from 1H NMR peak intensity based on the free and bound polymer ratios from triplicate experiments. Similar EnF values were obtained from the initial weight of each polymer, the total weight loss from TGA and the polymer ratio on the surface of Fe3O4 NPs from 1H NMR.

Additionally, the enhanced binding affinity toward Fe3O4 NPs (diameter: ~6 nm) of P(NDMA-co-NIPAM) was further examined with thermo- and magneto-responsive polymer-Fe3O4 ferrogels fabricated by cross-linking polymers via Fe3O4 NPs. The enhanced stability of P(NDMA-co-NIPAM)-Fe3O4 pre-gel suspension on a magnet and its corresponding robust ferrogel in the presence of DMA support the former EnF, ~40.

Furthermore, the improved binding affinity by introducing the nitro group toward a small organic molecule, 4-fluorophenylboronic acid (FPBA), was investigated in PBS solutions using 1H NMR. The binding constants of P(NDMA-co-NIPAM) to FPBA are about 20 times and 5 times higher than P(DMA-co-NIPAM) at pH 6.5 and pH 7.4, respectively.

Chapter 3 describes the preparation of stable PVA magneto-responsive ferrogels with covalent Si-O bonds between PVA and iron oxide NPs. PVA magneto-responsive ferrogels have gained increasing interest in important fields such as drug delivery. However, the reported PVA magneto-ferrogels without covalent bonds between PVA and iron oxide NPs face potential leakage of iron oxide NPs. Herein, to obtain the Fe3O4 NPs covalently coated with PVA, vinyl functionalized Fe3O4 NPs using vinyl trimethoxyl silane were copolymerized with vinyl acetate, followed by hydrolysis of ester groups in the presence of sodium hydroxide. The resultant PVA-Fe3O4 NPs were dispersed in water. PVA-Fe3O4 magneto-responsive gels formed after freezing at -15 oC and thawing at room temperature for three cycles. No leakage of iron oxide NPs and gel decomposition was observed after long standing.

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