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Date of Award

9-2012

Document Type

Campus Access

Degree Name

Doctor of Philosophy (PhD)

Degree Program

Polymer Science and Engineering

First Advisor

Gregory N. Tew

Second Advisor

E. Bryan Coughlin

Third Advisor

Dhandapani Venkataraman

Subject Categories

Materials Chemistry | Polymer Chemistry

Abstract

Nanostructured magnetic materials are important for various applications, and hence their development is critical for the advancement of science and technology. Coupling self-assembly to the generation of magnetic materials in a simple, straight-forward manner has remained a challenge. Here, a series of novel cobalt-functionalized block copolymers (BCPs) with various block ratios were synthesized using ring-opening metathesis polymerization (ROMP). These BCPs self-assembled into different nanostructured morphologies, including cylindrical, lamellar, and inverted cylindrical phases. Upon a simple heat treatment, all these nanostructured materials exhibited room temperature ferromagnetic (RTF) behavior due to the nanoconfinement of the cobalt species within one phase. The effect of dimensionality, or the degree of nanoconfinement, on the macroscopic magnetic properties was studied using superconducting quantum interference device (SQUID) magnetometer. The most highly constrained cylindrical morphology yielded the highest coercivity. The inverted cylindrical morphology, analogous to antidot materials, in which a 3D magnetic matrix is confined between diamagnetic cylinders, showed the second highest coercivity, while the least confined lamellar morphology exhibited the lowest coercivity value.

A series of metal-containing block-random copolymers composed of an alkyl-functionalized homo block (C16 ) and a random block of cobalt complex- (Co) and ferrocene-functionalized (Fe) units was synthesized via ROMP. Taking advantage of the block-random architecture, the influence of dipolar interactions on the magnetic properties of these nanostructured BCPs was studied by varying the molar ratio of the Co units to the Fe units, while maintaining the cylindrical phase-separated morphology. DC magnetic measurements, including magnetization versus field, zero-field-cooled and field cooled, as well as AC susceptibility measurements, showed that the magnetic properties of the nanostructured BCPs could be easily tuned by diluting the cobalt density with Fe units in the cylindrical domains. Decreasing the cobalt density weakened the dipolar interactions of the cobalt nanoparticles, leading to the transition from a room temperature ferromagnetic to a superparamagnetic material. These results confirmed that dipolar interactions of the cobalt nanoparticles within the phase-separated domains were responsible for the RTF properties of the nanostructured BCPs.

The effect of domain size on the magnetic properties of these RTF materials was investigated using a series of five cobalt-containing BCPs with various molecular weights and constant block ratios. The BCPs self-assembled into cylindrical morphologies with different domain sizes upon solvent annealing, and then were converted to RTF materials upon a simple heat treatment. The domain sizes of these RTF materials did not show a significant impact on their coercivity values, possibly because the domain size range investigated was not large enough and the cobalt-cobalt dipolar interactions were nearly constant throughout. At the same time, this study confirms that the RTF materials generated from these novel BCPs are robust.

The first metal cation-based anion exchange membranes (AEMs) were synthesized by copolymerization and cross-linking of a norbornene monomer functionalized with a water-soluble bis(terpyridine)ruthenium(II) complex and dicyclopentadiene. Each ruthenium complex has two associated counter anions, unlike most ammonium- and phosphonium-based membranes with single cation-anion pairs. The resulting AEMs show comparable anion conductivities and mechanical properties to traditional quaternary ammonium-based AEMs, as well as good alkaline stability and methanol tolerance. These results suggest that metal cation-based polymers hold promise as a new class of materials for anion-conducting applications.

Metal-cation-based polyelectrolytes with Cl- and HCO3- as counterions were synthesized from the water-soluble bis(terpyridine)Ru(II) complex-functionalized norbornene monomer via ROMP. The Ru complex-functionalized homopolymer with conventional non-coordinating counterion, PF6-, was synthesized for comparison. The integrity of the Ru complex in all the molecules was confirmed by UV-Vis spectroscopy. The solubility and conductivity properties of the Cl-, HCO3-, and PF6- polymers were studied and compared. The Ru-cation-based polyelectrolyte not only showed the ability to exchange its counterion with either another anion, or an anionic surfactant, but also possessed similar conductivity to traditional organic polycations.

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