Metal-containing functional polymers: (I) Room temperature magnetic materials and (II) Anion exchange membranes

Yongping Zha, University of Massachusetts Amherst

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. (Abstract shortened by UMI.)

Subject Area

Polymer chemistry|Materials science

Recommended Citation

Zha, Yongping, "Metal-containing functional polymers: (I) Room temperature magnetic materials and (II) Anion exchange membranes" (2012). Doctoral Dissertations Available from Proquest. AAI3545999.
https://scholarworks.umass.edu/dissertations/AAI3545999

COinS