46575-UFS
Multinuclear High-Resolution Solid State NMR Investigations of Polymer Brush-Clay Nanocomposites
The goal of my sabbatical with Professor Hans Spiess at the Max Planck Institute for Polymer Research (Mainz, Germany), was to use multinuclear solid-state NMR techniques to study the composition, structure, and bonding within organic–inorganic hybrid materials based on clay-like layered materials, and to investigate the potential relevance of these materials in the field of proton conductivity. My research group at Amherst College had previously developed novel polymer–clay nanocomposites in which polymer chains are end-tethered to the clay lamellae by covalent bonds; brushes of poly(methyl methacrylate) are grown by atom transfer radical polymerization from initiator sites (R*) that are covalently bonded to the lamellae of synthetic zirconium mixed-phosphonate clays (Zr(O3PR)2–x(O3PR*)x).
My sabbatical research focused on elucidating the distribution of two types of organic groups within individual lamellae of zirconium mixed-phosphonates, which is relevant to the distribution of initiator sites (R*) and non-reactive groups (R) within the zirconium mixed‑phosphonate hosts for polymerization. X‑ray diffraction shows that the two types of phosphonate groups are not segregated into different layers, but it is difficult to probe the distribution of the two types of groups within a layer. The distribution of phosphonate groups within zirconium mixed-phosphonate materials reported in the literature is assumed to be random, but no direct evidence has been reported. In collaboration with Dr. Gunther Brunklaus (project leader in the Spiess group), we applied a two-dimensional 31P double-quantum solid‑state NMR technique that was sensitive to 31P homonuclear dipolar couplings and thus indicated the spatial proximity of the two different types of phosphonate groups. Using zirconium and zinc mixed-phosphonate materials (Zr(O3PCH2CH2X)(O3PCHCH2), X = Cl, Br, and Zn(O3PCH2CH2Br)0.5(O3PCHCH2)0.5.H2O) designed and synthesized specifically for study by this technique, but amenable to future polymerization work, the 31P double-quantum studies, confirmed by 31P R-TOBSY (total through-bond correlation) experiments, revealed that the two different groups were intimately mixed but not ordered, with no substantial clustering. This insight, which was unattainable by other techniques, is consistent with the desired distribution of initiator and non‑reactive sites in the polymer brush composites. A manuscript is currently in preparation.
As an initial foray into the proton conductivity of organic–inorganic hybrid layered materials, a family of zirconium phosphonates (Zr(O3P(CH2)nCOOH)2; n = 1–5) and a family of zinc phosphonates (Zn(O3P(CH2)nCOOH).H2O; n = 1–5) were synthesized, and the structures and proton dynamics of the interlayer hydrogen bonding networks were studied by one- and two-dimensional 1H double-quantum solid-state NMR spectroscopy. The rigid hydrogen bond arrays detected by 1H NMR were consistent with the observed low proton conductivity of the materials. Although significant differences were observed in the strength and orientation of the hydrogen-bonded networks as a function of chain length (n), the pattern was more complicated than the simple even–odd dependences observed previously for the proton conductivity and chemical reactivity of the acid groups in the zirconium phosphonate materials. A manuscript that describes these hydrogen bond networks is currently in preparation. In addition, the synthesis of zirconium phosphonates that have pendant phosphonic acid groups as better potential proton-conducting moieties has been initiated with a current Amherst College undergraduate honors thesis student (Tasha Drake) as a consequence of these 1H NMR studies of proton conductivity, and 1H NMR investigations in collaboration with Prof. Spiess and Dr. Brunklaus will follow.
At the beginning of the sabbatical, well-ordered samples of the polymer brush–clay nanocomposites with sufficiently high polymer loadings for study by solid-state NMR were not available. Although research efforts during the sabbatical were focused on the solid-state NMR techniques, a collaborative effort was established with a Ph.D. student (Anne Bohle) in the laboratory of Dr. Brunklaus and Prof. Spiess to synthesize a related family of polymer–clay nanocomposites that can be used to begin development of the NMR techniques for study of the composition, structure, and polymer chain dynamics of our unique polymer brush–clay nanocomposites. Synthetic efforts in my laboratory by a current undergraduate honors thesis student (Evelyn Auyeung) are focused on a new strategy for the original goal of preparing brushes of poly(methyl methacrylate) by atom transfer radical polymerization from initiator sites (R*) that are covalently bonded to the lamellae of synthetic zirconium mixed-phosphonate clays (Zr(O3PR)2–x(O3PR*)x). Because of the collaborations established during the sabbatical, detailed NMR investigation of the composition, structure, and polymer chain dynamics of these unique materials will be possible. The sabbatical was a productive year in which I was able to pursue a vexing question about the structure of organic–inorganic hybrid materials, take existing research interests in new directions, have hands-on experience with advanced solid-state NMR techniques, and establish collaborations that will continue well after the sabbatical and may lead to research opportunities for my students in Mainz. Two manuscripts are in preparation as a result of this work. In addition, I gained insight into the field of proton conductors and fuel cells, which has led to a new research effort in my laboratory.
