Christina A. Bauer

Laser and Electron‐Beam Induced Growth of Nanoparticles for 2D and 3D Metal Patterning

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Control of Grafting Density and Distribution in Graft Polymers by Living Ring-Opening Metathesis Copolymerization

Control over polymer sequence and architecture is crucial to both understanding structure-property relationships and designing functional materials. In pursuit of these goals, we developed a new synthetic approach that enables facile manipulation of the density and distribution of grafts in polymers via living ring-opening metathesis polymerization (ROMP). Discrete endo,exo-norbornenyl dialkylesters (dimethyl DME, diethyl DEE, di-n-butyl DBE) were strategically designed to copolymerize with a norbornene-functionalized polystyrene (PS), polylactide (PLA), or polydimethylsiloxane (PDMS) macromonomer mediated by the third-generation metathesis catalyst (G3). The small-molecule diesters act as diluents that increase the average distance between grafted side chains, generating polymers with variable grafting density. The grafting density (number of side chains/number of norbornene backbone repeats) could be straightforwardly controlled by the macromonomer/diluent feed ratio. To gain insight into the copolymer sequence and architecture, self-propagation and cross-propagation rate constants were determined according to a terminal copolymerization model. These kinetic analyses suggest that copolymerizing a macromonomer/diluent pair with evenly matched self-propagation rate constants favors randomly distributed side chains. As the disparity between macromonomer and diluent homopolymerization rates increases, the reactivity ratios depart from unity, leading to an increase in gradient tendency. To demonstrate the effectiveness of our method, an array of monodisperse polymers (PLAx-ran-DME1-x)n bearing variable grafting densities (x = 1.0, 0.75, 0.5, 0.25) and total backbone degrees of polymerization (n = 167, 133, 100, 67, 33) were synthesized. The approach disclosed in this work therefore constitutes a powerful strategy for the synthesis of polymers spanning the linear-to-bottlebrush regimes with controlled grafting density and side chain distribution, molecular attributes that dictate micro- and macroscopic properties.

Dependence of amine-accelerated silicate condensation on amine structure

Diatoms are known to grow elaborate nano- and microstructured silica shells by depositing material from precursor-containing vesicles at mild temperature and pH. Oligo(1-methylazetane) and related moieties, in some cases attached to proteins, are believed to facilitate this process. To complement prior studies of more complex amines, we aim to understand why such a unique structure has evolved through a systematic study of a set of simple model compounds. The degree to which a series of diamines in solution enhances condensation of phosphate-buffered silicic acid at neutral pH increases with increasing alkylation, a factor more important than amine pKa. This suggests why diatoms often use methylated oligomers. Bis(quaternary ammonium) salts result in even greater reactivity enhancement, constituting a new class of compounds that promote condensation under mild conditions. Methods are presented for incorporation of these new moieties into artificial peptides or other template-forming molecules, which should allow for more effective production of tailored silica nanostructures.

One- and two-photon induced growth of ligand-coated nanoparticles for 2D and 3D metal patterning

Ligand-coated metallic nanoparticles are powerful new materials for nano-electronic and photonics applications. They can be readily synthesized and their stability and solubility allows them to be cast in complex composite materials or self-assembled into quasi-ordered films. We demonstrate that, in the presence of reducing dyes with a large two-photon cross-section and metal salts, it is possible to induce the growth of metal nanoparticles in composite films under optical excitation or irradiation with electrons. We demonstrate further that continuous metal structures can be obtained via laser irradiation of the composites and that, with two-photon excitation, 3D structures can be fabricated. Silver, copper, and gold microstructures have been fabricated via two-photon excitation. The composition of our polymeric precursor is described in detail. In order to achieve highly-homogenous solid-state solutions of nanoparticles dissolved in polyvinylcarbazole (PVK), we have synthesized tailor-made nanoparticles on which we have introduced a mixture of carbazoyl-terminated octylthiol and simple octylthiols.. Preliminary experiments on e-beam lithography show that thin films of nanoparticles doped with suitable dyes and metal salts are efficient precursors form metal structures.

Two-photon 3D lithography: materials and applications

This study demonstrates the fabrication of a representative set of microstructures based on the three classes of two-photon photoactive materials compositions. Such structures can be used directly for suitable applications but can also be used as templates for the growth or molding of other materials as needed for a specific application. Given the ability of the two-photon lithography to produce 3D patterns of arbitrary structure, there is potential for the fabrication of an essentially limitless number of micro- and nano-structures in a wide variety of materials compositions.