UC Santa Barbara

In the first part of this dissertation, a library of polyester-based A(BA′)n asymmetric miktoarm star polymers was synthesized with A, A′ = poly(L-lactide) (PLLA) as the semi-crystalline hard blocks and B = poly(4-methylcaprolactone) (PMCL) as the soft segment using a grafting-through synthetic platform known as µSTAR. The synthetic versatility of µSTAR enabled a systematic investigation of architectural design parameters, in particular the number of BA′ arms (n), while holding the A, A′, and B block lengths constant. The value of n has a pronounced impact on the mechanical properties of these high molecular weight miktoarm materials. Tensile toughness increases with n, an effect likely related to bridging, as the modulus drops because the hard-block volume fraction decreases. These insights expand our understanding of architecture effects in sustainable thermoplastic elastomers.

Other architectures can have a large impact on material properties, including bottlebrush polymers. Extreme examples, where each sidechain also contains densely grafted polymeric segments, are brush-on-brush polymers. Synthesizing these materials often relies on multiple steps, requiring protection–deprotection, and low yields or grafting densities. Here, we use ring-opening metathesis (ROMP) to simplify the synthesis of these materials. Increasing the length of the alkyl spacer between a polymerizable norbornene group and brush-like polymer chain has a large impact on both the rate of polymerization and the maximum obtainable backbone length (NBB). This enabled the synthesis of poly(oligo ethylene glycol acrylate) brush-on-brush polymers using ROMP up to at least a NBB = 200 in almost quantitative conversion (99%) utilizing a ten carbon spacer. In stark contrast, shorter spacers of 2 and 6 carbons which result in low conversions even after multiple days.

In the last part of this dissertation, the self-assembly of statistical bottlebrush polymers is explored including Frank–Kasper phases. The discovery of Frank–Kasper (FK) A15 and σ phases in block copolymer systems in recent years have renewed the interest to study the phase behavior of different copolymer systems. These phases belong to a class of topologically close-packed sphere phases with a high coordination number and low symmetry. Using a combination of SCFT simulations and experiments, we systematically investigated the self-assembly behavior of conformationally asymmetric A-stat-B statistical bottlebrushes with asymmetric statistical segment lengths bA > bB. SCFT simulations predict the stabilization of the FK A15 phase at both low volume fraction fA < 0.5 of A and high fA > 0.5. At high fA, we find that the statistical bottlebrushes form B-rich spheres in an A-rich matrix. To our best knowledge, this is the first time such “inverse” FK phases have been predicted. SCFT simulations at fA < 0.5 have been experimentally verified and there are ongoing experiments to investigate the FK A15 phase at fA > 0.5.