[Thomas A. Desjardin, These Honored Dead: How the Story of Gettysburg Shaped American Memory. Cambridge, Mass: Da Capo Press, 2003. 246 pp. $26.00 (Hard). ISBN:0306812673.]

This study examines the three generations of Civil War reenactors: the veterans, the centennial reenactors, and the modern community. It argues that they are an excellent case study for examining the evolving memory of the Civil War, particularly when considered in the context of five interpretative traditions: the “Lost Cause,” the “Union Cause,” the “Reconciliationist Cause,” the “Emancipationist Cause,” and the “White Supremacist Cause.” At the same time, a careful analysis of the modern community illustrates the myriad ways in which contemporary individuals interact with and utilize the past.

Polymers containing a backbone of repeated Si–O bonds, referred to as siloxanes or silicones, are a ubiquitous class of materials due to their collection of unique properties. The exceptionally wide Si–O bond angle and high deformability of the backbone bonds results in a low glass transition temperature of –120 °C and a crystallization temperature below room temperature. Additionally, the high bond dissociation energy gives rise to decomposition temperatures above 300 °C and results in a chemically inert polymer. The combination of these features result in a massive temperature window while maintaining inert. With a market size of over $4 billion, polydimethylsiloxane (PDMS) is the most common siloxane and has been the subject of decades of research. These research and development efforts have led to the material being used in numerous environments ranging from cosmetics to aerospace. The utility of PDMS arises from the sophisticated methods used to tune the polymer chemistry, which result in a range of properties appropriate for numerous different applications. Robust chemical reactions, like hydrosilylation, have led to an extremely broad scope of functional groups that can be synthetically bonded to the siloxane backbone. This same strategy has been used to attach polymers, polymerization initiators, or polymerizable moieties to the siloxane chain, leading to a broad scope of possible siloxane-based copolymer architectures. One architecture that is lacking in synthetic versatility, however, is graft copolymers with a siloxane backbone and polymeric grafts based on vinyl monomers. Chapter 2 of this dissertation outlines a methodology to synthesize graft copolymers with siloxane backbones using a combination of thiol–ene click chemistry and atom-transfer radical polymerization. Optimization of polymerization conditions enabled the synthesis of graft copolymers with a range of grafting densities and molecular weights > 5 MDa. Degradation of the siloxane backbone enabled characterization of the polymerized grafts, showcasing dispersities around 1.2 and molecular weights close to the targeted values. In order to form a freestanding polymeric material, the siloxane must be crosslinked. The network architecture is an important parameter that can be manipulated to control the properties of the material. Additionally, novel functionality can be incorporated through the usage of a dynamic crosslinker. An example of such a material is Silly Putty, which is PDMS crosslinked through borosiloxane bonds. The method of synthesizing this material, however, has remained largely unchanged since the 1940s and remains a time and energy intensive process with limited tunabilty. In Chapter, 3 we demonstrate a novel methodology to synthesize polyborosiloxane networks using hydrosilylation in minutes. While the current state-of-the-art is limited to end-functionalized PDMS chains, our developed method gives access to backbone-functionalized PDMS starting materials, which drastically increases the degree of tunability over the final network properties. In Chapter 4, we expand upon this method by synthesizing boronate esters containing varying functional groups to investigate the impact of the crosslinker chemical environment on the stress relaxation dynamics. We demonstrate an unprecedented level of control over the rate of stress relaxation for polyborosiloxane materials while maintaining a facile and robust crosslinking method. The newly developed methodologies for graft copolymers and functional polymeric networks will enable future investigations into the properties of these materials and their incorporation into complex composite systems to expand upon the current availability of silicone materials.

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.

Asymmetric miktoarm star polymers, comprising a single A arm coupled to a multitude of B or AB′ arms, produce unique material properties. In particular, morphologies at significantly elevated volume fraction of the A material can be obtained. For instance, the performance of thermoplastic elastomers (TPEs) stands to significantly improve from increasing the volume fraction of A while maintaining an elastic matrix. However, existing synthetic strategies are beleaguered by complicated reaction schemes restricted in both the monomer scope and yield. Bottlebrushes copolymers, on the other hand, are easily synthetically accessible using Grubbs third-generation catalyst via the ring-opening metathesis co-polymerization (ROMP) of macromonomers. The well controlled synthesis of bottlebrush copolymers at low backbone degrees of polymerization (n) was demonstrated. Self-assembly in the bulk was studied as a function of molecular composition, arm stoichiometry, and n. Insights generated from scattering experiments and self-consistent field theory simulations indicate these materials behave as disperse miktoarm stars at low n with a transition to brush-like conformations as n increases. The star-to-bottlebrush transition is quantifiable for both statistical and diblock sequences by unique signatures in the experimental scaling of domain spacing and simulated distribution of backbone/side-chain density within lamellar unit cells. These findings represent a conceptual framework that simplifies the synthesis of miktoarm star polymers.These conclusions were used to develop a second-generation synthetic approach, coined “μSTAR”, miktoarm synthesis by termination after ring-opening metathesis polymerization, wherein asymmetry is introduced by first homopolymerizing a macromonomer followed by in situ enyne-mediated termination to install a single mikto-arm with exceptional efficiency. This modular μSTAR platform cleanly generates ABn and A(BA′)n miktoarm star polymers with unprecedented versatility in the selection of A and B chemistries as demonstrated using many common polymer building blocks. The average number of B or BA′ arms (n) is easily controlled by the equivalents of Grubbs catalyst. While these materials are characterized by dispersity in n that arises from the statistics of polymerization, they self-assemble into mesophases at elevated volume fractions of the A material that are identical to those predicted for precise miktoarm stars. In particular phases at significantly elevated volume fraction of the A material were obtained. The μSTAR technique provides a significant boost in design flexibility and synthetic simplicity while retaining the salient phase behavior of precise miktoarm star materials. The usefulness of μSTAR for generating high performing materials was demonstrated by synthesizing sustainable and high molecular weight asymmetry miktoarm star polymers and examining their performance as TPEs. Stiff, tough, and elastic TPEs at greater than 50% volume fraction were tested. The versatility and efficiency of μSTAR at generating a library of stars facilitated the elucidation of structure-property relationships in this unique class of materials. Comparisons to literature examples of thermoplastic elastomers show that these sustainable stars offer advantages over other sustainable linear as well as petroleum derived TPEs.