Browsing by Subject "Bottlebrush"

Now showing 1 - 5 of 5
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    Bottlebrush Polymers: Synthesis, Rheology, and Self-Assembly
    (2015-09) Dalsin, Sam
    Bottlebrush polymers are comb-like molecules with a high density of side chains grafted along a central backbone. Due to their unique conformational properties, bottlebrush polymers have become attractive candidates for developing new photonic bandgap materials, nanotubes and nanowires, or drug delivery vehicles, to name a few. This dissertation primarily investigates the rheological properties and self-assembly behavior of bottlebrush polymer molecules made using a variety of different polymerization routes. A considerable portion of the work is directed towards the linear rheology of model, polyolefin-based bottlebrush polymers with independently varied branch and backbone lengths. These studies demonstrate how the tight spacing between branch points effectively precludes backbone entanglement in the polymer melts, but it does not inhibit the formation of entanglements among the branched side chains. Furthermore, the relaxation profiles reveal transient scaling behavior in which the dynamics transition from Zimm-like to Rouse-like at increasing relaxation times. These results highlight the distinct conformational character of bottlebrushes at different length scales. The latter parts of this work report on the self-assembly behavior of bottlebrush diblock polymers composed of atactic polypropylene and polystyrene side chains. The diblock samples are analyzed using small-angle X-ray scattering and atomic force microscopy. Nearly all of the samples display strong segregation between the two blocks, owing to the large molar mass of typical bottlebrush polymers. Consequently, only one experimental sample displays an accessible order-disorder transition temperature. The strong segregation is also shown to affect the ability of large bottlebrush diblocks to readily achieve well-ordered nanostructures by self-assembly. Finally, results of the most symmetric (by volume fraction) diblock samples are compared with predictions of a newly developed self-consistent field theory model, yielding remarkable quantitative agreement. The theory is further utilized to conclusively establish the molecular origins of the domain scaling behavior in lamellar forming diblock bottlebrush polymers.
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    Eeffective pDNA and CRISPR RNP delivery promoted by design of cationic bottlebrush and combinatorial polymers synthesis
    (2022-07) Dalal, Rishad
    The field of gene therapy has grown in response of the millions of people who suffer from genetic diseases worldwide. As genetic payloads need a delivery carrier, cationic polymeric vectors have grown in promise as delivery vehicles that are more cost-effective, scalable, and stable in comparison to viral vectors. The field of polymeric gene delivery has focused on improving delivery efficiency through chemical and structural modifications. Herein, we have made steps towards understanding how architectural modifications and how structure property relationships can improve the field of gene delivery. Initially it was found that when comparing cationic homopolymers, a bottlebrush architecture outperformed a linear analog in over pDNA delivery efficacy. Follow-up studies explored how bottlebrush end-group hydrophilicity can play a role in balancing colloidal stability, gene expression, and cellular viability. In addition to architectural understanding, studies to understand how structure-property relationships within linear polymers were explored in which a combinatorial library of 36 polymers was synthesized and used to deliver pDNA and CRISPR-Cas9 RNP. Machine learning aided in optimizing and analyzing structural relationships relative to expression outputs. Overall, we were able to create guides in improving gene expression through the optimization of polymer macromolecular structure and unique chemical understanding per biological payload.
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    Molecular bottlebrushes: new routes to self-assembled morphologies with small periodicities
    (2021-09) Karavolias, Michael
    Linear A/B block polymer self-assembly offers exciting opportunities for bottom-up nanostructured materials design. For any chemically incompatible A/B monomer pair, there is a minimum degree of polymerization (N) required for melt self-assembly that sets the smallest microdomain (d) periodicity that such a material can form. This thermodynamic limitation impacts some block polymer applications, especially sub-10 nm templating for microelectronic device manufacture. Nonlinear polymer architectures offer opportunities to manipulate A/B block polymer self-assembly thermodynamics without resorting to new monomer chemistries. This thesis compares the morphologies and self-assembly thermodynamics of parent poly(lactide)-block-poly(𝜀-decalactone)-block-poly(lactide) (LDL) polymers bearing a midchain norbornene functionality to those of daughter core-shell bottlebrushes (csBBs) with varied backbone degrees of polymerization (Nbb), which derive from living “grafting through” polymerization of the norbornyl moieties. We specifically quantify how the d-spacings and order-disorder transition temperatures of the parent LDL triblocks change on enchainment into csBBs using small-angle X-ray scattering. Across lactide volume fractions fL = 0.27–0.73, we demonstrate that the csBB architecture thermodynamically stabilizes microphase separated melts and that it can drive ordering of disordered parent LDL triblocks. On this basis, we develop general phenomenological relationships for the composition-dependent critical degree of polymerization of the LDL triblock arms (Narm) for melt self-assembly in terms of Nbb, which enable new materials designs with ever smaller d-spacings. In compositionally asymmetric LDL triblocks and their daughter csBBs, we establish that the csBB architecture also alters the observed microphase separated morphologies. When the total volume fraction of the L block is fL = 0.27-0.34, the parent LDL polymers form hexagonally-packed cylinders and micellar Frank-Kasper A15, σ, and dodecagonal quasicrystal phases. However, enchainment into csBBs induces a spheres-to-cylinders transition with increasing Nbb. When fL = 0.73, the parent LDL triblocks form hexagonally-packed cylinders yet the csBBs form both double gyroid and metastable modulated lamellar phases. Thus, the csBB architecture induces an interfacial curvature reduction in the observed morphology relative to the parent triblock. Thus, this work establishes that the csBBs of A/B block polymers offer new opportunities to tune both the accessible morphologies and their thermodynamic stabilities.
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    Supporting Data for Mesoscopic Morphologies in Frustrated ABC Bottlebrush Block Terpolymers
    (2025-02-03) Cui, Shuquan; Murphy, Elizabeth A; Santra, Subrata; Bates, Frank S; Lodge, Timothy P; lodge@umn.edu; Lodge, Timothy P; University of Minnesota Department of Chemistry
    Bottlebrush block polymers, characterized by densely grafted side chains extending from a backbone, have recently garnered significant attention. A particularly attractive feature is the accessibility of ordered morphologies with domain spacings exceeding several hundred nanometers, a capability that is challenging to achieve with linear polymers. These large morphologies make bottlebrush block polymers promising for various applications, such as photonic crystals. However, the structures observed in AB diblock bottlebrushes are generally limited to simple lamellae and cylindrical phases, which restricts their use in many applications. In this study, we synthesized a large library of 50 ABC bottlebrush triblock terpolymers, poly(DL-lactide)-b-poly(ethylene-alt-propylene)-b-polystyrene (PLA-PEP-PS), spanning a wide range of compositions using ring-opening metathesis polymerization (ROMP) of norbornene-functionalized macromonomers. This constitutes a frustrated system, in that the mandatory internal interfaces (PLA/PEP) have larger interfacial energies than PLA/PS. We systematically explored phase behavior using small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). Morphological characterization revealed a series of intriguing mesoscopic structures, including layered microstructures, core-shell hexagonally packed cylinders (CSHEX, plane group p6mm), alternating tetragonally packed cylinders (ATET, plane group p4mm), and an unprecedented morphology, rectangular centered cylinders-in-undulating-lamellae (RCCUL, plane group c2mm). Adjustments in molecular weight resulted in a wide range of unit cell dimensions (exemplified by RCCUL), from 40 nm to over 130 nm. This work demonstrates that multiblock bottlebrushes offer promising opportunities for developing materials with novel diverse structures and a broad range of domain dimensions.
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    Understanding Polymer-Lipid Bilayer Interactions
    (2023) Hassler, Joseph
    Cell membrane instability is a common feature to Duchenne Muscular Dystrophy, heart attacks, strokes, and traumatic brain injury, which together affect over a million people in the United States every year and currently have no clinical treatment. In 1992, it was discovered that poloxamers, a class of biocompatible block polymer amphiphile stabilized cell membranes under stress, thereby having therapeutic potential. Unfortunately, the stabilization mechanism is not fully understood, hindering the engineering of more effective treatments.Bottlebrush polymers have a wide parameter space and known relationships between architectural parameters and polymer properties, enabling their use as a tool for mechanistic investigations of polymer−lipid bilayer interactions. In this thesis, I report a synthetic strategy for making grafted block polymers with poly(propylene oxide) and poly(ethylene oxide) side chains, “bottlebrush poloxamers (BBPs).” Combined anionic and sequential ring-opening metathesis polymerization yielded low dispersity polymers, at full conversion of the macromonomers, with control over graft length, graft end-groups, and overall molecular weight. Dynamic light scattering and transmission electron microscopy were used to characterize micelle formation in aqueous buffer. The critical micelle concentration scales exponentially with overall molecular weight for both linear and bottlebrush poloxamers; however, the scaling coefficient is two orders of magnitude smaller in the bottlebrush architecture compared to the linear architecture, suggesting that micellization of BBPs is less sensitive to molecular weight. I then employed this synthetic platform to create a set of BBPs over a range of molecular weight, with two PEO block side chain lengths, and with block and statistical architectures. Then, this set of molecules was used to interrogate the effects of bottlebrush architectural parameters on binding to, and protection of, phospholipid bilayers using pulsed-field-gradient NMR and an in vitro osmotic stress assay, respectively. I found that the binding affinity of a bottlebrush poloxamer (BBP) ("B-" "E" _"10" ^"43" "P" _"5" ^"15" , Mn = 26 kDa) is about 3 times higher than a linear poloxamer with a similar composition and number of PPO units (L-E93P54E93, Mn = 11 kDa). Furthermore, BBP binding is sensitive to overall molecular weight, side-chain length, and architecture (statistical versus block). Finally, all tested BBPs exhibit a protective effect on cell membranes under stress at sub-μM concentrations. As the factors controlling membrane affinity and protection efficacy of bottlebrush poloxamers are not understood, these results provide important insight into how they adhere to and stabilize a lipid bilayer surface. The final two chapters of this thesis return to commercially available, linear poloxamers and seek to understand the effect of temperature and the role of lipid phase coexistence on poloxamer-liposome interactions. Hydrogen bonding between water and oxygen atoms in PEO and PPO units results in thermoresponsive behavior because the bound water shell around both blocks dehydrates as temperature increases. This motivates an investigation of poloxamer-lipid bilayer interactions as a function of temperature and thermal history. Pulsed-field gradient NMR spectroscopy measurements revealed that the fraction of chains bound to 1-palmitoyl-2-oleoyl-glycero-3-phosphocholine (POPC) liposomes increased by 11 (± 3)× at 37 °C relative to 27 °C. Moreover, following incubation at 37 °C, it takes weeks for the system to re-equilibrate at 25 °C. Such slow desorption kinetics suggests that at elevated temperatures polymer chains can pass through the bilayer and access the interior of the liposomes, a mechanism that is inaccessible at lower temperatures. We propose a molecular mechanism to explain this effect, which could have important ramifications on the cellular distribution of ABPs and could be exploited to modulate mechanical and surface properties of liposomes and cell membranes. The lipid raft and picket fence models assert that the cell membrane contains liquid ordered domains (Lo) among a matrix of liquid disordered domains (Ld). These domains have different structural and physical properties, affecting protein conformation, cell signaling, and cellular processes. Therefore, I employed a liposome model consisting of a saturated lipid, an unsaturated lipid, and cholesterol that has a well-documented phase space to explore how lipid phase behavior affects polymer binding. I found that polymer binding is maximized in a window of the phase space coinciding with coexistence of the two liquid domains. This is likely because the borders between the Lo and Ld domains are attractive binding sites. The proximity between bound polymer and lipid rafts could provide a non-specific mechanism by which flexible, non-polar amphiphilic block polymers affect cell signaling.