Browsing by Author "Li, Xianyu"

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    Properties and applications of bottlebrush polymers
    (2014-04-21) Li, Xianyu; Verduzco, Rafael; Chapman, Walter G.; Li, Qilin
    This work presented in this dissertation aims to understand physical and chemical properties of bottlebrush polymers and determine their potential for applications including for anti-fouling surfaces and for reduction in oil-water interfacial tension. This dissertation focus on the following four parts: the synthesis of well-defined bottlebrush polymers and investigation of stimuli-responsive film properties; analysis of the solution phase behaviors and conformation of bottlebrush polymers with thermally-responsive side chains; the behavior of bottlebrush polymers in blends with linear polymers; and the development of bottlebrush polymers for antifouling surfaces and for reducing the oil-water interfacial tension. In chapter 1, we provide an introduction to bottlebrush polymers. Details on the synthesis, conformation, phase behavior, potential applications, and recent work with bottlebrush polymers are discussed. Chapter 2 describes the preparation of stimuli-responsive bottlebrush thin films with hydrophobic polystyrene (PS) and hydrophilic poly(ethylene oxide) (PEO) amphiphilic side-chains. We find that due to the conformational flexibility of the polymeric side-chains, bottlebrush polymer films exhibit a processing dependent contact angle. This behavior is analogous to that observed for polymer brush films and enables a more scalable approach to responsive, brushy polymer films. In Chapter 3, we explore the solution phase behavior of bottlebrush polymers with thermoresponsive poly(N-isopropylacrylamide) (PNIPAAM) side-chains. PNIPAAM is a water-soluble polymer that exhibits a lower critical solution temperature (LCST). Due to the unique structure of bottlebrush polymers, PNIPAAM bottlebrush polymers exhibit very different solution phase behavior compared with linear PNIPAAM. We show that the LCST depends on the side-chain length and side-chain endgroup, and the bottlebrush polymer side-chains collapse on approaching the LCST. PNIPAAM bottlebrush polymers form lyotropic liquid crystal phases for sufficiently long PNIPAAM side-chains. In Chapter 4 we explore the properties of bottlebrush polymers in blends with linear polymers. This work is aimed at investigating the potential of bottlebrush polymers to be used as additives for modifying polymer thin films. We show that bottlebrush polymers are interfacially active and spontaneously segregate to the film-substrate and film-air interface. The interfacial segregation of bottlebrush polymer additives was systematically studied for varying lengths of linear polymers and bottlebrush polymer side-chains through secondary ion mass spectrometry (SIMS), neutron reflectivity and optical microscopy. These results demonstrate that relatively small amounts of bottlebrush polymer additives can be used to tailor interfaces. Chapter 5 presents preliminary results on the antifouling properties and oil-water interfacial properties of bottlebrush polymers. Stable films were prepared by introducing cross-linker into the bottlebrush side-chains, and we demonstrate that PNIPAAM bottlebrush thin films have comparable cell resistance to that PEO polymer brushes. To study bottlebrush polymers at the oil-water interface, both pure bottlebrush polymers and a mixture of surfactant 4,5-orthoxylene sulfonate (OXS) were used as additives in water-chloroform blends. Experimental measurement of the interfacial tension show that the presence of bottlebrush polymer can reduce the interfacial tension significantly, larger than that for commercial surfactant. Although more work is needed, preliminary results are promising for bottlebrush polymers as interfacial tension reducer and anti-fouling surface materials.
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    Structure, function, self-assembly, and applications of bottlebrush copolymers
    (Royal Society of Chemistry, 2015) Verduzco, Rafael; Li, Xianyu; Pesek, Stacy L.; Stein, Gila E.
    Bottlebrush polymers are a type of branched or graft polymer with polymeric side-chains attached to a linear backbone, and the unusual architectures of bottlebrushes provide a number of unique and potentially useful properties. These include a high entanglement molecular weight, enabling rapid self-assembly of bottlebrush block copolymers into large domain structures, the self-assembly of bottlebrush block copolymer micelles in a selective solvent even at very low dilutions, and the functionalization of bottlebrush side-chains for recognition, imaging, or drug delivery in aqueous environments. This review article focuses on recent developments in the field of bottlebrush polymers with an emphasis on applications of bottlebrush copolymers. Bottlebrush copolymers contain two (or more) different types of polymeric side-chains. Recent work has explored the diverse properties and functions of bottlebrush polymers and copolymers in solutions, films, and melts, and applications explored include photonic materials, bottlebrush films for lithographic patterning, drug delivery, and tumor detection and imaging. We provide a brief introduction to bottlebrush synthesis and physical properties and then discuss work related to: (i) bottlebrush self-assembly in melts and bulk thin films, (ii) bottlebrushes for photonics and lithography, (iii) bottlebrushes for small molecule encapsulation and delivery in solution, and (iv) bottlebrush micelles and assemblies in solution. We briefly discuss three potential areas for future research, including developing a more quantitative model of bottlebrush self-assembly in the bulk, studying the properties of bottlebrushes at interfaces, and investigating the solution assembly of bottlebrush copolymers.
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    Thermoresponsive PNIPAAM bottlebrush polymers with tailored side-chain length and end-group structure
    (The Royal Society of Chemistry, 2014) Li, Xianyu; ShamsiJazeyi, Hadi; Pesek, Stacy L.; Agrawal, Aditya; Hammouda, Boualem; Verduzco, Rafael
    We explore the phase behaviour, solution conformation, and interfacial properties of bottlebrush polymers with side-chains comprised of poly(N-isopropylacrylamide) (PNIPAAM), a thermally responsive polymer that exhibits a lower critical solution temperature (LCST) in water. PNIPAAM bottlebrush polymers with controlled side-chain length and side-chain end-group structure are prepared using a ﾓgrafting-throughﾔ technique. Due to reduced flexibility of bottlebrush polymer side-chains, side-chain end-groups have a disproportionate effect on bottlebrush polymer solubility and phase behaviour. Bottlebrush polymers with a hydrophobic end-group have poor water solubilities and depressed LCSTs, whereas bottlebrush polymers with thiol-terminated side-chains are fully water-soluble and exhibit an LCST greater than that of PNIPAAM homopolymers. The temperature-dependent solution conformation of PNIPAAM bottlebrush polymers in D2O is analyzed by small-angle neutron scattering (SANS), and data analysis using the Guinier-Porod model shows that the bottlebrush polymer radius decreases as the temperature increases towards the LCST for PNIPAAM bottlebrush polymers with relatively long 9 kg mol1 sidechains. Above the LCST, PNIPAAM bottlebrush polymers can form a lyotropic liquid crystal phase in water. Interfacial tension measurements show that bottlebrush polymers reduce the interfacial tension between chloroform and water to levels comparable to PNIPAAM homopolymers without the formation of microemulsions, suggesting that bottlebrush polymers are unable to stabilize highly curved interfaces. These results demonstrate that bottlebrush polymer side-chain length and flexibility impact phase behavior, solubility, and interfacial properties.