Browsing by Author "Qi, Luqing"

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    Amphiphilic Bottlebrush Block Copolymers: Analysis of Aqueous Self-Assembly by Small-Angle Neutron Scattering and Surface Tension Measurements
    (American Chemical Society, 2019) Alaboalirat, Mohammed; Qi, Luqing; Arrington, Kyle J.; Qian, Shuo; Keum, Jong K.; Mei, Hao; Littrell, Kenneth C.; Sumpter, Bobby G.; Carrillo, Jan-Michael Y.; Verduzco, Rafael; Matson, John B.
    A systematic series of 16 amphiphilic bottlebrush block copolymers (BCPs) containing polystyrene and poly(N-acryloylmorpholine) (PACMO) side chains were prepared by a combination of atom-transfer radical polymerization (ATRP), photoiniferter polymerization, and ring-opening metathesis polymerization (ROMP). The grafting-through method used to prepare the polymers enabled a high degree of control over backbone and side-chain molar masses for each block. Surface tension measurements on the self-assembled amphiphilic bottlebrush BCPs in water revealed an ultralow critical micelle concentration (cmc), 1–2 orders of magnitude lower than linear BCP analogues on a molar basis, even for micelles with >90% PACMO content. Combined with coarse-grained molecular dynamics simulations, fitting of small-angle neutron scattering traces (SANS) allowed us to evaluate solution conformations for individual bottlebrush BCPs and micellar nanostructures for self-assembled macromolecules. Bottlebrush BCPs showed an increase in anisotropy with increasing PACMO content in toluene-d8, which is a good solvent for both blocks, reflecting an extended conformation for the PACMO block. SANS traces of bottlebrush BCPs assembled into micelles in D2O, a selective solvent for PACMO, were fitted to a core–shell–shell model, suggesting the presence of a partially hydrated inner shell. Results showed an average micelle diameter of 40 nm with combined shell diameters ranging from 16 to 18 nm. A general trend of increased stability of micelles (i.e., resistance to precipitation) was observed with increases in PACMO content. These results demonstrate the stability of bottlebrush polymer micelles, which self-assemble to form spherical micelles with ultralow (<70 nmol/L) cmc’s across a broad range of compositions.
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    Bottlebrush Copolymer Additives for Immiscible Polymer Blends
    (American Chemical Society, 2018) Mah, Adeline Huizhen; Afzali, Pantea; Qi, Luqing; Pesek, Stacy; Verduzco, Rafael; Stein, Gila E.
    Thin films of immiscible polymer blends will undergo phase separation into large domains, but this behavior can be suppressed with additives that accumulate and adhere at the polymer/polymer interface. Herein, we describe the phase behavior of polystyrene/poly(methyl methacrylate) (PS/PMMA) blends with 20 vol % of a bottlebrush additive, where the bottlebrush has poly(styrene-r-methyl methacrylate) side chains with 61 mol % styrene. All blends are cast into films and thermally annealed above the glass transition temperature. The phase-separated structures are measured as a function of time with atomic force microscopy and optical microscopy. We demonstrate that subtle changes in bottlebrush architecture and homopolymer chain lengths can have a large impact on phase behavior, domain coarsening, and domain continuity. The bottlebrush additives are miscible with PS under a broad range of conditions. However, these additives are only miscible with PMMA when the bottlebrush backbones are short or when the PMMA chains are similar in length to the bottlebrush side chains. Otherwise, the limited bottlebrush/PMMA miscibility drives the formation of a bottlebrush-rich interphase that encapsulates the PMMA-rich domains, stabilizing the blend against further coarsening at elevated temperatures. The encapsulated domains are aggregated in short chains or larger networks, depending on the blend composition. Interestingly, the network structures can provide continuity in the minor phases.
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    Interfacial Properties of Polymeric Nanomaterials
    (2017-12-01) Qi, Luqing; Verduzco, Rafael; Hirasaki, George J.
    Polymeric nanomaterials such as nanoparticles and branched polymers are interfacially active and can be used to stabilize or increase the viscosity of an emulsion, which is potentially useful for enhanced oil recovery (EOR) process. However, nanoparticles can be limited in terms of compatibility with different environments, such as elevated salinities and temperature and present challenges for demulsification. The research presented in this dissertation mainly focuses on my work on the development of polymer-coated nanoparticles (PNPs) which are amphiphilic, surface active, and stimuli-responsive. Polymer-coated nanoparticles are prepared by covalently grafting polymers to the surface of the nanoparticles. The interaction of the PNPs with the environment can be tailored through variation of the polymer attached to the nanoparticle surface. In Chapter 2 is the work on the development of carbon black which can segregate to oil-water-surfactant bicontinuous microemulsions. These PNPs are prepared by attaching hydrophobic and hydrophilic chains to the surface of carbon black nanoparticles. By tuning the surface charge through variation of the degree of sulfation, the hydrophobic/hydrophilic properties and stability can be tuned. We find that highly-sulfated PNPs are stable to elevated salinities and spontaneously segregate to oil-water-surfactant microemulsions. Chapter 3 is the development of pH-responsive PNPs. These are prepared by grafting a pH-responsive polymer to silica nanoparticles. These nanoparticles can emulsify crude oil, and be quickly demulsified through a change in pH. We demonstrate that the use of 0.1 wt % of pH-responsive PNPs enhance the recovery of crude oil in a sandpack. This work demonstrates the versatility and potential of PNPs for a variety of applications, including oil recovery, controlled emulsification and demulsification, and reduction of interfacial tension. Chapter 4 is the study of aqueous self-assembly of bottlebrush block copolymers, which have similarity to polymer-coated nanoparticles in many aspects. It is able to self-assemble rapidly to form structures with large periodic domains and form stable micelles with very low critical-micelle concentrations (CMC). In this work, we have studied a library of amphiphilic bottlebrush polymers with different ratio of hydrophilic and hydrophobic chains, and demonstrated a lower CMC of bottlebrush polymer over linear copolymers. Some primary work to quantify the self-assembly of a model library of amphipilic bottlebrush block polymers through small-angle neutron scattering (SANS) measurement is discussed.
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    Segregation of Amphiphilic Polymer-Coated Nanoparticles to Bicontinuous Oil/Water Microemulsion Phases
    (American Chemical Society, 2017) Qi, Luqing; ShamsiJazeyi, Hadi; Ruan, Gedeng; Mann, Jason A.; Lin, Yen-Hao; Song, Chen; Ma, Yichuan; Wang, Le; Tour, James M.; Hirasaki, George J.; Verduzco, Rafael
    Polymer-coated nanoparticles are interfacially active and have been shown to stabilize macroscopic emulsions of oil and water, also known as Pickering emulsions. However, prior work has not explored the phase behavior of amphiphilic nanoparticles in the presence of bicontinuous microemulsions. Here, we show that properly designed amphiphilic polymer-coated nanoparticles spontaneously and preferentially segregate to the bicontinuous microemulsion phases of oil, water, and surfactant. Mixtures of hydrophilic and hydrophobic chains are covalently grafted onto the surface of oxidized carbon black nanoparticles. By sulfating hydrophilic chains, the polymer-coated nanoparticles are stable in the aqueous phase at salinities up to 15 wt % NaCl. These amphiphilic, negatively charged polymer-coated nanoparticles segregate to the bicontinuous microemulsion phases. We analyzed the equilibrium phase behavior of the nanoparticles, measured the interfacial tension, and quantified the domain spacing in the presence of nanoparticles. This work shows a novel route to the design of polymer-coated nanoparticles which are stable at high salinities and preferentially segregate to bicontinuous microemulsion phases.