10 research outputs found

    Emulsion/Surface Interactions from Quiescent Quartz Crystal Microbalance Measurements with an Inverted Sensor

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    Interactions of three oil-in-water emulsion types with polystyrene-coated quartz crystal microbalance (QCM) sensor surfaces were probed with the QCM cell in both the conventional orientation (i.e., polystyrene surface on the bottom, “looking up”) and the inverted orientation (polystyrene on top interior surface of sensor chamber, “looking down”). With the conventionally oriented QCM sensors, the adsorption of soluble and/or dispersed species quickly gave steady-state frequency and dissipation outputs. By contrast, the inverted sensors gave changing responses at long times because of the gravity driven buildup of a viscous consolidation layer next to but not necessarily bound to the sensor surface. Three emulsion types (a simple hexadecane/phosphatidylcholine emulsion, 2% homogenized milk, and a diluted commercial ophthalmic emulsion) displayed a wide range of behaviors. We propose that quiescent QCM measurement made with an inverted sample chamber is a new approach to probing emulsion behaviors near solid surfaces

    Degradable Microgel Wet-Strength Adhesives: A Route to Enhanced Paper Recycling

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    Demonstrated is a new approach to cellulose fiber-based materials that are strong when wet, yet can be recycled after exposure to a weak reducing agent. Poly­(<i>N</i>-isopropylacrylamide-<i>co</i>-acrylic acid) microgels were transformed into wet cellulose adhesives by incorporation of hydrazide groups that can form hydrazone linkages to oxidized cellulose. Reductant responsivity was obtained by introducing cleavable disulfide linkages, either in the chains tethering the adhesive hydrazide groups, or by using disulfide cross-links in the microgels. Both types of disulfide derivatives gave about 75% reduction in cellulose wet adhesion after exposure to a reductant. Truly sustainable wood-fiber replacements for plastic packaging must be insensitive to water while being fully recyclable; this work demonstrates two routes to reversible wet cellulose adhesives, facilitating recycling

    Stable Aqueous Foams from Cellulose Nanocrystals and Methyl Cellulose

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    The addition of cellulose nanocrystals (CNC) greatly enhanced the properties of methylcellulose (MC) stabilized aqueous foams. CNC addition decreased air bubble size, initial foam densities and drainage rates. Mixtures of 2 wt % CNC + 0.5 wt % MC gave the lowest density foams. This composition sits near the onset of nematic phase formation and also near the overlap concentration of methylcellulose. More than 94% of the added CNC particles remained in the foam phase, not leaving with the draining water. We propose that the nanoscale CNC particles bind to the larger MC coils both in solution and with MC at the air/water interface, forming weak gels that stabilize air bubbles. Wet CNC-MC foams were sufficiently robust to withstand high temperature (70 °C for 6 h) polymerization of water-soluble monomers giving macroporous CNC composite hydrogels based on acrylamide (AM), 2-hydroxyethyl methacrylate (HEMA), or polyethylene glycol diacrylate (PEGDA). At high temperatures, the MC was present as a fibrillar gel phase reinforced by CNC particles, explaining the very high foam stability. Finally, our CNC-MC foams are based on commercially available forms of CNC and MC, already approved for many applications. This is a “shovel-ready” technology

    Relating Nanoparticle Shape and Adhesiveness to Performance as Flotation Collectors

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    Cationic polystyrene-core-poly­(<i>n</i>-butyl methacrylate)-shell (PS–PB) nanoparticles perform as flotation collectors as they spontaneously adsorb onto 43 ÎŒm glass beads in water, promoting glass bead attachment to air bubbles. Under our flotation conditions at room temperature, polystyrene is a hard plastic, whereas, with glass transition near room temperature, poly­(<i>n</i>-butyl methacrylate) is a soft polymer. Colloidal probe atomic force microscopy measurements revealed that the pull-off forces and the work of adhesion of PS–PB nanoparticles to glass were significantly higher than observed with harder PS particles. Glass bead recovery in laboratory flotation experiments increased significantly with thickness of the soft PB shells on the PB–PS core/shell nanoparticles. Ninety-two nm Janus particles consisting of one PS and one PB lobe were also very effective collectors. We propose that high nanoparticle/glass bead adhesion minimizes nanoparticle removal by bead/bead collisions (nanoscale ball milling) during mixing and flotation

    Phase Behavior of Aqueous Poly(acrylic acid‑<i>g</i>‑TEMPO)

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    Poly­(acrylic acid) with grafted TEMPO moieties, PAA-T, phase separates over the pH range 2–4, whereas at lower and higher pH, the polymer is water-soluble. The pH range of the two-phase region increases linearly with the content of grafted TEMPO moieties. As evidenced by surface tension measurements, the PAA-T molecules have little hydrophobic character. We propose that at low pH PAA-T is an amphoteric polymer that, in addition the anionic carboxyl groups, contains cationic species resulting from the disproportion of TEMPO at low pH. Furthermore, we propose that two-phase regions occur near the isoelectric points of the polymer, suggesting that phase separation is due to electrostatically driven association of polymer molecules

    A Colloidal Stability Assay Suitable for High-Throughput Screening

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    A library of 32 polystyrene copolymer latexes, with diameters ranging between 53 and 387 nm, was used to develop and demonstrate a high-throughput assay using a 96-well microplate platform to measure critical coagulation concentrations, a measure of colloidal stability. The most robust assay involved an automated centrifugation–decantation step to remove latex aggregates before absorbance measurements, eliminating aggregate interference with optical measurements made through the base of the multiwell plates. For smaller nanoparticles (diameter <150 nm), the centrifugation–decantation step was not required as the interference was less than with larger particles. Parallel measurements with a ChemiDoc MP plate scanner gave indications of aggregation; however, the results were less sensitive than the absorbance measurements

    Hydrazide-Derivatized Microgels Bond to Wet, Oxidized Cellulose Giving Adhesion Without Drying or Curing

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    Hydrazide-derivatized poly­(<i>N</i>-isopropylacrylamide-<i>co</i>-acrylic acid) microgels gave strong adhesion to wet, TEMPO oxidized, regenerated cellulose membranes without a drying or heating step. Adhesion was attributed to hydrazone covalent bond formation with aldehyde groups present on the cellulose surfaces. This is one of only three chemistries we have found that gives significant never-dried adhesion between wet cellulose surfaces. By contrast, for cellulose joints that have been dried and heated before wet testing, the hydrazide-hydrazone chemistry offers no advantages over standard paper industry wet strength resins. The design rules for the hydrazide-microgel adhesives include: cationic microgels are superior to anionic gels; the lower the microgel cross-link density, the higher the adhesion; longer PEG-based hydrazide tethers offer no advantage over shorter attachments; and, adhesion is independent of microgel diameter. Many of these rules were in agreement with predictions of a simple adhesion model where the microgels were assumed to be ideal springs. We propose that the unexpected, high cohesion between neighboring microgels in multilayer films was a result of bond formation between hydrazide groups and residual NHS-carboxyl esters from the preparation of the hydrazide microgels

    Low Molecular Weight Shuttle Molecules Enhance Polychloramide Antimicrobial Activity

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    Investigated were the influences of succinimide (SI), 5,5-dimethyl­hydantoin (DMH), and 3-(hydroxy­methyl)-5,5-dimethyl­hydantoin (HDMH) on the biocidal activity of chlorinated, water-soluble polyamide prepared by the reaction of isopropylamine with poly(styrene-alt-maleic anhydride). The resulting polymer was a negatively charged, water-soluble polymer bearing a carboxylic acid and an isopropylamide moiety on nearly every repeat unit. Subsequent treatment with NaOCl chlorinated the polymers to up to 4.4% Cl while inflicting some polymer chain scission. SI, DMH, or HDMH increased the biocidal activity of polychloramides toward planktonic Escherichia coli and Staphylococcus aureus. Independent solution studies confirmed that oxidative chlorine spontaneously transferred from aqueous polychloramides to small molecules. We concluded that SI, DMH, and HDMH acted as shuttles that extracted oxidative Cl from the polymer chloramides and transported oxidative Cl more efficiently to microbial surfaces. Succinimide was the most effective shuttle. These results warn that some low molecular weight soluble molecules in antimicrobial testing solutions may exaggerate the effectiveness of the polymer or immobilized antimicrobial agents

    Redox Properties of Polyvinylamine‑<i>g</i>‑TEMPO in Multilayer Films with Sodium Poly(styrenesulfonate)

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    Layer-by-layer (LbL) assemblies of polyvinylamine with grafted TEMPO moieties (PVAm-T) with sodium polystyrenesulfonate (PSS) were prepared on gold-sulfonate surfaces, and the redox properties were measured by cyclic voltammetry. LbL compositions were probed by quartz crystal microbalance (wet) and ellipsometric (dry) film measurements. Approximately 30% of the TEMPO moieties in the LbL assemblies were redox-active when the total TEMPO coverage was varied up to 6 ÎŒmol/m<sup>2</sup>, by either varying the TEMPO content in PVAm-T or by varying the number of LbL bilayers. Three non-redox-active PVAm/PSS blocking bilayers were required to prevent the electrode from oxidizing PVAm-T in the exterior LbL layer. This suggests significant intermixing between the layers in the LbL film. In addition to contributing to the small but growing body of work on redox polymers based on grafted TEMPO, this work serves as a reference point for understanding the redox properties of colloidal PVAm-T-laccase complexes in future work

    Design Rules for Fluorocarbon-Free Omniphobic Solvent Barriers in Paper-Based Devices

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    The utility of hydrophobic wax barriers in paper-based lateral flow and multiwell devices for containment of aqueous solvents was extended to organic solvents and challenging aqueous surfactant solutions by preparation of a three layer barrier, consisting of internal pullulan impregnated paper barriers surrounded by external wax barriers. When paper impregnated with pullulan solution dries, the polymer forms solvent blocking lenses in the paper structure. Lens formation was illustrated by forming pullulan lenses in glass capillaries. The lens shapes were less curved compared to the predictions of a model based upon minimizing surface area. For barriers on Whatman # 1 filter paper, the pullulan molecular weight must be greater than ∌70 kDa, the mass fraction of pullulan in the barrier zone must be at least 32%, and there are restrictions on the minimum width of the pullulan impregnated zone
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