Showing papers by "Kenneth R. Shull published in 2014"

Upper-critical solution temperature (UCST) polymer functionalized graphene oxide as thermally responsive ion permeable membrane for energy storage devices

Abstract: A thermally responsive membrane separator, suitable for use in non-aqueous electrolytes, was constructed by grafting a UCST polymer, poly(sulfobetaine), onto graphene oxide sheets. When heated from 20 to 80 °C, it decreased the specific storage capacity of the electrode for Li by >50% reversibly, compared to 30% increase without polymer modification.

High-Frequency Rheological Characterization of Homogeneous Polymer Films with the Quartz Crystal Microbalance

Abstract: We utilize quartz crystal resonators operating at multiple resonant harmonics to measure the high-frequency rheological properties of materials with a broad range of viscoelastic properties. The technique is demonstrated with poly(t-butyl acrylate) films in the vicinity of the calorimetrically determined glass transition and with rubbery polyisoprene films. The technique is a noncontact technique that can be used to quantify the temperature or time-dependent viscoelastic response in homogeneous films with thicknesses in the micrometer range. This work complements the ability of the resonators to quantify the viscoelastic behavior of viscoelastic polymer solutions and simple Newtonian liquids. For each material we obtain the density-shear modulus product and the viscoelastic phase angle at frequencies of 5 and 15 MHz. A standardized analysis protocol is described that enables this information to be obtained reliably and accurately. The polyisoprene data are found to be in good agreement with measurements obtained by dynamic mechanical analysis using extrapolated temperature shift factors.

Formation and Mechanical Characterization of Ionically Crosslinked Membranes at Oil/Water Interfaces

Abstract: Here we report on the preparation and mechanical characterization of a 2-D self-assembled membrane formed by ionically crosslinking the polyelectrolyte parts of a gradient amphiphilic copolymer at oil and water interfaces To fabricate these membranes, chloroform solutions of styrene–acrylic acid copolymers were suspended as pendant drops in an aqueous embedding phase Due to the amphiphilic nature of these molecules, the copolymer chains migrate to the oil–water interface creating an interfacial layer Upon the addition of zinc acetate to the embedding phase, crosslinks between copolymer molecules are formed via zinc–carboxylate complexes While ionically crosslinked block copolymer membranes were critically damaged after one expansion cycle, ionically crosslinked gradient copolymers formed durable membranes that maintained their physical integrity through multiple expansion–compression–expansion cycles This difference in mechanical behavior is attributed to the fact that gradient copolymers are more effective interfacial modifiers and have a significantly different molecular alignment at the oil–water interface Additionally by changing the incubation time from 20 to 30 minutes, the low-strain dilatational modulus of these membranes was significantly increased due to higher interfacial coverage and crosslinking density Longer incubation times also led to a distinct yield point and plastic deformation behavior at larger strains Further mechanical characterization of the membranes showed that they can be quite robust and that by replacing the internal oil phase with an aqueous solution, future testing of membrane filtration and permeation may be possible