Tuning the Photoexcitation Response of Cyanobacterial Photosystem I via Reconstitution into Proteoliposomes

The role of natural thylakoid membrane housing of Photosystem I (PSI), the transmembrane photosynthetic protein, in its robust photoactivated charge separation with near unity quantum efficiency is not fundamentally understood. To this end, incorporation of suitable protein scaffolds for PSI incorporation is of great scientific and device manufacturing interest. Areas of interest include solidstate bioelectronics, and photoelectrochemical devices that require bio-abio interfaces that do not compromise the photoactivity and photostability of PSI. Therefore, the surfactant-induced membrane solubilization of a negatively charged phospholipid (DPhPG) with the motivation of creating biomimetic reconstructs of PSI reconstitution in DPhPG liposomes is studied. Specifically, a simple yet elegant method for incorporation of PSI trimeric complexes into DPhPG bilayer membranes that mimic the natural thylakoid membrane housing of PSI is introduced. The efficacy of this method is demonstrated via absorption and fluorescence spectroscopy measurements as well as direct visualization using atomic force microscopy. This study provides direct evidence that PSI confinements in synthetic lipid scaffolds can be used for tuning the photoexcitation characteristics of PSI. Hence, it paves the way for development of fundamental understanding of microenvironment alterations on photochemical response of light activated membrane proteins

Hybrid Nanocomposites of Nanostructured Co3O4 Interfaced with Reduced/Nitrogen-Doped Graphene Oxides for Selective Improvements in Electrocatalytic and/or Supercapacitive Properties

Performance enhancements in next-generation electrochemical energy storage/conversion devices require the design of new classes of nanomaterials that exhibit unique electrocatalytic and supercapacitive properties. To this end, we report the use of laser ablation synthesis in solution (LASiS) operated with cobalt as the target in graphene oxide (GO) solution in tandem with two different post treatments to manufacture three kinds of hybrid nanocomposites (HNCs) namely, (1) Co3O4 nanoparticle (NP)/reduced graphene oxide (rGO), (2) Co3O4 nanorod (NR)/rGO, and (3) Co3O4 NP/nitrogen-doped graphene oxide (NGO). FTIR and Raman spectroscopic studies indicate that both chemical and charge driven interactions are partially responsible for embedding the Co3O4 NPs/NRs into the various GO films. We tune the selective functionalities of the as-synthesized HNCs as oxygen reduction reaction (ORR) catalysts and/or supercapacitors by tailoring their structure–property relationships. Specifically, the nitrogen doping in the NP/NGO HNC samples promotes higher electron conductivity while hindering aggregation between 0D CoO NPs that are partially reshaped into Co3O4 nanocubes due to induced surface strain energies. Our results indicate that such interfacial energetics and arrangements lead to superior ORR electrocatalytic activities. On the other hand, the interconnecting 1D nanostructures in theNR/rGO HNCs benefit charge transport and electrolyte diffusion at the electrode–electrolyte interfaces, thereby promoting their supercapacitive properties. The NP/rGO HNCs exhibit intermediate functionalities towards both ORR catalysis and supercapacitance

Self-similar shear-thickening behavior in CTAB/NaSal surfactant solutions

The effect of salt concentration Cs on the critical shear rate required for the onset of shear thickening and apparent relaxation time of the shear-thickened phase, has been investigated systematically for dilute CTAB/NaSal solutions. Experimental data suggest a self-similar behavior of the critical shear rate and relaxation time as functions of Cs. Specifically, the former ~ Cs^(-6) whereas the latter ~ Cs^(6) such that an effective Weissenberg number for the onset of the shear thickened phase is only weakly dependent on Cs. A procedure has been developed to collapse the apparent shear viscosity versus shear rate data obtained for various values of Cs into a single master curve. The effect of Cs on the elastic modulus and mesh size of the shear-induced gel phase for different surfactant concentrations is discussed. Experiments performed using different flow cells (Couette and cone-and-plate) show that the critical shear rate, relaxation time and the maximum viscosity attained are geometry-independent. The elastic modulus of the gel phase inferred indirectly by employing simplified hydrodynamic instability analysis of a sheared gel-fluid interface is in qualitative agreement with that predicted for an entangled phase of living polymers. A qualitative mechanism that combines the effect of Cs on average micelle length and Debye parameter with shear-induced configurational changes of rod-like micelles is proposed to rationalize the self-similarity of SIS formation.Comment: 27 pages, 17 figure

Flow History - Morphology - Property Interactions in Poly(vinylidene Fluoride)

235 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1987.A numerical and experimental analysis of the melt flow history of poly(vinylidene fluoride), PvF\sb2, in an extensional flow, film-processing geometry as well as morphological characterization and property measurements of the melt extruded films are presented.Numerical computations were carried out for both Newtonian and powerlaw flow in an impinging channels die using both finite difference and element techniques. Computations demonstrated that a strong extensional flow exists in the region from the stream impingement point to a distance about 0.75 D downstream where D is the channel height at the impingement point. Measurements of the stress fields using the technique of flow birefringence showed that in consequence of the changing flow kinematics from the stream impingement region to the downstream converging channels region, both the isochromatics and isoclinic patterns exhibit a pronounced axial positional dependence. Excellent qualitative and reasonably good quantitative agreement was found between measured and calculated stress fields.Row nucleated structures were observed at two-dimensional flow rates higher than 0.01 cm\sp2/sec and the resultant extruded films exhibited either $\alpha$-phase of (PvF\sb2) or a mixture of $\alpha$ and $\beta$ phases of (PvF\sb2) as shown by DSC, x-ray diffraction, FTIR and hot stage microscopy, depending on the processing history. Films with draw ratios less than 5 exhibited only the $\alpha$-polymorph while above the critical draw ratio a mixture of both polymorphs existed. The extent of the conversion of the $\alpha$-phase to $\beta$ is shown to be a very strong function of the draw temperature. Furthermore, the tensile property of the extruded films is an increasing function of draw ratio.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD

Modeling and Simulation of Dynamics of Polymeric Solutions: Progress and Challenges

Presented on September 17, 2008, from 4-5 pm in room G011 of the Molecular Science and Engineering Building on the Georgia Tech Campus.Runtime: 60:11 minutesQuantitative understanding of the influence of physico-chemical parameters on the dynamic evolution of microstructure in polymeric solutions plays a central role in processing of wide variety of micro-structured materials. Over the past decade tremendous progress has been made in development of kinetic theory bases coarse- grained micro-mechanical models for polymeric solutions as well as robust and highly accurate continuum and multi-scale simulation techniques for flow simulation of this class of fluids in complex kinematics flows. In this presentation, I will briefly review the progress made in these areas as well the remaining challenges in development of a unified approach for predicting dynamics of polymeric solutions in prototypical complex kinematics flows. Specifically, I will address the following issues: A detailed evaluation of various coarse-grained kinetic theory based micro-mechanical models for polymeric solutions in terms of their ability to predict detailed polymer configuration states observed via single molecule microscopy as well as measured rheological properties will be given. In addition, new computationally tractable models for macromolecular scission and chain dynamics will be discussed. An overview of existing continuum and multi-scale simulation techniques for steady and dynamic simulations of polymeric solutions in complex kinematics flows will also be presented. In turn, the ability of current simulation techniques and constitutive equations/micro-mechanical models for polymeric solutions to capture the experimentally observed flow kinematics, polymeric stresses in a number of complex kinematics flow geometries will be discussed. Based on these comparisons a unified approach for predicting dynamics of polymeric solutions in complex kinematics flows will be suggested

Molecular Processes Leading to Shear Banding in Well Entangled Polymeric Melts

We have performed hi-fidelity dissipative particle dynamics (DPD) simulations of shear flow of polymeric melts in a broad range of system sizes and two entanglement densities to determine the critical conditions for occurrence of both transient and steady shear banding. Here, we report, for the first time, simulation results that clearly demonstrate the consecutive steps leading to shear banding, that is, the stress overshoot drives locally inhomogeneous chain deformation and thus spatially inhomogeneous chain disentanglement; in turn, the localized jump in the entanglement density along the velocity gradient direction results in a considerable jump in normal stress and viscosity, which ultimately leads to shear banding. Overall, our observations are consistent with prior experimental studies, and an explanation for the stability of steady and transient shear banded flows is postulated based on the well-known interfacial stability mechanism of stratified polymeric fluids

Characterization of the Flory-Huggins interaction parameter of polymer thermodynamics

Flory-Huggins theory is the main basis of polymer solution and blend thermodynamics. A key piece of this theory is a parameter quantifying the enthalpic interactions between the components; however, experiments have revealed that this parameter is not composition independent, as originally assumed. This composition dependence has been attributed by some theorists to experimental error; others have tried to explain it based on several competing hypotheses. Here, we use atomistic simulations of isotopic blends based on realistic potentials to study this parameter without making any prior hypotheses. Simulations reveal a composition dependence of this parameter that compares well with experimental data, and serve to verify theoretical relationships between the various forms of this parameter

Individual Molecular Dynamics of an Entangled Polyethylene Melt Undergoing Steady Shear Flow: Steady-State and Transient Dynamics

The startup and steady shear flow properties of an entangled, monodisperse polyethylene liquid (C1000H2002) were investigated via virtual experimentation using nonequilibrium molecular dynamics. The simulations revealed a multifaceted dynamical response of the liquid to the imposed flow field in which entanglement loss leading to individual molecular rotation plays a dominant role in dictating the bulk rheological response at intermediate and high shear rates. Under steady shear conditions, four regimes of flow behavior were evident. In the linear viscoelastic regime ( γ ˙ < τ d − 1 ), orientation of the reptation tube network dictates the rheological response. Within the second regime ( τ d − 1 < γ ˙ < τ R − 1 ), the tube segments begin to stretch mildly and the molecular entanglement network begins to relax as flow strength increases; however, the dominant relaxation mechanism in this region remains the orientation of the tube segments. In the third regime ( τ R − 1 < γ ˙ < τ e − 1 ), molecular disentangling accelerates and tube stretching dominates the response. Additionally, the rotation of molecules become a significant source of the overall dynamic response. In the fourth regime ( γ ˙ > τ e − 1 ), the entanglement network deteriorates such that some molecules become almost completely unraveled, and molecular tumbling becomes the dominant relaxation mechanism. The comparison of transient shear viscosity, η + , with the dynamic responses of key variables of the tube model, including the tube segmental orientation, S , and tube stretch, λ , revealed that the stress overshoot and undershoot in steady shear flow of entangled liquids are essentially originated and dynamically controlled by the S x y component of the tube orientation tensor, rather than the tube stretch, over a wide range of flow strengths