Glass Transition Behavior of Polymer Films of Nanoscopic Dimensions

Glass transition behavior of nanoscopically thin polymer films is investigated by means of molecular dynamics simulations. A thin polymer film that is composed of bead-spring model chains and supported on an idealized, fcc lattice substrate surface is studied in this work.Comment: in review, macromolecule

Grafting of Poly(methyl methacrylate) Brushes from Magnetite Nanoparticles Using a Phosphonic Acid Based Initiator by Ambient Temperature Atom Transfer Radical Polymerization (ATATRP)

Poly(methyl methacrylate) in the brush form is grown from the surface of magnetite nanoparticles by ambient temperature atom transfer radical polymerization (ATATRP) using a phosphonic acid based initiator. The surface initiator was prepared by the reaction of ethylene glycol with 2-bromoisobutyrl bromide, followed by the reaction with phosphorus oxychloride and hydrolysis. This initiator is anchored to magnetite nanoparticles via physisorption. The ATATRP of methyl methacrylate was carried out in the presence of CuBr/PMDETA complex, without a sacrificial initiator, and the grafting density is found to be as high as 0.90 molecules/nm2. The organic–inorganic hybrid material thus prepared shows exceptional stability in organic solvents unlike unfunctionalized magnetite nanoparticles which tend to flocculate. The polymer brushes of various number average molecular weights were prepared and the molecular weight was determined using size exclusion chromatography, after degrafting the polymer from the magnetite core. Thermogravimetric analysis, X-ray photoelectron spectra and diffused reflection FT-IR were used to confirm the grafting reaction

Simulation of Electron Transfer and Electron Transport in Molecular Systems at Surfaces

We have investigated electron transfer and transport processes in several molecular systems adsorbed at metal surfaces using a methodology that combines first-principles electronic structure methods with quantum dynamics and transport approaches. Specifically, we have analysed the molecular factors that control electron transfer in a series of nitrile-substituted alkanethiolate self-assembled monolayer models adsorbed at the Au(111) surface that differ in the size of the aliphatic spacer chain. In addition, we have analysed the possibility of using a proton transfer reaction triggered by an external electrostatic field as a novel mechanism for switching a molecular junction. To demonstrate the feasibility of the process, we have investigated electron transport in a junction containing a molecular bridge that can exist in two tautomeric forms, [2,5-(4-hydroxypyridine)] and 2,5-[4(1H)-pyridone], that exhibit very different conductance properties

The polymer-supported phospholipid bilayer: Tethering as a new approach to substrate-membrane stabilization

We present a new molecular engineering approach in which a polymer-supported phospholipid bilayer is vertically stabilized by controlled covalent tethering at both the polymer-substrate and polymer-bilayer interfaces. This approach is based on lipopolymer molecules, which not only form a polymer cushion between the phospholipid bilayer and a solid glass substrate but also act as covalent connections (tethers) between the bilayer and cushion. Our approach involves Langmuir-Blodgett transfer of a phospholipid-lipopolymer monolayer followed by Schaefer transfer of a pure phospholipid monolayer and is capable of varying the tethering density between the polymer layer and the phospholipid bilayer in a very controlled manner. Further stabilization is achieved if the glass substrate is surface-functionalized with a benzophenone silane. In this case, a photocross-linking reaction between the polymer and benzophenone group allows for the covalent attachment of the polymer cushion to the glass substrate. This approach is similar to that recently reported by Wagner and Tamm in which double tethering is achieved via lipopolymer silanes (Wagner, M. L.; Tamm, L. K. Biophys. J. 2000, 79, 1400). To obtain a deeper understanding of how the covalent tethering affects the lateral mobility of the bilayer, we performed fluorescence recovery after photobleaching (FRAP) experiments on polymer- tethered bilayers at different tethering densities (lipopolymer/phospholipid molar ratios). The FRAP data clearly indicate that the hydrophobic lipopolymer moieties act as rather immobile obstacles within the phospholipid bilayer, thereby leading to hindered diffusion of phospholipids. Whereas the high lateral diffusion coefficient of D = 17.7 mum(2)/s measured at low tethering density (5 mol % lipopolymer) indicates rather unrestricted motion within the bilayer, corresponding values at moderate (10 mol % lipopolymer) and high (30 mol % lipopolymer) tethering densities of D = 9.7 mum(2)/s and D = 1.1 mum(2)/s, respectively, show significant hindered diffusion. These results are contrary to the recent findings on similar membrane systems reported by Wagner and Tamm. in which no significant change in phospholipid diffusion was found between 0 and 10 mol % lipopolymer. Our experimental report leads to a deeper understanding of the complex problem of interlayer coupling and offers a path toward a compromise between stability of the whole system and lateral mobility within the bilayer. Furthermore, the FRAP measurements show that polymer-tethered membranes are very interesting model systems for studying problems of restricted diffusion within two-dimensional fluids

Growth of tungsten bronze phase out of niobate perovskite phase for opto-ferroelectric applications

Abstract Engineering the optical bandgaps of classic ferroelectrics from the typical ultraviolet range down to the visible range is an emerging methodology of developing the next-generation optoelectric and opto-ferroelectric devices including ferroelectric solar cells, light-driven transistors and modulators, and multi-sensors/energy harvesters. Recently, a material interface comprised of a pseudo-morphotropic phase boundary between the tungsten bronze and perovskite phases of the KNBNNO [(K,Na,Ba)x(Ni,Nb)yOz] has been reported to be an effective approach for bandgap engineering while retaining excellent ferroelectricity and piezoelectricity of the perovskite-phased KNBNNO. However, this approach requires the compositions of the materials to be determined at the synthesis stage, leaving little room for any further modification of the microstructure and functional properties at the post-processing stage. This paper presents a post-processing method, that is, atmospheric annealing in N₂ and O₂, to grow the necessary tungsten bronze phase out of the perovskite phase in the KNBNNO. This method is advantageous over the previously reported because it enables to grow the tungsten bronze–perovskite interface region independent of the initial composition. The distinctive electrical properties and the giant tunability of photoconductivity of the tungsten bronze phase, the perovskite phase, and the interface are characterized in detail in this paper, supporting the exploitation of fabricating opto-ferroelectric devices using the reported method which is compatible and comparable with some of the post-processing methods applied in the silicon industry