Thermally Induced Demixing in an LCST Mixture in the Presence of Densely Grafted Nanoparticles: Tuning the Graft Chain Length To Induce Thermodynamic Miscibility

The demixing in an LCST mixture of PS/PVME (polystyrene/poly(vinyl methyl ether)) was probed here by melt rheology in the presence of gold nanoparticles which were densely coated with varying graft lengths of PS. The graft density for the gold nanoparticles coated with 3 kDa PS was ca. Sigma = 1.7 chains/nm(2), and that for 53 kDa PS was ca. Sigma = 1.2 chains/nm(2). The evolution of morphology, as the blends transit through the metastable and the unstable envelopes of the phase diagram, and the localization of the gold nanoparticles upon demixing were monitored using in situ hot-stage AFM and confocal Raman imaging. Interestingly, gold nanoparticles coated with 3 kDa polystyrene (PS(3 kDa)-g-nAu) were localized in the PVME phase, whereas gold nanoparticles coated with 53 kDa polystyrene (PS(53 kDa)-g-nAu) were localized in the PS phase of the blend. While the localization of PS(3 kDa)-g-nAu in the PVME phase can be expected to be of entropic origin due to expulsion from the PS phase as R-g,R-matrix chains > R-g,R-grafted chains (where R-g is the radius of gyration of the polymer chain), the localization of PS(53 kDa)-g-nAu in the PS phase is believed to be facilitated by favorable melt/graft interactions. The latter nanoparticles also delayed the demixing by 12 degrees C with respect to the neat mixture. The observed changes were addressed in context to enthalpic interactions between the grafted PS and the free PS, the entropic losses (deformational entropic losses on blending, translational entropic loss of the free PS, and the conformational entropic loss of the grafted PS), and the interface of the grafted and the free chains.

Thermally developing combined electroosmotic and pressure-driven flow of nanofluids in a microchannel under the effect of magnetic field

In the present study, the heat transfer characteristics of thermally developing magnetohydroclynamic flow of nanofluid through microchannel are delineated by following a semi analytical approach. The combined influences of pressure driven flow, electroosmotic transport and magnetic field is taken into account for the analysis of the complex microscale thermal transport processes. Solutions for the normalized temperature distributions and the Nusselt number variations, considering the simultaneous interplay of electrokinetic effects (electroosmosis), magnetic effects, Joule heating and viscous dissipation are obtained, for constant wall temperature condition. Particular attention is paid to assess the role of nanolluids in altering the transport phenomena, through variations in the effective nanoparticle volume fractions, as well as the aggregate structure of the particulate phases. It is observed that magnetohydrodynamic effect reduces advective transport of the liquid resulting in gradual reduction of heat transfer. Increase in nanoparticle volume fraction shows decrease in heat transfer. Similar effects are observed with increase in aggregate sizes of the nanoparticles. The effect of the nanofluids on system irreversibility is also studied through entropy generation analysis due to flow and heat transfer in the microchannel. Total entropy generation is found to be dominant at the thermally developing region of the microchannel, whereas it drops sharply at the thermally developed region. Presence of nanoparticles in the base fluid reduces the total entropy generation in the microchannel, thereby indicating decrease in thermodynamic irreversibility with increasing nanoparticle volume fraction. (C) 2014 Elsevier Ltd. All rights reserved.

Thermally switchable metamaterial absorber with a VO2 ground plane

A tri-layer metamaterial absorber, composed of a metal structure/dielectric spacer/vanadium dioxide (VO2) ground plane, is shown to switch reversibly between reflective and absorptive states as a function of temperature. The VO2 film, which changes its conductivity by four orders of magnitude across a insulator-metal transition at about 68 degrees C, enables the switching by forming a resonant absorptive structure at high temperatures while being inactive at low temperatures. The fabricated metamaterial shows a modulation of the reflectivity levels of 58% at a frequency of 22.5 THz and 57% at a frequency of 34.5 THz. (C) 2015 Elsevier B.V. All rights reserved.

Thermally flexible epoxy/cellulose blends mediated by an ionic liquid

Blends between the widely used thermoset resin, epoxy, and the most abundant organic material, natural cellulose are demonstrated for the first time. The blending modification induced by charge transfer complexes using a room temperature ionic liquid, leads to the formation of thermally flexible thermoset materials. The blend materials containing low concentrations of cellulose were optically transparent which indicates the miscibility at these compositions. We observed the existence of intermolecular hydrogen bonding between epoxy and cellulose in the presence of the ionic liquid, leading to partial miscibility between these two polymers. The addition of cellulose improves the tensile mechanical properties of epoxy. This study reveals the use of ionic liquids as a compatible processing medium to prepare epoxy thermosets modified with natural polymers.

Dual Stress and Thermally Driven Mechanical Properties of the Same Organic Crystal: 2,6-Dichlorobenzylidene-4-fluoro-3-nitroaniline

An elastic organic crystal, 2,6-dichlorobenzylidine-4-fluoro-3-nitroaniline (DFNA), which also shows thermosalient behavior, is studied. The presence of these two distinct properties in the same crystal is unusual and unprecedented because they follow respectively from isotropy and anisotropy in the crystal packing. Therefore, while both properties lead from the crystal structure, the mechanisms for bending and thermosalience are quite independent of one another. Crystals of the low-temperature (a) form of the title compound are bent easily without any signs of fracture with the application of deforming stress, and this bending is within the elastic limit. The crystal structure of the a-form was determined (P2(1)/c, Z = 4, a = 3.927(7) angstrom, b = 21.98(4) angstrom, c = 15.32(3) angstrom). There is an irreversible phase transition at 138 degrees C of this form to the high-temperature beta-form followed by melting at 140 degrees C. Variable-temperature X-ray powder diffraction was used to investigate the structural changes across the phase transition and, along with an FTIR study, establishes the structure of the beta-form. A possible rationale for strain build-up is given. Thermosalient behavior arises from anisotropic changes in the three unit cell parameters across the phase transition, notably an increase in the b axis parameter from 21.98 to 22.30 angstrom. A rationale is provided for the existence of both elasticity and thermosalience in the same crystal. FTIR studies across the phase transition reveal important mechanistic insights: (i) increased pi...pi repulsions along 100] lead to expansion along the a axis; (ii) change in alignment of C-Cl and NO2 groups result from density changes; and (iii) competition between short-range repulsive (pi...pi) interactions and long-range attractive dipolar interactions (C-Cl and NO2) could lie at the origin of the existence of two distinctive properties.

Optically Stimulated Luminescence (OSL) Dating of Sediments from Himalaya

Optically Stimulated Luminescence (OSL) dating gives the age of most recent daylight exposure or heating of samples to >400 degrees C or the formation events of authigenic minerals. These correspond to the age of sedimentation and burial, ages of thermal events like contact heating by lava flows and heating during faulting and sand dyke formation, and the formation of a mineral via chemical precipitation. With the first observation of OSL in 1985, this method now occupies centre stage in Quaternary Geochronology. The use of OSL method for sediments from Himalaya began over three decades ago. The method has since provided chronology for a variety of events, such as past glaciation events, formation ages of river terraces, paleo-lacustrine deposits, landslides, floods, seismic events with substantive new insights into timing and style of geological processes. Theoretically, the dating range of method is present to a Million years, and this critically depends on two factors, viz, luminescence properties of mineral and their radiation environments. The general working range using quartz is 200ka, and using feldspars is up to Brunhes Matuyam Boundary. Extensions beyond this limit are currently being explored.

Designer porous antibacterial membranes derived from thermally induced phase separation of PS/PVME blends decorated with an electrospun nanofiber scaffold

We report the development of porous membranes by thermally induced phase separation of a PS/PVME (polystyrene/polyvinylmethyl ether]) blend, which is a typical LCST mixture. The morphology of the membrane after etching out the PVME phase was characterized by scanning electron microscopy. To give the membrane an antibacterial surface, polystyrene (PS) and polyvinyl(methyl ether)]-alt-maleic anhydride (PVME-MAH) with silver nanoparticles (nAg) were electrospun on the membrane surface. Pure water flux was evaluated by using a cross-flow membrane setup. The microgrooved fibers changed the flux across the membrane depending on the surface properties. The antibacterial properties of the membrane were confirmed by the reduction in the colony count of E. coli. The SEM images show the disruption of the bacterial cell membrane and the antibacterial mechanism was discussed.

Void Geometry Driven Spin Crossover in Zeolite-Encapsulated Cobalt Tris(bipyridyl) Complex Ion

The cobalt(II) tris(bipyridyl) complex ion encapsulated in zeolite-Y supercages exhibits a thermally driven interconversion between a low-spin and a high-spin state-a phenomenon not observed for this ion either in solid state or in solution. From a comparative study of the magnetism and optical spectroscopy of the encapsulated and unencapsulated complex ion, supported by molecular modeling, such spin behavior is shown to be intramolecular in origin. In the unencapsulated or free state, the [Co(bipy)(3)](2+) ion exhibits a marked trigonal prismatic distortion, but on encapsulation, the topology of the supercage forces it to adopt a near-octahedral geometry. An analysis using the angular overlap ligand field model with spectroscopically derived parameters shows that the geometry does indeed give rise to a low-spin ground state, and suggests a possible scenario for the spin state interconversion.

Colossal dielectric behavior of semiconducting Sr2TiMnO6 ceramics

Manganitelike double perovskite Sr2TiMnO6 (STMO) ceramics fabricated from the powders synthesized via the solid-state reaction route, exhibited dielectric constants as high as similar to 10(5) in the low frequency range (100 Hz-10 kHz) at room temperature. The Maxwell-Wagner type of relaxation mechanism was found to be more appropriate to rationalize such high dielectric constant values akin to that observed in materials such as KxTiyNi(1-x-y)O and CaCu3Ti4O12. The dielectric measurements carried out on the samples with different thicknesses and electrode materials reflected the influence of extrinsic effects. The impedance studies (100 Hz-10 MHz) in the 180-300 K temperature range revealed the presence of two dielectric relaxations corresponding to the grain boundary and the electrode. The dielectric response of the grain boundary was found to be weakly dependent on the dc bias field (up to 11 V/cm). However, owing to the electrode polarization, the applied ac/dc field had significant effect on the low frequency dielectric response. At low temperatures (100-180 K), the dc conductivity of STMO followed a variable range hopping behavior. Above 180 K, it followed the Arrhenius behavior because of the thermally activated conduction process. The bulk conductivity relaxation owing to the localized hopping of charge carriers obeyed the typical universal dielectric response.

Variable-Temperature EPR and Transport Studies on Poly(4,4'-methylenedianiline): Evidence for Bipolarons

The correlation between magnetic and transport properties is examined by studying poly(4,4'-methylenedianiline)(PMDA) salts and their bases using EPR and conductivity measurements. Five different PMDA salts (doped polymers)were prepared by chemical polymerization of 4,4'-methylenedianiline using different protonic acids. The PMDA bases were obtained by dedoping the salts using ammonium hydroxide. Ambient temperature electrical conductivity measurements show evidence for the doped PMDA system to be highly disordered. The EPR spectra of the samples were recorded in the range 20-200 "C, and the results were analyzed on the basis of the polaron-bipolaron model, which is typical of nondegenerate systems. Both PMDA salts and their bases consist of self-trapped, highly mobile polarons or radical cations. EPR studies on PMDA salts show evidence for the presence of thermally activated and temperature independent (or Pauli type) paramagnetism while the bases show thermally activated, Pauli and Curie-Weiss types of paramagnetism. The paramagnetism arises due to polarons.It is proposed that charge transport takes place through both polarons and bipolarons.

A combined experimental and theoretical study of the charge-transfer compound between C60Br8 and tetrathiafulvalene

C60Br8, unlike C60Br6 and C60Cl6, forms a solid charge-transfer compound with tetrathiafulvalene (TTF), the composition being C60Br8(TTF)(8). The unique complex-forming property of C60Br8 can be understood on the basis of the electronic structures of the halogenated derivatives of C-60. Molecular orbital calculations show that the low LUMO energy of C60Br8 compared with the other halogen derivatives renders the formation of the complex with TTF favourable, the four virtual LUMOs being able to accept 8 electrons. The Raman spectrum of C60Br8(TTF)(8) shows a marked softening of the bands (-46 cm(-1) on average) with respect to C60Br8 suggesting that indeed 8 electrons are transferred per C60Br8 molecule, one from each TTF molecule. The complex is weakly paramagnetic and shows a magnetic transition around 80 K.

Colossal dielectric behavior of semiconducting $Sr_{2}TiMnO_{6}$ ceramics

Manganitelike double perovskite Sr2TiMnO6 (STMO) ceramics fabricated from the powders synthesized via the solid-state reaction route, exhibited dielectric constants as high as similar to 10(5) in the low frequency range (100 Hz-10 kHz) at room temperature. The Maxwell-Wagner type of relaxation mechanism was found to be more appropriate to rationalize such high dielectric constant values akin to that observed in materials such as KxTiyNi(1-x-y)O and CaCu3Ti4O12. The dielectric measurements carried out on the samples with different thicknesses and electrode materials reflected the influence of extrinsic effects. The impedance studies (100 Hz-10 MHz) in the 180-300 K temperature range revealed the presence of two dielectric relaxations corresponding to the grain boundary and the electrode. The dielectric response of the grain boundary was found to be weakly dependent on the dc bias field (up to 11 V/cm). However, owing to the electrode polarization, the applied ac/dc field had significant effect on the low frequency dielectric response. At low temperatures (100-180 K), the dc conductivity of STMO followed a variable range hopping behavior. Above 180 K, it followed the Arrhenius behavior because of the thermally activated conduction process. The bulk conductivity relaxation owing to the localized hopping of charge carriers obeyed the typical universal dielectric response.

Defect-selective photothermal imaging: a sensitive tool for damage mapping in optical coatings

A defect-selective photothermal imaging system for the diagnostics of optical coatings is demonstrated. The instrument has been optimized for pump and probe parameters, detector performance, and signal processing algorithm. The imager is capable of mapping purely optical or thermal defects efficiently in coatings of low damage threshold and low absorbance. Detailed mapping of minor inhomogeneities at low pump power has been achieved through the simultaneous action of a low-noise fiber optic photothermal beam defection sensor and a common-mode-rejection demodulation (CMRD) technique. The linearity and sensitivity of the sensor have been examined theoretically and experimentally, and the signal to noise ratio improvement factor is found to be about 110 compared to a conventional bicell photodiode. The scanner is so designed that mapping of static or shock sensitive samples is possible. In the case of a sample with absolute absorptance of 3.8 x 10(-4), a change in absorptance of about 0.005 x 10(-4) has been detected without ambiguity, ensuring a contrast parameter of 760. This is about 1085% improvement over the conventional approach containing a bicell photodiode, at the same pump power. The merits of the system have been demonstrated by mapping two intentionally created damage sites in a MgF2 coating on fused silica at different excitation powers. Amplitude and phase maps were recorded for thermally thin and thick cases, and the results are compared to demonstrate a case which, in conventional imaging, would lead to a deceptive conclusion regarding the type and location of the damage. Also, a residual damage profile created by long term irradiation with high pump power density has been depicted.

Stability of Dimeric Interface in Banana Lectin: Insight from Molecular Dynamics Simulations

Banana lectin (Banlec) is a homodimeric non-glycosylated protein. It exhibits the b-prism I structure. High-temperature molecular dynamics simulations have been utilized to monitor and understand early stages of thermally induced unfolding of Banlec. The present study elucidates the behavior of the dimeric protein at four different temperatures and compares the structural and conformational changes to that of the minimized crystal structure. The process of unfolding was monitored by following the radius of gyration, the rms deviation of each residue, change in relative solvent accessibility and the pattern of inter- and intra-subunit interactions. The overall study demonstrates that the Banlec dimer is a highly stable structure, and the stability is mostly contributed by interfacial interactions. It maintains its overall conformation during high-temperature (400–500 K) simulations, with only the unstructured loop regions acquiring greater momentum under such condition. Nevertheless, at still higher temperatures (600 K) the tertiary structure is gradually lost which later extends to loss of secondary structural elements. The pattern of hydrogen bonding within the subunit and at the interface across different stages has been analyzed and has provided rationale for its intrinsic high stability.