Thermal properties of multilayer graphene and hBN reinforced copper matrix composites

The unique properties of graphene make it a very attractive application, although there are still no commercial products in which graphene would play a key role. Good thermal conductivity is undoubtedly one of the attributes which can be easily used both in materials involving large monoatomic layers, that are very difficult to obtain, as well as multilayer graphene flakes, which have been commercially available on the market for several years. The article presents the results of tests on the characteristic thermal properties of composites with the addition of 2–15% of multilayer graphene (MLG) crystals. The motivation of the study was literature reports showing the possibility of increasing the thermal conductivity of composites with MLG participation in the copper matrix. Since the production of composites with increased properties is associated with obtaining a strong orientation of the flakes in the structure, composites with hBN flakes exhibiting significantly worse but also directional thermal properties were produced for comparison. The paper showed a strong influence of flake morphology on the possibility of creating a directional structure. The obtained Cu/MLG composites with the addition of only 2% MLG were characterized by an increase in the thermal conductivity coefficient of about 30% in relation to sinters without the participation of MLG.

     

Nonwoven Releasing Propolis as a Potential New Wound Healing Method—A Review

Wound healing poses a serious therapeutic problem. Methods which accelerate tissue regeneration and minimize or eliminate complications are constantly being sought. This paper is aimed at evaluation of the potential use of biodegradable polymer nonwovens releasing propolis as wound healing dressings, based on the literature data. Propolis is honeybee product with antioxidant, antibacterial, antifungal, anticancer, anti-inflammatory, analgesic, and regenerative properties. Controlled release of this substance throughout the healing should promote healing process, reduce the risk of wound infection, and improve aesthetic effect. The use of biodegradable aliphatic polyesters and polyester carbonates as a propolis carrier eliminates the problem of local drug administration and dressing changes. Well-known degradation processes and kinetics of the active substance release allows the selection of the material composition appropriate to the therapy. The electrospinning method allows the production of nonwovens that protect the wound against mechanical damage. Moreover, this processing technique enables adjusting product properties by modifying the production parameters. It can be concluded that biodegradable polymer dressings, releasing a propolis, may find potential application in the treatment of complicated wounds, as they may increase the effectiveness of treatment, as well as improve the patient’s life quality.     

Hydrogen Bonding Directed Self-Assembly of a Binuclear Ag(I) Metallacycle into a 1D Supramolecular Polymer

An Ag(I) metallacycle obtained unexpectedly during the preparation of Pd(II) complexes of the bifunctional ligand 5-([2,2′-bipyridin]-5-yl)pyrimidine-2-amine (L) has been characterized using X-ray structure determination as a binuclear, metallacyclic species [Ag₂L₂](SbF₆)₂, where both the bipyridine and pyrimidine-N donors of L are involved in coordination to the metal. The full coordination environment of the Ag(I) defines a case of highly irregular 4-coordination. In the crystal, the Ag-metallacycles assemble into one-dimensional supramolecular metalladynamers linked together by hydrogen-bonding interactions.     

Measuring sub nanometre sizes using dynamic light scattering

Dynamic light scattering (DLS) measures time-dependent fluctuations in the scattering intensity arising from particles undergoing random Brownian motion. Diffusion coefficient and particle size information can be obtained from the analysis of these fluctuations. This paper discusses the factors which will influence the lower size limit of DLS and reports the use of sucrose as a test sample to probe this lower limit of the technique. Hydrodynamic diameter values of less than 1 nm are obtained by the use of 173° backscatter detection that is applied to increase the sensitivity of DLS. The peak means (with standard deviations) obtained for the intensity and volume data from a series of sucrose concentrations, ranging from 5 to 35% w/v, were measured as D_I,Mean = 0.82 nm (0.11 nm) and D_V,Mean = 0.62 nm (0.05 nm), respectively. These sucrose results suggest that sub nanometer measurements are achievable with a precision of 0.1 nm. Evidence to support these size results for sucrose is discussed.     

Resolving Concentrated Particle Size Mixtures Using Dynamic Light Scattering

Dynamic light scattering (DLS) is a technique used for measuring the size of molecules and particles undergoing Brownian motion by observing time‐dependent fluctuations in the intensity of scattered light. The measurement of samples using conventional DLS instrumentation is limited to low concentrations due to the onset of a phenomenon called multiple scattering. The problems of multiple scattering have been addressed in a light scattering instrument incorporating non‐ invasive backscatter optics (NIBS). This novel optic arrangement maximizes the detection of scattered light while maintaining signal quality and allows for measurements of turbid samples. This paper discusses the ability of backscatter detection to accurately determine particle sizes at 1 %w/v sample concentrations and demonstrates the correct resolution of different size populations using a series of latex standard mixtures with known volume ratios. The concentration of 1 %w/v is much higher than can be measured on conventional dynamic light scattering instruments.     

A direct assessment of refractive indices of nematic liquid crystals at broad VIS - MWIR range

We developed an interference method for determination refractive indices of nematic liquid crystals in a broad electromagnetic spectrum. Values of ordinary and extraordinary refractive indices were measured in VIS (from 0.4 to 0.8 μm), NIR (from 0.8 to 1.4 μm), SWIR (from 1.4 to 3.0 μm) and at an edge of MWIR (from 3.0 to 4.2 μm) regions. The main task of this work was to develop the efficient and accurate interference method. The absolute error of these measurements is not higher than 0.02. The method was tested on two newly designed mixtures. The first one was a high birefringence liquid crystal mixture for the laser rangefinder, while the second one is intentionally prepared for breathalyser application. The mixture does not possess high absorption bands at 3.4 μm. To measure a dispersion of the refractive indices cells with various thicknesses and dielectric mirrors were prepared. The paper presents a theoretical discussion and experimental results.     

Energy transfer in oligofluorene-C60 and C60-oligofluorene-C60 donor-acceptor conjugates

Two complementary series of C(60)-(Fl)(n) and C(60)-(Fl)(n)-C60 (Fl = 9,9-dihexylfluorene-2,7-diyl; n = 1-5) derivatives with terminal N-methylfulleropyrrolidine units have been synthesized from CHO-(Fl)(n) and CHO-(Fl)(n)-CHO precursors via 1,3-dipolar cycloaddition reaction of in situ generated azomethine ylides with an excess of C60. In solution electrochemical experiments, these conjugates give rise to amphoteric redox behavior. Three consecutive quasireversible reduction waves have been observed at the expected potentials for the N-methylfulleropyrrolidine cores. For the C(60)-(Fl)(n)-C(60) series, each reduction wave is a two-electron process with no observable interaction between the C(60) units. Two or, in some cases, three oxidation waves--most of them irreversible--are ascribed to the oligofluorene system. These waves are cathodically shifted with an increasing number of fluorene units and anodically shifted by the conjugated terminal aldehyde units, compared to the N-methylfulleropyrrolidine termini. Steady-state and time-resolved photolytic techniques show that an efficient transduction of singlet excited-state energy transfer prevails from the photoexcited oligofluorene to the energy accepting fullerene.     

Wide temperature range of an electrically tunable selective reflection of light by oblique helicoidal cholesteric

ABSTRACT

Oblique helicoidal cholesteric liquid crystals (Ch_OH) offer an unprecedented opportunity to tune selective reflection of light in a broad spectral range from ultraviolet to infrared by an electric field. The major problem is that the temperature range of stable Ch_OH is typically above the room temperature and is relatively narrow, a few degrees. In this work, we demonstrate that by using a mixture of flexible dimeric and rod-like molecules, one can significantly expand the temperature range of intense Bragg reflection, from 16°C to 27°C. We demonstrate that the selective reflection peak, reflection intensity, bandwidth and threshold electric field are all temperature dependent and discuss the associated mechanisms. The results show that both the electric field and temperature can be used to tune the structural colour of oblique helicoidal cholesteric structures. The proposed material can be used in switchable optical devices based on liquid crystals, such as light modulators, indoor smart windows, and filters.     

Application of Sugar Phosphonates for the Preparation of Higher Carbon Monosaccharides

ABSTRACT

Reaction of sugar derived phosphonates [Sug-C(O)CH₂P(O)(OMe)₂] with sugar aldehydes (Sug'-CHO) provides the higher enones of the general formula Sug-C(O)CH=CH-Sug' with the trans configuration of the double bond. The phosphonate method is superior to the previously used phosphorane methodology [Sug-C(O)CH=PPh₃ + Sug'-CHO] since sugar phosphonates can be prepared in much higher yields and are much more nucleophilic than corresponding phosphoranes. The sugar enones are reduced to appropriate allylic alcohols with zinc borohydride; the stereoselectivity of this process is >97:3 (with the D-glycero isomer predominating) when the carbonyl group is placed at the α-position to the sugar ring. CD spectroscopy was used for the determination of the configuration of higher sugar allylic alcohols.     

Precise Control of Intramolecular Charge‐Transport: The Interplay of Distance and Conformational Effects

A new series of donor–bridge–acceptor (D–B–A) compounds consisting of π‐conjugated oligofluorene (oFL) bridges between a ferrocene (Fc) electron‐donor and a fullerene (C₆₀) electron‐acceptor have been synthesized. In addition to varying the length of the bridge (i.e., mono‐ and bi‐fluorene derivatives), four different ways of linking ferrocene to the bridge have been examined. The Fc moiety is linked to oFL: 1) directly without any spacer, 2) by an ethynyl linkage, 3) by a vinylene linkage, and 4) by a p‐phenylene unit. The electronic interactions between the electroactive species have been characterized by cyclic voltammetry, absorption, fluorescence, and transient absorption spectroscopy in combination with quantum chemical calculations. The calculations reveal exceptionally close energy‐matching between the Fc and the oFL units, which results in strong electronic‐coupling. Hence, intramolecular charge‐transfer may easily occur upon exciting either the oFLs or Fcs. Photoexcitation of Fc–oFL–C₆₀ conjugates results in transient radical‐ion‐pair states. The mode of linkage of the Fc and FL bridge has a profound effect on the photophysical properties. Whereas intramolecular charge‐separation is found to occur rather independently of the distance, the linker between Fc and oFL acts (at least in oFL) as a bottleneck and significantly impacts the intramolecular charge‐separation rates, resulting in beta values between β_CS 0.08 and 0.19 Å^?1. In contrast, charge recombination depends strongly on the electron‐donor–acceptor distance, but not at all on the linker. A value of β_CR (0.35±0.01 Å^?1) was found for all the systems studied. Oligofluorenes prove, therefore, to be excellent bridges for probing how small structural variations affect charge transport in D–B–A systems.