Blue phase liquid crystals affected by graphene oxide modified with aminoazobenzol group

Liquid-crystalline blue phases (BPs) are stable only for very narrow temperature range between the isotropic and the chiral nematic phase that severely hinders their applicability. Herein, the aminoazobenzol group was chemically grafted onto epoxy group of graphene oxide (GO) via addition reaction. Successful grafting of aminoazobenzol group was confirmed using X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), UV–vis absorption spectra and thermogravimetric analysis (TGA). The resultant aminoazobenzol group–modified GO sheets, which is reduced (RGO-Az), were easily redispersable in common organic solvents or liquid crystals (LCs). By doping different contents of RGO-Az, nanosheets could stabilise BP and increase the BP range. When doped with 0.5 wt% RGO-Az, the mixtures show the wider range with 5.9°C than the range with 3.6°C of BPLCs without RGO-Az. Meanwhile, the phase sequence and the range of the aforementioned phases are reproducible upon heating and cooling, which shows that the BPs doped with RGO-Az nanosheets are thermodynamically stable.     

Synthesis and study the liquid crystalline properties of compounds containing benzoxazole core and terminal vinyl group

Terminal vinyl-based benzoxazole liquid crystalline compounds, 2-(3-fluoro-4?-alkoxy-1,1?-biphenyl ?4-yl)-5-(2-propenyloxymethyl)-benzoxazole (nPPF(2)BP), were synthesised and their structures were confirmed by infrared (IR) spectra, proton nuclear magnetic resonance (¹H-NMR) spectra, gas chromatography with electron impact-mass spectrometry (GC/EI-MS), matrix-assisted laser desorption/ionisation-time of flight (MALDI-TOF) mass spectrometry and elemental analysis (EA). The compounds show enantiotropic smectic/nematic phases with mesophase ranges are 71–97 °C and 87–136°C on heating and cooling processes for nPPF(2)BP, respectively. They give low melting points due to lateral fluoro substituent and flexible terminal 2-propenyloxymethyl chain. It is found that the compounds nPPF(2)BP with shorter alkoxy chain (n = 3, 4) exhibit a wide range of nematic mesophase, which is ascribed to enhanced π–π interaction caused by terminal vinyl moiety, whereas further elongation of the terminal alkoxy chain results in supressing nematic phase and increasing smectic mesophase. Compared with methyl terminated analogues, 2-propenyloxymethyl terminated compounds nPPF(2)BP display much lower melting points and wider or comparable mesophase range both in heating and cooling.     

The effects of silica nanoparticles on blue-phase liquid crystals

We investigate the effects of hydrophobic silica nanoparticles (SNs) on blue-phase liquid crystals (BPLCs). The optical microscope and reflection spectra observation reveal that a tiny hydrophobic SN dopant stabilises the BPLC phase, and widens the temperature range of the BP I phase. Furthermore, the doped dilute SNs can fine-tune their positions to relax the formation stress of the BPLC lattices, and slightly increase the platelet sizes of the BPLCs. The doped SNs also decrease the driving voltage and response time of the BPLC cell, because the added SNs decrease the elastic constant of the LC host and the relaxation time constant of the BPLC mixture.     

PbS nanoparticles stabilised blue phase liquid crystals

A kind of blue phase liquid crystal (BPLC), consisting of nematic liquid crystal, E7, and chiral dopants, CB15 and R1011, was investigated by doping PbS nanoparticles. The blue phase temperature range was extended from 3^oC to 4.6°C by doping PbS nanoparticles with diameters around 9.6 nm. A kind of porous texture was observed both in the forming process of PbS nanoparticles doped BPLCs as well as in the BPLCs (with/without PbS nanoparticles) under assisting electric field. The porous texture may indicate that the liquid crystals molecule should be reoriented during the formation process of PbS nanoparticles doped BPLCs.     

Thermal stability and nonisothermal crystallization of short fiber-reinforced poly(ether ether ketone) composites

The thermal stability of a short carbon-fiber-reinforced PEEK composite was assessed by thermogravimetry and by a Rheometrics dynamic analyzer. The results indicated that holding for 10 min at 380°C was a suitable melting condition to avoid the thermooxidative degradation under air. After proving that the heating rate of 50°C/min can be used to evaluate the crystallinity, a heating stage was used to prepare nonisothermally crystallized specimens using cooling rates from 1 to 100°C/min after melting at 400°C for 3 or 15 min. The degree of crystallinity and the melting behavior of these specimens were investigated by DSC at a heating rate of 50°C/min. The presence of three or four regions indicated that the upper melting temperature, T_m, changed with the crystallization temperature. The first region with the highest T_m, which corresponded to the cooling rate of 1°C/min, can be associated with the crystallization in regime II. There was a second region where T_m decreased as the amount of crystals formed in regime II decreased with increasing cooling rate from 5 to 20°C/min. The third region, a plateau region, corresponded to regime III condition in which the crystals were imperfect. In the fourth region, the cooling was so fast that crystallization was incomplete during the cooling for the melting condition of 400°C for 15 min. © 1998 John Wiley & Sons, Inc. J. Polym. Sci. B Polym. Phys. 36: 2225–2235, 1998     

Thermoelectric‐based temperature‐controlling system for in‐tube solid‐phase microextraction

A temperature‐controlling device for in‐tube solid‐phase microextraction was developed based on thermoelectric cooling and heating. This device can control the temperature of the capillary column from 0 to 100°C by applying a voltage to a Peltier cooler or stainless steel tube. The extraction temperatures for angiotensin I, propranolol, and ranitidine were optimized. In all cases, setting the temperature to 10°C for extraction achieved the best extraction efficiency. Desorption showed minimum peak broadening at 70°C, contributing to better chromatographic performance. Propranolol was selected as a model compound to compare the performance of temperature‐controlled in‐tube solid‐phase microextraction at optimized conditions. Calibration curves exhibited good linearity (R² > 0.999) over the studied range, and the limit of detection and limit of quantification were about three times lower than those obtained at standard conditions (30°C extraction and desorption).     

Low-voltage polymer-stabilised blue-phase liquid crystals with oleic acid (OA)-modified LaF3 nanoparticles

The resultant oleic acid (OA)-modified LaF₃ (OA-LaF₃) NPs were easily dispersed well in common organic solvents or liquid crystals (LCs). Usually, the stability of BPs depends on a well-arranged double-twisted alignment and a stable defect. Theoretical simulation shows that the BPLCs is likely to trap the doped nanoparticles due to the elastic interactions.By means of the OA-LaF₃ NPs we synthesized and doping into conventional polymer-stabilized blue phase liquid crystals (PS-BPLCs), it was observed that the on-state voltage can be reduced by 41% when the PS-BPLCs doped with 0.8wt. % OA-LaF₃ NPs compared with that PS-BPLCs without OA-LaF₃ NPs. It showed significant effects on the driving voltage when doping OA-LaF₃ NPs to PS-BPLCs. Our studies will certainly provide an effective and simple method to stabilize cubic BP structures and improve the electronic-optical performances, leading to potential applications in areas of displays as well as optoelectronic devices.     

Thermal and kinetic study of the ferroelectric phase transition in deuterated triglycine selenate

The specific heat and the enthalpy variation of a highly deuterated crystal of ferroelectric triglycine selenate have been measured around its first-order phase transition using the technique square modulated differential thermal analysis (SMDTA). The low temperature variation rate has allowed analyzing the kinetics of the phase transition. Due to an internal crack in the sample, the transition is carried out in two steps and an intermediate region where the transition is blocked and both phases coexist without transformation has been found. The latent heat on cooling (L _c=1.32±0.02 J g^–1) is higher than on heating (L _h=1.08±0.02 J g^–1) due to the thermal hysteresis and the great difference between the specific heat in both phases. Nevertheless, the enthalpy balance is fulfilled on heating and on cooling.     

Persistent polarization in polymers. I. Relationship between the structure of polymers and their ability to become electrically polarized

The ability of various polymers to acquire persistent polarization (i.e., to become electrets) was investigated. Polarization was induced in the polymers by two methods: (a) by cooling under a voltage drop and (b) by cooling with flow under pressure. There was found to be an optimum temperature for electret formation by either method. This optimum temperature was roughly 37°C. above T_g when polarization was produced by application of a voltage drop and roughly 57°C. when polarization was produced by flow under pressure. Crystallinity and the nonhomogeneities present in blended polymers were harmful to electret formation, but a small critical amount of ionic impurity was helpful.     

Study on the microstructure and liquid–solid correlation of Al–Mg alloys

Based on the available structural models and theories of electrical resistivity (ER) of liquid alloys, the structure and the liquid–solid correlation of Al _(100-x) Mg_x (x = 0, 10, 20, 30, 40, 50) alloys have been qualitatively studied by measuring the ER during the heating/cooling process using the direct-current (DC) four-probe method, as well as by characterizing the solidification morphology and testing the hardness. The result shows that the ER of Al–Mg alloys increases with the increasing temperature and the Mg content; thermal state and history have an effect on the solidification structure and properties: the ER of Al–Mg alloys exhibits a lag phenomenon of structure change during the heating/cooling process. A higher heating/cooling rate contributes to the more obvious relaxation effect of ER and the more uniform structure. Furthermore, higher pouring temperature (PT) leads the melts and solidification structure to be more homogeneous, which increases the hardness.     

Determination of thermophysical properties of high temperature alloy IN713LC by thermal analysis

The presented paper deals with the study of thermophysical properties of cast and complex alloyed nickel based on superalloy Inconel 713LC (IN713LC). In this work, the technique of Differential Thermal Analysis was selected for determination of the phase transformation temperatures and for the study of the effect of varying heating/cooling rate at these temperatures. The samples taken from as-received state of superalloy were analysed at heating and cooling rates of 1, 5, 10, 20 and 50?°C?min^?1 with the help of the experimental system Setaram SETSYS 18_TM. Moreover, the transformation temperatures at zero heating/cooling rate were calculated. The recommended values for IN713LC after correcting to a zero heating rate, are 1,205?°C (T _?á?,solvus), 1,250?°C (solidus) and 1,349?°C (liquidus). Influence of heating/cooling rate on shift of almost all temperatures of phase transformations was established from the DTA curves. Undercooling was observed at the cooling process. The samples before and after DTA analysis were also subjected to the phase analysis by scanning electron microscopy using the microscope JEOL JSM-6490LV equipped with an energy dispersive analyser EDAX (EDS INCA x-act). Documentation of the microstructure was made in the mode of secondary (SEI) and backscattered (BEI) electron imaging. On the basis of DTA analysis and phase analysis it may be stated that development of phase transformations of the alloy IN713LC will probably correspond to the following scheme: melting?????? phase; melting???????+?MC; melting????eutectics ??/?á?; melting???????+?minority phases (e.g. borides); and matrix ???????á?.     

New cloud vapor zone (CVZ) coupled headspace solid-phase microextraction technique

A new cloud vapor zone (CVZ)-based headspace solid-phase microextraction (HS-SPME) technique has been demonstrated with the capability of heating the sample matrix and simultaneously cooling the sampling zone. A bi-temperature-controlled (BTC) system, allowing 10 mL of test sample heating and headspace external-cooling, was employed for the CVZ formation around the SPME-fiber sampling area. In the CVZ procedure, the heated headspace vapor undergoes a sudden cooling near the SPME to form a dense cloud of analyte–water vapor, which is helpful for adsorption or absorption of the analyte. The device was evaluated for the quantitative analysis of aqueous chlorothalonil. Parameters influencing sampling efficiency, e.g., SPME fiber coating, SPME sampling temperature and time, solution modifier, addition of salt, sample pH, and temperature, were investigated and optimized thoroughly. The proposed BTC-HS-SPME method afforded a best extraction efficiency of above 94% accuracy (less than 4.1% RSD, n = 7) by using the PDMS fiber to collect chlorothalonil in the headspace at 5 °C under the optimized condition, i.e., heating sample solution (added as 10% ethylene glycol and 30% NaCl, at pH 7.0) at 130 °C for 15 min. The detection was linear from 0.01 to 80 μg L⁻¹ with a regression coefficient of 0.9998 and had a detection limit of 3.0 ng L⁻¹ based on S/N = 3. Practical application was demonstrated by analyzing chlorothalonil in farm water samples with promising results and recoveries. The approach provided a very simple, fast, sensitive, and solvent-free procedure to collect analytes from aqueous solution. The approach can provide a new platform for other sensitive HS-SPME assays.     

Electrical and self‐heating properties of UHMWPE‐EMMA‐NiCF composite films

Conductive polymer composites (CPC) containing nickel‐coated carbon fiber (NiCF) as filler were prepared using ultra‐high molecular weight polyethylene (UHMWPE) or its mixture with ethylene‐methyl methacrylate (EMMA) as matrix by gelation/crystallization from dilute solution. The electrical conductivity, its temperature dependence, and self‐heating properties of the CPC films were investigated as a function of NiCF content and composition of matrix in details. This article reported the first successful result for getting a good positive temperature coefficient (PTC) effect with 9–10 orders of magnitude of PTC intensity for UHMWPE filled with NiCF fillers where the pure UHMWPE was used as matrix. At the same time, it was found that the drastic increase of resistivity occurred in temperature range of 120–200 °C, especially in the range of 180–200 °C, for the specimens with matrix ratio of UHMWPE and EMMA (UHMWPE/EMMA) of 1/0 and 1/1 (NiCF = 10 vol %). The SEM observation revealed to the difference between the surfaces of NiCF heated at 180 and 200 °C. Researches on the self‐heating properties of the composites indicated a very high heat transfer for this kind of CPCs. For the 1/1 composite film with 10 vol % NiCF, surface temperature (T_s) reached 125 °C within 40 s under direct electric field where the supplied voltage was only 2 V corresponding to the supplied power as 0.9 W. When the supplied voltage was enough high to make T_s beyond the melting point of UHMWPE component, the T_s and its stability of CPC films were greatly influenced by the PTC effect. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1253–1266, 2009     

Voltage-induced pseudo-dielectric heating and its application for color tuning in a thermally sensitive cholesteric liquid crystal

ABSTRACT

We previously proposed an electrical approach enabling the tuning of the center wavelength λ_c of the cholesteric liquid crystal (CLC) bandgap in the full-color visible spectrum based on the electro-thermal effect. The idea involved the design of a negative CLC with a thermally sensitive bandgap feature and the use of a sandwich-type cell with finite electrode conductivity, allowing the control of cell temperature by applied voltage via pseudo-dielectric heating. It has been demonstrated experimentally that the induced pseudo-dielectric heating is predominated by the pseudo-dielectric relaxation originating from the designated cell geometry. On the basis of this technique, key factors determining the tuning efficacy of the temperature and thus λ_c are primarily investigated in this study. Our results suggest that lowering the electrode resistivity and the specific heat conductivity of the cell can promote the maximum tunable temperature range. Expectedly, optimizing the electrode area, cell gap and dielectric permittivity of the CLC favors a decreased relaxation frequency and, in turn, reducing the voltage as well as the frequency required for λ_c tuning.     

Sintered composite materials on the basis of Fe/FePO4‐coated powders

Composite materials with a network microstructure of α‐Fe areas surrounded by continuous layer of iron phosphate compounds were prepared on the basis of Fe/FePO₄ precipitation coated powder. Spherically shaped particles of this powder were compacted and sintered at 912, 930, 980, 1075 and 1130 °C for 3 min in air. Sufficiently fast heating rate, rapid formation of a liquid phase and fast cooling suppress the development of diffusion processes and consequently the formation of Fe–Fe metallic connections. Microscopic observations showed that, after appropriate processing conditions, an even and continuous layer, comprising a mixture of ferrous orthophosphate and iron oxide compounds, was achieved. Sintering at 1130 °C and fast cooling at ~25 000 °C/min resulted in the formation of a phosphate layer consisting of very fine lamellar structure – graftonite. The phase composition of the phosphate layer was correlated with processing conditions. Corrosion resistance of the sintered composites was measured and related to microstructure and processing technique variants. Copyright © 2014 John Wiley & Sons, Ltd.     

Phase transition of YBO<Subscript>3</Subscript>

Yttrium orthoborate crystallizes in the vaterite-type structure and has two polymorphous forms, viz. a low- und a high temperature one. DTA measurements of YBO₃ confirmed a reversible phase transition with a large thermal hysteresis. The phase transition has been accurately characterized by the application of different heating and cooling rates (β). Consequently, the extrapolation of the experimental data to zero β yields the transition points at 986.9°C for the heating up and at 596.5°C for the cooling down cycle. These values correspond to samples just after treatment at 1350°C. For samples with a different ‘thermal history’ other phase transition temperatures are observed, (e.g. after having performed several heating and cooling cycles). The linear relationship between the associated DTA signal ΔT=T _onset–T _offset and the square root of the heating rate β was confirmed, but the relation between T _onset and square root of β is not found here. From the empirical data a good linear fitting between T _onset and ln(β+1) can be derived. From the kinetic analysis (Kissinger method) of the phase transformation of YBO₃ an apparent activation energy of about 1386 kJ mol^–1 for heating and of about 568 kJ mol^–1 for cooling can be determined     

GC-MS headspace analysis of the volatile components of soya oil without heating the sample

The analysis of volatile compounds, and particularly of certain hydrocarbons, e.g. pentane, and some aldehydes, e.g. hexanal, has been used to estimate the degree of oxidation of vegetable oil. We have tested a dynamic headspace technique without heating the sample during the purge time of the analysis. Some tests have been performed after heating the sample at 180°C for a specified time before the analysis, to compare the results of analysis with and without heating. The best results were obtained at room temperature.     

Non-isothermal unfolding/denaturing kinetics of egg white protein

Egg protein is an important part of our food to get protein in our daily diet, and makes this protein more important to researchers to understand its kinetic behavior to understand the energy involved in the digestion of the egg protein. Hence, the present study explores the denaturing kinetics of the protein obtained from the hen??s egg white (EW) using high resolution calorimetric technique. Fresh EW was scanned for heating and cooling to see the thermodynamics from 10 to 100?°C at different heating ramp rates varying from 1 to 20?°C?min^?1. An endothermic peak was found on heating scan showing denaturing of protein which was found absent at the cooling indicating the absence of any residue after heating. The denature peak shifted towards higher temperature as ramp rate increases following Arrhenius behavior and shows an activated denaturing kinetics of the egg protein. This peak was also compared with the water to avoid water effects. Behavior of denaturing peak can be explained in terms of Arrhenius theory and further discussed to get the energy involved in digestion.     

Spectroscopic studies on the effect of field strength upon the curie transition of a VDF/TrFE copolymer

Infrared spectroscopic studies of the effects of field strength upon the ferroelectric phase transition behavior of a VDF/TrFE (75/25) copolymer upon heating and cooling in an electric field have revealed new findings. The paraelectric phase in the absence of an electric field resembles the α phase of PVDF with a trace of short trans sequences distributed randomly along the chain axis. The paraelectric phase in a high electric field is very different from that in the absence of an electric field. The paraelectric phase under an electric field has much longer trans sequences. The Curie transition temperature upon heating is a first-order transition temperature (T^↑_c) and is dramatically elevated from 120 to 135°C under the field of more than 0.4 MV/cm. Upon cooling, the paraelectric phase in an electric field does not show a clear transition. The field-induced phase transition and the loss of dipole switchability observed below a cooling temperature of 120°C, and their dependence on time and filed strength when exposed to a cyclic bipolar electric field are discussed. © 1993 John Wiley & Sons, Inc.     

Optical activity in CE6 A new metastable blue phase?

Abstract

The optical rotation of a mixture of 60 per cent of chiral and 40 per cent of racemic CE6 as an 18 μm thick sample placed between glass plates treated with PI has been measured. This mixture exhibits one blue phase (BP1) on heating over a temperature range of about 0·1°C. On cooling the sample on the other hand, the BP region is expanded to 0·6°C and is divided into two regions. One region (BP1) (of range about 0·38°C below the isotropic–blue phase transition) shows two Bragg wavelengths which increase with decreasing temperature. For the second region (BPS) (of range about 0·22°C above the cholesteric–blue phase transition), one Bragg wavelength decreases with decreasing temperature, and a third Bragg wavelength appears. At constant temperature both phases remained stable for a period of several days.