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1.
The display parameters and the stability of the various mesophases of a newly synthesised racemic mixture of anti-ferroelectric liquid crystals (AFLCs) have been boosted with the help of the electron beam irradiation. The mixture has phase sequence of isotropic–smectic C–smectic CA crystal. The effective macroscopic polarisation and transition temperatures of the various mesophases of the irradiated mixture have increased as compared with the pure mixture. The macroscopic polarisation of the mixture has increased from 21 to 27 nC/cm2 due to the irradiation. From various studies, we have observed that the temperature range of the smectic CA phase has increased by 20.6°C with marginal decrease in the temperature range of the smectic C phase. 相似文献
2.
Poly(N-vinylcaprolactam) (PVCL) connected to aminopropyl silica is a new stationary phase for temperature responsive liquid chromatography (TR-LC). PVCL shows a transition from hydrophilic to hydrophobic interaction between 30 and 40 °C. The synthesis is described in detail. The temperature responsive characteristic of the phase is illustrated with a mixture of steroids using pure water as mobile phase. An increase in retention is observed when raising the temperature. H–u plots at different temperatures were constructed. Below the lower critical solution temperature (LCST), no optimal velocity could be measured because of substantial resistance to mass transfer. Above the LCST, u opt was ca. 0.3 mm s?1 with reduced plate heights from 4 at 45 °C to 3 at 65 °C. The temperature responsive nature of the polymer is lost in green chromatography with ethanol as modifier in concentrations above 5%. 相似文献
3.
Seven polymorphic modifications of doxazosin mesylate, designed as forms A, D, E, F, G, H, I, and the amorphous state were
studied by thermal methods (TG and DSC), temperature resolved X-ray powder diffractometry, hot stage and scanning electron
microscopy and by FT-IR spectroscopy. Amorphous form was obtained either by fast evaporation of the solvent or by fast cooling
of the melt in the DSC. Polymorphs A and F were found to be stable in the temperature range from room temperature to their
melting points at 277.9 and 276.5°C, respectively. Form G, which melts at 270.8°C, was found to be hygroscopic. Polymorph
D undergoes irreversible solid–liquid–solid phase transition at 235.5°C to polymorph I which melts at 274.9°C. Form H, which
melts at 258.0°C, was found to be unstable at high temperatures. DSC examinations revealed that form H is irreversibly transformed
to polymorph F during heating above the temperature of about 240°C. The amorphous state was found to be stable at room temperature
but when heating above the glass transition (T
g=144.1°C) it crystallizes at 221.6°C, what leads into a mixture of polymorphic forms. The new polymorphic form designed as
E was identified in the mixture. The polymorph E is converted by heating to the more stable form F. The solubilities at 25°C
for forms A, and F in methanol are 3.5 and 7.7 mg mL−1and in water they are 3.8 and 6.2 mg mL−1, respectively.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
4.
Shujing Li Yimin Wei Yanqiang Fang Wei Zhang Bo Zhang 《Journal of Thermal Analysis and Calorimetry》2014,115(2):1633-1638
The use of soybean protein isolates (SPI) and corn starch (CS) for the manufacturing of textured protein by thermo-mechanical means requires a characterization of their thermal properties. SPI and CS mixtures were examined at starch mass fractions from 0 (pure SPI) to 100 (pure CS). The blends were determined by means of differential scanning calorimetry, with water content of 30, 50, and 70 % and heating rate of 5 and 10 °C min?1 over 20 to 130 °C. The results obtained showed that protein in the blend increased the onset (T o) and peak (T p) temperatures of the starch gelatinization, while starch in the blend decreased the ΔH and ΔT1/2 of the protein. T o , T p, and ΔT1/2 of SPI and CS decreased significantly with the increase of water content. T p and ΔT1/2 of SPI and CS had a marked increase with an increase of heating rate from 5 to 10 °C min?1. These results suggested that there was no chemical reaction between SPI and CS when they were heated from 20 to 130 °C. SPI in the blend restricted the CS gelatinization, while the presence of CS protected the SPI from denaturation. The increasing water content did promote thermal transition of the mixture. Higher heating rate leads to higher transition temperature. 相似文献
5.
In this study, as cast (AC, at 30 °C) and annealed (AN, at 130 °C, 3 h) samples of polyvinylidene fluoride (PVDF) and PVDF/hyperbranched polyester (HBP) (90/10) blend ultrathin films were subjected to heating-cooling (30 → 210 → 30 °C) cycle, and studied for their changes in crystalline phase transition behavior using in-situ Fourier transform infrared-transmission spectroscopy (FTIR-TS) and grazing incident reflection absorption spectroscopy (FTIR-GIRAS) techniques. Factor analysis was employed to extract the pure crystalline and amorphous spectra as well as the percentage content of ferroelectric crystallinity for both the samples. Irrespective of the thermal treatment (AC or AN) and spectral measurement (FTIR-TS or GIRAS) techniques, neat PVDF sample exhibited irreversible phase transitions during heating-cooling cycle associated with the transformation from ordered β-crystalline (1276 cm−1) into disordered amorphous (1234 cm−1) form. Interestingly, annealed PVDF/HBP blend sample measured using FTIR-GIRAS exhibited reversible crystalline phase transition behavior similar to a ‘dipole memory effect’ even after heating to 210 °C (>Tm) and then cooled to 30 °C. Compared to neat PVDF, higher ferroelectric crystallinity and reversible phase transition in PVDF/HBP blend may be attributed to (i) the existence of H-bonding between HBP (CO and OH groups) and PVDF (CH2 and CF2) and/or (ii) HBP acting as a nanoparticle in PVDF matrix. 相似文献
6.
(NH4)3[M2NCl10] (M = Nb, Ta): Synthesis, Crystal Structure, and Phase Transition The nitrido complexes (NH4)3[Nb2NCl10], and (NH4)3[Ta2NCl10] are obtained in form of moisture-sensitive, tetragonal crystals by the reaction of the corresponding pentachlorides with NH4Cl at 400 °C in sealed glass ampoules. Both compounds crystallize isotypically in two modifications, a low temperature form with the space group P4/mnc and a high temperature form with space group I4/mmm. In case of (NH4)3[Ta2NCl10] a continuous phase transition occurs between –70 °C and +60 °C. For the niobium compound this phase transition is not yet fully completed at 90 °C. The structure of (NH4)3[Nb2NCl10] was determined at several temperatures between –65 °C und +90 °C to carefully follow the continuous phase transition. For (NH4)3[Ta2NCl10] the structure of the low temperature form was determined at –70 °C, and of the high temperature form at +60 °C. The closely related crystal structures of the two modifications contain NH4+ cations and [M2NCl10]3– anions. The anions with the symmetry D4h are characterized by a symmetrical nitrido bridge M=N=M with distances Nb–N = 184.5(1) pm at –65 °C or 183.8(2) pm at 90 °C, and Ta–N = 184.86(5) pm at –70 °C or 184.57(5) pm at 60 °C. 相似文献
7.
The fungicide triadimenol consists of a mixture of two diastereoisomers. Diastereoisomer A (1RS,2SR) could be obtained from the mixture by fractionated crystallization from ethanol/water and toluene, successively, whereas
diastereoisomer B (1RS,2RS) could be separated by column chromatography on a silica gel column using ethylacetate as eluent. Four different crystal
forms of diastereoisomer A could be derived. The modifications were characterized by means of thermal analysis (thermomicroscopy,
DSC), FTIR-spectroscopy, FT-Raman-spectroscopy and powder X-ray diffraction, as well as pycnometry. The thermodynamic relationships
are illustrated in a semischematic energy/temperature-diagram which provides information about the relative thermodynamic
stabilities and physical properties of the four crystal forms. Mod. II (m.p. 132 °C, ΔHf 33.1±0.2 kJ mol−1, density 1.271±0.001 g cm−3) was obtained from toluene after the separation of diastereoisomer A and is enantiotropically related to mod. I (m.p. 138 °C,
ΔHf 32.0 ± 0.2 kJ mol−1, density 1.243±0.001 g cm−3). The transition point of mod. II with mod. I was determined between 30 and 40 °C, which means that mod. II is thermodynamically
stable at ambient conditions. Mod. III (m.p. 112 °C, ΔHf 25.1±0.5 kJ mol−1) and mod. IV were obtained from the melt. Furthermore, the phase diagrams of the binary systems of diastereoisomer B and
the four modifications of diastereoisomer A were calculated by means of the experimentally obtained thermodynamical data.
Received September 30, 1999. Revision July 30, 2000. 相似文献
8.
Akiteru Watanabe 《Journal of solid state chemistry》1982,41(2):160-165
The polymorphism of Bi2WO6 has been studied using differential dilatometry and differential thermal analysis with polycrystalline specimens prepared by sintering the oxides Bi2O3 and WO3. Two reversible polymorphic transitions were observed, one at 662°C and one at 962°C. The former transition showed a very small change of enthalpy and very little dimensional change, while the latter showed a large thermal hysteresis, had a large change of enthalpy, and was accompanied by a sizable volume change. The high-temperature powder X-ray data indicated that the intermediate phase as well as the low-temperature form had orthorhombic symmetry and the high-temperature form had monoclinic symmetry. The 662°C transition is displacive and the 962°C transition reconstructive. A crystal structure of the high-temperature form is proposed and discussed in comparison with that of the low-temperature form. 相似文献
9.
J. M. Igartua G. Aguirre-Zamalloa I. Ruiz-Larrea M. Couzi A. López-Echarri T. Breczewski 《Journal of Thermal Analysis and Calorimetry》1994,41(6):1211-1215
The specific heat of N(CH3)4CdBr3 from 50 to 300 K has been measured by adiabatic calorimetry, using both static and dynamic methods. The obtained results have permitted a careful study of the ferro-paraelectric phase transition the crystal shows at 160 K. The available spectroscopic data have been used to generate a reliable baseline which accounts for the normal lattice contribution to the specific heat. These results allow for an accurate estimation of the phase transition thermodynamic functions: ΔH=2620 J·mol?1 and ΔS=18.04 J·(mol°C)?1. These high values are in agreement with the predictions of the 6 well potential Frenkel model. 相似文献
10.
G. A. M. Hussein A. K. H. Nohman K. M. A. Attyia 《Journal of Thermal Analysis and Calorimetry》1994,42(6):1155-1165
Thermogravimetry, differential thermal analysis, X-ray diffractometry and infrared spectroscopy showed that Ni(CH3COO)2·4H2O decomposes completely at 500°C, giving rise to a mixture of Nio and NiO. The results revealed that the compound undergoes dehydration at 160°C and melts at 310°C. The water thus released hydrolyses surface acetate groups, acetic acid being evolved into the gas phase. At 330°C, the anhydrous acetate is converted into NiCO3, releasing CH3COCH3 into the gas phase. The carbonate subsequently decomposes (at 365°C) to give NiO(s), CO2(g) and CO(g). On further heating up to 373°C, a mixture of Nio and NiO is formed. Other gas-phase products were detected at 400°C, viz. CH4 and (CH3)2CH=CH2, which were formed in surface reactions involving initial gas-phase products. Non-isothermal kinetic parameters (A and ΔE) were calculated on the basis of temperature shifts experienced in the various decomposition processes as a function of heating rate (2–20 deg·min?1). 相似文献
11.
Synthesis, Crystal Structure and Properties of Cesium Ozonide By reaction between CsO2 and mixtures of O2 and O3 in the temperature range from 25°C to ?70°C and subsequent extraction with liquid ammonia pure CsO3 was obtained in grammeamounts. By X-ray powder and thermal techniques a reversible, structural phase transition was detected at +8°C, and decomposition into CsO2 and O2 at +53°C. The low-temperature form (T? CsO3) is isostructural to RbO3 (P21/c; a = 675.1(2), c = 901.5(3) pm, β = 120.74(3)°l Z = 4), the crystal structure of H? CsO3, which shows orientational disorder with respect to the ozonide ion, corresponds to the CsCl-type of structure (a = 436.06(3) pm). Using the geometry as determined for KO3 and RbO3, and the vibrational frequencies of different isotopomeres, the force constants of O3? have been redetermined. 相似文献
12.
Ming Li 《Journal of Polymer Science.Polymer Physics》2019,57(8):465-472
This work describes the melting and polymorphic behavior of poly(decamethylene terephthalamide) (PA 10T). Both solution‐crystallized (SC) and melt‐crystallized (MC) PA 10T show double melting endotherms in DSC. The SC crystal form melts at 260–300°C giving the first melting endotherm, and meanwhile undergoes a polymorphic transition forming the MC crystal form. The subsequent melting of the MC crystal form gives the second melting endotherm at 300–325°C. This irreversible polymorphic transition is confirmed by variable‐temperature WAXD and IR. Dynamic mechanical thermal analysis (DMTA) shows a glass transition temperature (Tg) at 127°C and the presence of an α′ transition at 203°C (0.1 and 1 Hz). This transition could be confirmed by DSC and variable‐temperature WAXD experiments. The α′ transition correlates with a reversible thermal process and a sudden change in intersheet spacing. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 465–472 相似文献
13.
On Na3PO4: Preparative Investigations, Crystal Structure of High Temperature Form Possibilities for preparing Na3PO4 by solid state reactions have been investigated. The existence of two modifications was proved by means of DTA and X-ray powder methods (temperature range 25–800°C). The phase transition is of first order and occurs reversibly at 325°C. The crystal structure of the high temperature form has been determined using single crystals which have been quenched to room temperature. The structure of H? Na3PO4 contains orientationally disordered PO3?4 anions and derives from the Li3Bi type of structure (Fm3m, a = 742,3 pm). 相似文献
14.
Atsuhiro Fujimori Hideki Saitoh Yoshio Shibasaki 《Journal of polymer science. Part A, Polymer chemistry》1999,37(20):3845-3853
The polymorphic behavior of 1-octadecyl vinyl ether was investigated by DSC and X-ray diffraction measurements under various temperatures. In DSC measurement of 1-octadecyl vinyl ether in the temperature range of −30 to approximately 50°C, four transition peaks were observed on heating, whereas three transition peaks appeared on cooling. The phase-transition behavior was investigated by the repeating scanning DSC measurements. It was concluded that this compound exhibited four crystalline modifications: α, sub α, β0, and β1. It was confirmed by the temperature-controlled X-ray diffraction measurement that these phase transitions are attributed to the change of crystal systems from hexagonal packing (α form) to a distorted orthorhombic (O⟂′) system (β1 form) via orthorhombic (O⟂) (sub α form) and intermediate β0 form, although the β0 form has not yet been clarified. In the γ-ray-irradiation solid-state polymerization for these crystal forms of this compound, the polymerizability of the sub α form is higher than that of other forms, and that of the α form is lowest. The polymerizability demonstrated an unusual increase at a temperature of −83.6°C, probably because the cationic polymerization mechanism is dominant over that of the free radical. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 3845–3853, 1999 相似文献
15.
《Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy》1988,44(12):1457-1460
In this work, the Raman intensities of the librational mode of the bifluoride ion and the translational mode of the K+ ion are considered as the order parameters for the phase transition of KH1−xDxF2 since group theoretical consideration shows that the Raman active translational and librational modes in the low temperature phase will be inactive in the high temperature phase if the latter is of symmetry group O or higher. The measured Raman intensities are correlated with the temperature from 163°C to 196°C to elucidate the exponent β which relates the Raman intensity to the temperature close to Tc through (Tc-T)β for various deuteration compositions. The β value of KHF2 is very close to that predicted by the Ising model of three-dimensional space with a one-dimensional order parameter, suggesting that the nearest-neighbour interaction of the translational modes can play a very important role for the phase transition. Experiments also show that the phase transition temperature is slightly isotope composition dependent. 相似文献
16.
Jahar L. Mukherjee Chandra P. Khattak Kedar P. Gupta Franklin F.Y. Wang 《Journal of solid state chemistry》1978,24(2):163-167
Ba2NaNb5O15 (BSN) exhibits a large thermal contraction in its c-axis between 350° and 750°C. This behaviour contributes to a serious cracking problem during its crystal growth. The substitutions of Gd for Ba and/or Na in BSN can be made with either anion or cation compensation. The former, namely, type A, compositions were found to eliminate the thermal contraction behaviors, and had small variations in their linear thermal expansion coefficients, α, between 50° and 650°C. With good ferroelectric and dielectric properties, they showed potentials to replace the BSN crystal in electro-optic devices. 相似文献
17.
The crystal structure of CaZr(PO4)2 has been revised by ab initio Rietveld analysis of X-ray powder diffraction data. At room temperature, CaZr(PO4)2 crystallizes in the orthorhombic space group Pna21 (Z = 4). Differential thermal analysis suggests a reversible second order transition at 1000 °C confirmed by high temperature XRD analysis that brings out the existence of a high temperature form, very similar to the room temperature one, but more symmetrical (Pnma, Z = 4). Analysis of the crystal parameters evolution during heating reveals that CaZr(PO4)2 exhibits a quite low thermal expansion coefficient of 6.11·10−6 K−1. This value stems from a combination of several mechanisms, including Coulombic repulsion and bridging oxygen rocking motion. 相似文献
18.
The non-isothermal devitrification of Na2O · 2 CaO · 3 SiO2 glass has been studied by differential thermal analysis in order to evaluate, from DTA curves, the temperature of maximum nucleation rate, Tm, and the activation energy values, Ec, for crystal growth.The temperature, Tm=580°C, is very close to the glass transition temperature, Tg=570°C, and the value of Ec=78 Kcal mole?1 for the surface crystal growth is nearly the same as the value Ec=89 kcal mole?1 for the bulk crystal growth; both are consistent with the activation energy for viscous flow. It is also pointed out that the nucleation rate—temperature curve and the crystallization rate—temperature curve are partially overlapped. 相似文献
19.
When the anatase form of TiO2 was heated at a constant rate of 6°C/min to 450°C it crystallized from hydrated amorphous TiO2 gel at 170°C in pure water or at <150°C in NaOH solutions. The uptake of Na+ ions into crystallized anatase affected the reactions subsequent to this initial crystallization while only anatase crystals continued to grow with increasing temperature in pure water. Immediately after the nearly amorphous second stage at 325°C, conversion from colloidal anatase particles to square sheet-shaped bronze-type TiO2 crystals began at 350°C and was complete at 425°C in 0.5 M NaOH. This conversion was considered to proceed via crystallographic shear rather than via dissolution and precipitation since this also happened with thermal treatment to 700°C in air. 相似文献
20.
S. R. Challa S. Q. Wang J. L. Koenig 《Journal of Thermal Analysis and Calorimetry》1995,45(6):1297-1312
The phase behaviour of thermoplastic polymer-dispersed liquid crystal system is studied with particular emphasis on the various transitions that occur within the system. The extent of plasticization of the polymer(polymethyl methacrylate) by the low molecular weight liquid crystal(E7) along with the several transitions of theLC(Liquid Crystal) are determined by modulated DSC. Optical microscopy was used to construct the temperature versus composition phase diagram. Our study indicates the existence of a limiting temperature of 40°C around which the PMMA matrix turns glassy irrespective of the initial composition within the phase separated region, suggesting the intersection of the glass transition curve with the coexistence curve. A slight depression of theN-I(Nematic to Isotropic) transition of theLC is observed with increasing composition of PMMA whereas theS-M(Smectic to Nematic) transition and theT g (Glass transition temperature) of theLC remain unaffected. The one phase mixture remains isotropic until phase separation at a lower temperature where theLC rich domains become nematic. The growth ofLC rich domains is studied as a function of temperature and time. 相似文献