首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献,搜索用时 31 毫秒
1.
Isothermal physical aging and the glass transition temperature (T g) of PMMA thin films were investigated by means of differential scanning calorimetry (DSC). Freestanding thin films of different molecular weights (M w = 120,000, 350,000, 996,000 g/mol) and film thicknesses (40–667 nm) were obtained by spin coating onto a silicon wafer substrate and then releasing the coated film using a water floating technique. The thin films were stacked in a DSC pan and isothermally aged for different aging times (t a = 1 and 12 h) and aging temperatures (T a = 105, 110, and 115 °C) below but near T g. Enthalpy relaxation (ΔH Relax), resulting from the isothermal physical aging, initially increased with increasing ΔT (T g − T a, driving force of aging), reached a maximum value, and then decreased with further increase in ΔT. Below ~100 nm film thickness, ΔH Relax of samples aged near their T g (i.e., T a = 110 and 115 °C) decreased with decreasing film thickness, indicating the suppression of physical aging. Up to 9.9 °C depression in T g was observed for thinner films (~40 nm), when compared to the thicker films (~660 nm) in this study. The decrease in ΔH Relax with decreasing film thickness at a given T a appears to be associated with the reduction in T g.  相似文献   

2.
There is still lack of the insight into the storage stability of dry probiotics produced by vacuum drying. Therefore, in this study we assessed the stability of a vacuum-dried Lactobacillus paracasei F19 under varying storage conditions. L. paracasei F19 was vacuum-dried with and without sorbitol and trehalose. The dried cells were stored at 4, 20 and 37 °C, and at aw = 0.07, 0.22 and 0.33. The survival was determined by viable counts on MRS agar plates. The inactivation rate constants were determined for each storage condition. The survival after drying of cells dried without and with trehalose and sorbitol was 29, 70 and 54%, respectively. All vacuum-dried cells were very stable at 4 °C. However, high stability at non-refrigerated temperatures was obtained only in the presence of sorbitol. In contrast to sorbitol, the supplementation of trehalose did not stabilize cells during storage. This is supposedly due to the rapid crystallization of trehalose during storage. While glass transition temperatures of dry cell-sorbitol increased from ?32 °C to 12 °C during storage at 37 °C and aw = 0.07, Tg of dry cell-trehalose (?15 °C after drying) could not be determined after storage for only 24 h. In conclusion, we showed that high stability of probiotic cells at non-refrigerated temperatures could be obtained by vacuum drying process with appropriate protectant.  相似文献   

3.
The synthesis of feruloylated coconut oil and feruloylated shea butter were demonstrated in 0.5-L scale, shaken, batch reactions. Ethyl ferulate and the vegetable oil/fat were combined in a 1.0:1.3 mol ratio in the presence of Candida antartica lipase B immobilized on an acrylic resin (Novozym 435) at 60 °C. The transesterification of ethyl ferulate with coconut oil and shea butter reached equilibrium conversions, after 22 days, of 63 and 70%, respectively, with the shea butter transesterifications producing a white precipitate not observed in the coconut oil transesterifications. The faster transesterification rates, equilibrium conversions and white precipitate were shown to result from di- and monoacylglycerols (DAG and MAG) present in the shea butter. The transesterification of ethyl ferulate and coconut oil was also tested in a continuous, enzymatic, packed-bed bioreactor using Novozym 435 at 60 °C to produce feruloylated coconut oil at rates of 0.5–0.9 kg/day over 4.5 months. The feruloylated coconut oil acylglycerol species were identified by LC–MS analysis of transesterification reactions of ethyl ferulate with medium chain triacylglycerol (TAG) standards, C8–C14. The feruloylated vegetable oils possessed an ultra violet (UV) absorbing λ max 328 nm, making them good UVAII absorbers, as defined by the U.S. Food and Drug Administration. The feruloylated coconut oil possessed a 17.5% higher absorption capacity than feruloylated shea butter on a per weight basis. All the feruloylated vegetable oils possessed rapid antioxidant capacity (50% reduction of initial radical concentration <5 min) at the concentrations tested, 0.5–2.5 mM. Feruloylated coconut oil possessed chemical and physical characteristics that suggested it would be fungible for feruloylated soybean oil in current retail formulations.  相似文献   

4.
The use of HPLC-MS to separate and identify the feruloylated acylglycerols formed during the transesterification of ethyl ferulate with TAG was examined. Novozym 435 (Candida antarctica lipase B)-catalyzed transesterifications of ethyl ferulate and soybean oil resulted in a mixture of feruloylated MAG, DAG, and TAG and diferuloylated DAG and TAG. These feruloylated acylglycerols have recently garnered much interest as cosmeceutical ingredients. The ratio of the various feruloylated acylglycerol species in the resultant oils is presumed to affect the oil's cosmetic efficacy as well as its physical (formulation) properties. Thus, it was desirable to develop an analytical method to separate, identify, and quantify the individual feruloylated acylglycerols to determine their relative ratios. The feruloylated acylglycerols were successfully separated and identified by HPLC-MS using a phenyl-hexyl reversed-phase column developed with a water/methanol/1-butanol gradient. The chromatograms of the feruloylated acylglycerols from soybean oil were convoluted by myriad fatty acids; therefore, feruloylated acylglycerols from triolein were studied as a model reaction. Hydrolysis of the feruloylated acylglycerols from triolein catalyzed by Lipase PS-C “Amano” I (Burkholderia cepacia), which showed no hydrolysis reactivity toward ethyl ferulate, allowed for the chromatographic assignment of the feruloyl acylglycerol positional isomers.  相似文献   

5.
Novozym® 435-catalyzed transesterification of ethyl 4-hydroxy-3-methoxy cinnamate (ethyl ferulate, EF) with triolein to form the ultraviolet (UV)-absorbing lipids monoferuloylmonooleoyl-glycerol (FMO) and feruloyl-dioleoyl-glycerol (FDO) has been conducted using supercritical CO2 (SC?CO2) batch reactions. The alcoholysis of 0.1 M EF with 0.1 M 1-octanol in SC?CO2 to form octyl ferulate was used as a model reaction to optimize pressure and temperature conditions. Conditions ranging from 45 to 80°C and 10.3 to 34.5 MPa (1,500 to 5,000 psi) were tested with a maximal conversion of 53% of the EF being achieved at 13.8 MPa (2,000 psi) and 80°C after 24 h. These optimized conditions applied to the transesterification of EF with triolein effected a combined FMO and FDO yield of 69%. Triolein exhibits higher solubilities in SC?CO2 at higher pressures; therefore, the transesterification was performed at 80°C over a range of pressures from 13.8 to 34.5 MPa (2,000 to 5,000 psi). Results showed that a maximal yield of 74% of FMO and FDO was reached at 80°C and 24.1 MPa (3,500 psi) after 48 h. Compared to the FMO and FDO synthesis conducted neat or in toluene, the synthesis of the UV-absorbing lipids in SC?CO2 affords higher yields within a shorter amount of time. Therefore, the transesterification of EF with triolein in a SC?CO2 batch reaction is a viable route to UV-absorbing lipids that could be used as active ingredients in sunscreen formulations.  相似文献   

6.
Turner C  He X  Nguyen T  Lin JT  Wong RY  Lundin RE  Harden L  McKeon T 《Lipids》2003,38(11):1197-1206
The objective of this study was to find the optimal parameters for lipase-catalyzed methanolysis of triricinolein to produce 1,2(2,3)-diricinolein. Four different immobilized lipases were tested, Candida antarctica type B (CALB), Rhizomucor miehei (RML), Pseudomonas cepacia (PCL), and Penicillium roquefortii (PRL). n-Hexane and diisopropyl ether (DIPE) were examined as reaction media at three different water activities (a w), 0.11, 0.53, and 0.97. The consumption of triricinolein and the formation of 1,2(2,3)-diricinolein, methyl ricinoleate, and ricinoleic acid were followed for up to 48 h. PRL gave the highest yield of 1,2(2,3)-diricinolein. Moreover, this lipase showed the highest specificity for the studied reaction, i.e., high selectivity for the reaction with triricinolein but low for 1,2(2,3)-diricinolein. Recoveries of 93 and 88% DAG were obtained using PRL in DIPE at a w of 0.11 and 0.53, respectively. Further, NMR studies showed that a higher purity of the 1,2(2,3)-isomer vs. the 1,3-isomer was achieved at higher a w (88% at a w=0.53), compared to lower a w (71% at a w=0.11). The DAG obtained was acylated by the DAG acyltransferase from Arabidopsis thaliana. Therefore, this enzymatic product is a useful enzyme substrate for lipid biosynthesis. Accordingly, the use of PRL in DIPE at a w 0.53 is considered optimal for the synthesis of 1,2(2,3)-diricinolein from triricinolein.  相似文献   

7.
Micromolding in capillaries has been used to fabricate alumina stripes on smooth (Ra = 0.5 nm) and rough substrates (Ra = 900 nm). Different lateral (10–500 μm) and vertical stripe dimensions (8–27 μm) were used to study the influence of substrate roughness (smooth and rough sapphire) and substrate material (platinum coated and plain sapphire) on sintering behavior. Alumina stripes experience edge delamination during sintering on a rigid substrate independent of substrate roughness. However, enhanced substrate roughness reduced delamination length by half and lowered lateral strains by up to 0.10. Grooves in the rough substrate were found to be responsible for this feature as they act as crack propagation barriers and generate local density minima. Accompanying discrete element simulations revealed a localized triaxial stress state at the grooves of a sinusoidal‐shaped substrate interface as the main cause of the density minima. Platinum interlayers also resulted in reduced delamination by 40% in some stripe geometries while density was enhanced by 4% and lateral strains doubled in some geometries. Creep of the metal layer during sintering is thought to be the reason for this seemingly contradictory behavior.  相似文献   

8.
Novel triazole-based aluminum complex {O,O′-[4,5-P(O)Ph2tz]-AlMe2 was studied as the catalyst for the ring-opening polymerization of caprolactone (ε-CL) in chlorobenzene. In the presence of methanol, isopropanol, and bifunctional poly(ethylene glycol), the catalytic system produced polymers with high conversion (81–85 %) but broader distribution (M w/M n = 1.5–1.8). The system of catalyst and benzyl alcohol produced relative monodisperse PCLs (M w/M n ~ 1.2) with defined molecular weight at 1/1ratio, 60 °C and an initial concentration of ε-CL equal to 0.5 mol/L.  相似文献   

9.
Dry prickly palm cactus (Nopalea cochenillifera) husk was investigated as a substrate for Rhizopus sp. cultivation in the solid state, aiming at the production of laccase (Lac), lignin peroxidase (LiP), and manganese peroxidase (MnP). The optimization of fermentation was evaluated by an experimental design and it was obtained, for each enzyme, maximum productivities (U g?1 h?1) of: 0.085 ± 0.02 (MnP), 0.066 ± 0.001 (LiP), and 0.023 ± 2.3.10?4 (Lac), at the conditions of 10 g of substrate, 72 h of fermentation, aw = 0.865, and 30°C. The enzymes thermal and pH stabilities were evaluated and it was observed better results at temperatures no higher than 60°C and pH of 5.0; in addition, the storage of these enzymes was better at ? 25°C than at 4°C. Since the prickly palm cactus is an agricultural substrate and specially because of its low cost, it is important to propose different applications for it as, for example, an alternative substrate for biotechnological processes.  相似文献   

10.
A hexagonal form of tungsten trioxide (h‐WO3, particle size: 15.9‐57.1 nm) was found to be formed by a direct reaction between metallic tungsten powder (W, particle size: 0.45‐0.59 μm) and 15%‐30% hydrogen peroxide (H2O2) aq solution. Oxide film on the powder surface having the similar crystal structure as h‐WO3 was essential for the formation, and the surface oxide film was formed by aging the powder in air at 45°C, a relative humidity of 100% (PH2O 96 hPa) for 3‐28 days or in ambient atmosphere at room temperature for 12 years. The Rietveld analysis performed in the space group P63/mcm (Z = 6) indicated the crystal structures were the same as those of the reported h‐WO3 and that the crystallographic characteristic was as follows: a = 0.74219 nm, c = 0.77198 nm for h‐WO3 from the 28‐day aged powder, and a = 0.74538 nm, c = 0.77194 nm for h‐WO3 from the 12‐year aged powder.  相似文献   

11.
The Ag-Pd internal electrode of multilayer piezoelectric ceramics needs to be sintered below 1000°C, and lead wires and components need to be welded with lead-free solder at 260°C. PNN–PMW–PZT–xSr piezoelectric ceramics with high Curie temperature (Tc > 260°C) were synthesized at a low sintering temperature (960°C) to meet the requirements of multilayer piezoelectric devices. The relationship between structures (phase, domain, and microstructures) and electrical properties (piezo/ferroelectric properties, and dielectric relaxation) in the Sr2+ substituted ceramics was investigated. Rietveld refinement and Raman spectra show that Sr2+ substitution can cause the phase change and increase the force constant of [BO6] octahedron. The piezoelectric response increases with increasing the content of the tetragonal phase (CTP) in the rhombohedral-tetragonal (R-T) coexisted ceramics. The ceramics with 0.6 mol% Sr2+ substitution have minimum activation energy for domain wall movement (Ea) of 0.0362 eV which favors the formation of nanometer-sized domains, and possess excellent electrical properties (d33 = 623 pC/N, d33* =783 pm/V, Tc =295°C). The higher the CTP, the lower the Ea. The lower Ea favors the rotation of polarization direction and extension, and is beneficial to the generation of the nanometer-size domains, resulting in high piezoelectric properties.  相似文献   

12.
Cellulase from Penicillium funiculosum was immobilized on functionalized MCF (Meso Cellular Foam) silica by imine bond formation followed by reduction using NaBH4. The specific activities of free and immobilized enzyme were measured for hydrolysis of soluble carboxymethyl cellulose (CMC). The highest activity of MCF immobilized and native enzyme was obtained at optimum pH 5 and 4.5 respectively. Kinetic parameters, Michaelis–Menten constant (Km) and maximum reaction velocity (Vmax), were calculated as Km = 0.025 × 10−2 mg/mL, Vmax = 5.327 × 10−3 U/mg for the free enzyme and Km = 0.024 × 10−2 mg/mL, Vmax = 9.794 × 10−3 U/mg for MCF immobilized enzyme respectively. The reusability of immobilized enzymes showed that 66% of its activity is retained even after 15 cycles. The availability of polar groups (–NH–, –OH) and large pore size of surface modified MCF could be electrostatically stabilizing the cellulase. Functionalized MCF was found to be a promising material for stabilizing cellulase with 16.4 wt% loading of enzyme.  相似文献   

13.
Cubic niobium nitrides δ-NbNx with different x = 0.87–1.015 were prepared by the self-propagating high-temperature synthesis (SHS) under nitrogen pressures of P(N2) = 48–230 MPa.Nitrogen composition x as a function of P(N2) was determined in weight gain experiments and compared with that determined by chemical (Kjeldahl) analysis. For powders with different x, the superconducting transition temperature Tc was measured as a function of lattice parameter a. The Tc values were found to grow linearly with increasing a. A maximum value of Tc = 15.0 K for cubic niobium nitride corresponded to a maximum value of a = 4.3901 Å.  相似文献   

14.
The 10 mol% ZnO–2 mol% B2O3–8 mol% P2O5–80 mol% TeO2 (ZBPT) glass was prepared by quenching as well as slowly cooling the melt. The ZBPT glass prepared by both methods show similar microwave dielectric properties. ZBPT glass has an εr of 22.5 (at 7 GHz), Qu × f of 1500 GHz, and τf of ?100 ppm/°C. The ceramic‐glass composites of Sr2ZnTeO6 (SZT) and ZBPT is prepared through two convenient methods: (a) conventional way of co‐firing the ceramic with ZBPT glass powder and (b) a nonconventional facile route by co‐firing the ceramic with precursor oxide mixture of ZBPT glass at 950°C. In the former route, SZT + 5 wt% ZBPT composite sintered at 950°C showed moderately good microwave dielectric properties (εr = 13.4, Qu × f = 4500 GHz and τf = ?52 ppm/°C). Although the SZT + 5 wt% ZBPT composite prepared through the nonconventional method also showed similar microwave dielectric properties (εr = 13.8, Qu × f = 5300 GHz and τf = ?50 ppm/°C), the synthesis procedure is much simplified in the latter case. The composites are found to be chemically compatible with Ag. The composite containing 5 wt% ZBPT prepared through conventional and nonconventional ways shows linear coefficients of thermal expansion of 7.0 ppm/°C and 7.1 ppm/°C, respectively. Both the composites have a room‐temperature thermal conductivity of 2.1 Wm?1 K?1.  相似文献   

15.
The article reports on low-temperature high-rate sputtering of hydrophilic transparent TiO2 thin films using dc dual magnetron (DM) sputtering in Ar + O2 mixture on unheated glass substrates. The DM was operated in a bipolar asymmetric mode and was equipped with Ti(99.5) targets of 50 mm in diameter. The substrate surface temperature Tsurf measured by a thermostrip was less than 180 °C for all experiments. The effect of the repetition frequency fr was investigated in detail. It was found that the increase of fr from 100 to 350 kHz leads to (a) an improvement of the efficiency of the deposition process that results in a significant increase of the deposition rate aD of sputtered TiO2 films and (b) a decrease of peak pulse voltage and sustaining of the magnetron discharge at higher target power densities. It was demonstrated that several hundreds nm thick hydrophilic TiO2 films can be sputtered on unheated glass substrates at aD = 80 nm/min, Tsurf < 180 °C when high value of fr = 350 kHz was used. Properties of a thin hydrophilic TiO2 film deposited on a polycarbonate substrate are given.  相似文献   

16.
Lipase (E.C. 3.1.1.3) from walnut seed was purified 28.6-fold with 31% yield using Sephadex G-100 gel chromatography. Olive oil served as good substrate for the enzyme. The optimum pH and temperature were 9.0 and 70 °C, respectively. The lipase was stable between 30 and 80 °C for 5 min. K m and V max values were determined as 48 mM and 23.06 × 10−3 U/min mg for triolein as substrate. Lipase activity was slightly reduced by Cu2+, Ca2+, Hg2+, Mn2+, and Ni2+ ions, while Mg2+ and Zn2+ had no effects. Anionic surfactant sodium dodecyl sulfate stimulated lipase activity while non-ionic surfactants Tween-80 and Triton X-100 had negligible effects on enzymatic activity. The enzyme activity was not affected by 50 mM urea and thioacetamide. Potassium ferricyanide, n-bromosuccinamide and potassium cyanide reduced the enzyme activity. The enzyme showed a good stability in organic solvents, the best result being in n-hexane (113% residual activity). The activity of dialysate was maintained approximately 80% for 1 year at −20 °C.  相似文献   

17.
The title compound, [Sm2(C3H2O4)3(H2O)6], was investigated by X-ray diffraction. It crystallizes in the monoclinic space group C2/c with cell parameters a = 17.1650(8) ?, b = 12.3010(5) ?, c = 11.1420(4) ?, β = 127.5161(10)°, Z = 4 and V = 1866.04(14) ?3. The Sm atom lies on a two-fold axis and has nine-coordination with six oxygen atoms from carboxylate groups and three water molecules. The compound forms a layer-type polymeric structure. The layers are formed by samarium and one independent malonate group to give a three-dimensional framework. The extensive network of hydrogen bonds and bridge bonds observed in this structure enhances the structural stability. The thermal dehydration of the compound was investigated by thermogravimetric analysis.  相似文献   

18.
In this work, we demonstrate that the introduction of a small amount of NiO (1.8 wt%) to the proton-conducting perovskite yttrium-doped barium (BaCe0.9Y0.1O3−δ, BCY10) can radically improve its chemical stability, even in conditions of very high carbon dioxide partial pressure (pO2 = 1 atm) and wet conditions (pH2O = 0.033 atm). To this end, we test sets of unmodified and NiO-modified BCY samples sintered at different temperatures to achieve different grain sizes. Long-term stability measurements up to 720 h at 400°C, under these conditions, reveal a noticeable drop in conductivity for the unmodified samples, scaling with decreasing grain size, due to the formation of barium carbonate. Conversely, the NiO-modified samples show no apparent degradation, with a stable conductivity performance retained over 720 h, irrespective of grain sizes. We tentatively attribute this unusual behavior to the increased chemical resistance of the perovskite phase due to an increase in the aNiO/aBaO activity ratio at the bulk surfaces, which can prevent surface attack. Such an effect is supported by an observed increase in the Schottky barrier height, revealing a change in the specific grain boundary properties of the NiO-modified samples. Conductivity measurements in wet O2 (pH2O = 0.033 atm) underscore that both the bulk and grain boundary terms of the conductivity of the NiO-modified sample, sintered at 1350°C, are competitive with the unmodified BCY sample, sintered at 1450°C, even at temperatures as low as 400°C. The results here reported, thus, unlock a different perspective for these transition metal additives, to improve the chemical resistance of proton-conducting ceramics perovskites.  相似文献   

19.
Two novel metal–organic coordination polymers {[Pb2(MIP)2(BDC)2]·H2O} n 1 and [Cd(MIP)(NDC)] n 2 [MIP = 2-(3-methoxyphenyl)-1H-imidazo [4,5-f] [1, 10] phenanthroline, BDC = terephthalic acid, NDC = naphthalene-1,4-dicarboxylic acid] were obtained from hydrothermal reaction and characterized by elemental analysis, thermogravimetric (TG) analysis, infrared spectrum (IR) and single-crystal X-ray diffraction. The coordination polymers crystallize in triclinic, space group P-1 with a = 1.0335(2), b = 1.4224(3), c = 1.8156(4) nm, β = 106.088(3)° for complex 1 and the complex 2 crystallizes in monoclinic, space group P2(1)/n with a = 1.2562(8), b = 1.4800(9), c = 1.3723(8) nm, β = 97.257(1)°, respectively. The metal ions Pb(II) are located in [:PbN2O4] pentagonal bipyramidal geometry in complex 1. The metal ions Cd(II) in complex 2 act as distorted octahedral geometry, being surrounded by four carboxylate oxygen atoms from three NDC ligands and two donor nitrogen atoms from one MIP molecule. Moreover, there are hydrogen bonds in the two complexes, and it is noteworthy that the existence of hydrogen bonds and π–π interactions reinforce the structural stability of the title complexes, which have been proved by TG analysis. The luminescent properties for the ligand MIP, NDC and complex 2 are also discussed in detail.  相似文献   

20.
Dense Bi2Te2W3O16 ceramics were prepared by the conventional solid‐state reaction route. X‐ray diffraction data show the room‐temperature (RT) crystal symmetry of Bi2Te2W3O16 to be well described by the centrosymmetric monoclinic C2/c space group [a = 21.280(5) Å, b = 5.5663(16) Å, c = 12.831(3) Å and β = 124.014(19)° and Z = 4]. Raman spectroscopy analyses are in broad agreement with space group assignment, but also revealed the presence of Bi2W2O9 as a secondary phase. This phase is present as plate‐like grains embedded on a fine‐grained equiaxed matrix, as revealed by scanning electron microscopy. From the fitting of infrared reflectivity data the relative permittivity, εr, was estimated as 34.2, and the intrinsic quality factor, Qu × f as 57 500 GHz. At RT and microwave frequencies, Bi2Te2W3O16 ceramics sintered at 720°C for 6 h exhibit εr ~ 34.5, Qu × f = 3173 GHz (at 7.5 GHz), and temperature coefficient of resonant frequency, τf = ?92 ppm/°C. This shows a good agreement between the estimated and measured εr values, but also shows that, in principle, the dielectric losses of the ceramics are of extrinsic origin.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号