Phase separation dynamics of aqueous solutions of thermoresponsive polymers studied by a laser T-jump technique

Poly(N-isopropylacrylamide) and poly(vinyl methyl ether) are well-known thermoresponsive polymers. The aqueous solutions of these polymers exhibit a phase transition followed by phase separation with LCST approximately 305-310 K. In the present study, the dynamic behavior of the phase separation was analyzed by a laser T-jump method. Two different T-jump methodologies were employed: the first was a dye-photosensitized T-jump technique (indirect heating) using 532 nm laser pulses, while the other was a direct heating T-jump technique using 1.2 mum laser pulses. Both methods gave similar results. The time constants (tau) of the phase separation were systematically determined for 1-10 wt % aqueous solutions of the polymers, and a hydrodynamic radius (R) dependence for tau was clearly observed. The values of tau increased linearly with increasing square of R. The present behavior is interpretable in the framework of Tanaka's model for the volume phase transition of a gel, since each of the polymer chains are entangled in the present sample solutions, which can be regarded as approximating to a gel in solution.     

Study of the chemical mechanism involved in the formation of tungstite in benzyl alcohol by the advanced QEXAFS technique

Insight into the complex chemical mechanism for the formation of tungstite nanoparticles obtained by the reaction of tungsten hexachloride with benzyl alcohol is presented herein. The organic and inorganic species involved in the formation of the nanoparticles were studied by time-dependent gas chromatography and X-ray diffraction as well as by time-resolved in situ X-ray absorption near-edge structure and extended X-ray absorption fine structure spectroscopy. Principal component analysis revealed two intermediates, which were identified as WCl(4) and WOCl(4) by using linear combination analysis. Quick-scanning extended X-ray absorption fine structure spectroscopy enabled the time-dependent evolution of the starting compound, the intermediates and the product to be monitored over the full reaction period. The reaction starts with fast chlorine substitution and partial reduction during the dissolution of the tungsten hexachloride in benzyl alcohol followed by the generation of intermediates with W=O double bonds and finally the construction of the W-O-W network of the tungstite structure.     

Strain criterion in nonlinear creep and recovery in concentrated polymer solutions

Transient and steady-state rheological data are reported for several anionic polystyrene solutions in tritolylphosphate (1. 6 < cM/ρM_c < 7). Here c is the concentration of the solution, M is the molecular weight, ρ the density of the undiluted polymer, and M_c the molecular weight between entanglements as determined from zero-shear viscosity. The polystyrene used had M_w = 410,000 and M_w/M_n < 1.06. Data are also given for solutions of polyisobutylene and poly(vinyl acetate) with larger M_w/M_n. The results give a critical strain γ′ ∝ c⁻¹ such that linear viscoelastic behavior was obtained in a simple shear deformation with shear less than γ′. A simplified version of the constitutive equation of Bernstein, Kearsley, and Zapas is used with an empirical strain function F (γ) which contains γ′ as a parameter to discuss transient and steady-state behavior in terms of the distribution of relaxation (or retardation) times determined for linear viscoelastic responce. Features of the dependence of the steady-state viscosity η_k, recoverable compliance R_k, the first-normal stress function N_k⁽¹⁾ on shear rate k are discussed in terms of F (γ) and the distribution of relaxation times to conclude that the latter plays a dominant role in the behavior observed in the range of k usually studied. The results predict that the reduced functions η_k/η₀, R_k/R₀, and N_k⁽¹⁾/N₀⁽¹⁾ should depend on η₀R_0k, and that the functional form depends markedly on the distribution of relaxation times, at least in the range η₀R₀k < 10². Comparison with the mechanistic model of Doi and Edwards shows a similar F (γ) but substantial differences in the reduced functions caused by a very narrow distribution of relaxation times in the model.     

A discrete complex compliance spectra model of the nonlinear viscoelastic creep and recovery of microcellular polymers

The present work reports a discrete stress‐dependent, complex compliance spectra method that may be used to predict the mechanical response of nonlinear viscoelastic polymers during creep and recovery processes. The method is based on the observation that the real and imaginary parts of a discrete complex compliance frequency spectra obtained from creep and recovery measurements are smooth, easily fit functions of stress. The new method is applied to a set of microcellular polycarbonate materials with differing relative density. The nonlinear viscoelastic characteristics of a microcellular polycarbonate material system are very sensitive to relative density and therefore, this material system is a particularly difficult modeling challenge. However, the present model was able to exhibit excellent quantitative agreement with the basis creep and recovery measurements at all experimental stress levels for each of the experimental relative density material types. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 691–697, 2000     

Ultrafast T-jump in water: studies of conformation and reaction dynamics at the thermal limit

We report the development of ultrafast T-jump with time resolution reaching the fundamental time scale of water thermalization time ( approximately 5 ps). The T-jump heats the bulk water up to 20 degrees C via the overtone absorption of the OH- stretch at 1.5 mum. The example given here shows the application of the methodology, for the first time, and the results demonstrate distinct time scales for solvation, conformational change, and thermal reaction.     

Hydrogen desorption mechanism in a Li-N-H system by means of the isotopic exchange technique

The hydrogen desorption mechanism in the reaction from LiH + LiNH2 to Li2NH + H2 was examined by thermal desorption mass spectrometry, thermogravimetric analysis, and Fourier transform IR analyses for the products replaced by LiD or LiND2 for LiH or LiNH2, respectively. The results obtained indicate that the hydrogen desorption reaction proceeds through the following two-step elementary reactions mediated by ammonia: 2LiNH2 --> Li2NH + NH3 and LiH + NH3 --> LiNH2 + H2, where hydrogen molecules are randomly formed from four equivalent hydrogen atoms in a hypothetical LiNH4 produced by the reaction between LiH and NH3 according to the laws of probability.     

Determination of creep compliance,recovery and Poisson's ratio of elastomers by means of digital image correlation (DIC)

This work aims to determine the creep compliance, creep recovery and Poisson's ratio of three common sealing elastomers by means of the digital image correlation (DIC). The tests were conducted by stressing specimens under three different constant stresses during short duration experiments (3 h) to see the prospective of DIC for this application. The strains were measured in x and y axes with time. Thus, the behavior of creep compliance, creep recovery, and the Poisson's ratio of each elastomer were obtained. The creep results exhibited repeatability, as well as, the mean Poisson's ratios estimated were close to reported values for elastomers. Finally, despite of some limitations from the DIC equipment, it was found that this procedure can be implemented as a suitable alternative for the characterization of creep compliance, creep recovery and Poisson's ratio of elastomers. Also, it may be enhanced by following some recommendations given.     

基于代谢组学技术的辛伐他汀对磷脂类物质作用的初探

用超高效液相色谱和高分辨飞行时间质谱（UPLC／Q-TOF MS）联用技术,跟踪小鼠给予药物辛伐他汀前后血清中的内源性代谢物的变化,开展基于代谢组学技术的辛伐他汀对磷脂类物质作用的研究．研究结果显示：辛伐他汀药物对血清中磷脂类物质有较大影响,本文对其中6种具有显著性差异的标记物进行了结构确认,并对这些物质与降脂作用的关系进行了讨论．研究结果表明,将代谢组学技术应用于药物的作用效果研究,对新药研发和合理临床用药具有重要的指导意义．     

Study on the mechanism of Schiemann reaction by mass spectrometry

Schiemann reaction of 12 aryl diazonium fluoborates was studied by mass spectrometry. The products and by-products of the reactions were identified and the possible pathways were explored. It is shown that Schiemann reaction proceeds by free-radical mechanism.     

Microsecond melting of a folding intermediate in a coiled-coil peptide, monitored by T-jump/UV Raman spectroscopy

A truncated version of the GCN4 coiled-coil peptide has been studied by ultraviolet resonance Raman (UVRR) spectroscopy with 197 nm excitation, where amide modes are optimally enhanced. Although the CD melting curve could be satisfactorily described with a two-state transition having Tm = 30 degrees C, singular value decomposition of the UVRR data yielded three principal components, whose temperature dependence implicates an intermediate form between the folded and unfolded forms, with formation and melting temperatures of 10 and 40 degrees C. Two alpha-helical amide III bands, at 1340 and 1300 cm(-1), melted out selectively at 10 and 40 degrees C, respectively, and are assigned to hydrated and unhydrated helical regions. The hydrated regions are proposed to be melted in the intermediate form, while the unhydrated regions are intact. Time-resolved UVRR spectra following laser-induced temperature jumps revealed two relaxations, with time constants of 0.2 and 15 mus. These are suggested to reflect the melting times of hydrated and unhydrated helices. The unhydrated helical region may be associated with a 14-residue "trigger" sequence that has been identified in the C-terminal half of GCN4. Dehydration of helices may be a key step in the folding of coiled-coils.     

Simulation of the T-jump triggered unfolding and thermal unfolding vibrational spectroscopy related to polypeptides conformation fluctuation

We review in this article our recent simulation works on modeling peptide T-jump and thermal unfolding Fourier transform infrared spectroscopy(FTIR) and two-dimensional infrared(2DIR) spectra. The theoretical and computational techniques used,including Markov state model(MSM), integrated tempering sampling(ITS) and nonlinear exciton propagation(NEP), are first briefly introduced. The protocols for simulating the thermal unfolding as well as T-jump unfolding are then summarized in details. The simulated spectral features, such as the intensity and ellipticity, are demonstrated to agree well with the experimental observations.     

Study of molecular mobility in ultraoriented polypropylene by the spin-probe technique

The rotational mobility of nitroxide radicals in polypropylene samples with draw ratios varying from 1 to 18 has been studied between 30 and 110°C by the ESR technique. The drop in the rotational frequency with increasing orientation correlates very well with the behavior of various other measures of molecular mobility such as organic vapor sorption, proton spin–spin relaxation times from NMR, and dynamic mechanical loss factor. This implies that each of these parameters is a good indicator of the amount of free volume in the amorphous regions. Annealing at high temperature relaxes the amorphous chain segments, thereby increasing the spin-probe rotational frequency in the drawn samples to values even higher than that in the quenched isotropic material.     

Physical aging of a polyetherimide: Volume recovery and its comparison to creep and enthalpy measurements

Volume recovery measurements have been used to study the physical aging behavior of a polyetherimide. Isothermal aging temperatures near T_g were studied with aging times ranging up to several days. The volume decreases during physical aging and levels off at equilibrium. For comparison purposes, the data are normalized to yield the departure from equilibrium which varies from unity at very short aging times to zero when equilibrium is reached. As the aging temperature decreases, the normalized curves are shifted to longer times without a significant change in shape. Hence, the data can be reduced by aging time—temperature superposition. The temperature dependence of the shift factors used to reduce the volume recovery data and the times to reach equilibrium for the volume recovery follow the WLF equation and agree within experimental error with the values from enthalpy and creep measurements obtained in previous work. However, the approach to equilibrium for volume appears to differ from that of enthalpy, with volume recovery being faster than the enthalpy recovery at short times. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35 : 929–936, 1997     

纳米尺度分散的TiO2光催化降解甲醛的机理

纳米TiO2具有小尺寸效应,表面效应和宏观量子隧道效应等[1],广泛用于防水处理、空气净化、抗菌防霉等方面。本文对其能带结构及其消除甲醛的性能及机理进行分析。1　纳米TiO2光催化剂1 1　制备与表征以四氯化钛为主要原料,氨水、醇类化合物、盐酸等为主要反应助剂,采用亚稳态氯化法制备工艺,用相应的溶胶反应形成溶胶液体,低于100℃加热该液体,除去多余的水分,得到溶胶-凝胶体。过滤所得溶胶-凝胶体,并用蒸馏水反复洗涤数次,使其pH呈中性。将洗涤后的溶胶-凝胶体进行真空干燥,得到白色微粉,1000℃高温煅烧,将煅烧后得到的纳米TiO2基…     

Thermal decomposition mechanism of aluminum nitrate octahydrate and characterization of intermediate products by the technique of computerized modeling

Thermal decomposition of aluminum nitrate hydrate was studied by thermogravimetry, differential scanning calorimetry, and infrared spectroscopy, so that all mass losses were related to the exactly coincident endothermic effects and vibrational energy levels of the evolved gases. The process starts with the simultaneous condensation of two moles of the initial monomer Al(NO₃)₃·8H₂O. Soon after that, the resulting product Al₂(NO₃)₆·13H₂O gradually loses azeotrope HNO₃ + H₂O, then N₂O₃ and O₂ and, through the formation of Al₂O₂(NO₃)₂, is transformed into aluminum oxide. The molecular mechanics method used for comparison of the potential energies of consecutive products of thermal decomposition permits an evaluation of their structural arrangement. On the basis of the results obtained, a probable mechanism for the overall decomposition of Al(NO₃)₃·8H₂O has been proposed.     

高效液相色谱手性拆分药物对映体机理研究

于Whelk-O1(R,R)手性固定相上直接拆分药物萘普生和丙卡特罗。两药物的对映体在该手性固定相上均可以实现基线分离,分离因子达1.4以上。根据相关手性色谱分离热力学数据,进行了手性识别机理研究,并据此成功预测了药物克仑特罗对映体在Whelk-O1(R,R)手性柱上的拆分。     

Study of the extrinsic properties of ZnO:Al grown by SILAR technique

Aluminum-doped ZnO thin films with pebble-like structures have been successfully deposited on glass substrates by successive ionic layer adsorption reaction method. The effect of percentage composition of the aluminum dopant on the flower-like clusters of the ZnO nanostructures on the structure, morphology, and optical properties was investigated. The ZnO thin films which were crystallized in hexagonal wurtzite structures with crystallite sizes of 44, 51, 56, and 43 nm for the intrinsic and 1, 3, and 5% Al-doped ZnO thin films, respectively. Preferred orientation of crystallites is in all cases in [001] direction perpendicular to the sample surface The Raman spectroscopy revealed decrease in the intensity of the ZnO characteristic peak due to the substitution of the Zn²⁺ atoms by the Al³⁺ and attributed to potential fluctuations of the alloy disorder. The introduction of the Al³⁺ dopant significantly increased the optical band gap.

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