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 46 毫秒
1.
Small-angle X-ray scattering (SAXS) was employed to study the nanostructural properties of poly(ethylene glycol)(PEG)/silica hybrid wet gels prepared from hydrolysis of tetraethoxysilane (TEOS) in mixtures with PEG/(PEG + TEOS) molar ratio in the nominal range between 0 and 0.8. The SAXS pattern was found to be well fitted by the scattering from a polymeric particle of radius of gyration R g with an internal structure of primary silica particles described by a polymeric constraint of a mass-fractal of dimension D. R g increases with the PEG quantity while D first increases from 2.24 for the pure TEOS-derived gel to reach values between 2.46 and 2.40 with the additions of PEG. The correlation volume V c as determined by SAXS for such a polymeric structure behaves as a volume-fractal and accordingly scales with R g as V c ~ R g D . An overall mean value of about 1 nm could be estimated as an upper limit for the radius of the primary silica particle building up the structure of all the wet gels, independent of the PEG addition.  相似文献   

2.
Critical fluctuations were studied in polymer mixtures of poly(dimethylsiloxane) and poly(ethylmethylsiloxane), which exhibit an upper critical temperature at Tc ⋍ 57 °C. The measurements were performed in a broad temperature range at three compositions in the miscible region close to the coexistence line. The temperature dependence of the static structure factor S(q=0) can be described by a mean field behaviour except for temperatures in the range of 6 K above Tc. There, a turnover to an Ising behaviour is observed according to a modified Landau-Ginzburg criterion. The mean field spinodal temperature Ts was determined by extrapolation of S(0)−1 to zero. From the Ornstein-Zernike representation of the angular dependence of S(q), the correlation length ζ of the concentration fluctuations can be determined and leads to a critical amplitude ζ0 = lim ζ(T→∞) = 20.5 Å. The interdiffusion dynamics described by the mutual diffusion constant D has been measured by quasielastic light scattering. It shows for the critical composition ϕc a critical slowing down as T approaches the critical temperature Tc. Furthermore, the q2 scaling of the relaxation rate of the interdiffusion dynamics changes to q3 behaviour close to Tc according to the mode coupling theory by Kawasaki.  相似文献   

3.
The small-angle X-ray scattering (SAXS) in micellar sodium dodecyl sulfate solutions has been studied in the range of overall concentrations c from 8 mM (CMC1) to 300 mM and the absolute values of scattering vector q from 0.07 to 3.0 nm–1. The total intensity of isotropic scattering has been revealed to increase with solution concentration. At c > 27 mM, the SAXS spectra have been found to exhibit an interference peak, which testifies a correlation in the arrangement of micelles in the bulk solution. This peak corresponds to the magnitude of q close to 1.55 nm–1. Using the position of this maximum, average distance r0 between the centers of micelles has been determined, which is equal to 4.1 nm and remains almost unchanged upon an increase in the overall concentration of sodium dodecyl sulfate. The observed regularities have been explained in terms of the DLVO theory taking into account the electrostatic and molecular intermicellar interaction.  相似文献   

4.
Summary: The effect of monovalent/divalent cation exchange on the structure and osmotic properties of chemically cross-linked polyacrylate and DNA gels swollen in near physiological salt solutions has been investigated. Both systems exhibit a reversible volume phase transition in the presence of calcium ions. The small-angle neutron scattering spectra of these gels display qualitatively similar features. At low values of q surface scattering is observed, while in the intermediate q range the signal is characteristic of scattering from rod-like elements. At high values of q the scattering intensity is governed by the local (short-range) geometry of the polymer chains. The competition between monovalent and divalent cations has been studied by anomalous small-angle X-ray scattering (ASAXS). The ASAXS results reveal that the local concentration of the divalent counter-ions in the vicinity of the polymer chains significantly exceeds that of the monovalent counter-ions.  相似文献   

5.
Poly(vinyl chloride) (PVC)/bis(2‐ethylhexyl)phthalate (DOP) gels were prepared at room temperature from tetrahydrofuran solutions of PVC and DOP. PVC/DOP gels of different molecular weights at various PVC concentrations (c) were investigated with small‐angle X‐ray scattering (SAXS). The mean distance between two neighboring inhomogeneities (D) and two characteristic lengths, the intrainhomogeneity distance (d1) and interinhomogeneity distance (d2), were evaluated from Bragg's law and the distance distribution function, respectively. Both D and d2 can be expressed by a power‐law relation (e.g., D and d2c?0.5). After a period of rapid cooling to 25 °C from the sol state, the structural evolution was examined with time‐resolved SAXS measurements. An Avrami analysis with the SAXS invariant data revealed that the growth kinetics of PVC/DOP gels was one‐dimensional growth from predetermined nuclei, regardless of c. These results suggest that the PVC/DOP gels are constructed from a fibrillar structure that forms gel structures at high concentrations or low temperatures. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2340–2350, 2001  相似文献   

6.
We investigate the morphology, segmental dynamics, and conductivity of 1‐ethyl‐3‐methylimidazolium trifluoromethanesulfonate (EMI‐Tf) swollen short side chain perfluorosulfonate ionomer (Aquivion) over a broad uptake range using small angle X‐ray scattering (SAXS), dielectric relaxation spectroscopy, and transient current measurement. The SAXS data indicate that the absorbed EMI‐Tf is mainly bounded in the ionic region of Aquivion. At low uptakes, EMI‐Tf acts as an effective plasticizer lowering the cluster Tg and markedly shifting the segmental relaxation to a high frequency; however, at high uptakes, the additional EMI‐Tf acts like a filler instead. A time–domain model was employed to quantify the conductivity of these membranes containing two mobile ion species, that is, cations and anions. The conductivity of both neat EMI‐Tf and EMI‐Tf swollen membranes exhibits Vogel‐Fulcher‐Tamman relation, revealing different activation parameters for ionic conduction. Furthermore, membranes containing different EMI‐Tf uptakes have similar conductivity over the reduced Tg/T axis and also follow Debye‐Stokes‐Einstein relation. Therefore, despite the abrupt change in conductivity near the critical uptake (29 wt %), both cluster Tg and segmental motion remain the key factors for the ionic conduction in these EMI‐Tf swollen membranes. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1273–1280  相似文献   

7.
Quasielastic light scattering measurements are reported for experiments performed on mixtures of gelatin and glutaraldehyde (GA) in the aqueous phase, where the gelatin concentration was fixed at 5 (w/v) and the GA concentration was varied from 1×10−5 to 1×10−3 (w/v). The dynamic structure factor, S(q,t), was deduced from the measured intensity autocorrelation function, g 2(τ), with appropriate allowance for heterodyning detection in the gel phase. The S(q,t) data could be fitted to S(q,t)=Aexp(−D f q 2 t)+Bexp(−tc)β, both in the sol (50 and 60 C) and gel states (25 and 40 C). The fast-mode diffusion coefficient, D f showed almost negligible dependence on the concentration of the crosslinker GA; however, the resultant mesh size, ξ, of the crosslinked network exhibited strong temperature dependence, ξ∼(0.5−χ)1/5exp(−A/RT) implying shrinkage of the network as the gel phase was approached. The slow-mode relaxation was characterized by the stretched exponential factor exp(−tc)β. β was found to be independent of GA concentration but strongly dependent on the temperature as β=β01 T2 T 2. The slow-mode relaxation time, τc, exhibited a maximum GA concentration dependence in the gel phase and at a given temperature we found τc(c)=τ01 c2 c 2. Our results agree with the predictions of the Zimm model in the gel case but differ significantly for the sol state. Received: 25 May 1999 /Accepted in revised form: 27 July 1999  相似文献   

8.
 The analysis of the interaction of micelles formed by a blockcopolymer is given by means of small-angle X-ray (SAXS) and small-angle neutron scattering (SANS). The blockcopolymer consists of poly(styrene) and poly(ethylene oxide) (molecular weight of each block: 1000 g/mol) and forms well-defined micelles (weight-association number: 400, weight-average diameter: 15.4 nm) in water. The internal structure has been studied previously (Macromolecules 29:4006 (1996)) by SAXS. There it has been shown that the micelles are spherical objects. The structure factor S(q) as a function of the scattering vector q (q=(4π/λ) sin (θ/2); λ: wavelength of the radiation in the medium; θ: scattering angle) can be extracted from both sets of small-angle scattering data (SANS: q≤0.4 nm-1; SAXS: q≤0.6 nm-1). It is shown that particle interaction in the present system can be described by assuming soft interaction which is modeled by a square-step potential. Received: 12 May 1997 Accepted: 9 July 1997  相似文献   

9.
Crystal and phase morphologies and structures determined by self-organization of crystalline-amorphous diblockcopolymers, crystallization of the crystallizable blocks, and vitrification of the amorphous blocks are reviewed through asystematic study on a series of poly(ethylene oxide)-b-polystyrene (PEO-b-PS) diblock copolymers. On the base ofcompetitions among these three processes, molecular and supramolecular ordering in confined environments can beinvestigated. In a concentration-fluctuation-induced disordered (D_(CF)) diblock copolymer, the competition between crystalli-zation of the PEO blocks and vitrification of the PS blocks is momtored by time-resolved simultaneous small angle X-rayscattering (SAXS) and wide angle X-ray diffraction (WAXD) techniques. In the case of T_c相似文献   

10.
This paper gives two examples of experiments that demonstrate the power of small angle scattering techniques in the study of swollen polymer networks. First, it is shown how the partly ergodic character of these systems is directly detected by neutron spin echo experiments. The observed total field correlation function of the intensity scattered from a neutral gel allows the ergodic contribution to be directly distinguished from the non ergodic part, at values of transfer wave vector q that lie well beyond the range of dynamic light scattering. The results can be compared with those obtained at much lower q from visible light scattering. Second, a recent application of small angle X-ray (SAXS) and neutron (SANS) scattering is described for a polyelectrolyte molecule, DNA, in semi-dilute solutions under near-physiological conditions. For these observations, the divalent ion normally present, calcium, is replaced by an equivalent ion, strontium. The comparison between SANS and SAXS yields a quantitative picture of the cloud of divalent counter-ions around the central DNA core. At physiological conditions, the cloud is thinner than that predicted on the basis of the Debye screening length but thicker than if the counter-ions were condensed on the DNA chain.  相似文献   

11.
The molecular motion of crosslinked polyepichlorohydrin (PECH) is studied qualitatively by NMR techniques. The results of temperature dependence of 1H T2 and T1 indicate that the crosslinking (crosslink density < 3%) restricts molecular motions of the polymer even far above its Tg. The 1H T1 minimum, corresponding to the large-scale chain-motion of crosslinked PECH, shifts to higher temperatures with increasing crosslink density. 1H T2 data also show that the crosslinking hinders free chain motions of the polymer above its Tg. The 13C T1 relaxation time is sensitive to such motional changes as well. 13C linewidths of crosslinked PECHs vary with the crosslink density in both the swollen state and the solid state. The mechanism of 13C linewidth broadening of crosslinked polymers is discussed in detail. In the case of PECH, the linewidth broadening is caused by changing molecular environment due to crosslinking (such as presence of various chemical shift structures and freezing effects in conformational environment as chain mobility decreases), rather than increasing correlation times, which shorten the relaxation time (T2) of polymer chains. © 1994 John Wiley & Sons, Inc.  相似文献   

12.
The internal dynamics of semiflexible fibrin gels has been investigated by means of dynamic light scattering (DLS). Fibrin gels, grown at room temperature from fibrinogen solutions at different NaCl concentrations, exhibit rather different structural features. High salt concentrations produce “fine” gels characterized by thin fibers and small mesh sizes, while low salt concentrations give rise to “coarse” gels with thick fibers and much larger mesh sizes. The observed dynamics of these two kinds of gels are quite different as well. “Fine” gels behave as typical semiflexible polymer gels, in which the dynamic structure factor f(q,t) shows an initial monoexponential decay followed by a stretched exponential decay, f(q,t) ∼ exp ‐(Γqt)β. Conversely, “coarse” gels exhibit a highly arrested dynamics, in which the dynamic structure factor does not relax to zero, but decays to a plateau whose value depends on the scattering wavevector q. Moreover, only the stretched exponential decay is observed at the fastest decay times, with the exponent β = 0.63±0.07 being independently of q. This behaviour can be interpreted as given by the contributions of the internal elastic modes of many different length scales.  相似文献   

13.
A functionalized cyclam was synthesized by the attachment of a polymerizable acryloyl group to one of the four nitrogens on the cyclam molecule. The polymerization of the functionalized cyclam was performed with N‐isopropylacrylamide and N,N′‐methylene bisacrylamide, and the gels obtained were studied in the presence of different transition‐metal‐ion solutions. There was a drastic difference in the phase‐transition temperature (Tc) of the poly(N‐isopropylacrylamide) (PNIPAAm)/cyclam gel in comparison with the pure PNIPAAm gel. For the described system, a Tc shift of 15 °C was obtained. The presence of functionalized cyclam increased the hydrophilicity and Tc of the aforementioned polymer gels in deionized water (at pH 6) because of the presence of protonated amino moieties. The PNIPAAm/cyclam gels showed a dependence of the swelling behavior on pH. Tc of the pure PNIPAAm gel was weakly influenced by the presence of any transition‐metal ions, such as Cu2+, Ni2+, Zn2+, and Mn2+. The addition of Cu2+ or Ni2+ to the PNIPAAm/cyclam gel reduced Tc of the polymer gel, and a shift of approximately 12 °C was observed. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 1594–1602, 2003  相似文献   

14.
The internal and overall motions of 1,3- and 1,4-diethylpyridinium bromides have been studied by 13C relaxation. The enthalpies of activation for the rotation of the ethyl groups in positions 1 and 3 are deduced from the temperature dependence of the T1 values of the methylene and methine carbons. The 14N quadrupolar relaxation time, Tq, together with 13C relaxation data provide an estimate of the 14N quadrupolar coupling constants.  相似文献   

15.
Aerogels are well suited as transparent insulation materials in solar architecture and collector systems. Their nanoporous structure provides a high solar transmittance and a low thermal conductivity, generally below 0.02 W m–1 K–1. Transparent aerogels with densities above 80 kgm–3 can easily be prepared at room temperature via a one-step sol-gel process with subsequent supercritical drying. Separating hydrolysis and condensation via a two-step method allows the preparation of transparent ultra-low density SiO2-aerogels. To optimize the optical properties, characterized by the scattering coefficient of the gels, we have investigated the influence of preparation parameters, such as pH-value of the sol-gel starting solution and macroscopic density, on the gel structure. To determine the nanostructure we performed spectral light scattering as well as small angle X-ray scattering (SAXS) measurements.  相似文献   

16.
Dynamic mechanical properties of styrene‐based ionomers containing varying amounts of either 15‐crown‐5 ether (CE) or pentaethylene glycol (PG) are compared with those of ionomers of varying degree of neutralization (ND). The cluster Tg (Tg,c) and ionic modulus of the ionomers decrease with increasing amount of CE or PG or decreasing ND. Thus, we propose that the CE binds Na+ strongly to form a large‐sized complex. Thus, the electrostatic interactions between charges decrease, leading to lower Tg,c. For the PG‐containing ionomers, the PG acts as polar plasticizer, further lowing the Tg,c. In the case of the underneutralized ionomers, the Tg,c is reduced by the existence of both relatively weak hydrogen bonds between carboxylic acid groups and relatively strong ionic bonds between ion pairs in the multiplets. The small‐angle X‐ray scattering results are also supportive of the above interpretations. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1358–1367  相似文献   

17.
Frozen aqueous chlorides (≤50 mM ) are characterized by using confocal fluorescence microscopy and small angel X‐ray scattering (SAXS). The former method allows us to determine the size of a liquid inclusion formed in the ice matrix at temperatures above the eutectic point of the system (teu). Isolated liquid inclusions of a uniform size are formed when the temperature of a frozen electrolyte increases past teu. The size of the liquid inclusions depends on the observation temperature as well as on the concentration (csalt) and type of salt dissolved in the original unfrozen solution. However, the number density of liquid inclusions is almost constant and independent of these experimental parameters, particularly when an electrolyte is frozen in liquid nitrogen. Salt accumulation can then occur at the imperfections of the ice crystals. The occurrence probability of the imperfections is independent of the nature of an incorporated salt. The amount of a salt confined in each inclusion ranges from 7 to 240 fmol, depending on csalt. SAXS measurements provide information on the size of individual salt crystals formed at temperatures below teu. The radius of gyration of a salt crystal ranges from 2 to 2.8 nm, and does not depend significantly on csalt. Thus, each inclusion is formed from 106–109 nanocrystals, which can act as seeds. When doped ice is prepared at higher temperatures, for example ?16 °C, the isolation of liquid inclusions is not sufficient and coalescence occurs more easily upon an increase in temperature or csalt. However, when csalt is lower than 10 mM , the number density of liquid inclusions is almost constant, irrespective of the freezing temperature.  相似文献   

18.
The Gibbs free enthalpy, chemical activity across the transition temperature to superconductivity, Tc in YBa2Cu3O7 − δ is obtained from reciprocally enhanced X-Ray absorbance, XAS and diffraction, XRD data near the Ba L3,2 edges' energy Ea, and orientations in the X-ray beam for preferred Miller indexed [HKL] planes' scattering, that are enhanced near Tc. The standard enthalpy and entropy for the formation of mixed normal metal/superconducting domains above Tc, determined individually across the two Ba L3,2 edges, to better than a percent accuracy: ΔH = − 220 meV, and ΔS = − 2 meV/K when 121 ≥ T ≥ 92 K ≈ Tc1, indicate that energy is gained, with an increased order by forming a mixture with reduced entropy. Below Tc the standard enthalpy and entropy to form at least two mixed superconducting phases are halved to ΔH = − 86 meV, ΔS = − 1 meV/K when 92 K ≥ T ≥ Tc2 ≈ 72 K. Thus reciprocal XAS/XRD enhancement at orientations in the X-ray field, of preferred 2D planes, induced by the transition to superconductivity in the layer solid, indicates one, the importance of the 2D-plane electron density scattering near Tc, and two, identifies the 2D-plane chemical activity, by the electron density leading to symmetry allowed excitations, a role similar to that of the electron density in linear bonds for molecular activity.  相似文献   

19.
The results of an experimental study of the kinetics of structural relaxation of amorphous poly(ethylene terephthalate) are reported. Samples were prepared by ultraquenching the melt on rotating stainless-steel discs. Two types of measurements by differential scanning calorimetry were made: (1) the dependence of the “fictive” (or “structural”) temperature Tf(q?) introduced by Tool, on the cooling rate q? and (2) the dependence of the glass transition temperature Tg on the heating rate q+. In this way the value x = 0.47 was obtained for the dimensionless parameter proposed by Narayanaswamy.  相似文献   

20.
The scaling of the polyelectrolyte scattering peak in chitosan solutions, as deduced from the relation qmaxcpα was studied by synchrotron SAXS as a function of the charge density of the polymer. We observe a variation in the α exponent corresponding to the limit of the ionic condensation, by varying the degree of acetylation of the polymer. The nature of the solution medium also affects the polyelectrolyte peak, and it is shown that in alcoholic/water mixtures, the lower dissociation of the acid induces a lower charge density, thus influencing the polyelectrolyte ordering.  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