共查询到20条相似文献,搜索用时 15 毫秒
1.
The self-diffusion coefficients of polystyrene dissolved in C6D6 were measured by means of pulsed field gradient n.m.r.. The dependence on the molecular weight was of the power of -2 in accordance with the reptation model. The concentration dependence was theoretically calculated within the blob concept which shows that in the crossover region the self-diffusion coefficient is determined only by the blob dimensions and the monomeric friction coefficient is constant. The calculated concentration dependence agrees well with that observed by the authors. 相似文献
2.
Transport in ternary polymer1, polymer2, solvent systems has been investigated using an n.m.r. spin-echo technique. The dependence of the self-diffusion coefficient of poly(ethylene oxide) polymers on the concentration and molecular size of dextran in aqueous solution has been measured. Monodisperse poly(ethylene oxide) fractions (, 2.8·105 and 1.2·106) and dextrans (, 1·105 and 5·105) have been employed over a range of concentration up to the miscibility limit in each system. It is found that when the molecular size of the diffusant is commensurate with or exceeds that of the matrix polymer, a relationship of the form: is applicable, where C[η] refers to the dextran component and is considered to describe the extent of coil overlap in concentrated solution. () is independent of the molecular size of the poly(ethylene oxide), at least in the range studied (Mw<300 000). 相似文献
3.
The motion of perdeuterated pyridine imbibed in Argonne premium high-volatile coal (no. 601) has been studied by wide-line 2H n.m.r. spectroscopy. The coal sample had been saturated with perdeuterated pyridine for several days, then evacuated until the pyridine reached a concentration of 3.5%. The entrapped pyridine molecules undergo two tyes of rotation about the twofold symmetry axis: rapid diffusional rotation with a correlation time of 1 × 10−7 s; and 180 ° jumps with a residence time of 6 × 10−6 s. It is estimated that roughly half of the pyridine molecules execute each kind of motion. There is no evidence for a pyridine fraction that executes rapid isotropic rotation. The anisotropic nature of the rotation indicates that the interaction between pyridine and the coal structure is strong enough to prevent isotropic rotation. 相似文献
4.
The 1H and 13C nuclear magnetic resonance (n.m.r.) spectra of a delayed coking feedstock and products, presented in a previous paper, are analysed here in detail by matching results from inspection of both nuclei. The conventional spectra of whole samples are used in combination with elemental analysis data, and substantial agreement is obtained. Differences are explored to yield valuable information. Interpretation of some band assignments is modified to account for new results. This approach results in a detailed quantitative estimation of a few key structures that contain the main functional groups that characterize these petroleum fractions. 相似文献
5.
R. Kimmich 《Polymer》1984,25(2):187-194
As shown in previous papers, chain fluctuations in polymer melts can be analysed into three consecutive components (local segment re-orientation, reptation within the tight tube and tube renewal). The intensity function derived on this basis leads to a series of limiting cases, which can be realized by frequency and/or molecular weight variation. The occurrence of the characteristic molecular weights MAB, MBC and MC separating these limits have been verified by the aid of nuclear magnetic relaxation of linear polyethylene and atactic polystyrene melts. The MC-values are equivalent to the critical molecular weights as known from rheology. An expression for the zero-shear viscosity in the whole range of molecular weights has been derived. 相似文献
6.
Changes in chemical reactions during coal liquefaction of Yallourn brown coal and Akabira bituminous coal were monitored by using high temperature and high pressure 1H n.m.r. A simple high temperature and high pressure e.s.r. cell was constructed and the variation in concentration of free radicals during coal liquefaction reactions was monitored under the same experimental conditions as for n.m.r. With the liquefaction and coking reaction, typical spectra of n.m.r. and e.s.r. were obtained and could distinguish the reaction steps: swelling; liquefaction; coking. The combination of high temperature and high pressure n.m.r. and e.s.r. is promising to the chemistry of coal liquefaction reactions. 相似文献
7.
Temperature and frequency-dependent proton relaxation measurements, in conjunction with spin concentrations determined by e.s.r., have shown that for fresh and oxidized coal the relaxation mechanism involves spin-diffusion to paramagnetic centres and to rotating methyl groups on alkyl chains. 1H relaxation in semicokes is more complex; probably both direct relaxation to paramagnetic centres as well as spin-diffusion contribute. 相似文献
8.
G. Fleischer 《Polymer》1985,26(11):1677-1682
The echo attenuation in the n.m.r. pulsed field gradient technique for polydisperse samples ( and 10) was calculated, which, for the smallest polydispersity, is already markedly nonexponential. The echo attenuations of solutions of a polydisperse polystyrene, mixtures of monodisperse polystyrenes and the melt of a commercial polyethylene were measured. In the solutions, depending on the concentration regime, the echo attenuations were considerably less nonexponential than calculated and often exponential within experimental error giving one averaged self-diffusion coefficient. An averaging of the self-diffusion process does exist which is not in accordance with the reptation concept. In the entangled melt the echo attenuation is the sum of the contributions of the different molar masses, where no averaging is present, but, due to the strong dependence of the nuclear relaxation times on molar mass, the smaller molar masses are more strongly weighted and a dependence of the echo attenuation on the diffusion time arises. 相似文献
9.
Ronald A. Greinke 《Fuel》1984,63(10):1374-1377
A new proton n.m.r. technique has been developed for the chemical characterization of polymerized petroleum pitches with pyridine-insoluble contents as high as 80%. The polymerized pitches were completely solubilized for conventional 1H n.m.r. evaluation in a mixed solvent system consisting of S2Cl2-SO2Cl2. Although solubilization occurs via the addition of chlorine to the polynuclear aromatic rings, the n.m.r. spectra can still be quantitatively interpreted. Several applications are discussed. 相似文献
10.
Colin E. Snape Alan M. Kenwright Jenaro Bermejo Julio Fernandez Sabino R. Moinelo 《Fuel》1989,68(12):1605-1608
Rapid magic angle spinning has been used to obtain sideband-free high field 13C n.m.r. spectra of coal tar pitches in the solid state. However, the unfavourable cross-polarization and 1H relaxation characteristics make it difficult to accurately estimate concentrations of quaternary aromatic carbon using the dipolar dephasing method. The concentrations of bridgehead aromatic carbon in the whole pitch and toluene-insoluble fraction investigated were deduced using a combination of solid and solution state n.m.r. and elemental analyses. 相似文献
11.
A new method for analysing complex mixtures, 1H 2D n.m.r., was used to determine polyaromatics in crude gas oil mixtures. 2D NMR overcomes the lack of resolution due to crowded 1D spectra and provides structural information. In particular, TOCSY (total correlation spectroscopy) 2D n.m.r. is well suited to polyaromatics because these molecules give specific 2D fingerprints which can be easily recognized. These patterns were selected and analysed in two ways: (1) specific fingerprint recognition using a pure compound library; (2) using the mixing time τm of the TOCSY sequence. Variation of τm gives a change in cross-peak intensities. The intensity variation curves are characteristic of spin systems and hence of the structures of compounds. Alkylated substitutions were also studied. The compounds were quantified. This strategy was used to analyse crude gas oils and measure the contents of separated alkylated naphthalene isomers, biphenyls, anthracene, phenanthrene and benzothiophene. 相似文献
12.
Comments on the use and relative merits of gated spin echo (GASPE) and spin echo broad band off-resonance decoupling (SEBBORD) selected multiplet 13C n.m.r. methods, for the identification and quantitation of CHn (n = Q?3) groups in fossil fuel materials, are given in response to recently published reports. 相似文献
13.
Aromatic C and CH carbon sites in a variety of petroleum and coal derived materials have been investigated using a 13C n.m.r. technique termed spin echo broad band off-resonance decoupling (SEBBORD). Only resonances due to non-protonated aromatic carbon sites are observed in SEBBORD spectra such that comparison with conventional 13C n.m.r. spectra enables differentiation between aromatic C and CH group resonances. Relative abundances of non-protonated aromatic carbon sites calculated from SEBBORD data are in good agreement with values derived from a combination of conventional1H n.m.r., 13C n.m.r. and elemental analysis data. The occurrence of significant proportions of aromatic C intensity to high field of 129–130 ppm and of aromatic CH intensity to low field of 129–130 ppm has been found to be quite common. Consequently attempts to determine aromatic C and CH group abundances by partitioning conventional 13C n.m.r. spectra in the vicinity of 129–130 ppm can lead to considerable quantitative errors. SEBBORD provides more detailed information about aromatic carbon sites than can be obtained from conventional 13C n.m.r. spectra. 相似文献
14.
The quantitative aspects of cross-polarization (CP), which is used in conjunction with dipolar decoupling and magic-angle rotation to obtain high resolution 13C n.m.r. spectra of coals, have been studied using a bituminous coal (82 wt% C, dmmf basis) and asphaltenes from an extract of the same coal. The condition for obtaining reliable quantitative data, that rotating frame 1H relaxation times (T1p′ these govern the extent of CP) are much longer than the time required to polarize the carbons present (≈1 ms), was met for the asphaltenes. In contrast, about half the protons in the coal have T1p5 of ≈ ? 1 ms, these times being too short to allow CP of all the carbons. Although the aromaticities obtained for this coal were fairly constant (≈0.75) using (CP) contact times > 0.5 ms, the total peak intensity decreased markedly as the contact time was increased and was much less than that for the asphaltenes. These results indicate that not all the carbons in bituminous coals are observed by CP and, as a consequence, aromaticities reported in the literature for some bituminous coals appear to be low. 相似文献
15.
The use of 1H and 13C nuclear magnetic resonance (n.m.r.) spectroscopy to evaluate the performance of a delayed coking process is presented in a series of papers, of which this is the first. A meaningful analysis requires an accurate balance, which in turn requires analysis of the various types of atoms in the feed and in all products, including solid coke, as well as the amounts of each. Product yields were determined in a delayed coking pilot plant. Complete n.m.r. (1H and 13C) characterization of feed and all delayed coking products, using a consistent classification, was combined with pilot plant data to obtain the total distribution of carbon and proton atoms in the delayed coking process. Changes in carbon and hydrogen distributions (products versus feedstock) reveal the nature of the thermal cracking reactions involved; the main change is a substantial increase in the number of aromatic and paraffinic carbons at the expense of naphthenic carbons. 相似文献
16.
Factors affecting the spin-lattice relaxation times T1 of 13C resonances at 62.9 MHz were investigated for simple mixtures of tetralin, ethylbenzene, pentane and phenol and the prominent resonances of an anthracene oil and a coal-derived hydrogenation oil. Parameters varied were solvent temperature and nature—Cr(acac)3, Cr(dpm)3, Cr(hfac)3, Fe(acac)3, Fe(dpm)3—and concentration of a paramagnetic relaxation agent (PARR). Effects of PARRs on relaxation rates are strongly dependent on neighbouring acidic or basic centres, but are much more uniform for Cr(hfac)3 than for Cr(acac)3. The presence of basic or acidic compounds does not significantly affect the relaxation of 13C of the neutral components. Optimum experimental conditions are characterized by nearly uniform T1 values of all resonances to be detected. While 2D spectra allow disentanglement of overlapping lines, choosing optimum conditions for the neutral constituents may lead to a loss of signals from the acidic or basic compounds. 相似文献
17.
On the basis of correlation of data obtained by broad-line n.m.r. and magic angle spinning n.m.r. analysis as well as conformational analysis, conclusions have been drawn concerning the nature of molecular motion occurring in a polycarbonate chain in the vicinity of ?60°C. It is postulated that this motion is connected with conformational reorientation of bisphenol in unordered chains with a random conformation. 相似文献
18.
Michel Bouquet 《Fuel》1985,64(2):226-228
A low resolution pulsed n.m.r. spectrometer has been used to determine total hydrogen content for a wide range of petroleum cuts. Criteria for selecting experimental conditions are described with respect to three kinds of petroleum-derived products. A hydrogen pulsed n.m.r. instrument was suitable for a small-coil technique (constant volume) experiment when used with a sample of known density. Excellent agreement has been found with microanalysis technique results, and the accuracy is as good as the combustion method. Problems of magnetic contaminants have also been simulated. 相似文献
19.
David T. Alien Leonidas Petrakis David W. Grandy George R. gavalas Bruce C. Gates 《Fuel》1984,63(6):803-809
A new method of structural analysis was applied to a group of hydroliquefied coal samples. The method uses elemental analysis and n.m.r. data to estimate the concentrations of functional groups in the samples. The samples included oil and asphaltene fractions obtained in a series of hydroliquefaction experiments, and a set of nine fractions separated from a coal-derived oil. The structural characterization of these samples demonstrates that estimates of functional group concentrations can be used to provide detailed structural profiles of complex mixtures and to obtain limited information about reaction pathways. 相似文献
20.
Broad line n.m.r. measurements have been carried out over a wide range of temperatures on poly(methylmethacrylate) (PMMA) and several selectively deuterated forms of this polymer. The line shapes and second moments for the different samples were examined in the light of theoretical calculations based on possible structural models. A spectrum subtraction procedure was particularly useful in confirming the conclusions drawn from consideration of second moment data. A consistent set of conclusions was obtained for the motion of the methyl groups. 相似文献