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131.
Anne Kari Nyhus Steinar Hagen Arvid Berge 《Journal of polymer science. Part A, Polymer chemistry》1999,37(16):3345-3359
The kinetics of the polymerization of pure meta-divinylbenzene (DVB) and pure para-divinylbenzene at 70°C have been studied in the presence of toluene and 2-ethylhexanoic acid. The apparent rate constant ratios (kp/kt)1/2 for these systems have been calculated. meta-Divinylbenzene polymerizes at a higher rate than the para-isomer in both toluene and 2-EHA, and the polymerization rates of meta-DVB and para-DVB before the gel point were both higher in the presence of 2-EHA than in toluene. The monomer conversion at the visual gel point is higher for para-DVB than for meta-DVB. The gel point has also been determined indirectly by size exclusion chromatography, and these results are consistent with the gel times observed visually. The conversion of pendant vinyl groups during the polymerization has been determined by bromination. It is found that the homopolymers of poly(para-DVB) have a substantially higher content of pendant vinyl groups than poly(meta-DVB) both during and at the end of the polymerization. The molecular weight distribution (MWD) prior to gelation has been determined by size exclusion chromatography (SEC). Weight average (M̄w); and number average (M̄n) molecular weight prior to gelation and of the sol fractions after gelation have also been measured by SEC. There are larger fractions of high molecular weight polymers prior to gelation, when the polymerization was run in the presence of toluene, than in 2-EHA, mainly due to the differences in solvating power of the two diluents. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 3345–3359, 1999 相似文献
132.
Monodisperse porous particles of poly(divinylbenzene) prepared by the activated swelling method have been investigated by solid‐state 13C crosspolarization magic‐angle spinning (CPMAS) nuclear magnetic resonance (NMR) relaxation measurements. Homopolymeric combinations of two porogens (toluene and 2‐ethylhexanoic acid) and two monomers (meta‐ and para‐divinylbenzene) were studied. Residual vinyl groups were systematically reacted with increasing amounts of bromine, producing 20 different polymers samples for which we measured crosspolarization times, TCH, proton rotating frame spin‐lattice relaxation, T, 13C spin‐lattice relaxation, T, and proton spin‐lattice relaxation, T. These parameters were chosen to reflect expected changes in a wide range of frequencies of motion as a function of structure. Relative differences in the molecular mobility of the major functional groups (aromatic, vinyl and aliphatic) is related to initial reactants used, vinyl concentration, relative reactivity of vinyl groups, distribution of vinyl groups, pore structure, and degree of crosslinking. Variable temperature 1H combined rotation and multiple pulse NMR (CRAMPS) was used to derive activation energies for selected samples via measurement of the proton spin‐lattice relaxation time, T. Irreversible thermal effects were observed in ambient temperature relaxation after heating to temperatures in the range of 393–418 K. Simple univariate statistical analyses failed to reveal consistent correlations among the known variables. However, the application of more sophisticated multivariate and neural network analyses allowed excellent structure–property predictions to be made from the relaxation time data. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1307–1328, 1999 相似文献
133.
134.
Aromatic hydrocarbons can be selectively recognized by four endo-functionalized molecular tubes through C/N-H...π interactions in nonpolar media with binding constants up to 1580 L/mol. 相似文献
135.
136.
Sampling powder flows in industrial grinding circuits are discussed. Work was done with different types of sampler and with different minerals and sampling objectives. It was found that all the samplers showed marked autocorrelation in sample mass for all the minerals ground. Typical autocorrelation could be found at both high (1 · 10?3–15 · 10?3 Hz) and low (1 · 10?4 – 4 · 10?4 Hz) frequencies. Higher frequencies were attributed to powder transport and low frequencies to mill circuit variations. The effects of the found autocorrelation on systematic errors and on industrial sampling schemes are discussed. Systematic sampling produced large errors if operated at sampling intervals that were high-frequency autocorrelation frequencies or small multiples of these. Low-frequency autocorrelation was less detrimental to the sampling results. 相似文献
137.
Marko Mattila Kari Saloheimo Kari Koskinen 《Particle & Particle Systems Characterization》2007,24(3):173-183
The robustness of online particle size analysis in wet processes is improved by applying data based modeling methods to the control of the sample preparation and measurement sequence of the particle size analyzer. The aim is to find a more accurate and reliable method of determining the end of the particle size integration period using multivariate statistical process control (MSPC). The studied approach is tested on analyzers installed at two mineral processing plant sites and validated using two validation tests. Research shows that the proposed method works with two very different slurry types. The main advantage of the adapted approach is that there are no adjustable parameters that have to be set by the user. 相似文献
138.
We consider a two-field model for inflation where the second order metric perturbations can be amplified by a parametric resonance during preheating. We demonstrate that there can arise a considerable enhancement of non-Gaussianity sourced by the local terms generated through the coupled perturbations. We argue that the non-Gaussianity parameter could be as large as f(NL) approximately 50. Our results may provide a useful test of preheating in future cosmic microwave background experiments. 相似文献
139.
Rissanen K 《Angewandte Chemie (International ed. in English)》2005,44(24):3652-3654