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131.
We have investigated the effect of crosslink density on shear and elongational flow properties of alkali-swellable acrylic
thickener solutions using a mixing series of the two commercial thickeners Sterocoll FD and Sterocoll D as model system. Linear
viscoelastic moduli show a smooth transition from weakly elastic to gel-like behavior. Steady shear data are very well described
by a single mode Giesekus model at all mixing ratios. Extensional flow behavior has been characterized using the CaBER technique.
Corresponding decay of filament diameter is also well fitted by the Giesekus model, except for the highest crosslink densities,
when filament deformation is highly non-uniform, but the non-linearity parameter α, which is independent of the mixing ratio, is two orders of magnitude higher in shear compared to elongational flow. Shear
relaxation times increase by orders of magnitude, but the characteristic elongational relaxation time decreases weakly, as
gel content increases. Accordingly, variation of gel content is a valuable tool to adjust the low shear viscosity in a wide
range while keeping extensional flow resistance essentially constant. 相似文献
132.
Saeid Kheirandish Ilshat Guybaidullin Wendel Wohlleben Norbert Willenbacher 《Rheologica Acta》2008,47(9):999-1013
We investigate the effect of hydrophobic aggregation in alkali-swellable acrylic thickener solutions on shear and extensional
flow properties at technically relevant polymer concentrations using the commercial thickener Sterocoll FD as model system.
Apparent molecular weight of aggregates in water is M
w
≈ 108 g/mol and decreases by more than an order of magnitude in ethanol. Zero shear viscosity η
0 is low and shear thinning is weak compared to the high molecular weight of the aggregates. Linear viscoelastic relaxation
is described by the Zimm theory up to frequencies of 104 rad/s, demonstrating that no entanglements are present in these solutions. This is further supported by the concentration
dependence of η
0 and is attributed to strong association within the aggregates. Extensional flow behavior is characterized using the capillary
break-up extensional rheometry technique including high-speed imaging. Solutions with ϕ ≥ 1% undergo uniform deformation and show pronounced strain hardening up to large Hencky strains. Elongational relaxation
times are more than one order of magnitude lower than the longest shear relaxation times, suggesting that aggregates cannot
withstand strong flows and do not contribute to the elongational viscosity.
相似文献
Norbert WillenbacherEmail: |
133.
This work investigated the repeatability of column preparation for a reversed-phase C18 monolith, namely stearyl methacrylate-co-ethylene glycol dimethacrylate (SMA-EDMA). The columns were thermally polymerised using three commonly available heating devices (GC oven, hot air oven and water bath) and their chromatographic performance evaluated using micro-liquid chromatography for separation of five test compounds. Precision in terms of %RSD of retention times were 9.0, 6.5, and 12.5 using GC oven, hot air oven and water bath, respectively. Between-batch precision for the hot air oven (n = 3 days) was less than 10.4% for retention time. The SMA-EDMA monolith was applied to the separation of tocopherol homologues by capillary electrochromatography. Usually tocopherol homologues cannot be completely separated by conventional reversed-phase C8- or C18-packed bed or C18-silica based monolithic columns. Polymer monolith has been shown to give remarkable selectivity towards the tocopherols compared to the conventional microparticulate phase and silica based monolith. Successful separation of the tocopherol isomers was achieved on the SMA-EDMA monolith without any column modification. 相似文献
134.
This article shows that there is great interest in using an electrochromatographic microchip made of hexyl acrylate (HA) based porous monolith cast within the channel of a cyclic olefin copolymer (COC) device. The monolith is simultaneously in situ synthesized and anchored to the inner walls of the channel in less than 10 min. By appropriate choice of light intensity used during the synthesis, the separation efficiency obtained for nonpolar solutes such as polycyclic aromatic hydrocarbons (PAH) is increased up to 250 000 plates/m. The performance of this HA‐filled COC microchip was investigated for a wide range of analytes of varying nature. The reversed‐phase separation of four aflatoxins is obtained in less than 2 min. The baseline separation of a mixture of neurotransmitters including six amino acids and two catecholamines is possible thanks to the superimposition of the differences in electrophoretic mobility on the chromatographic process. The durability of the system at pH 13 allows the separation of five biogenic amines and the quantitative determination of two of them in numerous wine samples. The feasibility of on‐line preconcentration is also demonstrated. Hydrophilic surface modification of COC channel via UV‐photografting with poly(ethylene glycol) methacrylate (PEGMA) before in situ synthesis of HA, is necessary to reduce the adsorption of very hydrophobic solutes such as PAH during enrichment. The detection limit of fluoranthene is decreased down to less than 1 ppb with a preconcentration of 4.5 h on the HA‐filled PEGMA functionalized COC microchip. 相似文献