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31.
L. H. Tung A. T. Hu S. V. McKinley A. M. Paul 《Journal of polymer science. Part A, Polymer chemistry》1981,19(8):2027-2039
Polystyrene with random long chain branches has been difficult to prepare. A new approach using small amounts of chain-transferring monomers to copolymerize with styrene free radically was examined in this work. Of the several comonomers examined, vinylbenzylthiol yielded polystyrene with branched structure. But because of the high chain trnsfer constant, the branches occurred mainly in the low-molecular-weight end of the distribution. As a side interest, vinylbenzylthiol was found to be an effective agent for the broadening of molecular weight distribution. 相似文献
32.
High shear rate viscometry 总被引:1,自引:0,他引:1
We investigate the use of two distinct and complementary approaches in measuring the viscometric properties of low viscosity complex fluids at high shear rates up to 80,000 s?1. Firstly, we adapt commercial controlled-stress and controlled-rate rheometers to access elevated shear rates by using parallel-plate fixtures with very small gap settings (down to 30 μm). The resulting apparent viscosities are gap dependent and systematically in error, but the data can be corrected—at least for Newtonian fluids—via a simple linear gap correction originally presented by Connelly and Greener, J. Rheol, 29(2):209–226, 1985). Secondly, we use a microfabricated rheometer-on-a-chip to measure the steady flow curve in rectangular microchannels. The Weissenberg–Rabinowitsch–Mooney analysis is used to convert measurements of the pressure-drop/flow-rate relationship into the true wall-shear rate and the corresponding rate-dependent viscosity. Microchannel measurements are presented for a range of Newtonian calibration oils, a weakly shear-thinning dilute solution of poly(ethylene oxide), a strongly shear-thinning concentrated solution of xanthan gum, and a wormlike micelle solution that exhibits shear banding at a critical stress. Excellent agreement between the two approaches is obtained for the Newtonian calibration oils, and the relative benefits of each technique are compared and contrasted by considering the physical processes and instrumental limitations that bound the operating spaces for each device. 相似文献
33.
Edward T. Kopesky Stephen G. Boyes Neil Treat Robert E. Cohen Gareth H. McKinley 《Rheologica Acta》2006,45(6):971-981
Two distinct oligomeric species of similar mass and chemical functionality (M
w≈2,000 g/mol), one a linear methyl methacrylate oligomer (radius of gyration R
g≈1.1 nm) and the other a hybrid organic–inorganic polyhedral silsesquioxane nanocage (methacryl-POSS, r≈1.0 nm), were subjected to thermal and rheological tests to compare the behaviors of these geometrically dissimilar molecules over the entire composition range. The glass transition temperatures of the blends varied monotonically between the glass transition temperatures of the pure oligomer (T
g=−47.3°C) and the pure POSS (T
g=−61.0°C). Blends containing high POSS contents (with volume fraction φ
POSS≥0.90) exhibited enhanced enthalpy relaxation in differential scanning calorimetry (DSC) measurements, and the degree of enthalpy relaxation was used to calculate the kinetic fragility indices m of the oligomeric MMA (m=59) and the POSS (m=74). The temperature dependences of the viscosities were fitted by the free-volume based Williams–Landel–Ferry (WLF) and Vogel–Fulcher–Tammann (VFT) framework and a dynamic scaling relation. The calculated values of the fragility from the WLF–VFT fits were similar for the POSS (m=82) and for the oligomer (m=76), and the dynamic scaling exponent was similar for the oligomeric MMA and the POSS. Within the range of known fragilities for glass-forming liquids, the temperature dependence of the viscosity was found to be similarly fragile for the two species. The difference in shape of the nanocages and oligomer chains is unimportant in controlling the glass-forming properties of the blends at low volume fractions (φ
POSS<0.20). However, at higher volume fractions, adjacent POSS cages begin to crowd each other, leading to an increase in the fractional free volume at the glass transition temperature and the observed enhanced enthalpy relaxation in DSC. 相似文献
34.
35.
Lapere KM LaMacchia RJ Quak LH Kettner M Dale SG McKinley AJ Wild DA 《The journal of physical chemistry. A》2012,116(14):3577-3584
Anion photoelectron spectra are reported for the iodide-carbon monoxide clusters, with supporting ab initio calculations for the 1:1 dimer anion and neutral complexes. A C(s) minimum geometry is predicted for the anion complex, while for the neutral complex two linear van der Waals minima are predicted differing in the attachment point of the iodine, that is, I···CO and I···OC. The predicted adiabatic photodetachment energy agrees well with the experimental spectrum. The photoelectron spectra feature a vibrational progression in the CO stretching mode, which becomes more pronounced for the larger clusters. 相似文献
36.
Timothy R. Corkish Christian T. Haakansson Peter D. Watson Prof. Allan J. McKinley Dr. Duncan A. Wild 《Chemphyschem》2021,22(1):69-75
A combined experimental and theoretical approach has been used to investigate X−⋅⋅⋅CH2O (X=F, Cl, Br, I) complexes in the gas phase. Photoelectron spectroscopy, in tandem with time-of-flight mass spectrometry, has been used to determine electron binding energies for the Cl−⋅⋅⋅CH2O, Br−⋅⋅⋅CH2O, and I−⋅⋅⋅CH2O species. Additionally, high-level CCSD(T) calculations found a C2v minimum for these three anion complexes, with predicted electron detachment energies in excellent agreement with the experimental photoelectron spectra. F−⋅⋅⋅CH2O was also studied theoretically, with a Cs hydrogen-bonded complex found to be the global minimum. Calculations extended to neutral X⋅⋅⋅CH2O complexes, with the results of potential interest to atmospheric CH2O chemistry. 相似文献
37.
We present detailed experimental results examining “negative wakes” behind spheres settling along the centerline of a tube
containing a viscoelastic aqueous polyacrylamide solution. Negative wakes are found for all Deborah numbers (2.43≤De(˙γ)≤8.75) and sphere-to-tube aspect ratios (0.060≤a/R≤0.396) examined. The wake structures are investigated using laser-Doppler velocimetry (LDV) to examine the centerline fluid
velocity around the sphere and digital particle image velocimetry (DPIV) for full-field velocity profiles. For a fixed aspect
ratio, the magnitude of the most negative velocity, U
min
, in the wake is seen to increase with increasing De. Additionally, as the Deborah number becomes larger, the location of this minimum velocity shifts farther downstream. When
normalized with the sphere radius and the steady state velocity of the sphere, the axial velocity profiles become self-similar
to the point of the minimum velocity. Beyond this point, the wake structure varies weakly with aspect ratio and De, and it extends more than 20 radii downstream. Inertial effects at high Reynolds numbers are observed to shift the entire
negative wake farther downstream. Using DPIV to investigate the transient kinematic response of the fluid to the initial acceleration
of the sphere from rest, it is seen that the wake develops from the nonlinear fluid response at large strains. Measurements
of the transient uniaxial extensional viscosity of this weakly strain-hardening fluid using a filament stretching rheometer
show that the existence of a negative wake is consistent with theoretical arguments based on the opposing roles of extensional
stresses and shearing stresses in the wake of the sphere.
Received: 10 November 1997 Accepted: 1 May 1998 相似文献
38.
When the shear stress measured in large amplitude oscillatory shear (LAOS) deformation is represented as a 2-D Lissajous–Bowditch
curve, the corresponding trajectory can appear to self-intersect and form secondary loops. This self-intersection is a general
consequence of a strongly nonlinear material response to the imposed oscillatory forcing and can be observed for various material
systems and constitutive models. We derive the mathematical criteria for the formation of secondary loops, quantify the location
of the apparent intersection, and furthermore suggest a qualitative physical understanding for the associated nonlinear material
behavior. We show that when secondary loops appear in the viscous projection of the stress response (the 2-D plot of stress
vs. strain rate), they are best interpreted by understanding the corresponding elastic response (the 2-D projection of stress
vs. strain). The analysis shows clearly that sufficiently strong elastic nonlinearity is required to observe secondary loops on the conjugate viscous projection. Such a strong elastic nonlinearity physically corresponds to a nonlinear viscoelastic shear stress overshoot
in which existing stress is unloaded more quickly than new deformation is accumulated. This general understanding of secondary
loops in LAOS flows can be applied to various molecular configurations and microstructures such as polymer solutions, polymer
melts, soft glassy materials, and other structured fluids. 相似文献
39.
Olds WJ McKinley AR Moore MR Kimlin MG 《Journal of photochemistry and photobiology. B, Biology》2008,93(2):88-93
Vitamin D deficiency is a major health concern worldwide. Very little is understood regarding its production in the human body by exposure to UV radiation. In particular, we have no means of predicting how much vitamin D (cholecalciferol) will be produced in the skin after exposure to sunlight. Using a refined in vitro model, we found that there is a nonlinear relationship between UV dose and cholecalciferol synthesis. Two minimal erythemal doses (MED) of UV radiation produced 1.84 microg/mL of cholecalciferol whereas 4 MED produced 2.81 microg/mL. We also found that the production of cholecalciferol is restricted by the initial concentration of its precursor (7-dehydrocholesterol, 7-DHC). For example, using an initial concentration of 7-DHC of 102 microg/mL, the resultant cholecalciferol production was 1.05 microg/mL after receiving 4 MED exposure. Under the same exposure conditions, an initial concentration of 305 microg/mL yielded 2.81 g/mL of cholecalciferol. The data presented in this paper has important implications for humans, including: (1) increasing UV exposure does not result in a proportionate increase in the amount of cholecalciferol that is produced; and (2) the initial concentration of 7-DHC in the skin may impact the amount of cholecalciferol that can be synthesized. When translating these results to population groups, we will discuss how the sun exposure message needs to be carefully formulated to account for such considerations. 相似文献
40.
Timothy R. Corkish Christian T. Haakansson Peter D. Watson Hayden T. Robinson Prof. Allan J. McKinley Dr. Duncan A. Wild 《Chemphyschem》2021,22(13):1316-1320
The anion photoelectron spectra of Cl−⋅⋅⋅CD3CDO, Cl−⋅⋅⋅(CD3CDO)2, Br−⋅⋅⋅CH3CHO, and I−⋅⋅⋅CH3CHO are presented with electron stabilisation energies of 0.55, 0.93, 0.48, and 0.40 eV, respectively. Optimised geometries of the singly solvated species featured the halide appended to the CH3CHO molecule in-line with the electropositive portion of the C=O bond and having binding energies between 45 and 52 kJ mol−1. The doubly solvated Cl−⋅⋅⋅(CH3CHO)2 species features asymmetric solvation upon the addition of a second CH3CHO molecule. Theoretical detachment energies were found to be in excellent agreement with experiment, with comparisons drawn between other halide complexes with simple carbonyl molecules. 相似文献