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1.
本文研究了一种非牛顿流体-高聚物水溶液在垂直上升管内强制流动沸腾过程中的壁温沿管长变化及临界热负荷的特点,并考察了高聚物种类、浓度、质量流速等对临界热负荷和壁温工况的影响。  相似文献
2.
Free energy of semiflexible polymers and structure of interfaces   总被引:1,自引:0,他引:1  
The free energy of semiflexible polymers is calculated as a functional of the compositional scalar order parameter and the orientational order parameter of second-rank tensor Sij on the basis of a microscopic model of wormlike chains with variable segment lengths. We use a density functional theory and a gradient expansion to evaluate the entropic part of the free energy, which is given in a power series of .The interaction term of the free energy is derived with a random phase approximation. For the rigid rod limit, the nematic-isotropic transition point is given by , N and w being the degree of polymerization and the anisotropic interaction parameter, respectively, and the degree of ordering at the transition point is 0.33448. We also find that the contour length of polymer chains becomes larger in a nematic phase than in an isotropic phase. Interface profiles are obtained numerically for some typical cases. In the neighborhood of isotropic-isotropic interfaces, polymer chains tend to align parallel to the interface on the polymer-rich side and perpendicular on the poor side. When an isotropic region and a nematic region coexist, orientational order parallel to the interface is preferred in the nematic region. Received: 28 May 1998 / Revised: 12 August 1998 / Accepted: 8 September 1998  相似文献
3.
The single-chain motion of flexible bead polymer chains in solution was investigated taking into account hydrodynamic screening. The Oseen-like tensor was found within the Debye–Bueche–Brinkman effective-medium model, which describes the polymer solution. Two dynamical regimes in time scales were predicted: Zimm-like motion and Rouse-like motion. The time dependence of the hydrodynamic screening length was considered.  相似文献
4.
Stationary ultrasonically induced birefringence in various polymer solutions was measured in order to investigate its relation to the anisotropy in polarizability of a polymer chain. From the concentration dependence of stationary ultrasonically induced birefringence in polystyrene–toluene, polycarbonate–chloroform and polybutadiene–toluene solutions, the intrinsic values of the birefringence were obtained. A linear relationship between the intrinsic value of the birefringence per segment and the segmental anisotropy in polarizability was obtained. In aqueous solutions of polyelectrolytes, sodium polystyrenesulfonate and tetramethylammonium polystyrenesulfonate, the birefringence decreased with ionic strength by the addition of the salts. The stationary birefringence per ultrasonic intensity for all polymer solutions investigated here decreased with increasing frequency.  相似文献
5.
Gravity driven instabilities in model porous packings of 1 mm diameter spheres are studied by comparing the broadening of the displacement front between fluids of slightly different densities in stable and unstable configurations. Water, water–glycerol and water–polymer solutions are used to vary independently viscosity and molecular diffusion and study the influence of shear-thinning properties. Both injected and displaced solutions are identical but for a different concentration of NaNO3 salt used as an ionic tracer and to introduce the density contrast. Dispersivity in stable configuration increases with polymer concentration – as already reported for double porosity packings of porous grains. Gravity-induced instabilities are shown to develop below a same threshold Péclet number Pe for water and water–glycerol solutions of different viscosities and result in considerable increases of the dispersivity. Measured threshold Pe values decrease markedly on the contrary with polymer concentration. The quantitative analysis demonstrates that the development of the instabilities is controlled by viscosity through a characteristic gravity number G (ratio between hydrostatic and viscous pressure gradients). A single threshold value of G accounts for results obtained on Newtonian and non-Newtonian solutions.  相似文献
6.
The appearance of spherical particles resulting from phase separation in dilute solutions of poly(N-isopropylacrylamide) has been observed by dynamic light scattering (DLS). The particles have a relatively narrow size distribution. The size of particles increases with increasing concentration of polymer, and decreasing heating speed. Electron microscopy confirms the existence of spherical particles with size and polydispersity in agreement with DLS.  相似文献
7.
We construct a model for a slippage plane in a sheared melt, based on a balance between reptation bridging and shear debonding. The resulting state could show up at rather low shear rates and be locally stable. But it is not easy to nucleate: the conventional entangled state is also locally stable. We propose that slippage occurs on solid walls: either at the container surface, or on dust particles floating in the melt. Slippage at solid/melt interfaces was studied (experimentally and theoretically) long ago. There is a critical stress for slippage: our estimate (for strong adsorption of melt chains on the solid) gives (plateau modulus) for typical cases. Thus, melt fracture is expected at moderate stresses, in agreement with observations by S.Q. Wang and coworkers.  相似文献
8.
XPS and AFM have been used to investigate surface modifications produced by acrylic acid (AA) vapor plasma treatment of silicon (Si)(1 0 0) substrates and polyurethanes (PUs) membranes. XPS analyses of Si and PUs treated substrates at low plasma power (5-20 W) revealed the formation of a thin film on the surfaces, which chemically resembles the poly(acrylic acid) film conventionally synthesised. No signal of the Si substrate could be seen under these low plasma power applications on silicon. However, when the plasma power is higher than 30 W one can clearly see XPS silicon signatures. AFM measurements of silicon substrates treated with AA plasma at low power (5-20 W) showed the formation of a thin polymer film of about 220-55 nm thickness. Further, applications of high plasma power (30-100 W) displayed a marked difference from low plasma modifications and it was found sputtering of the silicon substrate. Pervaporation results of AA plasma treated PUs membranes revealed that the selectivity for the separation of methanol from methyl-t-butyl ether is higher at 100 W and 1 min treatment time, than the other conditions studied. This last finding is discussed concerning the surface modifications produced on plasma treated silicon substrates and PU membranes.  相似文献
9.
The Langevin dynamics of a self-interacting chain embedded in a quenched random medium is investigated by making use of the generating functional method and one-loop (Hartree) approximation. We have shown how this intrinsic disorder causes different dynamical regimes. Namely, within the Rouse characteristic time interval the anomalous diffusion shows up. The corresponding subdiffusional dynamical exponents have been explicitly calculated and thoroughly discussed. For the larger time interval the disorder drives the center of mass of the chain to a trap or frozen state provided that the Harris parameter, (Δ/b d)N 2 - νd≥1, where Δ is a disorder strength, b is a Kuhnian segment length, N is a chain length and ν is the Flory exponent. We have derived the general equation for the non-ergodicity function f (p) which characterizes the amplitude of frozen Rouse modes with an index p = 2πj/N. The numerical solution of this equation has been implemented and shown that the different Rouse modes freeze up at the same critical disorder strength Δ cN - γ where the exponent γ ≈ 0.25 and does not depend from the solvent quality. Received 17 December 2002 Published online 23 May 2003 RID="a" ID="a"e-mail: vilgis@mpip-mainz.mpg.de  相似文献
10.
When a glassy polymer film is formed by evaporation, the region near the free surface is polymer rich and becomes glassy first, as noticed long ago by Scriven et al. We discuss the thickness of this “crust” and the time interval where it is present --before freezing of the whole film. We argue that the crust is under mechanical tension, and should form some cracks. This may be the source of the roughness observed on the final, dry films, when the solvent vapor pressure is high (and leads to thin crusts). Received 13 November 2001  相似文献
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