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81.
A. Milchev J.P. Wittmer D.P. Landau 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,12(2):241-251
We use an off-lattice microscopic model for solutions of equilibrium polymers (EP) in a lamellar shear flow generated by means
of a self-consistent external field between parallel hard walls. The individual conformations of the chains are found to elongate
in flow direction and shrink perpendicular to it while the average polymer length decreases with increasing shear rate. The
Molecular Weight Distribution of the chain lengths retains largely its exponential form in dense solutions whereas in dilute
solutions it changes from a power-exponential Schwartz distribution to a purely exponential one upon an increase of the shear
rate. With growing shear rate the system becomes increasingly inhomogeneous so that a characteristic variation of the total
monomer density, the diffusion coefficient, and the center-of-mass distribution of polymer chains of different contour length
with the velocity of flow is observed. At higher temperature, as the average chain length decreases significantly, the system is shown to undergo an order-disorder transition
into a state of nematic liquid crystalline order with an easy direction parallel to the hard walls. The influence of shear
flow on this state is briefly examined.
Received 22 October 1998 and Received in final form 12 April 1999 相似文献
82.
T. Charitat E. Bellet-Amalric G. Fragneto F. Graner 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,8(4):583-593
Single and double phosphocholine (DPPC and DSPC) bilayers adsorbed at the silicon-water interface have been prepared and characterised.
The second bilayer, called “free bilayer”, is a novel highly hydrated system floating at above the first one. Robust and reproducible preparation has been possible thanks to a combination of Langmuir-Blodgett and
Langmuir-Schaeffer techniques. Carefully optimised neutron reflectivity measurements have allowed a precise non-destructive
characterisation of the structure, hydration and roughness of the layers. This work opens new possibilities for the investigation
of the interaction between membrane lipids and soluble proteins, in particular peptides too small to be visible with other
techniques.
Received 17 July 1998 相似文献
83.
E. Freyssingeas D. Antelmi P. Kékicheff P. Richetti A.-M. Bellocq 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,9(1):123-136
The compressibility modulus of a lamellar phase containing a neutral polymer guest molecule was measured directly using a
surface force apparatus. The system studied consisted of sodium dodecyl sulphate (SDS), pentanol, water and polyethylene glycol
(PEG) . The lamellar phase was induced from a micellar phase in situ via a confinement induced isotropic to lamellar phase transition. This avoided problems resulting from the viscosity and turbidity
normally characteristic of these lamellar phase samples. Increasing the amount of PEG resulted in a marked decrease in the
layer compressibility modulus indicating a decrease in the repulsive forces between the lamellae. The origin of such a phenomenon is discussed in terms
of different mechanisms including depletion interactions, bridging interactions and modification of the electrostatic interaction
between the lamellae by the polymer.
Received 2 February 1998 相似文献
84.
W. Essafi F. Lafuma C.E. Williams 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,9(2):261-266
We show experimentally that Manning counterion condensation also leads to a renormalization of the charge density at high
concentrations of highly charged, flexible, hydrophilic polyelectrolytes. Investigations by small angle neutron and X-ray
scattering of semi-dilute solutions of poly(acrylamide-co-sodium-2-acrylamido-2-methylpropane sulfonate) at different charge densities above the condensation threshold, show that
the scattering function is invariant with the charge density.
Received 16 June 1998 相似文献
85.
Y.O. Popov A.N. Semenov 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,6(2):245-256
Nematic ordering in anisotropic non-Gaussian elastomers is considered theoretically using mean field approximation. We focus on the effect of anisotropy during network
cross-linking on the system elasticity and, in particular, on the so-called soft deformation mode. As the main result, we
calculate the dependence of the elastomer free energy on the angle between the axis of “frozen” anisotropy and the nematic
director. The dependence of the isotropic-nematic transition point on the orientational field acting on the monomers during
the cross-linking process is also calculated.
Received: 5 November 1997 / Revised and Accepted: 29 June 1998 相似文献
86.
A.N. Semenov M. Rubinstein 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,1(1):87-94
The effect of excluded-volume interactions on the reptation dynamics of long polymer chains is considered theoretically. It
is shown that interactions give rise to an exponential increase of the reptation time, , if polymer chains are long enough: , where is the number of monomers per entanglement. We propose a novel dynamical mechanism of activated reptation implying that neighboring
chains exchange conformations of their terminal fragments. It is shown that the exchange mechanism is compatible with the
equilibrium polymer chain statistics and that it provides a bridge between the previous theories.
Received: 25 July 1997 / Accepted: 8 October 1997 相似文献
87.
We discuss the influence of polymer adsorption on the curvature energy of an interface. Following an article by Clement and
Joanny (J. Phys. II 7, 973 (1997)), a mean-field theory is used to calculate the surface tension, rigidity constants and spontaneous curvature
associated with both reversible and irreversible polymer adsorption. In the case of irreversible polymer adsorption it is
assumed that the amount of adsorbed polymer remains constant upon curving the interface. Unfortunately, constraining the amount
of polymer by adding a Lagrange multiplier affects the thermodynamic state of the (free) polymer far away from the interface.
Clement and Joanny solve this problem by removing the polymers in the bulk. We allow for the presence of free polymers, but
to achieve this we have to apply a local external field to keep the adsorbed amount fixed. The results of the two approaches
are compared and a physical interpretation is given.
Received 25 July 2001 and Received in final form 5 December 2001 相似文献
88.
Dynamical heterogeneity (DH) in high-density Al2O3·2SiO2 melts has been studied in a model containing 3025 atoms via molecular dynamics (MD) simulation and at the fixed density of 4.0 g/cm3. Non-Gaussian parameter of atomic species in the system has been found and discussed. We found a clear evidence of the existence of DH in high-density Al2O3·2SiO2, which has specific features differed from those observed in the lower-density one. The most mobile and immobile atoms in the system have a tendency to form clusters and temperature dependence of their mean cluster size was found. On the other hand, diffusion constant of atomic species in the system has been calculated at temperatures ranged from 3150 to 7000 K. Calculations show that at relatively not high temperatures, temperature dependence of diffusion constant shows an Arrhenius law and at higher temperatures it shows a power law: D∝(T−TC)γ. Diffusion data of high-density melts have been compared with those for the low-density ones. Diffusion mechanism in the system has been discussed via the temperature dependence of diffusion constant ratio and activation energy. And we found the existence of cooperative diffusion mechanism in the system. 相似文献
89.
We present here a theoretical study of the early kinetics of the microphase separation in crosslinked polymer blends, made of two incompatible polymers A and B, dissolved in a common good solvent. Use is made of an extended blob model used previously for the investigation of the static properties of such a transition. We are interested in the variation of the relaxation rate,
, versus the wave number q, in the vicinity of the spinodal temperature. We first show that kinetics is entirely dominated by local motions, which are of Rouse type. Slow motions are absent, because of the permanent presence of crosslinks. Second, we find that the characteristic frequency,
(q ) =
, increases with increasing wave number q according to a sixth power law, that is
(q )
q6{-9/4}, where is the overall monomer volume fraction. Therefore, the swelling of strands due to the excluded-volume forces leads to a renormalization of the characteristic frequency by a multiplicative factor scaling as {-9/4}. The main conclusion is that the presence of a good solvent necessitates relaxation rates less important than those relative to crosslinked mixtures in the molten state. 相似文献
90.
Semenov AN 《The European physical journal. E, Soft matter》2002,9(4):353-363
Adsorption of ideal polymers with stiff backbone onto a flat surface is considered theoretically. Both scaling approach and
quantitative theory are developed. We predict a self-similar monomer concentration profile c(x) ∼ x
-4/3 near the surface (when the distance to the surface x is much smaller than the chain persistence length l /2). The typical conformation of a weakly adsorbed chain can be viewed as a sequence of alternating flat (2-dimensional)
trains of wormlike short loops (flat blobs) and coil-like (3-dimensional) loops forming a triple-layer structure: contact
layer (x < Δ) of adsorbed fragments virtually laid on the surface, proximal layer (Δ < x < l) of flat blobs, and more dilute distal corona layer (x > l). Here Δ defines the range of monomer/surface attraction, Δ ≪ l. The adsorption transition is continuous. However, its relative width is small (T
* is the adsorption temperature, ΔT is the relevant temperature interval): ∼
, i.e. a discontinuous transition in the limit Δ/l↦ 0.
Received 10 October 2002 and Received in final form 22 November 2002
RID="a"
ID="a"Permanent address: Physics Department, Moscow State University, Moscow 119992, Russia. e-mail: semenov@polly.phys.msu.ru 相似文献