共查询到20条相似文献,搜索用时 218 毫秒
1.
采用非格点珠簧球链模型, 结合Monte Carlo方法, 研究了半刚性高分子链受限于无限长圆柱体的构象性质. 模拟结果表明: 在圆柱体内表面附近具有吸附能的情况下, 当弯曲能b由小到大变化时, 发现半刚性高分子链由开始时的无规则被吸附在圆柱体内表面, 到逐渐出现螺旋结构, 最后伸展成类似棒状的结构. 同时计算了不同弯曲能b时的半刚性高分子链的平均螺旋数Nt, 平均每条链单体的螺旋百分比Ph和能量涨落. 发现高分子链螺旋结构的形成与转变, 不仅与圆柱体半径R的大小有关, 还与弯曲能b的大小有关. 研究结果能有助于加深对受限生物大分子构象的认识. 相似文献
2.
3.
高功率激光通常具有振幅调制和相位畸变. 采用统计光学方法推导了截断的有振幅调制和位相畸变光束在大气湍流中传输 的等效曲率半径R的解析公式.研究表明:随着位相畸变参量、振幅调制参量和光 束截断参量的增大,光束在自由空间中的 R增大,但R受湍流的影响也会增大; 并且高斯光束在自由空间中的R最大, 但其受湍流影响也最大. 因此,在大气湍流中传输到足够远时, 截断的有振幅调制和位相畸变光束的R就要比高斯光束的大.特别地, 相对等效曲率半径Rr随传输距离为非单调变化, 存在一个最小值, 即在该位置处R受湍流的影响最大. 此外, 达到 Rr最小值所需传输距离随光束位相畸变和振幅调制的加剧而增大. 相似文献
4.
5.
6.
7.
8.
9.
第Ⅱ种强度不等的非对称两态叠加多模叠加态光场的偶数阶等阶N次方Y压缩 总被引:5,自引:0,他引:5
本文构造了由多模复共轭相干态的相反态|{-Zj(a)*}>q与多模虚共轭相干态的相反态|{-iZj(b)*}>q这两者的线性叠加所组成的第Ⅱ种强度不等的非对称两态叠加多模叠加态光场|ΨⅡ(ab)>q,利用多模压缩态理论研究了态|ΨⅡ(ab)>q的任意偶数阶等阶N次方Y压缩特性.结果发现:1)在压缩阶数N取偶数,即N=2p的条件下,无论p=2m(m=1,2,3,…,…),还是p=2m+1(m=0,1,2,3,…,…),只要构成态|ΨⅡ(ab)>q的两个不同的量子光场态中各对应模的强度(即平均光子数)和初始相位都不相等,亦即Rj(a)≠Rj(b)和φj(a)≠φj(b)(j=1,2,3,…,q),并且
,则当满足一定的量子化条件(或者在一些闭区间内连续取值)时,态|ΨⅡ(ab)>q总可呈现出周期性变化的、任意偶数阶的等阶N次方Y压缩效应.2)在N=2p且p=2m+1(m=0,1,2,3,…,…)的条件下,若Rj(a)=Rj(b)和φj(a)=φj(b)(j=1,2,3,…,q),态|ΨⅡ(ab)>q则可呈现出等阶N次方Y压缩简并现象. 相似文献
10.
两原子非简并双光子Jaynes-Cummings模型的辐射谱 总被引:2,自引:2,他引:0
本文研究了两原子非简并双光子Jaynes-Cummings模型的辐射谱,结果表明,当双模腔场处于不同数态时,辐射谱呈现如下新特性:当一腔场处于真空态,另一腔场为强场时,辐射谱为对称六峰结构,任两对称峰的间距均与√2n2g成正比;当双模腔场均为强场时,辐射谱为对称四峰结构,两内峰的间距(4g)与数态光子数n1及n2无关;当一腔场处于真空态,另一腔场处于数态|n2>时,辐射谱一般为对称六峰结构,当n2=1时,谱变为对称四峰结构。 相似文献
11.
We study isotropic-isotropic and isotropic-nematic phase transitions of fluid mixtures containing hard spherocylinders (HSC)
and added non-adsorbing ideal polymer chains using scaled particle theory (SPT). First, we investigate isotropic-nematic (I -N phase coexistence using SPT in the absence of polymer. We compare the results obtained using a Gaussian form of the orientational
distribution function (ODF) to minimize the free energy versus minimizing numerically. We find that formal numerical minimization gives results that are much closer to computer simulation
results. In order to describe mixtures of HSC plus ideal chains we studied the depletion of ideal chains around a HSC. We
analyze the density profiles of ideal chains near a hard cylinder and find the depletion thickness δ is a function of the
ratio of the polymer's radius of gyration Rg and the cylinder radius Rc. Our results are compared with a common approximation in which the depletion thickness is taken equal to the radius of gyration
of the polymer chain. We incorporate the correct depletion thickness into SPT and find that for R
g/R
c < 1.56 using ideal chains gives phase transitions at smaller polymer concentrations, whereas for R
g/R
c > 1.56 , which is a common experimental situation, the phase transitions are found at larger polymer concentrations with
respect to δ = R
g . The differences are significant, especially for R
g ≫ R
c , so we can conclude it is essential to take into account the properties of ideal polymer chains and the resulting depletion
near a cylinder. Finally, we present phase diagrams for rod-polymer mixtures which could be realized under experimental conditions. 相似文献
12.
It has been shown over the last few years that the dynamics close to the glass transition is strongly heterogeneous, both
by measuring the diffusion coefficient of tagged particles or by NMR studies. Recent experiments have also demonstrated that
the glass transition temperature of thin polymer films can be shifted as compared to the same polymer in the bulk. We propose
here first a thermodynamical model for van der Waals liquids, which accounts for experimental results regarding the bulk modulus
of polymer melts and the evolution of the density with temperature. This model allows us to describe the density fluctuations
in such van der Waals liquids. Then, by considering the thermally induced density fluctuations in the bulk, we propose that
the 3D glass transition is controlled by the percolation of small domains of slow dynamics, which allows to explain the heterogeneous
dynamics close to T
g. We show then that these domains percolate at a lower temperature in the quasi-2D case of thin suspended polymer films and
we calculate the corresponding glass transition temperature reduction, in quantitative agreement with experimental results
of Jones and co-workers. In the case of strongly adsorbed films, we show that the strong adsorption amounts to enhance the
slow domains percolation. This effect leads to 1) a broadening of the glass transition and 2) an increase of T
g in quantitative agreement with experimental results. For both strongly and weakly adsorbed films, the shift in T
g is given by a power law, the exponent being the inverse of that of the correlation length of 3D percolation.
Received 21 March 2000 and Received in final form 4 December 2000 相似文献
13.
《Current Applied Physics》2015,15(10):1188-1199
The effect of surface chemical functionalization of a single graphene layer on its thermodynamic work of adhesion (WA) with polyethylene (PE) chains has been investigated using molecular dynamics (MD) simulation. For this purpose, amine (NH2), carboxyl (COOH), hydroxyl (OH), and methyl (CH3) functional groups were distributed randomly throughout the graphene surface using a Monte Carlo (MC) algorithm to achieve graphene functionalized structures with minimized potential energies. The MD simulation results showed that the thermodynamic WA between the PE and the functionalized graphene was larger than that between the PE and the pristine graphene. In fact, the electronegativity of functional groups and Van der Waals forces play influential roles in the thermodynamic WA between the PE and the functionalized graphene. In addition, the amount of thermodynamic WA was increased with increasing the functional group surface density, except for the graphene functionalized with the methyl groups. The segmental density of the PE chains near the single sheet surface was determined based on the density profile calculation. The polymer segments exhibited strong ordering and sharp density variations near the PE/graphene interface. The dynamic of chains was quantitatively characterized by calculating mean square displacement (MSD). Furthermore, the influence of functionality on the glass transition temperature (Tg) of the PE at the PE/graphene interface region was investigated. The results showed that the Tg at the PE/graphene interface was much higher than that of the bulk polymer. In fact, the functionalization of the graphene surface seems to considerably enhance the Tg of the polymer due to lowering the chains mobility. 相似文献
14.
Zhai Jun-Xue Wang He Shi Xin-Yan Zhao Shu-Gao 《Journal of Macromolecular Science: Physics》2013,52(3):496-509
The structure of the bound rubber, the 1H NMR (nuclear magnetic resonance) relaxation time, and the crosslink density of the physical network and the glass transition, were studied for solution polymerized styrene-butadiene rubber (SSBR) filled by carbon black, to investigate the effects of carbon black on the chain mobility and dynamic mechanical properties. It was found by 1H NMR analysis that the rubber chains were adsorbed on the surface of carbon black to form physical crosslinks and restrict the mobility of the chains, especially for some high-mobility units such as chain ends. It was calculated, according to the molecular weight between adjacent crosslinks, that the main motion units of the tightly adsorbed chains appeared to be similar in size to the chain segments. The glass transition temperature (T g) obtained by differential scanning calorimetry (DSC) could not be used to judge the effect of carbon black on chain mobility, while the appearance and change of the loss-tangent (tan δ) peak at high temperature in dynamic mechanical thermal spectrometry (DMTS) test showed that there were three chain states: free chains, loosely adsorbed chains, and tightly adsorbed chains. The dynamic rheology test showed that the unfilled SSBR compound had the rheological characteristics of entangled chain networks; however the nonlinear viscoelasticities of the filled SSBR were related to the gradual disentanglement of adsorbed chains and free chains. The peaks in tan δ vs. temperature curves implied that the motion unit size decreased with the increase of bound rubber content, and the modulus vs. temperature curve showed an apparently lower mobility of adsorbed chains than that of free chains through the very low dependence of modulus on temperature for the highly filled compounds. The extremely high tensile modulus of the vulcanizate with 63.6% carbon black at room temperature also implied that the adsorbed chains were in the glass state due to their restriction by the carbon black. 相似文献
15.
S. Stepanow 《Annalen der Physik》1983,495(6):301-316
Quantum field theoretic regularization methods are used to regularize the set of end-to-end chain correlation functions. The renormalization group invariance of the polymer theory allows to calculate the parameters g and S which in the regularized theory take on the role of z and Nl2 of the two parameter theory accordingly. The second virial coefficient A2 and the mean-square end-to-end polymer distance R2 of a polymer solution are represented in powers of g. g and S are evaluated up to order ?2 (? = 4 - d, where d is the space dimension). A2 and R2 are calculated up to order g2 and g, respectively, and compared with results obtained by ELDERFIELD and des CLOIZEAUX . 相似文献
16.
P. D. Gujrati 《Journal of statistical physics》1982,28(3):441-472
Rigorous lower bounds on the entropy per particle as a function of the fractiong of thegauche bonds of a system of semiflexible polymer chains is obtained in the thermodynamic limit. Only square and cubic lattices are considered. For the case of a single chain havingl monomers, the bound is obtained for all gg=2/3. For the case of p>1 chains, each havingl monomers, wherel is a multiple of 4, the bound is obtained for all gg=13/90. In both cases, it is shown that the entropy is alwaysnonzero for all 0<gm(l), whereg
m(l) =(l-2)/l. Thiscontradicts the prediction from the Flory-Huggins approximations that the entropy is zero for allgg0, whereg
0 is some finite nonzero number. It is also pointed out that it isnot impossible to pack a lattice with disordered configurations of rodlike chains with finite entropy, again contradicting an assertion by Flory that it is impossible to do so. Finally, it is concluded that onecannot trust the Flory-Huggins approximations at least at low temperatures. The study also casts doubts on the validity of the Gibbs-DiMarzio theory of glass transitions in polymeric systems. 相似文献
17.
Flows around small colloidal particles of diameter b, or in thin films, capillaries, etc., cannot always be described in terms of the macroscopic polymer viscosity. We discuss
these features for entangled polymer melts, where two distinct regimes can be found: (a) the thin regime where b is smaller than the coil radius R0, but larger than the diameter of the Edwards tube; (b) the ultrathin regime, where . We consider (i) non adsorbing particles, where slippage may occur between the melt and the solid surface; (ii) “hairy” particles,
which carry some bound polymer chains. We obtain scaling predictions for mobilities of spheres, of needles, and of clusters
of particles. We also discuss translational and rotational diffusion of needles.
Received 19 April 1999 相似文献
18.
Adsorption of chlorinated poly(ethylene-co-vinyl acetate)-g-maleic anhydride copolymer and in situ-generated polyaniline (PANI) on thermally reduced graphene oxide (TRGO) platelets was studied in the current study. The adsorption was characterized structurally and morphologically through thermogravimetric analysis, differential scanning calorimetry (DSC), elemental analysis, infra-red and Raman spectroscopy, X-ray diffraction and microscopy. The amount of copolymer adsorption reached a plateau of 0.22 g per g of TRGO, when the initial copolymer to TRGO weight ratio of 1 was used. In the case of PANI modification, much higher extent of adsorption of 0.92 g/g of TRGO (without reaching plateau) was observed due to in situ polymer synthesis and the absence of any steric hindrance to the chains. Shift in the DSC melting transition temperatures of copolymer also indicated that some change in the polymer chain morphology took place after immobilization of polymer on TRGO. PANI modification led to significant reduction in peak melting point from 175C to 140 °C owing to the hindrance in polymer crystallization. The basal plane spacing in the TRGO platelets increased the copolymer adsorption as the 0?0?1 basal plane diffraction shifted from 27° 2Θ for pristine TRGO to 22.5° 2Θ for modified TRGO. For the PANI modified TRGO, no diffraction signal corresponding to TRGO was observed due to extensive adsorption of polymer on the surface. A much thicker polymer phase wrapping the TRGO platelets was observed for PANI modified TRGO. This was also observed through EFTEM and EDX, where the presence of Cl and N (along with other atoms) indicated layer of copolymer and PANI, respectively on the surface of the platelets. EELS analysis also confirmed the semi-crystalline nature of the modified TRGO resulting from the adsorption of semi-crystalline polymers on TRGO. The adsorption approaches used in the study demonstrate successful generation of the functional nanomaterials with tunable extent of surface coverage and potential of employing diverse surface modifications. 相似文献
19.
Vikas Mittal 《哲学杂志》2013,93(36):4498-4517
The resistance of modification molecules bound to montmorillonite platelet surfaces towards structural damage at high temperature is a major parameter guiding the formation of optimal interface between the filler and polymer phases in a nanocomposite material. As nanocomposites are generated by melt-blending of modified mineral and polymer, it is necessary to quantify the thermal resistance of the filler surface modification at the compounding conditions because different modifications differ in chain length, chemical structure, chain density, and thermal performance. A number of different alkyl ammonium modifications were exchanged on the montmorillonites with cation exchange capacities in the range 680–900?µequiv.?g?1 and their thermal behaviour was characterised using high resolution thermogravimetric analysis. Quantitative comparisons between different modified minerals were achieved by comparing temperature at 10% weight loss as well peak degradation temperature. Various factors affecting thermal stability, such as length and density (or number) of alkyl chains in the modification, presence of excess modification molecules on the filler surface, the chemical structure of the surface modifications, etc. were studied. The TGA findings were also correlated with X-ray diffraction of the modified platelets. 相似文献
20.
Glass transition for atactic poly(methyl methacrylate)
(a-PMMA) prepared in
nano-cells by microemulsion polymerization was measured at a
faster heating rate after slow cooling of the sample from a
temperature above Tg. An
additional enthalpy relaxation and glass transition were
observed at higher temperatures for the a-PMMA sample due to the partial
organization of the chain segments which occurred during
microemulsion polymerization. The re-precipitated
a-PMMA did not show any
self-organization under the same thermal conditions, although
there are no changes in molecular weight or tacticity of the
polymer chains. A depletion-interaction phenomenon was
understood to provide entropic force for the self-organization
of polymer chains inside the walls of the microemulsion cells. 相似文献