共查询到20条相似文献,搜索用时 31 毫秒
1.
Read DJ Auhl D Das C den Doelder J Kapnistos M Vittorias I McLeish TC 《Science (New York, N.Y.)》2011,333(6051):1871-1874
We present a predictive scheme connecting the topological structure of highly branched entangled polymers, with industrial-level complexity, to the emergent viscoelasticity of the polymer melt. The scheme is able to calculate the linear and nonlinear viscoelasticity of a stochastically branched "high-pressure free radical" polymer melt as a function of the chemical kinetics of its formation. The method combines numerical simulation of polymerization with the tube/entanglement physics of polymer dynamics extended to fully nonlinear response. We compare calculations for a series of low-density polyethylenes with experiments on structural and viscoelastic properties. The method provides a window onto the molecular processes responsible for the optimized rheology of these melts, connecting fundamental science to process in complex flow, and opens up the in silico design of new materials. 相似文献
2.
A radical polymerization process that yields well-defined polymers normally obtained only through anionic polymerizations is reported. Atom transfer radical polymerizations of styrene were conducted with several solubilizing ligands for the copper(I) halides: 4,4'-di-tert-butyl, 4,4'-di-n-heptyl, and 4,4'-di-(5-nonyl)-2,2'-dipyridyl. The resulting polymerizations have all of the characteristics of a living polymerization and displayed linear semilogarithmic kinetic plots, a linear correlation between the number-average molecular weight and the monomer conversion, and low polydispersities (ratio of the weight-average to number-average molecular weights of 1.04 to 1.05). Similar results were obtained for the polymerization of acrylates. 相似文献
3.
[目的]分析二甲胺-丙烯酰胺阳离子聚丙烯酰胺的合成与表征。[方法]以丙烯酰胺、二甲胺和甲醛为原料,以亚硫酸氢钠和过硫酸铵作为引发剂合成阳离子聚丙烯酰胺(CPAM),采用正交试验来确定CPAM合成的最优方案,并对CPAM进行阳离子度、分子量的测定及红外光谱表征,分析其对制胶废水处理的效果。[结果]制备CPAM的最佳工艺为m(丙烯酰胺)∶m(甲醛):m(二甲胺)=20∶15∶11、反应温度为35℃、反应时间为6 h、引发剂(亚硫酸氢钠和过硫酸铵)质量分数是0.4%;随高聚物的分子量和阳离子度增大,制胶废水絮凝效也增强;分子量检测并结合红外谱图表明由丙烯酰胺、二甲胺等在试验条件下合成了阳离子聚丙烯酰胺。[结论]CPAM对制胶废水有很好的絮凝效果。 相似文献
4.
Mass spectrometry has advanced with the renaissance of time-of-flight mass analysis, the use of ion traps as analyzers and reactors, the application of tandem mass spectrometers to problems in ionic reaction mechanisms and chemical analysis, and the development of new desorption ionization techniques. These developments have allowed determination of the molecular weight distributions for polymers through the 10,000-dalton range, as well as the molecular weight and partial sequence of biopolymers of similar size. Surfaces can be characterized by use of the mass, energy, and angle distributions of particles ejected by sputtering or by laser-induced desorption. Mass spectrometry has yielded new information on the kinetics of catalytic surface reactions and on the reactivity of metal clusters. 相似文献
5.
Golden JH Deng H Disalvoa FJ Fréchet JM Thompson PM 《Science (New York, N.Y.)》1995,268(5216):1463-1466
A general methodology is presented for the dispersion of an inorganic compound within an organic polymer host through the use of solubilizing and polymerizable ligands. The dispersion of metal cluster cations 10 angstroms in diameter within a polymer host is achieved by free-radical polymerization of the hexafunctional metal cluster [Mo(6)Cl(8)(NVI)(6)](triflate)(4) with bound polymerizable ligands in N-vinylimidazole (NVI) solutions. Copolymerization of the activated cluster-bound ligands with the surrounding medium probably plays a key role in preventing aggregation and produces near monodisperse molecular clusters within the polymer matrix. 相似文献
6.
Solid friction between a charged polymer and fixed gel points can dramatically affect polymer mobility in gel electrophoresis. The effect is present when a polymer chain is entangled over many gel points along a portion of its length, leading to significantly different behavior than predicted by conventional theory: the mobility of the chain decreases and exhibits a stronger length dependence, which separates long linear charged polymers of different molecular weights. 相似文献
7.
[目的]利用马铃薯残渣为原料合成乳酸预聚物,为聚乳酸的生产工艺改造提供理论依据。[方法]将由马铃薯残渣发酵所得的乳酸采用熔融缩聚法合成乳酸预聚物,并对此方法进行了优化。[结果]最佳工艺:催化剂辛酸亚锡的用量1%,熔融缩聚时间10 h,反应温度170℃,1,4-丁二醇用量1%。[结论]熔融缩聚法聚合所得的聚乳酸预聚物的最高分子量为3059 g/mol,用马铃薯残渣为原料合成乳酸预聚物简单易行,该方法提供了一种新的废物利用途径。 相似文献
8.
9.
A method of increasing cow milk production using new-type polymer additives has been tested. The objective of this research was to study the influence of a high-molecular water-soluble polymer with a molecular weight of 1 × 107 Da, a linear molecular structure, and a viscosity of 20 to 100 mm2/s on cow milk production. The polymer was introduced into the diet together with concentrated feed during the dry and postcalving periods. The additive increased the cow milk productivity by 9% and insignificantly increased the density of milk and the content of fat, protein, and some macro- and microelements. 相似文献
10.
11.
Mackay ME Tuteja A Duxbury PM Hawker CJ Van Horn B Guan Z Chen G Krishnan RS 《Science (New York, N.Y.)》2006,311(5768):1740-1743
Traditionally the dispersion of particles in polymeric materials has proven difficult and frequently results in phase separation and agglomeration. We show that thermodynamically stable dispersion of nanoparticles into a polymeric liquid is enhanced for systems where the radius of gyration of the linear polymer is greater than the radius of the nanoparticle. Dispersed nanoparticles swell the linear polymer chains, resulting in a polymer radius of gyration that grows with the nanoparticle volume fraction. It is proposed that this entropically unfavorable process is offset by an enthalpy gain due to an increase in molecular contacts at dispersed nanoparticle surfaces as compared with the surfaces of phase-separated nanoparticles. Even when the dispersed state is thermodynamically stable, it may be inaccessible unless the correct processing strategy is adopted, which is particularly important for the case of fullerene dispersion into linear polymers. 相似文献
12.
以制备麦芽糖浆的副产物米渣为原料,选取胰酶为改性用酶。在对加酶量、固液比、pH、温度的单因素试验基础上,采用4因素3水平的响应面设计方法,以水解度(DH)为评定指标,并综合考虑限制性酶解程度与产品分子量分布之间的相关性,得到了以寡肽为水解目标的最佳酶解工艺参数:加酶量为底物的1.6%,固液比为1︰9,pH值为8.5,温度为55℃,反应时间为3.0 h。经脱色与喷雾干燥后,水解产物的分子量分布基本上都处于1 000 u以下,且以二肽、三肽为主。 相似文献
13.
Electrical sources in human somatosensory cortex: identification by combined magnetic and potential recordings 总被引:7,自引:0,他引:7
Magnetic fields and electrical potentials produced by neuronal activity have different properties that can be used for the identification of electrical sources in the human brain. Fields and potentials occurring 20 to 30 milliseconds after median nerve stimulation in human subjects were compared in order to investigate the sources of evoked potential components that have been attributed by different investigators to the thalamus or thalamocortical afferents, to separate radial sources in somatosensory cortex and motor cortex, or to a tangential source in somatosensory cortex. The magnetic and potential wave forms were highly similar in morphology, and their spatial distributions were centered over sensorimotor cortex, were dipolar in shape, and differed in orientation by approximately 90 degrees; distances between the minimum and maximum of the magnetic distributions were about 60 percent of those of the potential distributions. These results cannot be accounted for by thalamic sources or radial cortical sources alone, but are consistent with a tangential source in somatosensory cortex, with an additional smaller contribution from radial sources. 相似文献
14.
The great advances made in polymeric structural materials over the past decade have led to their replacement of conventional materials in a wide range of uses including high-performance aerospace applications. This shift in choice of materials is based on economic advantages, simplified fabrication, freedom from corrosion, and lower weight. The trend toward use of polymeric materials will grow as the materials science of this new technology is developed. Better insight into such fundamental problems as the mechanisms by which forces are transferred to the polymer molecule, the surface interactions between polymer phases or polymer and fiber, and the molecular processes by which energy is absorbed during fracture will greatly stimulate the development of this field. 相似文献
15.
16.
Webster OW 《Science (New York, N.Y.)》1991,251(4996):887-893
Living polymerization techniques can be used to achieve a high degree of control over polymer chain architecture. Examples of the type of polymers that can be synthesized include block copolymers, comb-shaped polymers, multiarmed polymers, ladder polymers, and cyclic polymers. This control of structure, in turn, results in polymers with widely diverse physical properties, even though they are made from readily available low-cost monomers. 相似文献
17.
Conjugated organic polymers generally must include large substituents for stability, either contained within or appended to the polymer chain. In polydiacetylenes, the substituents fulfill another important role: During topochemical polymerization, they control the spacing between the diyne monomers to produce an ordered polymer. By using a co-crystal scaffolding, we have prepared poly(diiododiacetylene), or PIDA, a nearly unadorned carbon chain substituted with only single-atom iodine side groups. The monomer, diiodobutadiyne, forms co-crystals with bis(nitrile) oxalamides, aligned by hydrogen bonds between oxalamide groups and weak Lewis acid-base interactions between nitriles and iodoalkynes. In co-crystals with one oxalamide host, the diyne undergoes spontaneous topochemical polymerization to form PIDA. The structure of the dark blue crystals, which look copper-colored under reflected light, has been confirmed by single-crystal x-ray diffraction, ultraviolet-visible absorption spectroscopy, and scanning electron microscopy. 相似文献
18.
19.
20.
The squeezing of polymers in narrow gaps is important for the dynamics of nanostructure fabrication by nanoimprint embossing and the operation of polymer boundary lubricants. We measured stress versus strain behavior while squeezing entangled polystyrene films to large strains. In confined conditions where films were prepared to a thickness less than the size of the bulk macromolecule, resistance to deformation was markedly reduced for both solid-glass forging and liquid-melt molding. For melt flow, we further observed a complete inversion of conventional polymer viscosity scaling with molecular weight. Our results show that squeeze flow is accelerated at small scales by an unexpected influence of film thickness in polymer materials. 相似文献