On the Decomposition of Graphs into Complete Bipartite Graphs

In a complete bipartite decomposition π of a graph, we consider the number ϑ(v;π) of complete bipartite subgraphs incident with a vertex v. Let ϑ(G)= ϑ(v;π). In this paper the exact values of ϑ(G) for complete graphs and hypercubes and a sharp upper bound on ϑ(G) for planar graphs are provided, respectively. An open problem proposed by P.C. Fishburn and P.L. Hammer is solved as well.     

Obstructions for acyclic local tournament orientation completions

The orientation completion problem for a fixed class of oriented graphs asks whether a given partially oriented graph can be completed to an oriented graph in the class. Orientation completion problems have been studied recently for several classes of oriented graphs, including local tournaments. Local tournaments are intimately related to proper circular-arc graphs and proper interval graphs. In particular, proper interval graphs are precisely those which can be oriented as acyclic local tournaments. In this paper we determine all obstructions for acyclic local tournament orientation completions. These are in a sense minimal partially oriented graphs that cannot be completed to acyclic local tournaments. Our results imply that a polynomial time certifying algorithm exists for the acyclic local tournament orientation completion problem.     

Robust Proton Conduction against Mechanical Stress in Flexible Free-Standing Membrane Composed of Two-Dimensional Coordination Polymer

Introduction of mechanical flexibility into proton-conducting coordination polymers (CPs) is in high demand for future protonic applications such as fuel cells and hydrogen sensors. Although such mechanical properties have been primarily investigated in one-dimensional (1D) CPs, in this study, we successfully fabricated highly flexible free-standing CP membranes with a high surface-to-volume ratio, which is beneficial for enhanced performance in the aforementioned applications. We fabricated a layered CP, Cu₂(NiTCPP) (H₄(H₂TCPP); 5,10,15,20-tetrakis(4-carboxyphenyl) porphyrin), in which a two-dimensional (2D) square grid sheet composed of tetradentate nickel porphyrins and paddlewheel-type copper dimers was connected to each other by weak van der Waals forces. The mechanical flexibility was evaluated by bending and tensile tests. The flexural and Young's moduli of the membrane were significantly higher than those of conventional Nafion membranes. Electrochemical impedance spectroscopy analysis revealed that the in-plane proton conductivity of the membrane was maintained even under applied bending stress. Because the X-ray diffraction analysis indicates that the proton-conducting pathway through the hydrogen bonding network remains intact during the bending operation, our present study provides a promising strategy for the fabrication of new and advanced 2D CPs without using substrates or additional polymers for protonic devices.     

Oriented Ultrathin π-complexation MOF Membrane for Ethylene/Ethane and Flue Gas Separations

Rational design and engineering of high-performance molecular sieve membranes towards C₂H₄/C₂H₆ and flue gas separations remain a grand challenge to date. In this study, through combining pore micro-environment engineering with meso-structure manipulation, highly c-oriented sub-100 nm-thick Cu@NH₂-MIL-125 membrane was successfully prepared. Coordinatively unsaturated Cu ions immobilized in the NH₂-MIL-125 framework enabled high-affinity π-complexation interactions with C₂H₄, resulting in an C₂H₄/C₂H₆ selectivity approaching 13.6, which was 9.4 times higher than that of pristine NH₂-MIL-125 membrane; moreover, benefiting from π-complexation interactions between CO₂ and Cu(I) sites, our membrane displayed superior CO₂/N₂ selectivity of 43.2 with CO₂ permeance of 696 GPU, which far surpassed the benchmark of other pure MOF membranes. The above multi-scale structure optimization strategy is anticipated to present opportunities for significantly enhancing the separation performance of diverse molecular sieve membranes.     

Orientation effects on C2(5)-C2'(5')linked bioxazole isomers synthesized via regioselective and sequential C—H arylation

Bis(4-fluorophenyl) substituted oxazole(2,5-Oxz) and C2(5)-C2'(5') linked bioxazole isomers(C2-C2'_BOxz,C2-C5'_BOxz and C5-C5'_BOxz) were concisely synthesized via palladium-catalyzed regioselective and sequential C-H arylation in 1-3 reaction steps along with 20%-83% of total yields from oxazole and4-bromofluorobenzene.The linking orientation plays a key role in the packing geometry and photophysical properties of C2-C2'_BOxz,C2-C5'_BOxz and C5-C5'_BOxz.These bioxazole isomers in solid state showed significant differences in photoluminescence quantum yields(PLQY)(0.33,0.25 and 0.04,respectively),delayed fluorescence properties and powder X-ray diffraction(PXRD) patterns,suggesting the divergence in intermolecular interactions.The theoretically calculated gradient isosurfaces and complexation energies indicate the existence of intense π-π interactions between molecular layers,which are in good agreement with the variation trend of optical properties.     

取向晶种法制备沸石分子筛膜研究进展

近年来沸石分子筛膜因在分离、 催化、 传感和防腐等研究领域具有重要的应用价值而引起广泛关注. 其中, 具有优先取向微观结构的沸石分子筛膜由于能够显著降低客体分子在膜内的扩散路径并减少膜内的晶间界缺陷密度, 成为膜分离学科的研究重点. 本文以b-轴取向MFI沸石分子筛膜为主线, 综述了国内外制备取向沸石分子筛膜的最新研究进展; 详细介绍了取向晶种法制膜工艺, 侧重总结了MFI沸石纳米片合成、 b-轴取向沸石晶种单层制备以及二次生长调控等方面的研究成果; 在深入探讨各类取向沸石分子筛膜制备策略的基础上, 分析探讨了其中存在的关键问题与解决措施, 最后对取向沸石分子筛膜的发展方向进行了展望.     

Rheology of polymer layered silicate nanocomposites

Layered silicate based polymer nanocomposites have gained significant technological interest because of the recent commercialization of nylon 6 and polypropylene based materials. Aside from the natural interests in understanding and improving the processing of these hybrids, viscoelastic measurements have also proven to be a sensitive tool to probe the mesoscale structure and the strength of polymer–nanoparticle interactions.     

POLYMETHYLMETHACRYLATE FILMS IN THE STATE OF HIGH GLOBAL CHAIN ORIENTATION BUT NEARLY RANDOM SEGMENTAL ORIENTATION*

PMMA films in the high global chain orientation and nearly random segmental orientation(GOLR) state were prepared by uni-axially drawing at temperatures 20～30℃ above its glass transitiontempefature, T_g, and their isotropic and anisotropic properties were studied. Experimental results show thatthis kind of amorphous state, the GOLR state, is nearly isotropic in optical birefringence, IR-dichroism andX-ray diffraction patterns, but is very anisotropic in behaviors of thermal expansion, solvent-swelling andstress-strain.     

Coupled annealing temperature and layer thickness effect on strengthening mechanisms of Ti/Ni multilayer thin films

A systematic study was performed on mechanical and microstructural properties of Ti/Ni multilayers with layer thickness from 200 nm to 6 nm and annealing temperature from room temperature to 500 °C. Based on the observed hardness evolution, a coupled layer-thickness and annealing-temperature dependent strengthening mechanism map is proposed. For as-deposited films, the deformation behavior follows the traditional trend of dislocation mediated strengthening to grain boundary mediated softening with decreasing layer thickness. For annealed films, grain boundary relaxation is considered to be the initial strengthening mechanism with higher activation temperature required for thicker layers. Under further annealing, solid solution hardening, intermetallic precipitation hardening, and fully intermixed alloy structure continue to strengthen the thin layered films, while recrystallization and grain-growth lead to the eventual softening of thick layered films. For the films with intermediate layer thickness, a strong orientation dependent hardness behavior is exhibited under high temperature annealing due to mechanism switch from grain growth softening to intermetallic precipitation hardening when changing the loading orientation from perpendicular to parallel to the layer interfaces.     

高取向聚对苯二甲酸乙二酯纤维的热收缩应力

为得到具有更高拉伸强度和模量的半晶聚合物纤维,需要使分子链充分结晶和取向,然而这种高度取向样品受热时,随着温度的升高,取向的非晶态分子链熵力增大,解取向可以自发进行.在外加张力较小时,纤维产生热收缩;在定长状态下,表现为外加张力增大,此过程被视为取向材料中＂冻结＂内应力的释放,通常将这种内应力称为热收缩应力。