Packing structures and transitions in liquids and solids

Classification of potential energy minima-mechanically stable molecular packings-offers a unifying principle for understanding condensed phase properties. This approach permits identification of an inherent structure in liquids that is normally obscured by thermal motions. Melting and freezing occur through characteristic sequences of molecular packings, and a defect-softening phenomenon underlies the fact that they are thermodynamically first order. The topological distribution of feasible transitions between contiguous potential minima explains glass transitions and associated relaxation behavior.     

Ultrapermeable, reverse-selective nanocomposite membranes

Polymer nanocomposites continue to receive tremendous attention for application in areas such as microelectronics, organic batteries, optics, and catalysis. We have discovered that physical dispersion of nonporous, nanoscale, fumed silica particles in glassy amorphous poly(4-methyl-2-pentyne) simultaneously and surprisingly enhances both membrane permeability and selectivity for large organic molecules over small permanent gases. These highly unusual property enhancements, in contrast to results obtained in conventional filled polymer systems, reflect fumed silica-induced disruption of polymer chain packing and an accompanying subtle increase in the size of free volume elements through which molecular transport occurs, as discerned by positron annihilation lifetime spectroscopy. Such nanoscale hybridization represents an innovative means to tune the separation properties of glassy polymeric media through systematic manipulation of molecular packing.     

Photoinduced plasticity in cross-linked polymers

Chemically cross-linked polymers are inherently limited by stresses that are introduced by post-gelation volume changes during polymerization. It is also difficult to change a cross-linked polymer's shape without a corresponding loss of material properties or substantial stress development. We demonstrate a cross-linked polymer that, upon exposure to light, exhibits stress and/or strain relaxation without any concomitant change in material properties. This result is achieved by introducing radicals via photocleavage of residual photoinitiator in the polymer matrix, which then diffuse via addition-fragmentation chain transfer of midchain functional groups. These processes lead to photoinduced plasticity, actuation, and equilibrium shape changes without residual stress. Such polymeric materials are critical to the development of microdevices, biomaterials, and polymeric coatings.     

Multicomponent polymeric engineering materials

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.     

General strategies for nanoparticle dispersion

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.     

Molecular ordering of organic molten salts triggered by single-walled carbon nanotubes

When mixed with imidazolium ion-based room-temperature ionic liquid, pristine single-walled carbon nanotubes formed gels after being ground. The heavily entangled nanotube bundles were found to untangle within the gel to form much finer bundles. Phase transition and rheological properties suggest that the gels are formed by physical cross-linking of the nanotube bundles, mediated by local molecular ordering of the ionic liquids rather than by entanglement of the nanotubes. The gels were thermally stable and did not shrivel, even under reduced pressure resulting from the nonvolatility of the ionic liquids, but they would readily undergo a gel-to-solid transition on absorbent materials. The use of a polymerizable ionic liquid as the gelling medium allows for the fabrication of a highly electroconductive polymer/nanotube composite material, which showed a substantial enhancement in dynamic hardness.     

The interdiffision of polymers

In contrast to interdiffusion in simple liquids, interdiffusion of polymeric chains is dominated by their intertanglement and their large size. These properties profoundly reduce both the molecular mobilities and the role of entropy in driving the mixing. The resulting diffusional processes have only recently been studied. Such studies reveal a wide spectrum of behavior ranging from accelerated interdiffusion (for strongly compatible chains) to its suppression below the critical point for phase separation. Effects that are still poorly understood include the initial disposition at interfaces of the chains' ends (through which diffusion proceeds by reptation) and the need for cooperative motion, which can strongly magnify local friction.     

刺云实胶对魔芋葡甘聚糖分子链拓扑缠结的分析

将魔芋葡甘聚糖(KGM)与刺云实胶(TG)以一定的比例复配,通过旋转流变仪测定KGM-TG复配体系的相关流变特性,结合分子动力学和拓扑分析学研究KGM和TG分子的作用方式,探讨刺云实胶对魔芋葡甘聚糖分子链拓扑缠结的作用机理。结果表明,刺云实胶与KGM分子间存在协同增效作用,分子链间的主要作用是氢键作用力,刺云实胶的添加使得KGM分子链间的拓扑缠结强度与密度的增强,复配体系的力学性能得到加强。从分子作用本质上验证了刺云实胶与KGM之间流变性能的理化实验结果。     

Effects on pesticide spray drift of the physicochemical properties of the spray liquid

This research was on the effect of the physicochemical properties of the spray liquid on pesticide spray drift. Ten pesticide spray liquids with various physicochemical properties were selected for study. Some of these spray liquids were also examined with the addition of a polymer drift-retardant. In the first part, laboratory tests were performed to measure surface tension, viscosity, evaporation rate and density of the spray liquids. Subsequently, drift experiments were performed in a wind tunnel. From the results it was found that the dynamic surface tension is a major drift-determining factor, and also that the addition of a polymer drift-retardant can reduce drift significantly by increasing the viscosity. Drift reduction was found to be less effective with spray liquids of emulsifiable and suspendable formulation types than with spray liquids of water-dispersible granules and powders.     

Polymer brushes

Polymers attached by one end to an interface at relatively high coverage stretch away from the interface to avoid overlapping, forming a polymer "brush." This simple picture may serve as the basis for models in diverse interfacial systems in polymer science, such as polymeric surfactants, stabilized suspensions of colloidal particles, and structures formed by block copolymers. The structure and dynamics of polymer brushes have been the subject of considerable theoretical and experimental activity in recent years. An account is given of recent advances in theoretical understanding of stretched polymers at interfaces, and the diverse experimental probes of systems modeled by brushes are briefly reviewed.     

In vitro study of complexing properties of high-molecular water-soluble polymer in relation to model protein

The objective of this paper was to study in vitro the complexing ability of anionic polyelectrolyte in relation to model protein to develop a new type of polymeric feed additive for agricultural animals. It has been established that the investigated polymer with a molecular weight of 1 × 10⁶ forms interpolymer complexes with model protein, namely, bovine serum albumin (BSA). The degree of the formation of these complexes significantly depends on the acidity of the medium and the ratio of components. It has been established that these complexes are formed in a wide interval of the ratio of components, i.e., both in the excess of protein in relation to polymer and in the excess of polymer in relation to protein. It has been revealed that 2.0–2.5% of BSA is complexed under conditions that are similar to the conditions of ruminant forestomachs at reaction medium pH 5.0–6.5 and a polymer-to-protein weight ratio of approximately 0.3 × 10^–3.     

Multicompartment micelles from ABC miktoarm stars in water

By combining three mutually immiscible polymeric components in a mixed-arm star block terpolymer architecture, we have observed the formation of a previously unknown class of multicompartment micelles in dilute aqueous solution. Connection of water-soluble poly(ethylene oxide) and two hydrophobic but immiscible components (a polymeric hydrocarbon and a perfluorinated polyether) at a common junction leads to molecular frustration when dispersed in aqueous solution. The incompatible hydrophobic blocks form cores that are protected from the water by the poly(ethylene oxide) blocks, but both are forced to make contact with the poly(ethylene oxide) by virtue of the chain architecture. The structures that emerge depend on the relative lengths of the blocks and can be tuned from discrete multicompartment micelles to extended wormlike structures with segmented cores.     

Linking models of polymerization and dynamics to predict branched polymer structure and flow

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.     

新型的分子印迹固相萃取小柱分离富集饲料中的三聚氰胺

[目的]研制一种新型的分子印迹固相萃取小柱,用于分离富集饲料中的三聚氰胺。[方法]采用本体聚合法制备了三聚氰胺分子印迹聚合物,以该聚合物为填料制作三聚氰胺分子印迹固相萃取(MIP-SPE)小柱,优化了固相萃取条件,应用高效液相色谱(HPLC)法测定浓缩液中的三聚氰胺含量。[结果]自制的MIP-SPE小柱可以从饲料中选择性地分离、富集三聚氰胺,有效地去除饲料中的复杂基质,加标回收率约为95%。MIP-SPE小柱与空白印迹固相萃取小柱(NMIP-SPE)的对比试验表明,前者的萃取效率明显高于后者。[结论]自制的新型三聚氰胺MIP-SPE小柱可用于饲料提取液中的三聚氰胺高选择性分离富集,使用简便,具有广阔的应用前景。     

Representing Topological Relationships Among Heterogeneous Geometry-Collection Features

Topological relationships between two spatial features represent important knowledge in Geographical Information Systems (GIS). In the last few years, many models that represent topological relationships have been proposed. But these models cannot represent the topological relationships between heterogeneous geometrycollection features, which are composed of different dimensional geometries. In this paper, the formal definition of regular heterogeneous geometrycollection and regularization rules are given. Based on the spatial model, two methods for representing topological relationships between these complex features are proposed. The first method is Dimensionally Extended Nine-Intersection Model Based on Components (DE-9IMBC) that extends Dimensionally Extended Nine-Intersection Model (DE-9IM) and takes into account the topological relationships between components of these complex features. The advantage of DE-9IMBC is that a large number of different topological relationships can be checked. The second method extends the definitions of topological relationships in Open Geodata Interoperability Specification (OpenGIS), and redefines the seven named topological relationships: {Disjoint, Touches, Within, Crosses, Overlaps, Contains and Equal}, to represent the topological relationships between heterogeneous geometrycollection features. It is proven that the seven extended topological relationships are complete and mutually exclusive, and they are suitable for being embedded in spatial query languages.     

Pattern formation in homogeneous polymer solutions induced by a continuous-wave visible laser

We report an unexpected nonphotothermal material organization induced by continuous-wave visible laser light at low power levels. This effect is observed along the laser beam propagation direction in fully transparent entangled solutions of common homopolymers featuring sufficiently high molecular mass and optical anisotropy along the chain backbone. The resulting formation of long-lived stringlike or dotlike patterns on the micrometer scale, probed by dark-field coherent imaging, depends on the molecular mass, architecture, solvent nature, and polymer concentration. Electrostrictive and alignment forces as well as chain cooperativity are responsible for the osmotic compression of the polymer solute. Subsequent waveguiding effects induce autoamplification and "pattern writing" upon prolonged illumination. This wave-medium coupling could potentially lead to photorefractive, microoptics, and nanotechnology applications.     

Mechanism and dynamics of ion transfer across a liquid-liquid interface

A detailed molecular model for ion transfer across the interface between water and 1,2-dichloroethane provides insight into the mechanism of the transfer across the interface between two immiscible liquids. Extensive molecular dynamics computer simulations underscore the roles of surface roughness and capillary distortions in this process, demonstrate that ion transfer is an activated rather than a simple diffusive process, and provide a test for the applicability of theoretical models.     

Gas-phase polymerization: ultraslow chemistry

The mechanism of formation of polymer molecules in the gas phase is difficult to study because the involatile polymers tend to condense out of that phase. However, new techniques, involving the use of cloud chambers, have enabled workers to use the nucleation of liquid drops in supersaturated monomer vapors to detect single polymer molecules and therefore to work with so few simultaneously growing polymers that aggregation and condensation are avoided. Chain polymerization in which the chain carriers are either radicals or ions can therefore be studied in the vapor. Furthermore, the ability to work with such small concentrations of growing polymeric radicals, for example, makes it possible to avoid encounters between them that lead to recombination and the formation of "dead" polymers that are incapable of further growth. Many aspects of gas-phase polymerization can be studied including, besides radical and ion chains, ring-opening polymerization, initiation, radiation-induced polymerization, and especially "ultraslow" chemistry.     

天然气管道的减阻与天然气减阻剂

依据流体管道的流动规律和减阻增输机理,指出天然气管道的减阻原理是抑制贴壁薄层(厚度等于粗糙度)内的气体脉动。认为管壁内涂、壁面薄液膜、凝析液中注入液体减阻剂和在管道内壁上吸附气体减阻剂是天然气管道有效的减阻方法。研究结果表明,气体减阻剂应是一种具有极性端和非极性长链的分子聚合物或化合物。     

Influence of solid friction on polymer relaxation in gel electrophoresis

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.