Flexible Branches Research Articles

The influence of flexible side chains on the dimensions of flexible polymers

Molecular dynamics simulations of single chains in vacuo have been used to investigate the role of flexible branches on the dimensions and in particular the backbone rigidity of flexible polymers. A series of isotactic poly-α-olefin chains was chosen as a model system to study because of their simplicity of structure. Θ-conditions were simulated for each branched chain by careful adjustment of the Van der Waals parameters, and properties such as end-to-end length, radius of gyration and persistence length were measured. As expected the backbone rigidity of the polymers was found to be determined by a balance between the coiling of the backbone induced by a gauche state at the branch point and the size of the side chain. Trends in persistence length with branch content compared favourably to the previous Rotational Isomeric State and experimental measurements of the characteristic ratio. The stiffening of the backbone under good solvent conditions was also investigated and it was found that the persistence length could be systematically increased by increasing the size of the side group, with the shape of the side group also making a small difference to the sampled backbone conformations.

Design of the ITER Shielding Blanket

The ITER blanket system removes the surface heat flux from the plasma and from bulk heating by the neutrons, reduces the activity in the vacuum vessel (W) structural material to the level allowable to ensure vessel reweldability for the ITER fluence goal and, in combination with the vacuum vessel, protects the superconducting coils and other ex-vessel components from excessive nuclear heating and radiation damage. The blanket system contributes with its eddy currents to the passive stabilization of the plasma motion. It minimizes the effects of electromagnetic loads on the VV due to plasma disruptions, and provides a well defined load path to the VV for net vertical and horizontal loads arising from vertical displacement events (VDE's). The system is designed to allow the possibility of replacing the shield with a breeding blanket, within the same dimensional, maintenance, and coolant constraints, to provide the tritium to meet the technical objectives of the Enhanced Performance Phase.The basic blanket system concept as well as the arrangement and function of its components is essentially unchanged from that established in 19951. However, as discussed in this paper, the design of each component has progressed significantly as a result of the detail design and technical analysis efforts of the last two years. The main components of the blanket system are:• A back plate: a structure comprising a double wall shell that supports the first wall/shield modules and routes the coolant water to them.• First wall/shield modules: comprising a plasma facing first wall (FW) section, and a shielding (or later breeding) section. Primary wall and baffle modules are distinguishable by the function of their FW.• Limiters: define the plasma boundary during plasma start-up and shutdown and are located in equatorial ports.• Flexible connectors, electrical straps, and branch pipes: the remote handling compatible structural, electrical, and cooling connections between the modules and back plate.• Filler shields: shielding permanently mounted to the back plate in the triangular gaps between FW/shield modules.The system will use austenitic stainless steel 316L(N)-IG (ITER Grade) as the primary structural material cooled by water with inlet conditions of 3.8 MPa and 140°C. The plasma facing surface of the FW will be beryllium except the lower region of the baffles, where tungsten is used. The electrical straps and heat sink layer in the FW will be copper alloy. A titanium alloy is the prime candidate material for the flexible connectors.

Orientational ordering in some nematogens deviating from the classical rod-shape

Several new laterally substituted liquid crystalline compounds have been synthesized. They have the same main core which contains four rings (two aromatic, two alicyclic) with two lateral substituents introduced on the same side of one of the inner rings. One of the substituents is a 4-X-benzyloxy group (X=CH3,CN,Cl) and the other is a hexyloxy chain. The presence of the lateral aromatic substituent makes these compounds deviate markedly from the classical rod-shape. However, a wide enantiotropic nematic phase is present for all the compounds. The order parameters of the chain and the para -disubstituted aromatic rings were obtained by using a 2D 13C NMR technique with variable angle spinning. The temperature dependence of the order parameters was estimated using 13C chemical shifts with slow spinning of the sample parallel to the magnetic field. The results indicate that the two lateral substituents are more or less folded back along the mesogenic core. Thus, the flexible lateral chain is found to be roughly aligned with the molecular long axis, whereas the para axis of the less flexible aromatic branch makes a considerable angle with the molecular long axis imposed by the core, substantially increasing the mean width of the molecule. The core ordering does not seem to be influenced by the type and position of the substituents. The folding back of the lateral chain and the substantial tilt of the lateral aromatic branch with respect to the core main axis are confirmed by the X-ray structure of a parent compound.

Rheological, thermodynamic, and structural studies of linear and branched alkanes under shear

The rheological, thermodynamic, and structural behavior of linear and branched alkanes in simple shear is investigated using nonequilibrium molecular-dynamics simulation of united atom model fluids. Our results for the zero-shear viscosity of pure linear alkanes as well as mixtures of alkanes are in reasonable agreement with experiment. Simulation results for intermediate molecular weight linear alkanes indicate that the simple models employed here are capable of describing the pressure and temperature dependence of the viscosity. More importantly, our calculations indicate that addition of short, flexible branches to alkanes leads to a viscosity enhancement of a factor of 2 or more, thereby offering interesting possibilities for formulation of lubricants with specific properties.

On the Geometric Stiffness Matrices in Flexible Multibody Dynamics

A flexible branch body in a multibody chain undergoing large overall motions experiences geometric stiffening effects due to its own motions and interbody forces arising from bodies outboard of it. The self-motion effects are modelled through the use of 21 geometric stiffness matrices, in the literature, for an arbitrarily shaped flexible body. In this paper, the effect of interbody forces on the flexible body dynamics is modelled using up to 6 additional geometric stiffness matrices per outboard joint. Simulation results presented indicate that for a flexible body consisting of outboard bodies one may consider only the interbody force effects, without loss of accuracy, and thus significantly reduce the computational burden. This approach is especially suitable for multibody systems such as the Shuttle Remote Manipulator System, where the long boom masses are of similar magnitude, or when the payloads are several orders of magnitude larger than the link masses.

Design of miscible polyolefin copolymer blends

AbstractTheoretical guidelines are established for designing miscible blends of amorphous polyolefin copolymers. On the basis of calculations for an athermal and incompressible model of copolymer melts, limits are placed on the compositions and structural differences between blend components that are consistent with thermodynamic stability of a single liquid phase. Specific cases analyzed include binary blends of random copolymers containing short branches and blends of graft polymers with long flexible branches, either periodically or randomly placed. The predictions are shown to be in good agreement with recent experimental studies of miscibility in model polyolefin copolymer blends. © 1995 John Wiley & Sons, Inc.

Liquid crystal formation of multibranched polystyrene induced by molecular anisotropy associated with its high branch density

Polarization microscopic observation was carried out for solvent-cast films and solutions of multibranched polystyrenes. The multibranched polystyrenes were prepared by the radical polymerization of ω-methacryloyloxyethyl polystyrene macromonomers of different molecular weights. It was found that multibranched polystyrene, with long flexible polystyrene chains and without any mesogenic groups, form the mesomorphic phase depending on the branching architecture of the polymer molecule. The formation of the liquid crystalline phase indicates that the multibranched polystyrenes possessing large branch number and sufficient branch length behave as rod-like molecules. The large molecular anisotropy might be caused by the stretching of backbone chain to maximize the conformational entropy of the individual flexible branch chains. The cast films of the multibranched polystyrene/linear polystyrene blend (50/50 by weight) showed the phase-separated structure of the mesomorphic domains dispersed in the isotropic matrix.

Outboard body effects on flexible branch body dynamics in articulated multibody systems

Investigations addressing modeling of component flexible bodies in articulated multibody systems have typically been concerned only with the boundary conditions at the inboard end of a body. It is shown here that the boundary conditions at the end (or joint) leading to the outboard bodies of a branch body play an important role as well, especially in nonplanar multibody systems. An example for such a system is a uniform flexible beam fixed at one end and with a rotating end mass attached at the other. The end mass is offset from, and rotates in a plane perpendicular to, the beam axis. This results in a time-varying boundary condition on the beam bending moment. Shape functions that account for this time-varying boundary condition are presented in this paper, and using these, the associated dynamic equations are derived. The performance of these shape functions is compared to that of three traditional mode sets, using numerical simulations. These results demonstrate the limitation of the latter mode sets.

Hybrid molecular assemblies in combinations of a synthetic peptide lipid with macrocyclic cyclophanes

AbstractHybrid molecular assemblies were prepared as combinations of a synthetic lipid, involving an L‐alanine residue interposed between an anionic head group and a hydrophobic double‐chain segment, with macrocyclic hosts, cage‐type cyclophanes having a rigid molecular framework and peptide cyclophanes bearing four flexible branches. Each host was completely incorporated into a relatively polar domain close to the vesicular surface, as confirmed by ultrasensitive differential‐scanning calorimetry and dynamic‐light‐scattering measurements as well as by gel‐filtration chromatography and electronic‐absorption spectroscopy. Perturbation effects of the hosts on the membrane structure were characterized on the basis of steady‐state fluorescence polarization, evaluated using a fluorescent probe, 1‐[4‐(trimethylammonio)phenyl]‐6‐phenyl‐1,3,5‐hexatriene iodide, and thermodynamic parameters associated with the phase transition between gel and liquid‐crystalline states. The cage‐type cyclophanes caused a disturbance of the orderly bilayer lipid membrane in the liquid‐crystalline phase due to their rigid conformational framework, while the peptide cyclophanes generated little effect on the lipid fluidity. The hybrid molecular assembly formed with the lipid and the cage‐type host exercised effective molecular recognition toward α‐amino acids with an aromatic moiety through hydrophobic, π‐π and chirality‐based interactions; L‐ and D‐tryptophan were strongly incorporated into the host cavities as confirmed by 1H‐NMR spectroscopy. On the other hand, the hybrid assembly showed no capacity to bind aliphatic guests, such as L‐methionine and L‐threonine.

Self-bonding anisometric polymer masses and equipment for manufacturing them

An anisometric polymer mass (APM) is a set of particles of complex form and relatively small size. The form of the particles, depending on preparation conditions, may be fibrous, dendritic-fibrous, film type, split-film type, or transitional between these. The length of the main stem of the particles is 0.1-10 ram, but most often it does not exceed 2-3 ram; and the diameter (thickness) varies in the range 2-50/~m; the number and length of the branchings are variable. A basic property of APM is their ability to undergo self-bonding in bulk, caused by the specific morphology of the particles. Self-bonding is assured by interweaving of the anisometric particles due to the flexible main branch of the particles and of the side branches with formation of a strong steric network. Thanks to this there is a possibility of obtaining both macrotwo-dimensional (sheet or band) materials and also of macro-three-dimensional (bulk) particles from APM only or in a composition with maumade fibres or other highly dispersed fillers. In particular, based on APM the following have been prepared and are widely used: very high classes of electrical insulating materials with respect to heat resistance; heat-resistant honeycomb constructions for flying apparatus and other forms of transport; and filter materials, including those for microfiltration, and others. In view of the specific and diverse morphology of particles of these masses, and their ability to undergo self-bonding and the process, mainly by the so-called vapor-making method, in the literature they are sometimes called fibrids (a hybrid of fibre and film), or synthetic pulps, or fibrous film binders (FFB) [1]. A number of ways are known for preparing APM directly from monomers, solutions, or polymer melts, or from twophase disperse systems or macromonolithic fibres or films. Selection of a manufacturing method is determined primarily by requirements for consumer properties in the end articles, and depends on the features of the raw material being processed and technico--economic indices in manufacturing. At present two ways of preparing APM have been realized on an industrial or pilot-plant schedule, called in technological jargon either hydrodynamic or explosive. The hydrodynamic method of preparing APM, whose block scheme is shown in Fig. 1, makes it possible to prepare a product from all soluble polymers, including in heat-resistant and high-boiling solvents. If we seek an analog in manmade fibre technology, this method may be formally assigned to the wet metho~ts, since it proposes the use of precipitation to separate the polymer from the solution. A description of this process in the part of interaction within the polymer--solvent--precipitant system may be performed from the thermodynamic, kinetic, or phenomenological positions, just as in spinning fibres by the wet method. The fundamental difference between spinning an APM and spinning fibres consists in the hydrodynamics of the process. In a short exposition it comes down to this, that while in fibre technology hydrodynamic processes should not negatively "affect the stability of the liquid polymeric jet, in APM technology the converse is true: the hydrodynamic processes are aimed at breaking up the polymer jet and elements in its breakdown to form liquid particles which are desired in form and dimensions and fixing them as a result of polymer solidification processes. The characteristics of the APM may be assigned by the type (Puazelian or Cuette) or by the regime (laminar, laminar spiral, or turbulent) of precipitation bath flow. And in essence the main scientific problem consists in separating in time the hydrodynamic phenomena and the processes which ensure polymer solidification. The observable hydrodynamic phenomena may be described on the basis of concepts previously developed in classical hydrodynamics. But by virtue of the complexity of the phenomena which take place in the variable field of a number of solution parameters (viscosity, density, surface tension, etc.), the results obtainable give only a qualitative picture. Standard equipment is used to make up and prepare the polymer solution and precipitation bath for spinning. Special types of apparatus have been developed for carrying out the spinning process: cylindrical (tank) with stirrers of various

Physical Models for Exploring DNA Topology

Two types of physical models have been developed for treating DNA molecules whose topology is of interest. The two model motifs combine jacks-and-straws molecular representations with flexible tubing in different proportions. Both motifs present a low-resolution construct of DNA that retains helix axes, strand individuality and the distinguishability of the major and minor grooves. Molecules whose double helix axes are branched are modelled by stiff double helices and flexible branch sites. Supercoiled and knotted DNA molecules are modelled on a smaller scale, in a system in which a flexible backbone is supported by a series of stiff helical struts; removal of this scaffolding immediately reveals the linking of the strands. The models are light and easy to construct. They may be used either for demonstrations or as a research tool that assists the interpretation data.

Positional behavior, selective use of habitat substrate and associated non-positional behavior in free-rangingColobus guereza (Rüppel, 1835)

Positional behavior (posture and locomotion), associated social/maintenance behaviors and selective utilization of available habitat structure were recorded for free-rangingColobus guereza occupying a remant montane forest near Limuru, Kenya. This research design focuses on relationships between anatomy, behavior, and the environment and provides a framework for interpreting adaptations inColobus. Quantitative analyses show daily activity patterns. These monkeys spend most of their time in above branch postural activities, primarily in sitting postures. Locomotion accounts for less than one-fifth of positional behavior. In both postural and locomotor sequences, the forelimbs may be placed above the shoulder complex, often in abducted positions. Suspensory positional behavior is rare. Most activity occurs within Zone II, flexible branches of medium diameter and less than 45° in grade. Selective use of the substrate and preferred positional modes are related to variables such as social/maintenance activities, weight distribution of the link system and substrate configuration. These data suggest that, (a) black and white colobus do not exhibit behavior associated with so-called “semibrachiation” and (b) bio-behavioral adaptations involving foraging and feeding strategies may account, in part, for observed morphological differences among Old World monkeys.

Great Destruction of Birds' Eggs and Nestlings in the Sierra Nevada

AS WE approached Cisco, Placer County, California, toward evening of June 7, 1912, the air seemed filled with the songs and call-notes of mountain birds. Observing that we had reached a section where birds were more numerous than is usual at so high an altitude as 5900 feet, I stopped my automobile at the only hotel in that charming resort and engaged accommodation for Mrs. Ingersoll and self. Knowing of no locality in the Sierras where small birds nested more plentifully than in that particular place, I anticipated the pleasure of adding much choice material to my collection of eggs. A few days' search, however, convinced me that I was not the only nest hunter, and that the Bluefronted Jays had a great advantage over one who collects full sets only. Jays were no more abundant than in similar places elsewhere, but these particular birds doubtless had an extra strong desire for eggs and naked birds. No jays were detected in the act of eating well-feathered young. Other natural enemies were doubtless the cause of some of the nests being tenantless. But as the jays were the only robbers caught in the act of taking eggs and young, the principal havoc is attributed to them and to an unseasonable snow fall. It is to be hoped that birds in the surrounding localities were more fortunate in raising their young. For a wide spread destruction like that at Cisco would tend to wipe some species out of existence. Following a week of delightful weather, a cold rain began falling on the morning of June 22, by night turning to sleet. At six o'clock on the morning of June 23 there was a depth of three and one-half inches of snow on the level. This snow was of a wet, clinging nature, weighing down every leaf and twig, and causing large branches and limbs of some deciduous trees to break. Clumps of bushes were generally weighted down to about half their height on the previous day. Many nests that were on flexible branches had their contents spilled out, while those built against trunks of small trees or between the main stems of bushes were later in the day bombarded with huge chunks of snow dislodged by the wind. This permitted branches to spring violently up to their accustomed position, a further cause of destruction. Horizontal branches of large fir trees drooped and crushed nests that chanced to be located between them. It is easy to imagine that many sleeping birds were crushed to death as the snow-laden branches quietly settled on them. Personally I know of two instances. A brooding Audubon Warbler was killed, and two of her three eggs broken, in the nest situated thirty feet above the ground on the branch of a fir. The other instance was that of a Western Wood Pewee picked up from the ground with nest on dead aspen limb that had broken off and fallen from a height of some twenty feet. Another Wood Pewee's nest destroyed in the same manner, was found later in the day. While searching a large pine stub for the nest of a Sierra Creeper, I discovered a female Calaveras Warbler under a partially detached piece of bark. Her feathers were quite wet, and as the crevice was rather dry, I presume this ground-nesting bird was flooded out of her home and sought shelter as death approached.