Bimodal Networks Research Articles

Organizational Image Construction in a Fragmented Online Media Environment

This article examines the role of different online and social media channels in constructing organizational image. Bimodal network analysis was used to examine 6 months of self-presentation by natural supermarket chain Whole Foods through its online press room, blog, and microblogging account. The channels converged on a core set of terms, and overlapped in others, but each channel also addressed divergent aspects of the organization's projected image. This study suggests that the structural and social characteristics of these channels give them varying roles in the image construction process, creating new challenges for the public relations function in coordinating image management among various new media.

Drying Simulations of Various 3D Pore Structures by a Nonisothermal Pore Network Model

During drying of a porous medium, pores are subsequently invaded by gas upon evaporation of water, typically disintegrating the liquid phase into clusters. Pore network models describe this invasion process on the basis of void space geometry and physics of pore-scale events. In the recent modeling by Surasani et al., (2008) invasion percolation rules under nonisothermal conditions are described with strong coupling between heat and mass transfer. In this work, the drying of various three-dimensional (3D) cubic pore structures with mono- and bimodal pore radius distributions is simulated for convective heat and mass transfer boundary conditions. Drying rate curves differ significantly for mono- and bimodal networks. A pronounced first drying period is seen for the bimodal case; here, macropore channels empty first and water is evaporated from the still-wet micropores at the surface. Additionally, freely evolving temperature fields in conjunction with phase distributions are presented.

Rubber‐like thermosetting epoxy asphalt composites exhibiting atypical yielding behaviors

AbstractRubber‐like thermosetting epoxy asphalt composites (REACs) were prepared by curing epoxy resin (diglycidyl ether of bisphenol A) with curing agents system made up of maleated petroleum asphalt, adipic acid, and methylhexahydrophthalic anhydride. REACs' glass transition temperatures (Tg) and mechanics performances were investigated by differential scanning calorimetry (DSC) and universal tester, respectively. DSC results indicated that REACs had homogeneous phase structures, and theirTgwere lower than room temperature. Large elongation at break (εb), as high as 150–286%, and atypical yielding behaviors similar to so‐called “hard elastic” thermoplastics were observed simultaneously for the first time as to thermosetting epoxy composites at room temperature. The striking characteristics of REACs were ascribed to the formation of unique asphalt‐filled bimodal networks, consisting of simultaneous cured maleated asphalt short‐chain and dicarboxylic acid long‐chain interpenetrating bimodal networks and wherewith filled unmaleated asphalt. The experimental results suggest that these inexpensive REACs may find promising applications in many engineering fields. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

Extraction of Segment Orientation Distributions in Polymer Networks by Inversion of2H NMR Spectra through the Maximum-Entropy Method

We present a simple and computationally inexpensive strategy, to first estimate the average segment-orientation order parameter and then, based on the maximum-entropy method, extract distributions of this order parameter from the conventionally measured 2H NMR spectra. The proposed methodology allows for a more complete characterization of the segmental orientation behavior as compared to solely measuring the observed quadrupolar splittings. The latter approach only quantifies the segment orientation due to local excluded volume interactions, and does not account for the orientation arising from the network topology that is manifested through the width of the spectral “wings”. The application of the proposed strategy can be especially advantageous in polymer networks exhibiting highly heterogeneous segment-orientation responses (e.g., bimodal networks) and complex 2H NMR spectrum lineshapes. The methodology is validated with both molecular simulation and experimental results as well as applied to complex ...

Palladium supported on hierarchically macro–mesoporous titania for styrene hydrogenation

Hierarchically macro–mesoporous titania was synthesized by the simple dropwise addition of tetrabutyl titanate to an ammonia solution in the absence of surfactant molecules, and then calcined at different temperatures. The as-prepared titania was characterized by X-ray diffraction, N 2 adsorption–desorption analysis and scanning electron microscopy. The results showed that the hierarchically marcro–mesoporous structure of the titania was well preserved after calcination at 650 °C, indicating high thermal stability. The 500 °C calcined titania with the hierarchically bimodal pore network was impregnated with 0.5 wt.% of palladium and compared with 0.5 wt.% Pd/TiO 2 catalyst without such macrochannels. The novel macro–mesostructured Pd/TiO 2 catalyst exhibited higher catalytic activity for styrene hydrogenation due to the lower diffusion resistance of species inside the catalyst caused by the hierarchically macro–mesoporous bimodal structure.

Synthesis, structure and properties of a novel hybrid bimodal network elastomer with inorganic cross-links: The case of silicone–nanocrystalline titania

We describe an innovative concept of synthesizing a novel hybrid bimodal network elastomer with high strength–high ductility combination involving utilization of functionalized nanocrystalline titania as short-chain cross-links between neighboring elastomer chains. This subject is germane both fundamentally and from an application viewpoint. Silicone rubber is selected as the model elastomer. The short-chain cross-links are acrylic acid functionalized nanocrystalline titania that are an integral component of bimodal network structure of the elastomer. To delineate and separate the effects of functionalization from nanoparticle effects, a relative comparison is made between silicone rubber–titania nanocomposite (i.e. containing dispersion of titania as a reinforcement filler) and silicone rubber–titania hybrid network elastomer (i.e. titania as short chain cross-links). An important finding is that the effect of functionalized titania present as short chain cross-links is far more significant than non-functionalized titania present as reinforcement filler, on mechanical behavior. This is presently ascribed to the double bonds introduced to nanocrystalline titania via functionalization with acrylic acid that provide active sites for the cross-linking reaction resulting in inorganic bridging chains. The basic physical mechanisms that govern elastic recovery in hybrid bimodal network elastomer with short chain cross-links of functionalized nanocrystalline inorganic particles are discussed. The hypothesis of the study described here is that the hybrid bimodal network elastomer with short chain cross-links of functionalized nanocrystalline inorganic particles modifies the unimodal long chain network elastomer with consequent increase in modulus and high strength–ductility combination.

Hierarchical control of porous silica by pH adjustment: Alkyl polyamines as surfactants for bimodal silica synthesis and its carbon replica

Bimodal macro-mesoporous silica networks have been prepared in a simple one-pot synthesis using an inexpensive tetramine surfactant and tetraethoxysilane as a silica precursor. These novel materials show high pore volumes and templated mesopores (average pore size 3.0 nm) embedded in 20 nm thick walls forming interparticle large meso/macropores. The judicious control of the pH during the silica formation allows for the precise control of the interparticle condensation, likely due to the change in the interaction between the tetramine surfactant and the silica precursors. Finally, a highly porous carbon replica with bimodal porosity was prepared by using the bimodal silica as a hard sacrificial template. The microstructure of the silica template was accurately transferred to the carbon material obtaining high surface areas (up to 1300 m 2 g −1) and total pore volumes ≥2 cm 3 g −1.

Porosity and mineralogy evolution during the decay process involved in the Chellah monument stones

The objective of this work is to study the decay process involved in the historical Roman Chellah located in the Rabat city (Morocco). This monument is made up of porous calcarenite stone. Several samples, taken from altered and unaltered blocks, were analyzed by the water saturation, the mercury intrusion porosimetry techniques and using the scanning electron microscopy coupled to energy dispersive X-ray spectrometer (SEM–EDX). To perform a reliable chemical analysis, some samples were also analyzed by inductively coupled plasma-atomic emission spectroscopy (ICP-AES). The mercury porosimetry results show a bimodal porous network for this porous material, the deterioration process of these stones involved an increase in porous volume of about 2%. The lowest porosity observed in the unaltered block is connected to the presence of sparitic cement which causes a partial inter-granular porosity clogging. The highest porosity of the deteriorated block without crust is due to the increase in inter-granular space. SEM photographs of the unaltered sample show the presence of the porous primary grains, of ovoid forms and millimeter-length sizes, and of the secondary grains, of rhombohedric forms and micro size. Porosity is essentially located between the primary grains and can be completely clogged by secondary precipitations. Various forms of deterioration are observed on the altered samples such as the dissolution of the secondary grains edges, wells of dissolution and also the presence of argillaceous residues on the surface. This last was also detected by the ICP-AES and EDX analysis which show an increase of the silicon and aluminum contents toward the surface.

Evolutionary construction of geographical networks with nearly optimal robustness and efficient routing properties

Robust and efficient design of networks on a realistic geographical space is one of the important issues for the realization of dependable communication systems. In this paper, based on a percolation theory and a geometric graph property, we investigate such a design from the following viewpoints: (1) network evolution according to a spatially heterogeneous population, (2) trimodal low degrees for the tolerant connectivity against both failures and attacks, and (3) decentralized routing within short paths. Furthermore, we point out the weakened tolerance by geographical constraints on local cycles, and propose a practical strategy by adding a small fraction of shortcut links between randomly chosen nodes in order to improve the robustness to a similar level to that of the optimal bimodal networks with a larger degree O ( N ) for the network size N . These properties will be useful for constructing future ad hoc networks in wide-area communications.

Effect of meta-Carborane on Segmental Dynamics in a Bimodal Poly(dimethylsiloxane) Network

Bimodal networks of polydimethylsiloxane (PDMS) filled with varying amounts of icosahedral meta-carborane (m-CB) have been developed and characterized by broadband dielectric spectroscopy (BDS) and static 1H multiple quantum nuclear magnetic resonance (MQ NMR). Both BDS and MQ NMR showed evidence for a decrease in the polymer chain dynamics. BDS spectra quantified a normal-mode relaxation near 40 Hz at 40 °C. The frequency maximum observed for filled samples decreased with increasing m-CB content until contents greater than 5 wt %. The width of the relaxation spectrum increased with the addition of small quantities of filler and decreased with filler contents greater that 5 wt %. Agglomeration effects were observed at loadings greater than 5 wt % as manifest by the onset of low frequency Maxwell−Wagner−Sillars (MWS) processes. The MQ NMR data allowed the characterization of distributions of the residual dipolar couplings, <Ωd> and, thus, in the dynamic order parameter, Sb, consistent with the bimodal network architecture expected from the synthesis protocol used. Upon addition of less than 10 wt % m-CB filler, the mean <Ωd> for the longer chains increased by 46% and the width of the distribution increased by 33%. The mean <Ωd> for the shorter chains increased by much less, indicative of preferential dispersion of the filler particles in the long chain domains of the network structure. We conclude that the mechanism of reinforcement is likely free volume space filling at low loadings transitioning to complex molecular filler and polymer chain interaction phenomena at higher loadings.

Experiments and Simulations: Enhanced Mechanical Properties of End-Linked Bimodal Elastomers

We report on stress−strain and swelling results for polydimethylsiloxane bimodal networks, studied both experimentally and via Monte Carlo simulations. These end-linked networks were formed with negligible extent of soluble fractions, and reasonable agreement is found between experimental results and simulation data. We examine the variation in microstructure for networks with different concentrations of short chains. When the concentration of short chains is low, these chains aggregate during the end-linking process and lead to a heterogeneous network structure, while networks formed with higher short chain concentrations are more homogeneous. Short chains that are long enough to initially have a Gaussian conformation also produce the characteristic stress upturn and enhanced toughness previously reported in bimodal networks with non-Gaussian short chains. We find that it is the limited extensibility of the short chains at high concentrations and not the cluster formation of short chains at low concentrations that leads to the enhanced mechanical properties of these elastomers.

Examination of the effects of computationally determined network topology on an analytical constitutive model for bimodal elastomers

Bimodal elastomer networks, so called due to their nominal bimodal molecular weight distribution of starting oligomers, are of continued interest due to the enhanced strength and toughness seen in certain mixtures. Researchers have suggested that the enhanced properties stem from the particular micromechanics of the networks formed by these systems at these optimal compositions. This work extends an existing analytical constitutive model for bimodal elastomer networks by incorporating aspects of network topology, including network connectivity patterns and realistic chain length distributions, determined through computational simulations of the formation of the network structure. These factors are included as functions of bimodal composition and are shown to affect the predicted mechanical, optical, and orientation responses of the network. The extended model elucidates how the naturally occurring doubled connection topology creates a micro-mechanism that lowers overall chain orientation in the lower molecular weight component and recreates experimentally observed optical response phenomena. Specifically, the model predicts that the presence of the stiff, contractile, doubled connections forces the rest of the network to conform more to the macroscopic stretch ratio while reducing the measured average orientation of the lower molecular weight component in the system; the effect diminishes as the composition-dependent population of doubled connections in the system decreases.

Equity and Efficiency Analysis of Pricing Strategies in a Bimodal Network with Heterogeneous User Groups

This paper investigates the equity and efficiency of pricing strategies in a bimodal transport network with elastic demand and heterogeneous users on the basis of logit-based discrete choice models. The network consists of a railway line and a parallel highway with a bottleneck that provide daily commuting services between a residential area and a workplace. On the basis of two user groups differing in values of time, first-best pricing is shown to remain possible through the use of anonymous charges under the stochastic user equilibrium condition. Second-best pricing is also examined, in which an equity constraint is introduced to ensure that some users, if not all, are at least not worse off from implementing the pricing at expectation level. The efficiency, equity, demand realization, and modal split resulting from various pricing strategies are investigated and numerically validated in a simplified real network connecting the Kowloon urban area to Hong Kong, China, International Airport. It was found that the equity-constrained second-best pricing can result in a win-win situation for society and users. The efficiency and welfare gains depend heavily on the values of time of user groups and the number of users in each group.

Bimodality and Increased Amount of Crosslinker Enhances Tensile Properties of the Silicone Networks

AbstractThe effect of bimodality i.e. blending short and long chain (0 to 80 % w/w) silicone prepolymers, and that of concentration of the crosslinker on the tensile properties such as percent elongation at break (%Eb), ultimate tensile strength (UTS), 100 % modulus and elastic modulus (E) has been investigated. It was found out that the greater amount of crosslinker used for crosslinking provide an additional reinforcement to the silicone network whereas bimodality further significantly accentuates this effect. Remarkably, the %Eb of the silicone networks was found to increase to an average of 2.4 times than that of the monomodal network, cured in each of the three series of bimodal networks cured with 3.9, 9.1 and 12.3% of crosslinker. The optimum property in each case was observed at about 70 mol % of P100. The over all order of sensitivity of these properties up to about 70 mol % of short chain prepolymer has been observed to be as: %Eb &gt; UTS &gt; 100% modulus &gt; elastic modulus. It has been shown that in addition to the previously published reports the tensile properties are acutely dependent not only upon the degree of crosslinking and primary molecular weight; but also on bimodality and crosslinker concentration for preparation of the unfilled silicone polymer networks. The concept of phase inversion (or phase transition) associated with the optimum properties in polymer blends has for the first time been applied to the bimodal polymer networks. It has been observed that the maxima in tensile properties generally corresponds to phase inversion which takes place at 0.5 volume fraction (70 mol %) of short chain (P100) prepolymer which is in accordance with the literature for other systems.

Molecular traffic control for a cracking reaction

We investigate the conditions for reactivity enhancement of a catalytic cracking process in porous crystalline solids using the concept of molecular traffic control (MTC). Using dynamic Monte Carlo simulations we obtain a quantitative description of the MTC effect for a bimodal network of intersecting size-selective channels over a wide range of grain parameters, diffusivities and reaction constants, and external operating conditions. The results are compared with those obtained on a similar network but with only one type of channels, called the reference system. We find significant reactivity enhancement in the MTC system with respect to the reference system. This effect is observed to increase with the grain size, and also with longer segments between lattice intersections. We find that in certain regions of the microscopic reaction rate the reactivity enhancement is up to 68%.

Investigation of Network Heterogeneities in Filled, Trimodal, Highly Functional PDMS Networks by 1H Multiple Quantum NMR

The segmental order and dynamics of polymer network chains in a filled, tri-modal silicone foam network have been studied by static 1H Multiple Quantum (MQ) NMR methods to gain insight into the structure property relationships. The foam materials were synthesized with two different types of crosslinks, with functionalities, {phi}, of 4 and near 60. The network chains were composed of distributions of high, low, and medium molecular weight chains. Crosslinking was accomplished by standard acid catalyzed reactions. MQ NMR methods have detected domains with residual dipolar couplings ( ) of near 4 kRad/s and 1 kRad/s assigned to (a) the shorter polymer chains and chains near the multifunctional (f=60) crosslinking sites and to (b) the longer polymer chains far from these sites. Three structural variables were systematically varied and the mechanical properties via compression and distributions of residual dipolar couplings measured in order to gain insight in to the network structural motifs that contribute significantly to the composite properties. The partitioning of and the average values of the residual dipolar couplings for the two domains were observed to be dependent on formulation variable and provided increased insight into the network structure of these materials which are unavailable from swelling and spin-echo methods. The results of this study suggest that the domains with high crosslink density contribute significantly to the high strain modulus, while the low crosslink density domains do not. This is in agreement with theories and experimental studies on silicone bimodal networks over the last 20 years. In-situ MQ-NMR of swollen sample suggests that the networks deform heterogeneously and non-affinely. The heterogeneity of the deformation process was observed to depend on the amount of the high functionality crosslinking site PMHS. The NMR experiments shown here provide increased ability to characterize multimodal networks of typical engineering silicone foam materials and to gain significant insight into structure-property relationships.

Modeling an Elastic-Demand Bimodal Transport Network with Park-and-Ride Trips

This paper presents a network equilibrium formulation for modeling commuters' travel choices in a bimodal transport system with park-and-ride (P&R) trips while the total demand is elastic to the congestion level of the network. A super-network approach is adopted in the proposed model. It is assumed that commuters' trips are categorized into two types, auto mode only and a combined mode with both auto and transit modes. The former is referred to as the pure mode trip and the latter as the P&R mode trip. The proposed model simultaneously considers the commuter's choice of the pure mode versus the P&R mode, the choice of parking location for the pure mode, the choice of transfer point for the P&R mode, as well as the route choice for each mode. The demand elasticity of transport system, the capacity constraints of transport facilities, and the congestion interaction throughout the super-network are also explicitly incorporated into the proposed model. The results of the numerical experiment show the following key findings: (i) traditional parking/P&R models may overestimate or underestimate travel demand distribution over network; (ii) parking/P&R, transit scheduling, and carpooling schemes bring significant impacts on commuters' travel behavior and network performance; and (iii) different transport policies may be to some extent mutually substituted.

Elastomeric properties of polysiloxane networks: birefringence measurements on bimodal elastomers that are presumed to be spatially inhomogeneous

In-situ measurements of birefringence during simple extension were performed on a series of end-linked bimodal poly(dimethylsiloxane) elastomers that were templated to possess high cross-link density nanostructures. The strain birefringence showed marked deviations from predictions of linearized Gaussian rubber-like elasticity theory for a bimodal elastomer with well-developed structure (nanoscale to microscale). However, the stress-birefringence relation for this same network seemed to be described by Gaussian rubber-like elasticity theory. Reinforced bimodal networks that were macroscopically homogenous (although potentially possessing nanoscale structure) were observed to have strain birefringence scaling similar to theoretical predictions, although the magnitude was below the Gaussian prediction. Analysis of the strain birefringence shows that a non-linear, non-Gaussian effect (likely limited chain extensibility) can occur. However, in all cases the birefringence was found to have a significant linear regime over a large range of strains which suggests that limited chain extensibility may not be the sole contributor towards the well known “bimodal” reinforcement.

Optimizing transport in nanostructured catalysts: A computational study

A continuum modeling approach is used to compare the yield of catalyst particles called monodisperse, and containing only nanopores (i.e., micropores or narrow mesopores), to the yield of the same nanoporous particles to which larger diffusion channels have been introduced in various ways. Bidisperse and bimodal pore size distributions are considered; in the bimodal case, the large diffusion channels are allowed to vary in size throughout a particle, while in the bidisperse case they are all of the same size. Model-based optimization is performed with respect to the overall yield of a first-order reaction, optimizing the spatial distributions of both the volume fraction and the diameter of the diffusion channels. It is found that the yield of catalysts with an optimized bidisperse pore size distribution may be at least an order of magnitude increased with respect to the original monodisperse distribution, thanks to the optimized introduction of large pore channels. Remarkably, however, local variations in pore diameter and porosity in bimodal pore networks do not lead to significant further improvements. This calls for the synthesis of bidisperse catalysts with a carefully optimized size of the large pore channels and thickness of their nanoporous catalytic walls.

Elastomeric Properties of Polysiloxane Networks. Bimodal Elastomers that are Spatially Inhomogeneous and Others that are Very Broadly Multimodal

End‐linking poly(dimethylsiloxane) was used to prepare bimodal elastomers networks so as to have inhomogeneous nanostructures, and also to prepare others having very broadly multimodal chain‐length distributions. Macroscopic phase separation, probably high crosslink density clusters, was observed to occur in some of the bimodal networks. The mechanical properties in simple extension and in equilibrium swelling were measured. The bimodal elastomers that were not obviously inhomogeneous showed very good mechanical properties, but the macroscopically phase‐separated networks, and the broadly multimodal network were weak. Analysis of the Mooney‐Rivlin profiles suggests that the reinforcing mechanism could have a structural component in addition to that from the limited extensibilities of the short chains. The mechanical properties and the extents of swelling support the cluster conjecture, in accord with previous morphological studies on spatially‐inhomogeneous polysiloxane elastomers.