78 research outputs found
Non-extensivity of the chemical potential of polymer melts
Following Flory's ideality hypothesis the chemical potential of a test chain
of length immersed into a dense solution of chemically identical polymers
of length distribution P(N) is extensive in . We argue that an additional
contribution arises ( being the
monomer density) for all if which can be traced back to the
overall incompressibility of the solution leading to a long-range repulsion
between monomers. Focusing on Flory distributed melts we obtain for , hence,
if is similar to the typical
length of the bath . Similar results are obtained for monodisperse
solutions. Our perturbation calculations are checked numerically by analyzing
the annealed length distribution P(N) of linear equilibrium polymers generated
by Monte Carlo simulation of the bond-fluctuation model. As predicted we find,
e.g., the non-exponentiality parameter to decay
as for all moments of the distribution.Comment: 14 pages, 6 figures, submitted to EPJ
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POLLINATION ECOLOGY IN PENSTEMON: MECHANISMS OF REPRODUCTIVE ISOLATION IN PENSTEMON VIRENS AND PENSTEMON SECUNDIFLORUS
Two distantly related species of Penstemon, Penstemon virens and Penstemon secundiflorus frequently flower contemporaneously throughout an overlapping range in the foothills and Front Range of Colorado. In cases where populations are sympatric (co-occurring) and hybrids do not form, non-geographic and non-temporal boundaries must be present that limit or prohibit gene flow between species. I examined two possible contributors to such boundaries, pollinator activity and floral traits, through a field pollination experiment in the foothills of central Colorado in the summer of 2016 and found that both species exhibit distinctive floral traits that may affect both pollinator behavior and mechanical compatibility. Field collection of floral visitors (totaling 68 individuals) indicated that only five bee species from two genera (Lasioglossum and Hylaeus) were shared between P. virens and P. secundiflorus, suggesting that pollinator behavior contributes to limiting gene flow between these two species. Thus, the limited overlap in pollinator species and floral morphology likely severely limit the capacity for hybridization between these two species of Penstemon. Finally, I found that floral traits could not be used to predict floral visitors in a way that is consistent with prior research on pollination syndromes in the genus of Penstemon. Further field research in pollination ecology is needed to assess pollinator behavior in response to the variety of floral traits exhibited in the genus and to assess the fitness and life history of these two iconic flowering-plant species of the Colorado foothills
Why polymer chains in a melt are not random walks
A cornerstone of modern polymer physics is the `Flory ideality hypothesis'
which states that a chain in a polymer melt adopts `ideal' random-walk-like
conformations. Here we revisit theoretically and numerically this pivotal
assumption and demonstrate that there are noticeable deviations from ideality.
The deviations come from the interplay of chain connectivity and the
incompressibility of the melt, leading to an effective repulsion between chain
segments of all sizes . The amplitude of this repulsion increases with
decreasing where chain segments become more and more swollen. We illustrate
this swelling by an analysis of the form factor , i.e. the scattered
intensity at wavevector resulting from intramolecular interferences of a
chain. A `Kratky plot' of {\em vs.} does not exhibit the plateau
for intermediate wavevectors characteristic of ideal chains. One rather finds a
conspicuous depression of the plateau, ,
which increases with and only depends on the monomer density .Comment: 4 pages, 4 figures, EPL, accepted January 200
Introducing Fly Ash into LSI\u27s Recipe
There has been published data that states that concrete producers can use Fly Ash as an alternative to Portland cement and still generate similar or better strength properties. By applying these principles to our industry and products, what new processes will need to be put in place and followed? What is the correct combination/portion of Fly Ash that will produce similar and better results as well as the adjusted new recipes? What type of cost is associated with the introduction of this new ingredient? Lastly, how does this affect the quality and reputation of the product
Contribution à la caractérisation et à la modélisation de transistors bipolaires de puissance intégrés dans une filière BICMOS submicronique
L’émergence de la troisième génération de téléphone portable a fait évoluer les usages. Les utilisateurs peuvent désormais bénéficier d’un accès haut débit à l’Internet sans fil ce qui a rendu les services de communication multimedia: visiophonie, MMS, réception de programmes télévisés... Cette diversification du mode d’utilisation d’un portable a fait des amplificateurs de puissance des éléments prépondérants pour la transmission de l’information. C’est pourquoi cette étude propose une contribution à la caractérisation et à la modélisation de transistor de puissance RF intégrés dans une filière submicronique. Dans ce cadre d’application, il est important de tenir compte de l’influence de la température sur la réponse électrique d’un transistor bipolaire; cet effet est d’autant plus marqué pour les applications de puissance au regard des densités de courant mises en jeu. C’est pourquoi, une analyse des lois en température des paramètres du modèle compact HICUM/L2 est présentée, ainsi que les méthodes d’extraction associées. Puis, les phénomènes d’auto-échauffement et de couplage thermique dans les transistors de puissance sont étudiés à l’aide de simulations physiques et de caractérisations électriques pour mettre au point un modèle nodal SPICE. Finalement, les transistors de puissance sont caractérisés à l’aide de mesure load-pull en appliquant un signal à deux tons sur leur base. Ces caractéristiques sont comparées à des simulations Harmonic Balance de manière à valider le comportement grand signal du modèle HICUM/L2 et sa capacité à modéliser les phénomènes d’intermodulations fréquentielles.The emergence of the mobile phone third generation made evolved the uses. From now on the users can have accesses to the Internet, which made the services of communication multi-media: video-telephony, MMS... That diversification of mobile phone uses makes power amplifiers a preponderant element for the transmission of information. That’s why this study represents a contribution to the characterization and the modeling of RF power transistor integrated in a submicronic technology. For such applications, it is fundamental to consider the temperature influence on the electrical response of a bipolar transistor; this effect is even more important within the framework of power applications due to the current densities involved. Consequently, an analysis of the temperature laws of the HICUM/L2 model parameters is first presented, as well as the associated extraction methods. Then, self-heating and thermal coupling phenomena in powers transistors are studied using physical simulations and electrical characterization to develop a nodal SPICE model. Finally, the power transistors are characterized using load-pull measurement by applying a two-tone signal. These measurement are compared to Harmonic Balance simulations so as to validate the large signal behavior of HICUM/L2 model and its capacity to model frequency intermodulations
Static Rouse Modes and Related Quantities: Corrections to Chain Ideality in Polymer Melts
Following the Flory ideality hypothesis intrachain and interchain excluded
volume interactions are supposed to compensate each other in dense polymer
systems. Multi-chain effects should thus be neglected and polymer conformations
may be understood from simple phantom chain models. Here we provide evidence
against this phantom chain, mean-field picture. We analyze numerically and
theoretically the static correlation function of the Rouse modes. Our numerical
results are obtained from computer simulations of two coarse-grained polymer
models for which the strength of the monomer repulsion can be varied, from full
excluded volume (`hard monomers') to no excluded volume (`phantom chains'). For
nonvanishing excluded volume we find the simulated correlation function of the
Rouse modes to deviate markedly from the predictions of phantom chain models.
This demonstrates that there are nonnegligible correlations along the chains in
a melt. These correlations can be taken into account by perturbation theory.
Our simulation results are in good agreement with these new theoretical
predictions.Comment: 9 pages, 7 figures, accepted for publication in EPJ
Are polymer melts "ideal"?
It is commonly accepted that in concentrated solutions or melts
high-molecular weight polymers display random-walk conformational properties
without long-range correlations between subsequent bonds. This absence of
memory means, for instance, that the bond-bond correlation function, , of
two bonds separated by monomers along the chain should exponentially decay
with . Presenting numerical results and theoretical arguments for both
monodisperse chains and self-assembled (essentially Flory size-distributed)
equilibrium polymers we demonstrate that some long-range correlations remain
due to self-interactions of the chains caused by the chain connectivity and the
incompressibility of the melt. Suggesting a profound analogy with the
well-known long-range velocity correlations in liquids we find, for instance,
to decay algebraically as . Our study suggests a precise
method for obtaining the statistical segment length \bstar in a computer
experiment.Comment: 4 pages, 3 figure
Compression modulus of macroscopic fiber bundles
We study dense, disordered stacks of elastic macroscopic fibers. These stacks
often exhibit non-linear elasticity, due to the coupling between the applied
stress and the internal distribution of fiber contacts. We propose a
theoretical model for the compression modulus of such systems, and illustrate
our method by studying the conical shapes frequently observed at the
extremities of ropes and other fiber structures. studying the conical shapes
frequently observed at theextremities of ropes and other fiber structures
Single chain structure in thin polymer films: Corrections to Flory's and Silberberg's hypotheses
Conformational properties of polymer melts confined between two hard
structureless walls are investigated by Monte Carlo simulation of the
bond-fluctuation model. Parallel and perpendicular components of chain
extension, bond-bond correlation function and structure factor are computed and
compared with recent theoretical approaches attempting to go beyond Flory's and
Silberberg's hypotheses. We demonstrate that for ultrathin films where the
thickness, , is smaller than the excluded volume screening length (blob
size), , the chain size parallel to the walls diverges logarithmically,
with . The corresponding bond-bond
correlation function decreases like a power law, with
being the curvilinear distance between bonds and . % Upon increasing
the film thickness, , we find -- in contrast to Flory's hypothesis -- the
bulk exponent and, more importantly, an {\em decreasing}
that gives direct evidence for an {\em enhanced} self-interaction of chain
segments reflected at the walls. Systematic deviations from the Kratky plateau
as a function of are found for the single chain form factor parallel to the
walls in agreement with the {\em non-monotonous} behaviour predicted by theory.
This structure in the Kratky plateau might give rise to an erroneous estimation
of the chain extension from scattering experiments. For large the
deviations are linear with the wave vector, , but are very weak. In
contrast, for ultrathin films, , very strong corrections are found
(albeit logarithmic in ) suggesting a possible experimental verification of
our results.Comment: 16 pages, 7 figures. Dedicated to L. Sch\"afer on the occasion of his
60th birthda
Distance dependence of angular correlations in dense polymer solutions
Angular correlations in dense solutions and melts of flexible polymer chains
are investigated with respect to the distance between the bonds by
comparing quantitative predictions of perturbation calculations with numerical
data obtained by Monte Carlo simulation of the bond-fluctuation model. We
consider both monodisperse systems and grand-canonical (Flory-distributed)
equilibrium polymers. Density effects are discussed as well as finite chain
length corrections. The intrachain bond-bond correlation function is
shown to decay as for \xi \ll r \ll \r^* with being
the screening length of the density fluctuations and a novel
length scale increasing slowly with (mean) chain length .Comment: 17 pages, 5 figures, accepted for publication at Macromolecule
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