575 research outputs found
Segment Motion in the Reptation Model of Polymer Dynamics. I. Analytical Investigation
We analyze the motion of individual beads of a polymer chain using a discrete
version of De Gennes' reptation model that describes the motion of a polymer
through an ordered lattice of obstacles. The motion within the tube can be
evaluated rigorously, tube renewal is taken into account in an approximation
motivated by random walk theory. We find microstructure effects to be present
for remarkably large times and long chains, affecting essentially all present
day computer experiments. The various asymptotic power laws, commonly
considered as typical for reptation, hold only for extremely long chains.
Furthermore, for an arbitrary segment even in a very long chain, we find a rich
variety of fairly broad crossovers, which for practicably accessible chain
lengths overlap and smear out the asymptotic power laws. Our analysis suggests
observables specifically adapted to distinguish reptation from motions
dominated by disorder of the environment.Comment: 38 pages in latex plus 8 ps figures, submitted to J. Stat. Phys. on
September 18, 1997, please note part II on cond-mat/971006
Segment Motion in the Reptation Model of Polymer Dynamics. II. Simulations
We present simulation data for the motion of a polymer chain through a
regular lattice of impenetrable obstacles (Evans-Edwards model). Chain lengths
range from N=20 to N=640, and time up to Monte Carlo steps. For we for the central segment find clear -behavior as an
intermediate asymptote. The also expected -range is not yet developed.
For the end segment also the -behavior is not reached. All these data
compare well to our recent analytical evaluation of the reptation model, which
shows that for shorter times (t \alt 10^{4}) the discreteness of the
elementary motion cannot be neglected, whereas for longer times and short
chains (N \alt 100) tube renewal plays an essential role also for the central
segment. Due to the very broad crossover behavior both the diffusion
coefficient and the reptation time within the range of our simulation do not
reach the asymptotic power laws predicted by reptation theory. We present
results for the center-of-mass motion, showing the expected intermediate
-behavior, but again only for very long chains. In addition we show
results for the motion of the central segment relative to the center of mass,
where in some intermediate range we see the expected increase of the effective
power beyond the -law, before saturation sets in. Analysis and
simulations agree on defining a new set of criteria as characteristic for
reptation of finite chains.Comment: 19 pages in latex plus 13 ps figures, submitted to J. Stat. Phys. on
September 18, 199
Dynamics of unbinding of polymers in a random medium
We have studied the aging effect on the dynamics of unbinding of a double
stranded directed polymer in a random medium. By using the Monte Carlo dynamics
of a lattice model in two dimensions, for which disorder is known to be
relevant, the unbinding dynamics is studied by allowing the bound polymer to
relax in the random medium for a waiting time and then allowing the two strands
to unbind. The subsequent dynamics is formulated in terms of the overlap of the
two strands and also the overlap of each polymer with the configuration at the
start of the unbinding process. The interrelations between the two and the
nature of the dependence on the waiting time are studied.Comment: 7 pages, latex, 3 figures, To appear in J. Chem. Phy
Insertion and hairpin formation of membrane proteins: a Monte Carlo study
Some particular effects of a lipid membrane on the partitioning and the concomitant folding processes of model proteins have been investigated using Monte Carlo methods. It is observed that orientational order and lateral density fluctuations of the lipid matrix stabilize the orientation of helical proteins and induce a tendency of spontaneous formation of helical hairpins for helices longer than the width of the membrane. The lateral compression of the lipids on a hairpin leads to the extrusion of a loop at the trans side of the membrane. The stability of the hairpin can be increased by the design of appropriate groups of hydrophilic and hydrophobic residues at the extruded loop. It is shown that in the absence of lipids the orientation of proteins is not stable and the formation of hairpins is absent. Some analogies between the formation of helical hairpins in membranes and the formation of hairpins in polymer liquid crystals are discussed. The simulations indicate that the insertion process follows a well-defined pattern of kinetic steps
On the orientational ordering of long rods on a lattice
We argue that a system of straight rigid rods of length k on square lattice
with only hard-core interactions shows two phase transitions as a function of
density, rho, for k >= 7. The system undergoes a phase transition from the
low-density disordered phase to a nematic phase as rho is increased from 0, at
rho = rho_c1, and then again undergoes a reentrant phase transition from the
nematic phase to a disordered phase at rho = rho_c2 < 1.Comment: epl.cl
Intra-chain correlation functions and shapes of homopolymers with different architectures in dilute solution
We present results of Monte Carlo study of the monomer-monomer correlation
functions, static structure factor and asphericity characteristics of a single
homopolymer in the coil and globular states for three distinct architectures of
the chain: ring, open and star. To rationalise the results we introduce the
dimensionless correlation functions rescaled via the corresponding mean-squared
distances between monomers. For flexible chains with some architectures these
functions exhibit a large degree of universality by falling onto a single or
several distinct master curves. In the repulsive regime, where a stretched
exponential times a power law form (de Cloizeaux scaling) can be applied, the
corresponding exponents and have been obtained. The exponent
is found to be universal for flexible strongly repulsive coils
and in agreement with the theoretical prediction from improved higher-order
Borel-resummed renormalisation group calculations. The short-distance exponents
of an open flexible chain are in a good agreement with the
theoretical predictions in the strongly repulsive regime also. However,
increasing the Kuhn length in relation to the monomer size leads to their fast
cross-over towards the Gaussian behaviour. Likewise, a strong sensitivity of
various exponents on the stiffness of the chain, or on the number
of arms in star polymers, is observed. The correlation functions in the
globular state are found to have a more complicated oscillating behaviour and
their degree of universality has been reviewed. Average shapes of the polymers
in terms of the asphericity characteristics, as well as the universal behaviour
in the static structure factors, have been also investigated.Comment: RevTeX 12 pages, 10 PS figures. Accepted by J. Chem. Phy
Phase Transitions of Single Semi-stiff Polymer Chains
We study numerically a lattice model of semiflexible homopolymers with
nearest neighbor attraction and energetic preference for straight joints
between bonded monomers. For this we use a new algorithm, the "Pruned-Enriched
Rosenbluth Method" (PERM). It is very efficient both for relatively open
configurations at high temperatures and for compact and frozen-in low-T states.
This allows us to study in detail the phase diagram as a function of
nn-attraction epsilon and stiffness x. It shows a theta-collapse line with a
transition from open coils to molten compact globules (large epsilon) and a
freezing transition toward a state with orientational global order (large
stiffness x). Qualitatively this is similar to a recently studied mean field
theory (Doniach et al. (1996), J. Chem. Phys. 105, 1601), but there are
important differences. In contrast to the mean field theory, the
theta-temperature increases with stiffness x. The freezing temperature
increases even faster, and reaches the theta-line at a finite value of x. For
even stiffer chains, the freezing transition takes place directly without the
formation of an intermediate globule state. Although being in contrast with
mean filed theory, the latter has been conjectured already by Doniach et al. on
the basis of low statistics Monte Carlo simulations. Finally, we discuss the
relevance of the present model as a very crude model for protein folding.Comment: 11 pages, Latex, 8 figure
Barriers and Challenges for Visually Impaired Students in PE - An Interview Study With Students in Austria, Germany, and the USA
Physical education (PE) is an important part of school education worldwide, and at the same time, almost the only subject that explicitly deals with body and movement. PE is therefore of elementary importance in the upbringing of young people. This also applies to children with visual impairments. However, existing findings on participation and belonging in PE as well as on physical and motor development reveal that this group of children and adolescents is noticeably disadvantaged in this respect. Against this background, this paper aims to explore fundamental barriers and challenges across different types of schools, types of schooling, and countries from the perspective of visually impaired children. The qualitative interview study with 22 children with visual impairments at different types of schools in three countries (Austria, Germany, USA) reveals that none of the respondents could escape the power of social distinctions and related problematic and existing hierarchies. Hence, ideas of normality and associated values remain the main challenge for all of them. However, the type-forming analysis provides important insight across settings on how visually impaired children differ on this, allowing for greater sensitivity to the concerns of children with visual impairments
...And After That Came Me . Subjective Constructions of Social Hierarchy in Physical Education Classes Among Youth with Visual Impairments in Germany
The aim of this study was to reconstruct subjective constructions of experiences in PE and feelings of being valued within PE classes in Germany by students with visual impairment (VI). Two female and two male students (average age: 19.25 years) participated in the study from the upper level. For the reconstruction of experiences of feeling valued, episodic interviews with a semi-structured interview guide were used. The data analysis was conducted with MAXQDA 2020 based on content-related structuring of qualitative text analysis with deductive-inductive category formation. To structure the analysis, the main category, feelings of being valued, was defined by two poles (positive feelings of being valued as opposed to bullying). As a main finding, respondents primarily reported negative feelings and experiences characterized by instances of bullying, discrimination, and physical and social isolation, perpetuated by both their peers and teachers. In search of a deeper understanding, we identified social hierarchy as an underlying structure determining the students\u27 perceived positioning within the social context and thus directing their feelings of being (de-)valued. It became evident that it is not the setting per se that determined social hierarchy, but that it is more about the concrete manifestation of social hierarchy
- …