Search results for "polymers"
showing 10 items of 3567 documents
Untersuchungen zur Ursache der Entstehung von “Extended Chain”-Kristallen des Polyäthylens
1969
Als Mechanismus fur die Bildung von “extended chain”-Kristallen bei der Kristallisation von Polyathylen unter hohen Drucken ist vorgeschlagen worden, das sich zunachst Kristallite mit gefalteten Ketten bilden sollen, die sich dann infolge des Dickenwachstum zu Kristallen mit gestreckten Ketten umwandeln. Um diese Deutung zu uberprufen, wurden Polyathylen-Einkristalle und orientierte Polyathylenproben bei Drucken zwischen 1 und 4200 atm und bei Temperaturen zwischen 115 °C und 220 °C getempert. Die Proben wurden mit Hilfe der Rontgenkleinwinkelstreuung, der Elektronenmikroskopie und der Differential-Thermoanalyse untersucht. In keinem Fall konnte ein Hinweis auf eine Umwandlung zu “extended …
Coarse-grained models and collective phenomena in membranes: Computer simulation of membrane fusion
2003
We discuss the role coarse-grained models play in in- vestigating collective phenomena in bilayer membranes and place them in the context of alternative approaches. By reducing the de- grees of freedom and applying simple effective potentials, coarse- grained models can address the large time scales and length scales of collective phenomena in mem- branes. Although the mapping from a coarse-grained model onto chemi- cally realistic models is a challenge, such models provide a direct view on the phenomena that occur on the length scales of a few tens of nano- meters. Their relevance is exempli- ied by the study of fusion of model membranes. ' 2003 Wiley Periodicals,
Critical phenomena in polymer mixtures: Monte Carlo simulation of a lattice model
1987
A lattice model of a symmetrical binary (AB) polymer mixture is studied, modelling the polymer chains by self-avoiding walks withN A =N B =N steps on a simple cubic lattice. If a pair of nearest neighbour sites is taken by different monomersAB orBA, an energye ab is won; if the pair of sites is taken by anAA or aBB pair, an energye is won, while the energy is reduced to zero if at least one of the sites of the pair is vacant. To allow enough chain mobility, 20% of the lattice sites are vacancies. In addition to local motions of the chain segments we use a novel “grand-canonical” simulation technique:A chains are transformed intoB chains and vice versa, keeping the chemical potential differe…
Intra- and Interchain Correlations in Semidilute Polymer Solutions: Monte Carlo Simulations and Renormalization Group Results
2000
We investigate the intra- and intermolecular correlations in semidilute polymer solutions by large-scale computer simulations and renormalization group calculations. In the framework of the bond fluctuation model we study polymers with chain lengths up to N = 2048 monomers and determine the intermolecular pair correlation function, the coherent scattering intensity, and its distinct part at all length scales. The simulations are compared quantitatively to renormalization group calculations of the universal crossover scaling function. Special attention is paid to length scales smaller than the density screening length ξ, where the distinct part of the scattering function in the simulations i…
Electronic momentum distribution in the one-dimensional extended Hubbard model: determinantal Monte Carlo study
2002
Abstract The effect of electron–electron (e–e) interaction on trans -polyacetylene ( t -PA) properties is investigated within the framework of an extended Hubbard model in one dimension. For numerical calculation, we use the determinantal version of quantum Monte Carlo approach, which provides a breakthrough to simulate statistical fluctuations in the systems with many degrees of freedom, in order to obtain mean values for observables of physical interest. This allows one to analyze the discrete system of fermions without encountering the numerical instabilities that generally occur from the original problem involving anticommuting fermion operators. We calculate the electronic momentum dis…
Phase Equilibria of Lattice Polymers from Histogram Reweighting Monte Carlo Simulations
1998
Histogram-reweighting Monte Carlo simulations were used to obtain polymer / solvent phase diagrams for lattice homopolymers of chain lengths up to r=1000 monomers. The simulation technique was based on performing a series of grand canonical Monte Carlo calculations for a small number of state points and combining the results to obtain the phase behavior of a system over a range of temperatures and densities. Critical parameters were determined from mixed-field finite-size scaling concepts by matching the order parameter distribution near the critical point to the distribution for the three-dimensional Ising universality class. Calculations for the simple cubic lattice (coordination number z…
The escape transition of a compressed star polymer: Self-consistent field predictions tested by simulation
2013
The escape transition of a polymer "mushroom" (a flexible chain grafted to a flat non-adsorbing substrate surface in a good solvent) occurs when the polymer is compressed by a cylindrical piston of radius $R$, that by far exceeds the chain gyration radius. At this transition, the chain conformation abruptly changes from a two-dimensional self-avoiding walk of blobs (of diameter $H$, the height of the piston above the substrate) to a "flower conformation", i.e. stretched almost one-dimensional string of blobs (with end-to-end distance $\approx R$) and an "escaped" part of the chain, the "crown", outside the piston. The extension of this problem to the case of star polymers with $f$ arms is c…
Scattering from concentration fluctuations in polymer blends: A monte carlo investigation
1989
The collective scattering function Scoll( $$\vec q$$ ), which describes light (neutron-, x-ray) scattering under wavevector $$\vec q$$ , is obtained from Monte Carlo simulations for a symmetrical polymer mixture. The polymers are modelled by self-avoiding walks ofN A=NB=N steps on a simple cubic lattice, where a fractionφ V of sites is left vacant, and an attractive energye occurs if two neighboring sites are taken by the same kind of monomer. Spinodal curves are estimated from linear extrapolation of S coll −1 (0) vs.e/k B T, whereT is the temperature. Also the single chain structure factor is obtained and the de Gennes random phase approximation (RPA) can thus be tested. Unexpectedly, str…
Bottom-up construction of dynamic density functional theories for inhomogeneous polymer systems from microscopic simulations
2020
We propose and compare different strategies to construct dynamic density functional theories (DDFTs) for inhomogeneous polymer systems close to equilibrium from microscopic simulation trajectories. We focus on the systematic construction of the mobility coefficient, $\Lambda(r,r')$, which relates the thermodynamic driving force on monomers at position $r'$ to the motion of monomers at position $r$. A first approach based on the Green-Kubo formalism turns out to be impractical because of a severe plateau problem. Instead, we propose to extract the mobility coefficient from an effective characteristic relaxation time of the single chain dynamic structure factor. To test our approach, we study…
New Development of Monte Carlo Techniques for Studying Bottle-brush Polymers
2011
Due to the complex characteristics of bottle-brush polymers, it became a challenge to develop an efficient algorithm for studying such macromolecules under various solvent conditions or some constraints in the space by using computer simulations. In the limit of a bottle-brush polymer with a rather stiff backbone (straight rigid backbone), we generalize the variant of the biased chain growth algorithm, the pruned-enriched Rosenbluth method, for simulating polymers with complex architecture, from star polymers to bottle-brush polymers, on the simple cubic lattice. With the high statistics of our Monte Carlo results, we check the theoretical predictions of side chain behavior and radial monom…