Search results for "Gyration"

Monte Carlo simulation of polymers at interfaces

1993

Abstract Polymers at interfaces pose challenging problems to statistical physics because their configurations often differ greatly from the bulk. Computer simulation of coarse-grained models then gives valuable insight and allows stringent tests of various theoretical predictions. Three examples are briefly treated: chain configurations of B-chains in the surface-enriched B-rich layer of an (AB) binary polymer mixture; “frustrated” lamellar ordering in ultra-thin block-copolymer films; and the collapse of polymer brushes in bad solvents.

Conformation of a chain polyelectrolyte in solution with low molecular weight salt: small angle neutron scattering measurements

1986

Small angle neutron scattering measurements have been carried out on the tetramethylammonium salt of the polystyrenesulfonic acid withDP w =310 and 1060 in water solution with tetramethylammonium chloride with ionic strength between 0.02 M and 1.0 M. The scattering curves in the scattering vector range 0.05 nm−1≤Q≤1.8nm−1 have been fitted using the form factor of a worm-like chain of finite thickness. The conformational parameters mean square radius of gyration, statistic chain element, mass per unit length and mean square radius of the cross-section have been determined experimentally and used for describing the conformation of the coils. By these molecular weights and ionic strengths, exc…

Structure relaxation after temperature jumps in homogeneous polystyrene/poly(styrene-co-bromostyrene) blends

1988

Concentration fluctuations in polymer blends and their change after a temperature jump were studied by time-dependent small angle X-ray scattering experiments. Measurements were conducted on homogeneous mixtures of polystyrene and a partially brominated derivative. Structure factors in thermal equilibrium show the form given by the random phase approximation, thus enabling a direct determination of theχ-parameter and the mean radius of gyration. TheT-dependence ofχ can be understood as the result of superposed enthalpic contributions and a free volume term. In theT-jump experiments, samples were quenched to temperatures near Tg. Relaxation occurs on the time scale of minutes and is nonexpon…

Computer Simulation Studies of Chain Dynamics in Polymer Brushes

2012

Center-of-mass and single monomer motion in grafted chains comprising a strongly stretched polymer brush in thermal equilibrium are studied by large scale molecular dynamics and Monte Carlo simulations of a coarse-grained model. Good solvent conditions are assumed. Our findings seriously question earlier theoretical predictions about the relaxation described by Rouse dynamics of brush coatings. Thus, the correlation functions of parallel and perpendicular components of the mean distance of the center-of-mass from the grafting site, the squared gyration radius and end-to-end distance, are found to deviate strongly from a simple exponential decay. While the relaxation times extracted from the…

Lösungseigenschaften von äthylcellulose. Vergleichende bestimmungen von molekulargewichten und zweiten virialkoeffizienten mittels osmose, lichtstreu…

1973

Es wurden osmotische und Lichtstreuungsmessungen an Athylcellulose in Aceton, Athylacetat, Butanol-(2), n-Butylchlorid und Benzol (nur Osmose) ausgefuhrt und die Ergebnisse in einigen Fallen mit Hilfe der Sedimentation/Diffusion kontrolliert. Durch Vergleich der verschiedenen Mesmethoden konnten die von Gelen unbeeinflusten Mesergebnisse gegen die durch Gele verfalschten Werte abgegrenzt werden. Die osmotisch und hydrodynamisch bestimmten A2-Werte sind hoch im Vergleich zu denen von Vinylpolymeren. Im Gegensatz dazu steht der niedrige Exponent e der Tragheitsradius-Molekulargewichtsbeziehung der sich fur negative Exponenten e nur durch Geleinflus deuten last. fur die zweiten Virialkoeffizie…

Hydrodynamische eigenschaften von methylcellulosen in lösung

1961

Technische Methylcellulose wurde fraktioniert und auf ihr Viskositats-, Sedimentations und Diffusionsverhalten in wasriger Losung untersucht. Die Viskositats-Molekularge-wichts-Beziehung fur Methylcellulosen von etwa 28% Methoxylgehalt ergab sich zu Der Exponent 0,63 besagt, das Kette relativ eng geknauelt ist. Ihr Verhalten last sich durch die Theorien der Viskositat gut deuten. So ergibt sich nach DEBYE-BUECHE-PETERLIN fur die Knaueldimension (R = Tragheitsradius in A) was eine nur schwache Abweichung vom GAUSSschen Knauel mit M1,00 bedeutet. Das Verhalten der gelosten Methylcellulose wird zusammen mit Messungen an Athylhydroxyathylcellulosen und Cellulosenitraten betrachtet, bei denen st…

Expansionskoeffizient (α) und zweiter virialkoeffizient (A2) der lösungen von cellulosetrinitraten in aceton

1968

Durch Streulichtmessungen werden an Acetonlosungen von zwei Reihen fraktionierter Cellulosetrinitrate (CTN 13,9 und CTN 12,9 entsprechend dem %-Gehalt N) Molekulargewicht, Tragheitsradius r und 2. osmotischer Virialkoeffizient A2 gemessen. Der Expansionskoeffizient gehorcht der Gleichung α5 − α3 = 2,6 z. Sowohl z wie α sind niedrig. Thermodynamisch gibt die, h(z)-Funktion von CASASSA die Versuchsresultate am besten wieder. Die Unterschiede zwischen Losungen von Cellulosenitraten und Vinylpolymeren beruchen auf der Verschiedenheit des einzigen Parameters K0 = r/M, der bei CTN 13,9 um den Faktor 12 hoher liegt als z. B. bei Polystyrol (Tab. 8). Dadurch sind unter vergleichbaren Bedingungen in…

EFFECTS OF SENSITIZED AND UNSENSITIZED LONGWAVE U.V.-IRRADIATION ON THE SOLUTION PROPERTIES OF DNA

1971

— Two types of photoreactions occur in DNA irradiated in aqueous systems with longwave u.v.-light (Λ > 295 nm), namely, (a) thymine dimerization, and (b) single- and double-strand breakage of the sugar phosphate backbone; these two reactions are unrelated. The presence of acetophenone as a photosensitizer caused an increase in dimerization by a factor of 16, and an increase in single-strand breaks by a factor of 4. The number of thymine dimers per single-strand break is about 100 in the sensitized and 25 in the unsensitized reaction. The alteration of the radius of gyration of DNA molecules is that expected by the degradation observed. At the same time the change in hyperchromicity is very …

Molecular dynamics of supercooled polymer films

2000

We present results of molecular dynamics simulations for a supercooled polymer melt confined between two smooth and purely repulsive walls. The thickness D of the film is about 7 times the bulk radius of gyration. For all temperatures studied, a significant increase of the monomer and chain mobilities with respect to the bulk is observed. Preliminary results suggest that structural relaxation times exhibit a power-law behavior in the vicinity of a critical temperature T c (D) 0.39 (in Lennard-Jones units). This estimate of T c (D) is about 14% smaller than the corresponding bulk value. Despite this significant decrease the time dependence of various mean-square displacements seems to be una…

1988

Monte Carlo simulations are presented for binary (AB) symmetric polymer mixtures (chain lengths NANBN) for the case that an attractive interaction ϵ exists between monomers of the same kind, and the limiting case that one species (B) is very diluted. It is shown that with increasing interaction strength ϵ/(kB · T), T being the absolute temperature, the minority chains collapse to a very dense configuration, while the majority chains stay nearly Gaussian. Both chain radii, structure factors and numbers of nearest neighbour contacts are discussed.