Search results for "Gyration"

A synchrotron radiation X-ray scattering study of aqueous solutions of native DNA

1999

Synchrotron radiation small-angle X-ray scattering (SAXS) was used to investigate solutions of native DNA at different ionic strengths and temperatures. The mass per unit length, radius of gyration of the cross-section of DNA and apparent second virial coefficient (A2) were obtained from Zimm plots in the rodlike particle approximation. The values of A2 obtained in this way are positive and almost constant indicating that the repulsive interactions still influence the scattering patterns at resolutions as high as 5-8 nm. SAXS measurements in continuous temperature scans indicate that the rod approximation is valid over a wide temperature range during DNA melting and confirm that the rodlike…

Advances in contrast variation for macromolecular structure determination by polarized neutron scattering and anomalous dispersion of synchrotron X-r…

1988

Contrast variation for macromolecular structure determination is usually achieved by isomorphous replacement of 1-H by 2-H (D) using small-angle neutron scattering (SANS). This is particularly easy in aqueous solvents. By adding heavy water the contrast of dissolved proteins, nucleic acids and membranes changes drastically. It is the region inaccesible to solvent molecules, which acts as a label. Measurements of the scattering intensity at three different scattering densities of a solvent yields the three basic scattering functions. The contrast dependence of the radius of gyration receives particular interest. More recently smaller labels have been used. Their dimensions are smaller than t…

Evidence for the time-temperature superposition principle from Monte-Carlo simulations of the glass transition in two-dimensional polymer melts

1992

The bond fluctuation model on a square lattice with a bond-length dependent potential exhibits in simulations of slow cooling a kinetic glass transition where the system falls out of equilibrium. Extending previous work, the relaxation functions of gyration radius and end-to-end distance, and the bond autocorrelation function of the polymers are presented and related to the time-dependent displacements of inner monomeric units and center of gravity of the whole chains, respectively. Over a wide temperature range the data can be collapsed on master curves satisfying the time-temperature superposition principle for Rouse dynamics.

Formation of Micelles in Homopolymer-Copolymer Mixtures:  Quantitative Comparison between Simulations of Long Chains and Self-Consistent Field Calcul…

2006

Using Monte Carlo simulations of the bond fluctuation model and self-consistent field calculations, we study the formation of micelles in a mixture of homopolymers and asymmetric AB-diblock copolymers with composition, fA = 1/8. Both types of molecules are fully flexible and have identical length. We work in the semi-grand-canonical ensemble, i.e., we fix the monomer density and incompatibility, χN ≃ 100 (strong segregation regime), and control the composition of the mixture via the exchange chemical potential, δμ ≡ μAB − μB between the copolymers and homopolymers. The Monte Carlo simulation comprises moves that allow homopolymers to mutate into AB-diblock copolymers and vice versa. These m…

1988

Light scattering, neutron scattering and viscosity measurements on a nematic and a smectic polymethacrylate demonstrate the presence of molecularly disperse coils in solution. For the nematic polymer, toluene is a theta solvent, the behaviour familiar from conventional polymers (unperturbed coil conformation) being exhibited at the theta point. The isotropic melts likewise contain unperturbed coils; in the case of the nematic polymers, however, the coils in the melt have a radius of gyration which is about 30% smaller than that in solution at the theta point. In the liquid-crystalline phase, the mean radii of gyration in both cases are identical to the corresponding parameters in the isotro…

Monte Carlo simulation of micelle formation in block copolymer solutions

1998

Short block copolymers in selective solvents (bad for A-block, good for B-block) are modeled by flexible bead-spring chains, where beads interact with short range Morse potentials of variable strength. It is shown that already very short chains (N A = N B = 2) exhibit a rather well-defined critical micelle concentration (cmc). The mass distribution of the micelles and their gyration tensor components as well as their internal structure are studied. It is shown that the relaxation time increases exponentially with the strength E AA of the attractive energy between the A-monomers, and thus frozen-in micelles of medium size are obtained when E AA is chosen too large. Our results are compared t…

A light-scattering photometer for use at elevated pressures

1970

A new light scattering photometer has been designed in order to perform light-scattering measurements at various temperatures and under pressures of 1–1000 atm. Since the scattering angle is constant (90°C), the dimensions of the dissolved molecules are determined by measuring the intensity at different wavelengths. The method of measurement, the calibration of the instrument, and the treatment of data are described. By a series of test measurements on polystyrene in trans-decalin solutions it is shown that one obtains reliable results and finds considerable effects of pressure on the radius of gyration and the fundamental thermodynamic properties of the system.

Statics and Dynamics of Bidisperse Polymer Melts:  A Monte Carlo Study of the Bond-Fluctuation Model

1998

As a first step toward the computer simulation of polydisperse polymeric melts, a lattice model containing two types of chains with lengths N1 = 20 − x and N2 = 20 + 4x (0 ≤ x ≤ 10 ) is studied. This variation of x, together with the fixed composition of 80% of short and 20% of long chains, leads to a polydispersity of 1 ≤ Nw/Nn ≤ 2 (Nw, Nn:  weight-, number-average chain lengths). To represent dense melts, the bond-fluctuation model at a volume fraction, φ = 1/2, of occupied lattice sites is used. The simulation treats both the athermal case (chain connectivity and excluded volume interaction only) and a thermal case, where additionally a choice for the bond length and bond angle potential…

Moleküldimensionen von polymeren in verschiedenen lösungsmitteln auf grund hydrodynamischer messungen

1960

Ein fraktioniertes Polystyrol vom Molekulargewicht Mw ≈ 540000 wurde in Chloroform, Toluol, Methylathylketon und Cyclohexan sowie in drei Mischungen von Toluol und Methylathylketon untersucht. Da das Chloroform ein gutes, das Cyclohexan dagegen ein Θ-Losungsmittel fur Polystyrol darstellt, nehmen die Dimensionen des Fadenmolekuls beim Ubergang zwischen den Losungsmitteln ab. Mit den Knaueldimensionen andern sich Reibung und Viskositat des Polystyrols. Messungen der Sedimentationskonstanten, der Diffusionskonstanten, der Viskositatszahl sowie der Dichte und der Zahigkeit fur die gemischten Losungsmittel wurden ausgefuhrt. Die Ergebnisse der Messungen werden benutzt, um aus der Viskositatsthe…

Solution behavior of ethylcellulose in organic solvents

2007

Ethylcellulose has been investigated by osmotic pressure, sedimentation-diffusion, and light scattering measurements in dilute solutions of acetone, ethylacetate, 2-butanol, n-butylchloride, and benzene. Combining the results of different measuring techniques allowed the recognition and elimination of the influence of microgels present in the solution. The observed second virial coefficients are high as compared with those of the vinyl polymers and do not show a pronounced dependence on the type of solvent. On the other hand, the radius of gyration is nearly proportional to the root of the molecular weight. This behavior is discussed in connection with results on other cellulose ethers repo…