Large Persistence Length Research Articles

Configurational properties of polypyrrole chains

Conformational energy surfaces are determined as functions of torsional angles of bonds between rings for an α-α pyrrole trimer by using semiempirical quantum-mechanical calculations (AM1). Isomeric states for torsional rotations are identified as positions of minimum energy. Statistical weights determined on this basis are used in calculating the average dimensions of polypyrrole chains by the rotational isomeric state (RIS) model and the matrix multiplication scheme. Results of calculations indicate that the chains have unusually large characteristic ratios and persistence lengths with very strong temperature dependence. Characteristic ratios obtained by the RIS model show perfect agreement, in a wide temperature range, with the predictions of the worm-like chain model.

Anomalous dipolar flexoelectric effect in a nematic main-chain polymer.

An anomalously large flexoelectric effect is predicted in the nematic phase of thermotropic main-chain polymers, which have a defined orientation of subsequent monomers bonded on the chain. We identify two different ranges of order parameters. When the persistence length of the chain in the direction parallel to the order parameters is larger than a monomer length but not too long (small order parameter), the longitudinal flexoelectric coefficient ${\mathit{f}}_{\mathrm{\ensuremath{\parallel}}}$ is proportional to this exponentially large persistence length, assuming that the polymers are many persistence lengths long. This result corresponds to the simple estimation that relates flexoelectric coefficients to the Frank elastic constants: ${\mathit{f}}_{\mathrm{\ensuremath{\parallel}}}$\ensuremath{\sim}${\mathit{K}}_{\mathit{s}}$. When the nematic ordering approaches its saturation (strongly extended chains), the flexoelectric effect is essentially independent of the persistence length and is either determined by a competition between this effect and the tendency of the chain segments to align antiparallel, or the system is ferroelectric in the absence of deformations.

The molecular weight distribution and conformation of citrus pectins in solution studied by hydrodynamics

The molecular weight distribution of a citrus pectin has been analysed by a combined approach using gel-permeation chromatography with low-speed sedimentation equilibrium. (1) A pectin preparation from citrus fruit was fractionated on Sepharose CL-2B/Sepharose CL-4B. (2) Weight average molecular weights of the fractions were determined by low speed sedimentation equilibrium in multichannel cells. (3) An absolute calibration for the column for this material was thereby defined. (4) The (lognormal) molecular weight distribution thus obtained is consistent with a weight average of (90000±10000) g/mol, obtained separately on unfractionated material, and consistent with a distribution obtained on the same material but using light scattering as the molecular weight probe. The conformation of the pectin fractions in solution was studied in terms of: (1) the Wales-Van Holde parameter, k s [η] ; (2) Mark-Houwink-Kuhn-Sakurada plots of sedimentation coefficient and intrinsic viscosity data versus molecular weight; (3) rod models and (4) worm-like-coil models. The sedimentation data is consistent with a rod model (or a worm-like-coil with a large persistence length) with mass per unit length ∼ 430 g mol −1 nm −1. The intrinsic viscosity data is also consistent with a rod model but shows some anomalous features which may be suggestive of worm-like-coil behaviour at higher molecular weight, although it is not possible to fit this data with a realistic value of the mass per unit length.

Elasticity and swelling behaviour of chemically crosslinked cellulose ethers in aqueous systems

The mechanical properties and the swelling behaviour of hydrogels based on sodium car☐ymethylcellulose and hydroxyethylcellulose were investigated. The gels were prepared by crosslinking the cellulose ethers via divinylsulphone in alkaline solution. The outstanding feature of such materials is their increase of modulus with increasing swelling, which starts already at rather low degrees of swelling (around 10). In this respect, the cellulosic gels are superior to the hydrogels based on acrylic polyelectrolytes currently in use for superabsorbent products. The reason for these properties is seen in the large persistence length of the cellulose backbone, giving rise to non-Gaussian behaviour at much lower chain deformations than with more flexible polymers.

Dynamic bending rigidity of DNA

Rapidly relaxing components in the decay of the transient electric dichroism of DNA restriction fragments were reported by Diekmann et al. [(1982) Biophys. Chem. 15, 263-270] and Pörschke et al. [(1987) Biopolymers 26, 1971-1974]. These are analyzed using a new normal mode theory for weakly bending rods and assigned to bending. The longest bending relaxation times for fragments with 95-250 base pairs coincide with the theoretical curve calculated for a dynamic bending rigidity corresponding to a dynamic persistence length Pd = 2100 A. Analysis of the relative amplitudes of fast and slow components following weak orienting pulses is also consistent with a rather large dynamic persistence length. The enhancement of the relative amplitude of the fast component in large electric fields is attributed to steady-state bending of initially perpendicular DNAs by the field. Several reasons are proposed why the dynamic bending rigidity is 4 times larger than the apparent static bending rigidity inferred from equilibrium persistence length measurements on the same fragments.