J. P. Cotton

Improvement of data treatment in small-angle neutron scattering

Small-angle neutron scattering has been successfully used since the 1970s. As a general rule, methods to extract the useful signal from that received by the detector are well known and give good results. At the Laboratoire Leon Brillouin, for example, these methods have been employed for a long time. However, the data reduction software has been changed for the following reasons. Problems are encountered if the container of the sample gives a spurious signal or if the scattering angle is so large that its cosine cannot be approximated by 1. In the present paper, generalizations of formulas are made in order to account for these difficulties. The decrease of scattered intensity delivered by an incoherent sample that is often observed at large angles is shown to be only due to a geometrical effect. The consequent modifications of the relations used for the normalization of cells of position-sensitive detectors and for the absolute calibration are given. As for the inherent background of the sample, the contribution of density fluctuations is usually neglected. This contribution is formally given as a function of the contrast lengths and of the isothermal compressibility of the sample. This new result allows numerical evaluations of the different terms of the inherent sample background. Practical examples are given. Finally, several methods, developed at the Laboratoire Leon Brillouin, are given to determine the background properly. They are based on systematic measurements of transmissions and background levels of suitably prepared blank samples.

On the structure and the chain conformation of side-chain liquid crystal polymers

Abstract Backbone anisotropy and the structure of the mesophases of a series of side-chain liquid crystal polymers have been studied in the bulk by neutron scattering. The backbone conformation is obtained by small-angle neutron scattering on mixtures of hydrogenous polymers with deuteriated backbones. The components of the radius of gyration parallel, R ⊥ and perpendicular, R ∥ to the magnetic field are determined as a function of temperature for both the nematic phase and the smectic phase. It is shown that the polymer backbone is deformed in both phases. When the polymer exhibits only a nematic phase, it adopts a prolate conformation, where the average backbone direction is more or less parallel to the aligned mesogenic groups. Upon transition from the smectic phase to a nematic phase, the backbone in the nematic phase assumes a slightly oblate shape. This tendency towards oblate shape is due to the smectic fluctuations which are always present in such nematic phases. The exentricity of the oblate back...

A direct measurement of the polyion conformation in aqueous solutions at different temperatures. Small angle neutron scattering of PSSNa using zero average and full contrast

The intrachain correlations in polystyrene sulfonate of sodium (PSSNa) have been measured as a function of the temperature, in semidilute solutions, using the zero average contrast technique. No temperature effect is observed. We also display and discuss data on interchain correlation, which are obtained from a scattering experiment at full contrast.

Are nematic side-on polymers totally extended? A SANS study

The strong prolate anisotropy of the backbone conformation observed in the nematic phase of ‘side-on fixed’ LCP is found to be as great as for main chain LCP. From this remark originated this SANS study of a ‘jacketed’ polymer PA4,4,4. A form factor of cylinders is used to model the experimental scattering data as in the case of main chain LCPs. The data are well fitted by this model and the backbone appears as a long and very thin rigid rod.

Neutron scattering study and discussion of the backbone conformation in the nematic phase of a side chain polymer

Abstract A small angle neutron scattering study of a mesomorphic side chain polyacrylate in the nematic phase shows that the backbone adopts a prolate conformation. This observation is discussed in relation to the previous determinations of backbone anisotropy in the nematic phase of other side chain polymers. On the basis of the comparison with these previous determinations and also with the results obtained on other related systems, the assumption is made that, provided that the spacers are long enough, the natural tendency of the backbones is to adopt a prolate shape in any nematic phase devoid of smectic fluctuations.

Polyelectrolyte persistence length in semidilute solution as a function of the ionic strength

Abstract The persistence length of a sodium sulfonated polystyrene (PSSNa) is measured by small-angle neutron scattering in the semidilute regime for 2 values of c, the polymer concentration, as a function of cs, the added salt concentration. The results suggest that this persistence length lp depends only on the ionic strength I = c + 2cs, and questions the role of the counterion condensation at the nanometric scale. The variation l p ∼ I −1 3 found corresponds to the distance between free ions for concentrated solutions where the Debye-Huckel prediction for the screening length is no longer valid.

Neutron scattering studies of molecular conformations in liquid crystal polymers

A comblike liquid crystal polymer (LPC) is a polymer on which mesogenic molecules have been grafted. It exhibits a succession of liquid crystal phases. Usually the equilibrium conformation of an ordinary polymeric chain corresponds to a maximum entropy, i.e., to an isotropic spherical coil. How does the backbone of a LCP behave in the nematic and smectic field? Small-angle neutron scattering may answer this question. Such measurements are presented here on four different polymers as a function of temperature. An anisotropy of the backbone conformation is found in all these studied compounds, much more pronounced in the smectic phase than in the nematic phase: the backbone spreads more or less perpendicularly to its hanging cores. A comparison with existing theories and a discussion of these results is outlined.

Backbone conformation study on “side-on fixed” liquid crystal polymers

Abstract Recent studies on the backbone conformation of “side-on fixed” thermotropic LCPs tend to focus on the following question: what about the anisotropy of the backbone in the orientational medium imposed by its mesogenic groups in the nematic and the smectic C phases? By means of small-angle neutron scattering (SANS), different features are qualitatively and quantitatively discussed. The chain anisotropy is essentially driven by thermodynamical, structural and chemical consideration and is compared to the behaviour of other LCPs as “side-end fixed” and main chain LCPs.

Liquid crystal polymers: evidence of hairpin defects in nematic main chains, comparison with side chain polymers

Abstract This article describes the conformation of two species of liquid crystalline polymers as revealed by small angle neutron scattering. The results obtained with side chain polymers are recalled. The procedure used to analyze the scattering data of main chains in the nematic phase is reported in this paper. It permits a demonstration of the existence of hairpins. Comparison of both polymer species shows that in the isotropic phase, the two polymers adopt a random coil conformation. In the nematic phase, the conformations are very different; the side chains behave as a melt of penetrable random coils whereas the main chains behave as a nematic phase of non penetrable cylinders.

Polymer-Polymer Correlations in the Solution State

We examine the average 2-point monomer correlation function of distinct polymer chains in solution, and discuss the effect of repulsion when two polymer clouds are close to each other. A prediction is given for the asymptotic behaviour of the distinct structure function, which varies as q-d in good solvents. This dependence is discussed in terms of the field-theoretical analog, revealing also the contribution of a higher non-analytic q-dependent term. The prediction for the asymptotic behaviour is confronted to a neutron small-angle scattering experiment in a semi-dilute polymer solution.