Physics Abstracts 61.12 1. Introduction.-That some molecules dissolved in water can spontaneously aggregate is well known : examples are the formation of micelles of amphiphilic molecules [1], the complexation of small molecules by cage-like structures [2] or by macromolecules [3], and also the association of 2 types of macromolecules [4]. With these few examples the subject is usually considered to be exhausted, at least for artificial systems. Biological systems, on the other hand, show a limitless ability to produce complex particles such as viruses, lipoproteins, or mixed bile salt + lecithin micelles. Thus it is of interest to find out whether artificial particles of comparable complexity can be formed through (+) Laboratoire associe au C.N.R.S.

2014 On améliore la résolution obtenue en diffusion des neutrons aux petits angles pour déterminer la structure des micelles de dodécylsulfate de sodium dans l'eau. L'expérience mesure la distribution moyenne des distances entre tous les noyaux d'une micelle, ainsi que les distributions de distances entre des groupes deutérés attachés aux positions 03C9 et 03B3 des chaînes de SDS. A basse résolution (15 Å) on n'observe que la structure moyenne de la micelle : il s'agit d'une sphère contenant N = 74 molécules de SDS; son coeur hydrocarboné a un rayon de 18,4 Å ; il contient très peu d'eau, en tout cas moins qu'une molécule d'eau par molécule de SDS. A haute résolution (5 A) on voit surtout les fluctuations spontanées qui écartent la micelle de cette structure moyenne. Ces fluctuations produisent une dispersion des nombres d'agrégation (03C32/N2 = 0,1) et des déviations par rapport à la forme sphérique. La structure interne du coeur, mesurée par les distances entre groupes deutérés, est également distordue par chacune de ces fluctuations : les queues de chaînes (03C9) ne sont pas concentrées près d'un « centre », et les méthylènes en position 03B3 ne restent pas dans une coquille sphérique. Abstract 2014 An improvement over previous determinations of the structure of sodium dodecyl sulfate micelles has been obtained from small angle neutron scattering. The experiment measures the average distribution of distances between all nuclei within a micelle, and also those between deuterium labels attached at positions 03C9 or 03B3 on the SDS chains. At low resolution (15 Å), only the average structure of the micelle is observed : this is a dense sphere containing N = 74 SDS molecules; the hydrocarbon core has a radius of 18.4 Å and contains less than one water molecule per SDS molecule. At high resolution (5 Å), fluctuations away from the average structure are observed. If the internal structure of the core is not resolved, one sees mainly the dispersion in the aggregation numbers (03C3N/N = 0.33) or radii (03C3R/R = 0.1) of the micelle. When the distributions of distances between deuterium labels are observed, shape fluctuations show up through distortions of the internal structure : indeed the chain ends (03C9) are not concentrated near a « centre » of the micelle, and the 03B3 methylene groups do not remain in a spherical shell.

MOLECULAR WEIGHT(X10" =)Figure 7. Dependence of the percentage of the y spherulites in the crystalline regions on the molecular weight for PVF2 samples crystallized at 433 K for 24 h.longitudinal propagation is increased. On the other hand, the transformation is impeded by the interlamellar amorphous phase, which acts as a barrier to the transverse propagation. Both the percentage of the y spherulites in the crystalline regions and the content of the interlamellar amorphous phase increase with the head-to-head defect concentration and the molecular weight. As a result, the degree of the a -* y transformation appears to be a complicated function of both of these molecular parameters.

SynopsisThe physical structure of Nafion membranes has been investigated by small-angle neutron scattering (SANS) and small-angle x-ray scattering (SAXS). Samples in the acid form may exhibit two scattering peaks. The first, observed by SANS at an angle corresponding to a Bragg spacing of 180 A, is shown to arise from structures in crystalline regions. A second peak at larger scattering angles is shown to arise from ion-containing regions which may be swollen with water. Salt-form samples made by soaking the acid form in an aqueous salt solution can also exhibit the same two scattering signals. But in amorphous salt-form samples produced by quenching from the melt the first peak is absent. This permits a more accurate study of the second peak by SAXS, which shows that the second scattering component is present as a maximum over a wide range of water contents but is absent in a sample dried at 200OC. The position of the peak shifts to lower scattering angles (or larger spacings) at higher water contents. Possible structural models that might give rise to the maximum are discussed. A calculation of the SAX invariant is made and results are consistent with a phase separation of a large fraction of the water.