Isotactic polystyrene/cis-decaline mixtures: phase diagram and molecular structures

Organized structures grown from iPS/cis-decalin solutions (ranging from 5% to 50% w/w) have been studied by several techniques. From DSC experiments the thermal behavior has been investigated as a function of the annealing temperature. The temperature-concentration phase diagram has been established. From neutron diffraction experiments the short range molecular order has been determined. We show that between the gel state formed at high undercooling and the crystalline state obtained at low undercooling there exist two intermediate phases: thes-phase, already described by Klein et al., and thep-phase. Except for the crystalline state all the other phases contain intercalated solvent molecules. Thegel state displays nematic-like order, whereas thes-phase is reminiscent of smectic arrangements. Thep-phase is less solvated and can be described as a peritectic system. Preliminary neutron scattering experiments show that chain-folding reappears in thes-phase, whereas it was shown to be absent in thegel phase.Formerly Laboratoire de Spectrométrie et d'imagerie Ultrasonores     

Anomalous behaviour of comb-like dextran-polyacrylamide copolymers in mixed solvents

Copolymers obtained by grafting polyacrylamide (PAA) onto dextran (DEX) have been synthesised by ceric-ion-reduced redox initiation method. These copolymers have been characterized by light scattering and self-exclusion chromatography. Viscometry measurements in mixed solvents, water/dimethyl formamide (DMF) have revealed an anomalous behaviour reminiscent of a polyelectrolyte behaviour, namely a strong upturn when decreasing polymer concentration, effect which is absent in pure water. The origin of this phenomenon has been studied by determining by means of SAXS the short-range molecular structure of these copolymers in the binary solvents. Comparison has been also performed with water/methyl formamide (MF) and water/formamide (FA) binary solvents where the pseudo-polyelectrolyte effect is absent. A molecular model is discussed on the basis of the results from SAXS that throws some light on the viscometry effect. This model considers the occurrence of an interdigitation process between the PAA grafts together with the formation of a PAA-DMF complex.     

Plant delta(15)N associated with arbuscular mycorrhization, drought and nitrogen deficiency

It has long been evident that plant (15)N chiefly reflects the processes which fractionate (15)N/(14)N rather than the (15)N of plant N source(s). It has emerged recently that one of the most important fractionating processes contributing to the whole plant (15)N is the presence/absence, type or species of mycorrhiza, especially when interacting with nutrient deficiency. Ecto- and ericoid mycorrhizas are frequently associated with (15)N-depleted foliar (15)N, commonly as low as -12 per thousand. As shown by the present study, plants having no mycorrhiza, or those infected with various species of arbuscular mycorrhiza (AM)-forming fungi, interact with varying concentrations of soil nitrogen [N] and moisture to enrich plant (15)N by as much as 3.5 per thousand. Hence the lack of a mycorrhiza, or variation in the species of AM-forming fungal associations, can account for about 25% of the usually reported variations of foliar (15)N found in field situations and do so by (15)N enrichment rather than depletion. Copyright 1999 John Wiley & Sons, Ltd.     

Physical gels from PVC: molecular structure of pregels and gels by low-angle neutron scattering

In this paper we report on investigations into the short-range molecular structure of pregels (aggregates formed in dilute solutions) and gels as a function of ageing time, temperature and solvent type. In some solvents (diethyl oxalate and bromobenzene) it is found that the structure of the pregels is cylinder-like or ellipsoid-like. The structure is seen to be the same for the gels and the pregels in the investigated range of transfer momentum which leads us to conclude that the gels are probably fiber-like, a statement in agreement with the morphology observed by Yang and Geil. The fiber section is also about the same order of magnitude as that reported by these authors. Also, further evidence is given for the existence of a second type of physical knot, different from those formed by the more syndiotactic sequences. The validity of the molecular structures deduced from these experiments is discussed in the light of macroscopic properties such as compression modulus and swelling.