Kinetic study of silica gels by a new rheological ultrasonic investigation.

The last decades have seen the development of sol-gel (SG) process currently used to develop new materials in a wide range of scientific applications. The SG process leads to an oxide macromolecular network through a sol (liquid phase) to gel transition. To optimize this process, the control of the kinetic of the chemical reaction is required. This kinetic can be deduced from the temporal evolution of the viscoelastic parameters. Upto date no complete investigation during the SG formation can be achieved by a unique non-destructive technique. In this paper, we present an ultrasonic technique to measure the viscoelastic parameters (storage G' and loss G'' shear moduli) of the gel material during its formation. By using a suitable model which takes into account the mass loading on the surface, the viscoelastic parameters of these materials are accurately deduced. In order to study the efficiency of this technique, silica gels transition is monitored at various formation temperatures and for different initial hydrolysis molar ratio (h). In addition, the monitoring is performed at different oscillatory shear measurements in the 6-54 MHz frequency range to determine a new characteristic time t(vs) corresponding to the moment when the material is no more a newtonian liquid. This characteristic time is then compared to the gelation time t(g) determined by rheological or acoustic audible range methods. Thus the new characteristic time is also a good criterion to characterize earlier the SG matrix transition. Our AT-cut quartz technique using our model can also be used as a high frequency rheometer for the sol-gel materials.

[Approaching the gene of mucoviscidosis. New data]. / Approche du gène de la mucoviscidose. Nouvelles données.

We used 5 polymorphic probes strongly linked to the gene of cystic fibrosis (CF) to perform the genotypical study of 48 families with at least one child presenting with the disease. The last Km19 and XV2c probes showed a very important linkage imbalance with the CF gene (allele 2 = 6.6 kb of Km19/Pstl, chi 2 = 56; allele 1 = 2.1 kb of XV2c/Taql, chi 2 = 21). These two markers define a B haplotype which confers a relative risk of 55 to be gene carrier. From these data, the predictive value for an individual presenting with this haplotype to be heterozygous was computed to be 1/5. Presently, the risk of 1/20 for a randomized subject to be gene-carrier should be reexamined after study of this genotype. These results are very important practically, as they modify the classical data of genetic counselling concerning cystic fibrosis for the couples with a risk higher than 1/4.