Melting, Solidification, and Crystallization of a Thermoplastic Polyurethane as a Function of Hard Segment Content

Thermoplastic polyurethanes (TPU) with varying hard segment contents (HSC) are monitored during melting and solidifying (20 K/min , Tmax = 220 ° C) by small-angle and wide-angle X-ray scattering (WAXS and SAXS). Hard segments: MDI/BD. Soft segments: PTHF1000. The neat materials are injection-molded, having small amorphous hard domains (chord length d⎯⎯h ∼ 35% show sharp Bragg peaks and larger hard domains ( d⎯⎯h > 7 nm ). When heated, small domains melt, but crystallization in the remaining large domains is not detected. Upon cooling, large agglomerates segregate first, which crystallize immediately. Segregation starts for HSC = 42% at 160 °C and for HSC = 75% at 210 °C. When HSC ≤ 30%, the morphologies before and after are similar, but afterward, many hard blocks are dissolved in the soft phase at the expense of the hard domain fraction. In heating and cooling the melts, multiple homogenization and segregation processes are observed, which are explained by the agglomeration of hard blocks of different lengths in the colloidal fluid

Chord distribution functions of three-dimensional random media: Approximate first-passage times of Gaussian processes

The main result of this paper is a semi-analytic approximation for the chord distribution functions of three-dimensional models of microstructure derived from Gaussian random fields. In the simplest case the chord functions are equivalent to a standard first-passage time problem, i.e., the probability density governing the time taken by a Gaussian random process to first exceed a threshold. We obtain an approximation based on the assumption that successive chords are independent. The result is a generalization of the independent interval approximation recently used to determine the exponent of persistence time decay in coarsening. The approximation is easily extended to more general models based on the intersection and union sets of models generated from the iso-surfaces of random fields. The chord distribution functions play an important role in the characterization of random composite and porous materials. Our results are compared with experimental data obtained from a three-dimensional image of a porous Fontainebleau sandstone and a two-dimensional image of a tungsten-silver composite alloy.Comment: 12 pages, 11 figures. Submitted to Phys. Rev.

Étude par électrodéposition de la morphologie des montmorillonites. Effet des cations calcium

Le présent travail est l'application à l'étude de la morphologie des montmorillonites, d'une nouvelle méthode de préparation des échantillons pour le microscope électronique. Cette méthode de dépôt des particules, basée sur la charge électrique de ces dernières, permet de connaître sans ambiguïté la forme des microcristaux au sein de la suspension. Les auteurs étudient l'effet des cations échangeables dans le cas de la montmorillonite de Camp Berteaux et de la montmorillonite de Wyoming. Les expériences permettent de relier les faciès observés, aux propriétés rhéologiques de ces minéraux.Méring Jacques, Oberlin Mathieu-Sicaud Agnès, Villière J. Étude par électrodéposition de la morphologie des montmorillonites. Effet des cations calcium. In: Bulletin de la Société française de Minéralogie et de Cristallographie, volume 79, 10-12, 1956. pp. 515-522