La microfiltration tangentielle avec décolmatage est étudiée sur une membrane céramique tubulaire en alumine pure à structure anisotrope dont la couche filtrante possède des pores de diamètre 0,2 um, Un appareil de microfiltration tangentielle original de très faible volume mort et dénué de boucle de recirculation est présenté. Il permet d'obtenir en tout point de la membrane et rapidement des conditions identiques de pression, vitesse tangentielle et concentration. Le décolmatage est assuré par un organe refoulant une fraction de perméat sous des conditions ajustables.

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We report in this paper detailed measurements of the linear and nonlinear optical properties of thin films of P(VDF-TrFE) copolymers (with composition 70/30 mol%). The polymer films are cast from solutions using the spin coating technique and poled at saturation using very low-frequency a.c. fields. The refractive index (n = 1.42) is determined from measurements of the Brewster incidence, and the thickness from measurements of the optical path close to the normal incidence. Recording of the Maker fringes and calibration of the set-up with a quartz plate allow us to determine the second-harmonic coefficients: d33 = 4.3, d31 = 0.22 · 10−12 m/V and the optical dispersion n(2ω) − n(ω) = 0.0077 (ω is the frequency of the incident laser beam at 1060 nm). The spin-coated copolymer films are optically unaxial due to a preferential orientation of the chain axes parallel to the plane of the film, and measurement of their birefringence gives ne − no = −0.0083. Therefore, this feature allows us to find a direction of phase matching along which the first- and second-harmonic waves have an infinite coherence length. It is observed using an index-matching liquid.

The ferroelectric polarization of vinylidene fluoride-trifluoroethylene copolymers (70/30 mol %) is investigated using complementary measurements of the surface charge and the second harmonic intensity (at 530 nm) generated in the polymer from a Nd:YAG laser beam (at 1060 nm). Due to the nonzero electrical conductivity of the polymer (especially above room temperature), the nonlineear optical technique provides better measurement of the bulk polarization, its changes with time, the applied electric field, and temperature. First, we present optical results obtained on polarized films after removal of the electrodes: they confirm the proportionality between the second harmonic intensity and the square of the ferroelectric polarization, and the centrosymmetric character of the paraelectric phase; they also show the temperature dependence of the remanent polarization in the crystal phase. Second, we present simultaneous measurements of the surface charge and of the second harmonic intensity under very low-frequency applied voltage through transparent electrodes. The field dependence of the measured polarization is discussed in terms of a microstructural analysis of the dielectric properties in the semicrystalline material. It is also shown that unipolar voltages applied during long periods of time are able to produce charge injection and space charge in the vicinity of the electrodes which can result in screening of the applied electric field.