Relations entre la structure et les proprietes des membranes de polytetrafluoroethylene greffe—II. Membranes bifonctionnelles

Bifunctional membranes containing both carboxylic and pyridine groups are obtained by successive grafting of acrylic acid and 4-vinylpyridine onto thin films of PTFE. Homogeneous membranes with a random distribution of grafting chains have been prepared by the direct radiation method. Mosaic membranes containing geometrically well defined acid and basic zones are made by pre-irradiation with X-rays using two absorbing shields; the two monomers are grafted in successive steps. The equilibrium and transport properties of these membranes have been determined. They can either result from the addition of the properties of each component (in the case of conductivity) or give rise to a new properties (e.g. abnormal negative osmosis). Whatever the structure of the bifunctional membrane (homogeneous or mosaic), this negative osmosis occurs within a pH range in which the membrane potential varies greatly i.e. when the ionic character of the membrane changes. Experimental data are discussed on the basis of thermodynamics of irreversible phenomena. The phenomenological coefficients characteristic for a membrane belonging to each of the two types (mono- and bi-functional) have been calculated. The reflection coefficient σ is negative: this suggests a potential application of these membranes in piezodialysis.