Chemical etching introduction

The scope of applications of MF has been broadened in recent years by the introduction of inert membranes, particularly polypropylene, polycarbonate, and Teflon . In general, these materials cannot be made by the methods developed for the cellulosics because of their insolubility. Both Teflon and pol)propylene MF membranes have been made by a controlled stretching procedure in which microtears are introduced Microporous polycarbonate membranes have been prepared by a unique radiation-track-etch method A thin polycarbonate film is exposed to ionizing radiation which leaves labile sites that can later be chemically etched to produce straight-throi channels. The pore size can be controlled by the etching conditions. The pores in these membranes, contrary to those in cellulosic membranes, are quite uniform in diameter. 

The surface chemical analysis made possible by XPS has proved useful in a number of areas. Simple detection of surface contamination has already been mentioned another technically important area is the detection of changes in surface functionality introduced by treatments such as plasma and flame modification and chemical etching. These treatments are extensively used in practice to modify surface properties such as adhesion and wettability and the use of XPS and other surface analysis techniques permits one to associate these changes with the introduction of specific chemical functionality at the surface. Excellent entries into the extensive literature in this area may be found in the monograph by Garbassi et al. (1994) and the review by Briggs (1990). 

In the materials with DBDI (Fig. 2.14(b)) the overall etched texture is much flatter. The regions of chemical segregation are more or less equally attacked. The material with DBDI does not show the precipitated crystals. A more detailed SEM description on the morphology of MDI and DBDI based PUs and mixtures of them, is made in section 2.3.2.2. where the materials are characterized and compared from two perspectives (a) effect of the SS macrodiol nature (polyester or polyether) (b) effect of type, and number of diisocyanates (crystallizing or not) and their order of introduction in the reaction synthesis. 

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