Publisher's Synopsis
Microporous membranes are commonly used in separation processes such as battery separators or medical applications to control the permeation rate of chemical components due to the wide range of chemical structures, optimum physical properties, and low cost of polymers and polymer blends. These materials are known as the best candidates for the fabrication of microporous membranes. This book provides ample explanation for the production of microporous polypropylene membranes through the MEAUS process. The two main techniques to develop polymeric membranes are solution casting and extrusion followed by stretching. High cost and solvent contamination are the main drawbacks of the solution technique. Techniques to make porous membranes from polymers without using any solvent developed in the seventies of the last century for some applications, but most of the information on these processes remains proprietary to the companies and are not available to the scientific community. One of the techniques to fabricate microporous membranes is MEAUS (melt extrusion - annealing - uniaxial strain). It is based on the stretching of a polymer film containing a row-nucleated lamellar structure. Then, three consecutive stages carried out to obtain porous membranes: (1) creating a precursor film having a row-nucleated lamellar structure by mechanisms of shear and elongation-induced crystallization, (2) annealing the precursor film at temperatures near the melting point of the resin to remove imperfections in the crystalline phase and to increase lamellae thickness, and (3) stretching at low and high temperatures to create and then enlarge pores, respectively. In fact, in this process, the material variables, as well as the applied processing conditions, are key parameters that control the structure and the final properties of the fabricated microporous membranes. The material variables include molecular weight, molecular weight distribution, and chain structure of the polymer. These factors mainly influence the row-nucleated structure in the precursor films at the first step of the formation of microporous membranes. In the MEAUS process, the extrusion and production of the precursor films is a delicate process since the samples should be produced under a high draw ratio, and cooling rates. Obtaining a very uniform film is a major concern since any non-uniformity and thickness variations cause irregularities in the stress distribution. This method is relatively less expensive and there is no solvent contamination. Some drawbacks of the technology and industrial market of these products are the difficulty to have a homogenous distribution of the pore morphology created, and the narrow range of polymers that can create the initial row-lamellar structure.
