Polyethylene microporous membranes

Figure 2.29 Scanning electron micrographs at approximately the same magnification of four microporous membranes having approximately the same particle retention, (a) Nuclepore (polycarbonate) nucleation track membrane (b) Celgard (polyethylene) expanded film membrane (c) Millipore cellulose acetate/cellulose nitrate phase separation membrane made by water vapor imbibition (Courtesy of Millipore Corporation, Billerica, MA) (d) anisotropic polysulfone membrane made by the Loeb-Sourirajan phase separation process...

In some cases, the rate-controlling polymeric membrane is not compact but porous. Microporous membranes can be prepared by making hydrophobic polymer membranes in the presence of water-soluble materials such as polyethylene glycol), which can be subsequently removed from the polymer matrix by dissolving in aqueous solution. Cellulose esters, loosely cross-linked hydrogels and other polymers given in Table 4.2 also give rise to porous membranes. 

From outward appearance membrane contactors look similar to other membrane devices. However, functionally the membranes used in contactors are very different. They are mostly nonselective and microporous. Membrane contactors can be made out of flat sheet membranes and there are some commercial apphcations. Most common commercial membrane contactors are, however, made from small-diameter microporous hollow fiber (or capillary) membranes with fine pores (illustrated in Figure 2.1) that span the hoUow fiber wall from the fiber inside surface to the fiber outside surface. The contactor shown as an example in Figure 2.1 resembles a tube-in-sheU configuration with inlet/outlet ports for the shell side and tube side. The membrane is typically made up of hydrophobic materials such as Polypropylene, Polyethylene, PTFE, PFA, and PVDF. 

Most of the available commercial microporous membranes such as polysulfone, polyethersulfone, polyamide, cellulose, polyethylene, polypropylene, and polyvinylidene difluoride are prepared by phase inversion processes. The concept of phase inversion in membrane formation was introduced by Resting [75] and can be defined as follows a homogeneous polymer solution is transformed into a two-phase system in which a solidified polymer-rich phase forms the continuous membrane matrix and the polymer lean phase fills the pores. A detailed description of the phase inversion process is beyond the scope of this section as it was widely discussed in Chapters 1 and 2 nevertheless a short introduction of this process will be presented. 

Stretched Membranes. Another relatively simple procedure for preparing microporous membranes is the stretching of a homogeneous polymer film of partial crystallinity. This technique is mainly employed with films of polyethylene or polytetrafluoroethylene which have been extruded from a polymer powder and then stretched perpendicular to the direction of extrusion.10 11 This leads to a partial fracture of the film and relatively uniform pores with diameters of 1 to 20jum. A typical stretched membrane prepared from tetrafluoroethylene is shown in the scanning electron micrograph of Figure 1.2. 

One of these types is the membrane-controlled transdermal therapeutic system, which is outlined in Figure 18.12. These systems consist of the following parts i) covering membrane, ii) drug reservoir, iii) micropore membrane controlling drug delivery, and iv) adhesive contact surface. (Further types of transdermal systems are going to be described in Chapter 16.2.4.3.3). The most commonly used membranes are polyethylene vinyl acetate and polyethylene [60-62]. 

Microporous polymeric membrane separators are characterized by pore sizes in the micrometer scale. Microporous polymeric membrane separators are mainly made of polyethylene (PE), polypropylene (PP), and the combinations of them (PE/PP and PP/PE/PP) because of their high chemical and mechanical stabilities. According to the number of layers, they can be classified into monolayer and multilayer polymeric microporous membranes. 

Most of the available commercial microporous membranes such as PSf, PES, polyamide, cellulose, polyethylene, polypropylene, and PVDF are prepared by phase inversion (phase separation) processes. The concept of phase separation in... 

In particular, MD is a thermally driven membrane operation in which a temperature gradient is applied between the two sides of a microporous membrane. This temperature difference results in a vapour pressure difference, leading to the transfer of water in vapour form through the membrane to the condensation surface. Hydrophobic membranes made in polyvi-nylidenefluoride (PVDF), polypropylene (PP), polyethylene (PE) and poly-tetrafluoroethylene (PTFE) with pore sizes of 0.2-1.0 pm are typically used. 

By laminating conventional polyethylene and polypropylene microporous membranes together, it is possible to obtain a separator with the desired shutdown function together with protection from rupture. Microporous membrane of liquid crystalline polyester, polyphenylene ether, aromatic polyamide, polyimide, polyamide imide resin, acrylic resin, and cross-linked polymer are now being studied as candidates for lamination with polyethylene in order to gain even greater heat resistance. 

This study is the extension of our previous study. Here, a variety of microporous ion-exchange membranes was prepared by plasma grafting various ionic monomers, such as acrylic acid (AA), methacrylic acid (MAA) and 2-(N,N-dimethyl)aminoethyl methacrylate (DAMA) on several lands of substrate membranes such as microporous polyethylene (PE) membranes and polytetrafluoroethylene (PTFE) membranes. These membranes were evaluated as supports for facilitated transport of CO2 using several kinds of monoprotonated amines as carriers. N,N-dimethylaminoethyl methacrylate (DAMA) grafted membranes prepared by a similar technique were also evaluated as a fixed carrier membrane for the facilitated transport of CO2. 

Microporous polyethylene (PE) membranes with various pore diameters and porosities and microporous polytetrafluoroethylene (PTFE) membrane were used as substrates for the plasma graft polymerization (Table II). Besides these porous substrates, homogeneous poly[ l-(trimethyl si lyl)-l-propyne] (PTMSP), which has the highest gas permeability among polymeric materials, was used as the substrate. The poly[l-(trimethylsilyl)-l-propyne] was synthesized from l-(trimethylsilyl)-l-propyne according to the literature procedure (19). Films were prepared by casting polymers from toluene solutions. Hereafter, the respective substrate membranes will be abbreviated as shown in Table II. 

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