Polyimides glassy polymers

Glassy polymers with much higher glass transition temperatures and more rigid polymer chains than rubbery polymers have been extensively used as the continuous polymer matrices in the zeolite/polymer mixed-matrix membranes. Typical glassy polymers in the mixed-matrix membranes include cellulose acetate, polysul-fone, polyethersulfone, polyimides, polyetherimides, polyvinyl alcohol, Nafion , poly(4-methyl-2-pentyne), etc. 

The second key factor determining permeability in polymers is the sorption coefficient. The data in Figure 2.18 show that sorption coefficients for a particular gas are relatively constant within a single family of related materials. In fact, sorption coefficients of gases in polymers are relatively constant for a wide range of chemically different polymers. Figure 2.25 plots sorption and diffusion coefficients of methane in Tanaka s fluorinated polyimides [23], carboxylated polyvinyl trimethylsiloxane [37] and substituted polyacetylenes [38], all amorphous glassy polymers, and a variety of substituted siloxanes [39], all rubbers. The diffusion... 

Alentiev, A. Yu., Loza, K. A., and Yampol skii, Y. P. (2000). Development of methods for prediction of gas permeation parameters of glassy polymers Polyimides as altenating copolymers, J. Membrane Sci. 167, 91. 

In a rubbery polymer with flexible macromolecular chains (PDMS for example) the cavities forming the free-volume are clearly separated from each other. The detailed evaluation of the movement of a penetrant particle from cavity (1) to the neighboring (2), did not show any immediate back jumps (2) — (1). This is mainly do to the fact that the channel between (1) and (2) closes quiet quickly. In a polymer with stiff chains (glassy polyimide (PI) for example) the individual cavities are closer to each other and a rather large number of immediate back jumps ocurred during the time interval simulated (120). This indicates that once a channel between two adjacent cavities in a stiff chain polymer is formed it will stay open for some 100 ps. This makes the back jump (2) - (1) of the penetrant more probable than a jump to any other adjacent hole (3). This process seems to be one cause for the general tendency that the diffusion coefficient of small penetrants in stiff chain glassy polymers is smaller than in flexible chain rubbery polymers. 

The development of asymmetric membrane technology in the 1960 s was a critical point in the history of gas separations. These asymmetric structures consist of a thin (0.1 utol n) dense skin supported on a coarse open-cell foam stmcture. A mmetric membranes composed of the polyimides discussed above can provide extremely high fluxes throuj the thin dense skin, and still possess the inherently hij separation factors of the basic glassy polymers from which they are made. In the early 1960 s, Loeb and Sourirajan described techniques for producing asymmetric cellulose acetate membranes suitable for separation operations. The processes involved in membrane formation are complex. It is believed that the thin dense skin forms at the... 

FIGURE 9.2 Dependence of hydrocarbon solubility coefbcient in glassy polymers on hydrocarbon Lennard-Jones force constant, ejk, at T — 323 K and pressure of 2 atm (6FDA-TrMPD is polyimide based on dianhydride of 4,4 -hexafluoroisopropylydene diphthalic acid and 2,4,6-trimethyl-l,3-phenylenediamine PPO is polyphenylene oxide). (From Tanaka, K., Taguchi, A., Flao, J., Kita, FI., Okamoto, K., J. Membr. Sci., 121, 197, 1996. With permission.)... 

The polymer materials mainly used for the membranes are glassy polymers, the first and foremost polyimides. The use of glassy polymers having a rigid ensemble of macromolecules results in high separation effectiveness. Separation effectiveness in pervaporation processes is characterized by the separation factor, /3p, which is determined by the diffusion component, /3d, and the sorption component, /3s [8,55]. Let us consider the effect of chemical composition of polymer membranes on their transport properties with respect to aromatic, alicyclic, aliphatic hydrocarbons and analyze ways to improve these properties. 

The result for PET (poly(ethylene terephthalate)) is also shown in Figure 4. PET is a glassy polymer at room temperature and we would expect that t 3 and I3 behave in a similar way as in the polyimide. In fact I3 decreased, but the rise of t 3 is not what was expected. Apparently there is something more than the simple picture present thus far of the Langmuir- and Henry-type processes... 

Argon, A. S. and Bessonov, M. I. (1977) Plastic deformation in polyimides with new implications on the theory of plastic flow of glassy polymers, Phil. Mag., 35, 917-933. 

Snch shape of the Pip) curves is typical for glassy polymers. The left, low pressure branch of the curve is explained by sani-empirical dual-mode sorption and mobility model. The right branch that shows increases in permeability with increasing pressure is usually explained by the plasticization effects (see Ref. 19, p. 24). There is, however, one interesting peculiarity of PTMSN in conventional glassy polymers such as polyimides, polycarbonates, etc. the pressure p a where the curve passes through a minimum amount... 

Figure 5.4 Transition State Theory for diffusion in condensed media, (a) General representation of the transition state theory, (b) Diffusive jump in glassy polymer [ 17j. Reprinted from journal of Membrane Science, 73, E. Smit, M. H. V. Mulder, C. A. Smolders, H. Karrenbeld, j. van Eerden and D. Eeil, Modelling of the diffusion of carbon dioxide in polyimide matrices by computer simulation, 247 257, Copyright (1992), with permission from Elsevier, (c) Diffusive jump in microporous silica, reprinted with permission from AlChE, Theory of gas diffusion and permeation in inorganic molecular-sieve membranes by A. B. Shelekhin, A. C. Dixon and Y. H. Ma, 41, 58 67, Copyright (1995) AlChE...

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