Glassy polymer membranes diffusion

Dual-Mode Gas Sorption and Diffusion in Glassy Polymer Membranes. 97... 

So far, it appears that the gas transport properties of glassy polymer membranes, manifested in a decreasing P(a), or increasing D(C), function can be adequately represented by the above dual diffusion model with constant diffusion coefficients Dl5 D2 (or Dtj, DX2). We now consider the implications of this model from the physical point of view ... 

The above results indicate that the general characteristics of gaseous diffusion in glassy polymer membranes can be represented reasonably well in terms of the dualmode concept. The basic reason for the observed increasing D(C) function is seen to be the concurrent increasing proportion of less strongly sorbed (and hence more easily activated) penetrant molecules. The model is, no doubt, highly idealized, but is nevertheless shown to be physically reasonable and consistent with the correspond-... 

S. The diffusion coefficients of gases in glassy polymer membranes are strong functions of the penetrant gas concentration in the membranes (or of the gas pressure), and depend also on polymer morphology (crystallinity, orientation), crosslinking, and chain mobility. The chain mobility depends, in turn, on the polymer free volume, the... 

It is known that glassy polymer membranes can have a considerable size-sieving character, reflected mainly in the diffusive term of the transport equation. Many studies have therefore attempted to correlate the diffusion coefficient and the membrane permeability with the size of the penetrant molecules, for instance expressed in terms of the kinetic diameter, Lennard-Jones diameter or critical volume [40]. Since the transport takes place through the available free volume in the material, a correlation between the free volume fraction and transport properties should also exist. Through the years, authors have proposed different equations to correlate transport and FFV, starting with the historical model of Cohen and Turnbull for self diffusion [41], later adapted by Fujita for polymer systans [42]. Park and Paul adopted a somewhat simpler form of this equation to correlate the permeability coefficient with fractional free volume [43] ... 

Poly(substituted acetylene)s such as PTMSP and PMP, amorphous fluoro-polymers like Teflon AF and Hyflon AD, polymers with intrinsic microporosity, and thermally rearranged (TR) polymers are the candidate polymers for highly permeable glassy polymer membranes. The high free volume in glassy polymers contributes to enhanced diffusion and permeation of small gas molecules. The gas permeation performances of these highly permeable polymers even surpass upper bounds for CO2/N2, CO2/CH4 and H2/CO2 separations. 

It has been shown that olefin/paraffin selectivity of glassy polymer membrane materials was mainly of diffusive nature." Indeed, with the pressure and temperature conditions currently used in the literature, sorption isotherms of C2/C3 olefins and paraffins in glassy polymer are very similar." " " 114,125... 

Glassy polymer membranes are a contrast to rubbery ones. These consist of ultrathin polymer layers that are supported on inert microporous layers. The ultrathin layer, which is responsible for the membrane s selectivity, contains no large crystals, but smaller structures sometimes described as fringed micelles. In these membranes, the partition coefficient H correlates closely with the critical temperature of the diffusing gas, as shown in Fig. 18.2-1. Thus much of the selectivity between species with similar critical temperatures must be dominated by differences in diffusion. The diffusion coefficients in glassy polymers are smaller than in rubbery polymers, characteristically around 10 ° cm /sec. These small diffusion coefficients are of course why these membranes must be used as ultrathin layers. 

Fig. 38. Permeability as a function of molar volume for a mbbery and glassy polymer, illustrating the different balance between sorption and diffusion in these polymer types. The mbbery membrane is highly permeable the permeability increases rapidly with increasing permeant size because sorption dominates. The glassy membrane is much less permeable the permeability decreases with increasing permeant size because diffusion dominates (84).

In the limit of zero-concentration, gas-glassy polymer systems behave "ideally. As the gas concentration in the membrane approaches zero the solubility coefficient becomes constant with the value ao [eqs. (1), (3) and (4)]. In the same limit, the diffusion coefficients are constant and equal to the diffusion coefficient Do, [eqs. (5), (7), (8) and (9)]. As typical of limiting values, ao and Do have no correspondence to... 

Permeation of gases in glassy polymers can also be described in terms of the dual sorption model. One diffusion coefficient (Do) is used for the portion of the gas dissolved in the polymer according to the Henry s law expression and a second, somewhat larger, diffusion coefficient (DH) for the portion of the gas contained in the excess free volume. The Fick s law expression for flux through the membrane has the form... 

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