Permeability of polymers to gases

Permeability of Polymers to Gases and Vapors Brochure P302-335-79, The Dow Chemical Company, Midland, Mich., 1979. 

Stern SA, Krishnakumar B and Nadakatti SM, "Permeability of Polymers to Gases and Vapours", in Mark JE (Ed), "Physical Properties of Polymers Handbook", AIP Press, Woodbury, NY, 1996, Chap. 50. 

Permeability of Polymers to Gases, Vapors, and Liquids, Alexander Leborits, Modern... 

Polymers are permeable to liquids having similar solubility parameters. Several different celts have been designed to measure the permeability of membranes to gases and vapors. 

Methods u.sed to measure the permeability of polymers to vapors fall broadly into two categories, namely those for water vapor and those for volatile liquids. In both cases the most common measuring technique is the weight change or gravimetric method. Several test standards are based on this. Other methods include techniques similar to those used for gases. 

Permeability of plastics to gases and liquids depends on the affinity of the gas or liquid to the polymer. For example, blow molded polyethylene... 

Vamac polymers are halogen- tee and therefore do not promote corrosion and are suitable for use in noncorrosive, flame-retardant applications. The permeability of Vamac to gases is low, compared with that of butyl mbber. In addition, AEM compounds have good adhesion to a variety of metals, fibers, and other elastomers. Mineral-filled compounds of Vamac can be brightly colored and are capable of maintaining their color quality after thermal aging. 

Transport properties of SPPO and its comparison with that of PPO dense films studied by Polotskaya and coworkers are shown in Table 10. The authors related the lower permeability of SPPO to gases compared to PPO to the reduction in free volume of the polymer on sulfonation. 

Hence polyethylene will be more permeable to liquids of similar solubility parameter, e.g. hydrocarbons, than to liquids of different solubility parameter but of similar size. The permeabilities of a number of polymers to a number of gases are given the Table 5.77. ... 

We test the permeability of polymer films or sheets to various vapors and gases by mounting the polymer between chambers that contain different concentrations of the migrant molecules. We can determine the permeability from pressure changes, volumetric changes, or by microanalytical techniques that measure the concentration of the migrant molecules in a stream of gas flowing across the low concentration side of the barrier. 

The factors affecting the selectivity and permeability of polymer membranes to different gases are best discussed on the basis of Eqs. (12) and (14). As noted in Eq. (12), the permeability coefficient, P, of a penetrant gas in a polymer membrane is the product of a (concentration-averaged) diffusion coefficient, D, and of a solubility coefficient,... 

Contrary to other atmospheric gases like N2, O2 or CO2, the permeability of polymer films for SO2 is very high (Table III). Obviously the molecular size of SO2 is not the dominating factor for its permeation rate. As the permeability coefficient P is defined by the product of the diffusion coefficient, D, and the solubility, S, the SO2 solubility of the polymer films plays an Important role. In fact the few data published on solubility of SO2 in polymers corroborate this expectation. Equilibrium solubility of SO2 in polyacrylate was found to be as high as 21.5% by weight at 760 mmHgl. In case of a bisphenol A polycarbonate. 

The permeability of a crystalline polymer to gases depends on its crystallinity index and degree of order. These depend on processing conditions, so the preparation conditions and thermal history of the samples to be measured must be clearly stated. 1... 

Table 9-20. Permeability of Polymer Membranes with Respect to Different Individual Gases...

To understand gas transport phenomenon it is critical to consider the interactions between the gas molecules and the pore wall. The van der Waals interactions between particles are explained well by the Lennard-Jones function containing two parameters, the kinetic diameter g (the distance where the potential energy between the particles is zero) and the well depth e (the deepest potential minimum between the particles). These parameters were used, e.g. by Freeman [52], to establish a theoretical basis for the relationship between selectivity and permeability for a range of polymers and gases (so-called first upper bonnd empirically determined by Robeson in 1991 [53] see also more recent paper by Robeson [54]). The rate of diffnsion of a gas is dependent on its kinetic diameter while its solnbility mainly depends on the condensability of the gas and consequently on the well depth for gas-gas interactions. 

A very large body of data on the gas permeability of many rubbery and glassy polymers has been published in the literature. These data were obtained with homopolymers as well as with copolymers and polymer blends in the form of nonporous dense (homogeneous) membranes and, to a much lesser extent, with asymmetric or composite membranes. The results of gas permeability measurements are commonly reported for dense membranes as permeability coefficients, and for asymmetric or composite membranes as permeances (permeability coefficients not normalized for the effective membrane thickness). Most permeability data have been obtained with pure gases, but information on the permeability of polymer membranes to a variety of gas mixtures has also become available in recent years. Many of the earlier gas permeability measurements were made at ambient temperature and at atmospheric pressure. In recent years, however, permeability coefficients as well as solubility and diffusion coefficients for many gas/polymer systems have been determined also at different temperatures and at elevated pressures. Values of permeability coefficients for selected gases and polymers, usually at a single temperature and pressure, have been published in a number of compilations and review articles [27—35]. 

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