Penetrant glassy polymers, effective

The swelling mechanism is well established in rubbers (Flory, 1943). The expansion force generated by the solvent penetration is equilibrated by the entropic force linked to chain stretching. Little is known, in contrast, for the case of glassy polymers where plasticization effects are not sufficient to induce a devitrification. Swelling can be defined by... 

Odani, H., J. Hayashi and M. Tamura Diffusion in glassy polymers. II. Effects of polymer-penetrant interaction diffusion of methyl ethyl ketone in atactic polystyrene. Bull. Chem. Soc. Japan 34, 817 (1961). 

The predicted effect of du mode sorption on the time lag and permeability vras derived by Paul using the total immobilization transport model and experimentally verified by Paul and Kemp using molecular sieves embedded in a silicone mbber. This was an excellent model system which fulfilled the postulate of complete inunobilization of the Langmuirian mode penetrant. The possibility that gas molecules sorbed in the Langmuirian mode may not necessarily be completely immobilized in glassy polymers was first raised by Petropoulos in 1970 Equations were developed and the possibility of these being used to check the assumption of immobilization by sorption and permeation data were described. The relaxation of the... 

Recently Koros and Hopfenberg have considered explicitely the effect of dual mode sorption on the local effective concentration dependent diffusion coefficient for low activity penetrant migration in glassy polymers. They showed that... 

The effect is especially significant in the case of glassy polymers [8,9,32]. It was shown in several studies that diffusion coefficients of penetrants, including hydrocarbons, decrease with the decrease in molar fraction of free volume of the polymer (see, e.g.. Figure 9.8b). 

The models most frequently used to describe the concentration dependence of diffusion and permeability coefficients of gases and vapors, including hydrocarbons, are transport model of dual-mode sorption (which is usually used to describe diffusion and permeation in polymer glasses) as well as its various modifications molecular models analyzing the relation of diffusion coefficients to the movement of penetrant molecules and the effect of intermolecular forces on these processes and free volume models describing the relation of diffusion coefficients and fractional free volume of the system. Molecular models and free volume models are commonly used to describe diffusion in rubbery polymers. However, some versions of these models that fall into both classification groups have been used for both mbbery and glassy polymers. These are the models by Pace-Datyner and Duda-Vrentas [7,29,30]. 

Chapter 4(71) focuses on the characterization of sorption kinetics in several glassy polymers for a broad spectrum of penetrants ranging from the fixed gases to organic vapors. The sorption kinetics and equilibria of these diverse penetrants are rationalized in terms of the polymer-penetrant interaction parameter and the effective glass transition of the polymer relative to the temperature of measurement. The kinetic response is shown to transition systematically from concentration independent diffusion, to concentration dependent diffusion, and finally to complex nonFickian responses. The nonFickian behavior involves so-called "Case II" and other anomalous situations in which a coupling exists between the diffusion process and mechanical property relaxations in the polymer that are induced by the invasion of the penetrant (72-78). ... 

Figure 7. The effective diffusion coefficient for small molecules in glassy polymers as a function of penetrant concentration. Dp is the mutual diffusion coefficient for the penetrant population dissolved by the Henry s-law mode. Other symbols are defined in the text.

The concept of two or more modes of sorption of penetrants in polymers is very familiar to cellulose and protein chemists for the case of water vapor. In fact combined Langmuir and Henry s law sorption was proposed and correctly formulated by Matthes in 1944 for water in cellulose29. The discovery of dual mode sorption of gases in glassy polymers and the subsequent realization that diffusion constants determined by the time lag method did not have the same ample fundamental significance associated with these parameters for rubbery polymers was of profound importance. Not only were the many carefully determined diffusion coefficients in the literature of questionable value for polymers below their glass transition but a good deal of the careful speculation about solution and diffusion and the effect of... 

---