Glassy polymers transport properties

Atomistically detailed models account for all atoms. The force field contains additive contributions specified in tenns of bond lengtlis, bond angles, torsional angles and possible crosstenns. It also includes non-bonded contributions as tire sum of van der Waals interactions, often described by Lennard-Jones potentials, and Coulomb interactions. Atomistic simulations are successfully used to predict tire transport properties of small molecules in glassy polymers, to calculate elastic moduli and to study plastic defonnation and local motion in quasi-static simulations [fy7, ( ]. The atomistic models are also useful to interiDret scattering data [fyl] and NMR measurements [70] in tenns of local order. 

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 ... 

Unrelaxed Volume - Relation To Sorption And Transport Properties Of Glassy Polymers... 

Nonlinear, pressure-dependent sorption and transport of gases and vapors in glassy polymers have been observed frequently. The effect of pressure on the observable variables, solubility coefficient, permeability coefficient and diffusion timelag, is well documented (1, 2). Previous attempts to explain the pressure-dependent sorption and transport properties in glassy polymers can be classified as concentration-dependent and "dual-mode models. While the former deal mainly with vapor-polymer systems (1) the latter are unique for gas-glassy polymer systems (2). 

The preceding structural characteristics dictate the state of polymer (rubbery vs. glassy vs. semicrystalline) which will strongly affect mechanical strength, thermal stability, chemical resistance and transport properties [6]. In most polymeric membranes, the polymer is in an amorphous state. However, some polymers, especially those with flexible chains of regular chemical structure (e.g., polyethylene/PE/, polypropylene/PP/or poly(vinylidene fluoride)/PVDF/), tend to form crystalline... 

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. 

M. E. Rezac, J. D. Le Roux, H. Chen, D. R. Paul, and W. J. Koros, Effect of mild solvent post-treatments on the gas transport properties of glassy polymer membranes. Journal of Membrane Science 90, 213-229 (1994). 

Although no commercial examples exist currently in the gas separation field, thin film composite membranes such as those pioneered by Cadotte and co-workers (10) may ultimately permit the use of novel materials with unique transport properties supported on standard porous membranes. Therefore, the focus in this paper will be on suggesting a basis for understanding differences in the permeability and selectivity properties of glassy polymers. Presumably, if such materials prove to be difficult to fabricate into conventional monolithic asymmetric structures, they could be produced in a composite form. Even if thin film composite structures are used, however, the chemical resistance of the material remains an important consideration. For this reason, a brief discussion of this topic will be offered. 

An overview of the physics of glassy polymers and the relationships between molecular mechanisms and macroscopic physical, mechanical and transport properties of polymer glasses is presented. The importance of local translational and/or rotational motions of molecular segments in the glass is discussed in terms of the implications for thermodynamic descriptions of the glass (configurational states and energy surfaces) as well as history dependent properties such as expansivity, refractive index, gas permeability, and viscoelastic mechanical behaviour. 

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