Barrier polymeric structures

The most important characteristic of nonporous membranes is that they are hydrophobic and contain no pores in the polymeric structure. This means that these membranes not only selectively act as a barrier to particles and polar species, but they also provide unique selectivity and specificity for the permeation and transport of a specific group of compounds that can readily solubilize and diffuse in the membrane material. The analyte extraction rate (permeability) in a nonporous membrane separation process is governed by the solution-diffusion mechanism, as commented on earlier. 

S.H. Jacobson, Molecular modeUng studies of polymeric gas separation and barrier materials structure and transport mechanisms, Polym. Adv. Technol., 5 (1994) 724-732. 

The charge carrier balance problem has been minimized by the introduction of multilayered polymeric structures that produce potential barriers at the internal interfaces. These potential barriers impose restrictions to charge carrier transport through the device and enhance the recombination probability and, consequently, the device efficiency. One example of such a structure is the rrO/PPV/CN-PPV/metal LED. CN-PPV presents a higher electroafiinity and ionization potential than PPV, so that there is a potential barrier for electron transport from the CN-PPV to the PPV and a potential barrier for holes in the opposite direction [218]. Several other conjugated polymers and molecules are also used in combination with PPV in heterolayer LEDs [213]. 

Lagaron, J. M., Catala, R.,and Gavara, R. Structural characteristics defining high barrier polymeric materials. Mater. Sci. TechnoL, 20, 1-7 (2004). 

Tethering may be a reversible or an irreversible process. Irreversible grafting is typically accomplished by chemical bonding. The number of grafted chains is controlled by the number of grafting sites and their functionality, and then ultimately by the extent of the chemical reaction. The reaction kinetics may reflect the potential barrier confronting reactive chains which try to penetrate the tethered layer. Reversible grafting is accomplished via the self-assembly of polymeric surfactants and end-functionalized polymers [59]. In this case, the surface density and all other characteristic dimensions of the structure are controlled by thermodynamic equilibrium, albeit with possible kinetic effects. In this instance, the equilibrium condition involves the penalties due to the deformation of tethered chains. 

Polymeric carbon refers to chains of carbon monomers (surface carbide) that are connected by covalent bonds. It has been shown recently47 that the barrier for C-C coupling on flat surfaces (1.22 eV) is half that for a step site (2.43 eV), and may indicate that the growth of these polymeric species is favored on terraces. Polymeric carbon may also refer to carbon chains that contain hydrogen. In the case of CO hydrogenation on ruthenium catalysts, polymeric carbon has been identified as a less reactive carbon that forms from polymerization of CHX and has an alkyl group structure.48... 

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