Penetrant molecules transport networks

For NR based IPNs, Mathew et al. had investigated the sorption and diffusion of three aromatic solvents (benzene, toluene, and xylene) through NR/polystyrene semi-, and full IPNs, and reported that as the polystyrene content increased, the solvent uptake value decreased. This was attributed to the fact that the introduction of plastic phase decreased the chain flexibility of the network. The study further showed that the nature and size of the penetrant molecule affected the transport behaviour. Temperature, it was observed, affected the transport properties. The solvent uptake was found to inerease with temperature up to 65 °C, and at 70 °C, a deerease in uptake was observed (Figure 22.4). The values of sorption and permeation eoeffieients obtained showed a direct dependency on sample characteristics, blend composition, and crosslink level. The study further showed that as the number of crosslinks increased in the blends, the resistance offered to solvent uptake increased since the solvent molecules have to overcome the dense barrier of polymer cross-linking and entanglements to diffuse into the blend. 

Despite the few reported research being carried out on the transport of penetrant molecules through NR based IPNs, data obtained from the transport studies can be used to advantage in determining parameters of importance as they relate to rubber based networks. The following parameters can be determined (i) The molecular weight between crosslinks (Me) can be calculated as ... 

Adsorption of molecules proceeds by successive steps (1) penetration inside a particle (2) diffusion inside the particle (3) adsorption (4) desorption and (5) diffusion out of the particle. In general, the rates of adsorption and desorption in porous adsorbents are controlled by the rate of transport within the pore network rather than by the intrinsic kinetics of sorption at the surface of the adsorbent. Pore diffusion may take place through several different mechanisms that usually coexist. The rates of these mechanisms depend on the pore size, the pore tortuosity and constriction, the cormectivity of the pore network, the solute concentration, and other conditions. Four main, distinct mechanisms have been identified molecular diffusion, Knudsen diffusion, Poiseiulle flow, and surface diffusion. The effective pore diffusivity measured experimentally often includes contributions for more than one mechanism. It is often difficult to predict accurately the effective diffusivity since it depends so strongly on the details of the pore structure. 

As illustrated in Figure 2.1b, ideal locations of Pt particles are at the true triple-phase boundary, highlighted by the big star. Catalyst particles with nonoptimal double-phase contacts are indicated by the smaller stars. Pt gas interfaces are inactive due to the inhibited access to protons. Bulky chunks of ionomer on the agglomerate surface build the percolating network for proton conduction in secondary pores. Only individual or loosely connected ionomer molecules seem to be able to penetrate the small primary pores. It is unlikely that they could sustain notable proton conductivity. They merely act as a binder. Proton transport inside agglomerates, thus, predominantly occurs via water-filled primary pores, toward Pt water interfaces. 

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