Binary diffusion, polymer transport properties

In accordance with theoretical predictions of the dynamic properties of networks, the critical concentration of dextran appears to be independent of the molecular weight of the flexible polymeric diffusant although some differences are noted when the behaviour of the flexible polymers used is compared e.g. the critical dextran concentrations are lower for PEG than for PVP and PVA. For ternary polymer systems, as studied here, the requirement of a critical concentration that corresponds to the molecular dimensions of the dextran matrix is an experimental feature which appears to be critical for the onset of rapid polymer transport. It is noteworthy that an unambiguous experimental identification of a critical concentration associated with the transport of these types of polymers in solution in relation to the onset of polymer network formation has not been reported so far. Indeed, our studies on the diffusion of dextran in binary (polymer/solvent) systems demonstrated that both its mutual and intradiffusion coefficients vary continuously with increasing concentration 2. ... 

The relationship between the gas-transport properties and composition of semicrystalline binary blends of cellulose (CELL) and PVA has been assessed by following the kinetics of CO2 sorption [95]. The blends are thermodynamically miscible with Xai = —0.985, consistent with the presence of favorable interactions due to hydrogen bonding between the two different polymers. As sorption takes place only in the amorphous regions, the absolute level of CO2 equilibrium sorption is relatively low with the highest value for pure CELL The sorption curves for the blends lie intermediate to those for the pure components. Accordingly, both the diffusion coefficient and permeabUity were increased in line with the CELL content, with little or no pressure dependency. 

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