Swelling behavior equilibrium forces

Through the uptake of a substance in a polymer matrix, time dependent changes in the polymer matrix can take place, particularly at high concentrations. As a consequence, the diffusion coefficient can be time- as well as concentration-dependent. One observes such behavior for example by the sorption of substances that lead to swelling at temperatures below the Tg. After a relatively rapid approach to an apparent state of equilibrium one observes a slow change towards the actual equilibrium (Fig. 9-2e). These two-step processes are caused by a gradual loosening of the cohesive forces between the macromolecules. 

Molecular simulations of ionomer systems that employ classical force fields to describe interactions between atomic and molecular species are more flexible in terms of system size and simulation time but they must fulfill a number of other requirements they should account for sufficient details of the chemical ionomer architecture and accurately represent molecular interactions. Moreover, they should be consistent with basic polymer properties like persistence length, aggregation or phase separation behavior, ion distributions around fibrils or bundles of hydrophobic backbones, polymer elastic properties, and microscopic swelling. They should provide insights on transport properties at relevant time and length scales. Classical all-atom molecular dynamics methods are routinely applied to model equilibrium fluctuations in biological systems and condensed matter on length scales of tens of nanometers and timescales of 100 ns. 

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