Swelling and Tensile Properties of Tetra-Polyethylene glycol via Coarse-Grained Molecular Models

Coarse-grained (CG) implicit-solvent potentials are developed for tetra-polyethylene glycol (PEG) at different water concentrations using the iterative Boltzmann inversion (IBI) technique. The resulting potentials are used to study the swelling and tensile properties of tetra-PEG gels at various swelling degrees phi(m). Two types of network topologies are considered, one "ideal" with a defect-free diamond connectivity and the other "realistic" as simulated from an experimentally based cross-linking process. Equilibrium swelling results for the realistic Tetra-PEG networks are consistent with available experimental data, while those for the ideal tetra-PEG networks exhibit much larger swelling. The realistic networks have higher Young's modulus Em at the same fm than ideal networks due to the presence of trapped entanglements. Uniaxial deformation results of realistic networks show that Em increases with degree of swelling, in accord with experimental results. The Young's moduli of gels at different fm confirm that the CG potentials developed by IBI are most suited to predict swelling states commensurate with the fm values at which the potentials were calibrated. A more generic, coarser potential, based on matching the persistence length of atomistic PEG chains in water, is able to produce a similar swelling behavior of an ideal diamond network.