Microphase behaviors and shear moduli of double-network gels: The effect of crosslinking constraints and chain...

By performing coarse-grained molecular dynamics simulations, we study the effect of crosslinking and chain uncrossability on the microphase behaviors and mechanical properties of the double-network gels. The double-network systems can be viewed as two separate networks interpenetrating each other uniformly, and the crosslinks in each network are generated, forming a regular cubic lattice. The chain uncrossability is confirmed by appropriately choosing the bonded and nonbonded interaction potentials. Our simulations reveal a close relation between the phase and mechanical properties of the double-network systems and their network topological structures. Depending on the lattice size and the solvent affinity, we have observed two different microphases: one is the aggregation of solvophobic beads around the crosslinking points, which leads to locally polymer-rich domains, and the other is the bunching of polymer strands, which thickens the network edges and thus changes the network periodicity. The former is a representation of the interfacial effect, while the latter is determined by the chain uncrossability. The coalescence of network edges is demonstrated to be responsible for the large relative increase in the shear modulus. Compressing and stretching induced phase transitions are observed in the current double-network systems, and the sharp discontinuous change in the stress that appears at the transition point is found to be related to the bunching or debunching of the network edges. The results suggest that the regulation of network edges has a strong influence on the network mechanical properties.