Effect of polymer solvent on the mechanical properties of entangled polymer gels: Coarse-grained molecular simulation

Polymer gels are composed of a chemically or physically cross-linked polymer that is highly swollen with solvent. Two important limitations for the practical application of polymer gels are low toughness and a limited ability to tailor the strain-rate dependent mechanical response. Both these limitations are due to the high loadings of small molecule solvents that are typically incorporated into the gel formulation. Here, we provide insight into the role of physical entanglements on the performance of polymer gels, when the solvent molecular weight is large enough to entangle with the polymer network. Our simulations demonstrate that the solvent entanglements dominate the time-dependent elastic modulus of polymer gels with high-molecular-weight solvent. We have found that entanglement contribution to the modulus is essentially equal for the entangled polymer melt and loosely cross-linked entangled polymer gel at high strain rate. At lower strain rate, the modulus of the polymer gels develops a long-lived plateau that originates from “semi-trapped” entanglements formed between network strands and polymer solvent. The simulations were validated with selected experiments of chemically cross-linked polydimethylsiloxane (PDMS) elastomers loaded with a non-reactive silicone oil solvent, where the solvent molecular weight was varied. The simulations and experimental results demonstrate that highly entangled solvent can be used to tune the rate dependent modulus of polymer gels.