The mechanics of network polymers with thermally reversible linkages

Abstract

Network polymers with thermally reversible linkages include functionalities that continuously break and form covalent bonds. These processes dynamically change the network connectivity, which produces three distinct behaviors compared with conventional thermosetting polymers (in which the network connectivity is static): permanent shape evolution in the rubbery state; dependence of the number density of chains and associated thermal and mechanical properties on temperature and chemical composition; and a gel-point transition temperature above which the connectivity of the network falls below the percolation threshold, and the material response changes from a solid to liquid. This last property allows such materials to be non-mechanically removed, which is an attractive material capability for encapsulation in specialized electronics packaging applications wherein system maintenance is required. Given their complex, multi-physics behavior, appropriate simulation tools are needed to aid in their use.To meet this need, a thermodynamically consistent constitutive model is developed that accounts for the thermal–chemical–mechanical behavior of such materials. This model includes a representation for the permanent shape evolution that accompanies the dynamic network connectivity as well as non-equilibrium viscoelastic behavior to represent the material's glassy response. Analytic solutions in the rubbery state are derived to show the effects of competing time scales in the material, and the model is calibrated and validated against experimental data published in the literature. Finally, simple encapsulation scenarios are examined that demonstrate a substantial difference in behavior between conventional polymer networks and those with thermally reversible linkages under thermal–mechanical cycling.