Crosslinked polyisoprene networks: linker architecture and elastomer properties

Crosslinking macromolecules is an important process that aims to modulate mechanical properties of elastomers and meet desired specifications based on the application sought. The impact of crosslinking density on rubber moduli has been well established by theory, experiments and computational studies. However, several reports imply a role for the length of the sulfide bond and the attachment location. In this study, we construct all-atom models of polyisoprene (PI) networks using equilibrated precursor melts and sulfide crosslinks of a specific chemical architecture. We first examine network characteristics which follow expectations based on our random crosslinking approach. We report the presence of a substantial number of intramolecular connections formed. Thermodynamics and microscopic dynamics of the resulting networks are also probed. Comparing systems with the same number of crosslinks, we find that local mobility is most decelerated in the presence of long quaternary connections. To resolve any impact on mechanical properties we resorted to extensive characterization of moduli via equilibrium (stress-stress fluctuations) and non-equilibrium processes (constant-rate and oscillatory deformations). Both equilibrium and non-equilibrium simulations (at times/frequencies accessible to our all-atom models) confirm that quaternary linkages provide for the highest moduli with linker length holding a secondary role during moderate deformations.