Cross-Linked Polyisoprene Networks: Linker Architecture and Elastomer Properties

Cross-linking macromolecules is an important process that aims to modulate the mechanical properties of elastomers and meet desired specifications based on the application sought. The impact of cross-linking 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 cross-links of a specific chemical architecture. We first examine network characteristics that follow expectations based on our random cross-linking 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 cross-links, 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 nonequilibrium processes (constant rate and oscillatory deformations). Both equilibrium and nonequilibrium 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.