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. The influence of the linker architecture on the dynamics and mechanical properties with the information from the detailed atomistic model will be helpful in tuning the design of elastomers with fillers. In this study, we construct numerous all-atom models of polyisoprene (PI) networks using well-equilibrated precursor melts and sulfide crosslinks of a specific chemical architecture. We examine network characteristics that follow expectations based on our random crosslinking approach and report the presence of a substantial number of intramolecular connections formed. Thermodynamics and microscopic dynamics of the resulting networks are also probed. Comparing systems at 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). At frequencies accessible to our all-atom models, simulations confirm that quaternary linkages provide for the highest moduli with linker length holding a secondary role during deformations.