Distinct Packing, Mobility and Stiffness in Sulfur- and TAIC-Crosslinked Styrene–Butadiene Rubber at Matched Interchain Joining Counts

At matched interchain joining counts, sulfur-crosslinked solution-polymerized styrene–butadiene rubber (SSBR) exhibited tighter packing, lower molecular mobility and greater local stiffness than triallyl isocyanurate (TAIC)-crosslinked SSBR. Atomistic molecular dynamics and small-strain calculations compared three matched parent sequences, each with 30 interchain joining contributions per crosslinked cell, alongside uncrosslinked controls. The smaller sulfur cells yielded higher joining-count density. Relative to TAIC, sulfur reduced the geometric free-volume fraction (FFV) by 5.67–6.30 percentage points and rubber-atom mean-squared displacement (MSD) at 250 ps by 60.0–64.8%. Its Hill Young’s modulus was 2.24–2.86 times that of TAIC. Sulfur also retained higher cooling-density breakpoints under all nine common analysis rules. These results jointly connect the spatial concentration of joining contributions, free volume and molecular motion to differences in network constraints between the two formulations. Equal joining counts or higher node functionality did not confer equal or greater local stiffness. A joint assessment of connectivity, packing and motion identified the defining formulation characteristics: greater local stiffness and restricted motion in sulfur, versus more free space and mobility in TAIC.