Effect of cross-linking degree on thermal-mechanical properties of vulcanized SBR

Styrene-butadiene rubber (SBR) has attracted widespread attention due to its excellent thermal-mechanical properties, which critically govern its processing and service performance. While existing research predominantly focuses on blending and filling modifications, the influence of vulcanization characteristics on these properties and their underlying micro-mechanisms remain underexplored. This work systematically investigates how cross-linking degree ( D c) modulates the thermal-mechanical properties of vulcanized SBR, including shear viscosity η , bulk viscosity η _b , specific heat capacity( C _p , C _v ), as well as the thermal conductivity( κ ). Key findings reveal that increasing shear rate (γ&) enhances the susceptibility of molecular chains to orientation, thereby decreasing η by 83.4–93.2%. Moreover, greater γ& resulted in heightened intermolecular forces, constraining the movement of the molecular chains and consequently increasing η _b by 88.6-100.9%. Conversely, higher D c intensifies intermolecular constraints, elevating η and η _b by 65.5–23.3% and 6.6–11.1%, respectively, within the same shear regime. Thermally, a 48.4% increase in κ is observed at D c = 8.0 compared to D c = 1.0, attributed to amplified low-frequency phonon density in tightly cross-linking networks. Meanwhile, restricted chain mobility at elevated D c suppresses heat absorption, decreasing C _p and C _v by 8.9% and 8.6%, respectively, across 300-330K. Critically, these insights provide actionable guidelines for tailoring vulcanization parameters to balance processability and service performance in SBR-based products. The elucidated micro-mechanisms that spanning phonon transport modulation and chain dynamics restriction offer fundamental principles for designing cross-linked elastomers with targeted multifunctional properties.