Investigation on the impact of molecular weight distribution (unimodal and bimodal) on the viscoelastic properties of styrene-butadiene rubber via dynamic mechanical thermal analysis: A comparative study of emulsion and solution polymerized SBR

This research work is devoted to a thorough study of the viscoelastic properties of styrene-butadiene rubber (SBR) elastomers, with a specific focus on their applicability in tire tread compounds. Dynamic Mechanical Thermal Analysis (DMTA) was employed to evaluate three distinct SBR types: emulsion-grade SBR (E-SBR-1502) and two solution-polymerized SBR grades (SOL-5270M and SOL-5270H), which were further compounded with varying N330 carbon black loadings (30, 50, and 70 phr). Gel permeation chromatography (GPC) was employed to analyze the molecular weight distributions of the raw rubbers, showing distinct differences in polydispersity index (PDI), with E-SBR exhibiting a unimodal, wide distribution, and SSBRs with narrow and bimodal distributions. The dynamic mechanical thermal analysis assessed the viscoelastic properties, revealing that at low temperatures, E-SBR-1502 demonstrated a higher elastic modulus compared to S-SBRs due to greater contributions of short-chain macromolecules in the generation of occluded rubber and filler interactions. However, E-SBR-1502 exhibited lower energy loss and performance in traction applications at cold temperatures due to lower polymer portions. A detailed discussion on storage and loss moduli, as well as the impact of carbon black content at higher temperatures (60°C), showed how molecular structure, especially the high mobility of short-length polymer chains and the chain-end effects, influences stiffness and energy dissipation and thus affects the rolling resistance. Finally, it is concluded that both SSBR compounds give better viscoelastic properties (high at low temperature for wet-grip and low at high temperature for rolling resistance) in comparison to the E-SBR-1502 polymer. However, the SOL-5270M provides better processability than the SOL-5270H, while it also has a very low amount of high molecular weight chains to guarantee achieving the desired mechanical properties.