Vulcanization Accelerators and Silica Coupling Agents in Polyisoprene Melts

Area of Science:

• Polymer Science and Engineering
• Materials Chemistry
• Computational Materials Science

Background:

• Effective dispersion of vulcanization accelerators is essential for optimizing cross-linked elastomer properties.
• Current elastomer formulations utilize complex mixtures involving silica particles functionalized with coupling agents.

Purpose of the Study:

• To investigate the molecular properties and self-association behavior of key accelerators: (N-cyclohexyl)-2-benzothiazole sulfenamide (CBS), 1,6-bis(N,N-dibenzylthiocarbamoyldithio)hexane (DBTH), and diphenyl guanidine (DPG) in polyisoprene melts.
• To determine the affinity of these accelerators for bare and functionalized silica surfaces, considering various coupling agents like MPTES, NXT, and TESPD.
• To explore the role of DPG in binary DPG-CBS formulations for improved CBS solubility.

Main Methods:

• Extensive all-atom molecular dynamics simulations were employed to study accelerator behavior in polyisoprene melts.
• Free energy calculations were used to assess accelerator-substrate affinity at infinite dilution and finite concentrations.
• Simulations examined interactions with silica surfaces modified with different coupling agents at varying grafting densities.

Main Results:

• CBS and DBTH exhibit low self-association in polyisoprene, while DPG forms small hydrogen-bonded aggregates.
• All accelerators show high affinity for silica surfaces, with DPG > CBS > DBTH.
• Coupling agents can displace accelerators from the silica surface; displacement increases with grafting density and coupling agent size.
• In DPG-CBS mixtures, DPG acts as a covering agent, enhancing CBS solubility in the polymer matrix.

Conclusions:

• The molecular interactions and surface affinities of vulcanization accelerators significantly influence elastomer network formation.
• Understanding accelerator self-association and substrate interactions is crucial for designing high-performance rubber compounds.
• DPG's ability to solubilize CBS offers a potential strategy for improved accelerator dispersion and elastomer properties.