Exploring the Role of Hydroxy- and Phosphate-Terminated cis-1,4-Polyisoprene Chains in the Formation of Physical Junction Points in Natural Rubber: Insights from Molecular Dynamics Simulations

Abstract

This study elucidates the pivotal role of terminal structures in cis-1,4-polyisoprene (PI) chains, contributing to the exceptional mechanical properties of Hevea natural rubber (NR). NR’s unique networking structure, crucial for crack resistance, elasticity, and strain-induced crystallization, involves two terminal groups, ω and α. The proposed ω terminal structure is dimethyl allyl-(trans-1,4-isoprene)₂, and α terminals exist in various forms, including hydroxy, ester, and phosphate groups. Among others, we investigated three types of cis-1,4-PI with different terminal combinations: _HPI_H (pure PI with H terminal), _ωPI_α6 (PI with ω and α6 terminals), and _ωPI_PO4 (PI with ω and PO₄ terminals) and revealed significant dynamics variations. Hydrogen bonds between α6 and α6 and PO₄ and PO₄ residues in _ωPI_α6 and _ωPI_PO4 systems induce slower dynamics of hydroxy- and phosphate-terminated PI chains. Associations between α6 and α6 and PO₄ and PO₄ terminals are markedly stronger than ω and ω, and hydrogen terminals in _HPI_H and _ωPI_α6,PO4 systems. Phosphate terminals exhibit a stronger mutual association than hydroxy terminals. Potentials of mean force analysis and cluster-formation-fraction computations reveal stable clusters in _ωPI_α6 and _ωPI_PO4, supporting the formation of polar aggregates (physical junction points). Notably, phosphate terminal groups facilitate large and highly stable phosphate polar aggregates, crucial for the natural networking structure responsible for NR’s outstanding mechanical properties compared to synthetic PI rubber. This comprehensive investigation provides valuable insights into the role of terminal groups in cis-1,4-PI melt systems and their profound impact on the mechanical properties of NR.