Exploring the Role of Hydroxy- and Phosphate-Terminated <i>cis</i>-1,4-Polyisoprene Chains in the Formation of Physical Junction Points in Natural Rubber: Insights from Molecular Dynamics Simulations
Mayank Dixit, Takashi Taniguchi
Abstract
High Resolution Image Download MS PowerPoint Slide 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: H PI H (pure PI with H terminal), ω PI α6 (PI with ω and α6 terminals), and ω PI PO 4 (PI with ω and PO 4 terminals) and revealed significant dynamics variations. Hydrogen bonds between α6 and α6 and PO 4 and PO 4 residues in ω PI α6 and ω PI PO 4 systems induce slower dynamics of hydroxy- and phosphate-terminated PI chains. Associations between α6 and α6 and PO 4 and PO 4 terminals are markedly stronger than ω and ω, and hydrogen terminals in H PI H and ω PI α6,PO 4 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 ω PI α6 and ω PI PO 4, 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.