Investigation of Homogeneous and Heterogeneous Cluster Formation in Mixtures of Ester and Hydroxy-Terminated cis-1,4-Polyisoprene Chains in Oligomers of Natural Rubber

Abstract

The terminal structures of cis-1,4-polyisoprene (PI) chains significantly influence the exceptional mechanical properties of Hevea natural rubber (NR), including high toughness, wet skid resistance, and strain-induced crystallization. We conducted all-atom molecular dynamics simulations to investigate the structural and dynamic properties of PI melt systems with various terminal group combinations. These included single-component melts and binary mixtures of chains with ester- and hydroxy-terminated α-terminal groups. The study revealed that hydrogen bonding between α-terminal groups drive the formation of stable homogeneous and heterogeneous clusters. In single-component systems, hydroxy-terminal groups promoted homogeneous clusters, while in binary mixtures, heterogeneous clusters formed between ester and hydroxy terminals. These clusters, ranging in size from 2 to 5 chains, serve as physical junction points, slowing chain dynamics and enhancing network stability. Dynamic properties, such as rotational relaxation, Rouse mode times, and stress–stress autocorrelation, were significantly influenced by cluster formation, particularly in mixed systems. The stress–stress autocorrelation function, G(t), exhibits a Rouse-type relaxation behavior (G(t) ∼ t^–1/2) in the intermediate time range for PI₀. In contrast, the mixed melt systems PI₀ show a slower relaxation compared to the pure components. This slower relaxation is attributed to the formation of stable, well-ordered heterogeneous clusters, driven by hydrogen bonding between ester and hydroxy-terminal groups. These findings provide evidence for the formation of physical junction points between hydroxy- and ester-terminated polyisoprene chains through their respective α1, α2, α3, α4, α5, and α6 terminals. These physical junction points might be crucial for superior properties of NR such as high toughness, crack growth resistance, and strain-induced crystallization.