Substantial Effect of Terminal Groups in cis-Polyisoprene: A Multiscale Molecular Dynamics Simulation Study

Hevea natural rubber (NR) consists of 99% cis-polyisoprene with dimethyl allyl-(trans-1,4-isoprene)2 (ω) and α terminal groups. These terminal groups provide excellent mechanical and physical properties to NR. Oochi et al. (Oouchi, M.; Ukawa, J.; Ishii, Y.; Maeda, H. Biomacromolecules2019, 20, 1394–1400) have elucidated the structures of six types of α terminals of NR by using a solid -state NMR study. We examine four types of cis-1,4-polyisoprene (PI) melt systems with different combinations of dimethyl allyl-(trans-1,4-isoprene)2 (ω) and hydroxylated isoprene (α6) terminal groups, i.e., pure PI (no terminal) (PII), ω-PI-ω (PIII), ω-PI-α6 (PIIII), and α6-PI-α6 (PIIV). From the obtained equilibrated systems, we computed the end-to-end vector autocorrelation function (C(t)), average relaxation time (τ), end-to-end distance (Re), radius of gyration (Rg), self-diffusion coefficients of polymer chains, radial distribution functions (RDFs) between terminal residues, and survival probability (P(τ)) for terminal groups. From the analysis of C(t), τ, and self-diffusion coefficients, we found that the presence of a hydrogen bond between α6 residues makes the dynamics of polymer chains slower in PIIII and PIIV melt systems. From the analyses of RDFs and the potentials of mean force (W(r)), the association between α6 terminals in ω-PI-α6 and α6-PI-α6 is significantly stronger than the isoprene–isoprene association in PII and the ω–ω association in PIII. By the analysis based on the obtained potential of the mean field, we calculated the cluster-formation fraction of terminal groups associated in clusters of a given size in the four systems. We found that in the PII and PIII systems no firm cluster formation is observed, but in the ω-PI-α6 and α6-PI-α6 systems, stable clusters of α6 terminals with a size s (2 ≤ s ≤ 5) are observed, which supports the formation of multiple branching points in between polyisoprene chains through their α6 terminals. We also found that the cluster-formation fraction in the α6-PI-α6 system is significantly enhanced compared to that in the ω-PI-α6 system.