Role of terminal groups of cis-1,4-polyisoprene in the formation of physical cross-linking in NR --- All-atom simulation study ---

Hevea natural rubber (NR) consists of 99% cis- polyisoprene with dimethyl allyl-(trans-1,4-isoprene) (ω) and α terminal groups. These terminal groups provide excellent mechanical and physical properties to NR. Oochi et al[1] 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)(ω) and hydroxylated isoprene (α6) terminal groups, i.e., pure PI (no terminal) (PI_I), ω-PI-ω (PI_II), ω-PI-α6 (PI_III), and α6-PI-α6 (PI_IV). From the analysis of the end-to-end vector autocorrelation function C(t), average relaxation time τ, and self-diffusion coefficients of polymer chains, we found that the presence of a hydrogen bond between α6 residues makes the dynamics of polymer chains slower in PI_III and PI_IV 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 PI_I and the ω−ω association in PI_II. 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 [1] that in the PI_I and PI_II systems no firm cluster formation is observed, but in the ω-PI-α6 and α6-PI-α6 systems, on the other hand, 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.

[1] M. Oouchi, J. Ukawa, Y. Ishii, H. Maeda, Biomacromolecules 20, 1394−1400 (2019).

[2] D. Mayank and T. Taniguchi, Macromolecules in press ( 10.1021/acs.macromol.2c01414 )

We are grateful to the computing resources provided by the High Performance Computing Infrastructure of the University of Tokyo (project no. hp220128o) and by the Tohoku University Computer Center (202112-SCKXX-0017).