Molecular Insights into End-Group Cluster Formation in Natural Rubber Induced by l -Quebrachitol, a Major Component of Natural Rubber Serum Waste

Natural rubber (NR) contains l-Quebrachitol (Q) as a major component of its serum waste, and its mechanical properties is strongly influenced by interactions between Q and the terminal groups of cis-1,4-polyisoprene. To elucidate these effects, we investigate hydrogen-terminated polyisoprene (PI₀), ester-terminated PI (PIV), hydroxy-terminated PI (PIVI), and their corresponding Q-associated systems (PIV + Q and PIVI + Q). Using equilibrated molecular dynamics simulations, we examine static, dynamic, and structural properties, including the end-to-end distance, radius of gyration, end-to-end vector autocorrelation, stress autocorrelation, diffusion coefficients, radial distribution functions of terminal groups around Q, and survival probabilities of terminal groups around Q. The incorporation of Q markedly slows rotational and stress relaxation in both ester- and hydroxy-terminated PI melts through the formation of stable Q-α5 and Q-α6 clusters mediated by a strong bifurcated hydrogen bonding. These results demonstrate that Q-induced physical junctions play a central role in enhancing the mechanical performance of NR.