Role of Terminal Groups of cis-1,4-Polyisoprene Chains in the Formation of Physical Junction Points in Natural Rubber

Abstract

The terminal structures of cis-1,4-polyisoprene (PI) chains play a vital role in the excellent comprehensive performance of Hevea natural rubber (NR) with properties such as high toughness, tear-resistance, and wet skid resistance. The cis-1,4polyisoprene chain constituting NR exhibits a distinct composition of terminal groups comprising two distinct types, namely, the ω and α terminal groups. The structures of the ω terminal [dimethyl allyl (DMA)-(trans-1,4-isoprene) 2 ] and six kinds of α end groups of the polymer chain of NR have been explored by utilizing a newly developed 2D NMR method. In the present work, we examine different kinds of PI melt systems, and we choose various combinations of terminal groups: Hydrogen, one DMA unit with two trans isoprene units as ω end groups and ester-terminated isopentene (α1), hydroxy-terminated isopentene (α2), ester-terminated isobutane (α3), hydroxy-terminated isobutane (α4), esterterminated 1,4-cis-isoprene (α5), and hydroxy-terminated 1,4-cis-isoprene (α6), i.e., H PI H (PI 0 )−pure PI (Hydrogen terminal), ω PI α1 (PI I ), ω PI α2 (PI II ), ω PI α3 (PI III ), ω PI α4 (PI IV ), ω PI α5 (PI V ), and ω PI α6 (PI VI ). We evaluated dynamic and static properties of PI chains such as the end-to-end vector autocorrelation function (C(t)), its average relaxation time (τ), end-to-end distance (R ee ), and radius of gyration (R g ). We also estimated the diffusion coefficients of polyisoprene chains and pair correlation functions [radial distribution functions (RDFs)], potentials of mean force (PMFs) in between end residues, and survival probability (P(τ)) of end groups around the end group by analyzing the equilibrated trajectories of full-atom MD simulations. As per the examination of C(t), rotational relaxation time τ, and RDFs, we discovered that the existence of a strong hydrogen bond in α2−α2, α4−α4, and α6−α6 residues makes the dynamics of hydroxy-terminated polyisoprene chains in ω PI α2,α4,α6 melt systems slower. From the analyses of RDFs and PMFs (W(r)), the association between [α2]−[α2], [α4]−[α4], and [α6]−[α6] terminals in ω PI α2,α4,α6 melt systems is significantly stronger than in [ISO]−[ISO] [Hydrogen terminated 1,4-cis-isoprene:(ISO)] in H PI H and ω−ω, [α1]−[α1], [α3]−[α3], and [α5]− [α5] in ω PI α1,α3,α5 systems. We quantified the fraction of cluster formation of terminal groups of a given size in the seven PI melt systems by employing the criteria of PMFs. It is revealed that no stable cluster exists in the H PI H , ω PI α1 , ω PI α3 , and ω PI α5 melt systems. Conversely, in the ω PI α2 , ω PI α4 , and ω PI α6 systems, we perceived stable clusters of [(α2) p ] [(α4) p ] and [(α6) p ] end groups where p (2 ≤ x ≤ 6). These stable clusters validate the presence of physical junction points in between hydroxy-terminated polyisoprene chains through their α2, α4, 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.

Effect of Impurities on the Formation of End-Group Clusters in Natural Rubber: Phenylalanine Dipeptide as an Impurity Protein

“…The polymer chains in NR consist of cis -1,4-polyisoprene with ω-terminals and α-terminals. It has been reported that the ω-terminals contain a dimethylallyl group (DMA) and two to three trans isoprene units, ,, while the α-terminals are considered to consist of hydroxy, ester, ,,,− and phosphate groups . The resistance of rubber to deformation and fracture before vulcanization, known as green strength, has also been explored .…”

Ionic Pairing and Selective Solvation of Butylmethylimidazolium Chloride Ion Pairs in DMSO–Water Mixtures: A Comprehensive Examination via Molecular Dynamics Simulations and Potentials of Mean Force Analysis

“…In our framework, the interactions governing solvent–solvent, solvent–solute, and solute–solute dynamics are considered to be pairwise additive, encompassing both Lennard-Jones and Columbic components. Complete details of methodology and simulation protocols can be found in our previous work. , The PMF are extensively used to determine the stability of the contact pair, solvent-assisted pair, solvent-shared pair, and solvent-separated pair. − ,− The computation of the ion pair’s PMF, denoted as W ( r ), within the solvent milieu is achievable through the approach, which has been discussed extensively in our previous work and defined as − W ( r ) = prefix− ∫ r 0 r ′ normalΔ F ( r ′ ) normald r ′ + 2 k B T ln r r 0 …”

“…The potentials of mean force (PMF) has been extensively utilized to unveil the ion pairing within ILs in aqueous solutions as well as ion pairing in mixed polar mixtures. − Notably, the structural and dynamic attributes of water-alkyl-3-methylimidazolium IL mixtures exhibit dependence on both anion hydrophobicity and cation chain length . Meanwhile, Chen et al have extensively employed molecular force fields to simulate both the crystalline and liquid states of [BMIM]Cl as well as its binary mixtures with ethanol, shedding light on their structural characteristics.…”

Interactions between Water and a Hydrophobic Polymer

“…Rubber materials are widely used in various polymer composites in our daily life. − For instance, polymer rubbers are the main components in tires of vehicles. For safe driving of vehicles, the high braking performance of tire rubber on wet roads (wet-skid resistance) is essential.…”