Probe the terminal interactions and their synergistic effects on polyisoprene properties by mimicking the structure of natural rubber

Abstract

Biosynthetic residues in vulcanized natural rubber (VNR) create a blend end-linking network through two kinds of terminal groups dispersed in the covalent network, leading to excellent mechanical properties, such as high tensile strength and high crack growth resistance. However, due to the complex composition of natural rubber (NR), it is not likely to rebuild the terminal structures directly. In this circumstance, this work designs the terminal structures of synthetic polyisoprene by mimicking the structure of NR at the principal level. In detail, oligopeptide groups and phosphate groups were attached at the end of polyisoprene respectively, to form similar end-linking network as found in VNR. Three terminal functionalized model polymers containing oligopeptides (4A), phosphates (P) or both oligopeptides and phosphates (PA) were synthesized and vulcanized to generate V4A, VP and VPA, respectively. The oligopeptides and phosphates interact with each other to form new blended aggregates, which makes the end-linking network integrated. Blended aggregates are captured by the CLSM and TEM. The rheological tests demonstrate that the formation of blend aggregates is the foundation of synergistic effect on shear modulus, which will disappear at high temperature or solution state. It is also found that the mechanical properties of VPA are superior to that of VP and V4A, which is attributed to the more integrated end-linking network structure. Further analysis showed that the synergistic effect between terminals makes the entanglements near the blended aggregates difficult to unwrap and become permanent entanglements, thereby increasing the crosslinking density. The more complete end-linking network endow VPA higher crystallization ability than VP and V4A. These results expand our understanding of how terminals work in NR and provide new insight into the generic design of mechanically strong elastomer.