Stretching-Induced Crystallization in Less Entangled Rubbers

Compared to natural rubber (NR), petroleum-derived synthetic rubbers (SR) generally exhibit greater structural diversity and thermal stability. However, due to the lack of nonrubber components such as phospholipids and proteins, the mechanical properties of SR, such as strength and toughness, are far inferior to those of NR. It is reasonable to assume that the mechanical properties of rubbers are largely governed by chain networks. The coexistence of physical entanglements and chemical cross-links makes it difficult to distinguish their respective contributions. We elucidate the influence of physical entanglement using a series of model mixtures consisting of ultrahigh-molecular-weight (UHMW) uncross-linked rubber with its chemically identical unentangled oligomers. The concentration of physical entanglement can be quantitatively controlled and determined by rheological measurements. Uniaxial tensile results reveal that an intense entanglement network limits the maximum draw ratio of rubber, while slightly reducing the entanglement concentration could promote its elongation behavior. Due to the lack of stress transmitters, mixtures with a higher amount of oligomer fraction become brittle and soft. Combined with true stress–strain curve analysis, in situ synchrotron wide-angle X-ray scattering results provide conclusive evidence that the stretching-induced crystallization is delayed at higher strains with decreasing UHMW polymer content, and the onset critical strain ε c is inversely proportional to the entanglement concentration. Our observations are believed to provide new perspectives and understanding for designing rubbers with high-performance mechanical properties.