Research on architecture and composition of natural network in natural rubber

Though the superior properties of natural rubber (NR) have been attributed to its special network architecture, the currently accepted model "naturally occurring network" is far from describing its authentic network structure. In this paper, we focused on the composition of the chain entanglements in the network structure of unvulcanized NR. By using synchrotron wide-angle X-ray diffraction (WAXD), the evolution of its strain-induced crystallization (SIC) behaviors was real-time traced, and the stress-strain behaviors at various strain rates and temperatures were also tested. The results demonstrated that the entanglements can act as crosslinking points to increase the network chain density and lead to the easier SIC behavior. By applying the tube model to analyze the stress-strain curves of unvulcanized NR, we found that the contribution of the entanglement network to the stress is about an order of magnitude larger than that of the network formed by two terminal groups. Via double-quantum nuclear magnetic resonance (1H-DQ NMR), we further detected two types of entanglement in NR and deproteinized NR (DPNR), i.e. transiently trapped entanglements (TTEs) and permanently trapped entanglements (PTEs), among which TTEs are the main composition of the entanglement network. Moreover, the network formed by two terminals causes more PTEs in NR, while due to lacking the constraint of proteins, more TTEs exist in DPNR. Based on the research, we proposed a novel network model for unvulcanized NR.