Inhomogeneous Natural Network Promoting Strain-induced Crystallization: A Mesoscale Model of Natural Rubber

Although synthetic rubbers show continuously improved mechanical properties, natural rubber (NR) remains irreplaceable in the rubber family due to its superior mechanical properties. A mainstream viewpoint regarding the superiority of NR is that NR possesses a natural network formed by linking the poly(cis-1,4-isoprene) chain terminals to protein and phospholipid aggregates; after vulcanization, the natural network additionally contributes to rubber mechanics by both increasing the network density and promoting the strain-induced crystallization (SIC) behavior. However, the reason why the natural network promotes SIC is still unclear; in particular, only using the increased network density cannot explain our finding that the NR shows smaller onset strain of SIC than Gel (the gel component of NR with higher network density) and even vulcanized NR. Herein, we point out that the inhomogeneous chain deformation is the alternative reason why SIC of NR takes place at smaller strain than that of Gel. More specifically, although the natural network is homogenous on the subchain length scale based on the proton double-quantum NMR results, it is essentially inhomogeneous on mesoscale (100 nm), as revealed by the small angle X-ray scattering analysis. This inhomogeneous network also leads to the mesoscale deformation inhomogeneity, as detected by the orientation of stearic acid (SA) probe, thus resulting in the smaller onset strain of SIC of NR. Based on the experimental results, a mesoscale model is proposed to qualitatively describe the crucial roles of inhomogeneous structure and deformation of natural network in NR’s mechanical properties, providing a clue from nature to guide the development of high-performance rubbers with controlled structures at mesoscale.