Probing the structural evolution in deformed isoprene rubber by in situ synchrotron X-ray diffraction and atomic force microscopy

Developing a better understanding of the structural evolution in deformed polymers is key to designing new materials and structures that achieve superior mechanical properties. Here, we used in situ synchrotron wide-angle X-ray diffraction (WAXD) and atomic force microscopy (AFM) nanomechanical mapping (AFM-NM) to assess the strain-induced crystallization (SIC) and the associated structural evolution and mechanical properties of peroxide vulcanized isoprene rubber (IR) as a function of crosslink density (nu) and strain. The WAXD and AFM-NM results show agreement in the onset strain of SIC. Crystalline reflections appears in the WAXD while a nanofibrillar structure is found by AFM-NM. The higher nu, the smaller is the onset strain of a steep upturn in the stress-strain curves, the smaller is the onset strain of SIC, the higher is the crystallinity as evidenced in the WAXD, and the larger is the amount of nanofibrils seen by AFM-NM. Both WAXD and AFM-NM results show the SIC occurs rapidly at high strains while most chains remain in the amorphous state. The elastic modulus of the formed nanofibrils that range in diameter from several to a hundred nanometers, is two times higher than that of the amorphous regions. From the WAXD and AFM-NM results, a schematic model of structural evolution is proposed and used to illustrate the self-reinforcement mechanism in IR.