Strain-Induced Molecular Orientation and Crystallization in Natural and Synthetic Rubbers under Uniaxial Deformation by In-situ Synchrotron X-ray Study

In-situ synchrotron wide-angle X-ray diffraction (WAXD) studies and simultaneous measurements of stress and strain during uniaxial stretching of various vulcanized rubbers were carried out (at room temperature and 0°C) to reveal the strain-induced molecular orientation and crystallization relationships. Rubbers evaluated included natural rubber (NR), synthetic poly-isoprene rubber (IR), poly-cis-1,4-butadiene rubber (BR) and butyl rubber (IIR). Some universal features were observed in these systems: (i) At high strains (> 5.0), the majority of the chains (up to 50 ≈ 75%) in natural and synthetic rubbers remained in the un-oriented amorphous state with only a small amount of crystalline fraction formed (10–20%). The rest of the chains were in the oriented amorphous state. (ii) During deformation, the oriented amorphous chains acted as precursors to strain-induced crystallization. A network of micro-fibrillar crystallites is formed within the closely populated vulcanization points, leading to the enhancement of mechanical properties at high strains. Different rubbers exhibited different behaviors during strain-induced crystallization. For example, poly-isoprenes (NR and IR vulcanized with sulfur and peroxide) showed strain-induced crystallization at a low strain of 2.5, resulting in larger crystalline but smaller oriented amorphous fractions. In contrast, BR and IIR crystallized at a higher strain of 4.0 lead to higher molecular orientation, higher oriented amorphous, but smaller crystalline fractions. The relationship between the molecular orientation and crystallization in strained rubber depends on the intrinsic crystallizability of the chains and the topology of the crosslinked network.