Effect of molecular weight on the molecular chain orientation and strain-induced crystallization behaviors of HNBR under uniaxial stretching

Strain-induced crystallization (SIC) could be promoted by the presence of physical entanglement network within polymer systems with high regularity in molecular chain structure. However, such effect is not fully investigated within systems with lower chain structural regularity and inferior crystallization ability. In this study, hydrogenated nitrile butadiene rubber (HNBR) samples with varying molecular weights but share a similar acrylonitrile content of ∼ 37 wt% were used to investigate the influence of molecular weight on stretching orientation and SIC behaviors of HNBR by using polarized Fourier transform infrared spectroscopy (polarized FTIR) and two dimensional wide angle X-ray diffraction (2D WAXD) technique. Different chemical units within HNBR chain exhibited similar orientation degrees during stretching, implying a uniform orientation of molecular chains under strain. A positive correlation was obtained between the degree of whole chain orientation and molecular weight of HNBR. HNBR samples with Weight-average Molecular Weight (MW) of 14.6 × 1 0 4 g/mol and 32.5 × 1 0 4 g/mol could not crystallize under strain. When MW was high enough (52.7 × 1 0 4 g/mol), SIC occurred and presenting improved crystallization ability, where the crystal was composed of hydrogenated butadiene-acrylonitrile alternating copolymer segments. Higher degree of molecular chain orientation under strain induced by physical entanglement network of long-chain molecules was thought to account for the enhancement of SIC in HNBR-37. • Hydrogenated nitrile rubber (HNBR) with a moderate acrylonitrile content ( ∼ 37 wt%) is generally considered to have poor crystallization ability. It’s observed for the first time that the SIC of HNBR-37 is largely enhanced at high molecular weight level. • The relationship between molecular weight and SIC behavior in HNBR-37 is further discussed.