Thermal-oxidative aging behaviors of shape memory nitrile butadiene rubber composite with dual crosslinking networks

Abstract

Inspired by natural materials, sacrificial bonds have been successfully applied in the design of flexible shape memory polymers (SMPs). However, thermal oxidative stability of these SMPs is worth to be concerned because it is inevitable for the applications. Herein, model shape memory rubber composite was prepared by adding ZnCl2 and sulfur into nitrile butadiene rubber (NBR). Dual crosslinking networks with sacrificial bonds, composed of Zn2+-CN coordination bonds, was constructed in NBR composite, which endowed itself a good shape memory property in air, water and even oil. Through 20-day thermal oxidation at 100 °C, Zn2+-CN coordination bonds were partially dissociated and ZnCl2 agglomerations were gradually dissolved into rubber matrix. The growth of covalent crosslinking density accompanied with chains scission during aging. Excessive growth in covalent crosslinking density and the loss of coordination bonds induced a rapid ductile-brittle transition of NBR composite in mechanical tests. Ruptured chains developed an additional relaxation with Zn2+ ions to broaden glassy transition of rubber matrix through aging, which increased the shape fixing ratio of NBR composite and descended its recovery ratio. Moreover, the shape recovery processes were obviously slowed down owing to aging. These novel findings reveal that thermal-oxidative aging processes change the architecture of dual crosslinking networks and affect shape memory behaviors of modeled rubbers which are practically useful for the lifetime of their products.