Effect of chain extender on microphase structure and performance of self-healing polyurethane and poly(urethane-urea)

In this work, four aliphatic chain extenders, hexanediol (HDO), hexane diamine (HDA), cystamine (CY), and cystine dimethyl ester (CDE), were chosen to synthesize four kinds of polyurethane and poly(urethane-urea)s (PUs), respectively. HDO extended polyurethanes, HDA extended poly(urethane-urea), CY extended poly(urethane-urea), and CDE extended poly(urethane-urea) were denoted as OPU, APU, CPU, and SPU, respectively. The effect of chain extender type on microphase structure and performance of four PUs was investigated. Our research showed that mechanical strength increased in the following order: OPU < SPU < CPU < APU, and self-healing performance increased in the opposite direction. This result is attributed to the increasing degree of microphase separation: OPU < SPU < CPU < APU. The optimal sample SPU has not only excellent mechanical properties (tensile strength of 27.1 MPa and elongation at break of 397.7%), but also exhibits superior self-healing performance (self-healing efficiencies of 95.3% and 93.5% based on tensile strength and elongation at break). The moderate degree of microphase separation between the soft segments and the hard segments, the introduction of disulfide bonds and low degree of hydrogen bonding are responsible for preparing a polyurethane or poly(urethane-urea) system with high mechanical strength and excellent self-healing performance simultaneously. This work provides useful information for us to develop self-healing polyurethane or poly(urethane-urea) materials in the future.