Programming and healing temperature effects on the efficiency of confined self-healing polymers

Shape memory polymers are smart materials capable of fixing a temporary shape and returning to their initial shape in response to an external stimulus. Since the discovery and acknowledgment of their importance in 1960s, shape memory polymers have been the subject of tremendous and continuous attention. In a previous study conducted on a biomimetic shape memory polymer (SMP), the ability of a self-healing composite to heal, and to repair and restore structural-length scale damage using a close-then-heal (CTH) self-healing mechanism was examined and validated. The present study is purposed with investigation of the effects on healing efficiencies of the variation of temperature during both thermo-mechanical programming and shape recovery under three-dimensional (3-D) confinement. The polymer considered was a polystyrene shape memory polymer with 6% by volume of thermoplastic particle additives (copolyester) dispersed in the matrix. After fabrication, and determination of their glass transition temperature using DSC, the specimens were allowed to go through a strain-controlled programming at a wide range of temperatures (20°C, 45°C, 60°C, 82°C, 100°C and 140°C), at a pre-strain level of 15%. Fracture was imposed using a three-point flexure apparatus, and was followed by shape recovery at multiple temperatures (73°C, 100°C, 122°C and 148°C). The self-healing efficiency was evaluated per flexural strength immediately after programming and following healing. The results and deductions attained were verified using EDS analysis and SEM inspection. It is inferred from the study that the programming temperature only very slightly affects the recovered strength. Programming the specimen above its glass transition temperature provided a marginal gain in strength recovery. Shape recovery (healing) temperature, however, was found to have a significant impact on the self-healing efficiency. A sudden “boost” was noted around the melting temperature of the thermoplastics, with a significant increase in the healing efficiency past the bonding temperature of the copolymer. It was observed that programming above the glass transition temperature of the composite and healing above the melting point of the thermoplastic addives ensured a maximum healing efficiency of up to 63% for the material considered