Engineering applications of current thermoset shape memory polymers are limited by three critical issues: demanding fabrication conditions (from 70 to 300 °C temperatures for hours or days), lack of reprocessability or recyclability, and low recovery stress and energy output.
Biomass derivative and recyclability are two major factors in developing sustainable epoxy thermosets that cope with the global oil crisis and climate change. However, the major bottleneck persists in comparatively low strength of biobased epoxy thermosets and a complex synthesis route of recyclable epoxy thermosets. Here, we broke through the bottleneck by simply applying the biomassbased tannic acid as a multifunctional curing agent to a rigid epoxy monomer. Due to the diverse bonding abilities and particular topological structure of the tannic acid, the as-prepared epoxy thermosets exhibited a hierarchical molecular structure with high mechanical strength and tunable functionalities such as damping, shape memory, and recyclability. Specifically, two types of tannic acid cross-linked epoxy thermosets with raw hydroxyl/epoxide ratios of 0.5 and 1.0, respectively, were prepared to investigate the influence of feedstock ratios. When less tannic acid was used, a more complex topological thermoset network and better damping capability were achieved, and the effective damping temperature range was extended to 38.4 °C. On the other hand, the equalstoichiometric epoxy thermoset possessed a higher tensile strength (∼98.4 MPa), better shape memory ability, and good recyclability. This dual sustainability concept (biomass feedstock and recyclability) may provide a promising opportunity for developing high-performance multifunctional thermoset polymers.
A shape memory and healable epoxy was prepared based on esterification between diglycidyl ether of bisphenol A and tricarballylic acid. The healing was achieved through transesterification at the fracture surface between two epoxy blocks in a confined space assisted by shape recovery force. The shape recovery of the compression programmed epoxy at elevated temperature tightly closed the gap between the two epoxy blocks, whose fracture surfaces are saw-cut and not perfectly matched during healing, which is the worstcase scenario for the healing test. A healing efficiency of ~60% was achieved. This suggests that, for naturally cracked surfaces, which have smoother fracture surfaces and better surface alignments during healing, higher healing efficiencies could be expected. We believe that the combination of shape memory and intrinsic healing capability within one network will broaden the applications of thermosets and enable recycling.
While thermosets with high mechanical properties and shape memory capabilities have been developed in recent years, two main bottlenecks persist in their intrinsic nonreusability and flammability, especially for those shape memory thermosets used in recycling-required field with high glass transition temperatures (T g ) and thus risky high temperatures to trigger shape recovery. Here, we report a new shape memory epoxy thermoset integrated with excellent fire retardancy, recyclability, high mechanical performance, and 100% shape recovery ratio. The shape memory effect of this new thermoset was directly triggered by high-temperature flame for the first time. Furthermore, the survival thermoset can be recycled by a simple solid-state recycling method and reused as reinforcing fillers for polyester. The highest recycling efficiency reached 85.4%, and the reinforced composite presented about four times higher storage modulus compared to that of neat sample. This work may open a door for application of thermoset shape memory polymers in many lightweight engineering structures and devices where fire hazard is a concern. The newly proposed concept of flame-triggered shape memory effect may also find applications in fire-protecting system.
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