Tunable reversible deformation of semicrystalline polymer networks based on temperature memory effect

Two-Way Reversible Shape Memory Behavior of Chitosan/Glycerol Film Triggered by Water

Reversible shape memory polymers (SRMPs) have been identified as having great potential for biomedical applications due to their ability to switch between different shapes responding to stimuli. In this paper, a chitosan/glycerol (CS/GL) film with a reversible shape memory behavior was prepared, and the reversible shape memory effect (SME) and its mechanism were systematically investigated. The film with 40% glycerin/chitosan mass ratio demonstrated the best performance, with 95.7% shape recovery ratio to temporary shape one and 89.4% shape recovery ratio to temporary shape two. Moreover, it shows the capability to undergo four consecutive shape memory cycles. In addition, a new curvature measurement method was used to accurately calculate the shape recovery ratio. The suction and discharge of free water change the binding form of the hydrogen bonds inside the material, which makes a great reversible shape memory impact on the composite film. The incorporation of glycerol can enhance the precision and repeatability of the reversible shape memory effect and shortens the time used during this process. This paper gives a hypothetical premise to the preparation of two-way reversible shape memory polymers.

Hierarchical network relaxation of a dynamic cross-linked polyolefin elastomer for advanced reversible shape memory effect

Reversible shape-memory polymers (RSMPs) are highly desired for soft actuators due to the repeatability of deformation. Herein, a polyolefin elastomer vitrimer (POEV) was prepared by constructing a dynamic cross-linked network based on boronic ester bonds. POEV showed varied network relaxation in a wide temperature range due to hierarchical network relaxation, and then the entropy decreased and the relaxation of POEV chains was facilely controlled by temperature. The controllable relaxation of POEV by programming the temperature enabled the actuation domain with a reduction in entropy and the skeleton domain with a relatively high entropy can be built in POEV, greatly affecting the reversible shape memory effects (RSMEs). The topological rearrangement resulted from the activated exchange of dynamic covalent bonds, which enables POEV with good shape reconfigurability, and allows for complicated 3D shapes and shape-shifting on demand. More interestingly, combining the decreasing entropy of POEV chains and fully topological rearrangement tailored by temperature, hybrid aligned carbon nanotubes (CNTs) can be constructed in POEV via a two-stage training. Then, the aligned CNTs can enhance the elasticity and act as a hybrid skeleton for RSMEs, avoiding the negative impact of CNTs on the reversible actuation strain. The hierarchical network relaxation facilitates combining all these unusual properties in one shape memory network synergistically, paving new avenues for realizing smart materials with advanced RSME.

Configurational Entropy Regulation in Polyolefin Elastomer/Paraffin Wax Vitrimers by Thermally Responsive Liquid-Solid Transition for Force Storage

The work output of shape memory polymers during shape shifting is desired for practical application as actuators. Herein, a polyolefin elastomer (POE) and paraffin wax (PW) are co-cross-linked by dynamic boronic ester bonds to enhance the network elasticity and the stress transfer between the two phases, endowing high force storage capacity to the prepared vitrimers. Depending on the phase of PW, one-way force storage is realized by programming at a low temperature (25 °C), owing to which solid PW can promote the locking of POE chains in a low-entropy state, while reversible force storage can be realized by programming at a high temperature (75 °C), owing to which the relaxation of chains facilitated by liquid PW can promote the construction of a stable structure. Based on one-way force storage, a weight-lifting machine with a weight of 20 mg prestrained at 25 °C can lift a 100 g weight, showing a lifting ratio of no less than 5000, with a high work output of 0.98 J/g. A high-temperature alarm can be triggered at varied temperatures (43-56 °C) through controlled force release by adjusting the PW content and programmed prestrains. Based on the reversible force storage, crawling robots and artificial muscles with a work output of 0.025 J/g are demonstrated. The dynamic cross-linking network also confers mold-free self-healing capability to POE/PW vitrimers, and the repair efficiency is enhanced compared with the POE vitrimer due to the improved POE chain motion by liquid PW. The realized one-way and reversible force storage and self-healing by POE/PW vitrimers pave the way for the application of SMPs in the fields of soft robotic actuators.

Self-Sensing Actuators Based on a Stiffness Variable Reversible Shape Memory Polymer Enabled by a Phase Change Material

Soft actuators with integrated mechanical and actuation properties and self-sensing ability are still a challenge. Herein, a stiffness variable polyolefin elastomer (POE) with a reversible shape memory effect is prepared by introducing a typical phase change material, i.e., paraffin wax (PW). It is found that the variable stiffness of POE induced by PW can balance the reversible strain and load-bearing capability of actuators. Especially, carbon nanotubes (CNTs) are concentrated in a thin surface layer by spraying and hot pressing in the soft state of POE/PW blends, providing signal transductions for the strain and temperature perception for actuators. Taking advantage of tunable reversible deformation and mechanical transformation of the POE/PW actuator, different biomimetic robotics, including grippers with high load-bearing capability (weight-lifting ratio > 146), walking robots that can sense angles of joints, and high-temperature warning robots are demonstrated. A scheme combining the variable stiffness and electrical properties provides a versatile strategy to integrate actuation performance and self-sensing ability, inspiring the development of multifunctional composite designs for soft robotics.