Shape memory polymers (SMPs) represent an intriguing class of cross-linked polymers renowned for their unique ability to undergo programmable shape morphing when subjected to external stimuli. This remarkable feature positions SMPs as versatile materials with vast potential across a multitude of applications, ranging from biomedical devices and smart actuators to robotics, electronic components, and automation equipment.
SMPs, in their essence, exhibit dual-shape changing behavior, which hinges on the activation of specific polymer chain segments at temperatures surpassing their glass transition temperature (Tg). This activation effectively allows for deformation under the influence of external forces, culminating in the temporary fixation of the polymer’s shape upon cooling. The elasticity inherent to SMPs plays a pivotal role in their capacity to recover their original configuration upon reheating above Tg—a phenomenon underpinned by the inherent entropic nature of chain conformation alterations.
In contrast, polymers endowed with dynamic exchangeable bonds demonstrate plasticity, bestowing them with the capability to permanently reshape in response to external forces. This permanent restructuring is orchestrated by programming the polymer at temperatures exceeding a designated threshold (Tv), where dynamic covalent bonds facilitate the rearrangement of the polymer network under the influence of external loads. As the polymer cools, it adopts a new permanent shape, impervious to recovery and resistant to temperature fluctuations, provided there is no external stress applied.
While traditional SMPs offer a single shape-changing cycle, the contemporary landscape demands multifunctional devices with intricate geometries—a challenge addressed by vitrimers. These materials deftly blend the attributes of elasticity and plasticity, enabling them to be programmed with multiple shape memory effects. This adaptability lends itself to the continuous transformation of materials into complex 3D structures, opening vistas of possibility in applications as diverse as aerospace engineering and soft robotics, where the need for adaptability and complexity is paramount.
One exciting avenue of application for shape memory polymers is found in the realm of soft actuators. These devices are preconfigured with specific shapes and can undergo controlled motion in response to a medley of external stimuli—ranging from heat and light to solvents and electricity. Soft actuators have piqued considerable interest in domains like soft robotics, energy generation, motor systems, and fluid propulsion.
Liquid crystalline elastomers (LCEs) have also made significant contributions to the creation of intelligent 3D materials. These polymers can morph their shapes reversibly, with their constituent chains and mesogens gradually aligning and subsequently locking in place through cross-linking. Recent evolution in this field has come in the form of exchangeable LCEs, opening doors to the fabrication of intricate 3D actuators boasting robust mechanical properties, recyclable attributes, reprogrammability, and heightened actuation efficiency.
However, a substantial challenge in the development of thermal-responsive soft actuators is reconciling reprogrammability with stability. The actuation process in these materials is fueled by a transformation in network topology, which is facilitated by the exchange of linking moieties upon exposure to heat. Yet, excessive heating can precipitate undesired changes in network topology, jeopardizing operational stability and actuation performance. The judicious selection of exchangeable reactions that align with the actuator’s operational temperature stands as a pivotal consideration in designing advanced actuation systems.
In recent years, dynamic exchangeable reactions such as transesterification, transcarbamoylation, and boronic ester exchange reactions have gained prominence in the development of advanced actuator systems. These reactions promise a harmonious balance between re-programmability and stability in thermal-responsive soft actuators, signifying a significant stride forward in the field.
In summation, shape memory polymers represent a domain of tremendous potential, poised to revolutionize diverse industries by providing materials capable of adaptation, reshaping, and responsiveness to external stimuli. From the realm of biomedical devices to the world of soft robotics, the applications of these versatile materials continue to expand as researchers explore the far-reaching possibilities inherent to the captivating world of shape memory polymers.
