Sensitive Multi-Stimuli Responsive Actuating Films Driven by Submolecular Switching

Many stimuli-responsive polymeric systems and composites have been developed in recent years with growing interest in their potential for applications, including as remote-controlled actuators, intelligent soft robotics, biomedical devices, and in energy harvesting. Here, we describe aramid-based multi-stimuli responsive films that produce programmable movement driven by submolecular switching. Films are responsive to near infrared (NIR) light, humidity, solvent, and heating, e.g., less than 5 degrees above room temperature. Moreover, in tandem with poly (vinylidene fluoride) (PVDF), film deformation can generate electricity, which provides a pathway to low-grade thermal energy harvesting.<br/>Aramid films contain a small amount of crosslinking dibenzocycloocta-1,5-diene (DBCOD) and are covalently bonded to a thin sheet of aligned carbon nanotubes (CNTs). Coordinated switching of submolecular DBCOD units from ‘twist-boat’ to ‘chair’ conformation upon thermal stimulation manifests as a macroscopic shape change. This design takes advantage of the self-assembly of DBCOD units along the CNT direction to generate anisotropic thermal contraction. As a result, this deformation can be pre-programmed with the selection of CNT/cut pattern. Additionally, this unique thermal contraction offers the distinct advantages of being inherently fast, repeatable, low-energy driven, and does not require the presence or control of moisture. While the polymer layer exhibits a large negative thermal expansion, the coefficient of thermal expansion of CNTs is nearly zero. This mismatch results in a large deflection of the film with small changes in temperature. Furthermore, films exhibit excellent cycle stability, owing to the covalent interface and reversible nature of the conformational change. Such low-energy stimulus induced systems could enable development of ultrasensitive sensors, soft bio-robotics and wearable devices, as well as low-grade energy harvesting.<br/><br/>The authors acknowledge funding from NSF CHE-1900647, NSF DMR-1309673, National Aeronautics and Space Administration (NASA) grant numbers NNX15AQ01A and NNH18ZHA008CMIROG6R.