Research

1. Reprocessible and Recyclable Thermosetting Polymers and Their Composites

Our group aims to develop innovative material and processing conditions to recycle thermosetting polymers and their composites, primarily by leveraging the recently emerged covalent bond exchange reactions (BERs).

As a notable example, we pioneered a primary recycling method for epoxy thermosets and their composites using a low-toxic organic solvent. When thermosets are immersed in solvents, small molecules diffuse into the network, break the polymer chains, and eventually decompose the network. The decomposition product can be re-polymerized into new thermosets with identical network structures and mechanical properties. The developed recycling method involves simple heating in solvents and can realize full recyclability of thermosets and their composites with low cost and easy implementation. It will significantly ease environmental and economic concerns associated with traditional disposal techniques (i.e., landfills, incineration, and harsh chemicals).

The recycling method can be extended to enable other desirable properties of thermosets and their composites, such as malleability, surface welding, repairing, and reprocessing. Our research work aims to understand the evolution of network structure and interfacial properties of CANs under various material and processing conditions. The understanding is used to assist the rational design of material systems and processing conditions.

The solvent-assisted recycling method has been incorporated into 3D printing to enable its recyclability, where the thermosetting printouts are fully depolymerized into a new ink and recycled for the next round of 3D printing. This work realizes green and sustainable 3D printing of engineering polymers with full recyclability.

  • Funding Agency: NSF

2. Liquid Crystal Elastomers

Liquid crystalline elastomers (LCEs) have attracted recent attention due to their ability to achieve free-standing two-way shape memory behavior, which is more desirable to many engineering applications that require continuous processing control. In addition, the coupling between the viscoelastic relaxations of the mesogens and polymer chains within the LCE networks leads to extraordinary energy dissipation behavior and fracture toughness. These properties can be exploited to design new transformative biomaterials, protective materials, and structures.

Our research aims to establish mechanics understandings on the viscoelastic relaxation mechanism of LCEs, their free-standing actuation, and energy dissipation behaviors, and apply the knowledge to the design of new material and functional structures. In our recent work, dynamic networks with BERs are incorporated into LCEs. The adaptable LCEs exhibit actuation, protection, and self-healing properties, which mimics the major functionalities of biological muscle. When applying this new LCE network to the design of soft robots, it has the great potential to bring man-made machine closer to the natural capabilities of humans and greatly extending their potential applications.

  • Funding Agency: NSF; Sandia National Lab; Department of Energy

4. 3D Printing of Fiber-reinforced Composites

3D printing allows for the moldless fabrication of complex structures with tailored fiber distribution and orientation. Various functions can be readily incorporated by printing with functional fibers. The substantially promoted design freedom and low manufacturing cost make it ideal for rapid prototyping and product development.

Existing 3D printing methods of continuous fiber-reinforced composites require the polymer matrix to quickly solidify during manufacturing, so the printable resins are limited to thermoplastics or UV-curable thermosets. However, most industrial-grade composites use thermally curable thermosets as the matrix.

In our recent work, we developed a new printing method for thermoset composites leveraging the shear stress imposed on the continuous fibers to enable the extrusion of composite filaments, which is applicable to most thermally curable and UV-curable resins. The designed printer head is integrated with a six-axis robotic arm to enable the printing and surface composite laying in 3D space.

  • Funding Agency: Air Force Office of Scientific Research; Office of Navy

3. Shape-changing Polymers and 4D Printing

Shape changing polymers can recover their original shape when a proper stimulus is applied. Our research in this area aims to advance the conventional understanding of shape-changing polymers by examining some new and interesting material behaviors. We have identified the unique role of “reduced time” in predicting the shape fixity and recovery ratio. We found that no matter how complicated the programming conditions are, the polymer shape fixity is only dependent on the reduced programming time. In addition, once we know the shape fixity, the shape recovery ratio is only determined by the reduced recovery time. Therefore, a master shape memory performance map can be constructed from some standard polymer rheological experiments to predict the fixity and recovery.

After 3D printing, active shape-changing polymers can transform into new shapes upon activation. This technique is referred as 4D printing with time being the 4th dimension of shape formation. While such spontaneous shape changing of printed active polymers have been intensively demonstrated and studied in recent years, we want to further explore the feasibility of controlling their shape changing sequence, which is expected to enable novel devices, such as sensors and actuators that can be widely applied in microsystem actuation components, as well as biomedical devices and aerospace deployable structures.

  • Funding Agency: NSF