Abstract
Structures made from shape memory polymers (SMPs) can be programmed into different configurations by temperature change. To pattern the SMPs at micron and nano scale, and explore the applications in the high resolution required fields such as sensors, micro-robotics, and optical devices, new fabrication methods need to be developed. Additive manufacturing (AM) is a promising approach to achieve this. Here, two specific AM technologies, namely digital light processing (DLP) and two-photon lithography (TPL) are studied to print SMPs at micron and nanoscale and new functions are achieved, such as micro-actuators with self-healing effect, muti-colour invisible inks for submicron AM, and high aspect ratio stiff nanopillars for nanoscale reversible configuration transformations. The main contributions are summarized below: (1) Most of the existing SMP-based AM materials are (meth)acrylate thermosets which have permanently cross-linked covalent networks and cannot be repaired if any damage occurs. To address the unrepairable nature of SMP-based AM materials, we reported a double-network self-healing SMP (SH-SMP) system for high-resolution (up to 30 µm) self-healing additive manufacturing. In the SH-SMP system, the semicrystalline linear polymer polycaprolactone (PCL) is incorporated into a methacrylate-based SMP system which has good compatibility with the DLP-based 3D printing technology and can be used to fabricate complex structures with high resolution. The PCL linear polymer imparts the self-healing ability to the printed structures, and the mechanical properties of a damaged structure can be recovered to more than 90% after adding more than 20 wt % of PCL into the SH-SMP system. We investigated the effects of PCL concentration on the thermomechanical behaviour, viscosity, and self-healing capability of the SH-SMP system and performed the computational fluid dynamics simulations to study the effect of the SH-SMP solution’s viscosity on the 3D printing process. We printed micro grippers and stents to show the potential applications. (2) The minimum feature size of SMPs by traditional AM process was at micron and millimetre scale. To further improve the resolution, we explored AM of an SMP in the submicron length scale, extending its applications to nanophononics. We reported a new SMP photoresist based on Vero Clear achieving print features at a resolution of ~300 nm half-pitch using TPL. Prints consisting of grids with size-tunable multi-colours enabled the study of shape memory effects to achieve large visual shifts through nanoscale structure deformation. As the nanostructures are flattened, the colours and printed information become invisible. Remarkably, the shape memory effect recovers the original surface morphology of the nanostructures along with its structural colour within seconds of heating above its glass transition temperature. The high-resolution printing and excellent reversibility in both microtopography and optical properties promise a platform for temperature-sensitive labels, information hiding for anti-counterfeiting, and tunable photonic devices. (3) Traditional SMPs have a low storage modulus of less than 10 MPa in the rubbery state, which is not enough to overcome the stiction caused by van der Waals (VDW) forces at the nanoscale. To tackle this issue, we introduced a stiff self-recovery material system based on acrylic acid and applied it to high-aspect-ratio structures, where recovery is weakest. The storage modulus of ~5.2 GPa at room temperature (22 ?) and ~90 MPa at the rubbery state (150 ?) was designed. A high-resolution photoresist for TPL was developed based on this polymer system, enabling AM of nanopillars with a diameter of ~400 nm and an aspect ratio as high as ~10. The self-recovery effect which overcomes stiction caused by the VDW force at the nanoscale was experimentally verified and theoretically analysed. Reversible structural colour print and hologram were demonstrated, indicating the potential applications of the self-recovery nanopillars.