Conventional fabrication techniques can produce photoresponsive cellulose nanocrystal (CNC)-based composites that are capable of reversibly altering their optical, mechanical, or chemical properties when exposed to light. However, conventional methods are usually limited to 2D films or simple shapes that do not take full advantage of the directional alignment of CNCs to achieve spatial mechanical modulation. Researchers at ETH Zurich have proposed a new fabrication method to produce CNC-based 3D composite nanomaterials with fast and reversible light-responsive behavior and tailored mechanical properties.
By grafting pseudo-stilbene dispersed red 1 (DR1) onto the CNC surface, the researchers prepared multipurpose activity-enhanced particles that responded to light at a wavelength of 475 nm. These modified particles retained their elongated shape, enabling them to impart the rheological properties required for filament extrusion and particle alignment in Newtonian resin DIW. By dispersing the functionalized CNC into a polyurethane acrylate matrix, the printed composites can be reversibly softened and exhibit shape memory effects when irradiated at appropriate wavelengths.
Fig. 1 Preparation of photoresponsive 3D printed cellulose nanocomposites.
DR1 was grafted onto the CNC surface to endow the cellulose nanocomposites with photoresponsive functionality. The successful grafting of DR1 onto the CNC surface was confirmed by characterisation such as IR spectroscopy, appearance map and UV-Vis spectroscopy. In addition, XRD data indicated that grafting did not have a significant effect on the crystalline structure of CNC. In order to ensure the strong shear-thinning behaviour of the ink and the successful alignment of CNC during extrusion, the modified CNC needs to maintain the original aspect ratio. Atomic force microscopy pictures and transmission electron microscopy images confirm that the chemical treatment does not change the morphology of the individual particles.
Ink Direct Printing of 3D Structures
Oscillatory rheological studies were carried out on three different concentrations of CNC-DR1, and the resulting inks exhibited strong shear-thinning properties and rheological behavior at all concentrations tested. The ink concentration chosen for the DIW printer in this study was 15 wt%, a concentration that allows for maximizing the CNC content in the system while satisfying a wider range of print pressures to optimize print quality.
Dynamic Response of Printed Materials and Structures
At a concentration of 15%, CNC-DR1 particles provide printability on uncured PUA-HEMA substrates. Increases in elastic modulus and strength, intense light softening, and light-triggered shape memory behavior were the main effects on the cured composites. The energy absorption characteristics of the 3D printed structures were characterized by quasi-static compression tests. Comparing samples compressed in the dark with samples compressed under selective light, there is a significant difference in the energy absorption capacity of the hexagonal honeycomb. In addition, thanks to the shape memory behavior, light illumination 5 minutes after compression triggered the shape recovery of the test samples.
Fig. 2 Preparation of photoresponsive 3D printed cellulose nanocomposites.
Conclusion
This study was conducted to fabricate composites by DIW technique and to equip the obtained complex 3D printed structures with photoresponsive behavior. DR1 was grafted on CNC for chemical modification, which led to changes in the color of the nanoparticles without altering the crystal structure or morphology of the particles while maintaining their anisotropy. Modified nanoparticles can be introduced into PUA matrices to influence their rheological characteristics and promote the formation of gel-like inks to meet printability requirements. A strong photothermal effect imparts photo-softening and shape memory effects to 3D-printed structures.
Innovation Points
Combining photoresponsiveness with DIW technology to achieve local alignment of CNCs to create 3D complex shapes.
Inspiration
- It is possible to tailor the mechanical behavior of structures under compressive forces by temporarily illuminating them during printing.
- By introducing the same type of nanoparticles into liquid crystal elastomers, photomechanical effects will lead to dynamic and reversible shape changes.
The applications of cellulose nanocrystals have been greatly expanded by the above methods, providing new directions and ideas for future nanomaterials research and development.
Reference
- Functionalized Cellulose Nanocrystals as Active Reinforcements for Light-Actuated 3D-Printed Structures. ACS Nano (2022).
