Periodic Reporting for period 1 - FloR3D (Flow-Reactor Coupled 3D Printing: Achieving Voxel-Level Control of Out-of-Equilibrium Materials)
Reporting period: 2024-02-01 to 2025-01-31
Nature is able to synthesize materials with spatially patterned optical, mechanical, and chemical properties by designing the local nano or microscale structure of the materials. For example, using a single chemical building block, such as cellulose in plants and keratin in bird feathers, natural materials tightly control and vary the arrangement of these building blocks to obtain tailor-made properties locally. This approach yields multifunctional materials that are highly optimized for their application: the helicoidal nanoscale arrangement of cellulose in the cell wall of the Pollia Condensata fruit provide enhanced mechanical strength while causing an iridescent blue coloration.
While such locally variable nanostructure is difficult and cumbersome to achieve through traditional manufacturing methods, additive manufacturing, or 3D printing, offers a method to bypass this limitation. By careful design of the print path, the property of the material can be controlled down to a ‘voxel’ (3D pixel) level. However, current 3D printing approaches that capable of generating nanostructured materials are primarily limited by the ‘thermodynamic’ assembly of the ink. This means that the resultant nanostructure is an inherent product of the ink’s chemical structure, and varying the nanostructure requires using a different feedstock. As such, on-the-fly variations of the microstructure are very difficult to achieve.
In FloR3D, we aim to develop an approach to combine a flow reactor with a 3D printing system. In our custom-built FloR3D setup, the to-be-printed materials are synthesized in situ from a mixture of two starting materials and mixed in the flow reactor. The product of this reaction can then be extruded using a ‘direct-ink writing’ system to create spatially patterned nanostructures with arbitrary geometry. Importantly, the material’s properties can be designed by varying the ratio of the starting materials and mixing time during the printing process.
While such locally variable nanostructure is difficult and cumbersome to achieve through traditional manufacturing methods, additive manufacturing, or 3D printing, offers a method to bypass this limitation. By careful design of the print path, the property of the material can be controlled down to a ‘voxel’ (3D pixel) level. However, current 3D printing approaches that capable of generating nanostructured materials are primarily limited by the ‘thermodynamic’ assembly of the ink. This means that the resultant nanostructure is an inherent product of the ink’s chemical structure, and varying the nanostructure requires using a different feedstock. As such, on-the-fly variations of the microstructure are very difficult to achieve.
In FloR3D, we aim to develop an approach to combine a flow reactor with a 3D printing system. In our custom-built FloR3D setup, the to-be-printed materials are synthesized in situ from a mixture of two starting materials and mixed in the flow reactor. The product of this reaction can then be extruded using a ‘direct-ink writing’ system to create spatially patterned nanostructures with arbitrary geometry. Importantly, the material’s properties can be designed by varying the ratio of the starting materials and mixing time during the printing process.
To obtain controlled bicontinuous networks, which possesses hollow channels of well-defined lengthscales (Part A in attached figure), for photonic glass, we investigated polymerization-induced phase separation. The lengthscale of such bicontinuous networks are notoriously difficult to control precisely, but if done correctly can generate optical effects like in feathers of the eastern bluebird, for which the blue color arises from a bicontinuous network formed using keratin. Synthetically, such a structure is difficult to replicate as the structure will gradually broaden, known as coarsening, until the lengthscales are no longer well-defined. Indeed photonic glasses from a bicontinuous structure have yet to be prepared synthetically.
Choice of chemical system:
For the first part of this study, we investigated polymerization-induced phase separation of a diethylene glycol methacrylate/dimethacrylate based polymer (chemical structures in Part B of attached figure) to prepare arrested photonic bicontinuous structures. Over the course of polymerization from a mixture of monomers, the growing polymer gradually becomes insoluble in the monomer mixture. At this point, the polymer phase separates from the monomer by spinodal decomposition, which results in a bicontinuous network. We hope to be able to precisely control the lengthscales and prevent coarsening by chemical crosslinking to obtain a photonic glass system. In addition, to be compatible with our FloR3D setup, the investigated two-component system was designed such that the length-scale of the bicontinuous network can be tuned by changing the ratio of two sets of reactants.
Optimization of polymerization kinetics:
The lengthscale and timescale of spinodal decomposition and subsequent coarsening (and even whether spinodal decomposition can occur) depend critically on the monomer composition and polymerization speed. As such, we experimented extensively with changing the initiation and propagation rate of the polymerization as well as how the polymerization was triggered. For example, we used controlled polymerization techniques based on redox chemistry, such as ATRP and RAFT, to target different reaction rates. However, these initial tests invariably resulted in reaction speeds that were too slow. The fastest speed to gelation (when the material solidifies) achieved using these controlled methods was ~1 hr. The slow polymerization speed coupled with the stringent reaction condition (oxygen free among others) meant that the initial approaches were not compatible with our ultimate 3D printing approach.
Photopolymerization yields novel photonic glasses:
We discovered that rapid photopolymerization with UV light result in phase separated materials that exhibit photonic properties. With the inclusion of difunctional crosslinkers, the polymerized material was too crosslinked to undergo further coarsening, thus bypassing one of the major limitation of bicontinuous networks. Furthermore, we found that the colour can be tuned by varying the ratio between difunctional crosslinker and monofunctional monomers. Green and red samples prepared with different crosslinker ratios are shown in Part C of the attached figure. While we are still in the process of experimentally confirming the photonic behavior definitively originates from a bicontinuous network, we believe that this is the first synthetic demonstration of a photonic glass arising from a bicontinuous network. We are currently integrate our reaction scheme into a flow-reactor coupled 3D printing system (our FloR3D setup).
Choice of chemical system:
For the first part of this study, we investigated polymerization-induced phase separation of a diethylene glycol methacrylate/dimethacrylate based polymer (chemical structures in Part B of attached figure) to prepare arrested photonic bicontinuous structures. Over the course of polymerization from a mixture of monomers, the growing polymer gradually becomes insoluble in the monomer mixture. At this point, the polymer phase separates from the monomer by spinodal decomposition, which results in a bicontinuous network. We hope to be able to precisely control the lengthscales and prevent coarsening by chemical crosslinking to obtain a photonic glass system. In addition, to be compatible with our FloR3D setup, the investigated two-component system was designed such that the length-scale of the bicontinuous network can be tuned by changing the ratio of two sets of reactants.
Optimization of polymerization kinetics:
The lengthscale and timescale of spinodal decomposition and subsequent coarsening (and even whether spinodal decomposition can occur) depend critically on the monomer composition and polymerization speed. As such, we experimented extensively with changing the initiation and propagation rate of the polymerization as well as how the polymerization was triggered. For example, we used controlled polymerization techniques based on redox chemistry, such as ATRP and RAFT, to target different reaction rates. However, these initial tests invariably resulted in reaction speeds that were too slow. The fastest speed to gelation (when the material solidifies) achieved using these controlled methods was ~1 hr. The slow polymerization speed coupled with the stringent reaction condition (oxygen free among others) meant that the initial approaches were not compatible with our ultimate 3D printing approach.
Photopolymerization yields novel photonic glasses:
We discovered that rapid photopolymerization with UV light result in phase separated materials that exhibit photonic properties. With the inclusion of difunctional crosslinkers, the polymerized material was too crosslinked to undergo further coarsening, thus bypassing one of the major limitation of bicontinuous networks. Furthermore, we found that the colour can be tuned by varying the ratio between difunctional crosslinker and monofunctional monomers. Green and red samples prepared with different crosslinker ratios are shown in Part C of the attached figure. While we are still in the process of experimentally confirming the photonic behavior definitively originates from a bicontinuous network, we believe that this is the first synthetic demonstration of a photonic glass arising from a bicontinuous network. We are currently integrate our reaction scheme into a flow-reactor coupled 3D printing system (our FloR3D setup).
The successful preparation of a photonic glass from a bicontinuous network represents a major advance in the design of nanostructured photonic systems. Compared to photonic crystals (such as liquid crystals or synthetic opals), photonic glasses appear more angularly independent. For a fixed incident light and observation angle, the color of photonic glasses appears fixed regardless of the orientation of the material, which is highly beneficial for pigments or other diffusive optical applications. Notably, since the nanostructure can be easily tuned by varying the ratio of monomer to crosslinker, this synthetic technique provides a much more facile and flexible method to prepare photonic glasses compared to previous approaches based on synthesizing block copolymer or colloidal particles of precise sizes. A manuscript is currently under preparation for an upcoming publication.