Materials Horizons Blog

3D printing has revolutionized the production of polymer-based materials, with the development of numerous printing techniques that continuously enhance speed, resolution, and accessibility. Among the most significant advancements is light-based 3D printing, which utilizes photopolymerization reactions to solidify inks, offering precise spatiotemporal control over the printing process. However, light-based 3D printing primarily relies on uncontrolled polymerization of (meth)acrylate and epoxide monomers/ cross-linkers, which yield non-functional, non-recyclable materials. To fully exploit the potential of 3D printing, there is growing interest in integrating advanced chemistries, such as dynamic chemistry, that enable the fabrication of functional materials capable of post-printing modifications in shape, properties, and functionality. Most polymer-based objects created through current 3D printing techniques cannot be chemically recycled due to the chemical inertness of the materials and the irreversible polymerization processes involved. Thus, it is imperative to incorporate new chemistries into 3D printing that enable the fabrication of advanced, functional, and chemically recyclable materials at the end of their lifecycle.

In a recent work by Du Prez and Nguyen, a closed-loop 3D printing process utilizing dynamic chemistry was reported. The authors synthesized an acrylate photocurable polymeric material based on dynamic β-amino ester cross-linkers, which can be used as an ink for 3D printing. Importantly, this material can undergo decross-linking via a transesterification reaction, resulting in the depolymerization of the formed polymer network. This process is not only reversible, enabling the making and unmaking of polymers, but it also operates under highly appealing and relatively mild conditions.

Figure 1: a) Chemical overview of the polymerization and depolymerization reactions of the dynamic network and b) closed-loop 3D printing. Image reproduced from DOI: 10.1039/D4MH00823E with permission from the Royal Society of Chemistry.

The team first synthesized an acrylate-terminated β-amino ester-containing cross-linker, which serves as a building block for the formation of a dynamic polymer network. This cross-linker was prepared via a one-step aza-Michael addition reaction. The process is straightforward and utilizes commercially available reagents, making the material easily adaptable for other researchers. These low-molecular-weight diacrylate polymers undergo photo-induced free radical polymerization, resulting in the formation of a dynamic polymer network.

This dynamic nature of the polymeric network enables thermomechanical reprocessing of the material. To demonstrate this aspect, the authors used compression molding (at 150 °C under 2 tons of pressure for 30 minutes) for reprocessability. The recycling procedure was repeated three times, with the material showing a slight increase in shear modulus and activation energy, while maintaining stable thermal properties and chemical integrity. The increase in temperature results in a reduction of cross-link density, demonstrating the role of dissociative exchange via (retro) aza-Michael addition. In contrast, polymeric networks without dynamic bonds did not exhibit any reprocessability, highlighting the promise of dynamic chemistry in enabling reprocessable materials.

In the next step, to demonstrate more advanced chemical recycling of this dynamic network, the team employed another reaction (transesterification) using a commercially available methacrylate with a hydroxyl moiety (hydroxyethyl methacrylate) as a decross-linker. The hydroxyl group of this functional methacrylate reacts with the β-amino ester, resulting in the decross-linking of the network and its depolymerization. This reaction is temperature-dependent and requires an optimized amount of decross-linker to achieve a high depolymerization yield. Notably, the depolymerization reaction proceeds without the need for any catalyst or solvent, making the procedure even more appealing. Upon depolymerization of the polymer network, the resulting mixture consists of methacrylate-terminated compounds, as confirmed by electrospray ionization mass spectrometry. These compounds can be further employed for another cycle of photo-curing, enabling the development of curable-depolymerizable dynamic materials.

In the final part, the team demonstrated the translation of this concept into closed-loop 3D printing using the dynamic photocurable resin. Multiple printing cycles were performed with recycled photocurable resin to assess the repeatability of this procedure. The resulting samples exhibited consistent shapes and appearances for at least three cycles, thus demonstrating repeatable printability after chemical recycling.

Overall, this study elegantly combines fundamental aspects of dynamic polymer chemistry and its application in 3D printing, further enhancing interest in dynamic chemistry for both functional and recyclable materials.

To find out more, please read:

Direct restoration of photocurable cross-linkers for repeated light-based 3D printing of covalent adaptable networks
Loc Tan Nguyen and Filip E. Du Prez
Mater. Horiz., 2024, DOI:10.1039/D4MH00823E

About the blogger

Dr. Kostas Parkatzidis is a Swiss National Science Foundation Postdoc Fellow in the group of Professor Zhenan Bao at Stanford University (United States), working on the molecular design of polymer-based skin-inspired materials for various applications. Kostas obtained his PhD from ETH Zurich (Switzerland) under the supervision of Professor Athina Anastasaki where he focused on the development of advanced polymer synthesis and chemical recycling methodologies. He also holds MSc in Organic Chemistry and BSc in Materials Science and Engineering obtained from the University of Crete (Greece). Since 2023, Kostas has served as a Materials Horizons Community Board member.

Perovskite oxynitrides are intriguing materials with significant relevance in photoelectrochemistry and catalysis. Their crystallinity is often simplified using high-symmetry unit cells as a convenient approximation. However, this approach fails to capture the complex local atomic displacements of anions and neighboring cations caused by the differing ionic radii of oxygen and nitrogen anions. A recent study by the Seitz Group provides a more nuanced understanding of perovskite oxynitrides’ true crystallinity, using a newly synthesized calcium tungsten oxynitride, CaW(O,N)₃, as a model system. The authors leverage experimental and computational techniques to explore its crystal structure, focusing on distortion, anion ordering, and symmetry. This study highlights four key findings:

CaW(O,N)₃ exhibits structural distortion. The study marks the first successful synthesis of CaW(O,N)₃. Using a combination of density functional theory (DFT), X-ray diffraction (XRD), and neutron diffraction (ND), the authors reveal that this material adopts an orthorhombic Pnma average structure. This structure exhibits significant octahedral distortion and preferential anion site occupancy, highlighting its deviation from a purely symmetric model.

Symmetry models for perovskite oxynitrides need to account for disorder. The authors demonstrate that traditional high-symmetry models fail to capture the full complexity of CaW(O,N)₃. Instead, a low-symmetry model with a P1 space group—which incorporates a larger unit cell and random anion positioning—better represents the disorder inherent in the crystal. This disorder forms a “polymorphous network,” where the observed high symmetry is only an average, masking the true lower local symmetry of CaW(O,N)₃.

The structural disorder can be extended to other perovskite oxynitrides. The structural disorder observed in CaW(O,N)₃ was also identified in its analog, SrW(O,N)₃. Like CaW(O,N)₃, the authors obtained a better representation of the structural disorder in SrW(O,N)₃ by applying similar low-symmetry models, suggesting that such disorder is a common characteristic across perovskite oxynitrides.

Structural disorder in perovskite oxynitrides causes changes in electronic properties. Comparative X-ray Absorption Spectroscopy (XAS) revealed differences in electronic structures between Ca- and Sr-based perovskite oxynitrides. SrW(O,N)₃ exhibits stronger covalency between its anions and W 5d orbitals. These differences are attributed to the distinct electronic configurations and ionic radii of Ca²⁺ and Sr²⁺, which alter the interaction of W with neighboring anions.

Figure 1. summarises the study using CaW(O,N)3, as a model system. Experimental and computational techniques are used to explore its crystal structure, focusing on distortion, anion ordering, and symmetry. Image reproduced from DOI: 10.1039/D4MH00317A with permission from the Royal Society of Chemistry.

In summary, this study provides a comprehensive evaluation of the crystallinity and electronic properties of perovskite oxynitrides, emphasizing the importance of structural disorder. This work is a must-read for researchers studying heteroanionic materials, offering guidance on characterizing their structural and electronic properties through essential experimental and computational methods. Furthermore, it underscores the need for more sophisticated models to accurately represent these materials’ symmetry and local structure, accounting for the inherent complexities introduced by mixed anion occupancy. These findings are crucial for optimizing perovskite oxynitrides’ performance in catalytic and photoelectrochemical applications, where electronic and structural properties are pivotal in their performance.

To find out more, please read:

Synthesis and symmetry of perovskite oxynitride CaW(O,N)₃
Matthew E. Sweers, Tzu-chen Liu, Jiahong Shen, Bingzhang Lu, John W. Freeland, Christopher Wolverton, Gabriela B. Gonzalez Aviles, and  Linsey C. Seitz
Mater. Horiz., 2024,11, 4104-4114 DOI: 10.1039/D4MH00317A

About the blogger

Raul A. Marquez is a Ph.D. in Chemistry candidate working with Prof. C. Buddie Mullins at The University of Texas at Austin and a Materials Horizons Community Board member. His research focuses on understanding the transformations of electrocatalytic materials and developing advanced characterization methods for energy storage and conversion technologies. Follow Raul’s latest research on LinkedIn.