Advances in Photopolymer Design

The UV additive manufacturing techniques discussed in this column have focused primarily on hardware approaches to additive manufacturing or the chemistry developed in combination with those approaches. 1 The current column is a brief survey of chemistries developed to function within the existing printer hardware landscape to explore novel areas of material performance. These material developments are intended to address shortcomings in existing materials and unmet market needs.

Bottlebrush Polymers

There are a variety of application areas for which highly flexible, highly stretchable materials are of interest. Biosensors that utilize a chemical, electrical or thermal sensor encapsulated within a polymer matrix in direct contact with the skin require a high degree of flexibility and elasticity while still maintaining mechanical strength.

Solvents or plasticizers have been used to increase the flexibility of more rigid polymers, thereby enhancing flexibility and elasticity, but these additives also tend to migrate or evaporate over time, leading to undesirable performance degradation.

Figure 1. Compressive modulus (left plot) of the PDMS bottlebrush elastomer compared with a Poly(acrylamide) hydrogel with similar volume fractions of backbone and polymer chains. The G values are the shear moduli of the Poly(dimethylsiloxane) and Poly(acrylamide) samples. The images illustrate the crumbling of the acrylamide hydrogel and the elasticity of the bottlebrush structure under compression. 2

An alternative to hydrogel structures in which the polymer chains do not coalesce or interpenetrate has the geometry of a bottle brush, with a linear polymer backbone to which long side chains are attached. These sidechains typically contain hydrophobic, low-glass-transition structures such as silanes, similar to those reported by Sheiko and coauthors. 2 The comparison of the compressive stress of a Polyacrylamide hydrogel to a Poly(dimethylsiloxane) (PDMS) is shown in Figure 1.

To enable commercial applications of bottlebrush elastomers, Chabinyc and coauthors have developed a PDMS bottlebrush structure incorporating small amounts (0.04-0.06 volume fractions) of short polyethylene oxide chains that reversibly self-assemble into cubic sphere structures and can be printed into three-dimensional parts using direct ink writing (DIW), as shown in Figure 2. 3,4

Figure 2. Copolymers of structure A with NSC sidechain repeat units and NBB bottlebrush repeat units reversibly reorder and self-assemble. 3

As reported by Chabinyc et al, the structures created with DIW are soft and flow under stress. As shown in Figure 3, the structure can be locked into place to generate a supersoft elastomer with a shear modulus less than 50 kPa by incorporating a photocrosslinker into the formulation that covalently links the self-assembled spheres upon irradiation at 365 nm. 3

Figure 3. Overview of photocrosslinked bottlebrush polymer design. A) The formulation combines the reversibly self-assembled bottlebrush structure with the oligomeric benzophenone photocrosslinker. B) The photocrosslinking process locks in the cubic sphere structure to create a stable elastic microstructure within the geometry defined in the DIW process. The stretch and elastic recovery of the photocrosslinked bowl structure are shown on the right. 3

Inorganic Nanomaterials

There currently is a high level of interest in 3D-printed ceramics for electronic applications due to their insulating and mechanical properties. Parts are generated in a process in which ceramic particles are dispersed in UV formulations, printed and subsequently sintered free of the organic matrix at elevated temperatures. 5

Li and coauthors recently have reported an approach in which quantum dots (QD), also referred to as colloidal nanocrystals (NC), are bonded directly to each other photochemically through the layer of ligands attached to their surfaces using two-photon 780 nm laser excitation to achieve high spatial resolution. 6 As described by Li, the interparticle distance is driven by the sub-nanometer thickness of the nanocrystal ligand layer, leading to the formation of a dense material with compressive strength and density comparable to ceramics. Because the 3D printing process itself has a negligible effect on the emission wavelengths of the nanocrystals, this approach enables the generation of a wide range of colors and geometries.

The self-assembled bottlebrush elastomer and the nanocrystals are photocrosslinked non-acrylate materials with mechanical properties on opposite ends of the hardness scale. The bottlebrush will more likely find opportunities in biocompatible applications. The ceramic-like nanocrystal structures that luminesce in any color of the visible spectrum may find a home in communications or display applications.

Both non-acrylate chemistries represent technologies that provide new opportunities for additive manufacturing enabled by photochemistry.

For feedback, or if there are specific topics readers would like to see discussed, contact me at pshare@admatdesign.com.

Paul Share, Ph.D.
Principal Consultant
Advanced Materials
Design LLC

 

References:

  1. This column is focused on photopolymer applications in additive manufacturing, in which a reservoir or vat of liquid resin is photopolymerized to generate a part. When a layer is cured using a narrow laser beam in a series manner, one point at a time, it is referred to as stereolithography or SLA. Simultaneous illumination of a large area to cure one full layer at a time can be accomplished with a digital light projector (DLP) or a liquid crystal display (LCD) screen.
  2. Daniel, W., Burdynska, J., Vatankhah-Varnoosfaderani, M., Matyjaszewski, K., Paturej, J., Rubinstein, M., Dobrynin, A., & Sheiko, S. (2016). Solvent-free, supersoft and superelastic bottlebrush melts and networks. Nature Materials, 15(2), 183–189.
  3. Xie, R., Mukherjee, S., Levi, A., Reynolds, V., Wang, H., Chabinyc, M., & Bates, C. (2020). Room temperature 3D printing of supersoft and solvent-free elastomers. Science Advances, 6(18), 1–10.
  4. Share, P. (2025). Direct ink writing in multi-material AM. UV+EB Technology, 11(4), 22–23.
  5. Share, P. (2025). Additive manufacturing for electronics applications. UV+EB Technology, 11(2), 14–15.
  6. Li, F., Liu, S., Liu, W., Hou, Z., Jiang, J., Fu, Z., Wang, S., Si, Y., Lu, S., Zhou, H., Liu, D., Tian, X., Qiu, H., Yang, Y., Li, Z., Li, X., Lin, L., Sun, H., Zhang, H., & Li, J. (2023). 3D printing of inorganic materials by photochemically bonding colloidal nanocrystals. Science, 381(6661), 1468–1474.

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