Daniel A. Rau’s research while affiliated with University of Virginia and other places

Additive manufacturing of elastomers enables the fabrication of many technologically important structures and devices. However, it remains a challenge to develop soft and stretchable elastomers for stereolithography (SLA) printing, one of the most used additive manufacturing techniques for producing objects with relatively high-resolution and smooth finishes. Here, we report a modular, soft, stretchable, and low-cost elastomer resin for SLA printing. The resin consists of mainly commodity acrylates and can be photocured to form a dual-crosslinked network containing covalent and reversible crosslinks. Controlling the ratio of covalent and reversible crosslinks, we create elastomers with an exceptional combination of softness and stretchability (Young’s modulus of 20-150 kPa and tensile breaking strain of 510-1350%) that cannot be achieved by existing SLA resins. We demonstrate printing this resin to produce high-resolution three-dimensional (3D) structures with extreme dissipative properties. Further, we develop a setup to show that the 3D structures can protect brain-like soft gels from impact damage in reducing the severity of impact by 75%. Together with the low-cost of raw chemicals and modular nature of the design, our soft and stretchable elastomer resins provide a new class of soft materials for high-fidelity additive manufacturing of functional architectures.

Three-dimensional (3D) printing of elastomers enables the fabrication of many technologically important structures and devices. However, there remains a critical need for the development of reprocessable, solvent-free, soft elastomers that can be printed without the need for post-treatment. Herein, we report modular soft elastomers suitable for direct ink writing (DIW) printing by physically cross-linking associative polymers with a high fraction of reversible bonds. We designed and synthesized linear-associative-linear (LAL) triblock copolymers; the middle block is an associative polymer carrying amide groups that form double hydrogen bonding, and the end blocks aggregate to hard glassy domains that effectively act as physical cross-links. The amide groups do not aggregate to nanoscale clusters and only slow down polymer dynamics without changing the shape of the linear viscoelastic spectra; this enables molecular control over energy dissipation by varying the fraction of the associative groups. Increasing the volume fraction of the end linear blocks increases the network stiffness by more than 100 times without significantly compromising the extensibility. We created elastomers with Young’s moduli ranging from 8 kPa to 8 MPa while maintaining the tensile breaking strain around 150%. Using a high-temperature DIW printing platform, we transformed our elastomers to complex, highly deformable 3D structures without involving any solvent or post-print processing. Our elastomers represent the softest melt reprocessable materials for DIW printing. The developed LAL polymers synergize emerging homogeneous associative polymers with a high fraction of reversible bonds and classical block copolymer self-assembly to form a dual-cross-linked network, providing a versatile platform for the modular design and development of soft melt reprocessable elastomeric materials for practical applications.

Additive manufacturing (AM) of aerogels increases the achievable geometric complexity, and affords fabrication of hierarchically porous structures. In this work, a custom heated material extrusion (MEX) device prints aerogels of poly(phenylene sulfide) (PPS), an engineering thermoplastic, via in situ thermally induced phase separation (TIPS). First, pre‐prepared solid gel inks are dissolved at high temperatures in the heated extruder barrel to form a homogeneous polymer solution. Solutions are then extruded onto a room‐temperature substrate, where printed roads maintain their bead shape and rapidly solidify via TIPS, thus enabling layer‐wise MEX AM. Printed gels are converted to aerogels via postprocessing solvent exchange and freeze‐drying. This work explores the effect of ink composition on printed aerogel morphology and thermomechanical properties. Scanning electron microscopy micrographs reveal complex hierarchical microstructures that are compositionally dependent. Printed aerogels demonstrate tailorable porosities (50.0–74.8%) and densities (0.345–0.684 g cm⁻³), which align well with cast aerogel analogs. Differential scanning calorimetry thermograms indicate printed aerogels are highly crystalline (≈43%), suggesting that printing does not inhibit the solidification process occurring during TIPS (polymer crystallization). Uniaxial compression testing reveals that compositionally dependent microstructure governs aerogel mechanical behavior, with compressive moduli ranging from 33.0 to 106.5 MPa.

Three-dimensional (3D) printing of elastomers enables the fabrication of many technologically important structures and devices. However, there remains a critical need for the development of reprocessable, solvent-free soft elastomers that can be printed without the need for post-treatment. Here, we report modular soft elastomers suitable for direct ink write (DIW) printing by physically crosslinking associative polymers with a high fraction of reversible bonds. We design and synthesize linear-associative-linear (LAL) triblock copolymers; the middle block is an associative polymer carrying amide groups that form double hydrogen bonding, and the end blocks aggregate to hard glassy domains that effectively act as physical crosslinks. The amide groups do not aggregate to form nanoscale clusters and only slow polymer dynamics without changing the shape of the linear viscoelastic spectra; this enables molecular control over energy dissipation by varying the fraction of the associative groups. Exploiting the more ordered microstructures afforded by block copolymer self-assembly increases the network stiffness by >100 times without significantly compromising extensibility. We use a high-temperature DIW printing platform to print these LAL polymers and manufacture complex, highly deformable 3D structures. Our printing process uses melt processing and is solvent-free, and the printed parts do not require any post-print processing. We create elastomers with Young’s moduli ranging from 8 kPa to 8 MPa while maintaining tensile breaking strain around 150%. Our elastomers represent the softest melt reprocessable materials for DIW printing. The developed LAL polymers synergize emerging homogeneous associative polymers with high fraction of reversible bonds and classical block copolymer self-assembly to form a dual-crosslinked network, providing a versatile platform for the modular design and development of soft, melt reprocessable elastomeric materials for practical applications.

Vat photopolymerization (VP) and direct ink write (DIW) additive manufacturing (AM) provide complex geometries with precise spatial control employing a vast array of photo‐reactive polymeric systems. Although VP is recognized for superior resolution and surface finish, DIW provides versatility for higher viscosity systems. However, each AM platform presents specific rheological requirements that are essential for successful 3D printing. First, viscosity requirements constrain VP polymeric materials to viscosities below 10 Pa s. Thus, this requirement presents a challenging paradox that must be overcome to attain the physical performance of high molecular weight polymers while maintaining suitable viscosities for VP polymeric materials. Second, the necessary rheological complexity that is required for DIW pastes requires additional rheological measurements to ensure desirable thixotropic behavior. This manuscript describes the importance of rheological measurements when designing polymeric latexes for AM. Latexes effectively decouple the dependency of viscosity on molecular weight, thus enabling high molecular weight polymers with low viscosities. Photo‐crosslinking of water‐soluble monomers and telechelic oligomeric diacrylates in the presence of the latex enables the fabrication of a scaffold, which is restricted to the continuous aqueous phase and effectively surrounds the latex nanoparticles enabling the printing of otherwise inaccessible high molecular weight polymers. Rheological testing, including both steady and oscillatory shear experiments, provides insights into system properties and provides predictability for successful printing. This perspective article aims to provide an understanding of both chemical functionality (photo‐ and thermal‐reactivity) and rheological response and their importance for the successful design and evaluation of VP and DIW processable latex formulations.