Creation of three-dimensional composite architectures via high-intensity focused ultrasound inside of foams

Free-form creation of 3-dimensional (3D) structures, such as in additive manufacturing (AM) and 3D printing (3DP), typically requires a direct line-of-sight or physical contact between an energy source and a build material. By stepping away from this equipment paradigm, we discovered a method to achieve 3D composites inside of opaque, open-cell foams that enables unprecedented access to bicontinuous, interlocked composite structures. We found that high-intensity focused ultrasound (HIFU) provided efficient, localized heating at a focal point that could be spatially controlled within a foam matrix. Foam specimens were infused with thermally curable acrylate resin formulations, which enabled free-form creation of 3D structures as the HIFU focal point was moved throughout the interior of the foam. The 3D structure was created entirely based upon the toolpath, without any build plate or inherently sequenced layer-by-layer processes. Since the foam and cured resin were mechanically interlocked in the process, HIFU curing achieves bicontinuous composites seemingly independent of surface compatibilities between the foam and resin. Starting with commercially available polyurethane foams, we investigated combinations with different resin systems to achieve a range of mechanical properties from the final composite structures. For example, using poly(ethylene glycol) diacrylate (PEGDA) resulted in stiff, hard composite domains within the foam, whereas resins comprising 2-hydroxyethyl acrylate (HEA) led to soft, elastomeric composite structures. Multimaterial composites were also achieved, simply by displacing uncured resin from the foam and exchanging it with a different resin formulation. Control over the shape and orientation of internal structural features within the foam scaffolds also enabled controllable anisotropic mechanical responses from the composites.

Design, testing, and application of an open-source powder material extrusion 3D printer

Additive Manufacturing, Volume 81, 5 February 2024, 104014

Powder material extrusion (PME) additive manufacturing (AM) is a convenient and practical method to study novel materials by circumventing the need for filamentation or compounding of materials. Avoiding additional processing steps can be an enabler for research with exploratory materials or those that otherwise display thermal instabilities. In this work, we present the design, development, and testing of an open-source PME printer. The open-source PME 3D printer achieves print quality on par with commercial material extrusion 3D printers in terms of dimensional accuracy, print quality, and mechanical performance. Additionally, we demonstrate this system to be versatile and robust through printing of recycled materials, polymer composites, polymer blends, and functional polymers with thermally sensitive moieties. The broad range of build materials illustrates the diverse capabilities accessible with this system, which enabled access to properties and functions such as phosphorescence, ferromagnetism, shape memory, and mechanochromism. By improving the accessibility of PME AM and demonstrating its versatility, we hope to enable others to explore novel material systems.

Access to Functionalized Materials by Metal-Free Ring-Opening Metathesis Polymerization of Active Esters and Divergent Postpolymerization Modification

ACS Macro Lett. 2024, 13, 2, 144–150

Metal-free ring-opening metathesis polymerization (MF-ROMP) is an emerging polymerization strategy that provides access to ROMP materials by using organic initiators and photoredox catalysts. Unlike metal-mediated ROMP, MF-ROMP is not highly tolerant toward functionalized monomers. Herein, we report that pentafluorophenyl esters are polymerizable under MF-ROMP conditions to produce homopolymers, statistical copolymers, and block copolymers. Amine coupling agents were then used to install a range of functional groups via acyl substitution including alkynes, amino acid derivatives, fluorophores, and redox active moieties. Overall, these findings provide a framework to prepare functionalized ROMP polymers without the risk of metal contamination.

Block copolymer additives for toughening 3D printable epoxy resin

Giant, Volume 17, March 2024, 100204

We explore the potential for using a brush-coil triblock copolymer to enhance the mechanical properties of epoxy resin for 3D printing applications. Epoxy resins are widely used in structural material and adhesive and have great potential for 3D printing. However, the highly brittle nature of epoxy resins requires the use of large concentrations of toughening agents that pose significant challenges in meeting rheological requirements of 3D printing. We report a reactive brush-coil block copolymer with three distinct blocks that can phase separate and chemically crosslink with the base epoxy resin to form spherical aggregates. Detailed scanning electron microscopy imaging shows that these aggregates can arrest and deflect cracks during propagation and can synergistically strengthen (∼ 1.5×) and toughen (∼ 2×) the epoxy resin with even 1 wt% of the BCP additive to the base resin. Importantly, both the modulus and the glass transition temperatures are preserved. Direct ink writing (DIW) and digital light processing (DLP) 3D printing of the modified resins also shows the same strengthening and toughening effects seen in mold-cast samples, demonstrating its compatibility with 3D printing processes. These findings suggest that brush-coil triblock copolymers additives at very low concentrations can synergistically improve the mechanical properties of epoxy resin for 3D printed parts.

Flow Optimization of Photoredox-Mediated Metal-Free Ring-Opening Metathesis Polymerization

ACS Macro Lett. 2023, 12, 11, 1479–1485

Photoredox-mediated metal-free ring-opening metathesis polymerization (MF-ROMP) is a convenient metal-free method to produce a variety of ROMP polymers. Transitioning MF-ROMP from a batch to a continuous flow process has yet to be demonstrated and could potentially benefit the production efficiency, safety, and modularity of reaction conditions. We designed and evaluated continuous flow and droplet flow setups and compared the results for MF-ROMP across a short series of common monomers. By using the droplet flow reactor setup, we achieved flow conversions comparable to that of batch and circumvented issues with diffusion-limited mixing and air exposure.

Dependence of the kinetic energy absorption capacity of bistable mechanical metamaterials on impactor mass and velocity

Using an alternative mechanism to dissipation or scattering, bistable structures and mechanical metamaterials have shown promise for mitigating the detrimental effects of impact by reversibly locking energy into strained material. Herein, we extend prior works on impact absorption via bistable metamaterials to computationally explore the dependence of kinetic energy transmission on the velocity and mass of the impactor, with strain rates exceeding 10 2 s−1. We observe a large dependence on both impactor parameters, ranging from significantly better to worse performance than a comparative linear material. We then correlate the variability in performance to solitary wave formation in the system and give analytical estimates of idealized energy absorption capacity under dynamic loading. In addition, we find a significant dependence on damping accompanied by a qualitative difference in solitary wave propagation within the system. The complex dynamics revealed in this study offer potential future guidance for the application of bistable metamaterials to applications including human and engineered system shock and impact protection devices.

C-H Functionalization and Allylic Amination for Post-Polymerization Modification of Polynorbornenes

Post-polymerization modification (PPM) via direct C-H functionalization is a powerful synthetic strategy to convert polymer feed-stocks into value-added products. We found that a metal-free, Se-catalyzed allylic C-H amination provided an efficient method for PPM of polynorbornenes (PNBs) produced via ring-opening metathesis polymerization. Inherent to the mechanism of the allylic amination, PPM on PNBs preserved the alkene functional groups along the polymer backbone, while also avoiding transposition of the double bonds. Amination using a series of aryl sulfonamides led to good control over the degree of functionalization, access to a range of functionalities, and tunable thermal properties from the resulting polymers.

Additive Manufacturing by Heating at a Patterned Photothermal Interface

Direct additive manufacturing (AM) of commercial silicones is an unmet need with high demand. We report a new technology, heating at a patterned photothermal interface (HAPPI), which achieves AM of commercial thermoset resins without any chemical modifications. HAPPI integrates desirable aspects of stereolithography with the thermally driven chemical modalities of commercial silicone formulations. In this way, HAPPI combines the geometric advantages of vat photopolymerization with the materials properties of, for example, injection molded silicones. We describe the realization of the new technology, HAPPI printing using a commercial Sylgard 184 polydimethylsiloxane resin, comparative analyses of material properties, and demonstration of HAPPI in targeted applications.

Vat 3D Printing of Bioderivable Photoresins – Toward Sustainable and Robust Thermoplastic Parts

Vat photopolymerization 3D printing (3DP) of thermoplastic materials is exceedingly difficult due to the typical reliance on cross-linking to form well-defined, solid objects on timescales relevant to 3DP. Additionally, photoresin build materials overwhelmingly rely upon nonrenewable feedstocks. To address these challenges, we report the vat 3DP of bioderivable photoresins that produced thermoplastic parts with highly tunable thermal and mechanical properties. The photoresins were formulated from two monomers that are easily obtainable from lignin deconstruction: 4-propylguaiacyl acrylate (4-pGA) and syringyl methacrylate (SMA). These bioderivable materials generated printed parts that ranged from soft elastomers to rigid plastics. For example, for 4-pGA-based materials, the breaking stresses varied from 0.20 to 20 MPa and breaking strains could be tuned from 4.7% up to 1700%, whereas 3D-printed SMA-based materials resulted in higher breaking stresses (∼30 MPa) and Tgs (∼132 °C). Notably, parts printed from these bioderivable formulations exhibited thermoplastic behavior and were largely soluble in common organic solvents─expanding the application and repurposing of the 3D-printed parts. We highlight this feature by reusing a 3DP part via solvent casting. Overall, the tunable properties and thermoplastic behavior of the lignin-derivable photoresins showcase renewable lignin resources as promising biofeedstocks for sustainable 3DP.

Mechanoactivation of Color and Autonomous Shape Change in 3D-Printed Ionic Polymer Networks

Stimuli-responsive materials can enhance the field of three-dimensional (3D) printing by generating objects that change shape in response to external cues. While temperature and pH are common inputs for initiating a response in a 3D-printed object, there are few examples of using a mechanical input to afford a response. Herein, we report a suite of mechanochromic ionic liquid gel inks that can be used to fabricate 3D-printed objects that use a single mechanoactivation event to elicit both a mechanochromic response and an autonomous shape change. Direct-ink write 3D printing was used to deposit ionic liquid gel inks to create multimaterial objects that underwent a predetermined mechanoactivated shape change (mechanomorphic) when the sample was pulled and then released. When spiropyran was incorporated into the inks, the onset of spiropyran isomerization into its purple merocyanine form occurred at strains dependent upon the particular ion gel ink formulation. We suggest that the color onset could be used as a simple indicator for when the strain required to achieve a predetermined change in shape has been reached, potentially serving as real-time visual cue for the user. Such morphing 3D-printed structures with integrated instructions have potential application to areas including stowable structures as well as multiresponsive autonomous components and sensors.