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.
Additive Manufacturing
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.
The role of polymer mechanochemistry in responsive materials and additive manufacturing.
The use of mechanical forces to chemically transform polymers dates back decades. In recent years, the use of mechanochemistry to direct constructive transformations in polymers has resulted in a range of engineered molecular responses that span optical, mechanical, electronic and thermal properties. The chemistry that has been developed is now well positioned for use in materials science, polymer physics, mechanics and additive manufacturing. Here, we review the historical backdrop of polymer mechanochemistry, give an overview of the existing toolbox of mechanophores and associated theoretical methods, and speculate as to emerging opportunities in materials science for which current capabilities are seemingly well suited. Non-linear mechanical responses and internal, amplifying stimulus–response feedback loops, including those enabled by, or coupled to, microstructured metamaterial architectures, are seen as particularly promising.
Mechanical characterization and constitutive modeling of visco-hyperelasticity of photocured polymers
In this work, we study the nonlinear behavior of soft photocured polymers typically used in 3D-printing. We perform experimental testing of 3D-printed samples cured at various controlled light intensities. The experimental data show the dependency of the material elasticity and rate-sensitivity on the curing light intensity. To elucidate these relations, we develop a physically-based visco-hyperelastic model in the continuum thermodynamics framework. In our model, the macroscopic viscoelastic behavior is bridged to the microscopic molecular chain scale. This approach allows us to express the material constants in terms of polymer chain physical parameters. We consider different physical mechanisms governing hyperelasticity and rate-dependent behaviors. The hyperelastic behavior is dictated by the crosslinked network; whereas, the viscous part originates in the free and dangling chains. Based on our experimental data, we illustrate the ability of the new constitutive model to accurately describe the influence of the light intensity on photocured polymer viscoelasticity.
Not all PLA filaments are created equal: an experimental investigation.
Additive manufacturing (AM) methods such as material extrusion (ME) are becoming widely used by engineers, designers and hobbyists alike for a wide variety of applications. Successfully manufacturing objects using ME three-dimensional printers can often require numerous iterations to attain predictable performance because the exact mechanical behavior of parts fabricated via additive processes are difficult to predict. One of that factors that contributes to this difficulty is the wide variety of ME feed stock materials currently available in the marketplace. These build materials are often sold based on their base polymer material such as acrylonitrile butadiene styrene or polylactic acid (PLA), but are produced by numerous different commercial suppliers in a wide variety of colors using typically undisclosed additive feed stocks and base polymer formulations. This paper aims to present the results from an experimental study concerned with quantifying how these sources of polymer variability can affect the mechanical behavior of three-dimensional printed objects. Specifically, the set of experiments conducted in this study focused on following: several different colors of PLA filament from a single commercial supplier to explore the effect of color additives and three filaments of the same color but produced by three different suppliers to account for potential variations in polymer formulation.
100th Anniversary of Macromolecular Science Viewpoint: Integrating Chemistry and Engineering to Enable Additive Manufacturing with High-Performance Polymers
Additive manufacturing (AM) with high-performance polymers (HPPs) represents simultaneously one of the most desirable and challenging feats in the AM arena. The very properties that make HPPs so attractive in a broad range of applications also make them nearly impossible to process using common AM equipment. Furthermore, when AM is achieved, it often brings the caveat of compromised mechanical properties of the final parts, in comparison with those made via injection molding. The demand to have advanced fabrication methods, rapid prototyping, and customization of parts while maintaining high performance in the finished products has inspired creative innovations that integrate chemical synthesis, materials science, mechanical engineering, and other fields into a multidisciplinary approach to advance AM with the seemingly “unprintable” HPPs. In this Viewpoint, we summarize several standout developments in the area and offer our perspective on future directions and challenges.
Room Temperature Extrusion 3D Printing of Polyether Ether Ketone Using a Stimuli-Responsive Binder
We report our efforts toward 3D printing of polyether ether ketone (PEEK) at room temperature by direct-ink write technology. The room-temperature extrusion printing method was enabled by a unique formulation comprised of commercial PEEK powder, soluble epoxy-functionalized PEEK (ePEEK), and fenchone. This combination formed a Bingham plastic that could be extruded using a readily available direct-ink write printer. The initial green body specimens were strong enough to be manipulated manually after drying. After printing, thermal processing at 230 °C resulted in crosslinking of the ePEEK components to form a stabilizing network throughout the specimen, which helped to preclude distortion and cracking upon sintering. A final sintering stage was conducted at 380 °C. The final parts were found to have excellent thermal stability and solvent resistance. The Tg of the product specimens was found to be 158 °C, which is 13 °C higher than commercial PEEK as measured by DSC. Moreover, the thermal decomposition temperature was found to be 528 °C, which compares well against commercial molded PEEK samples. Chemical resistance in trifluoroacetic acid and 8 common organic solvents, including CH2Cl2 and toluene, were also investigated and no signs of degradation or weight changes were observed from parts submerged for 1 week in each solvent. Test specimens also displayed desirable mechanical properties, such as a Young’s modulus of 2.5 GPa, which corresponds to 63% of that of commercial PEEK (reported to be 4.0 GPa).
Stimuli-Responsive Materials in Additive Manufacturing
Additive manufacturing (AM) technologies are expanding the boundaries of materials science and providing an exciting forum for interdisciplinary research. The ability to fabricate arbitrarily complex objects has made AM technologies indispensable in personalized healthcare, soft electronics, and renewable energy. At the intersection of AM technologies and materials chemistry are stimuli-responsive polymers, which change their chemical and physical properties in response to specific environmental cues. The responsiveness of these “smart” polymers makes them suitable for AM and provides functionality to the additively manufactured objects. Furthermore, the type and degree of stimulus response of smart polymers can be regulated through precise synthetic design or via incorporation of additives. Herein, we review recently reported stimuli-responsive polymers used in AM, with a focus on the design and chemistry of the polymers. The materials are broadly classified by type of printing, and more specifically classified by type of stimulus response. Finally, we briefly consider existing challenges that stimuli-responsive materials in AM can address in the future.
Multimaterial Actinic Spatial Control 3D and 4D Printing
Production of objects with varied mechanical properties is challenging for current manufacturing methods. Additive manufacturing could make these multimaterial objects possible, but methods able to achieve multimaterial control along all three axes of printing are limited. Here we report a multi-wavelength method of vat photopolymerization that provides chemoselective wavelength-control over material composition utilizing multimaterial actinic spatial control (MASC) during additive manufacturing. The multicomponent photoresins include acrylate- and epoxide-based monomers with corresponding radical and cationic initiators. Under long wavelength (visible) irradiation, preferential curing of acrylate components is observed. Under short wavelength (UV) irradiation, a combination of acrylate and epoxide components are incorporated. This enables production of multimaterial parts containing stiff epoxide networks contrasted against soft hydrogels and organogels. Variation in MASC formulation drastically changes the mechanical properties of printed samples. Samples printed using different MASC formulations have spatially-controlled chemical heterogeneity, mechanical anisotropy, and spatially-controlled swelling that facilitates 4D printing.