Anion exchange membranes (AEMs) are integral to fuel cells and water electrolysis systems but suffer from poor durability under alkaline conditions. Ether cleavage is an important failure pathway of poly(arylene ether) based AEMs that compromises both mechanical stability and ion transport. While this degradation pathway is often studied in terms of polymer fragmentation, the role of newly formed hydrophilic groups has been largely overlooked. We show that polymer scission leads to reduced mechanical rigidity, while the introduction of hydrophilic groups partially mitigates this loss. Under alkaline conditions, phenoxide groups formed during ether cleavage neutralize the polymer cations, leading to a previously unreported loss of ion exchange capacity (IEC). This IEC loss mechanism exacerbates the reduction in ionic conductivity, emphasizing the severity of ether cleavage as a degradation pathway. Recognizing that ether cleavage introduces significant chemical changes beyond polymer fragmentation provides critical insights into its interplay with other degradation mechanisms, such as the direct reduction of cationic sites by E2 and SN2 and provides molecular-level interpretations for the concurrent effects of polymer scission and increased hydrophilicity on membrane performance.

Additive manufacturing of glass aims to change the paradigm of glass manufacturing by allowing for improved customization coupled with lower energy and post-processing needs. Fabrication of glass via volumetric additive manufacturing (VAM) involves printing in a silica-loaded photopolymer resin with subsequent thermal processing and adds additional advantages by allowing for rapid printing of parts with smooth surfaces and no supports. Previous work in glass VAM has demonstrated fabrication of optical quality microoptics with overall dimensions on the scale of tens of cubic millimeters. For applications requiring glass printed on the scale of cubic centimeters the rheology, scattering, and green part strength must be controlled via resin formulation. Here, we present insight into a novel glass photopolymer resin suitable for VAM and in the effect of tuning formulation on the ability to produce dense glass parts with volumes on the scale of cubic centimeters. image

In-situ monitoring and characterization systems play a pivotal role in advancing the state of the art in manufacturing and the associated qualification of processes and materials. Here we present a highly sensitive optical imaging method based on quantitative phase imaging for in-situ monitoring the tomographic volumetric additive manufacturing process. The proposed method can visualize the manufacturing process with higher sensitivity and contrast for materials with low refractive index change after polymerization. The information provided by quantitative phase imaging system will be useful for quantifying the underlying material properties and optimizing the polymerization process.

Visible light-based volumetric additive manufacturing (VAM) technology has recently enabled rapid 3D printing of optically transparent resins in a single step. There is now strong interest in extending the design space of VAM to include opaque, scattering and composite materials. Microwave energy can penetrate more deeply than visible light into a broader family of materials. For microwaves to be useful for VAM, however it is necessary to have a fundamental understanding of material dielectric properties, microwave field propagation and localization. Here we present a multi-physics microwave beam formed-thermal diffusion model that addresses these needs. The model demonstrates its ability to optimize power delivery and curing time to obtain better thermal control. We validate the model with a proof-of-concept single-antenna experimental system operating at 10 GHz that is able to cure a wide variety of materials, including both optically translucent and opaque epoxy resins loaded with conductive additives with a minimum curing spot of 5 mm. While available microwave hardware operating at 40 Watt power cures the resins in 2.5 min, the model estimates the ability to cure in as less as 6 s at 1 Kilowatt power levels. This computational model and experiments lay the foundation for a future multi-waveguide microwave-based VAM system.

Volumetric additive manufacturing is a novel fabrication method allowing rapid, freeform, layer-less 3D printing. Analogous to computer tomography (CT), the method projects dynamic light patterns into a rotating vat of photosensitive resin. These light patterns build up a three-dimensional energy dose within the photosensitive resin, solidifying the volume of the desired object within seconds. Departing from established sequential fabrication methods like stereolithography or digital light printing, volumetric additive manufacturing offers new opportunities for the materials that can be used for printing. These include viscous acrylates and elastomers, epoxies (and orthogonal epoxy-acrylate formulations with spatially controlled stiffness) formulations, tunable stiffness thiol-enes and shape memory foams, polymer derived ceramics, silica-nanocomposite based glass, and gelatin-based hydrogels for cell-laden biofabrication. Here we review these materials, highlight the challenges to adapt them to volumetric additive manufacturing, and discuss the perspectives they present.

Seamless multimaterial construction is a common motif in animal physiology. Such continuous mechanical gradients remain challenging to reproduce in engineered systems, as current resin chemistries typically result in a single fixed set of properties. As an alternative to single-property materials, we introduce a thiol-ene-epoxy-based photothermal reaction scheme that produces multimaterials by altering the polymer microstructure within a single resin. In this system, the photodosage during the first stage of processing dictates the extent of conversion for each subsequent reaction. As a result, our photochemistry can exhibit a diverse range of soft (Young's modulus, E similar to 400 kPa; elongation, dL/L-0 -300%) and stiff (E similar to 1.6 GPa; dL/L-0 similar to 3%) mechanical properties. Furthermore, we pattern photostable and mechanically robust modulus gradients (d [E-r,E- stiff/E-r,E- soft]/dx > 1,000 mm(-1)) that exceed those found in squid beaks and human knee entheses. We demonstrate the ability to build intricate multimaterial architectures including a soft, wearable braille display.

Alkaline anion exchange membranes (AAEMs) are an enabling component for next-generation electrochemical devices, including alkaline fuel cells, water and CO2 electrolyzers, and flow batteries. While commercial systems, notably fuel cells, have traditionally relied on proton-exchange membranes, hydroxide-ion conducting AAEMs hold promise as a method to reduce cost-per-device by enabling the use of non-platinum group electrodes and cell components. AAEMs have undergone significant material development over the past two decades; however, challenges remain in the areas of durability, water management, high temperature performance, and selectivity. In this review, we survey crosslinking as a tool capable of tuning AAEM properties. While crosslinking implementations vary, they generally result in reduced water uptake and increased transport selectivity and alkaline stability. We survey synthetic methodologies for incorporating crosslinks during AAEM fabrication and highlight necessary precautions for each approach.

Three-dimensional printing, or additive manufacturing (AM), is a broad term for a wide range of fabrication methods utilizing materials such as small-molecule, polymer, and metal feedstocks. Each method requires different chemical, physical, and engineering needs to be successful. This article will discuss some of the considerations for polymer-based AM methods. Ultimately, we focus on the chemistries of vat photopolymerization, in which light is used to cure a resin from liquid to solid, to provide an example of how chemical advancements have led to increased speed, resolution, and multimaterial printing capabilities not previously possible.

Computed axial lithography (CAL) is an emerging volumetric additive manufacturing technology which presents unique opportunities in layerless ultra-rapid fabrication. However, the required process control places particular demands on computing and delivering the appropriate 3D distribution of optical energy, as well as monitoring the solidifying structure within the photo-resin. For example, continued reaction after tomographic exposure is not currently accounted for and could lead to higher degree-of-conversion than designed and consequent feature dilations. Color Schlieren Tomography (CST) is developed as an in-situ metrology tool to monitor volumetrically the internal refractive index and the forming geometry. Major improvements of CST in real-time computation and processing of 3D reconstruction have enabled event-driven patterning control such as auto-termination. With this technique, we monitored the polymerization process in real-time during and after termination of the exposure period signaled by an index-volume termination criterion. Monitoring of continued polymerization after termination (dark polymerization) shows that the refractive index change can rise to 10 times higher than its value at termination. The time-resolved 3D reconstruction data provided by CST can be used for chemical kinetics modeling and development of compensation schemes.

Shape memory polymers (SMPs) capable of recovering from a deformed state through heating were 3D printed using volumetric additive manufacturing, which enabled the layerless, single-step fabrication of self-standing tripod and actuating gripper structures.

The recent development of optical volumetric additive manufacturing (VAM) technology has enabled rapid 3-D printing of optically transparent resins. However, there is growing interest in employing other materials that extend the design space of this technology. Here we leverage focused microwave radiation to expand VAM to a much broader family of resins, including those which are opaque. We use an X-band microwave applicator array coupled to a beam-steering algorithm to focus microwave energy into liquid resins. We performed numerical simulations to demonstrate this microwave energy localization, and will present supporting experimental results.

Polymer 3D printing is a broad set of manufacturing methods that permit the fabrication of complex architectures, and, as a result, numerous efforts focus on formulating processible chemistries that produce desirable material behavior in printed parts. However, current resin chemistries typically result in a single fixed set of properties once fully polymerized, a fact that poses significant engineering challenges to obtaining multimaterial devices. As an alternative to single-property materials, we introduce a ternary sequential reaction scheme that exhibits diverse multimaterial properties by profoundly altering the polymer microstructure from within a single resin composition. In this system, the photodosage during 3D printing sets both the shape and extent of conversion for each subsequent reaction. This different polymerization mechanisms of the subsequent stages yield disparate crosslink densities and viscoelastic properties. As a result, our materials possess Young's Moduli spanning over three orders of magnitude (400 kPa < E < 1.6 GPa) with smooth transitions between soft and stiff regions. We successfully pattern a 500x change in modulus in under a millimeter while the sequential assembly of our polymer networks ensures robust interfaces and enhances toughness by 10x compared to the single property materials. Most importantly, the final objects remain stable to UV and thermal aging, a key limitation to applications of previous multimaterial chemistries. We demonstrate the ability to 3D print intricate multimaterial architectures by fabricating a soft, wearable braille display.

Nanofabrication techniques that can generate large and complex 3D structures with nanoscale features are becoming increasingly important in the fields of biomedicine, micro-optics, and microfluidics. Direct laser writing via two-photon polymerization (DLW-TPP) is one such technique that relies on nonlinear absorption of light to form nanoscale 3D features. Although DLW-TPP provides the required nanoscale resolution, its built height is often limited to less than a millimetre. This height limitation is driven by the need to tightly focus the laser beam at arbitrary depths within the photopolymer. This requirement necessitates matching the photopolymer's refractive index to specific values but the required techniques have not been disseminated widely in the open scientific literature. To address this knowledge gap, we test two universal, different approaches to generate refractive index-matched polymeric and preceramic resins and demonstrate their performance by printing of fine submicron features in 3D structures as tall as 2.5 mm. Specifically, we achieve index-matching by mixing commercially-available resins or covalent modification of functional monomers. This work investigates the relationship of voxel shape to RI mismatch, and presents tuning of RI through mixing and covalent modification to a nonconventional material system of preceramic resin which has never been demonstrated before. We demonstrate the material flexibility by generating 3D silicon oxycarbide structures from preceramic resists while simultaneously eliminating the part-height limitation of conventional DLW-TPP.

In this study, we show that mechanochemical activation in responsive materials with designed, periodic microstructures can be achieved at lower applied strains than their bulk counterparts.

Purpose 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. Design/methodology/approach A set of five common mechanical and material characterization tests were performed on 11 commercially available PLA filaments in an effort to gain insight into the variations in mechanical response that stem from variances in filament manufacturer, feed stock polymer, additives and processing. Three black PLA filaments were purchased from three different commercial suppliers to consider the variations introduced by use of different feed stock polymers and filament processing by different manufacturers. An additional eight PLA filaments in varying colors were purchased from one of the three suppliers to focus on how color additives lead to property variations. Some tests were performed on unprocessed filament samples, while others were performed on objects three-dimensional printed from the various filaments. This study looked specifically at four mechanical properties (Young’s modulus, storage modulus, yield strength and toughness) as a function of numerous material properties (e.g. additive loading, molecular weight, molecular weight dispersity, enthalpy of melting and crystallinity). Findings For the 11 filaments tested the following mean values and standard deviations were observed for the material properties considered: pa = 1.3 ± 0.9% (percent additives), Mw = 98.6 ± 16.4 kDa (molecular weight), Ð = 1.33 ± 0.1 (molecular weight dispersity), Hm = 37.4 ± 7.2 J/g (enthalpy of melting) and = 19.6 ± 2.1% (crystallinity). The corresponding mean values and standard deviations for the resulting mechanical behaviors were: E = 2,790 ± 145 MPa (Young’s modulus), E’ = 1,050 ± 125 MPa (storage modulus), Sy = 49.6 ± 4.93 MPa (yield strength) and Ut = 1.87 ± 0.354 MJ/m^3 (toughness). These variations were observed in filaments that were all manufactured from the same base polymer (e.g. PLA) and are only different in terms of the additives used by the manufacturers to produce different colors or different three-dimensional printing performance. Unfortunately, while the observed variations were significant, no definitive strong correlations were found between these observed variations in the mechanical behavior of the filaments studied and the considered material properties. Research limitations/implications These variations in mechanical behavior and material properties could not be ascribed to any specific factor, but rather show that the mechanical of three-dimensional printed parts are potentially affected by variations in base polymer properties, additive usage and filament processing choices in complex ways that can be difficult to predict. Practical implications These results emphasize the need to take processing and thereby even filament color, into account when using ME printers, they emphasize the need for designers to use AM with caution when the mechanical behavior of a printed part is critical and they highlight the need for continued research in this important area. While all filaments used were marked as PLA, the feedstock materials, additives and processing conditions created significant differences in the mechanical behavior of the printed objects evaluated, but these differences could not be accurately and reliably predicted as function of the observed material properties that were the focus of this study. Originality/value The testing methods used in the study can be used by engineers and creators alike to better analyze the material properties of their filament printed objects, to increase success in print and mechanical design. Furthermore, the results clearly show that as AM continues to evolve and grow as a manufacturing method, standardization of feedstock processing conditions and additives would enable more reliable and repeatable printed objects and would better assist designers in effectively implementing AM methods.

Volumetric additive manufacturing (VAM) forms complete 3D objects in a single photocuring operation without layering defects, enabling 3D printed polymer parts with mechanical properties similar to their bulk material counterparts. This study presents the first report of VAM-printed thiol-ene resins. With well-ordered molecular networks, thiol-ene chemistry accesses polymer materials with a wide range of mechanical properties, moving VAM beyond the limitations of commonly used acrylate formulations. Since free-radical thiol-ene polymerization is not inhibited by oxygen, the nonlinear threshold response required in VAM is introduced by incorporating 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) as a radical scavenger. Tuning of the reaction kinetics is accomplished by balancing inhibitor and initiator content. Coupling this with quantitative measurements of the absorbed volumetric optical dose allows control of polymer conversion and gelation during printing. Importantly, this work thereby establishes the first comprehensive framework for spatial-temporal control over volumetric energy distribution, demonstrating structures 3D printed in thiol-ene resin by means of tomographic volumetric VAM. Mechanical characterization of this thiol-ene system, with varied ratios of isocyanurate and triethylene glycol monomers, reveals highly tunable mechanical response far more versatile than identical acrylate-based resins. This broadens the range of materials and properties available for VAM, taking another step toward high-performance printed polymers.

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.

Additive manufacturing, commonly referred to as 3D printing (3DP), has ushered in a new era of advanced manufacturing that is seemingly limited only by imagination. In actuality, the fullest potentials of 3DP can only be realized through innovative breakthroughs in printing technologies and build materials. Whereas equipment for 3DP has experienced considerable development, molecular-scale programming of function, adaptivity, and responsiveness in 3DP is burgeoning. This review aims to summarize the state-of-the-art in stimuli-responsive materials that are being explored in 3DP. First, we discuss stimuli-responsiveness as it is used to enable 3DP. This highlights the diverse ways in which molecular structure and reactivity dictate energy transduction that in turn enables 3D processability. Second, we summarize efforts that have demonstrated the use of 3DP to create materials, devices, and systems that are in their final stage stimuli-responsive. This section encourages the artistic license of advanced manufacturing to be applied toward leveraging, or enhancing, energy transduction to impart device function across multiple length scales.

New advancements in 3D printing enable manufacturing a solid part with spatially controlled and varying material properties; this research seeks to establish techniques for finding optimal designs that use this new technology for the greatest structural benefit. We describe the use of a sequential quadratic programming based optimization solver to find an optimal distribution of material properties that minimize strain energy gradients, as calculated using finite element analysis. This design method is applied to the case of a flat thin plate with a hole, and has been proven to successfully reduce strain energy gradients and therefore stress concentrations. The optimally designed plates are 3D printed using a novel technology that uses vat polymerization technology. The computational model is validated with experiments. Enabling design engineers to customize material properties around geometric discontinuities will provide greater flexibility in reducing stress concentrations without modifying geometry or adding additional supports.