Additive manufacturing in polymer science

Additive manufacturing, or 3D printing, has transformed the way that society creates. Relative to traditional subtractive manufacturing strategies and injection molding, 3D printing provides reduced design constraints, waste generation, and cost. It has enabled the rapid and efficient production of polymeric objects with hierarchical geometries having unique macroscopic properties from mechanical to thermal, optical, and electrical. This versatility has led to a myriad of applications from hobbyist printing of models to printing medical devices and even tissue constructs. However, despite the grandiose claims that 3D printing is only limited by our imaginations, several opportunities remain to expand the scope of printable materials. This special section on Additive Manufacturing in Polymer Science highlights research that has pushed the envelope on what is possible using different 3D printing techniques, from fused deposition modeling (FDM) to powder bed fusion (PBF) and various forms of vat photopolymerization. One of the most employed forms of 3D printing is FDM, where a filament is melted and deposited in a layer-by-layer fashion to reconstruct a 3D object from a digital file. However, plastic parts produced using FDM often suffer from poor mechanical properties, such as low tensile strengths. To address this limitation, Song and coworkers (pol.20230632) developed a process wherein carbon fiber can be uniformly dispersed and aligned within nylon filaments. This enabled the preparation of composite objects with high carbon fiber contents improving tensile strength and stiffness by ≥2× relative to pristine nylon controls. As a complementary strategy to improve mechanical properties Vogt and coworkers (pol.20230559) demonstrated how tuning the molecular weight distribution in poly(methyl methacrylate) (PMMA) filaments by mixing PMMA samples of different molecular weights could be used as a straightforward strategy to improve mechanical performance. Specifically, they found that the blended PMMA filaments enhanced tensile strength, stiffness, and strain at break over filaments solely comprised of the individual PMMA components. Another widely used form of 3D printing is PBF, where a fine (polymer) powder is fused (melted) upon applying a localized energy source, often a laser. After completing the fusion process for one layer, a fresh layer of powder is deposited onto it and then the process is repeated to build up a 3D structure. However, polymer feedstocks for PBF are limited given the strict requirements on particle size, flow, and scalability. To address this limitation, Dadmun and coworkers (pol.20230286) developed a thermally induced phase separation process to prepare polypropylene powders that are suitable for PBF. By systematically varying polymer concentration, molecular weight, and quench temperature the authors identified design principles for generating powders usable in PBF. Light-based 3D printing in the form of vat photopolymerization offers an unparalleled combination of build speed, feature resolution, and low cost. Several modalities of vat photopolymerization exist, effectively originating from stereolithographic apparatus (SLA) printing. Traditionally, SLA refers to the use of a rastering laser to cure (i.e., harden) a liquid resin on a 1D, point-by-point basis, followed by layering. In contrast, digital light processing (DLP) uses a 2D projection to cure an entire layer at once, making it considerably faster than SLA. However, none is faster than volumetric additive manufacturing (VAM), where an entire 3D object is cured within a single vat of resin without layering. Despite these technological differences, all vat photopolymerization techniques leverage the spatiotemporal control offered by light to rapidly cure resins to generate (predominantly) acrylic thermosets via an uncontrolled free radical polymerization mechanism. In turn, several opportunities exist to expand the compositional scope of resins for vat photopolymerization. To this end, Lalevée and coworkers (pol.20230327) leverage their expertise in the development of novel photoinitiators that respond to not only light, but also heat to induce curing. The photo/thermal curing enabled the rapid preparation of opaque composite structures, and the authors further demonstrated their potential utility in SLA-type 3D printing. In developing new materials for vat photopolymerization Schwartz, Porcincula and coworkers (pol.20230654) describe the fabrication of silica glass using VAM. This was accomplished by creating novel silica-loaded photopolymer resins, followed by sintering. Notably, their systematic study connecting photoresin composition to optical and rheological properties, photopolymerization kinetics, and thermal processing provide insight that will inform further advancements in this nascent space. While new additive manufacturing printing technologies continue to emerge, there is clearly a need for new chemical methods that are compatible with these machines. These developments include reactions that have tolerance to oxygen and water, thus enabling the printing to occur under ambient conditions while exposed to the open air environment. Seo and coworkers (pol.20230678) developed a photoinitiating system that enables cationic polymerization of vinyl ethers when irradiated with blue light. The dormant trithiocarbonate chain ends can then be photoactivated for radical polymerization of acrylate monomers. This chemical advancement will continue to expand the range of multimaterials that can be printed in the future. Beyond new chemical reactions, there is also a need to address how sustainable materials will fit into additive manufacturing. Most of the commercial resins available today are derived from petroleum sources. Bio-based resins provide a promising alternative as recently demonstrated by the Smaldone and coworkers (pol.20230026) and Voit and coworkers (pol.20230700). Interestingly, both teams showed that plant-sourced materials can be transformed into resins for vat photopolymerization processes. Smaldone and coworkers showed that methacrylated lignin could be developed into resins suitable for printing to afford constructs with 70% bio-based content. Additionally, the presence of ester bonds facilitated dynamic transesterification that enabled self-healing capabilities, increasing the printed object's lifetime of use and re-use. On the other hand, Voit and coworkers used methacrylated micro-crystalline cellulose as an additive in a soybean oil-based resin to enhance mechanical properties of the printed parts. Up to 20% of the cellulose derivative was added while still maintaining the printability of the resin for vat photopolymerization. A ten-fold enhancement in the modulus was observed with only a 3% reduction in the failure strain. The advances demonstrated by Smaldone and Voit are promising steps toward bio-based resins for vat photopolymerization. Finally, Peterson and coworkers (pol.20230649) provide a review wherein they discuss the incorporation of machine learning (ML) into additive manufacturing. ML is expected to serve a prominent role in materials discovery, process optimization, and design optimization. The review provides the reader with the challenges of integrating ML into the additive manufacturing of polymers and composites and identifies future areas of success in this field. Additive manufacturing has the potential to completely alter manufacturing paradigms across the globe. Now is the time to develop materials that will address the material needs of this industry in the future. New methodologies, material properties, and sustainability are all critical to expanding the capabilities of additive manufacturing processes. This special section on Additive Manufacturing in Polymer Science provides just a glimpse of more to come in this field! We would like to thank all the authors, reviewers, and editorial staff of Journal of Polymer Science for their significant contributions to make this special issue possible.