Viswanath Meenakshisundaram’s research while affiliated with Virginia Tech and other places

The availability of engineering polymers for vat photopolymerization (VP) additive manufacturing is limited. This limitation primarily stems from the inability of standard VP systems to recoat high-viscosity resins (> 3 Pa s). High-temperature vat photopolymerization is a new process-based VP platform that enables processing of viscous photopolymer resins (viscosity > 3 Pa s). Research in this area has been focused on demonstrating expanded access to new polymer families, and studying the effect of printing temperature on mechanical and esthetic performance of printed parts. However, methods to determine the printing temperature that prevents the occurrence of thermally induced polymerization (i.e., thermal stability) in the resin have not been established. In this work, the authors have applied characterization techniques such as thermogravimetric analysis, Rheology and differential scanning calorimetry to determine the printing temperature for processing viscous photopolymer resins. As a case study, the developed characterization techniques are applied to: (1) photopolymer that is solid at room temperature, (2) polymer with viscosity of 21 Pa s at room temperature, and the temperature at which the resins can be printed without triggering thermally induced polymerization is successfully determined. The results of this work will act as a materials’ characterization and process parameter development guide for high-temperature VP systems, thus enabling expansion of VP materials catalogue to engineering materials that were previously unprocessable.

Vat photopolymerization (VP) is an advanced additive manufacturing (AM) platform that enables production of intricate three‐dimensional (3D) monoliths that are unattainable with conventional manufacturing methods. In this work, modification of amorphous poly(arylene ether sulfone)s (PSU) allowed for VP printing. Post‐polymerization telechelic functionalization with acrylate functionality yielded photo‐crosslinkable PSUs across a molecular weight range. 1H NMR spectroscopy confirmed chemical composition and quantitative acrylate functionalization. Addition of diphenyl‐(2,4,6‐trimethylbenzoyl)phosphine oxide (TPO) photo‐initiator to 30 wt. % PSU solutions in NMP provided a photo‐curable composition. However, subsequent photo‐rheological studies elucidated rapid photo‐degradation of the polysulfone main chain, which was especially apparent in high Mn (15 kg·mol−1) PSU formulations. UV‐light intensity and wavelength range were altered to reduce degradation while allowing for efficient crosslinking. The addition of 0.5 wt. % of avobenzone photo‐blocker produced an ill‐defined structure with 6 kg·mol−1 PSU. For higher molecular weights (>12 kg·mol−1), solutions with a low molar mass reactive diluent, i.e., trimethylolpropane triacrylate, enabled the printing of an organogel with a storage modulus (>105 Pa) sufficient for vat photopolymerization. Employing multicomponent solutions provided well‐defined parts with complex geometries through vat photopolymerization. This article is protected by copyright. All rights reserved

Unsaturated polyester resins (UPRs) enjoy numerous applications as structural adhesives in glass fiber laminates, concrete flooring, and masonry repair. Typically, UPRs consist of unsaturated polyester (UPE) oligomers with up to 50 wt.% of a reactive diluent such as styrene. The ability of these resins to cure rapidly upon UV irradiation, in conjunction with a photoinitiator, enables vat photopolymerization (VP) 3D printing. However, the volatility and toxicity of styrene limits the use of traditional UPRs for VP. This report describes a nonvolatile ionic liquid reactive diluent for UPRs, which, in combination with the unreactive diluent dimethoxyethane, produce resins suitable for VP. Photorheological experiments help guide resin design for VP based on a series of synthesized UPEs and PIL concentrations. Photocured networks exhibit increasing degradation temperatures with increasing PIL incorporation from 215 to 279 °C. VP of a selected UPR composition demonstrated the ability to form self-supporting, geometrically complex 3D printed structures, suggesting the opportunity to utilize a common industrial feedstock as a component of a novel VP resin system. Dried and unextracted 3D printed test specimens exhibit ionic conductivities spanning from 10⁻⁸ to 10⁻⁵ S cm⁻¹ between 60 and 150 °C, which indicate a potential additional attribute for 3D printed UPE parts.

Unparalleled temporal and spatial control of colloidal chemical processes introduces immense potential for the manufacturing, modification, and manipulation of latex particles. This review highlights major advances in photochemistry, both as stimulus and response, to generate unprecedented functionality in polymer colloids. Light-based chemical modification generates polymer particles with unique structural complexity, and the incorporation of photoactive functionalities transforms inert particles into photoactive nanodevices. Latex photo-functionality, which is reflected in both the colloidal and coalesced states, enables photochromism, photoswitchable aggregation, tunable fluorescence, photoactivated crosslinking and solidification, and photomechanical actuation. Previous literature explores the capacity of photochemistry, which complements the rheological and processing advantages of latex, to expand beyond traditional coatings applications and enable disruptive technologies in critical areas including nanomedicine, data security, and additive manufacturing.

Industries such as orthodontics and athletic apparel are adopting vat photopolymerization (VP) to manufacture customized products with performance tailored through geometry. However, vat photopolymerization is limited by low manufacturing speeds and the trade-off between manufacturable part size and feature resolution. Current VP platforms and their optical sub-systems allow for simultaneous maximization of only two of three critical manufacturing metrics: layer fabrication time, fabrication area, and printed feature resolution. The Scanning Mask Projection Vat Photopolymerization (S-MPVP¹) system was developed to address this shortcoming. However, models developed to determine S-MPVP process parameters are accurate only for systems with an intensity distribution that can be approximated with a first order Gaussian distribution. Limitations of optical elements and the use of heterogeneous photopolymers result in non-analytic intensity distributions. Modeling the effect of non-analytic intensity distribution on the resultant cure profile is necessary for accurate manufacturing of multiscale products. In this work, a model to predict the shape of cured features using analytic and non-analytic intensity distribution is presented. First, existing modeling techniques developed for laser and mask projection VP processes were leveraged to create a numerical model to relate the process parameters (i.e. scan speed, mask pattern irradiance) of the S-MPVP system with the resulting cure profile. Then, by extracting the actual intensity distribution from the resin surface, we demonstrate the model's ability to use non-analytic intensity distribution for computing the irradiance for any projected pattern. Using a custom S-MPVP system, process parameters required to fabricate test specimens were experimentally determined. These parameters were then input into the S-MPVP model and the resulting cure profiles were simulated. Comparison between the simulated and printed specimens dimensions demonstrates the model’s effectiveness in predicting the dimensions of the cured shape with an error of 2.9%.

Vat photopolymerization (VP) additive manufacturing fabricates intricate geometries with excellent resolution; however, high molecular weight polymers are not amenable to VP due to concomitant high solution and melt viscosities. Thus, a challenging paradox arises between printability and mechanical performance. This report describes concurrent photopolymer and VP system design to navigate this paradox with the unprecedented use of polymeric colloids (latexes) that effectively decouple the dependency of viscosity on molecular weight. Photocrosslinking of a continuous-phase scaffold, which surrounds the latex particles, combined with in-situ computer-vision print parameter optimization, which compensates for light scattering, enables high-resolution VP of high molecular weight polymer latexes as particle-embedded green bodies. Thermal post-processing promotes coalescence of the dispersed particles throughout the scaffold, forming a semi-interpenetrating polymer network (sIPN) without loss in part resolution. Printing a styrene-butadiene rubber (SBR) latex, a previously inaccessible elastomer composition for VP, exemplified this approach and yielded printed elastomers with precise geometry and tensile extensibilities exceeding 500%.

Source Form is a stand-alone device capable of collecting crowdsourced images of a user-defined object, stitching together available visual data (e.g., photos tagged with search term) through photogrammetry, creating watertight models from the resulting point cloud and 3D printing a physical form. This device works completely independent of subjective user input resulting in two possible outcomes: 1. Produce iterative versions of a specific object (e.g., the Statue of Liberty) increasing in detail and accuracy over time as the collective dataset (e.g., uploaded images of the statue) grows. 2. Produce democratized versions of common objects (e.g., an apple) by aggregating a spectrum of tagged image results. This project demonstrates that an increase in readily available image data closes the gap between physical and digital perceptions of form through time. For example, when Source Form is asked to print the Statue of Liberty today and then print again 6 months from now, the later result will be more accurate and detailed than the previous version. As people continue to take pictures of the monument and upload them to social media, blogs and photo sharing sites, the database of images grows in quantity and quality. Because Source Form gathers a new dataset with each print, the resulting forms will always be evolving. The collection of prints the machine produces over time are cataloged and displayed in linear groupings, providing viewers an opportunity to see growth and change in physical space. In addition to rendering change over time, a snapshot of a more common object's web perception could be created. For example, when an image search for "apple" is performed, the results are a spectrum of condition and species from rotting crab apples to gleaming Granny-Smith's. Source Form aggregates all of these images into one model and outputs the collective web presence of an "apple". Characteristics of the model are guided by the frequency and order in response to the image web search. The resulting democratized forms are emblematic of the web's collective and popular perceptions.

This commentary discusses current capabilities of vat photopolymerization, an additive manufacturing (AM) technique also known as VP, with recent advances in the literature, current challenges/limitations, and future outlook in novel materials design. Current trends and recent research advances are broadly discussed covering a spectrum of material classes such as performance, medicine, energy, and active materials in parallel to their importance in diverse technologies. Current challenges and limitations of VP are also discussed in terms of material properties, photodegradation, and material toxicity with directions in future material design to overcome these challenges. This commentary paper is intended to be of broad interest to both chemists and engineers actively involved in the AM field, in terms of future material design and processing for further development of VP-based AM technology.