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An H-polymer has an architecture that consists of four branches symmetrically attached to the ends of a polymer backbone, similar in shape to the letter “H”. Here, a renewable H-polymer efficiently synthesized using only ring-opening transesterification is demonstrated. The strategy relies on a tetrafunctional poly(±-lactide) macroinitiator, from which four poly(±-lactide) branches are grown simultaneously. 1H NMR spectroscopy, size exclusion chromatography (SEC), and matrix-assisted laser desorption/ionization (MALDI) spectrometry were used to verify the macroinitiator purity. Branch growth was probed using 1H NMR spectroscopy and SEC to reveal unique transesterification phenomena that can be controlled to yield architecturally pure or more complex materials. H-shaped PLA was prepared at the multigram scale with a weight-average molar mass Mw > 100 kg/mol and low dispersity Đ < 1.15. Purification involved routine precipitations steps, which yielded products that were architecturally relatively pure (∼93%). Small-amplitude oscillatory shear and extensional rheology measurements demonstrate the unique viscoelastic behavior associated with the H-shaped architecture. 

Additive manufacturing, otherwise known as three-dimensional (3D) printing, is a rapidly growing technique that is increasingly used for the production of polymer products, resulting in an associated increase in plastic waste generation. Waste from a particular class of 3D-printing, known as vat photopolymerization, is of particular concern, as these materials are typically thermosets that cannot be recycled or reused. Here, we report a mechanical recycling process that uses cryomilling to generate a thermoset powder from photocured parts that can be recycled back into the neat liquid monomer resin. Mechanical recycling with three different materials is demonstrated: two commercial resins with characteristic brittle and elastic mechanical properties and a third model material formulated in-house. Studies using photocured films showed that up to 30 wt% of the model material could be recycled producing a toughness of 2.01 ± 0.55 MJ/m3, within error of neat analogues (1.65 ± 0.27 MJ/m3). Using dynamic mechanical analysis and atomic force microscopy-based infrared spectroscopy, it was determined that monomers diffuse into the recycled powder particles, creating interpenetrating networks upon ultraviolet (UV) exposure. This process mechanically adheres the particles to the matrix, preventing them from acting as failure sites under a tensile load. Finally, 3D-printing of the commercial brittle material with 10 wt% recycle content produced high quality parts that were visually similar. The maximum stress (46.7 ± 6.2 MPa) and strain at break (11.6 ± 2.3%) of 3D-printed parts with recycle content were within error the same as neat analogues (52.0 ± 1.7 MPa; 13.4 ± 1.8%). Overall, this work demonstrates mechanical recycling of photopolymerized thermosets and shows promise for the reuse of photopolymerized 3D-printing waste. 

3D printing is an essential tool for rapid prototyping in a variety of sectors such as automotive and public health. The 3D printing market is booming, and it is projected that it will continue to thrive in the coming years. Unfortunately, this rapid growth has led to an alarming increase in the amount of 3D printed plastic waste. 3D printing processes such as stereolithography (SLA) and digital light projection (DLP) in particular generally produce petroleum-based thermosets that are further worsening the plastic pollution problem. To mitigate this 3D printed plastic waste, sustainable alternatives to current 3D printing materials must be developed. The present review provides a comprehensive overview of the sustainable advances in SLA/DLP 3D printing to date and offers a perspective on future directions to improve sustainability in this field. The entire life cycle of 3D printed parts has been assessed by considering the feedstock selection and the end-of-use of the material. The feedstock selection section details how renewable feedstocks (from lignocellulosic biomass, oils, and animal products) or waste feedstocks ( e.g. , waste cooking oil) have been used to develop SLA/DLP resins. The end-of-use section describes how materials can be reprocessed ( e.g. thermoplastic materials or covalent adaptable networks) or degraded (through enzymatic or acid/base hydrolysis of sensitive linkages) after end-of-use. In addition, studies that have employed green chemistry principles in their resin synthesis and/or have shown their sustainable 3D printed parts to have mechanical properties comparable to commercial materials have been highlighted. This review also investigates how aspects of sustainability such as recycling for feedstock/end-of-use or biodegradation of 3D printed parts in natural environments can be incorporated as future research directions in SLA/DLP.