In the few decades since their debut, additive manufacturing (AM) processes have transformed product development workflows and select manufacturing capabilities. [1][2][3] Advances in polymer AM are of particular interest because of the prospect of low cost, rapid production of parts with highly tunable properties like stiffness, thermal and chemical stability, wear resistance, bio-compatibility, and more. 4,5 Yet, although polymer additive manufacturing significantly minimizes waste during production compared to subtractive manufacturing methods, [1][2][3] the life cycle of thermoset plastic parts, which cannot be easily reprocessed, presents many opportunities for improved sustainability.
and stereolithography (SLA) primarily produce thermoset polymers from acrylate and epoxide monomers. 1,6 While these chemistries enable high resolution, rapid production of parts with favorable chemical and mechanical attributes, the printed solid is typically a chemicallycrosslinked polymer network, whose irreversible bonds prevent melting or dissolution, rendering it non-recyclable through conventional and widespread mechanical methods. [6][7][8][9][10] Also, scrapped parts and support structures produced during printing cannot be easily recycled, and therefore VP-based manufacturing results in significant unrecyclable waste 5 that must be disposed of in landfills.
Researchers have explored alternative chemistries to improve the circularity of VP resins including strategies involving covalent adaptable networks (CANs) that allow for reprocessing or reconfiguration of printed parts using thermal and chemical stimuli, [11][12][13][14][15][16] or chemical upcycling of the thermoset polymer into useful precursors. 17 Yet, the proposed processes required for recovery and reuse of the printed polymers are not yet compatible with existing large-scale plastic recycling methods, which focus mainly on mechanical and thermal processing of thermoplastics. 9,18 The energy costs and lack of widespread adoption of chemical recycling methods 9 reduces the utility of these strategies in producing end-use parts.
Conversely, linear thermoplastic polymers are among the most recycled plastic materials. 9 At industrial scales, thermoplastic parts are manufactured by melt processing, typically through molding or extrusion. 19 Although these methods excel at cost effective, highthroughput production, they struggle to achieve spatial resolution and geometric freedom exhibited by light-based AM processes like vat-polymerization. 1,3 As a result, there has been development towards adapting thermoplastic materials for light-based 3D printing techniques. Researchers have demonstrated photopolymerization of both IBOA (isobornyl acrylate) and ACMO (acryloyl morpholine) monomers as well as reprocessing of the resulting polymers with heat and solvents. [20][21][22] These approaches rely on a single phase resin system and niche polymer chemistries, making them unsuitable for producing more widely used commodity and engineering polymers such as PMMA (polymethyl methacrylate) and PS (polystyrene), which dissolve easily in their monomer.
Here we present a novel process for light-based 3D printing of fully recyclable, linear thermoplastics that bridges the performance and sustainability gap while enabling the digital processing of polymers that readily dissolve in their monomers. Interfacial photopolymerization (IPP), first demonstrated by Chazot et al., 23 involves polymer films patterned by ultraviolet irradiation at an interface between two immiscible liquids containing a photoinitiator and monomer. Building upon that system, the present approach achieves high resolution, multilayer printing of PMMA, incorporates PEG (polyethylene glycol) polymer as a binder, and integrates the IPP process into commercial DLP hardware where it is used to produce small, complex three dimensional objects. While the IPP process for PMMA results in highly porous materials with a globular microstructure, post processing techniques are further developed to enhance part strength and improve dimensional stability, yielding functional and recyclable 3D printed parts via digital light processing.
The feedstock for the IPP process, summarized in Figure 1a, comprises two immiscible liquids: an organic phase containing a monomer and an aqueous phase containing a watersoluble photoinitiator. When UV light is directed at the liquid-liquid interface, nearby photoinitiator molecules dissociate and engage in free-radical polyaddition reactions with sparingly soluble monomer molecules that have crossed the interface. This interface effectively traps the propagating polymer chains, allowing them to grow and entangle in solution, preventing significant diffusion and inducing precipitation. The interaction between the nascent polymer and surrounding solvents is carefully controlled to ensure sufficient polymer chains grow and precipitate above a critical entanglement molecular weight, thereby generating a high resolution, cohesive polymer film in the exposed region. 23 Contrasting prior work that demonstrated the IPP concept for photoprinting of single Figure 1: (a) A solid build plate descends from the liquid-liquid interface by a fixed distance. Patterned UV light is projected onto the interface and induces the formation of a thin polymer layer in the space between the build plate and the interface. The build plate then descends again and the process is repeated until the desired height is achieved. (b) Within a reaction zone on the aqueous side of the liquid-liquid interface, UV radiation activates V-50 and LAP photoinitiators, which in turn initiate free radical polymerization of MMA monomer into PMMA. (c) An alternate embodiment of this technique enables UV-initiated free radical polymerization of AN monomer into PAN. (d) As polymer chains form and grow in the reaction zone, they entangle with one another as well as any previously printed polymer on the build plate below. Throughout the duration of the exposure, the newly formed layer gradually densifies. (e) Diagram of the custom adapter used to integrate the IPP system into a commercial DLP 3D printer (f) A model of MIT's Building 10, 3D-printed with PMMA using the IPP process, before post-processing and drying. (g) 3D printed PMMA parts are post-processed by infiltrating with additional PEG polymer followed by controlled drying to mitigate shrinkage and cracking as excess water leaves the structure. layer films, using a mask-based projection apparatus to print PAN (polyacrylonitrile), here we achieve IPP 3D printing with PMMA (Figure 1b-d). PMMA is highly soluble in its monomer, which makes it challenging to print in single phase systems. PMMA is of particular interest due to its commercial uses including consumer products, dental devices, and optical components. 24 To achieve IPP of 3D objects, custom hardware is built to adapt a commercial DLP printer for use with the IPP process (Figure 1e and Methods). The printer projects, with 385 nm UV light, cross-sections of a conventionally modeled and sliced 3D object, causing corresponding polymer films to form at the liquid-liquid interface. As each layer forms, the buildplate descends away from the interface, allowing the reaction zone near the interface to reform and repeat the reaction for subsequent layers. Figure 1a diagrams this sequential multilayer process, where polymer entanglement between the subsequent layers allows procession of the sequence to make digitally-defined 3D objects.
A first key consideration for 3D printing by IPP is control of light propagation through the printing medium. As developed, IPP 3D printing employs a top-down photopolymerization approach, where light is projected from above and the build plate progresses down and away from the interface. This arrangement is necessitated by the presence of photoinitiators in the aqueous phase, which, owing to its higher density, is stratified below the organic phase. The photoinitiator species, being far more soluble in water than the organic monomer, remain in solution without significant partitioning into the organic phase, resulting in a reaction zone on the aqueous side of the interface. To achieve high resolution printing and avoid unintended polymerization reactions, projected light must only activate radical generation near the interface within a shallow absorption depth. A large absorption coefficient or significant scattering in the light path would reduce the light intensity initiating radical formation and hinder further polymer chain growth. Therefore, the patterned light must reach the liquid-liquid interface through the optically transparent organic phase rather than the photoinitiator-rich aqueous phase.
Further, scattering of projected light through the aqueous solution and the printed object results in UV irradiation beyond the intended exposure area. This is undesirable because stray UV light can still activate photoinitiators and create polymer, a process often referred to as "overcure" or the "back-side effect" 25 in 3D printing. For IPP 3D printing, this is accounted for by the addition of a nonreactive, water-soluble dye (tartrazine) with a high absorption coefficient at the working wavelength, which prevents light from spreading too far from the exposed area. A visualization of the effect of dye concentration is shown in Figure S2.
Contrasting IPP, conventional thermoset resins used in VP rely on the rapid polymerization and dense crosslinking of (meth)acrylate and epoxide monomers to achieve phase separation, altogether typically taking just a few seconds in most commercial printers. Instead of covalent crosslinks, formation of thermoplastic polymers in solution relies on polymer chain entanglement forming a weakly-interacting physical gel governed by van der Waals interactions. To effectively print cohesive 2D films or 3D structures through the IPP process, linear polymer chains must be allowed to grow above their critical molecular weight for entanglement before termination by precipitation. Without sufficient entanglement, the formed polymer can diffuse or dissolve away from the exposure area, degrading the quality of the print. Further, in order to form a dense, dimensionally stable structure, it is essential to maximize the yield of long polymer chains within each exposed layer volume.
To this end, dye concentration and exposure time have an interrelated effect on the polymer printed by IPP. Raising the concentration of light-absorbing dye both improves printing resolution by reducing excess light leakage away from the exposed area and reduced the rate at which solid polymer forms. This is a consequence of dye competing with photoinitiators for incident light and consequently reducing the rate of initiation upon exposure, which corresponds to a reduced rate of active radicals introduced into the system and thus fewer polymer chains in a given time frame. Hence, IPP benefits from longer exposure times in order to produce sufficient polymer and yield dense layers. Figure S3 demonstrates the effect of exposure time on the polymer yield and resulting dimensional stability of printed parts.
Addition of high molecular weight PMMA to the organic monomer phase prior to printing mitigates cracking upon drying. This dissolved polymer is incorporated into the printed structure during exposure and layer formation. We hypothesize that the additive acts as a binder across the polydisperse network of globules that make up the part. This method can be especially well utilized for IPP printing of PMMA because of the high solubility of the polymer in its own monomer. Addition of PMMA into the system not only ensures that the polymer chains are present at large enough quantities within the reaction zone to yield the desired results, but also opens opportunities for process circularity.
Yet, choosing an appropriate molecular weight for the additive polymer is key to maximize its effectiveness. For the purpose of acting as a binder, a high average molecular weight is desired since a larger polymer chain leads to a greater number of interactions. However, larger PMMA molecules are also also less soluble in the MMA-isooctane organic phase mixture. This lower concentration results in low availability during printing and insufficient additive polymer incorporated into the print. Small PMMA molecules are more easily dissolved in the organic phase mixture but perform comparatively worse as binders. This trade-off is summarized in Figure S4.
Another important consideration is limiting the increase in viscosity of the organic phase resulting from the polymer additive, as higher viscosity can have adverse effects on the kinetics of the printing reactions. In the present method, isooctane as an anti-solvent both reduces the solubility of polymer chains formed during printing and limits the solubility of PMMA in the mixture. This simplifies processing and preparation as the organic solution can be fully saturated with the added polymer while still maintaining a workable viscosity.
Similarly, PEG is dissolved in the aqueous solution to control viscosity and improve layer formation. Because PEG remains homogeneously mixed with the printed PMMA and does not exhibit large-scale phase separation, it acts as an effective binder for the polymer during and after printing. While higher molecular weights of PEG were found to be better in this regard, the concentration of added PEG has competing effects. Increased viscosity of the aqueous phase reduces the diffusivity of chains, causing newly formed polymer to precipitate more quickly and closer to the site of initiation, improving confinement and allowing for sharper, higher resolution printed features. However, excessive viscosity also slows renewal of monomer and photoinitiator at the interface, which can hinder the formation of dense, interconnecting layers throughout the printed structure. A 20 wt.% solution of PEG 10k is found to offer an optimal balance. Because polymerization in IPP proceeds at a submerged liquid-liquid interface, we expect oxygen inhibition to only occur due to dissolved oxygen present after first mixing the printing solutions, most of which is exhausted during the initial setup exposure. During printing, because the solutions are not stirred between exposures, oxygen presence in the reaction zone is limited by what can dissolve from the atmosphere into the organic phase and diffuse to the interface. In practice, this was not found to have a noticeable effect on printing performance.
As polymer chains precipitate and entangle in UV exposed areas (Figure 1d), the growing network inevitably traps some aqueous PEG solution within the printed structure. After printing, the resulting material must be dried to remove the excess water. During drying, evaporation of water causes the polymer globules that make up the part to densify while leaving behind the dissolved PEG to act as a binder. Drying transforms the initially soft, gel-like printed structure (Figure 1f) into a solid polymer composite.
Densification of the polymer during drying also leads to volumetric contraction of the printed shape. Shrinkage is inherent in AM processes for both thermoplastic and thermoset polymers 26 and can be tolerated as long as the deformations are manageable or predictable.
Repeatable, uniform shrinkage can be mitigated through calibration and adjustment of part dimensions. Alternatively, when drying and shrinkage occur non-uniformly, unequal stresses induced within the volume of the part can distort the intended geometry. In IPP, warping during drying is caused by faster drying near surfaces, and asymmetry of drying-induced stresses.
To mitigate shrinkage and cracking during the drying step, printed specimens are submerged in a concentrated PEG solution and heated to slightly above the glass transition temperature of the PMMA, as depicted in Figure 1g. This approach is inspired by the use of PEG infiltration for improving the dimensional stability of fresh or delicate wood. The hydrophilic nature of the molecule allows it to diffuse well throughout the swollen cellulose structure and replace the volume occupied by water. With sufficient loading, it has been shown to act as an effective bulking agent once dried to prevent shrinkage and warping of the overall piece. [27][28][29] Holding the printed specimens at an elevated temperature facilitates transport of PEG into the printed object. Prior to thermal treatment, because of layer formation in the aqueous phase, printed structures already contain some of the 20 wt. % PEG solution used for printing, distributed throughout their bulk. With this low loading of PEG, the comparatively large volume of the aqueous solution compared to the volume of the PEG left behind after drying still leads to large shrinkage as surrounding structures densify to fill the space. Over the course of the thermal cycle, water evaporates from the bath, raising the concentration of PEG surrounding the printed structure. This, in turn, raises the concentration of the aqueous PEG solution inside the printed object through diffusion, forming a composite as schematically shown in Figure 2a. After drying, the final printed parts are composed of a mixture of 74.9% ± 0.4% by weight PEG and 25.1% PMMA with a total bulk density of 0.78 ± 0.01 g/cm 3 . This process is believed to also encourage diffusion of PMMA across the boundaries of the globular structures formed while printing, adding strength to the overall part by slight coalescence of the PMMA globules (Figure 2a-b). After thermal treatment, unbalanced drying can be palliated by slowing the rate of evaporation of water from the part, allowing sufficient time for moisture to redistribute through the volume of the part and preventing large gradients in capillary drying stress.
Printed IPP samples are therefore dried in simple humidity-controlled containers over a period of one to three days depending on the volume of the part. Such a container is fashioned by placing a wet print in a small glass container, sealing the container with parafilm, and creating small perforations in the film to allow moisture to gradually dissipate. The relatively higher humidity maintained in the container retards drying and reduces shrinkage and cracking of printed parts, compared to open air drying.
Micrographs of cross-sections of printed PMMA specimens (Figure 2b-d) show the typical internal structures resulting from the IPP process. At the micron scale, the printed material appears foam-like, with PMMA comprising globular structures characteristic of polymer precipitation through nucleation, growth, and coarsening. As the material was initially filled with an aqueous PEG solution during printing, a network of empty pores is left behind, interspersed with remanent PEG, after the print is dried.
At the millimeter scale, cracks and voids are observed throughout printed specimens.
While the micron scale porosity displayed in Figure 2c is inherent to the polymerization and precipitation process, these larger defects are affected by process parameters. The series of X-ray micrographs in Figure 2d specifically focuses on the effect on these voids due to the relative concentration of the photoinitator V-50.
We hypothesize that voids with round, organic forms originate from cavities present in the as-printed structure, typically from bubbles or nonuniform deposition of polymer during printing. As the print undergoes thermal treatment and drying, the surrounding polymer densifies around the still-present cavity, giving rise to similarly shaped, albeit elongated or deformed void geometries. On the other hand, cracks are believed to develop during drying, initiated by mechanical stress due to shrinkage as the included water leaves the polymer network.
The sample printed with 0.8 wt.% V-50, shown in shown in the first column of Figure 2d contains many round voids and few straight cracks, indicating a large amount of bubbleinduced porosity and comparatively little shrinkage internally. In contrast, both the 0.4 wt. % and 0.6 wt. % samples show more elongated internal voids, with the 0.4 wt. % samples showing the largest shrinkage during densification. This is consistent with the lower rate of photopolymerization and consequently lower density of layers with lower photoinitiator concentrations. The comparatively lower density layers in the 0.4 wt. % sample contain a larger fraction of the aqueous solution to printed polymer and thus experience greater shrinkage as the included water leaves the volume while drying, resulting in more voids after drying.
To assess the chemical structure of the printed materials, FTIR spectra were measured for the washed and as-printed samples, and were compared to spectra collected for pure commercial PMMA (96 kdA) and PEG (10 kDa). Figure 3a shows the resulting spectra normalized, stacked, and cropped to the relevant area for qualitative comparison. The spectrum of the as-printed sample appears as a blend of the pure PMMA and pure PEG spectra, showing, among others, a C=O stretching peak at ∼1720 cm -1 (characteristic of PMMA) and a strong C-O stretching peak at ∼1090 cm -1 (characteristic of PEG). The spectrum of the waterwashed sample is nearly identical to that of pure PMMA, further indicating that the PEG is dissolved by washing the printed material in water. The latter is an important consideration for the purity and recyclability of the printed parts. The full spectra are provided in Figure S6 for reference, and the spectra of the pure samples are in good agreement with those found in literature. 30 Upon heating, PMMA undergoes three stages of degradation, 31 whereas thermal decom-position of pure PEG typically occurs through a single step. 30 In Figure 3b, the former behavior can be clearly observed in the TGA curve obtained from the washed samples whereas the as-printed (unwashed) sample resembles a blend of the two, with more of its mass being lost in the final stage between 350°C -425°C.
To further assess thermal transitions in the printed composite polymer, dynamic mechanical analysis (DMA) and differential scanning calorimetry (DSC) are performed. A low temperature DMA sweep shown in Figure 3c captures a transition at -3°C likely contributed by the PEG, which makes up a large portion of the final part by weight. As the temperature increases to around 80°C, the melting point of the contained PEG is reached and the structure is compromised. For DSC, with a washed sample, a glass transition temperature of the PMMA is identified at approximately 114°C (Figure S7).
Measuring a washed sample using size exclusion chromatography reveals a very broad distribution of molecular weight of printed PMMA (Figure 3d). This spread is likely due to changes in polymerization kinetics as reaction-zone conditions change over the course of forming each layer. When exposed to UV light, much of the monomer that has partitioned into the aqueous side of the interface is quickly consumed to produce relatively long polymer chains. However, because diffusion into this area is limited, there is a rapid drop in monomer concentration, producing shorter chains as reaction rate decreases and the termination rate increases relative to the propagation rate.
The foam-like internal morphology of the IPP-printed structures governs their mechanical behavior. Under quasi-static compression, the stress-strain response Figure 4a appears to comprise four distinct regimes: a concave upward portion at low strain, a linear portion, a plateau, and another concave upward portion with a sharp rise. We hypothesize that the first concave section, up to around 0.8 MPa, corresponds to flattening of asperities on the surface of the model and closing of cracks. The subsequent portion is a period of linear elastic deformation where the structure is compressed without further densification.
Within this region, the material exhibits an average elastic modulus of 16 MPa. As load increases beyond 1.5 MPa, walls around the small micron-scale pores distributed throughout the structure begin to plastically deform and close, leading to a plateau region of increasing strain with nearly constant stress. Beyond about 2.0 MPa, the elastic modulus once again increases as material around these cavities collapses, leading to the final regime of additional densification. Following the initial regime, the response resembles well-known compression of polymeric foams, or cellular materials in general. 32,33 The influence of the porous morphology on the mechanical behavior of PEG/PMMA composite is further emphasized when compared to pure cast PEG 10k, which exhibits a stress-strain curve typical of a soft homogeneous thermoplastic material. Despite comprising approximately 75% PEG by weight, the mechanical properties of printed material are distinct from that of bulk PEG, and are governed by the porous, foam-like structure.
Further, strain-controlled cyclic loading (Figure 4b) at roughly 0.6% strain reveals a cyclic softening behavior as the stress amplitude drops with every cycle. No change in elastic modulus or hysteresis loss is apparent with respect to cycle number.
A representative 3D model fabricated using the 3D IPP process is shown in Figure 5a, highlighting the the capacity to resolve fine features with high fidelity. To assess practical resolution and geometry capabilities, a set of test specimens were designed and printed. The models shown in Figure 5b-g, begin with a uniform base structure (1.5 mm height) which ensures good adhesion to the buildplate, and end in 1 mm tall raised structures of varying lateral dimensions. All models were printed in PMMA and post processed with PEG thermal treatment and controlled air drying. In terms of XY resolution, towers as narrow as 0.4 mm in width can be seen standing fully formed in Figure 5b immediately after printing. Smaller widths are formed yet collapse, and in general, features less than 0.3 mm width do not appear to print properly. In Figure 5c, we see that step heights as small as 0.15 mm are resolved in the printed models, though none of the smaller (0.05 mm height) steps can be clearly discerned. All features that print successfully are maintained through thermal treatment and drying (Figure 5d-e) without noticeable degradation.
The 3D scan comparisons in Figure 5f-g reveal shrinkage in length, width and height of the parts. Further, due to constraint by the adhered buildplate, a taper is present on vertical faces of the part where areas farther from the base of the model are more deformed than those close to the base. While overall lateral shrinkage is limited, the top surface of the base structure in Figure 5g exhibits deviance of 0.2 to 0.4 mm, corresponding to approximately 13% to 26% shrinkage in the vertical direction.
Fully post-processed IPP materials as presented herein are composites of linear thermoplastic PMMA and PEG with trace amounts of dye and other additives used in the printing solutions, which are water soluble. Therefore, in addition to possible conventional recycling, there exists with this method an opportunity for low energy circular AM processing, which can reduce material waste and increase overall efficiency. Typically, the organic solution for IPP is saturated with dissolved commercial PMMA, which is incorporated into printed structures and acts as a binder. IPP printed parts can be easily be used as the source of PMMA for this step, allowing them to be processed into feedstock and reused in the same printing workflow. Printed structures can be washed with water to remove included PEG before simply dissolving back into MMA monomer in preparation of the organic phase, as summarized in Figure 6a. This workflow, for example, allows for the reuse of end-of-life PMMA parts, or recycling of support structures removed during manufacturing. It is also compatible with IPP printing as a means of reusing PMMA processed conventionally.
To validate the circularity of IPP 3D printing, five consecutive recycle-print cycles were conducted, in which water-washed printed polymer was dissolved in MMA and used to prepare the organic phase for printing in the next cycle. The initial sample (Recycle 0x) was produced using only commercial 96kDa PMMA dissolved in the organic solution. Subsequent prints (Recycle 1x through 5x) were conducted with only the polymer recovered from the previous cycle and no additional commercial PMMA. As shown in in Figure 6b, FTIR scans of polymer produced over five consecutive process cycles demonstrate good overlap, indicating no chemical degradation or changes resulting from reuse. Further, GPC measurements of molecular weight distribution after each cycle (Figure 6c), show a general increase in dispersity after each round and the appearance of higher molecular weight PMMA in the printed object, suggesting incorporation and potential additional growth of chains polymerized in the previous printing cycle as well as propagation of new PMMA chains. Dynamic mechanical testing shows no significant changes in storage and loss moduli of the printed specimens over the five cycles (Figure S8). Collectively, these results demonstrate that recycled PMMA is carried forward and incorporated into newly printed parts without measurable degradation. In practice, the observed shift in molecular weight suggests it would be effective to maintain a target ratio of used to virgin PMMA in IPP, as is common practice in reuse of thermoplastic powders in laser-based AM. 1,34
The objective of this study was to demonstrate interfacial photopolymerization (IPP) as a viable route to 3D print recyclable linear thermoplastics, using PMMA as a model system.
While the compatibility of the IPP process with commercial DLP hardware highlights the potential for additional development of the technology, further exploration of compatible polymer chemistries is necessary to enhance the speed, precision, and size of objects that can be produced. Challenges remain, including stability of the printing solutions, print size, part density, and mechanical performance.
At present, IPP printing of PMMA is best suited to applications where complex geometry and low, controlled density are primary design variables. The ability to recycle the PMMA further supports these use cases by reducing material waste and enabling potential routes to tailor material characteristics. Translation to broader, more demanding applications, like those where bulk PMMA is typically used, will require densification and targeted optimization of attributes like composition and porosity.
In addition to their intrinsic porous structure, PMMA parts currently produced via IPP contain voids caused by bubbles from the photoinitiator, along with shrinkage-induced cavities. These voids adversely affect the stiffness and strength of the printed material and create stress concentration areas where fracture can originate. Greater polymer yield per layer would improve mechanical properties and could be achieved with longer layer exposure times, with the trade off of longer overall printing times and smaller part size due to limited solution pot life. Alternatively, improvements in reaction kinetics, for example through the use of a more efficient photoinitiator, could speed up polymer formation and improve bulk density without excessively increasing print time. Vacuum infiltration postprinting offers another potential route to increase the density of parts and fill remaining voids.
Potential materials are a low viscosity thermoplastic, such as molten low molecular weight PEG, or another thermoplastic monomer that is capable of entering the porous network and polymerizing in-situ.
Removal of included PEG by dissolution in water severely impacts the mechanical properties of the printed structure, in some cases resulting in complete disintegration. Water incursion is detrimental to finished printed parts, rendering them unsuitable for many practical applications of a thermoplastic polymer. To address this issue it may be necessary to increase the polymer yield during printing to incorporate a higher fraction of PMMA within the part, utilizing polymer dissolution in the monomer to improve cohesion between individual layers, and/or explore different thermal post-processing methods to achieve a more dense and homogeneous polymer solid.
Further, tensile tests were not feasible due to specimen fragility and adhesion to a paper build plate. As density improves via higher polymer yield, improved kinetics, or post-print infiltration, we expect the behavior of the printed material will more closely resemble bulk polymers and standard tensile and fracture tests will become practical.
Beyond material properties, the physical configuration of the IPP setup also imposes constraints. As detailed earlier, the different densities of the printing solutions determine their order in the printing cuvette and the resulting optical pathway between the organic monomer phase and aqueous photoinitiator phase, which necessitates top-down exposure. This configuration, however, sets a finite build height determined by limited stage travel between the liquid-liquid interface and the bottom of the printing vessel. A bottom-up implementation, which is more typical in small resin printers, could alleviate the height constraint, but would require reconfiguring the two-phase system so that the optically transparent phase resides beneath the interface and the strongly absorbing or reactive phase above, while preserving immiscibility and interfacial kinetics. Possible routes include increasing the density of the organic phase, for example by selecting a denser monomer or adding an non-reactive densifier so that it resides below the aqueous layer, or alternatively relocating photoinitiators and dye to an upper phase composed of an even lower density solvent while ensuring the lower phase remains transparent at the working wavelength. While the current geometry sets a finite build height determined by the vessel and stage travel, in practice the dominant constraint on printable size is the short solution pot life.
Additionally, due to the use of a thermally activated azo photoinitiator, compositions remain viable for only a few hours at room temperature after mixing. While premixed solutions can be stored at lower temperatures to counteract this issue, thermally activated polymerization leads to excess polymer formation during the printing process, reducing the working time of the solutions and limiting the size of printable objects. This presents a challenge in scaling the process. Substituting the azo initiator with a more stable, less temperaturesensitive alternative will prolong the working lifespan of the printing solutions, improve the consistency of printed parts, and potentially increase the speed of the printing process by enabling higher reaction rates without concern for porosity in the polymer structure.
Owing to the relatively low viscosities of the two liquid phases and high interfacial tension between them, the interface was observed to remain stable and self-level very quickly after each re-coating move. For printing, a brief 3 s delay is applied before every exposure to allow the liquids to settle. This delay is functionally equivalent to the "light-off delay" widely used in vat photopolymerization which allows resin to squeeze from the interface and come to rest before the subsequent exposure. This short settling time suggests that IPP does not impose longer stabilization delays.
Last, embedded direct ink writing (DIW) of MMA in PEG-based support medium is a potential alternative approach to shaping linear thermoplastics. However, DIW relies on nozzle-delivered inks with tailored rheology and typically requires post-curing steps to achieve final properties. In contrast, IPP forms features optically at a uniform liquid-liquid interface, decoupling lateral resolution from nozzle diameter and leveraging full, layer-wise exposure for higher throughput across the cross section and intrinsically planar layers. A direct comparison of the two methods can be evaluated in future work.
In conclusion, interfacial photopolymerization (IPP) is demonstrated to achieve 3D printing of recyclable thermoplastic PMMA, in complex geometries with high resolution enabled by digital light processing. In contrast to chemical upcycling and depolymerization methods of reprocessing AM thermosets, the dissolution recycling of IPP-PMMA offers a simple path towards circular manufacturing, allowing for reuse of printed parts directly as feedstock to improve the material and energy efficiency of production. Binders, dyes, and other additives chosen with a focus on low toxicity and cost, are incorporated to improve the resolution, yield, and molecular weight distribution of polymer formed. Further advancements are made in post-processing techniques to enhance printed part strength, reduce shrinkage, and minimize variability caused by drying-induced damage. However, the printed materials are highly porous and have foam-like mechanical properties, and, it is yet to be determined if IPP can effectively form fully dense polymers, or if particular applications of foams and foam-like polymer components should be sought. Upon addressing these roadblocks, it will also be important to assess the costs of IPP AM technology at scale and evaluate its sustainability through analysis of the end-to-end material flows and their associated environmental considerations.
Solutions for the organic and aqueous phases are prepared separately, using the chemicals listed in the Supporting Information. To prepare the organic phase, in a 20 ml glass vial roughly 3 g of commercial 97 kDa PMMA powder is combined with 10 ml inhibitor-free methyl methacrylate. The vial and its contents are heated in a water bath at 50C for 1 to 3 h, or left to sit overnight at room temperature, until the solid has completely dissolved. Isooctane (10 ml) is added to this solution and vortex mixed for 30 seconds, during which time excess PMMA precipitates and clumps together. The saturated MMA-isooctane solution is set aside to rest overnight until the precipitated polymer settles to the bottom of the vial.
To prepare the aqueous phase, 200 mg of anhydrous calcium chloride (CaCl 2 ) is dissolved in a vial with 5 ml of 20 wt% PEG solution and set aside to cool. To another 20 ml vial is added 120 mg of V-50 photoinitiator, 50 mg of LAP photoinitiator, and 20 mg tartrazine dye along with 15 ml of 20 wt% PEG solution. The contents are vortex mixed for at least one minute to dissolve. If large pieces of solute remain, the vial is placed in a room temperature sonication bath for 15 seconds, or until no undissolved granules are visible. The solution is topped with 0.8 mL of 0.2 M hydrochloric acid (HCL) and the CaCl 2 solution prepared earlier, followed by vortex mixing until the solution is fully clear.
After preparation of the organic and aqueous phases, the aqueous phase, containing the photoinitiator, is first poured in the printing cuvette followed by the organic (monomer) solution. The aqueous and organic phases are gently stirred to facilitate mixing at the interface. The cuvette is capped with a UV transparent glass sheet and placed in a static projection setup, described in Chazot et al, to perform a setup exposure. 23 A UV LED (8 W, 365 nm) is directed through a mask and afocal projection optics to expose the contents of the cuvette with a small, roughly 16 cm 2 projected pattern with a intensity of around 20 mW cm -2 at the liquid-liquid interface. The exposure is held for 15 minutes or until visible polymer begins to form. This step increases the concentration of polymer chains to their saturation limit in the aqueous phase and allows time for the phases to separate and restabilize following mixing. This exposure step also serves to consume dissolved oxygen in both phases, reducing inhibition prior to printing. A relatively small projected pattern is used in this step to avoid excess activation of the photoinintiator. The solutions are then ready for printing using the procedure described below.
IPP printing is performed using a modified DLP printer (Envisiontec D4K) with a native projector resolution of 50 µm. The printer was inverted from its nominal orientation such that the light is projected from the top, and therefore travels through the organic phase first).
Projection power is adjusted to yield approximately 8 mW cm -2 at the exposure plane. A machined aluminum fixture plate replaces the PEI film in the printer build tray, to which a motorized z-stage, separate from the D4K's own, is mounted to advance the z-position of the IPP buildplate.
The build plate for IPP, upon which the printed polymer forms, is made of black filter paper (Thomas Scientific 4740C10), which, in addition to preventing stray light reflections and transmissions, provides an easily replaceable, textured surface onto which the polymer can securely attach. The filter paper is secured with No. 3 paper clips to a rigid carrier plate that is actuated by the independent motion stage.
With the build tray and fixture plate installed in the printer, a prepared cuvette filled with the two liquid phases can be inserted from the bottom through an opening in the center of the fixture plate. The cuvette is supported by a cradle that places the liquidliquid interface coplanar with the top surface of the fixture plate and the focal plane of the projector. More details about the build plate and printing adapter are provided in the Supporting Information (SI).
A 3D model of the object to be printed is then created in Autodesk Fusion 360 or similar CAD software, loaded into Envision One RP software where it sliced according to the set voxel height, and sent to the printer along with the exposure time settings for printing.
Before commencing the print, the stage is raised until the top surface of the buildplate meets the liquid-liquid interface. As each layer is exposed, a synchronized controller drives the buildplate down using the auxiliary motor.
Layers are advanced in two steps. After exposure, the buildplate first descends 10 mm from the interface to unpin the new layer and allow for recoating. The buildplate then returns to the interface stopping one layer height below the previous position. Following each descend-and-recoat motion, a 3 s delay is applied prior to illumination to allow the interface to settle.
Spectroscopy is performed with an OceanInsight Flame sensor and data acquisition is conducted through the accompanying OceanView software. The sample to evaluate is added to a quartz cuvette with 1 cm path length and placed into the holder (OceanInsight Square One). One side of the holder is coupled to the Flame sensor by a fiber optic cable and the opposing side is coupled to an OceanInsight DH-2000-BAL deuterium-halogen light source.
Micrographs are obtained with a Zeiss Gemini 450 scanning electron microscope (SEM) operating with 1 kV acceleration voltage and 100 pA probe current. Samples are prepared by freeze fracturing to expose the internal cross section, whereby printed polymer structures (along with the attached paper buildplate) are first submerged in liquid nitrogen and then split with pressure from a razor blade. The samples are mounted on 90°studs with the freshly cleaved surfaces facing up, using carbon tape and silver paste to create a conductive path from the polymer to the metal surface of the stud. After the silver paste is allowed to dry, the studs are then placed in a Pelco SC-7 Sputter Coater to receive a ∼5-10 nm coating of gold before mounting within the SEM.
3D scans of printed structures after post processing and drying are taken to analyze deviation in geometry resulting from the process. A 3D point cloud is gathered using a Keyence VK Laser Scanning Confocal Microscope. Scan data is compared against nominal CAD geometry using Zeiss Inspect 3D software.
FTIR spectroscopy is performed on the surface of printed samples using a Bruker ALPHA II equipped with a diamond ATR crystal. Bruker OPUS software is used to acquire data from the instrument and perform baseline correction and background subtraction.
TGA is performed on a TA Instruments Discovery TGA 5500 with an IR furnace. Samples are heated in a platinum (Pt) pan under nitrogen to 600 °C at a rate of 10°C/min and standard nitrogen flow rate.
DSC is performed on a TA Instruments DSC 2500. After printing, samples for DSC undergo thermal treatment and air drying followed by thorough washing with fresh HPLC-grade water to remove included PEG. The remaining polymer is allowed to dry fully before being loaded into the instrument, where it is cycled three times under nitrogen between 30°C and 160°C at a heating rate of 10°C/min.
Printed samples are analyzed by gel permeation chromatography (GPC) to determine the molecular weight of the formed polymers. First, samples are processed by briefly rinsing with isopropyl alcohol (IPA) before washing with water to remove included PEG contained in the composite solid. The remaining solid, comprised mostly of printed PMMA, is dried and dissolved in tetrahydrofuran (THF) to create a 1 mg/ml solution. Molecule size distributions are obtained on an Agilent 1260 Infinity GPC system equipped with a variable wavelength diode array (254, 450, and 530 nm) and refractive index detector and calibrated with narrowdispersity polystyrene standards between 1.7 and 3150 kg/mol.
X-ray micro CT is performed to examine the internal morphology of the printed polymer.
Rectangular structures are printed and post-processed with thermal treatment followed by air drying. Samples are then loaded into a ZEISS Xradia Versa 620. Image slices for each sample are reconstructed into 3D volumes and subsequently cropped, normalized, and segmented to select large-scale voids using Dragonfly software.
The viscoelastic properties of printed structures with respect to frequency and temperature are measured by DMA as per ASTM D5024. 35 For all mechanical tests, samples are prepared by printing 4x4x2 mm prisms (cross-sectional area of 16 mm 2 ) and post-processing with thermal PEG treatment followed by controlled air drying. The printed samples, with paper build plates still attached, are placed individually into a TA Instruments DMA Q850 equipped with compression clamps and a gas cooling accessory (GCA).
For frequency controlled testing, samples are pre-loaded with a compressive load of 2 N and then a cyclic 1 N load is applied. The oscillation frequency is stepped from 1 Hz to 200 Hz in 1 Hz increments and the storage modulus, loss modulus, and tan(delta) are calculated at each point.
For temperature controlled testing, samples are preloaded with a compressive load of 2 N and then a cyclic 0.5 N load is applied with a constant 1 Hz frequency. Temperature is ramped from -100°C to +100°C at a rate of 3°C/min and the storage modulus, loss modulus, and tan(delta) are calculated every 3 seconds.
The stress-strain behavior of the printed PEG-PMMA structures are further analyzed through quasi-static loading. Pure PEG samples are prepared for comparison by melting PEG 10k (identical to what is used in printing and thermal treatment) at 75°C and casting into cylinders with diameter 4.7 mm (cross sectional area of 17.3 mm 2 ). Once re-solidified, the PEG cylinder is cut with a razor to approximately 2 mm tall sections.
Stress strain curves are obtained for all samples under quasi-static compression on a Zwick T1-FR010TH material testing machine. Samples are loaded with a ramp from 0.1 N to 200 N at a rate of 0.5 N/s. The obtained force and displacement values for each sample type are averaged and the corresponding average stress and strain values are plotted as curves. For cyclic testing, samples are preloaded to 2 N and cycled between 0 and 0.01 mm compression travel at a rate of 2 N/s for 20 cycles.
This work introduces a 3D printing method using interfacial photopolymerization (IPP) to fabricate recyclable thermoplastic polymethyl methacrylate (PMMA) parts. Low toxicity additives and tailored post-processing techniques improve part integrity while enabling circular manufacturing through reuse of printed materials. Compatible with existing digital light processing (DLP) hardware, IPP offers a sustainable alternative to conventional, waste-intensive vat polymerization techniques.