3D Printing of Dual‐Cure Networks Based on (Meth)acrylate/Bispropargyl Ether Building Blocks

In recent years, dual‐cure chemistry has been exploited to realize interpenetrating networks (IPNs) that provide enhanced thermo‐mechanical properties. In this contribution, photoinduced curing of (meth)acrylates is used to build the desired 3D structure, whereas the thermally triggered polymerization reaction of 2H‐chromene functionalized building blocks is utilized to create the IPN. This strategy combines the advantages of traditional UV‐curable monomers with high‐performance thermosets. After the successful synthesis of the bispropargyl ether derivative, i.e., 4,4′‐(propane‐2,2‐diyl)bis((ethynyloxy)benzene), its thermally induced conversion to the corresponding 2H chromene functionalized prepolymer is studied by Fourier‐transform infrared spectroscopy and gel permeation chromatography. The network formation as well as the printability of various formulations containing different amounts of the thermo‐curable building block is investigated. The obtained IPNs provide enhanced thermo‐mechanical properties making these resins suitable for the additive manufacturing of functional 3D parts for high‐performance applications.


Introduction
Additive manufacturing technologies (AMTs) based on lightsensitive resins, e.g., stereolithography (SLA), digital light processing (DLP), liquid crystal display (LCD) 3D printing or 3D inkjet printing, allow the fast and very precise production of complex 3D parts with high surface quality. [1] State-of-the-art resins are mainly based on acrylate and methacrylate building blocks which are well established in the decorative and protecting coating industry. [2] Although they show fast curing rates and tuneable mechanical properties, cured materials offer limitations regarding toughness and heat deflection. [3] In addition to (meth)acrylates, UV-curable resins based on the cationic curing of epoxides are also used commercially. In general, radical systems react much faster than epoxides, which also makes them more attractive in terms of productivity. [4][5][6] Most of these AMTs process the resins in a layer-by-layer fashion, resulting in a nonuniform degree of cure in the printing direction and thus anisotropy of the thermomechanical properties. [7] To overcome these limitations UV postcuring of the printed parts is carried out. In this context, it must be mentioned that the UV light may not be able to penetrate into the deepest layers of the material, resulting in areas of incomplete curing and nonuniform properties. [8] In addition, it should be noted that an excessive degree of postcuring also has negative effects, e.g., embrittlement of photopolymers. [9] Variations in thermo-mechanical properties can be avoided by continuous SLA using an oxygen permeable membrane to generate an inhibition zone between the printed object and the vat. [10] However, this prevents the use of other UV-curing systems, e.g., those based on the cationic ring opening of epoxies [6] or on the thiol-ene/yne photo-click reaction. [11,12] These limitations described previously necessitate the exploration of alternative polymerization reactions or curing strategies. One approach that has been pursued in recent years is the combination of different reactions in a so-called dual-cure procedure to generate interpenetrating polymer networks (IPNs). [4,13] The formation of different polymer networks can be a complex phenomenon involving phase separation resulting in different structural and morphological features and properties depending on the polymerization sequence, composition, and polymer compatibility, among others. For this reason, IPNs are claimed to lead to interesting and/or superior properties compared to the individual polymer components. [7] In the dual cure strategy, the initial part geometry is often built by the photopolymerization reaction of (meth)acrylates, whereas in a second step, thermal polymerization of a separate functionality, e.g., cationic polymerization of epoxies, is initiated DOI: 10.1002/adem.202200901 In recent years, dual-cure chemistry has been exploited to realize interpenetrating networks (IPNs) that provide enhanced thermo-mechanical properties. In this contribution, photoinduced curing of (meth)acrylates is used to build the desired 3D structure, whereas the thermally triggered polymerization reaction of 2H-chromene functionalized building blocks is utilized to create the IPN. This strategy combines the advantages of traditional UV-curable monomers with highperformance thermosets. After the successful synthesis of the bispropargyl ether derivative, i.e., 4,4 0 -(propane-2,2-diyl)bis((ethynyloxy)benzene), its thermally induced conversion to the corresponding 2H chromene functionalized prepolymer is studied by Fourier-transform infrared spectroscopy and gel permeation chromatography. The network formation as well as the printability of various formulations containing different amounts of the thermo-curable building block is investigated. The obtained IPNs provide enhanced thermo-mechanical properties making these resins suitable for the additive manufacturing of functional 3D parts for high-performance applications.
to form an IPN. This two-step strategy enables a new class of photopolymers for lithography-based AMTs that can combine conventional UV-reactive monomers with high-performance thermally reactive monomers. Although dual cure strategies offer unique advantages for 3D printing, the majority of research efforts have focused on acrylic and epoxy systems. [8] The aim of this contribution is to evaluate a propargyl ether-terminated derivative of bisphenol A as a thermo-curable building block in dual-cure formulation with the aim to increase the heat resistivity of the formed photopolymers. The main advantage of this class of thermosets is its straightforward synthesis from comparably inexpensive starting compounds. [14][15][16] Moreover, the obtained networks provide good thermal stability and properties comparable to those of epoxides. [16,17] The curing of propargyl ether resins occurs by Claisen rearrangement, followed by addition polymerization of the resulting chromene groups. [18] In this work, a dipropargyl ether derivative of bisphenol A, i.e., 4,4 0 -(propane-2,2-diyl)bis((ethynyloxy)benzene) (PDEB), was synthesized and pre-polymerized by annealing for different periods of time. The obtained reaction mixture was characterized and added to a (meth)acrylic resin system. The two-step curing behavior was studied in detail and the thermo-mechanical properties of the obtained photopolymers were analyzed by dynamic mechanical analysis (DMA). Importantly, the 3D printability of such resins could be successfully shown by means of LCD printing. This study reveals the huge potential of this dual-cure system for the fabrication of 3D parts with enhanced thermo-mechanical properties.

Synthesis and Investigation of the Conversion of PDEB
PDEB was obtained via a one-step synthesis with a yield of 95% as described elsewhere. [14] It is well reported that a heat treatment of PDEB (T % 170-190°C; without catalyst) leads to a rearrangement reaction of the alkyne ether groups resulting in the formation of 2H-chromene moieties. [16] These heterocyclic moieties undergo a thermally induced polymerization (T > 200°C) reaction as shown in Scheme 1.
In the first step, the rearrangement reaction was investigated by FTIR spectroscopy following the conversion of the alkyne peak at 3300 cm À1 to chromene units, which can be detected at 1640 cm À1 (C═C stretching vibration). The formed peak at 1750 cm À1 is attributed to carbonyl-containing byproducts, which are generated by an oxidation reaction under ambient conditions at 170°C. Figure 1 displays the FTIR spectrum of PDEB prior to and after thermal treatment (170°C) for 5, 10, 15, and 18 h, respectively. After 18 h, 32% of the alkyne groups have been converted. It is well reported that the formed chromene units already undergo oligomerization and polymerization reactions, respectively, under these conditions. [14] The obtained Scheme 1. Overview of the thermally induced rearrangement reaction of PDEB to 2H-chromene-containing prepolymer and its polymerization reaction.
www.advancedsciencenews.com www.aem-journal.com prepolymer provides excellent shelf-life <100°C and can be crosslinked with an extraordinarily low shrinkage of below 1%. This behavior is of great importance for the intended application in dual cure resins. It is known that shrinkage during network formation leads to stresses which are responsible for the brittleness of (photo)polymers. [3] The prepolymerization reaction of PDEB can also be followed by GPC measurements, as shown from the molecular weight distribution curves in SI. The average molecular weight of the prepolymer increases and its distribution broadens with the annealing time from M n = 970 kg mol À1 (5 h; PDI = 1.52) to M n = 1810 kg mol À1 (18 h; PDI = 7.39). Figure 1 displays the observed change in the chemical composition of the reaction mixture during annealing. After 18 h, about 74% of the PDEB are oligomerized and polymerized, respectively.
For the realization of the dual cure concept, the formed prepolymer has to be well-soluble in (meth)acrylate monomers.
In this context, it was found that an annealing time up to 18 h at 170°C leads to non-crosslinked molecules which are sufficiently soluble in organic solvents and monomers, respectively. Above this period, crosslinking is observed, resulting in an insoluble thermoset.
To investigate the kinetics of the crosslinking reaction, the prepolymer-18 h (18 h of annealing at 170°C) was cured at different temperatures. Figure 2 shows the FTIR spectra of the prepolymer prior to and after thermal treatment at 208°C. A further decrease in the alkyne signal at 3300 cm À1 until complete conversion (13 h) is observed. Although changes in the C═C region of the chromene moiety can be observed, an interpretation of this signal is difficult due to the simultaneous occurrence of the formation and polymerization reaction. An increase in the temperature leads to a significant acceleration of the alkyne conversion reaction (see Figure 2, left). www.advancedsciencenews.com www.aem-journal.com

Investigation of the Network Formation by Photorheology
In the present dual cure strategy (see Figure 3), chromene functionalized prepolymers (obtained by the annealing of PDEB) are added to a (meth)acrylate-based resin forming an IPN in the photopolymer after thermal crosslinking. The photopolymerization of the (meth)acrylates will be performed in the LCD printer enabling the buildup of the desired 3D structure.
In this context, a resin-based on 70 wt% diurethane dimethacrylate (UDMA) and 30 wt% ACMO as reactive diluent was chosen. UDMA is widely used in the dental industry because it offers a good compromise between toughness and heat resistance in the cured state. [19] This can be attributed to the formation of physical cross-links in the polymer network through hydrogen bonds. ACMO lowers the viscosity of the formulation and significantly increases the solubility of the prepolymers in the reactive system. As photoinitiator, a mixture of ethyl(2,4,6-trimethylbenzoyl) phenylphosphinate (TPO-L, 0.5 wt%) and phenylbis(2,4,6trimethylbenzoyl)phosphine oxide (Omnirad 819, 1.0 wt%) has been used, since those phosphine oxide-based PIs provide a sufficiently high light absorption at the printing wavelength.
For the 3D printing process, the gelation time of the applied reactive system is crucial, as it determines both the printing speed and the thermo-mechanical properties of the photopolymers. One possibility to study the network formation (i.e., gel point) of photoreactive resins is provided by photorheology. [20] From this measurement the change in the storage (G 0 ) and loss modulus (G 00 ) with increasing light dose can be followed. The intersection of G 0 and G 00 is defined as the gel point and resembles the transition from liquid resin to solid polymer.
Using this technique, the influence of the molecular weight (i.e., annealing time of PDEB) of the prepolymer and its concentration in the resin, respectively, on the gel point of the photoreactive system was investigated. As shown in Figure 4 (left) the gelation time of the resins increases with the prepolymer content. Although this effect is not that pronounced for prepolymer-5 h and prepolymer-10 h, a significant increase in the gel time is observed for the prepolymer with higher molecular weight (i.e., prepolymer-15 h and prepolymer-18 h). In particular, the addition of the prepolymer-18 h increases the gel time from 14 s (0%) to 82 s (20 wt%) and 361 s (30 wt%), which can be explained by the UV-Vis absorption of the formed chromene units. In particular, the prepolymer-15 h and prepolymer-18 h contain a high concentration of chromene moieties leading to a significant increase in the UV-Vis absorption between 250 and 450 nm as revealed from Figure 4 (right). These groups act as an internal filter that reduces the penetration of light and thus slows down the rate of network formation.
Besides the network formation, a sufficiently low viscosity of the resin is crucial for the LCD 3D printing process, as it ensures that the resin moves quickly to the print area between the depositions of each layer. For LCD 3D printable resins, the viscosity is typically in the range between 250 and 5000 mPas. [21] It is important to note that the viscosity increases from 670 mPas (0 wt%) to 1000 mPas (10 wt%) and 2480 mPas (20 wt%) when the prepolymer-18 h is added, which is still within a printable range.

Investigation of the Thermo-Mechanical Properties
The glass transition temperature (T g ) of photopolymers is a crucial property as it affects the mechanical properties (due to flexibility of the polymer chains) and thus the field of application of 3D-printed parts. Besides the network density of photopolymers also the type of the monomer backbone exerts significant influence on the modulus and T g . [22] Although flexible spacers such as alkyl or oligo glycol chains are known to result in rather flexible and soft materials with low glass transition temperatures, molecules with rigid ring systems, e.g., aromatic systems, are www.advancedsciencenews.com www.aem-journal.com anticipated to be responsible for good mechanical properties at even higher temperatures. [12] Fully cured pristine PDEB networks are reported to provide a T g > 300°C depending on the (post-)curing conditions. [14] In the herein-investigated resin system, polymerization of the added chromene functionalized prepolymer leads to the formation of a rigid IPN, which exerts a noticeable influence on the T g of the obtained photopolymer. Figure 5 (left) shows the temperature profile of the loss factor of the photopolymerized UDMA/ ACMO-based resin containing different amounts of the prepolymer-18 h after thermal curing at 220°C for 4 h.
Although the UDMA/ACMO-based photopolymer provides a T g of 157 AE 0°C, the addition of 5%, 10%, and 20% of prepolymer-18 h leads to a shift of T g to 158 AE 0, 161 AE 1, and 172 AE 1°C, respectively. The observed shoulder of the peaks, especially at high concentrations, indicates that two independent networks are formed. However, the behavior of the storage modulus (see Figure 5, right) is hardly influenced by changing the content of the oligomeric/polymeric material.

3D Printing of the Dual-Cure Resin Formulation
Since the resin with 20 wt% of prepolymer-18 h showed the highest increase in the glass transition temperature and besides that still reasonable gelation time (82 s), this system was chosen for the evaluation of its 3D printability by an LCD printer.
The resolution of the LCD 3D printing process was evaluated by printing a comb-like structure with holes of gradually decreasing diameter ( Figure 6)  www.advancedsciencenews.com www.aem-journal.com resolution of 300 μm could be obtained, which is high enough to print complex 3D parts.

Conclusion
In this contribution, a photoreactive dual-cure resin system based on (meth)acrylates and bispropargylether building blocks has been explored for LCD-based additive manufacturing.
In the first step, a bispropargylether derivative of bisphenol A (PDEB) was synthesized (yield: 95%) and converted to a 2H chromene-based prepolymer by a thermal treatment at 170°C for different periods of time. This thermally induced reaction was followed by FTIR spectroscopy, which showed a decrease in alkyne signal and the formation of chromene moieties with rising annealing time.
In addition, GPC measurements revealed that the average molecular weight of the formed molecules increased, and its distribution broadened with the annealing time from M n = 970 kg mol À1 (5 h; PDI = 1.52) to M n = 1810 kg mol À1 (18 h; PDI = 7.39), which can be explained by the partial crosslinking of the chromene units.
The obtained oligomeric/polymeric mixtures showed a sufficiently high solubility in the used (meth)acrylate-based building blocks, i.e., ACMO and UDMA. The network formation of such mixtures was studied by photorheology, showing a strong dependence of the gelation time on the molecular weight of the added prepolymer and its concentration in the formulation, respectively. In particular, the addition of the prepolymer-18 h increases the gel time from 14 s (0%) to 82 s (20 wt%) and 361 s (30 wt%), which can be explained by the UV-Vis absorption of the formed chromene units. These groups act as an internal filter which reduces the penetration of light and thus slows down the rate of network formation.
In the investigated resin system, polymerization of the added chromene functionalized building blocks leads to the formation of a rigid IPN, which exerts a noticeable influence on the T g of the obtained photopolymer. Although the UDMA/ACMO-based photopolymer provides a T g of 157°C, the addition of 5%, 10%, and 20% of prepolymer-18 h leads to a shift of T g to 158, 161, and 172°C, respectively. The observed shoulder of the peaks, especially at high concentrations, indicates that two independent networks are formed.
Most importantly, the 3D printability of such resins could be successfully shown by means of LCD printing. This study reveals the huge potential of this dual-cure system for the fabrication of 3D parts with enhanced thermo-mechanical properties.
Gel Permeation Chromatography (GPC): The change of the molecular weight and the molecular weight distribution during prepolymer formation was determined by GPC measurements. Size exclusion chromatography was performed on a system provided by Shimadzu [equipped with two separating columns from MZ-Gel SD plus, 500 and 100 A, linear 5 μ; UV detector (SPD-20A) and RI detector (RID-20A)] using tetrahydrofuran (THF) as eluent. Poly(styrene) standards in the range of 350-17 800 g mol À1 purchased from polymer standard service were used for calibration. Priorly the solubility of prepolymers from different prepolymerization steps was investigated in THF, which was used as eluent. After more than 18 h at 170°C in the oven, the prepolymer was no longer completely soluble. Therefore, longer prepolymerization times were not considered in further experiments. For the measurement each solution was filtered through a syringe filter (0.45 μm), the concentration of all samples was 6 mg mL À1 . An additional reference measurement of the THF was made since the internal standard BHT (19.562 min) was added to the solvent.
Nuclear Magnetic Resonance (NMR) Spectroscopy: The 1 H-NMR spectroscopy was performed on a Bruker Avance III (USA) operating at 300 MHz. An amount of 20 AE 0.05 mg of each sample was dissolved in deuterated chloroform (signal at 7.25 ppm).
Fourier-Transform Infrared (FTIR) Spectroscopy: The FTIR measurements were conducted on a Spectrum One from PerkinElmer (USA). For each sample, 16 scans were measured between 4000 and 1000 cm À1 with a resolution of 4 cm À1 . To follow the prepolymer formation, the synthesized PDEB was melted and 1 μL was drop cast between two CaF 2 slides (8 mm diameter and 1 mm thickness). The sample was placed in an oven at 170°C and measured every hour until the maximum conversion was achieved. To analyze the final polymerization, 1 μL of the prepolymer received after 18 h of heat-treatment was drop cast between two CaF 2 slides, and the final conversion was investigated at different temperatures (208, 220, and 240°C). All spectra were baseline-corrected and the areas of the absorption peaks were calculated using Spectragryph 1.2 (Germany) and Opus 7.5 (USA) software.
Ultraviolet-Visible (UV-Vis) Spectroscopy: The UV-Vis absorption was investigated in the range of 250-450 nm with a Cary 50 spectrophotometer from Varian, Inc. (USA). All substances were dissolved in acetonitrile (HPLC grade) and measured in a quartz-glass high precision cell from Hellma Analytics (Germany). All provided spectra were measured at a concentration of 0.05 mg mL À1 .
Differential Scanning Calorimetry (DSC): The DSC measurements of the prepolymer received after 18 h at 170°C were performed on a DSC 1 from Mettler Toledo (USA) under nitrogen flow (50 mL min À1 ). An amount of 9 AE 0.1 mg of the samples was placed in aluminum crucibles and heated from 25 to 360°C with a heating rate of 10 K min À1 . Figure 6. Demonstration of the LCD 3D printability of the dual-cure resin system with a comb-like structure to evaluate the maximum resolution.
www.advancedsciencenews.com www.aem-journal.com Sample Preparation: As a reference system, 70 wt% of UDMA and 30 wt% of ACMO were mixed with 0.05 wt% Sudan II as an absorber and 1.0 wt% phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide as well as 0.5 wt% ethyl (2,4,6-trimethylbenzoyl) phenylphosphinate as photoinitiators. In addition, the prepared reference system was mixed with different concentrations of prepolymers from each prepolymerization step. All formulations were mixed on a magnetic stirrer for 2 h at 60°C.
Photorheology and Viscosity Measurements: The rheological properties were investigated with a modular compact rheometer MCR 102 from Anton Paar (Austria). The photorheological measurements were performed with a plate-plate set up (upper plate: "D-PP25" disposable aluminum plate with 25 mm diameter, lower plate: "P-PTD200/GL" fused silica plate with 65 mm diameter) with a measuring gap of 100 μm, a frequency of 1 Hz, a deformation of 1% and a temperature of 25°C. All samples were illuminated from underneath with a LED light source (405 nm) with an intensity of 0.15 mW cm À2 . The gel point was calculated at the intersection of storage modulus and loss modulus. The viscosity measurements were performed with a cone-plate set up (cone: "CP50-2" with 50 mm diameter and 2°opening angle, plate: "P-PTD200/80" with 80 mm diameter) with a measuring gap of 205 μm, at 25°C and a shear rate of 300 s À1 .
3D Printing Experiments: The printing of 3D structures was conducted on an Anycubic Photon Mono (China) LCD printer with an LED light source (405 nm, intensity 2.67 mW cm À2 ). The thickness of each layer was set to 10 μm. The bottom layer was irradiated for 950 s, whereas the normal layers were illuminated for 80 s. After printing, the samples were placed in the UV postcuring oven Form Cure from Formlabs (intensity 9.96 mW cm À2 ) for 30 min at 80°C. In a subsequent annealing step, the samples were heated to 220°C for 4 h.
Dynamic Mechanical Analysis (DMA): The thermomechanical properties were investigated using a DMA/SDTA 861 from Mettler Toledo (USA) in tensile mode. The printed specimens with a width of 3.5 mm, a thickness of 0.5 mm and a clamping distance of 10.5 mm were heated at a rate of 2 K min À1 in the temperature range from 25 to 225°C. The operating frequency was 1 Hz with a maximum amplitude of 10 μm. The glass transition temperature (T g ) was determined at the maximum of tan delta.