Photopolymerization of Cyclopolymerizable Monomers and Their

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

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Photopolymerization of Cyclopolymerizable Monomers and Their Application in Hot Lithography Gernot Peer,† Peter Dorfinger,§ Thomas Koch,§ Jürgen Stampfl,§,∥ Christian Gorsche,*,† and Robert Liska† †

Institute of Applied Synthetic Chemistry, Technische Universität Wien, Getreidemarkt 9/163 MC, 1060 Vienna, Austria Institute of Materials Science and Technology, Technische Universität Wien, Getreidemarkt 9/308, 1060 Vienna, Austria ∥ Cubicure GmbH, Photopolymer Development, Gutheil-Schoder-Gasse 17, 1230 Vienna, Austria Downloaded via KAOHSIUNG MEDICAL UNIV on November 13, 2018 at 15:28:27 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

§

S Supporting Information *

ABSTRACT: Cyclopolymerizable monomers (CPMs) were discovered about 70 years ago but are rarely described in the field of photopolymerization up to now. Herein, we present a class of tertiary amine-based CPMs that undergo complete cyclopolymerization, forming linear polymer chains with cyclic structures in the backbone. Compared to similar monofunctional methacrylates, they show significantly improved reactivity and high double-bond conversions. Because of their low viscosity and low volatility, they are also suitable reactive diluents for highly viscous or even solid urethane methacrylates usually applied in the field of Hot Lithography.

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biomaterials,13,14 and 3D-printing15,16). Resulting polymer backbones tend to exhibit high glass transition temperatures due to their hindered rotational freedom alongside the polymer backbone. To develop suitable CPMs that show high photoreactivity and good coreactivity with (meth)acrylates, ester activation of the polymerizable enes should be beneficial.17,18 Modern lithography-based additive manufacturing (L-AMT) provides precise 3D-structuring of parts with exceptional resolution (50 °C, has been a major motivation, not just in modern L-AMT but also in other advanced fields of application for photopolymers (e.g., dental medicine and tissue engineering).21 However, when one aims for tough, thermoplast-like properties, the reduction of final cross-link density, the introduction of functional groups (e.g., esters, amides, urethanes, and ureas) for the increase of intermolecular forces, and high molecular weight polymer chains exhibiting entanglements are crucial. The resulting resin mixtures, composing of interacting, long, nonreactive polymer fragments with high glass transition

INTRODUCTION Radical cyclopolymerization is a polymerization method that was discovered by Butler et al.1,2 about 70 years ago, yielding linear polymers from difunctional enes via intramolecular radical cyclization and subsequent intermolecular propagation. Quaternary diallylammonium salts surprisingly led to water-soluble polymers, which found wide application in wastewater treatment and paper industry.3 Since then, a vast amount of molecules that undergo cyclopolymerization was examined, whereas a number of prerequisites for efficient cyclopolymerization were disclosed. The reactivity of allyl- or vinyl-based molecules can be enhanced with electron-withdrawing groups adjacent to the radically reactive double bond. Dimethacrylamides,4 methacrylic anhydrides,5 or ester-functionalized diallylamines6 represent examples of activated monomers, while dimethacrylamides interestingly also show self-initiating behavior under UV-irradiation.7 Kodaira et al. state in an extensive review that it is crucial to suppress the intermolecular propagation step, while maintaining a high tendency toward the intramolecular propagation.8 Consequently, monomers that do not undergo homopolymerization but are capable of copolymerization are sought, as cyclopolymerization can be seen as a copolymerization-type reaction.9 Typically, bulky groups in between the two reactive enes promote the radical cyclization step, thus improving the reactivity of the respective cyclopolymerizable monomers (CPMs).10 Additionally, those bulky side groups hinder crystallization of the compound and can provide good thermomechanical properties of the final polymer backbone, which makes them exceptional candidates as reactive diluents for the fields of photopolymerization (e.g., coatings,11 adhesives,12 © XXXX American Chemical Society

Received: September 14, 2018 Revised: October 29, 2018

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DOI: 10.1021/acs.macromol.8b01991 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

Figure 1. (a) Motivation for the development of Hot Lithography. (b) Schematic illustration of the Hot Lithography setup. Methanol (MeOH; Donau Chemie) was used without further purification. Silica gel chromatography was performed with a Büchi MPLC system equipped with the control unit C-620, fraction collector C-660, and UV-photometer C-635. 1H NMR spectra were recorded on a Bruker Avance at 400 MHz (100 MHz for 13C); chemical shifts are given in ppm and were referenced to the solvent residual peak (CDCl3). Multiplicities are termed s (singlet), d (doublet), t (triplet), q (quartet), and m (multiplet). Coupling constants are given in hertz. Synthesis of Cyclopolymerizable Monomers CPM1−CPM3. The syntheses of all CPMs and NPM were performed in accordance with the literature.23 The educt amine together with triethylamine was diluted with dry methylene chloride (CH2Cl2), ethyl 2-(chloromethyl)acrylate (ECMA) was added, and the solution was stirred for 30 min. After aqueous work-up, the crude product was purified and isolated in a high yield (>80%). Further details for the syntheses are found in the Supporting Information. Photo-DSC. Photo-DSC measurements were conducted on a Netzsch DSC 204 F1 with autosampler. BMDG (1 mol %) was used as a PI in all polymerizations. All measurements were performed isocratic at 70 and 25 °C under a N2 atmosphere. Monomer formulations (11 ± 1 mg) were irradiated twice with filtered UV-light (400−500 nm) via an Exfo OmniCureTM series 2000 broadband Hg lamp under constant N2 flow (20 mL min−1). The light intensity was set to 1 W cm−2 at the tip of the light guide corresponding to ∼20 mW cm−2 on the surface of the sample. The heat flow of the polymerization reaction was recorded as a function of time. The times when the maximum of heat evolution was reached (tmax) and when 95% of the overall heat was evolved (t95%DSC) were determined. Polymerized samples were dissolved in CDCl3 (deuteration >99.5%) and analyzed via 1H NMR spectroscopy. The solvent signal was used as internal reference. The conversion was calculated from the integral ratio between the doublebond peaks before and after curing. The −CH2− signal from the ethyl ester group served as reference signal for the CPMs, while the proton attached to the tertiary carbon next to the carboxyl group was used as the internal reference signal for the polymerizations of IBMA. The accuracy for the conversion determination can be assumed with ±3%. All measurements were performed in duplicate with satisfactory reproducibility. GPC. Subsequent to NMR spectroscopy, CDCl3 was evaporated, and then the samples were diluted with 0.5 mL of THF for size exclusion chromatography (SEC). SEC was performed with a Waters GPC using three columns connected in series (Styragel HR 0.5, Styragel HR 3, and a Styragel HR 4) and a Waters 2410 RI detector. The columns were maintained at 40 °C, and a flow rate of 1.0 mL min−1 was set (THF). A calibration based on polystyrene standards was used for molecular weight calibration. The polymers were not precipitated prior to GPC characterization. RT-NIR-Photorheology. An Anton Paar MCR 302 WESP rheometer with a P-PTD 200/GL Peltier glass plate, an H-PTD 200 heating hood, and a disposable PP25 measuring system was used to conduct the RT-NIR-photorheology experiments. Additionally to the rheometer, a Bruker Vertex 80 FTIR spectrometer was used to analyze the conversion over time of the sample. Details for the setup and the measurement procedure are described in the literature.24 For each measurement, an exact amount of monomer formulation (130 μL) was placed at the center of the glass plate, and the measurements were conducted at 70 °C with a gap of 200 μm. A 5 min acclimatization

temperatures, inherently exhibit limited processability at ambient conditions (Figure 1a). Hot Lithography serves as a viable strategy to overcome these obstacles22 and opens the gate for the fabrication of highly viscous resins at elevated temperatures (∼50−150 °C), leading to improved toughness for the 3D-printed parts (Figure 1b). The viscous, photosensitive material is applied in thin layers via a heatable coating unit while the combination of a UV laser, and its placement below the heatable, transparent carrier plate guarantees curing of each layer in high resolution without oxygen inhibition. The reduction of overall cross-link density can be achieved by high molecular weight oligomers with the use of suitable, monofunctional reactive diluents. At the same time bulky substituents attached to the reactive diluent ensure high glass transition temperatures originating from the hindered rotational freedom alongside the polymer backbone. For those reasons, the most commonly applied reactive diluent in industry is isobornyl methacrylate (IBMA). Nevertheless, its high volatility hinders the employment of IBMA in Hot Lithography, which creates a demand for new reactive diluents. Herein, we present the synthesis of various cyclopolymerizable monomers CPM1−CPM3 (Figure 2) and investigate their homopolymerization behavior and copolymerization with methacrylates upon photoinitiation at elevated temperatures. Resins with the most promising candidate CPM3 and a commercial, highly viscous urethane-based difunctional methacrylate resin (Bomar XR-741MS, mentioned solely as Bomar throughout the manuscript) are formulated. The resin Bomar was selected for its relatively high molecular weight (Mw ∼ 1000 g mol−1) and strong intermolecular interactions, resulting in a high glass transition temperature Tg (well above 100 °C). The viscosity of the formulations within the processing window for Hot Lithography is assessed, and the formulations are characterized toward their photoreactivity and final thermomechanical and mechanical properties. Isobornyl methacrylate is added tothe study as commercial reference. Successful 3D-processing of a final formulation containing CPM3 and Bomar via Hot Lithography is described.

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EXPERIMENTAL SECTION

Materials and General Methods. Isobornyl methacrylate (IBMA; Sigma-Aldrich), Bomar XR-741MS (Mw ∼ 1000 g mol−1; Bomar; Dymax), triethylamine (Et3N; Sigma-Aldrich), benzylamine (Merck), aniline (Merck), N-benzylmethylamine (Fluka), 1-adamantylamine hydrochloride (TCI Chemicals), and the UV absorber 2,2′dihydroxy-4,4′-dimethoxybenzophenone (DHDMBP; TCI Chemicals) were purchased from the respective companies. The photoinitiator bis(4-methoxybenzoyl)diethylgermanium (BMDG) and the precursor ethyl 2-(chloromethyl)acrylate (ECMA) were kindly gifted from Ivoclar Vivadent AG. All reagents and monomers were used without further purification. Commercial grade methylene chloride (Donau Chemie) was dried using a PureSolv system (Inert, Amesbury, MA). B

DOI: 10.1021/acs.macromol.8b01991 Macromolecules XXXX, XXX, XXX−XXX

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placed in a 60 °C vacuum oven and dried until a constant weight was reached (mdry). The degree of swelling S (eq 1) and the gel fraction GF (eq 2) of the polymer networks were determined as follows:

time was conducted before starting the measurement. The formulations were sheered with a strain of 1% and a frequency of 1 Hz. UV light was used to initiate the reaction, which was emitted via an Exfo OmniCureTM 2000 device with a broadband Hg lamp (300 s, 400−500 nm, 1 W cm−2 at the tip of the light guide, ∼8 mW cm−2 on the surface of the sample measured with an Ocean Optics USB 2000+ spectrometer). The methacrylate double-bond conversion (DBC) was determined by recording a set of single spectra (time interval ∼0.26 s) with an OPUS 7.0 software and then integrating the respective double-bond bands at ∼6160 cm−1. The ratio of the double bond peak area at the start and after a certain irradiation period gave the respective DBC value. All measurements were performed in duplicate with satisfactory reproducibility (see the literature for a detailed discussion on experimental error24). Volatility Study. A Netzsch Jupiter STA 449 F1 thermal analysis instrument with autosampler was used to conduct combined TGA and DSC experiments, which were used to assess the volatility and thermal stability of the investigated reactive diluents. One set of measurements was performed with a temperature profile of −50 to 300 °C under synthetic air (10 °C min−1), and the second set was conducted under N2 atmosphere isocratic at 70 and 90 °C (2 h). All samples were accurately weighed (17 ± 1 mg) into aluminum DSC pans and measured at a constant gas flow rate (40 mL min−1). The mass loss and DSC signal were recorded. Formulations and Specimens Preparation. Resin formulations with Bomar and 50 and 75 mol % of reactive diluent (i.e., Bomar/ CPM3_50, Bomar/CPM3_75, Bomar/IBMA_50, or Bomar/ IBMA_75) were prepared. Pure Bomar resin was used as reference, and all formulations were mixed with 0.75 wt % BMDG as photoinitiator. The formulations were placed in an ultrasonic bath for at least 1 h at 75 °C. For photo-DSC and RT-NIR-photorheology studies the five formulations were used directly after mixing. RT-NIRphotorheology measurements were conducted analogously as described before. Additionally, a polyethylene tape (TESA 4668 MDPE) was applied on the optical window of the rheometer (lower rheometer plate) for a smooth removal of the photopolymerized specimens.24 For (thermo)mechanical and swelling tests, all five formulations were poured into silicone molds (5 × 2 × 40 mm3 for DMTA, ISO527 test specimens 5b dumbbell-shaped with a total length of 35 mm, and a parallel constriction region with dimensions of 2 × 2 × 12 mm3 for tensile tests and disc-shaped ϕ = 4 mm, h = 2 mm for swelling tests). The resins were cured with a Lumamat 100 light chamber (Ivoclar Vivadent, ∼75 °C, 400−580 nm, 20 mW cm−2 at the position of the silicone mold measured with an Ocean Optics USB 2000+ spectrometer). The samples were irradiated 7 min per side and afterward polished with sandpaper to ensure uniform geometries. Viscosity Study. Rheology tests of the mixtures with Bomar were conducted on a modular compact rheometer (MCR 300, Physica Anton Paar). The viscosity of the formulations was measured at 25, 50, and 70 °C (5 min acclimatization time before measurement) with a CP-25 measuring system (diameter 25 mm). A distance of 48 μm between Peltier plate and cone tip was set. Measurements were conducted in rotation mode with a constant shear rate of 100 s−1. Dynamic Mechanical Thermal Analysis. An Anton Paar MCR 301 device with a CTD 450 oven and an SRF 12 measuring system was used to perform the DMTA measurements. The prepared DMTA samples were tested in torsion mode with a frequency of 1 Hz and a strain of 0.1%. The temperature was increased from −100 to 200 °C with a heating rate of 2 °C min−1. The glass transition temperature was defined as the temperature at the maximum dissipation factor (tan δ). Tensile Tests. A Zwick Z050 equipped with a 1 kN load cell was used to conduct tensile tests. Five specimens per sample were measured. The specimens were fixed between two clamps and strained with a traverse speed of 5 mm min−1. A stress−strain plot was recorded simultaneously. Swelling Tests. The respective polymer specimens (disc-shaped) were weighed (mstart) and then submerged in ethanol. The samples were stored at ambient conditions for 7 days, and the ethanol was replaced once after 3 days. The polymer discs were dried using a paper towel and then weighed (mswollen). Afterward, the discs were

S=

mswollen mdry

GF =

(1)

mdry mstart

(2)

Hot Lithography. 3D-structuring via Hot Lithography was performed on a newly developed L-AMT setup. The Hot Lithography printer setup includes a galvanometer−scanner (IntelliSCAN 10, Scanlab) which is based between the 375 nm diode laser (Omicrometer) and the material vat. All parts that had direct contact with the formulation were heated to 70 °C; this includes the material vat, the building platform, and the recoating unit. For printing, the formulation Bomar/CPM3_50 was selected and 0.1 wt % of the UVabsorber DHDMBP were added to improve the final part resolution. All specimens were printed with a scan speed of 4500 mm s−1, a laser intensity of 70 mW, and a layer thickness of 100 μm. The specimens were built up by first scanning the contours of the layer and then scanning the area within the contours with a hatching distance of 20 μm in one direction. The laser spot on the surface of the material vat has a diameter of 20 μm. The preparation of the specimens was completed with a 1000 s postprocessing step in a UV-chamber (Intelli-Ray 600 UV-oven with a broadband Hg lamp; 600 W; UV-A: 125 mW cm−2; Vis: 125 mW cm−2). The emitted wavelength spectrum from the broadband mercury-based light source was ∼280−550 nm, and total irradiation intensity of 200 mW cm−2 was measured at the position of the samples using an Ocean Optics USB 2000+ spectrometer.

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RESULTS AND DISCUSSION Synthesis of Cyclopolymerizable Monomers CPM1− CPM3. Thermally initiated radical cyclopolymerization of aminelinked diacrylate monomers has been investigated by Avci et al.18 and showed high polymerization and cyclization tendencies. Therefore, three different derivatives that only differ in the group attached to the nitrogen atom were synthesized (Figure 2a). More rigid or bulky side groups should have a positive influence on the cyclopolymerization tendencies and also on the final (thermo)mechanical properties. Hence, a benzyl CPM1, a phenyl CPM2, and an adamantyl derivative CPM3 were targeted for synthesis. According to the literature, CPM2 and CPM3 have reported melting points at ∼60 °C, which additionally hints toward low volatility within the processing window of Hot Lithography. We have performed the syntheses of the three targeted CPMs (Figure 2b)diethyl 2,2′-((benzylazanediyl)bis(methylene))diacrylate (CPM1), diethyl 2,2′-((phenylazanediyl)bis(methylene))diacrylate (CPM2), and diethyl 2,2′-((adamantan1-yl)azanediyl)bis(methylene))diacrylate (CPM3)in accordance with literature.23 The CPMs were successfully synthesized in high yields (>80%, see the Supporting Information for detailed synthetic protocols). While CPM1 was isolated as colorless liquid, the obtained white crystals for CPM2 and CPM3 both exhibit melting points at ∼60 °C. Consequently, CPM2 and CPM3 can only be adapted as reactive diluent for reactions at temperatures above 60 °C. One key attribute of efficient CPMs is a hindered propagation after the initial radical addition step (Figure 2c).25 The formed intermediate radical needs to show a high tendency to first perform an intramolecular cyclization step and then undergo intermolecular propagation. This would ensure complete cyclization and prevent undesired cross-linking reactions. As a result, a linear polymer with a rigid polymer backbone composing of 5- and 6-membered rings is derived C

DOI: 10.1021/acs.macromol.8b01991 Macromolecules XXXX, XXX, XXX−XXX

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Figure 2. (a) Synthetic scheme for CPMs. (b) Structures of methacrylate IBMA, nonpolymerizable monomer NPM, and CPM1−CPM3. (c) Cyclopolymerization via intramolecular cyclization forming 5- or 6-membered rings, hindered initial intermolecular propagation step, resulting polymer backbone polyCPM.

(polyCPM). To verify the postulated hindered initial intermolecular propagation for the synthesized CPMs, ethyl 2-((benzyl(methyl)amino)methyl)acrylate was synthesized as the respective nonpolymerizable monomer (NPM, see the Supporting Information). The benzyl derivative was chosen over the rigid phenyl and the bulky adamantyl side group to minimize possible steric inhibition. Reactivity of CPMs in Photoinitiated Radical Homopolymerization. Photo-DSC. The photopolymerization

behavior of the synthesized CPM1−CPM3 in bulk at elevated temperatures (70 °C) was compared to the performance of IBMA as commercial reference. The nonpolymerizable monomer NPM was tested to prove the suppressed propagation of such amine-linked acrylates, which is a requirement for successful cyclopolymerization. As expected, photo-DSC experiments of NPM showed no significant photopolymerization behavior (Figure 3). While the radical attack of the photoinitiator radical onto the NPM double bond can be D

DOI: 10.1021/acs.macromol.8b01991 Macromolecules XXXX, XXX, XXX−XXX

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Subsequent GPC measurements of the photopolymerized specimens were performed. Photopolymerized CPM1 and CPM2 (polyCPM1 and polyCPM2) exhibit high molecular weights (17.9 and 22.4 kDa, respectively). The more bulky monomers CPM3 and IBMA yielded polymers with reduced molecular weights (∼10 kDa), which is explained by the lower mobility of the forming polymer backbones. Overall, the tested CPMs show superior reactivity toward photoinitiated radical polymerization at elevated temperatures. Besides the possible application in the novel 3D-printing method Hot Lithography, CPMs could also present a suitable alternative in generic photopolymerization technologies (e.g., coating industry). Therefore, photo-DSC measurements of IBMA and CPM1 (liquid at 25 °C) were conducted at ambient temperatures. CPM1 showed a fast polymerization reaction reaching high conversions, which confirms that elevated curing temperatures are not necessarily required for cyclopolymerization (Figure S1 and Table S1). RT-NIR-Photorheology. With an in situ rheological measurement for the characterization of photocuring and IR spectroscopic measurements for the evaluation of chemical conversion during photopolymerization, a more complete assessment of potential monomers for L-AMT can be conducted.24 For fast 3D-printing a fast gelation of the respective monomers is crucial as the time to reach the gel point tgel (intersection between storage G′ and loss modulus G″) determines the minimum irradiation time during 3D-printing (Figure 4a). Furthermore, a high double-bond conversion at the gel point (DBCgel) is desired. Photopolymerizations do not progress smoothly upon gelation, which is due to mobility restrictions in the solid state. With a more rigid and sterically stressed monomer (going from CPM1 to CPM3) the gel time can be significantly reduced and is reached already after 4.9 s with CPM3 (Table 1), which is critical for successful 3D-structuring. This inherently reduces the double-bond conversion at the gel point DBCgel and slightly lowers the overall conversion DBCfinal for CPM3 compared to the less sterically hindered monomers CPM1 and CPM2. However, when compared to the commercial reference IBMA, tgel for CPM3 could be reduced by a factor >3 and DBCgel is significantly increased while maintaining comparably high final conversion (DBCfinal = 87%, Figure 4b). Furthermore, the overall reaction time for CPM3 (characterized by t95%RHEO, meaning the time at which 95% of the final storage modulus G′final are reached) is with 21 s much faster compared to the methacrylate IBMA and the less sterically stressed CPMs (CPM1 and CPM2). Because of its fast gelation and good photoreactivity (lowest tgel and t95%RHEO and sufficiently high DBCfinal), the monomer CPM3 can be viewed as promising reactive diluent for Hot

Figure 3. Photo-DSC plots for IBMA (dash-dot), NPM (short dash), CPM1 (dash), CPM2 (dot), and CPM3 (solid).

observed, the radical intermolecular propagation is indeed suppressed for such amine-substituted acrylates. Subsequently, fast initial polymerization rates for all three tested CPMs (indicated by fast times to the maximum of polymerization rate tmax) compared to the methacrylate reference IBMA have been observed (Figure 3 and Table 1). The faster propagation reaction for CPM-based monomers can be contributed to the formation of primary radicals (upon favored formation of fivemembered rings, Figure 2c), which exhibit higher reactivity compared to the sterically stressed tertiary radicals for methacrylate propagation. Furthermore, the overall reaction (t95%DSC: time to 95% of heat evolution) is significantly faster for CPMs with