Efficient Photoinitiating System Based on Diaminoanthraquinone for

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An Efficient Photoinitiating System Based on Diaminoanthraquinone for 3D Printing of Polymer/Carbon Nanotube Nanocomposites Under Visible Light Guannan Wang, Nicholas Simon Hill, Di Zhu, Pu Xiao, Michelle L. Coote, and Martina H. Stenzel ACS Appl. Polym. Mater., Just Accepted Manuscript • DOI: 10.1021/acsapm.9b00140 • Publication Date (Web): 01 Apr 2019 Downloaded from http://pubs.acs.org on April 6, 2019

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ACS Applied Polymer Materials

An Efficient Photoinitiating System Based on Diaminoanthraquinone for 3D Printing of Polymer/Carbon Nanotube Nanocomposites Under Visible Light Guannan Wang,a Nicholas S. Hill,b,c Di Zhu,b Pu Xiao,a,b,* Michelle L. Coote,b,c,* Martina H. Stenzela,* a) Centre for Advanced Macromolecular Design (CAMD), School of Chemistry, University of New South Wales, Sydney, NSW 2052, Australia, b) Research School of Chemistry, Australian National University, Canberra, ACT 2601, Australia, c) ARC Centre of Excellence for Electromaterials Science, Australian National University, Canberra, ACT 2601, Australia * [email protected]; * [email protected]; * [email protected]

KEYWORDS: 3D printing, Photopolymerization, carbon nanoparticles, Three-component photoinitiator systems, Green light, LED

ABSTRACT: This work reports several 1,5-diaminoanthraquinone (1,5-DAAQ)-based visible-light-sensitive photoinitiating systems (PISs) which can efficiently initiate the free radical photopolymerization of (meth)acrylates under green light delivered from light-emitting diode. The PISs contain three components: 1,5-diaminoanthraquinone, an iodonium salt and aromatic amines. In one example, polymerization could be efficiently initiated and completed within 10 seconds with double bond conversions of approximately 80%. The initiating system could also be used in conjunction with reversible addition fragmentation chain transfer (RAFT) polymerization. Finally, the 1,5-DAAQ/iodonium salt/4-N,N-trimethylaniline PIS could be readily employed in the 3D printing process of polymer/carbon nanotube nanocomposites.

Introduction Additives manufacturing (AM), now commonly known as 3D printing, was first developed in the 1980s, when the stereolithography (SLA) patent was published by Charles Hull (1986). In 1987, The first commercially available SLA instrument (Model SLA-1) was produced by the 3D Systems company.1-2 In its present form, it is characterized by the use of raw materials to produce objects layer-by-layer, with the assistance of computer-assisted design (CAD) software.3-5 Compared with traditional subtractive manufacturing, 3D printing can produce objects in small quantities or with complex structures economically while reducing waste. At the same time, the CAD interface allows for easy and fast modification of the design.6 One important 3D printing technique is digital light processing (DLP). DLP 3D printing has a higher resolution in contrast to other 3D printing techniques, such as fused deposition modeling (FDM), selective laser sintering (SLS) or SLA.5, 7 Since the underpinning principle of DLP 3D printing is the photopolymerization of the liquid resins, the development of effi-

cient photoinitiators plays a central role to ensure a high rate of polymerization combined with high polymerization conversions of monomers to alleviate issues pertaining to unreacted vinyl functionalities. The two main types of photoinitiators that are commercially available are either sensitive to ultraviolet (UV) light (e.g. trimethylbenzoyl diphenylphosphine oxide (TPO), bis(2,4,6-trimethylbenzoyl)phenylphosphineoxide (BAPO), and isopropylthioxanthone (ITX) etc.) or visible light (e.g. camphorquinone (CQ)-based PISs, Eosin Y-based PISs, etc.).8-10 UV light-sensitive photoinitiators have the shortcoming that the UV light source used may not only be irritant to eyes and skin, but the generation of UV light is a high energy process, especially when using halogen and mercury lamps as light source.11-13 In particular the preparation of nanocomposites by photopolymerization in the presence of high-performance PISs is hampered as some nanoparticles such as carbon nanotubes absorb and scatter UV light and therefore slow down or even inhibit the polymerization.14 Visible light is moreover known to have

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higher penetration depth, which is particularly attractive when photocurable formulations contain insoluble inorganic fillers, which can often have a cloudy appearance. Polymer/carbon nanotube nanocomposites, which can be prepared by in-situ polymerization15 including UVphotopolymerization,16-18 have been widely investigated for their high mechanical and thermal stability, 17, 19 as conductors16 and as radar absorbing material.20-21 However, carbon nanotube can strongly absorb light of shorter wavelength. Therefore, the development of efficient long wavelength light sensitive photoinitiators for 3D printing nanocomposites is attractive for many applications such as dentistry.8 Nonetheless, most of the efficient visible light photoinitiators available are active at violet or near UV light. Radebner et al. pushed the light wavelength to induce free radical polymerization to 470nm by employing tetraacylstannane reaching high conversions of round 70% within 18 seconds.22 Al Mousawi et al. recently used Zinc Tetraphenylporphyrin (ZnTPP) as a cationic photoinitiator for 3D printing, polymerizing a monomer mixture within 40 to 50 seconds under 477nm light irradiation, with the rate of polymerization decreasing at 530 nm light.23 Moszner et al. used tetrabenzoylgermane and its derivatives as efficient visible light photoinitiators (400 to 500 nm) to cure dimethacrylate resins within 20 seconds with the finial monomers conversion reaching around 40% to 50%.24 Despite these advances, rapid curing at longer wavelengths remains elusive. In this work, a series of high performance 1,5diaminoanthraquinone (1,5-DAAQ)/iodonium salt/aromatic tertiary amines three-component PISs were developed for the free radical photopolymerization of (meth)acrylates under the irradiation of LED at 518nm (green light) (Scheme 1). Multicomponent PISs containing a photoinitiator, an iodonium salt and an amine have attracted a lot of attentions due to their high initiation ability (i.e. fast polymerization rate and high monomer conversion) for photopolymerization (ESI, Table S1).25-30 The three-component systems tested so far led to final conversion within a time frame seconds to minutes using predominantly high intensity light sources below 500 nm. Recently, 1,5-DAAQ has been identified as an efficient photoinitiator for polymerization under blue and green household LEDs, 31-36 which inspired us to further develop highly efficient PISs based on 1,5-DAAQthe 1,5DAAQ/Iod/aromatic tertiary amines combinations. Specifically, poly(ethylene glycol) diacrylate (PEGDA, 250 MW) and other (meth)acrylates were polymerized in the presence of four 1,5-DAAQ/Iod/aromatic tertiary amines [i.e. Ethyl 4-(dimethylamino)benzoate (EDMAB), 4(dimethylamino)benzaldehyde (DMAB), N,NDimethylaniline (DMA) and 4,N,N-trimethylaniline (TMA)] PISs, in which aromatic tertiary amines vary in their electron density in the amino groups as evidenced by NMR studies (Figure S1). It was identified that highly efficient

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PIS can initiate the photopolymerization and reach the final conversion within a few seconds upon exposure to the green LED at 518 nm, which was subsequently employed in DLP 3D printing of polymer/carbon nanotube nanocomposites. Initiation behavior of 1,5-DAAQ-based PISs Prior to the investigation of the 1,5-DAAQ/Iod/aromatic tertiary amines three-component PISs, the 1,5-DAAQ/Iod or 1,5-DAAQ/aromatic tertiary amines two-component PISs were first studied as control. As illustrated in Figure 1 (a), 1,5-DAAQ/Iod PIS showed the higher efficiency (i.e. higher polymerization rate and final conversion) than the 1,5-DAAQ/aromatic tertiary amines PISs. In addition, 1,5DAAQ/DMA is the most efficient PIS among all the investigated 1,5-DAAQ/aromatic tertiary amines PISs. Specifically, approximately 80% and 60% of double bond conversions can be attained in the presence of 1,5-DAAQ/Iod and 1,5-DAAQ/DMA, respectively, for the photopolymerization of PEGDA (250 MW) under the green LED irradiation. Scheme 1. Chemical structures of 1,5-DAAQ; diphenyliodonium hexafluorophosphate (Iod); Ethyl 4-(dimethylamino)benzoate (EDMAB); 4-(dimethylamino) benzaldehyde (DMAB); N,N-Dimethylaniline (DMA) and 4,N,Ntrimethylaniline (TMA).

As shown in Figure 1 (b), the addition of aromatic tertiary amines into 1,5-DAAQ/Iod PIS can accelerate the photopolymerization reactions. And ~80% of final double bond conversions can be achieved for the polymerization of PEGDA (250 MW) in the presence of 1,5-DAAQ/Iod/DMA (or TMA or EDMAB) PISs. More interestingly, 1,5DAAQ/Iod/DMA and 1,5-DAAQ/Iod/TMA can efficiently initiate the photopolymerization to reach the final double bond conversions within only 10 seconds. It can be seen that the polymerization rates are directly correlated with the electron densities at the aminoalkyl groups, which can be expressed by the 1H NMR chemical shift (Figure S1). Both the TMA and DMA systems are found to be highly efficient in contrast to amines with electron withdrawing substituents. The EDMAB system is still reasonably fast despite the electron-withdrawing effect. As discussed fur-

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ther below in the mechanism section, the electron-transfer step is faster in TMA and DMA, but the subsequent addition step is more favorable for EDMAB. To put the rate of polymerization into perspective, other three component systems are tabulated in ESI, Table S1 showing that this system is significantly faster, or it can reach similar rate of polymerizations as other efficient systems, but now with the use of a simple household LED. It should be noted that the light intensity chosen here (60 mW cm-2) will be a determining factor in achieving high rates of polymerization. Lower light intensities will lead to slower reaction rates. Same applies to the concentration of the initiator system as the reduction of initiator will reduce the amount of radical produced as it needs to be considered that a threecomponent reaction will follow high order kinetics and therefore the concentration will play a significant role. Reducing the initiator concentration ten-fold will however still lead to an efficient process (Figure S2)

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based three-component PISs with Iod (c = 0.052 M) and different aromatic tertiary amines (c = 0.164 M) as additives upon exposure to Green LED@518 nm (60 mW cm-2). Fluorescence quenching experiment The fluorescence emission behavior of 1,5-DAAQ was studied by fluorescence spectrometry. The fluorescence stems from the excited singlet state of 1,5-DAAQ with 3 ns of lifetime.34 With the addition of tertiary amines, the fluorescence has been quenched with different interaction rate constants. The interaction rate constants were calculated based on Stern-Volmer equation (Kq = 3.127×109 M-1s-1, 3.122×109 M-1s-1, 3.373×109 M-1s-1, and 4.707×109 M-1s-1 for EDMAB, DMAB, DMA, and TMA respectively) (ESI, Figure S3 and S4). The interaction rate constants show the same tendency as that the polymerization rates of 1,5DAAQ/aromatic tertiary amines PISs. TMA based twocomponent PIS has the highest Kq value and polymerization rate, while 1,5-DAAQ/DMAB PIS exhibited the lowest Kq value and polymerization rate. Compared to 1,5DAAQ/tertiary amines, 1,5-DAAQ/Iod has a relatively low Kq value (~1×109 M-1s-1)34, which implies the lower reactivity of 1,5-DAAQ/Iod interaction during the light irradiation. However, 1,5-DAAQ/Iod can initiate and regenerate the initiator while 1,5-DAAQ/aromatic amines can initiate (more rapidly) but not regenerate the initiator resulting in higher efficiencies of three component system over two component systems. The Stern-Volmer plots of 1,5-DAAQ with EDMAB, DMAB and TMA show a linear relationship with the increase of additive concentrations. EPR experiments

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Figure 1. Photopolymerization profiles (double bond conversions vs time) of PEGDA (250 MW) in laminate in the presence of a) 1,5-DAAQ (c = 0.023 M) -based twocomponent PISs with IOD, EDMAB, DMAB, DMA or TMA (c = 0.164 M) as additives and b) 1,5-DAAQ (c = 0.023 M) -

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Figure 2. EPR spectra of the radicals generated in 1,5DAAQ/Iod/EDMAB upon light exposure and trapped by



PBN in tert-butylbenzene: (a) experimental and (b) simulated spectra.

Photoinitiation Calculations Theoretical calculations suggest that the efficiency of the three-component systems arises from a number of desirable features. The first step in the photoinitiation process involves the excited 1,5-DAAQ molecule, which either reduces the Iod species or oxidizes the tertiary amines. Figure 3 shows the change in energies relative to the 1,5DAAQ excited state optimized geometry for these processes, before geometry relaxation of the product is allowed to take place. The energy changes for this process provide an estimate for the relative favorability for electron transfer to the various species, and suggest that, in the presence of both tertiary amine and Iod, the 1,5-DAAQ species is more likely to oxidize the amines. From Figure 3, of the four tertiary amine species DMA and TMA exhibit negative barriers to electron transfer, reflected experimentally in the highest overall conversion for these two species when paired with Iod. The DMAB and EDMAB tertiary amines radical cations appear more difficult to form, resulting in a lower conversion, however the Iod electron transfer is still out-competed for all amine species, hence the observed acceleration in initiation compared with the two component (1,5-DAAQ/Iod) system.

drogen atoms from their tertiary amine-bonded methyl groups. Upon their formation, the tertiary amine radicals can initiate free radical polymerization by reacting with the ethyl acrylate monomer, with each radical exhibiting a similar ΔG of activation (Table 1). From Table 1, the different remote groups for each of the tertiary amines have a limited impact on their ability to react with monomer units; the difference in observed initiation ability is therefore attributed to the ease with which the respective radicals can be formed. The reduction of 1,5-DAAQ by tertiary amine results in a 1,5-DAAQ radical, which can subsequently be oxidized by the Iod species to form a phenyl free radical. From here there are two likely steps involving the phenyl radical; either radical addition to monomer, i.e. initiation, or hydrogen atom abstraction (HA) from another tertiary amine molecule (Table 1). With ethyl acrylate as the monomer unit, phenyl radical-to-monomer addition is generally favored; however, the HA activation energy is calculated independently of the monomer of choice, so the relative HA vs radical addition energies are not necessarily experimentally relevant. The tertiary amine and phenyl radical addition activation energies are, however, within an order of magnitude of each other, suggesting that even if HA does take place to form benzene, the rate of initiation will not decrease. Table 1. Gibbs Free Energies (kJ mol-1) of activation for hydrogen abstraction and radical addition reactions with DMA/DMAB/EDMAB/TMA and phenyl radical, calculated at G3(MP2)-RAD. Hydrogen Abstraction (kJ mol-1) Radical addition (kJ mol-1)

Figure 3. Electronic and Gibbs Free Energy level diagram for electron/hydrogen transfer processes with 1,5-DAAQ, DMA/DMAB/EDMAB/TMA, and Iod

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The overall three-component redox process (Figure 4) results in the formation of reactive free radicals; the only likely competing reaction, i.e. HA between a tertiary amine and phenyl radical, also forms tertiary amine radicals that are as reactive as the phenyl radical. Together, the Iod and tertiary amine components allow for the reformation of neutral 1,5-DAAQ, which can then be re-excited to continue forming initiating radicals. The reader is referred to earlier work that investigated the mechanism of Iod with 1,5-DAAQ in the absence of amine.36

The second step in Figure 3 is hydrogen atom transfer from the tertiary amine species to 1,5-DAAQ with geometry relaxation allowed to take place. For 1,5-DAAQ/Iod system, the system simply relaxes and Iod dissociates to a phenyl radical and neutral iodo-phenyl species. Each of the tertiary amine species are found to preferentially lose hy-


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amount of MWCNTs (0 wt% or 1 wt% or 3 wt% or 5 wt% or 10 wt%) in laminate in the presence of 1,5DAAQ/Iod/TMA (0.5%/2%/2%, wt) upon exposure to Green LED@518 nm (60 mW cm-2).

The XY plane resolution of PEGDA with 1,5DAAQ/Iod/TMA PIS during photopolymerization was investigated by SEM (ESI, Figure S6). Specifically, photo masks with different diameter circle patterns were used to cover the light source. Both 80- and 40-micron samples display relatively good resolution, which can ensure the quality of the 3D printing of the formulation.

Figure 4. Photoinitiation mechanism for 1,5-DAAQ-based three-component systems 3D Priniting of polymer/multiwall carbon nanotube (MWCNT) nanocomposites To prepare polymer/MWCNT nanocomposites via 3D printing, the photopolymerization of PEGDA with different amounts of MWCNTs was monitored by FTIR-ATR, using 1,5-DAAQ/Iod/TMA PIS under the green LED irradiation. After adding up to 5 wt% of unfunctionalized MWCNTs to PEGDA, no obvious reduction in either polymerization rate or final conversion was observed (Figure 5). However, the polymerization rate and the final conversion were slightly decreased for the formulation containing 10 wt% of MWCNTs probably due to the scattering of the light by MWCNTs (ESI, Figure S5). But this is not considered a concern as a MWCNT content of 10 wt% is beyond what is typically used in various property enhancements such as improvement of mechanical properties and thermal stability.38-41

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Figure 6. a) Photo of “UNSW” logo pure PEGDA sample; b) characterization in 3D by profilometry of “UNSW” logo pure PEGDA sample; c) Photo of “UNSW” logo PEGDA with 0.5 wt% CNT sample. Sample was prepared by 3D printing PEGDA with TMA PIS (1,5-DAAQ: 0.5 wt%, Iod: 2.0 wt%, TMA: 2.0 wt%). 3D printed objects were obtained using PEGDA as monomer and TMA based PIS initiator and printed using DLP 3D printer with a 5 to 10 seconds per layer (30~60 microns) printing speed. For example, the “UNSW” logo in Figure 6 was printed and characterized by a profilometry showing a relatively smooth surface. More objects are shown in Figure S5. Moreover, objects based on nanocomposites with 0.5 wt% MWCNT as fillers were successfully manufactured by 3D printing (Figure 6c and ESI, Figure S7). The exposure time to print each layer for the sample with MWCNT is longer than that without MWCNT. The difference between the practical 3D printing time and photopolymerization kinetics (Figure 5) can be ascribed to the different irradiation sources used. The light intensity of the irradiation sources in the 3D printer (3.6 mW cm-2)36 is a much lower than that of the green LED@518nm (60 mW cm-2), which can be significantly blocked by the addition of fillers and subsequently decelerated the 3D printing process

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Potential applications of 1,5-DAAQ/Iod/aromatic tertiary amine PISs



To investigate the versatility of the 1,5-DAAQ-based PISs, different monomers, such as 1,6-hexanediol diacrylate (HDDA), 1,6-hexanediol dimethacrylate (HDDMA), 2Hydroxyethyl acrylate (HEA) and 2-Hydroxyethyl methacrylate (HEMA), were photopolymerizd using 1,5DAAQ/Iod/TMA PIS under the green LED irradiation. Similar to PEGDA, the acrylates HEA and HDDA, and to a lesser extent the methacrylates HDDMA and HEMA can be initiated and polymerized by 1,5-DAAQ/Iod/TMA PIS (ESI, Figure S8). Moreover, 1,5-DAAQ/Iod/TMA PIS can also be used in combination with reversible addition fragmentation chain transfer (RAFT) agent 3benzylsulfanylthiocarbonylsulfanyl-propionic acid (BSPA) for the RAFT photopolymerization poly(ethylene glycol) methyl ether acrylate (PEGMEA). The concentration of 1,5DAAQ/Iod/TMA (ESI, Table S2) was adjusted to suit the RAFT process, where the radical source is used in concentrations well below the RAFT agent concentration to avoid termination reactions.42 After 6 hours, the conversion of PEGMEA reached up to 90% resulting in polymers with low molecular weight distribution (Ð=1.15) (ESI, Figure S9). The molecular weight increased with conversions in a linear relationship highlighting that the PIS is orthogonal to the RAFT process. Since the light intensity used in this work is relatively strong, the potential possibility that RAFT agent can initiate polymerization itself cannot be ignored.44 In addition, it is possible that amine can also promote RAFT polymerization.45 In this research, however, no photopolymerization was observed for the PEGMEA with either RAFT agent (BSPA) alone or BSPA/TMA system under the irradiation of green LED@518 nm (60 mW/cm2) for 6 h (confirmed by 1H-NMR), which indicated the important role and necessity of 15-DAAQ in the photoinitiating system. Conclusion Novel three-component visible light sensitive photoinitiator systems were developed with the combination of 1,5DAAQ, iodonium salt (Iod) and different aromatic tertiary amines. The three-component PISs demonstrated much higher polymerization rates for free radical photopolymerization than two-component PISs (i.e. 1,5-DAAQ/Iod or 1,5-DAAQ/aromatic tertiary amines). Polymerization processes can be completed within 10 seconds when using the PISs 1,5-DAAQ/Iod/DMA or 1,5-DAAQ/Iod/TMA, and the final conversions of PEGDA can reach 80%. The acceleration mechanism was investigated by fluorescence quenching, EPR-ST, and quantum chemistry. The application of 1,5-DAAQ/Iod/TMA PIS for the 3D printing of photocurable resin (PEGDA) and polymer/carbon nanotube nanocomposites under visible light was also investigated which illustrated great potential. Further evaluation of 1,5-DAAQbased three-component PISs to produce MWCNTs reinforced 3D printed nanocomposites is underway.

ASSOCIATED CONTENT Supporting Information. The supporting Information is available free of charge via the Internet at http://pubs.acs.org.

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Comparison of different three-component PISs from literature can be found in Table S1. The concentration of different components during RAFT polymerization can be found in Table S2, while 1H-NMR spectrums of different tertiary amines can be found in Figure S1. Polymerization rate curve of PEGDA (250 MW) with different PIS concentration can be found in Figure S2. In addition, fluorescence quenching plots of 1,5DAAQ with different additives can be found in Figure S3 and Figure S4. UV-Vis absorption spectrum of MWCNT in water can be found in Figure S5. SEM images from photomask polymerization is available in Figure S6. 3D printed objects with or without CNT can be found in Figure S7. Potential applications including using different monomers and applying to RAFT polymerization can be found in Figure S8 and Figure S9. Finally, the computational methodology data are in Table S3.

AUTHOR INFORMATION Corresponding Author * Email: [email protected] * Email: [email protected] * Email: [email protected]

Author Contributions The manuscript was written through contributions of all authors. / All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT G. W. acknowledges the Chinese Scholarship Council (CSC) for scholarship support. P. X. acknowledges funding from the Australian Research Council Future Fellowship (FT170100301). MLC gratefully acknowledges a Georgina Sweet ARC Laureate Fellowship (FL170100041) and generous allocations of supercomputing time on the National Facility of the Australian National Computational Infrastructure.

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