PET-RAFT Facilitated 3D Printing of Polymeric Materials
Photopolymerization-based 3D printing process is typically conducted using nonliving free radical polymerization, which leads to fabrication of immutable materials. An alternative 3D printing of polymeric materials using trithiocarbonate (TTC) reversible addition-fragmentation chain transfer (RAFT) agents has always been a challenge for material and polymer scientists. Herein we report the first 3D printing of RAFTbased formulations that can be conducted fully open to air using standard digital light processing (DLP) 3D printer and under mild conditions of visible light at blue (λ max = 483 nm, 4.16 mW/cm 2 ) or green (λ max = 532 nm, 0.48 mW/cm 2 ) wavelength. Our approach is based on activation of TTC RAFT agents using eosin Y (EY) as a photoinduced electron-transfer (PET) catalyst in the presence of a reducing agent (tertiary amine), which facilitated oxygen tolerant 3D printing process via a reductive PET initiation mechanism.Re-activation of the TTCs present within the polymer networks enables post-printing monomer insertion into the outer layers of an already printed dormant object under second RAFT process, which provides a new pathway to design a more complex 3D printing. To our best knowledge, this is the first example of open-to-air PET-RAFT facilitated 3D printing of polymeric materials. We believe that our strategy is a significant step forward in the field of 3D printing.
INTRODUCTION<!>DSC<!>3D printing of the PET-RAFT based photocurable formulations under blue light<!>3D printing of the PET-RAFT based photocurable formulations under green light<!>Post-printing modification of the 3D printed objects<!>CONCLUSION
<p>3D printing (also known as additive manufacturing) is a rapid emerging technology, which has revolutionized manufacturing industry to produce customized products. Contribution of polymer chemistry to the 3D printing technology has extended its scientific and technological impact and enabled a wide range of applications, such as drug delivery, tissue engineering, dentistry, microfluidic and etc. 1,2,11,[3][4][5][6][7][8][9][10] In particular, photopolymerization is a widely utilized technique in 3D printing. [12][13][14][15][16] One shortcoming of photopolymerization-based 3D techniques is production of "dead" materials, due to the use of nonliving free radical polymerization. 17,18 Although 3D printed shape memory polymers are capable of evolving their shape and property with time in response to an external stimulus, [19][20][21][22][23][24][25][26][27] directly linking new polymers covalently from the surface of 3D printouts has not been achievable.</p><p>One promising approach to enable monomer insertion into an initially fabricated polymer networks [28][29][30][31] or to fabricate dynamic polymer networks 29,[32][33][34][35][36][37][38] , is by introduction of living functional groups such as trithiocarbonate (TTCs) iniferters-also known as reversible addition fragmentation chain transfer (RAFT) agents [39][40][41][42][43][44][45] into the strands of cross-linked networks. For instance, Johnson's group synthesized a gel network containing TTCs and demonstrated post-synthesis monomer insertion into these networks upon reactivation of TTC groups (under UV light). 37 Followed by the advancements on the transformable polymer networks, Johnson and co-workers introduced the concept of living additive manufacturing, in which an initially fabricated TTC-containing gels (consist of strands bearing TTC iniferters) could undergo postsynthesis transformation using visible light photoredox catalysis. 31,46 Recently, we 28 and Matyjaszewski's group 47 have exploited the iniferter process with TTC units 33,[48][49][50][51][52][53][54][55] present within photoexpandable/transformable-polymer networks (PET-PNs) to enable monomer insertion into crosslinked networks under visible light irradiation. Howerver, inteferter process under blue light irradiation is slow and reuqired deoxyination.</p><p>In this work, we propose to use a photoinduced electron-transfer RAFT (PET-RAFT) process without solvent in a 3D priting process. In general an efficient 3D printing process requires the following characteristics: (i) the presence of oxygen must not inhibit the polymerization reaction; and (ii) photopolymerization must be realized relatively with fast kinetics at room temperature. To develop a system that can be done fully open to air, an efficient oxygen tolerant photoRAFT system is demanded. Recently, oxygen tolerant reactions are rapidly moving to the forefront in polymerization systems to further extent the applicability of these systems and enable industrial viability. [56][57][58][59][60][61] Thus far, different strategies have been exploited for achieving oxygen tolerant controlled radical polymerization systems: polymerizing through oxygen, 59,60,[62][63][64] enzyme mediated deoxygenation [65][66][67] and the consumption of oxygen via a photoredox catalyst [68][69][70][71][72] ; where the latter strategy is the most commonly used approach in photoRAFT systems.</p><p>Photoredox catalysts with strong photoreducing nature, such as Ir(ppy)3, 73 Ru(bpy)3Cl2, 74 eosin Y (EY) 43,75,76 , and zinc tetraphenylporphine 72,77,78 can convert triplet oxygen into a singlet oxygen which can be trapped by a singlet oxygen quencher and therefore enable oxygen tolerant systems; which has been known as PET-RAFT polymerization. Furthermore, in recent years, there has been intense interest in using visible light in controlled radical polymerization systems [79][80][81][82][83][84][85][86][87][88] and in 3D applications. 11,16,18,[89][90][91][92][93][94] This is due to the advantageous of using visible light sources (mainly light emitting diodes ((LEDs)) as compared to the UV lights; e.g. low-energy visible light sources are eco-friendly with low thermal effect and minimal side reaction, and are preferable for bio-related applications. 18,95 In terms of 3D photopolymerization, visible light is also known for higher penetration depth that can facilitate higher 3D printing speed. Thus far, a number of visible light-sensitive photoinitiators that respond to light with longer wavelengths have been exploited to initiate the uncontrolled free radical-based 3D photopolymerization. 12 Among different PET catalysts, EY-previously employed for PET-RAFT polymerization of (meth)acrylates, 80,96 (meth)acrylamide 43,75 and vinyl ketones 97 presents an interesting opportunity for 3D printing process fully open to air and under mild low energy visible light irradiation.</p><p>Herein, we report the first example of RAFT-based 3D printing process using EY as a photocatalyst, triethylamine (TEA) as a reducing agent and 4-cyano-4-[(dodecylsulfanylthiocarbonyl) sulfanyl] pentanoic acid (CDTPA) as a RAFT agent under PET-RAFT polymerization conditions. This design offers several advantages: (i) the polymerization can be mediated in a fully open to air system, and (ii) longer wavelength visible light can be utilized to activate the polymerization due to the strong absorption of EY in the long wavelength range (green light); and consequently (iii) it can accelerate 3D printing process by reaching higher penetration depth (Figure 1). A modified digital light processing (DLP) 3D printer equipped with blue (λ max = 483 nm, 4.16 mW/cm 2 ) or green (λ max = 532 nm, 0.48 mW/cm 2 ) LED light was used. All 3D printings were conducted in open-to-air conditions with different formulations and printing parameters.</p><p>Finally, the post-printing modification of the printed dormant objects via re-activation of the TTC within the objects enabled second RAFT process to modify the surface of the printed objects with new properties. Proposed photopolymerization mechanism under visible light irradiation where the excited-state EY is reduced by a tertiary amine, leading to a reductive photoelectron transfer reversible addition−fragmentation chain transfer polymerization (PET-RAFT). This mechanism has been previously reported in both solution 80 and bulk 97 controlled living radical polymerization for preparing linear polymers 75 . Although the degenerative chain transfer (RAFT process) in a highly crosslinked networks is extremely difficult, initiation mechanisms should follow the same pathways irrespective of the network formations. c) Chemical structures of 4-cyano-4-[(dodecylsulfanylthiocarbonyl) sulfanyl] pentanoic acid (CDTPA), poly(ethylene glycol) diacrylate (PEGDA, Mn = 250 g/mol), eosin Y (EY) and triethylamine (TEA) employed in this study.</p><!><p>It has been reported that using EY involved a tertiary amine as a sacrificial reducing agent 43,96 enables oxygen tolerant photoinitiation and radical polymerization in both solution 80 and bulk 97 systems. Excited EY * converts the triplet oxygen into the singlet oxygen via a photo-energy transfer process. 56 Tertiary amine can act as an electron donor able to transfer electron to the excited EY * to produce an EY radical anion (EY• − ), 98 which can reduce the TTC by a PET process to generate a propagating carbon radical (Figure 1b). 96 (We refer readers to few comprehensive studies on PET-RAFT mechanism 43,75,76 ). In this work, we adopted the above mentioned EY/TEA system in combination with RAFT agent to prepare a visible lightcurable resin formulation without solvents. Our 3D printable PET-RAFT formulation contains the following ingredients: poly(ethylene glycol) diacrylate (PEGDA, Mn = 250 g/mol), EY (photocatalyst), TEA (reducing agent) and CDTPA as RAFT agent. [50][51][52][53]99 A photo-differential scanning calorimetry (photo-DSC; 400-500 nm light source) was employed to evaluate the photoreactivity of the formulations with different feed ratios. As it can be seen from the recorded DSC signals in Figure 2a, iniferter-based formulation with [PEGDA]/[CDTPA] = 200:1 showed negligible heat flow within 16 min of 400-500 nm blue light irradiation, while addition of EY and TEA enabled faster photopolymerization rate within tmax= ~ 3.8 min (tmax is the time to reach the maximum reaction rate which is an indication of the network formation) 100 These results confirm that the PET initiation pathway using EY/TEA is substantially faster than direct activation (photolysis) of TTC species under visible light irradiation, which were in agreement with previous studies 28,49,51,53,60,64,75,101 (Figure 2). Furthermore, a control experiment was carried out to distinguish the PET-RAFT initiation pathway from free radical initiation in the absence of TTC; using combination of EY and tertiary amine. 102,103 Slower photoinitiation and photopolymerization rate was observed when CDTPA was present in the formulations (Supporting Information, Figure S1) which confirms the activation of TTC units, corroborating other reports of using similar systems. 75,80 To evaluate the effect of TTC on the photopolymerization rate, three molar ratios of [ (Figure 2b). Increasing the content of TTC species slows down the overall photopolymerization rate of PEGDA and delays the crosslinking process which might be in part due to the initiation and induced chain transfer reactions resulted from the TTC units (Figure 1b). In addition, the increase in the content of CDTPA results in the competition between light absorbance by EY and TTC, which might also explain the slower reaction rate in presence of CDTPA. 104,105 We further assessed the effects of EY concentration on the photopolymerization process. We fixed the molar ratio of PEGDA, CDTPA and TEA at 200:1:2 while the concentration of EY was varied from 10 to 400 ppm relative to the PEGDA monomer (Figure 2c). An increase in the photopolymerization process was observed by increasing the EY concentration from 10 ppm to 200 ppm showing tmax varying from ~11.9 min to ~3.9 min. However, based on the photo-DSC results, increasing the EY concentration over ~ 300 ppm (relative to PEGDA) resulted in a decrease in the photopolymerization rate (Figure 2c). This can be attributed to detrimental EY self-quenching processes, which might include triplet−triplet annihilation, excimer formation, and dye−dye electron transfer. 106 We were also interested in the effect of TEA on the reactivity of the prepared formulations. As it can be seen from Figure 2d, increasing the amount of TEA relative to the CDTPA from 0.2 to 2 resulted in an increase in the photopolymerization rate. This increase in the reaction rate is due to the role of TEA to reduce the dissolved oxygen according to the mechanism presented in</p><!><p>Having demonstrated that PET initiation of TTC species under visible light facilitates faster photopolymerization rate as compared to the photolysis process and indeed offer tolerance to oxygen, we adopted EY/TEA-based resin formulations to investigate the printability of this formulation fully open to air. To demonstrate this, we first modified a bottom-up DLP printer where blue LED lights (λ max = 483 nm, 4.16 mW/cm 2 ) were focused on the bottom surface of the resin vat (Figure 3). The emission spectrum of the blue LED source was measured, showing some emission-absorption overlap with EY catalyst (Figure 3a). The 3D printing was then carried out in a layer-by-layer process by digital slicing software with pre-defined parameters. To begin with, a PET formulation with molar ratio of [PEGDA]/[CDTPA]/[EY]/[TEA] = 200:1:0.04:3 was prepared (PET 3D-1; Figure 3d), while the printing variables were set at: exposure (curing) times of 8 min per layer and layer thickness of 100 µm. Build speed of 430 µm/h was achieved using this processing parameters. We further attempted to lower the exposure time to 5 min, while keeping other parameters constant (PET 3D -2); which resulted in a higher build speed of 730 µm/h. Printability of a different PET-RAFT formulation with a molar ratio of [PEGDA]/[CDTPA]/[EY]/[TEA] = 50:1:0.01:3 was also studied by varying the processing parameters. For instance, 5 min exposure time per 100 µm layer (PET 3D -3, Figure 3d) and 20 µm layer (PET 3D -4, Figure 3d) resulted in build speeds of 658 µm/h and 138 µm/h respectively. A relative high build speed was obtained despite the presence of air.</p><p>We also attempted to reduce the exposure time to 3 min per 20 µm (PET 3D -5, Figure 3d) which resulted in a build speed of 282 µm/h. Nevertheless, 3 min exposure time was not sufficient to photopolymerize/solidify 200 µm layer (PET 3D-6, Figure 3d) to obtain adequate adhesion to the build platform. To enable 3D printing of [PEGDA]/[CDTPA]/[EY]/[TEA] = 50:1:0.01:3 formulation with 200 µm layer thickness, 8 min of blue LED (λ max = 483 nm, 4.16 mW/cm 2 ) light was required (PET 3D-7, Figure 3d). These results confirm that our PET formulations are efficient and robust. A microscope image of the printed object's cross section clearly shows uniform stacked layers of the printed object. To further determine the printing fidelity, scanning electron microscopy (SEM) was used to acquire image of the printed object; which confirmed layer-by-layer process with outstanding layer thickness uniformity Interestingly, the layer thicknesses were in good agreement with the predefined values using slicing software (Figure 3e).</p><!><p>Recently, Sumerlin, Boyer and co-workers elucidated the mechanism of PET-RAFT solution polymerization using EY involved a tertiary amine, in which blue light irradiation resulted in a multiple initiation mechanism and reversible-termination steps. 75,80 The study showed that use of green light enables mainly PET pathways and limits other possible initiation mechanisms (e.g., photoiniferter and tertiary amine reduction mechanisms). Therefore, using lower energy green light over blue light could enable only PET pathway and will pose less challenges during 3D printing of sensitive systems such as biomaterials. 43 To this end, we modified our 3D printer and used green LED light (λ max = 532 nm, 0.48 mW/cm 2 ) to perform 3D printing processes based on solely reductive PET pathway initiation. EY which has a high molar extinction coefficients at high wavelength (λ 𝑚𝑎𝑥 𝑎𝑏𝑠 = 543 nm) showed a good overlap with the emission spectrum of the commercial green LEDs (λ max = 532 nm, 0.48 mW/cm 2 ) (Figure 4a). Initially, a PET formulation using a [PEGDA]/[CDTPA]/[EY]/[TEA] = 200:1:0.04:3 was used in a printing process with 8 min exposure per 100 µm layer. The layer-by-layer 3D printing process was successful, in which a build speed of 528 µm/h was obtained (PET 3D-8; Figure 4b). As compared to its alike blue LED based printing process (PET 3D-1, Figure 3d), green LED light resulted in an even higher build speed despite its lower intensity (~10 fold). As it can be seen from Figure 4b, various formulations and 3D printing parameters were also tested, showing a similar trend as blue LED based printing processes; while in all cases, green LED light resulted in a higher build speed. We summaries this outcome resulted from: (i) better absorbanceemission overlap between EY and green LED emission, (ii) less contribution of homolytic photolysis to PET initiation 75 and (iii) higher penetration depth of green light as compared to the blue light 53 . To confirm higher penetration of green light over blue light, we carried out a curing depth measurement. A mask with a circular area (d =1cm) was used in a modified DLP 3D printer which was exposed to blue (λ max = 483 nm, 4.16 mW/cm 2 ) or green (λ max = 532 nm, 0.48 mW/cm 2 ) LED light for 12 hr. The cured discs were then removed from the vat and their thicknesses were measured. The light penetration depths for the blue and green corresponding energy dose were 0.64 mm and 1.23 mm respectively. Furthermore, to demonstrate the uniform stacked layers of the printed object, an optical image and a microscope image of a 3D object (printed using PET 3D-8 formulations and processing conditions) are presented in Figure 4c. SEM images of the representative printed objects further confirmed a layer-by-layer process with extremely high accuracy in layer uniformity and thickness (Figure 4d-g).</p><!><p>Having shown that our PET-RAFT-based formulations can be effectively utilized in a 3D printing process, we then studied whether the TTC units present within the 3D printed objects could be re-activated to either inset new monomers into an already printed object or chain extended on its surface. To achieve this, we first 3D printed an object using a formulation and printing conditions as PET 3D-10 (Figure 4b), where additional n-butyl acrylate (BA) and PEGDA 6000 (Mn = 6000 g/mol) were added and CDTPA was replaced with a dibenzyl trithiocarbonate (DBTTC) RAFT agent (monomer to RAFT ratio of [OEGDA 250]: [OEGDA 6000]: [BA]: [DBTTC] = 45:0.04:4:1). The BA co-monomer was used to reduce the glass transition temperature (Tg) value of the printed materials and thus facilitate high chain mobility for the enhancement of monomer diffusion into the network and subsequent monomer insertion process. 107 The PEGDA 6000 was also used in order to obtain higher swelling capacity and therefore enable better monomer diffusion into the networks. However, due to low solubility of PEGDA 6000, we were not able to increase its ratio to PEGDA 250. The addition of PEGDA 6000 resulted in a slight increase in the swelling ratio (from ~1.08 to ~1.4) as compared to its counterpart object without PEGDA 6000. Swelling ratio defined as Ww/ Wd, where Ww is the weight of a sample swollen in DMSO; Wd is the weight of sample in the dry state. Indeed symmetric DBTTC was used due to its ability in post-synthesis monomer insertion into highly crosslinked networks. 28 The new formulation was proved to be as effective as previous formulations in a 3D printing process. The object was then soaked into a "growth medium" containing a hydrophobic BA monomer dissolved in DMSO. Subsequently, a blue LED light (λ max = 460 nm, 0.7 mW/cm 2 ) was employed to reactivate the dormant DBTTC units within the printed object via an iniferter RAFT polymerization mechanism. The final modified 3D object was studied by water contact angle measurements. The average contact angle of the modified object increased from 32.8° to 68.1° compared to its original surface, confirming an increase in hydrophobicity resulted from increased poly(BA) on the surface (Figure 5). We believe that due to the high crosslinking density of the 3D printed materials, the formed pBA are mostly rope-shaped brushes for the outer layers of the modified object.  Reaction scheme of the post-printing BA insertion. Polymerization of BA or in another word, BA monomer insertion into the printed objects was performed under blue LED light (λ max = 460 nm, 0.7 mW/cm 2 ), without presence of external initiators or catalysts. Note that for simplicity, PEGDA 250 and PEGDA 6000 are presented as one polymer segment in the scheme presented here.</p><!><p>We report the first fully open to air PET-RAFT 3D printing process of polymeric materials. Our strategy is based on activation of TTC RAFT agents using EY as a PET catalyst in the presence of a TEA as a sacrificial and reducing agent. The EY/TEA system enabled oxygen tolerant 3D printing process via a reductive PET initiation mechanism under mild conditions of either blue (λ max = 483 nm, 4.16 mW/cm 2 ) or green (λ max = 532 nm, 0.48 mW/cm 2 ) LED light irradiation in a DLP printer. The advantage of using green light over blue light is higher light penetration depth and thus faster 3D printing process. The utility and versatility of our PET-3D approach was demonstrated using a variety of processing parameters and formulations. Re-activation of TTC species within the 3D printed objects allow post-printing monomer insertion into an already printed dormant object. Our strategy addresses two major challenges in 3D printing using controlled polymerization methods: oxygen tolerance and fast printing speed. We believe the contribution of this paper is a significant step forward in the field of complex 3D printing.</p>