Modular Elastomer Photoresins for Digital Light Processing Additive

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Modular Elastomer Photoresins for Digital Light Processing Additive Manufacturing Carl J Thrasher, Johanna Jesse Schwartz, and Andrew J Boydston ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13909 • Publication Date (Web): 17 Oct 2017 Downloaded from http://pubs.acs.org on October 23, 2017

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ACS Applied Materials & Interfaces

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Modular Elastomer Photoresins for Digital Light 5

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Processing Additive Manufacturing 9

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Carl J. Thrasher †, Johanna J. Schwartz †, and Andrew J. Boydston* 17

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Department of Chemistry, University of Washington, Box 351700, Seattle, WA, 98195, United States. Email: [email protected] 21

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Author Contributions

The authors Carl J. Thrasher and Johanna J. Schwartz contributed equally to this work. 26 27 28

KEYWORDS: 3D printing, elastomeric, flexible, stereolithography, digital light processing 29 30 31 32

A series of photoresins suitable for production of elastomeric objects via digital light processing 3 35

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additive manufacturing are reported. Notably, the printing procedure is readily accessible using 37

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only entry-level equipment under ambient conditions using visible light projection. The 38 39

photoresin formulations were found to be modular in nature and straightforward adjustments to 40 42

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the resin components enabled access to a range of compositions and mechanical properties. 4

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Collectively, the series includes silicones, hydrogels, and hybrids thereof. Printed test specimens 45 47

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displayed maximum elongations of up to 472% under tensile load, tunable swelling behavior in 49

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water, and Shore A hardness values from 13.7 to 33.3. A combination of the resins was used to 50 51

print a functional multi-material three-armed pneumatic gripper. These photoresins could be 52 54

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transformative to advanced prototyping applications such as simulated human tissues, stimuli56

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responsive materials, wearable devices, and soft robotics. 57 58 59 60 ACS Paragon Plus Environment

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INTRODUCTION 5

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Additive manufacturing (AM) has experienced tremendous growth at the interface of 6 7

chemistry and engineering.1–6 Recent breakthroughs have generated exciting opportunities to 10

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print a broad range of object geometries with impressive throughput.7–9 As printer capabilities 1 12

have rapidly evolved, the technologies for expanding the scope of build materials has started to 13 15

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follow suit. Innovations in synthetic macromolecular chemistry and nano-to-mesoscale 17

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molecular design have been vetted against, inspired by, and integrated with cutting-edge AM 18 19

equipment to produce unprecedented outcomes in multiple fields.10–15 A challenging and 20 2

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potentially highly rewarding target area for new materials development lies within elastomeric 24

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systems that display rapid customizability in molecular composition, to complement rapid 25 27

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prototyping available from AM. 29

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Integration of elastomers with AM technology offers exciting potential capabilities for 31

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multiple areas, including soft robotics, flexible electronics, biomimetic structures, and wearable 32 3

devices.6,11,13,14,16–23 Many current fabrication processes that incorporate elastomer materials into 36

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devices involve tedious and time-consuming iterative processing or assembly steps.17,18 AM, on 37 38

the other hand, can be a facile method to quickly fabricate complex 3D geometries in a single 39 41

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process. Among the various AM techniques available, digital light processing additive 43

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manufacturing (DLP-AM) is particularly attractive as it can offer low equipment costs, multi4 45

color input, high resolution print features, and relatively fast print speeds while enabling access 46 48

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to hollow object components, overhangs, and other challenging geometries.24 DLP-AM is a 50

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subset of vat photopolymerization and involves the repeated use of digitally generated 51 53

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photopatterns to irradiate a vat of photoactive resin, ultimately curing a 3D object in a layer-by5

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layer fashion (Figure S1). Since images are projected from a DLP system, an entire layer can be 56 57 58 59 60 ACS Paragon Plus Environment

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cured in parallel, in contrast to monochromatic laser-based systems that print a raster pattern for 5

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each layer. Recent advancements in DLP-AM show precision control over architectures with 6 8

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sub-micrometer resolution, 3D spatial control over cross-linking density, and continuous printing 10

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able to produce 10 cm objects in minutes.7–9,25 1 12

Options for using DLP-AM to print stretchable and flexible devices are currently 13 15

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restricted by a lack of appropriate printable materials, but recent advances have begun to answer 17

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this call.10–12 Commercially available “flexible” photoresins such as Formlabs Flexible and Spot18 19

E Elastic exhibit limited elongations from 90-100%, which is not sufficient for many advanced 20 2

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applications.26,27 Another impressive photoresin, Carbon’s Flexible Polyurethane resin, shows 24

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high mechanical toughness and elongations up to 300%, but is not yet available on open 25 26

platforms.28 Magdassi and Ge recently demonstrated UV-curing and AM of proprietary resins to 27 29

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produce objects capable of tensile strains of 270%.10 Other promising resins in their study 31

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displayed strains of up to 1100%, although their high viscosity necessitates specialized 32 34

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equipment to print. It should also be noted that commercial elastomer (or flexible) photoresins 36

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are often proprietary, challenging to modify, and can require extra equipment to achieve desired 37 38

printing characteristics (e.g., custom heating elements to combat high viscosities). For advanced 39 41

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prototyping applications such as simulated human tissues, stimuli-responsive materials, wearable 43

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devices, and soft robotics, tunable resins of known compositions and suitable performance are 4 45

needed. 46 48

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Motivated by the potential versatility of DLP-AM and utility of elastomer build 50

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materials, we sought user-friendly and readily-accessible methods for integration of the two. 51 53

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Moreover, we aimed to stay within the visible spectrum for optical input and used white light 5

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projection for our studies. This approach may enable broader adoption of common equipment for 56 57 58 59 60 ACS Paragon Plus Environment

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AM applications, wavelength-dependent printing outcomes, and broadened functional group 5

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compatibility for resins in comparison with UV-based curing methods. Herein, we report our 6 8

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results from visible light DLP-AM with various elastomer photoresin formulations. 9 1

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EXPERIMENTAL SECTION 12 13

General Considerations. 1H NMR spectra were recorded on a Bruker AVance 500 MHz 14 16

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spectrometer. Chemical shifts are reported in delta (δ) units, expressed in parts per million (ppm) 18

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downfield from tetramethylsilane using the residual protio-solvent as an internal standard 19 20

(CDCl3, 1H: 7.26 ppm). Tensile elongation was conducted according to ASTM D638 using type 23

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V specimen samples. Testing was accomplished with an Instron 5585H Universal Testing 24 25

System equipped with a 50 N load frame, pneumatic grips, and Bluehill 3 software. Elongation 26 28

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was conducted at a 100 mm/min extension rate and an Instron 2663-821 Advanced Video 30

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Extensometer was used to track strain. To investigate strain rate dependence, alternating 31 32

elongations with rates of 100 mm/min and 10 mm/min were conducted in triplicate on the same 3 35

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sample. Light intensity of projectors used for vat photopolymerization was measured using an 37

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Extech Instruments light meter (model HD450). Durometer measurements were taken in 38 40

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triplicate and reported as averages using a PCE Instruments PCE-DD-A Shore A Durometer. 42

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Rheology measurements were taken on a TA Instruments Discovery HR-2 hybrid rheometer 4

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using a stainless steel 20 mm Peltier plate. Data was collected from a strain sweep test from 10 45 47

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50,000 Pa at 25 °C and an angular frequency of 6.28 rad/s. Storage and loss modulus data were 49

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compared near 0.1 % oscillation strain. DSC measurements were taken on a TA DSC 250. Two 50 51

heating and cooling cycles were conducted under a nitrogen atmosphere, and the first heating 52 54

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cycle was used to remove the thermal history of the sample. Heating cycles were conducted at a 56

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rate of 10 °C/min to 250 °C or 300 °C, depending on the resin formulation. Cooling cycles were 57 58 59 60 ACS Paragon Plus Environment

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done at a rate of 5 °C/min to -90 °C. All samples were placed in sealed Tzero aluminum pans, 5

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and had masses between 3 – 8 mg. GC/MS measurements were accomplished with a combined 6 8

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Hewlett Packard 5973 Mass Selective Detector and HP 6890 Series GC System using an Agilent 10

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7683 Series Injector. 1 12

Bis(propylacrylamide)poly(dimethylsiloxane) (PDMSDMAA). Methacryloyl chloride (22 13 15

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mmol) was added dropwise into a solution of bis(propylamine)poly(dimethylsiloxane) (10 17

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mmol) and anhydrous triethylamine (22 mmol) in CH2Cl2 (150 mL) in an ice bath. The reaction 18 20

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mixture was then stirred for 24 h, during which time the ice batch expired. The CH2Cl2 was then 2

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removed under reduced pressure, and then hexanes (200 mL) was added to the reaction mixture. 23 24

The solution was then filtered through a fritted glass funnel. The filtrate was then washed with an 25 27

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80/20 v/v mixture of brine and saturated aqueous sodium bicarbonate solution (3 × 100 mL). A 29

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centrifuge was used to separate the emulsion when necessary. The organic phase was 30 31

subsequently separated and then dried with anhydrous calcium sulfate. This solution was then 32 34

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filtered through a fritted funnel and concentrated under reduced pressure to yield a viscous clear 36

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liquid (83% yield). 1H NMR (500 MHz, CDCl3): 5.85 (s, 1H), 5.67 (s, 1H), 5.30 (s, 1H), 3.30 (q, 37 39

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J = 6.5 Hz, 2H), 1.96 (s, 3H), 1.56 (m, J = 8 Hz, 2H), 0.55 (m, J = 4.5 Hz, 2H), 0.07 (s, 198H). 41

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Extraction of Uncured Material. Two printed discs (r = 10 mm, h = 1 mm) of each material 43

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were measured volumetrically using digital calipers and weighed. One disc was placed in a 4 46

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Supelco small Soxhlet extraction apparatus (50 mL extractor capacity, extractor I.D. 30 mm, 125 48

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mL flask capacity, glass thimble) and successive extractions were carried out over 6 h with 49 50

CH2Cl2. The mass and volume of the disc were measured after the extractions were completed 51 53

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and % swelling was calculated. The extracts were concentrated under reduced pressure and the 5

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mass of nonvolatile extracts was recorded. The second disc was soaked in 20 mL of CH2Cl2 for 2 56 57 58 59 60 ACS Paragon Plus Environment

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h. After removal from the solution, the swelled volume and mass were recorded. The solution 5

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was then concentrated under reduced pressure and the mass of nonvolatile extracts was recorded. 6 8

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The extracted mixtures from each extraction method were then analyzed by GC/MS as described 10

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below. 1 12

GC/MS analysis of materials extracted from printed disc samples. The extracted, 13 15

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concentrated material was redissolved in 5 mL of CH2Cl2 and then 1 mL of that solution was 17

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used for GC/MS analysis. To the 1 mL solution was added 10 µL of a 0.03 M solution of 1,3,518 19

trimethoxybenzene in CH2Cl2 as an internal standard. GC/MS was accomplished with injection 20 2

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volumes of 1 µL, an initial oven temperature of 40 °C held for 1 min, ramped 15 °C/min to 300 24

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°C and held for 3.5 min. From the GC/MS data, we calculated the wt % of unreacted monomer in 25 27

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the total extract. Apparent oligomeric species were not differentiated from one another. (In 29

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addition to the soak solutions, standard solutions of HEA and EEEA and potential crosslinkers in 31

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HEA were also analyzed, specifically ethylene glycol diacrylate (EGDA) and diethylene glycol 32 34

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diacrylate (DEDGA). For each, 0.1 mL of the analyte was dissolved in CH2Cl2 using a 5-mL 36

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volumetric flask, and then 0.1 mL of this solution was serially diluted to a final concentration of 37 38

0.4 mg/mL. To a GC/MS sample vial were then added 1 mL of this solution and 10 µL of a 39 41

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0.03M 1,3,5-trimethoxybenzene solution in CH2Cl2, and the same method as described above 43

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was used to conduct the GC/MS analyses. EGDA was detected in the commercial HEA solution. 4 45

DEDGA was not detected in the HEA solution, but a peak was found with a mass that correlated 46 48

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to an oligomeric tetraethylene glycol diacrylate crosslinker. 50

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Additive Manufacturing. Specimens were printed using a SeeMeCNC Droplit DLP 3D 51 53

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Printer (the build and vat plates were modified to accommodate leveling, as described 5

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previously)25 and an Acer X1161P projector with standard as received settings. The light 56 57 58 59 60 ACS Paragon Plus Environment

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intensity of white projector light displayed into the vat was 80 klx, as determined by light meter, 5

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and this intensity was used for all prints with this projector. The multi-material gripper, gyroid 6 8

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lattice, and octet truss were printed using an Optoma HD20 with the UV filter removed and the 10

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brightness decreased to 40% (via projector menu). The printer settings for each photoresin and 1 12

the light intensities used for the multi-material prints are described in the Supporting 13 15

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Information. Layer cure times were chosen to provide objects with visibly good printability, 17

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without under-cure, overgrowth, or build vat adhesion issues. Creation Workshop (version 18 19

1.0.0.75) software was used to operate/control the printer and projector as well as convert 3D 20 2

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model files (STL format) constructed in house using Google Sketchup (version 17.1.174) into 24

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image stacks for printing. The build vat consisted of a Pyrex petri-dish (d = 90 mm) with a layer 25 27

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of silicone elastomer (ca. 11 g of silicone applied to dish). Prints were conducted by repeating 29

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the process of projecting an image into the resin followed by raising the z-stage (Figure S1 and 31

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S2). Post-print parts were subjected to an excess of white light at printing intensity for 30 32 34

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seconds per side, repeated twice, to polymerize unreacted monomer and remove effects of under36

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curing that may come from the printing process. 37 38 40

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RESULTS AND DISCUSSION 42

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We report herein a series of elastomer photoresin compositions compatible with visible 4

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light DLP-AM that yield soft, flexible, durable, and highly elastic materials. High elongations up 45 47

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to 472 % were achieved without the necessity for extra equipment or processing steps. The 49

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photoresins used to produce these materials also have beneficial characteristics for printability 50 51

using DLP-AM including stability under ambient atmosphere, short cure times for each layer, 52 54

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and low viscosities (Table S4). These features, while not necessary for DLP-AM, impede the 56

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conversion of many other bulk photopolymerizations toward DLP-AM processes. This makes 57 58 59 60 ACS Paragon Plus Environment

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finding the appropriate bulk photopolymerizations for DLP-AM non-trivial. 5 The resin 5

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formulations (Table 1 and Table S1) enable access to a range of hydrophilicities, spanning 6 8

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hydrogel to silicone elastomer materials, and result in objects of varied composition, mechanical 10

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properties, and swelling behaviors. Irgacure 819 was chosen as the photoinitiator for each resin 1 12

based upon its high reactivity, absorption in the visible light region, low cost, and low 13 15

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cytotoxicity.29 For some experiments, the relative amount of Irgacure 819 was adjusted. To 17

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denote the amount of Irgacure 819 photoinitiator used in a particular photoresin, we include the 18 19

wt % in parenthesis after the name of the resin. For example, ThrashOHflex (1) refers to 20 2

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ThrashOHflex with 1 wt % of Irgacure 819. 24

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Table 1. Monomer composition of photoresins studied. 25 26 27 28 29 30 31 32 3 34 35 36 37 38

photoresin

resin composition (by weight)a

wt % Irgacure 819

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PDMSDMAA

PDMSDMAA (95%), toluene (5%)

0.25

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SilOHflex

HEA (56%), diacrylatesb (6%), BA (27%), PDMSDMAA (9%), cetrimonium bromide (2%)

0.25

ThrashOHflex

EEEA (60%), HEA (36%), diacrylatesb (4%)

0.25 or 1

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43 4 45 46

HydrOHflex 47 48

a

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b

b

HEA (91%), diacrylates (9%)

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Irgacure 819 reported as a wt % based on total monomer. All other materials reported as % by weight. Amount of diacrylates present in HEA, based on GC/MS analysis.

We initially focused on silicone-based elastomers as we were inspired by their material 50 52

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properties and widespread utility. Representative thermally-cured examples include tough and 54

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flexible specimens produced from Dragon Skin 30 (364% elongation at break) and Sylgard 184 56

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(140% elongation at break).30,31 To enable DLP-AM, we prepared a polydimethylsiloxane 57 58 59 60 ACS Paragon Plus Environment

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dimethacrylamide (PDMSDMAA) oligomer having Mn = 4.5 kDa based upon 1H NMR analysis. 5

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Printing with this material was initially met with limited success as the PDMSDMAA swelled 6 8

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into the silicone lining used to coat the vat. This lining was removed, but attempts to print using 10

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uncoated glass surfaces resulted in strong adhesion of the cured layers to the glass, causing 1 12

detachment of the print object from the rising build stage. Previous studies have avoided these 13 15

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issues by using a PTFE-lined vat.32 Alternatively, we found biphasic printing to be successful 17

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(Figure 1).33 Specifically, brine was used as a bottom layer in the vat to separate the immiscible 18 19

PDMSDMAA from the bottom surface. The build stage was then positioned just above the 20 2

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liquid-liquid interface, which defined the upper and lower boundaries of the build layer, 24

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respectively. Printing of successive layers was found to proceed smoothly and give rise to 25 27

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optically transparent elastomeric specimens (Figure 2). 28 29 30 31 32 3 34 35 36 37 38 39 40 41 42 43 4 45 46 47 48 49 50 51 52 54

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Figure 1. Schematic of a biphasic liquid on liquid vat photopolymerization process in which the printable resin rests on top of a layer of an immiscible higher density liquid. 56

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We also found 2-hydroxyethyl acrylate (HEA) containing small amounts of 5

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oligo(ethylene glycol) diacrylates to be an advantageous resin component for DLP-AM. HEA is 6 8

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known to polymerize faster than many other monofunctional acrylates, partly because of its 10

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greater density and partly from a reduced termination rate from hydrogen bonding effects.34 This 1 12

rapid polymerization helps to counter the deleterious inhibition effects of the ambient oxygen 13 15

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present during printing. The hydrogen bonding in poly(HEA) also facilitates printing by yielding 17

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solid layers at lower conversions, which can correspond to shorter cure times, in comparison 18 19

with non-hydrogen bonding resins.34 Crosslinked materials were achieved largely due to small 20 2

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amounts of diacrylate species in the HEA stock. Specifically, we found that commercial HEA 24

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contained 0.7 mol % (based on peak area analysis) of condensed ethylene glycol diacrylate 25 27

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(EGDA) and 3 mol % of an oligo(ethylene glycol) diacrylate. Hydroxyalkyl acrylates can also 29

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undergo a chain-transfer mechanism involving hydrogen atom abstraction alpha to the hydroxyl 31

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group, yielding a carbon centered radical that can react and create a crosslink.34 These existing 32 34

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diacrylate crosslinkers in HEA, and the potential for chain-transfer induced crosslinking during 36

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polymerization, add structural stability and fixity to the printed materials made through 37 38

photopolymerizations of HEA-based resins. 39 40 41

We next evaluated combinations of PDMSDMAA with common monofunctional 42 4

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acrylates and found that PDMSDMAA could be formulated with HEA and butyl acrylate with 46

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the inclusion of cetrimonium bromide surfactant. The use of surfactant mitigated phase 47 49

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separation of the different polarity acrylates, enabling longer total print times as the resin 51

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remained homogenous (based upon visual inspection) for hours. This photoresin, which we refer 53

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to as SilOHflex (Table 1), offered convenient printing properties and signifies a broad scope of 54 56

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resin component classes that can be used to tune the materials properties of the printed objects. 57 58 59 60 ACS Paragon Plus Environment

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Unlike printing with PDMSDMAA directly, this resin did not swell into the silicone lining of the 5

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vat, and exhibited good printability. The benefits of HEA in this resin led us to explore resin 6 7

formulations that would give rise to hydrogel products upon DLP-AM.35–40 A combination of 10

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HEA and 2-(2-ethoxyethoxy)ethyl acrylate (EEEA) also yielded a promising elastomer resin 1 12

(ThrashOHflex). Finally, we investigated DLP-AM with HEA (referred to here as HydrOHflex) 13 15

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directly and observed good printability to give clear hydrogel materials. 17

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The resin compositions evaluated herein were found to be remarkably printable. As low 18 19

viscosity liquids, they can be printed at room temperature and we achieved relatively short layer 20 2

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cure times (6 – 24 s). In recognition of the volatility and toxicity of some resin components, 24

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printing was conducted in a hood or ventilated workspace. Layer cure times were optimized for 25 27

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each resin to be close to the minimum time required to achieve shape fixity, keeping other 29

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printing settings unchanged (settings listed in Supporting Information). Post-curing with white 31

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light was used to mitigate any differences in mechanical properties that may results from under32 34

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curing of layers during printing. The resins used are optically clear and could be printed without 36

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dyes, although we note that addition of dyes appeared to improve resolution – likely by reducing 37 38

the effect of light scattering. Specifically, when printing hollow structures and unsupported 39 41

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crossbeams we recommend addition of dyes. Our standard post-cure procedure, which involved 43

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approximately 3 min of illumination with white light, seemed effective at increasing conversion 4 45

and fixing the final structures. The extent of curing was found to exceed 99% for HydrOHflex 46 48

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and ThrashOHflex, given that less than 10 mg of uncured monomer was extracted per gram of 50

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solid printed material during soaking with a good swelling solvent. As the extent of curing of 51 53

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these materials is high, the mechanical properties of the objects appear to be mainly resin5

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motivated and not print-parameter motivated. Within an acceptable range (based on visual 56 57 58 59 60 ACS Paragon Plus Environment

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inspection) of times used to print these objects (in which the objects showed good shape fixity, 5

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with no under-curing, outgrowth, or adhesion issues with the object and the build vat), 6 8

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differences in layer cure times did not affect the mechanical behavior of the tensile specimens 10

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(Figure S6, S7). In the case of SilOHflex and PDMSDMAA, the extent of curing still exceeded 1 12

97% and 88%, respectively. It is important to note that resins that do not achieve high extents of 13 15

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curing are expected to have more noticeable differences when varying printing parameters. 16

A) A)

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B)

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C)

D)

3 34 35 36 37 38 39 40 41 43

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Figure 2. A) Photos of printed objects of different resin compositions overlaid on printed text (font = Times New Roman 11 pt). Numbers in parentheses indicate wt % of Irgacure 819 in the feed. B) Octet truss unit cell printed from SilOHflex, ruler units = cm, cell dimensions = 1.5 × 1.5 × 1.5 cm3, print time = 31 min. C) Gyroid lattice printed from HydrOHflex, ruler units = cm, lattice dimensions = 4 × 4 × 3 cm3, print time = 49 min. D) Gyroid lattice being twisted. 49

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Representative examples of printed objects are depicted in Figure 2. A qualitative 51 53

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assessment of the optical clarity of each resin can be made from the overlay with printed text 5

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(Figure 2A). ThrashOHflex, HydrOHflex, and PDMSDMAA each give visually clear and 56 57 58 59 60 ACS Paragon Plus Environment

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colorless specimens. SilOHflex stood out as offering less clarity, which we speculate arises from 5

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the greater disparity in resin component properties and potential microphase separation in 6 8

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comparison with the other resins. In addition to simple test specimen geometries, more advanced 10

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structures were also easily obtained. Figure 2B depicts an octet truss unit cell printed from 1 12

SilOHflex, and Figure 2C shows a gyroid lattice from HydrOHflex (video of manipulation of 13 15

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gyroid lattice in Supporting Information, Video S1). In general, each of the resins we evaluated 17

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were successful in printing each of our attempted geometries. 18 19

Test specimens printed from each photoresin formulation were evaluated by tensile 20 2

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testing, rheology, durometry, and swelling experiments. While not exhaustively explored, 24

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variations in photoinitiator amounts of 0.25 wt % and 1 wt % were evaluated for ThrashOHflex, 25 27

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an intermediate polarity resin within our series. These initiator concentrations were chosen as 29

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they are within the range of photoinitiator amounts typically used in vat photopolymerization. 31

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Representative data are depicted in Figure 3 and summarized in Table 2. 32 34

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A) 35

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Tensile Stress (MPa)

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PDMSDMAA HydrOHflex SilOHflex ThrashOHflex (1) ThrashOHflex (0.25)

1.2 1.0 0.8 0.6 0.4 0.2 0.0

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0 50

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1

2 3 Tensile Strain (mm/mm)

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B) 4 5 6

1

10

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8

Load (N)

7

12 13 14 15

4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0

16

0.0 17

0.5

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C) 0.30 20

Tensile Stress (MPa) 32

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2.5

3.0

XY

21 2

1.0 1.5 2.0 Tensile Strain (mm/mm)

0.25

XZ ZX

0.20 0.15 0.10 0.05 0.00

3

0 34 35 36

1

2 3 4 Tensile Strain (mm/mm)

5

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Figure 3. Representative tensile test results for printed specimens (ASTM D638 Type V, strain rate = 100mm/min, gauge length = 9.53 mm): A) Comparison of stress-strain curves from various photoresin compositions. B) Cyclic tensile elongation (total cycles = 5) of HydrOHflex (1). Specimens were elongated to 250% strain. The first cycle is shown in black and cycles 2 – 5 are shown in red. C) Comparison of stress-strain curves from tensile elongation of specimens of ThrashOHflex (1) printed in the XY, XZ, and ZX orientations. 45

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Table 2. Summary of properties determined by tensile elongation, rheology, and durometry.a 49

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material

σmax [MPa]c

εmaxd

Shore A hardness

G' [MPa]e

G" [MPa]f

PDMSDMAA (0.25) SilOHflex (0.25)b ThrashOHflex (0.25) ThrashOHflex (1)

0.58 ± 0.09 1.01 ± 0.03 0.42 ± 0.02 0.24 ± 0.01

0.51 ± 0.06 3.38 ± 0.08 4.72 ± 0.22 3.29 ± 0.07

23.0 ± 2.0 22.0 ± 1.0 13.7 ± 0.6 15.0 ± 2.4

0.174 ± 0.022 0.228 ± 0.047 0.025 ± 0.004 0.023 ± 0.001

0.007 ± 0.001 0.117 ± 0.029 0.003 ± 0.0006 0.004 ± 0.0004

tan 𝛿g 0.04 ± 0.02 0.51 ± 0.002 0.13 ± 0.01 0.19 ± 0.02

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HydrOHflex (1) 4

1.23 ± 0.31

3.48 ± 0.26

33.3 ± 0.6

0.218 ± 0.021

0.106 ± 0.014

0.49 ± 0.03

a

Values are averages of three trials and standard deviations are reported. Samples were prone to slip out of the pneumatic grips used during tensile testing before failing due to strain. The strain at slippage was used to approximate the strain at break for these samples and is therefore likely to be an underestimated value. c σmax is the ultimate tensile strength determined from ASTM D638 testing. d εmax is the elongation at break determined from ASTM D638 testing. e G’ is the storage modulus determined by rheology. f G” is the loss modulus determined by rheology. g tan 𝛿 = G’’/G’. 5

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b

We found that the hardness, tensile strength, and storage moduli each increased with 15 16

increasing crosslink density and degree of hydrogen bonding, but were expectedly low as the 17 19

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cured materials were above their glass transition temperatures (Tg) and thereby the materials 21

20

behaved rubbery (Table 3, Table S2, Figures S20-24). Shore A hardness values ranged from 2 23

13.7 – 33.3 (Table 2) and were tunable within this range. For comparison, ThrashOHflex resins 24 26

25

have hardness values comparable to human skin41 whereas SilOHflex, HydrOHflex, and 28

27

PDMSDMAA, are similar in hardness to a solid rubber ball.42 Tensile testing (Figure 3A) 29 31

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revealed a relatively low elongation to break for the PDMSDMAA materials (51%), leaving 3

32

considerable room for improvement compared to thermally cured silicone elastomers. These low 34 35

elongations in comparison with thermally-cured silicones are most likely ascribed to the high 36 38

37

degree of chain-end chemical crosslinking in the photocured PDMSDMAA (Table S2). In 40

39

contrast, the other photoresin formulations produced test samples showing 329 – 472% 41 42

maximum strains (see SI for all tensile data). These latter materials each experience various 43 45

4

degrees of hydrogen bonding and covalent crosslinking, based upon the specific monomers used 47

46

in the formulation. SilOHflex and HydrOHflex were each found to have similar Mc values (see 48 50

49

Supporting Information), which likely dominates the ultimate strength of the test specimens. 52

51

Consistent with this, these two resins produced tensile test specimens with nearly identical stress53 54

strain curves. ThrashOHflex has both a lesser amount of covalent crosslinking (greater Mc value) 5 57

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and less physical crosslinking via hydrogen bonding. As a result, we observed the greatest 58 59 60 ACS Paragon Plus Environment

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elongation to break from ThrashOHflex while also observing the lowest strength within the 5

4

series. Maximum strain and tensile strength may also be sensitive to the amount of photoinitiator 6 8

7

used (comparing ThrashOHflex with 0.25 and 1 wt % Irgacure 819), but further studies are 10

9

needed to confirm this. Reducing the photoinitiator concentration in ThrashOHflex to 0.25 wt % 1 12

resulted in a longer elongation to break. 13 15

14

Table 3. Glass transition temperatures of compositions based on DSC analysis. 16

Resin ThrashOHflex (1) ThrashOHflex (0.25) HydrOHflex (1) SilOHflex (0.25) PDMSDMAA (0.25) 17 18 19 20 21 23

2 a

24

Tg ( °C) -30 -29 12 2 < -90a

A Tg was not visible above -90 °C, and is expected at colder temperatures.

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Each of the resins appeared to result in elastomeric printed specimens based upon 26 28

27

repeated tensile loading. Representative quantitative data for a printed HydrOHflex sample is 30

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depicted in Figure 3B. The results show very little hysteresis and no sign of creep or fatigue 31 3

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after 5 cycles, indicating a highly recoverable elastic response. This is relevant for soft robotics 35

34

and wearable devices where it is desirable to have controlled and repeatable deformation at high 37

36

frequency.17,23,37,43 Notably, this rapid recoverability is distinct from many other hydrogen 38 39

bonding materials, which often require a delayed “reset” period to allow for re-equilibration and 42

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40

obtain full property recovery.10,35,39,40 Similar results were obtained for a printed ThrashOHflex 43 4

sample, although some temporary deformation is visible (Figure S15). Consistent with cyclic 45 47

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tensile loading and qualitative macroscopic behavior, rheological experiments revealed G’ to be 49

48

greater than G” for each material (Table 2). Moreover, tan δ values (a measure of energy 50 51

dissipation efficiency) spanned one order of magnitude, from 0.04 to 0.51, signifying broad 52 54

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tunability of elastic response. The dynamic hydrogen bonds formed by HEA account for the 56

5

higher energy dissipation capabilities of SilOHflex and HydrOHflex, as well as their more 57 58 59 60 ACS Paragon Plus Environment

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viscoelastic tensile behavior. The viscoelasticity of HydrOHflex, SilOHflex, and ThrashOHflex 5

4

makes the stress-strain curves of these materials strain-rate dependent, with each materials’ 6 8

7

strength appearing higher at a higher strain rate (Figure 4, Table 4, Figure S17-19). At fast 10

9

strain rates, rates faster than the dissociation rate of the hydrogen bonds, higher mechanical stress 12

1

is necessary to dissociate the physical crosslinks.44 This results in an increase in network 13 15

14

modulus and toughness. Materials printed using PDMSDMAA did not exhibit a strain-rate 17

16

dependence as the polymer is elastic. 18 19 20 21

0.45 2

0.40

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Tensile Stress (MPa)

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0.35 0.30 0.25 0.20 0.15 0.10 0.05

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0.00 36

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0.0 39

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0.2

0.4

0.6 0.8 1.0 Tensile Strain (mm/mm)

1.2

1.4

1.6

Figure 4. Tensile test results for a printed HydrOHflex (1) specimen (ASTM D638 Type V, gauge length = 9.53 mm) taken to 150% strain at a rate of 100 mm/min (black), then taken back down and testing repeated at a strain rate of 10 mm/min (red). Graph shows this process done in triplicate on the same sample. 45

4

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Table 4. Percent difference between tensile stresses at 150% strain for HydrOHflex (1), SilOHflex (0.25), and ThrashOHflex (1) varying strain rate between 100 mm/min and 10 mm/min. Data for PDMSDMAA taken at 10% elongation. Values are an average of three tests at each strain-rate on the same sample. material tensile stress: 100 tensile stress: 10 % difference mm/min (MPa) mm/min (MPa) HydrOHflex (1) 0.388 ± 0.001 0.356 ± 0.001 8.7 SilOHflex (0.25) 0.410 ± 0.005 0.392 ± 0.004 4.5 ThrashOHflex (1) 0.116 ± 0.002 0.104 ± 0.001 10.8 PDMSDMAA (0.25) 0.191 ± 0.002 0.190 ± 0.002 0.25 57

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We also evaluated the effects of printing orientation on tensile stress-strain behavior 5

4

(Figure S8-10, Table S3). Many AM methods produce objects with anisotropic properties 6 7

dependent upon the print orientation used during fabrication.45,46 For example, differences in 10

9

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materials properties can emerge from incomplete interdigitation between layers during printing 1 12

processes. Achieving uniform properties throughout complex geometries is an area in which AM 13 15

14

tends to underperform when compared to traditional manufacturing methods. We chose 17

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ThrashOHflex (1) as a representative resin to evaluate the effects of layering during DLP-AM 18 19

(Figure 3C). We found that the tensile characteristics of printed dogbones of ThrashOHflex (1) 20 2

21

were largely independent of printing orientation, although XZ and ZX samples each achieved 24

23

greater elongation to break than XY samples (Table S3). Of particular note, samples printed in 25 27

26

the ZX orientation with layers perpendicular to the axis of elongation displayed the greatest 29

28

elongation to break within the series. This suggested to us that newly formed layers were not 31

30

completely cured and were capable of absorbing additional resin. In this way, interdigitation 32 34

3

could be achieved across layers. The samples printed in the ZX direction had the longest print 36

35

times, and thereby may have been able to cure to a greater extent than the other orientations, 37 38

increasing the effect of this interdigitation. 39 41

40

The disparate hydrophilicities of the resin materials suggested to us that the resulting 43

42

printed objects would display varied swelling behavior. To investigate, we printed 10-layer discs 4 45

(r = 10 mm, h = 1 mm) and recorded their swelling characteristics in water (Table 5). As 46 48

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expected, the silicone-containing resins displayed the least volumetric change, with 50

49

PDMSDMAA-based specimens showing essentially no swelling (ΔV = 1.01). ThrashOHflex and 51 52

HydrOHflex each showed considerably higher values of ΔV upon swelling in water, with 53 54 5 56 57 58 59 60 ACS Paragon Plus Environment

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ThrashOHflex exhibiting the highest increase in volume of the different formulations, while 5

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HydrOHflex exhibited the greatest increase in mass. 6 7

Table 5. Summary of swelling data.a 10

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material mass increaseb volume increasec PDMSDMAA (0.25) 1.01 ± 0.01 1.01 ± 0.01 SilOHflex (0.25) 1.55 ± 0.06 1.64 ± 0.20 ThrashOHflex (0.25) 2.98 ± 0.03 3.23 ± 0.07 ThrashOHflex (1) 2.85 ± 0.01 2.93 ± 0.23 HydrOHflex (1) 3.35 ± 0.07 2.84 ± 0.73 17

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1

a

Swelling behavior for printed discs with radius = 10 mm and height = 1 mm. Specimens were submerged in deionized water for 24 h, and then removed and patted dry. All values are an average of three trials, errors = 1 standard deviation. b Masses were recorded on an analytical balance. cVolumes were calculated based upon dimensions measured using a digital caliper. 21

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We also investigated the swelling behavior of our materials in dichloromethane, which 23 25

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also served to extract unreacted (or unbound) resin. To verify that all unreacted resin was 27

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extracted, both simple soaking (Table 6, Method A) and more rigorous Soxhlet extraction 28 29

(Method B) were performed and compared. In dichloromethane, ThrashOHflex exhibited a 30 32

31

significantly larger degree of swelling than all other formulations with increases in mass and 34

3

volume over 1000 %. HydrOHflex, by comparison, exhibited the lowest amount of swelling, 35 37

36

with increases in volume and mass around 140%. The disparate densities of HydrOHflex and 39

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ThrashOHflex in water, and the disparate swelling responses of the two materials in 41

40

dichloromethane, may be attributed to the greater presence of hydrogen bonding in HydrOHflex, 42 4

43

although we would like to emphasize that printing conditions and cross-link densities will also 46

45

impact the swelling behavior of printed products. The degree of swelling of these resins in both 47 48

water and dichloromethane suggest that these materials may not only be useful as hydrogels, but 49 51

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also as organogels. 53

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Table 6. Quantification of extractable monomer after printing for each photoresin composition in DCM. Method A corresponds to simple soaking for 2 h. Method B corresponds to Soxhlet extraction for 6 h. 57

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resin 6

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extraction method

% swell by volume

% swell by mass

mass of extract (mg)

wt % of object extracteda

SilOHflex (0.25) A 244 205 19 6.3 SilOHflex (0.25) B 219 202 20 6.9 HydrOHflex (1) A 124 129

Modular Elastomer Photoresins for Digital Light Processing Additive

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