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Production of Materials with Spatially-Controlled Crosslink Density via Vat Photopolymerization Gregory I. Peterson, Johanna Jesse Schwartz, Di Zhang, Benjamin Weiss, Mark A. Ganter, Duane W. Storti, and Andrew J Boydston ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b09768 • Publication Date (Web): 29 Sep 2016 Downloaded from http://pubs.acs.org on October 10, 2016

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ACS Applied Materials & Interfaces

Production of Materials with Spatially-Controlled Crosslink Density via Vat Photopolymerization

Gregory I. Peterson1, Johanna J. Schwartz1, Di Zhang2, Benjamin M. Weiss2, Mark A. Ganter2, Duane W. Storti2*, Andrew J. Boydston1*

1

Department of Chemistry, University of Washington, Seattle, WA, 98195, United States

2

Mechanical Engineering, University of Washington, Seattle, WA, 98195, United States *

e-mail: [email protected], *e-mail: [email protected]

1 ACS Paragon Plus Environment


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Abstract

We describe an efficient method to produce objects comprising spatially controlled and graded crosslink densities using vat photopolymerization additive manufacturing (AM). Using a commercially available diacrylate-based photoresin, 3D printer, and digital light processing (DLP) projector, we projected grayscale images to print objects in which the varied light intensity was correlated to controlled crosslink densities and associated mechanical properties. Cylinder and bar test specimens were used to establish correlations between light intensities used for printing and crosslink density in the resulting specimens. Mechanical testing of octet truss unit cells in which the properties of the crossbars and vertices were independently modified, revealed unique mechanical responses from the different compositions. From the various test geometries, we measured changes in mechanical properties such as increased strain-to-break in inhomogeneous structures in comparison with homogeneous variants.

Keywords: Additive Manufacturing, Graded Materials, Vat Photopolymerization, Functionbased Representations, Voxel Model


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Introduction Gradation in property or composition can produce significant enhancements in materials performance capabilities. For example, naturally occurring graded materials such as teeth, palm stems, and squid beaks display exceptional mechanical strength and toughness. This robust combination of properties manifests from materials compositions that enable a physical transition from low strength (soft) to high strength (hard) regions.1,2,3 Controlling gradation in synthetic structures could allow for tremendous advances in the development of metamaterials and functional devices.4,5,6 Current manufacturing techniques that are able to synthesize bulk graded materials are often limited by multiple steps in the build process, a narrow scope of output geometries and build materials, difficulty with precision in reproduction, and comparatively expensive or inaccessible equipment.7,8 A promising avenue for accessing graded object properties across a broad materials space, and with sophisticated geometries, is to further develop the capabilities of additive manufacturing (AM) or 3D printing (3DP). Research in this field continues to provide exciting breakthroughs for accessing complex and functional objects in a single printing procedure, with gradation of molecular, nanoscale, and microscale structure receiving considerable attention.9,10,11,12,13,14,15,16,17,18,19 Approaches to achieving property gradation via AM generally involves incorporating or blending multiple materials with varying properties,20,21,22,23,24 or varying the geometry along microscale regimes to impart graded porosity or density. 25,26,27 At smaller length scales, molecular engineering is achieved by controlling the chemical composition of the material during the print. An inspiring approach that has been demonstrated is to use photo-resin based AM processes that incorporate electronic dynamic masks. Masks, such as a liquid crystalline display (LCD) or digital micromirror device (DMD) chip, enable printing of entire layers at once that vary twodimensionally in grayscale light intensity.28,29 This incorporation of grayscale has been



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successfully demonstrated for fabrication of micro electromechanical systems (MEMS) and tissue

engineering

through

micro-stereolithography

and

photolithography.30,31,32,33,34

Particularly noteworthy to us was the use of projection micro-stereolithography to make sacrificial supports, where lower light intensity areas resulted in faster etch rates in the final objects.31 Direct correlation between graded molecular level structure and bulk properties are key to realization of the full potential of this approach. Inspired by these existing studies, we sought methods in which light intensity could be used to specifically control molecular-level structuring, such as crosslink density. Using an openly modifiable and readily available vat photopolymerization system could be broadly applicable to a number of photo-active materials. Moreover, vat photopolymerization involves conversion of monomeric building blocks into macromolecular structure during the printing process, thus enabling a bottom-up fabrication method that begins with molecular-scale building blocks and is directly influenced by the printing parameters. The ability to efficiently design and control gradation in the output properties of a structure through simple control of input energy has not yet been fully realized with current additive manufacturing techniques. By identifying the changes in crosslink density and their effect on mechanical properties of grayscale light intensity vat photopolymerized structures, we demonstrate the ability to achieve molecular-level control of the mechanical responses of bulk materials and guide anisotropic dynamic mechanical behavior.35 Experimental Methods Materials: The photo-resin G+ Yellow was obtained from MakerJuice and was used in all prints. Isopropyl alcohol (90%) was obtained from a retail store. The silicone elastomer (Cell Guard) was acquired from ML Solar as a two part kit (1:10 of Part B to Part A by mass).


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Vat Photopolymerization: Objects were printed using a SeeMeCNC Droplit DLP 3D Printer (the build and vat plates were modified to accommodate leveling, see Supplementary Information, NOTE: modifications are not necessary to print graded/heterogeneous objects) and an Acer P1500 projector with the brightness decreased to 25% (via projector menu). Creation Workshop (version 1.0.0.45) software was used to operate/control the printer and projector. The build vat consisted of a Pyrex petri-dish with a layer of UV-penetrable silicon elastomer (ca. 11 g of silicon applied to dish). Prints were conducted by repeating the process of projecting an image into the resin followed by raising the z-stage (Figure S1 and S2). The following print parameters were used: Layer height, 0.1 mm; exposure time, 12 s; bottom exposure time, 12 s; number of bottom layers, 1; z lift distance, 1 mm; z lift speed, 60 mm/min; z retract speed, 100 mm/min; slide tilt value, 0. Post-print parts were submerged briefly (less than one minute) in 90% isopropyl alcohol to remove residual resin from the surface, and then air dried. Other post-print procedures were also evaluated and found to have little impact on the specimen specific gravity and rubber plateau modulus, and thus little impact on calculated Mc values (see Supplementary Information).

Projector Intensity Calibration: Light intensity was measured using an Extech Instruments light meter (model HD450). Full screen images with the desired red/green/blue (RGB) values (as used in the printed object images) were projected into the empty polymerization vat. The light sensor was placed directly onto the silicon layer. The maximum and average light intensity was measured in three independent trials (see Supplementary Information).

Printable Files: Creation Workshop was used to slice a 3D homogeneous model file (e.g. STL file format) to produce a printer control file and associated stack of black and white images. To incorporate gradation, we replaced the set of black and white image with a set of



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grayscale images that were produced using custom in-house software (see below). After replacing the image stack, we executed the printer control file to fabricate the desired object.

Image Stack Generation: Voxel discrete function-based representation (F-rep) of the 3D objects were generated with software developed in-house.36,37,38 The geometry of the crossbar components of the truss structure specimens correspond to cylinders with spherical end caps or a union of two spherically-capped frustum cones. The crossbar geometry was described implicitly using functions sampled on a discrete voxel grid. The inhomogeneous crossbar model consists of the F-rep description of the geometry together with a material function that specifies a local grayscale light intensity. The truss construction was accomplished by applying a union operation to the set of crossbars by comparing the implicit geometry function values at each voxel in the working volume. The completed model of the truss was exported as a stack of grayscale images that replaced the black and white image stack in Creation Workshop.

Material Properties Characterization: Single cantilever testing was completed on a PerkinElmer dyamic mechanical analysis (DMA) 8000 machine. Single cantilever tests were conducted, with a clamp spacing of 17 mm, by applying a constant strain of 0.030 mm with a force multiplier of 1.5 and measurements taken at 1 Hz. Samples were held at 24 °C for 1 min, and then heated to 200 °C at a rate of 2 °C/min. Dimensions of each specimen were measured with calipers prior to mechanical testing. For determination of crosslink density of test bars printed at 80 and 200 klx, 20 specimens were printed at each light intensity, analyzed indepedently, and averaged. Thermomechanical analysis (TMA) was conducted on a Perkin Elmer TMA by applying a static force of 50 mN using a parallel plate-disc measuring system. The samples, 10 mm cylinders with diameters of 3 mm, were printed at


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each light intensity (80 klx and 200 klx) and were run in triplicate and measured from 25 to 200 °C at 2 °C/min. Compression testing of printed specimens was on an Instron 5500R load frame with a 100 kN load cell controlled using Bluehill 3.0 software. All tests were performed in triplicate using a crosshead rate of 1.3 mm/min. Compression tests for cylinders were stopped when a strain of 0.2 was reached. Compression tests for trusses were stopped after the first fracture event.

Density Calculations at Room Temperature and 40 °C above the Tg: Initial densities were calculated using rectangular bar samples and the simple mass over volume equation. These results were further verified using the Archimedes principle and a standard water displacement procedure.39 The difference between these two techniques was less than 1%, so simple volume calculations were deemed sufficient for higher temperature density calculations. For crosslink density calculations, the density at the temperature 40 °C above the average Tg for each light intensity was calculated using the linear expansion coefficient derived from thermomechanical analysis. The general equation used for expansion of each axis dimension (x, y, z, and in our case roughly 30 mm × 12 mm ×3.3 mm respectively) is: = + ( × × ∆)

where is the size of object dimension in millimeters at temperature T, is the original

size, is the linear expansion coefficient, and ∆ is the difference between the ascribed T and room temperature. The newly calculated dimensions were multiplied to get the expanded volume. Assuming the mass of the object stays the same, the expanded volume was used to calculate the new density for crosslink density calculations discussed in the results. Interpolation of Single Cantilever DMA Results: Using Microsoft Excel, each DMA run was linearly interpolated in order to get values with discrete temperature intervals of 0.5 °C. A moving average with a kernel size of 0.5 °C was also used to further improve the curve 7 ACS Paragon Plus Environment


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approximation. These 20 interpolated data tables for each sample were then averaged. When looking at the rubber plateau region, no moving average was used, and the interpolation increment was reduced to 0.1 °C to include more of the data.

Results To explore how the intensity of light influences the materials properties of printed parts, a series of cylindrical specimens were printed with different light intensities and constant image exposure times. We used a commercial acrylate photo-resin consisting of diacrylates, a photoinitiator, and an inorganic yellow pigment. This photo-resin can be initiated with wavelengths up to 440 nm and is compatible with the visible light output from the projector (Figure S5). Using 12-second exposure times a serviceable range of intensities was identified within which successful curing of 100 micron layers was observed. Specifically, a lower boundary of intensity near 50 klx was identified, below which specimens did not print, or did so with significant defects. On the high intensity end, heating of the resin appeared to be the limiting factor above 200 klx. Thus, initial test prints were performed by varying the intensity between 50 and 200 klx (Figure 1). Compression testing revealed that the elastic modulus and offset compressive yield strength each increased ca. 3-fold in going from lowest to highest light intensity (Figure 2). These results were consistent with greater extents of curing and crosslinking. With the exception of the cylinder printed at 50 klx intensity, the density of the cylinders were each within 2% of one another (Figure S6), indicating to us that the same amount of resin was being incorporated into the part and the differences in materials properties were likely due to varying degree of crosslinking. We thus chose to use light intensities of 80 klx or greater for subsequent prints to study the mechanical differences of samples with the same density in further testing. Overall, these results provided us with initial


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print parameters for exploring production of heterogeneous and graded objects using grayscale light exposure.

Figure 1. Images used to prepare cylinders (corresponding light intensity indicated below the image) and depiction of the image stack and the printed object.

Figure 2. Elastic modulus (black) and offset (0.2%) compressive yield strength (red) with changing light intensity. Error bars represent ± one standard deviation. To determine the correlation between light intensity and crosslink density, single cantilever DMA testing was done on multiple rectangular bars (30 × 12 × 3.3 mm) printed at 200 klx and 80 klx light intensities. At each of the light intensities, 20 individual beams were printed and analyzed; results from the sample averages are presented in Figure 3. As expected, samples printed at 200 klx light intensity had a higher overall storage modulus (934 MPa at 31 °C) and glass transition temperature (Tg = 81 °C) in comparison with samples printed at 80



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klx (578 MPa at 31 °C and a Tg of 64 °C). Notably, increases in storage modulus and Tg of a material are also consistent with increases in crosslink density.40 Importantly, the printed specimens were only briefly rinsed with isopropyl alcohol solution to clean their surfaces. Therefore, uncured monomer and network defects cannot be ruled out as contributing to the observed mechanical properties. Although the uncured resin is soluble in isopropyl alcohol solution, this solvent does not swell the cured resin. Our analyses revealed little to no impact of uncured resin, isopropyl alcohol, or drying conditions on the Tg or elastic storage modulus (glassy and rubber plateau regions). This indicated to use that the mechanical properties determined by DMA are primarly ascribed to changes in crosslink density between the specimens printed at different light intensities (see Supplementary Information). To estimate crosslink density in these samples, we assessed the rubber plateau region at 40 °C above each specimen’s Tg. 41 Using this rubber plateau region, it is possible to estimate the crosslink density of the material using the equations: 42,43

(1)

(2)

=

=

where Mc is the molecular weight of the polymer segments between crosslinks, is the density of the specimen, R is the gas constant, T is the absolute temperature, Eg is the storage modulus at the ascribed absolute temperature, and Vc is the crosslinking density.


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1000 Storage Modulus (MPa)

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100

10

1 30

60

90 120 150 Temperature (°C)

180

210

Figure 3. Average interpolated storage moduli of rectangular sample bars printed at 200 klx (•) and 80 klx (•) light intensity. Data points are an average of 20 runs, error bars represent ± one standard deviation. In order to estimate the density of the material in the rubber plateau region, thermomechanical analysis (TMA), was used to determine the linear thermal expansion coeffecient for the samples printed at 200 klx and 80 klx light intensities. It was found that the linear thermal expansion coefficient for each sample set was the same at 0.0002 mm/mm °C (Figure S7). This indicated that the density of the specimens was not dependent on the light intensity used during printing within this range of light intensities. Given that plasticization did not seem to be affecting this resin, the difference in storage moduli could then be primarily attributed to the differences in crosslink density. Based on the rubber plateau moduli (Figure 4), the Mc values for samples produced with 200 and 80 klx intensities were found to be 158 and 218 Da, respectively. These values correspond to estimated crosslinking densities of 7.25 × 103 mol m-3 and 5.29 × 103 mol m-3, respectively, signifying a 32% difference in crosslink density via straightforward modulation of the printing light intensity.



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Figure 4. Average storage modulus in the rubber plateau region, interpolated at 0.1-mm increments without a moving average. Data points are an average of 20 runs, error bars represent ± one standard deviation. Arrows designate temperature 40 °C above the Tg at which crosslink densities were compared (121 °C for 200 klx (•) and 104 °C for 80 klx (•)).

We next focused on incorporating graded crosslink densities throughout an object. Toward this end, cylinders with a light intensity gradient along the z-axis were prepared by printing a series of disc-shaped images using light intensities that were progressively lowered from 200 klx to 80 klx via 0.8-klx increments. Interestingly, the diameter of the cylinder was found to decrease as the intensity of light decreased, consistent with lesser extents of light scattering and outgrowth. To address this issue, the correlation between light intensity and geometric deviations was quantitatively determined. Specifically, a series of test parts of varying sizes and light intensities were printed (see Supplementary Information) and a calibration curve was generated (Figure 5) and used to correct for the discrepancies being observed. Between 80 and 200 klx intensities, a linear correlation was observed between light intensity and deviation in the printed object diameter in the x- and y- axes. Thus to print a graded 5 mm diameter cylinder, the diameter of the projected discs was linearly increased up to 5.25 mm for the final 80 klx image. This resulted in a graded cylinder with constant diameter. It is important to note that the calibration curve is expected to be geometry specific.


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Figure 5. Difference in x- and y- axes diameter dimensions of samples printed at lower light intensities in comparison to the specimen printed at 200 klx (see Supplementary Information for details). The R2 for the linear regression line is 0.9777. Having established a working range for light intensity and correlations between projected versus printed dimensions, we next aimed to explore gradation within more advanced geometries. Recognizing the potential to make intricate objects that are difficult to fabricate through other manufacturing means (e.g., injection molding), an octet truss unit cell structure was chosen. The octet truss compensates for mechanical loading via a complex distribution of stresses and strains upon anisotropic compression.45,46 We observed that the first fracture event upon compression of the truss specimens occurred within the four central beams on the equatorial plane (Figure 6), consistent with previous theoretical work.46 This suggested to us that strengthening the central beams might be advantageous to increasing the compressive strength of the truss. Moreover, we aimed to assess how heterogeneity and gradation within the truss might each impact the overall mechanical properties.

Figure 6. The truss structure (left) prior to compression and (right) under load showing the first fracture event. Each voxel-based truss structure model consisted of two parts: 1) a voxel set corresponding to a regular sampling of a function-based representation (referred to herein as an F-rep) 13 ACS Paragon Plus Environment


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describing the geometry,36 and 2) an appended material function to describe the local light intensity. The F-rep of a crossbar building block (cross-section depicted in Figure 7) for an octet truss is given by: = min (max #

$ %& |)| + $ 1, |)| $ *- , () $ *)' + ' $ %& ' , () + *)' + ' $ %& ' ) %' $ %& *

where ρ is the distance from the z-axis, L is length of the crossbar, R1 is the radius of the spherical caps, and R2 is the radius at the center of the crossbar.

Figure 7. Diagram of the cross-section of a crossbar for an octet truss unit cell.

Initially, we modeled an octet truss with cylindrical crossbars (R1 = R2) and graded light intensity specified by the material function: . = 0.7 + 0.3

|3|

456

. The geometry of the

cylindrical crossbar is shown in Figure 8A. The graded crossbar is shown in Figure 8B with blue and yellow shading represent light intensities from 80 to 200 klx. Union of crossbars yielded the graded octet truss model as shown in Figure 8C. The octet truss model was then converted into a grayscale image stack (Figure 8D) for export to the printer to produce the octet truss (Figure 8E).


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Figure 8. Generalized schematic for the modeling of a graded octet truss. (A) Cylindrical, homogeneous crossbar created by two unioned cones with R1 = R2 (see Figure 7). (B) Graded crossbar. (C) Graded octet truss created by unioning graded crossbars. (D) Representative grayscale image stack components. (E) Printed octet truss without empirically guided adjustments to R2.

As can be seen in Figure 8E, as-printed truss exhibited geometric discrepancies in the diameters of the component crossbars when compared with the design model (similar to our previous experiments with printing graded cylinders). Therefore, modified crossbar models (Figure 9A) were created with R2 > R1 and used to construct a modified octet truss (Figure 9B). The resulting image stack (Figure 9C) appropriately compensates for the geometric discrepancies by increasing crossbar thickness in regions of lower light intensity. This provided a systematic means (adjusting the ratio of R2/R1) by which graded octet truss structures were produced with uniform crossbar thickness (Figure 9D).

Figure 9. Generalized schematic for modeling and printing an altered octet truss. (A) Graded crossbar with R2 > R1. (B) Graded octet truss created by unioning graded crossbars. (C) 15 ACS Paragon Plus Environment


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Representative grayscale image stack components. (D) Printed octet truss with empirically guided adjustments to R2. Introducing gradation and spatial heterogeneity into the design of a series of octet truss structures (Figure 10A) revealed significant impacts on the mechanical properties of the printed objects. Homogeneous trusses were produced using 200 and 80 klx light intensity throughout (OT-1 and OT-2, respectively), whereas graded compositions were introduced such that either joint regions (OT-3) or center points within crossbars (OT-4) corresponded to low light intensity. Additionally, a heterogeneous truss structure (OT-5) in which the four equatorial crossbars were produced using 200 klx intensity and the rest of the truss was printed with 80 klx intensity was prepared and analyzed. The trusses were each subjected to compression testing using an Instron load frame, and representative engineering stress/strain curves are shown in Figure 10B (data summarized in Table 1). As expected, OT-1 (having the greatest crosslink density throughout the truss) exhibited higher elastic modulus, yield strength, and compressive strength in comparison with the less crosslinked homogeneous truss OT-2. Graded trusses OT-3 and OT-4 displayed intermediate values for these properties, and it appeared that lowering the crosslink density in joint regions (OT-3) offered some advantage over the graded composition of OT-4. Interestingly, analysis of the strain-to-break for each truss indicated to us that this property could be uniquely enhanced by judicious gradient or heterogeneous compositions. Specifically, OT-3 and OT-5 exhibited the highest strain-to-break values across all trusses in our series. In the case of OT-5, this was particularly noteworthy to us considering that this heterogeneous truss displayed lower elastic modulus, yield strength, and compressive strength than all other trusses in the series. The behavior of OT-3 and OT-5 indicates that strengthening the break points in the truss (crossbar beams) while maintaining greater flexibility in the joints can afford discernable increases in toughness. Collectively, the results demonstrate how simple modifications to AM techniques


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can be used to produce graded and heterogeneous structures with tunability and enhancement of mechanical properties in comparison with homogeneous materials.

3.0 Engineering Stress (MPa)

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2.5 2.0 1.5

OT-1 OT-2 OT-3 OT-4 OT-5

1.0 0.5 0.0 0.00

0.02

0.04 0.06 0.08 0.10 0.12 Engineering Strain (mm/mm)

0.14

Figure 10. (top) Models of homogeneous, graded, and heterogeneous truss structure designs, and the F-rep constituents. Note that the blue, 80 klx light intensity cylinder is larger than yellow, 200 klx light intensity cylinder. (bottom) Representative engineering stress/strain curves for all trusses. Black = 200 klx, OT-1; Red = 80 klx, OT-2; Purple = OT-3; Green = OT-4; Blue = OT-5.

Table 1. Material properties from engineering stress/strain curves for each truss structure. Values are an average of three independent compression tests ± one standard deviation. OT-1

OT-2

OT-3

OT-4

OT-5

Elastic Modulus (MPa)

63.29 ± 1.26

37.28 ± 1.04

49.11 ± 0.72

42.24 ± 4.39

32.55 ± 2.97

Offset Compressive Yield Strength (MPa)

1.98 ± 0.03

1.02 ± 0.10

1.38 ± 0.02

1.20 ± 0.17

0.97 ± 0.09

Compressive Strength (MPa)

2.88 ± 0.049

1.68 ± 0.011

2.30 ± 0.029

1.84 ± 0.24

1.64 ± 0.16

Stain-to-break (mm/mm)

0.097 ± 0.016

0.079 ± 0.002

0.11 ± 0.01

0.076 ± 0.010

0.12 ± 0.01

Conclusion In conclusion, we have demonstrated 3D printing of graded and heterogeneous structures by using gradient light intensities in a commercial resin and DLP printing system. We were able to relate and quantify the change in crosslinking density as a factor of light intensity to selectively tune the materials properties of printed objects by changing the projected light 17 ACS Paragon Plus Environment


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intensity. In certain embodiments, this led to a threefold increase in elastic modulus and offset compressive yield strength of the material by increasing the intensity from ca. 50 klx to ca. 200 klx. The impact of heterogeneous and graded materials was demonstrated by increasing the strain-to-break of octet truss structures to a value greater than that achieved by uniformly increasing the light intensity.

Acknowledgements We gratefully acknowledge financial support from the University of Washington, the College of Engineering Strategic Research Initiative (University of Washington), the Washington Research Foundation, Research Corporation for Science Advancement, the National Science Foundation (DMR-1452726), and the Army Research Office (Grant No. W911NF-15-10139). This material is based upon work supported by the National Science Foundation Graduate Research Fellowship Program (JJS and BMS) under Grant No. DGE-1256082. We would also like to acknowledge the University of Washington Student Technology Fee and the MSE User Facility. We are grateful to Dr. Daniel B. Knorr (U.S. Army Research Labs) for helpful discussion regarding DMA of test bars. Competing Financial Interests The authors declare no competing financial interests. Supporting Information Supporting information includes diagrams of digital light processing 3D printer, the printing process, moduli data for different post-processing conditions, projector emission spectrum data, density results with varying light intensity, thermomechanical analysis data, 3D printer modifications, intensity-object dimensions calibration, and projector intensity calibration Author Contributions


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