Formulation of a Model Resin System for Benchmarking Processing-Property Relationships in High-Performance Photo 3D Printing Applications

A well-defined resin system is needed to serve as a benchmark for 3D printing of high-performance composites. This work describes the design and characterization of such a system that takes into account processability and performance considerations. The Grunberg–Nissan model for resin viscosity and the Fox equation for polymer Tg were used to determine proper monomer ratios. The target viscosity of the resin was below 500 cP, and the target final Tg of the cured polymer was 150 °C based on tan-δ peak from dynamic mechanical analysis. A tri-component model resin system, termed DA-2 resin, was determined and fully characterized. The printed polymer exhibited good thermal properties and high mechanical strength after post-cure, but has a comparatively low fracture toughness. The model resin will be used in additive manufacturing of fiber reinforced composite materials as well as for understanding the fundamental processing–property relationships in light-based 3D printing.


Introduction
Three-dimensional (3D) printing is an additive manufacturing process in which successive layers of material are patterned and combined to form 3D shapes. 3D printing technologies are currently experiencing financial growth and are being increasingly adopted across industries. Factors driving this market growth are aggressive research and development and the growing demand for prototyping applications from industries such as healthcare, automotive, defense, and aerospace [1]. In fact, the aerospace 3D printing market was estimated to be USD 1.86 billion as of 2019-only 16.8% of the total 3D printing market-and is expected to grow annually at a rate of 16.9% over the next 7 years to reach USD 6.72 billion in 2027 [2]. For 3D patterning polymeric materials, extrusion or melt type techniques, such as fused deposition modelling and selective laser sintering, are common methods for the fabrication of thermoplastic parts. However, these techniques have the drawback of comparatively low resolution, weak layer adhesion, and slow processing. On the other hand, in light-based methods, the printing resolution and production speed are drastically improved due to the exceptional spatial control and versatility of photo-polymerization reactions [3]. Moreover, the mechanical properties of the printed objects are significantly enhanced due to better layer-to-layer cohesion. Thus, light-based technologies offer attractive routes for 3D printing of polymers and composites. Examples of technologies include stereolithography (SLA), digital light processing (DLP), and continuous liquid interface production (CLIP) [4][5][6]. In SLA, specific surface regions of photo-sensitive liquid resin undergo localized polymerization by exposure to a scanning spot light source. In DLP, all given portions of a layer are simultaneously photocured, significantly reducing

Resin Formulation and Printing
To prepare photo-polymerizable resins for DLP 3D printing, the monomers with determined weight ratios were evenly mixed, then 0.7 wt.% PPO (based on resin weight) was added as the photoinitiator to the resin. The mixture was stirred in an amber bottle to block UV light exposure until the photo-initiator completely dissolved. Upon degassing, the resin is ready for DLP printing. Samples were printed in an Anycubic Photon DLP printer (Shenzhen, China). The wavelength of the projected light by the printer is 405 nm. The light intensity is 0.45 ± 0.05 mW/cm 2 determined by a radiometer (ILT2400, International Light Technologies, Peabody, MA, USA). The default print settings for this study were 100 μm layer thickness and 100 s exposure time and 10 s off time between layers. Asprinted "green" parts were subject to the following post-processing procedure: samples first undergo photo post-cure in a blue light oven (Form Cure, Formlabs Inc., Somerville, MA, USA) at 80 °C for 2 h, followed by thermal post-cure in a conventional laboratory oven at 120 °C for 3 h, ramping to 180 °C in 60 min and isotherm at 180 °C for 30 min.

Working Curve
To create the working curve, several samples were 3D printed at different values of exposure time to achieve different thicknesses (cure depth). This requires the build platform to be removed from the vat to allow samples to have a variety of thicknesses. Initially, three coin-shaped samples with the approximate diameter of 2 cm were printed simultaneously at a specific exposure time and as a single print layer. Next, their thicknesses were measured using a ratchet micrometer and averaged to provide the average cure depth for that exposure time. This process was repeated for different exposure time durations to obtain different values of cure depth for each case. Finally, the Scheme 1. The chemical structures of the monomers and the photo-initiator.

Resin Formulation and Printing
To prepare photo-polymerizable resins for DLP 3D printing, the monomers with determined weight ratios were evenly mixed, then 0.7 wt.% PPO (based on resin weight) was added as the photo-initiator to the resin. The mixture was stirred in an amber bottle to block UV light exposure until the photo-initiator completely dissolved. Upon degassing, the resin is ready for DLP printing. Samples were printed in an Anycubic Photon DLP printer (Shenzhen, China). The wavelength of the projected light by the printer is 405 nm. The light intensity is 0.45 ± 0.05 mW/cm 2 determined by a radiometer (ILT2400, International Light Technologies, Peabody, MA, USA). The default print settings for this study were 100 µm layer thickness and 100 s exposure time and 10 s off time between layers. As-printed "green" parts were subject to the following post-processing procedure: samples first undergo photo post-cure in a blue light oven (Form Cure, Formlabs Inc., Somerville, MA, USA) at 80 • C for 2 h, followed by thermal post-cure in a conventional laboratory oven at 120 • C for 3 h, ramping to 180 • C in 60 min and isotherm at 180 • C for 30 min.

Working Curve
To create the working curve, several samples were 3D printed at different values of exposure time to achieve different thicknesses (cure depth). This requires the build platform to be removed from the vat to allow samples to have a variety of thicknesses. Initially, three coin-shaped samples with the approximate diameter of 2 cm were printed simultaneously at a specific exposure time and as a single print layer. Next, their thicknesses were measured using a ratchet micrometer and averaged to provide the average cure depth for that exposure time. This process was repeated for different exposure time durations to obtain different values of cure depth for each case. Finally, the working curve was constructed by plotting the average cure depth values against their corresponding energy doses on a logarithmic axis. Note that for each run, the energy dose is calculated by multiplying the exposure time of that run by the printer's light intensity. The slope and the x-intercept of the logarithmic curve fit are the resin properties known as depth of penetration (D p ) and critical energy dose (E c ) [23].

Characterization
Steady shear viscosity measurements were performed on TA Instruments AR2000 rheometer (New Castle, DE, USA) at 25 • C under steady state shear mode using a cone-plate geometry with shear rate ramping from 0.001 s −1 to 100 s −1 . For every resin, three viscosity measurements were taken. Fourier transform near infrared (FT-NIR) experiments were performed on a Nicolet iS50 FT-IR spectrometer (Thermo Fisher Scientific, Waltham, MA, USA), operating in transmission mode with a deuterated triglycine sulfate (DTGS) detector. FT-NIR spectra were recorded with 32 scans at a 4 cm −1 resolution in 4000-8000 cm −1 range. Dynamic mechanical analysis (DMA) experiments were performed on bar samples of size 35 mm × 12.7 mm × 3.2 mm using a TA Instruments Q800 Dynamic Mechanical Analyzer in a single cantilever mode with the oscillation frequency set to 1 Hz, the amplitude set to 10 µm, and temperature ramping at a rate of 2 • C/min. Resin densities were measured using an Anton Paar DMA 500 density meter (Graz, Austria), and a density value was obtained as an average over three measurements. Densities of cured polymers were determined using a density-gradient column in accordance with ASTM D1505-18 standard [24]. For each polymer, at least three samples were tested. The liquid system in the column consisted of water and sodium bromide. All mechanical testing was performed on an Instron tester, model #: A1740-3003 (Norwood, MA, USA). Tensile testing was carried out according to ASTM D638-14 standard [25]. At least five dog-bone specimens of type IV were tested at a test speed of 5 mm/min. An extensometer was applied to obtain accurate strain values, and the tensile modulus was calculated based on data up to 0.25% strain. Flexural properties were determined according to ASTM D790-17 standard [26]. At least three long rectangular bars of size 120 mm × 12.7 mm × 3.2 mm were tested in a three-point bending configuration and the span-to-depth ratio was kept at 16. Fracture toughness was measured using at least five single-edge-notch-bend (SENB) specimens according to ASTM D5045-14 standard [27]. Pre-cracks were initiated by tapping a fresh razor blade inserted in the notch.

Results and Discussion
In many resin formulations, the preferred major component is Bis-GMA resin because of its good final properties. The addition of Bis-EMA resin maintains a low cure shrinkage, but also reduces resin viscosity and final material moisture sensitivity significantly. In this work, a combination of Bis-GMA and Bis-EMA served as the base resin of the formulations. A reactive diluent is needed to further reduce the viscosity to make suitable formulations for DLP printing. Two reactive diluents, isobornyl methacrylate (IBMA) and 1,6-hexanediol dimethacrylate (HDDMA), were selected for the study after screening. HDDMA had been used in this research group as a reactive diluent in vinyl-ester resins [13]. HDDMA is known for its hydrophobic nature. A photo-cured polymer network containing HDDMA was found to uptake three to four times less water compared to the corresponding network containing the same amount of TEGDMA [28]. IBMA was selected for the high T g of its polymer [29][30][31][32]. Both diluents have high boiling points (258 • C for IBMA and 315 • C for HDDMA) and low vapor pressures (0.01 mmHg at 25 • C for IBMA and 0.02 mmHg at 100 • C for HDDMA) [33,34]. The use of monomers with low vapor pressures is essential for a safe work environment and minimizes the composition drift due to possible monomer evaporation during printings. Table 1 summarizes the measured room temperature viscosities of the component monomers, and the T g s of corresponding neat polymers available from the literature. Predictive models for resin viscosity and polymer T g were used as guiding tools to determine proper monomer ratios. The Fox equation, shown in Equation (1), has been widely used to predict the T g of a polymer mixture based on the T g s of the neat components [42]: where T g,i and ω i are the glass transition temperature and the mass fraction of component i, respectively. The simplest model for predicting the viscosity of liquid mixtures is the Arrhenius equation [43], but the additive model neglects thermodynamic parameters characteristic of the interactions between components and results in inaccurate predictions. The Grunberg-Nissan model, based on a modification of the Arrhenius equation to account for the excess free energy of mixing, shown in Equation (2), is commonly used to describe the viscosity of liquid mixtures [44][45][46]: where η is the viscosity of a mixture, η i and x i are the viscosity and the mole fraction of each component in the mixture, respectively, and G ij is an interaction parameter dependent on the components and temperature. A negative value of G ij indicates favorable mixing. Binary interaction parameters G ij were determined from the viscosity measurements of binary liquid mixtures of the monomers and calculated using Equation (2). The interaction parameters are found to be relatively constant with regards to monomer mixing ratios (see Table A2). Table 2 lists the averaged interaction parameters between Bis-GMA and Bis-EMA; Bis-GMA and a reactive diluent; and Bis-EMA and a reactive diluent such as G 12 , G 13 , and G 23 , respectively. The interaction parameters show that Bis-GMA mixed favorably with Bis-EMA and HDDMA, but IBMA did not efficiently mix with Bis-GMA or Bis-EMA, likely due to the bulky nature of the isobornyl group that hinders flow [47]. Interestingly, the excess free energy of mixing is close to zero when mixing Bis-EMA with HDDMA.  Figure 1a,b, shows the predictions of viscosity and the T g of final cured material using the Grunberg-Nissan model and the Fox equation, respectively, for ternary mixture system Bis-GMA/Bis-EMA/HDDMA. In Figure 1a, the shaded area denotes a predicted viscosity ≤500 cP. In Figure 1b, the shaded area denotes a predicted final T g higher than 150 • C. Figure 1c,d show the overlap area where the predicted viscosity is lower than 500 cP and T g is higher than 150 • C. The same graphs for Bis-GMA/Bis-EMA/IBMA system are shown in Figure A1 (Appendix C). Note the slightly higher cut-off viscosity, 600 cP. For the T g predictions, the final T g s of Bis-GMA, BisEMA, and IBMA polymers are 200 • C, 160 • C, and 140 • C, respectively [29][30][31][32][35][36][37][38][39]. The T g of the HDDMA polymer is not commonly reported in the literature, but it should be higher than the T g of its acrylate counterpart, 1,6-hexanediol diacrylate (HDDA), which was reported to be 93 • C [29,40]. Only one reference [41] reported 150 • C T g for the HDDMA polymer. To be conservative, the final T g of HDDMA polymer is assumed to be 110 • C for predictive calculations. T g is a primary consideration only once the viscosity values are satisfactory for facile 3D printing. In the predicted overlap areas shown in Figure 1c,d, higher T g s appears on the high viscosity side. Additionally, other factors need to be considered to obtain optimal material properties. Bis-EMA polymer has a relatively low Young's modulus, typically less than 2 GPa [22,48]. Also, impact resistance was reported to decrease as Bis-EMA was added to the Bis-GMA/Bis-EMA copolymer [49]. This can be explained by the decrease in the overall strength of intermolecular interactions; as the hydroxyl group concentration decreases, the number of physical crosslinking sites is reduced. On the other hand, if the Bis-GMA content is increased, more reactive diluent is needed, which will increase cure shrinkage. Therefore, to retain mechanical performance and minimize cure shrinkage, the formulations at the center of the high viscosity side of the overlap areas were selected for this study. These two formulations are called DA-1 and DA-2. Given in Table 3 Table 4. the overall strength of intermolecular interactions; as the hydroxyl group concentration decreases, the number of physical crosslinking sites is reduced. On the other hand, if the Bis-GMA content is increased, more reactive diluent is needed, which will increase cure shrinkage. Therefore, to retain mechanical performance and minimize cure shrinkage, the formulations at the center of the high viscosity side of the overlap areas were selected for this study. These two formulations are called DA-1 and DA-2. Given in Table 3 Table 4.    DA-1 and DA-2 green parts were printed using the default print settings (100 µm layer thickness and 100 s exposure time). Green parts created by room temperature DLP printing often need to be post-cured to promote additional conversion. The post-cure process is especially necessary for parts printed with high T g resins because these resins reach vitrification at low monomer conversions under the printing temperature [50]. When a DA-1 green part was directly placed in a conventional oven for thermal post-cure, cracks developed throughout the part (shown in the right picture of Figure 2) and some delamination was observed between layers. This is due to the unreacted monofunctional isobornyl methacrylate molecules, which diffuse out before they can react because the thermal activation of the methacrylate double bond reaction is relatively slow [47]. Such cracking and delamination phenomena would not happen if mono-functional monomers of higher reactivity were used, such as p-methyl styrene and N-vinylpyrrolidone. Another way to overcome this problem is to use multifunctional (≥2) monomers, proved by the case of DA-2. Direct thermal post-cure of DA-2 green parts did not cause cracks or delamination because most unreacted reactive diluent functionality exist as dangling chain ends. For this reason, the DA-2 formulation was chosen as the standard resin for the study.  DA-1 and DA-2 green parts were printed using the default print settings (100 μm layer thickness and 100 s exposure time). Green parts created by room temperature DLP printing often need to be post-cured to promote additional conversion. The post-cure process is especially necessary for parts printed with high Tg resins because these resins reach vitrification at low monomer conversions under the printing temperature [50]. When a DA-1 green part was directly placed in a conventional oven for thermal post-cure, cracks developed throughout the part (shown in the right picture of Figure 2) and some delamination was observed between layers. This is due to the unreacted monofunctional isobornyl methacrylate molecules, which diffuse out before they can react because the thermal activation of the methacrylate double bond reaction is relatively slow [47]. Such cracking and delamination phenomena would not happen if mono-functional monomers of higher reactivity were used, such as p-methyl styrene and N-vinylpyrrolidone. Another way to overcome this problem is to use multifunctional (≥2) monomers, proved by the case of DA-2. Direct thermal post-cure of DA-2 green parts did not cause cracks or delamination because most unreacted reactive diluent functionality exist as dangling chain ends. For this reason, the DA-2 formulation was chosen as the standard resin for the study. The fractional monomer conversion, α, is determined using Equation (3) from the decreasing integral of the characteristic near-IR absorption band of the methacrylate double bond, shown in Figure 3 [51]: where A0 and A1 are the area integrals of the absorption band from 6225-6105 cm −1 of the pristine resin and cured sample, respectively. Note that the calculated conversion is based on the absorbance averaged over the thickness of the tested sample. The fractional conversion is 0.67 for a DA-2 green The fractional monomer conversion, α, is determined using Equation (3) from the decreasing integral of the characteristic near-IR absorption band of the methacrylate double bond, shown in Figure 3 [51]: where A 0 and A 1 are the area integrals of the absorption band from 6225-6105 cm −1 of the pristine resin and cured sample, respectively. Note that the calculated conversion is based on the absorbance averaged over the thickness of the tested sample. The fractional conversion is 0.67 for a DA-2 green part. Figure 4 shows the DMA thermogram of a DA-2 green part with a T g of 22 • C identified as the peak of loss modulus, E". The T g value indicates that the vitrification conversion of the DA-2 resin is around 0.67 at room temperature. The tan δ curve shows a bimodal glass transition with a slightly more intense peak at 64 • C and a very broad second one at 120 • C. The E" curve also shows a shoulder above the main transition. Though this might be interpreted as a heterogeneous network morphology or spatial inhomogeneity of reaction, the likely cause of the behavior is a dark reaction by trapped radicals upon heating past the first T g resulting in partial revitrification and devitrification upon continued heating. The slight plateau in E following the first transition supports this explanation. After post-processing, the fractional conversion becomes 0.88. Figure 5 shows the DMA thermogram of a fully post-cured DA-2 sample. Based on three DMA tests, the T g of post-cured DA-2 material is 97 ± 3 • C and 165 ± 4 • C, identified as the maxima of loss modulus E" and of tan δ, respectively. The latter number agrees with the predicted 160 • C T g via the Fox equation.
Materials 2020, 13, x FOR PEER REVIEW 8 of 15 or spatial inhomogeneity of reaction, the likely cause of the behavior is a dark reaction by trapped radicals upon heating past the first Tg resulting in partial revitrification and devitrification upon continued heating. The slight plateau in E′ following the first transition supports this explanation. After post-processing, the fractional conversion becomes 0.88. Figure 5 shows the DMA thermogram of a fully post-cured DA-2 sample. Based on three DMA tests, the Tg of post-cured DA-2 material is 97 ± 3 °C and 165 ± 4 °C, identified as the maxima of loss modulus E″ and of tan δ, respectively. The latter number agrees with the predicted 160 °C Tg via the Fox equation.   or spatial inhomogeneity of reaction, the likely cause of the behavior is a dark reaction by trapped radicals upon heating past the first Tg resulting in partial revitrification and devitrification upon continued heating. The slight plateau in E′ following the first transition supports this explanation. After post-processing, the fractional conversion becomes 0.88. Figure 5 shows the DMA thermogram of a fully post-cured DA-2 sample. Based on three DMA tests, the Tg of post-cured DA-2 material is 97 ± 3 °C and 165 ± 4 °C, identified as the maxima of loss modulus E″ and of tan δ, respectively. The latter number agrees with the predicted 160 °C Tg via the Fox equation.     Successful printing of objects by stereolithography requires predetermined knowledge of the photo-curing properties of the starting material. Principles laid out by Jacobs to describe the photopolymerization process were used to create a working curve that provides two key parameters that Successful printing of objects by stereolithography requires predetermined knowledge of the photo-curing properties of the starting material. Principles laid out by Jacobs to describe the photo-polymerization process were used to create a working curve that provides two key parameters that govern the polymerization of a photo-sensitive resin: depth of penetration, D p, and critical energy of polymerization, E c [52]. Knowing D p and E c allows users to choose the appropriate settings for light exposure and z-axis increments, which optimizes the curing conditions to achieve the desired results. Here, the working curve is constructed for the DA-2 resin with 0.7 wt.% PPO photo-initiator, shown in Figure 6. Under irradiation wavelength of 405 nm, the depth of penetration D p is 550 ± 55 µm and the critical energy E c is 5.6 ± 0.5 mJ/cm 2 . The DA-2 resin has a large D p , which enables it to 3D print fiber-reinforced composite structures. The depth of penetration can be reduced by adding photo absorbers to allow for a better printing resolution.
Materials 2020, 13, x FOR PEER REVIEW 9 of 15 govern the polymerization of a photo-sensitive resin: depth of penetration, Dp, and critical energy of polymerization, Ec [52]. Knowing Dp and Ec allows users to choose the appropriate settings for light exposure and z-axis increments, which optimizes the curing conditions to achieve the desired results.
Here, the working curve is constructed for the DA-2 resin with 0.7 wt % PPO photo-initiator, shown in Figure 6. Under irradiation wavelength of 405 nm, the depth of penetration Dp is 550 ± 55 μm and the critical energy Ec is 5.6 ± 0.5 mJ/cm 2 . The DA-2 resin has a large Dp, which enables it to 3D print fiber-reinforced composite structures. The depth of penetration can be reduced by adding photo absorbers to allow for a better printing resolution. The DA-2 resin's density is 1105 g/cm 3 , and the fully cured DA-2 material (fraction conversion: 0.88) has a density of 1200 g/cm 3 . Cure shrinkage is, therefore, calculated based on the shrinkage in specific volume to be 7.9%. The tensile, flexural, and fracture toughness properties are measured for the fully cured DA-2 material. Table 5 and Scheme 2 summarize the properties and the composition information of the DA-2 material. Compared to the commercial resins shown in Table A1, DA-2 is a strong material with an elastic modulus around 3 GPa and a flexural strength over 100 MPa. It has a low viscosity and a high depth of penetration for blue lightcuring, both of which make the resin suitable for the additive manufacturing of fiber-reinforced composites. DA-2 has a comparatively low fracture toughness that is typical of free radical polymerization systems. Future work will focus on improving the fracture toughness of the resin, studying the effects of 3D printing parameters, as well as understanding the processing-property relationships in light-based 3D printing technologies.
The DA-2 resin's density is 1.105 ± 0.001 g/cm 3 , and the cured DA-2 material (fractional conversion: 0.88) has a density of 1.200 ± 0.002 g/cm 3 . Cure shrinkage is, therefore, calculated based on the shrinkage in specific volume to be 7.9%. The tensile, flexural, and fracture toughness properties are measured for the fully cured DA-2 material. Table 5 and Scheme 2 summarize the properties and the composition information of the DA-2 material. Compared to the commercial resins shown in Table A1, DA-2 is a strong material with an elastic modulus around 3 GPa and a flexural strength over 100 MPa. It has a low viscosity and a high depth of penetration for blue light curing, both of which make the resin suitable for the additive manufacturing of fiber-reinforced composites. DA-2 has a comparatively low fracture toughness that is typical of free radical polymerization systems. Future work will focus on improving the fracture toughness of the resin, studying the effects of 3D printing parameters, as well as understanding the processing-property relationships in light-based 3D printing technologies.

Conclusions
The additive manufacturing field lacks a well-defined photo-sensitive resin formulation for high performance composite applications. Current commercial resins produce parts with relatively low thermal and mechanical properties and contain proprietary composition information. The DA-2 formulation was herein developed as a model resin system for high performance 3D printing applications. Predictive models, namely the Grunberg-Nissan model for the prediction of resin mixture viscosity and the Fox equation for polymer Tg, were successfully applied to determine proper monomer ratios to give optimal material properties. The clear resin has a large depth of penetration suitable for the additive manufacturing of fiber-reinforced composites. The depth of penetration can be reduced for 3D printing with the addition of photo absorbers. The high Tg resin has to be postcured after printing to achieve maximum cure due to early vitrification of the resin at the room printing temperature. The post-cured polymer exhibited good thermal properties and high mechanical strength but has a comparatively low fracture toughness, typically observed in free radical polymerization systems. Future work will focus on improving the fracture toughness of the photo-polymerizable resin, applying the resin to the additive manufacturing of fiber-reinforced composites and understanding the fundamental processing-property relationships underlying lightbased 3D printing technologies.

Conclusions
The additive manufacturing field lacks a well-defined photo-sensitive resin formulation for high performance composite applications. Current commercial resins produce parts with relatively low thermal and mechanical properties and contain proprietary composition information. The DA-2 formulation was herein developed as a model resin system for high performance 3D printing applications. Predictive models, namely the Grunberg-Nissan model for the prediction of resin mixture viscosity and the Fox equation for polymer T g , were successfully applied to determine proper monomer ratios to give optimal material properties. The clear resin has a large depth of penetration suitable for the additive manufacturing of fiber-reinforced composites. The depth of penetration can be reduced for 3D printing with the addition of photo absorbers. The high T g resin has to be post-cured after printing to achieve maximum cure due to early vitrification of the resin at the room printing temperature. The post-cured polymer exhibited good thermal properties and high mechanical strength but has a comparatively low fracture toughness, typically observed in free radical polymerization systems. Future work will focus on improving the fracture toughness of the photo-polymerizable resin, applying the resin to the additive manufacturing of fiber-reinforced composites and understanding the fundamental processing-property relationships underlying light-based 3D printing technologies.
Author Contributions: Conceptualization, G.R.P. and N.J.A.; methodology, J.T. and K.M.; formal analysis, J.T. and G.R.P.; writing-original draft preparation, J.T.; writing-review and editing, K.M., N.J.A. and G.R.P.; supervision, G.R.P.; funding acquisition, G.R.P. and N.J.A. All authors have read and agreed to the published version of the manuscript.
Funding: Research was sponsored by the Army Research Laboratory and was accomplished under Cooperative Agreement Number W911NF-14-2-0227. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressed or implied, of the Army Research Laboratory or the U.S. Government. The U.S. Government is authorized to reproduce and distribute reprints for Government purposes not withstanding any copyright notation herein.

Conflicts of Interest:
The authors declare no conflict of interest.   (a) Predicted viscosity lower than 600 cP; (b) predicted T g higher than 150 • C; (c,d) predicted overlap area where viscosity is lower than 600 cP and T g is higher than 150 • C. The star in (c,d) marks the DA-1 composition.