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Application of Multivariate Adaptive Regression Splines (MARSplines) for Predicting Hansen Solubility Parameters Based on 1D and 2D Molecular Descript...
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Hansen Solubility Parameters of Surfactant-Capped Silver Nanoparticles for Ink and Printing Technologies Jacob B. Petersen,* Jeevan Meruga, James S. Randle, William M. Cross, and Jon J. Kellar* Materials Engineering and Science Program, South Dakota School of Mines and Technology, 501 East Saint Joseph Street, Rapid City, South Dakota 57701, United States S Supporting Information *

ABSTRACT: Optimal ink formulations, inclusive of nanoparticles, are often limited to matching the nanoparticle’s capping agent or surface degree of polarity to the solvent of choice. Rather than relying on this single attribute, nanoparticle dispersibility was optimized by identifying the Hansen solubility parameters (HSPs) of decanoic-acid-capped 5 nm silver nanoparticles (AgNPs) by broad spectrum dispersion testing and a more specific binary solvent gradient dispersion method. From the HSPs, solvents were chosen to disperse poly(methyl methacrylate) (PMMA) and nanoparticles, give uniform evaporation profiles, and yield a phaseseparated microstructure of nanoparticles on PMMA via film formation by solvent evaporation. The goal of this research was to yield a film that is reflective or transparent depending on the angle of incident light (i.e., optically variable). The nanoparticle HSPs were very close to alkanes with added small polar and hydrogenbonding components. This led to two ink formulations: one of 90:10 vol % toluene/methyl benzoate and one containing 80:10:10 vol % toluene/p-xylene/mesitylene, both of which yielded the desired final microstructure of a nanoparticle layer on a PMMA film. This approach to nanoparticle ink formulation allows one to obtain an ink that has desirable dispersive qualities, rheology, and evaporation to give a desired printed structure.

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results.9 While Hildebrand solubility can suffice for ink formulations of materials that are nonpolar, nanoparticlebased inks may require accounting for polar and hydrogenbonding components because of the nature of the capped nanoparticle’s surface chemistry.10,11 HSP-based formulations require knowledge of each parameter for each component in the ink system. A 3-D solubility space can be used to identify materials of interest, where the x, y, and z axes represent dispersive, polar, and hydrogen-bonding components, respectively. Solutes are plotted, and their interaction value is used to define a volume (typically modeled as a sphere) in this solubility space. Solvents that lie within this volume will dissolve the material of interest, while solvents outside the volume will not. This interaction condition can then be defined as12

INTRODUCTION Digital printing technologies are currently experiencing rapid growth in security printing, electronics, sensing, energy, and a number of other industries.1−3 Key components of these technologies are nanoparticle inks, typically composed of a functional nanomaterial dispersed in a solvent with additives to meet additional printing or device requirements. Metallic nanoparticles have large ranges of applications3,4 and can be incorporated into inks because of their small size and surface functionalization capabilities. These surface functionalizations have also been exploited to yield phase-separated nanoparticles in PMMA−PS block copolymer films,5,6 suggesting that unique printed microstructures are possible for optically variable inks. Inkjet and aerosol jet printing systems require unique ink properties that result in necessary drop-forming processes. A barrier to designing a functional ink that contains nanoparticles and other components is finding suitable solvents such that the ink will have the necessary rheology, dispersion lifetime, evaporation rate, and final print features.7,8 To approach this problem, our group has turned to Hansen solubility parameters (HSPs) to facilitate the design of such inks. HSPs divide interactions of a solvent with a solute into dispersive, polar, and hydrogen-bonding components. Solutes are also described with these components along with an interaction value. There are more parameters that can be used, but, in general, these three components provide reliable © 2014 American Chemical Society

Ra 2 = a(δd 2 − δd1)2 + b(δp2 − δp1 )2 + c(δh2 − δh1)2 RED =

Ra R0

where Ra is the distance from a solvent to the center of a solute volume, R0 is the interaction volume radius for the solute, RED Received: July 25, 2014 Revised: October 14, 2014 Published: December 3, 2014 15514

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and from two surfaces, a central point derived from these surfaces was used to estimate the nanoparticle HSPs and interaction radius. Hansen Solubility Parameters in Practice (HSPiP, Hansen-Solubility.com) software was used to simplify these calculations and was used extensively for multiple ink component developments. Similar dispersion tests were conducted via the binary solvent gradient method18 using mixtures of toluene with methyl benzoate, methyl benzoate with dodecane, and toluene with acetone.

is the relative energy difference, and a, b, and c are scaling variables. δd, δp, and δh are the dispersive, polar, and hydrogenbonding components, respectively. Typically, a = 4, while b = c = 1.6 If RED is less than one, the materials are dispersible; if RED is greater than one, the materials are nondispersible; and if RED is equal to one, the solvent lies on the volume surface.12 It is also important to note that solvent mixtures are treated as a new solvent with each component defined by

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INK FORMULATION HSPiP was used to determine an optimal ink formulation for the dispersion of nanoparticles after their HSPs were found. Solvent mixtures were chosen according to evaporation profile steps: the solvent mixture must initially disperse the AgNPs and the polymer. As components of this solvent mixture evaporate, the PMMA and AgNPs should begin to precipitate, forming a film by solvent evaporation.12 As previously mentioned, the immiscibility of AgNPs in PMMA5,6 will cause the AgNPs to phase-separate at full solvent evaporation, producing a thin film of AgNPs on the PMMA film. The majority solvent was chosen to evaporate quickly to ensure fast film drying, and secondary solvents were chosen to evaporate slowly to prevent the coffeering effect21 and alter the AgNP film structure. In addition, previous experience with inks incorporating PMMA printed with an aerosol-jet suggest using solvents that evaporate slower than chloroform (∼720 times nBuAc) to avoid clogging the print head assembly. This led to two final solvent mixtures of 80:10:10 vol % toluene/p-xylene/mesitylene (all species remain dispersible) and 90:10 vol % toluene/methyl benzoate (polymer remains soluble while nanoparticles become nondispersible). Given that toluene was the major component of the mixture, both inks were expected to behave very similarly to neat toluene during printing. This allowed simple use of parameter maps based on Reynolds, Weber, and Ohnesorge numbers8 to determine the printability of inks. These maps establish a range of rheological properties combined with printer transport mechanisms to predict drop profiles for a given ink. Regions in these maps then specify when an ink is optimal for inkjet or aerosol printing or can be used to tailor an ink for a specific printer. These parameter maps can be found in the Supporting Information. The final ink composition was then chosen as 1 wt % AgNPs and 1 wt % PMMA based on previous inks formulated in our group,22,23 dispersed in a total mixture volume of 10 mL for each solvent combination. PMMA was dissolved in the solvent mixture for 2 h, followed by the addition of nanoparticles, and mixed for another hour. The mixture was sonicated for 10 min before being transferred to a syringe for printing. Printing was accomplished with a Sono-Tek ExactaCoat aerosol jet printer using an Impact Edge print head (Sono-Tek, Milton, NY). The ink feed was set to 0.5 mL/min with a print head velocity of 20 mm/s, a line spacing of five mm, and a print head height of 10 cm from the substrate. The chosen substrate was a glass slide that had been cleaned beforehand with acetone and isopropyl alcohol. The substrate was allowed to remain on the platen for 5 min after printing to ensure evaporation of solvents.

n

δX mix =

∑ (δXi ·Vi ) i=1

where δX and V are the HSPs (dispersive, polar, or hydrogen bonding) and the volumetric fraction of the ith component in the mixture, respectively. This computation is performed for each parameter and can be used to develop solvents with finely tuned HSPs, evaporation profiles, and rheological properties. Furthermore, solvents with an RED greater than one can be mixed with other solvents to produce solvent mixtures with an RED less than one. Combining solvents and nonsolvents opens up a wide range of mixtures that may have previously been ignored in nanoparticle-based inks, where solvents are typically chosen simply to match the polarity of the capping agent used.13−17 Mixtures have also been used to identify an interaction surface of organic semiconductors by varying the composition of three solvent + nonsolvent mixtures.18 For nanoparticlebased inks this may require some knowledge of the nanoparticle dispersion beforehand but can be used to very specifically identify the interaction surface. As few as six solvents are required to approximate a nanoparticle’s HSPs. Developing an ink around the functional material of interest then becomes simpler, as all solvents and solvent mixtures must lie within the solubility sphere to be used. If multiple solutes are chosen, the overlap of their interaction volumes then defines a smaller selection of solvents. These results can then be further refined to choose final solvents and mixture volumes to obtain desired ink rheology, and evaporation rate, all in an attempt to print a desirable microstructure. For this research, we measured the HSPs of decanoic-acidcapped silver nanoparticles (AgNPs) by broad spectrum and binary solvent gradient dispersion testing approaches. These nanoparticles were then incorporated into inks containing polymers and solvents for aerosol-jet printing.

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EXPERIMENTAL SECTION

AgNPs were synthesized according to the method of Lee et al.19 and Ankireddy et al.20 In short, silver nitrate was first dissolved in n-butyl amine. During this time, decanoic acid was dissolved in toluene and then added to the n-butyl amine−silver mixture. Sodium borohydride was then added as a reductant, and the solution was refluxed for 1 h. Nanoparticles were then precipitated in equal parts acetone and methanol and run through a Büchner funnel, followed by being washed with acetone and then methanol. HSPs for these nanoparticles were determined by testing with ten solvents:15 chloroform, methylene dichloride, toluene, mesitylene, pxylene, acetone, methanol, ethanol, methyl benzoate, and dodecane. Three mg of nanoparticles was added to 1 mL of each test solvent and sonicated for 5 min. The mixtures were then observed for 3 days, which was sufficient time for any excess mixing energy from sonication to dissipate. If the nanoparticles settled during this time they were marked as outside the solubility sphere, and, conversely, if the nanoparticles stayed dispersed, they were marked as being within the solubility sphere. When one solvent dispersed nanoparticles, while another with slightly different HSPs did not, a surface point was found,

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RESULTS

Broad spectrum dispersion test results are condensed in Figure 1, showing a representative picture of dispersible and nondispersible mixtures. Over the course of 3 days the nanoparticles settled out of acetone, methyl benzoate, ethanol, 15515

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Table 1. HSPs for Decanoic-Acid-Capped AgNPs test type

δd

δp

δh

R0

broad spectrum binary gradient

16.5 17.5

3.6 0.3

3.5 1

5.2 2.5

evaporation. HSPs of mixtures change during evaporation because of the different evaporation rates of individual solvents in solution. The changes in HSPs over time for both the 90:10 and 80:10:10 inks were calculated to determine the RED of the nanoparticles in the ink throughout evaporation, as shown in Figure 3. HSPs and relative evaporation rates for solvents in each mixture are also shown in Table 2, along with surface tension and viscosity measurements.

Figure 1. Broad spectrum dispersion results are plotted in solubility space, where dispersion by toluene and methyl benzoate is shown. Red cubes are nondispersible results while blue spheres are dispersible.

and methanol, and remained dispersed in toluene, mesitylene, dodecane, p-xylene, chloroform, and methylene dichloride. Results for the gradient method are condensed in Figure 2,

Figure 3. RED over time during solvent evaporation for binary solvent gradient HSPs of AgNPs in 90:10 and 80:10:10 inks.

After printing, cross-sectional analysis of films was conducted with a Zeiss Supra 40VP field-emission scanning electron microscope (SEM) operating at 1.5 keV. The glass substrate was fractured and mounted such that the fracture surface was ∼30° incident to the beam. Micrographs collected (Figure 4) showed the desired separation of AgNPs from PMMA, forming distinct films on top of a glass slide for both ink mixtures. This was further confirmed with a Bruker AFM scanning along the top of dried printed lines for each ink deposition in noncontact mode to display nanoparticle topography. Optical microscope images showed radial cracking on formed droplets in the 90:10 ink, while these radially cracks were not present in the 80:10:10 ink. From a macroscopic perspective, the film appeared reflective when angled acutely to a light source and translucent with red shifting when oblique to a light source. Transmission spectra collected using a Foster + Freeman VSC6000/HS variable angle spectrometer showed absorption in the 400−600 nm regime for incident angles ranging from 0 to 70° (Figure 5). Four-point probe electrical conductivity measurements were also attempted; however, resistances for both printed inks were above our instrument’s detection limits.

Figure 2. Binary solvent gradient method results are plotted in solubility space. Lines A, B, and C are composed of individual dispersion tests for 10 vol % variations of each solvent. Each line has mixtures of solvent one/solvent two from 90:10 vol% to 10:90 vol %, where red squares indicate nondispersing mixtures and blue circles indicate dispersing mixtures.

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DISCUSSION Figure 4 shows cross-sectional SEM images of both ink formulations that appear very similar; however, optical microscope images show differing evaporated drop features, where radial cracks directed from the center to the drop edge appear for the 90:10 toluene/methyl benzoate ink. HSPs for both inks were calculated during solvent evaporation over 100 seconds alongside the AgNPs RED for each ink composition in Figure 3 to aid in understanding the dispersion of nanoparticles as solvents evaporate. For both inks, toluene evaporated rapidly and secondary solvents took much longer. The nanoparticles were nondispersible in methyl benzoate, while they were

where the solvent mixture ratios were varied in 10 vol % increments from 90:10 vol % to 10:90 vol % with acetone and toluene, methyl benzoate and toluene, and methyl benzoate and dodecane. The calculated nanoparticle HSPs for both methods are shown in Table 1. Once nanoparticle HSPs were determined, ink optimization was performed to determine solvent mixtures that would disperse both PMMA and nanoparticles, print via aerosol jet (see Supporting Information), evaporate uniformly, and enable phase separation of nanoparticles from PMMA during 15516

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Table 2. Solvent Evaporation Rate for Ink Components and Inks

a

solvent

δd

δp

δh

REDAgNP

REDPMMA

evap. rate (∝nBuAc)

surface tension (mN/m)

viscosity (cP)

toluene* methyl benzoate* mesitylene* p-xylene* 90:10 ink 80:10:10 ink

18.0 18.9 18.0 17.8 18.1 18.0

1.0 8.2 0.6 1.0 2.1 1.3

1.3 4.7 0.6 3.1 2.3 2.0

0.49 3.59 0.55 0.74 0.88 0.60

0.91 0.26 0.97 0.74 0.78 0.86

150 2 15 80

27.90 37.17 27.55 28.01 25.35 25.67

0.56 1.86 0.67 0.60 1.45 1.88

HSP values, surface tension, and viscosity for * were obtained from refs 12 and 24.

Figure 4. 90:10 toluene/methyl benzoate (top) and 80:10:10 toluene/p-xylene/mesitylene (bottom) with 1 wt % PMMA and 1 wt % nanoparticles. From left to right: Optical microscope images of evaporated drop microstructure with radial cracks indicated by white arrows, cross-sectional SEM indicating phase separation of polymer and AgNPs, and AFM showing dense nanoparticle topography on evaporated drop surfaces.

dispersible in both p-xylene and mesitylene. During evaporation of the 90:10 ink, the relative concentration of methyl benzoate would increase because of its much slower evaporation rate. AgNPs began to separate from the solvent as the methyl benzoate concentration approached 20 vol % (Figures 3 and 4), resulting in regions of evaporating methyl-benzoate-rich solvent that would cause rifts in the silver film. For the 80:10:10 ink, nanoparticles remained dispersible in p-xlyene and mesitylene, indicating that the nanoparticles would remain dispersed throughout solvent evaporation. The phase separation of nanoparticles from PMMA required analysis of how dispersion tests identify the interaction radius of a solute. Nanoparticles remaining in solution or settling identify the interaction radius over the course of 3 days. One of the caveats of this testing is the relative time frame used to identify settled nanoparticles. Hansen described the interaction radius as a variable term depending on the system requirements.12 In this case, if nanoparticles remain dispersed for 3 days (where we assume excess mixing energy from sonication has dissipated) they are deemed dispersible, but over a longer time period they may settle, leading to a different interaction volume approximation and corresponding HSPs. The interaction volume can be thought of as various HSPs that a solute can take in solution over a given time period. In the case of

polymers, conformational entropy, rotational hindrance, and other general properties of polymer motion in solution allow many different conformations, and thus different dispersive, polar, and hydrogen bonding forces are exerted by a molecule depending on its conformation. In the case of AgNPs capped with a 10 carbon chain pointing into the solvent, very few conformational changes are allowed on the same length scale as a polymer,25−28 as the nanoparticle is fairly rigid with short flexible molecules attached to its surface. Effective nanoparticle surface area would also decrease as aggregates form in solution. The combination of these two shape limiting factors then indicates a relatively small interaction volume compared with a polymer. When the ink first interacts with the substrate there is a large concentration of solvent relative to PMMA and AgNPs, enhancing diffusion of small molecules and nanoparticles to the film surface.12 The nanoparticle interaction sphere slightly overlaps with PMMAs, suggesting they would preferentially migrate with the solvent much closer to their HSPs rather than remain in the forming PMMA film. Diffusion of these nanoparticles to the film surface, as seen in AFM and SEM images presented in Figure 4, is then enabled by their small size, preferential dispersibility in the evaporating solvent instead of the polymer, and enhanced surface diffusion rate due to the 15517

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light. Future work could be inclusive of cross-sectional TEM analysis to determine distribution of nanoparticles in the PMMA film to better understand the kinetics of these nanoparticles at the end of solvent evaporation and testing with different nanoparticle−polymer ink systems and solute concentrations to yield similar phase separations with different microstructures.

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ASSOCIATED CONTENT

S Supporting Information *

Parameter maps based on Re, We, and Oh numbers. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

Figure 5. 90:10 toluene/methyl benzoate (top) and 80:10:10 toluene/ p-xylene/:mesitylene (bottom) with 1 wt % PMMA and 1 wt % nanoparticles. From left to right: Photographs of optical variance upon tilting and corresponding angular-dependent transmission spectra showing absorption and reflection in the 400 to 550 nm regime.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based on work supported by the National Science Foundation/EPSCoR Grant No. 0903804 and by the State of South Dakota. Some of this research was conducted under the auspices of the Center for Security Printing and AntiCounterfeiting Technology, supported by the South Dakota Board of Regents.

initially high concentration of solvent. Photographs of the films at nonreflective and reflective angles have transmission spectra and color characteristic of plasmon absorption from AgNPs (Figure 5), implying that some AgNPs became embedded in the film.20 The fast evaporation rate of toluene may have forced low mobility nanoparticles and nanoparticle aggregates to become locked in the forming PMMA film, as seen in crosssectional SEM images (Figure 5). The benefit of this evaporation-controlled diffusion of nanoparticles is then a printable polymer film with embedded AgNPs underneath a continuous film of AgNPs, yielding an optically variable product that appears translucent and red-shifted in transmission or reflective depending on the incident light angle. It is interesting to note that the films appears blue at reflecting angles under diffuse white light, as seen in Figure 5. The optical skin depth of silver for 400 to 500 nm light varies from 30 to 25 nm29 due to the wavelength-dependent refractive index. If the printed film of silver is thinner than 25 to 30 nm, wavelengths of light in the 400 to 500 nm regime can transmit through the film and reflect off of the AgNP film PMMA interface, resulting in an optically bulk-like metallic film with a blue hue at the reflecting angle.

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REFERENCES

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CONCLUSIONS HSPs have been used to predict silver nanoparticle ink formulations via broad spectrum solvent testing and binary solvent gradient methods. The final ink was loaded with PMMA and AgNPs dispersed in toluene and methyl benzoate or toluene, p-xylene, and mesitylene; both which yielded a film of phase-separated AgNPs on top of PMMA. These results suggest that the capped nanoparticles are immiscible with the polymer, and this effect may be predictable due to the difference in HSPs and molecular volumes for both species. As solvent molecules diffuse to the surface, most nanoparticles migrate with them, while some nanoparticles aggregate and become locked in the polymer, leading to an optically variable device that is reflective-dependent on the incident angle of 15518

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(11) Nath, S.; Jana, S.; Pradhan, M.; Pal, T. Ligand-Stabilized Metal Nanoparticles in Organic Solvent. J. Colloid Interface Sci. 2010, 341, 333−352. (12) Hansen, C. M. Hansen Solubility Parameters: A User’s Handbook; CRC Press: Boca Raton, FL, 2007 (13) Shankar, R.; Groven, L.; Amert, A.; Whites, K. W.; Kellar, J. J. Non-Aqueous Synthesis of Silver Nanoparticles Using Tin Acetate as a Reducing Agent for the Conductive Ink Formulation in Printed Electronics. J. Mater. Chem. 2011, 21, 10871. (14) Denneulin, A.; Bras, J.; Carcone, F.; Neuman, C.; Blayo, A. Impact of Ink Formulation on Carbon Nanotube Network Organization within Inkjet Printed Conductive Films. Carbon 2011, 49, 2603−2614. (15) Kamyshny, A.; Steinke, J.; Magdassi, S. Metal-Based Inkjet Inks for Printed Electronics. Open Appl. Phys. J. 2011, 19−36. (16) Zhao, D.; Liu, T.; Park, J. G.; Zhang, M.; Chen, J.-M.; Wang, B. Conductivity Enhancement of Aerosol-Jet Printed Electronics by Using Silver Nanoparticles Ink with Carbon Nanotubes. Microelectron. Eng. 2012, 96, 71−75. (17) Lin, J.-L.; Kao, Z.-K.; Liao, Y.-C. Preserving Precision of InkjetPrinted Features with Solvents of Different Volatilities. Langmuir 2013, 29, 11330−11336. (18) Machui, F.; Langner, S.; Zhu, X.; Abbott, S.; Brabec, C. J. Determination of the P3HT:PCBM Solubility Parameters via a Binary Solvent Gradient Method: Impact of Solubility on the Photovoltaic Performance. Sol. Energy Mater. Sol. Cells 2012, 100, 138−146. (19) Lee, K. J.; Lee, Y.-I.; Shim, I.-K.; Joung, J.; Oh, Y. S. Direct Synthesis and Bonding Origins of Monolayer-Protected Silver Nanocrystals from Silver Nitrate through in Situ Ligand Exchange. J. Colloid Interface Sci. 2006, 304, 92−97. (20) Ankireddy, K.; Vunnam, S.; Kellar, J.; Cross, W. Highly Conductive Short Chain Carboxylic Acid Encapsulated Silver Nanoparticle Based Inks for Direct Write Technology Applications. J. Mater. Chem. C 2013, 1, 572. (21) Deegan, R. Pattern Formation in Drying Drops. Phys. Rev. E: Stat. Phys., Plasmas, Fluids, Relat. Interdiscip. Top. 2000, 61, 475−485. (22) Blumenthal, T.; Meruga, J.; Stanley May, P.; Kellar, J.; Cross, W.; Ankireddy, K.; Vunnam, S.; Luu, Q. N. Patterned Direct-Write and Screen-Printing of NIR-to-Visible Upconverting Inks for Security Applications. Nanotechnology 2012, 23, 185305. (23) Meruga, J. M.; Cross, W. M.; Stanley May, P.; Luu, Q.; Crawford, G. A.; Kellar, J. J. Security Printing of Covert Quick Response Codes Using Upconverting Nanoparticle Inks. Nanotechnology 2012, 23, 395201. (24) Lide, D. R. CRC Handbook of Chemistry and Physics; CRC Press: Boca Raton, FL, 2005. http://www.hbcpnetbase.com. (25) Tortorich, R.; Choi, J.-W. Inkjet Printing of Carbon Nanotubes. Nanomaterials 2013, 3, 453−468. (26) Kango, S.; Kalia, S.; Celli, A.; Njuguna, J.; Habibi, Y.; Kumar, R. Surface Modification of Inorganic Nanoparticles for Development of Organic−inorganic nanocomposites  A Review. Prog. Polym. Sci. 2013, 38, 1232−1261. (27) Smith, G. D.; Bedrov, D. Dispersing Nanoparticles in a Polymer Matrix: Are Long, Dense Polymer Tethers Really Necessary? Langmuir 2009, 25, 11239−11243. (28) Studart, A. R.; Amstad, E.; Antoni, M.; Gauckler, J. Rheology of Concentrated Suspensions Containing Weakly Attractive Alumina Nanoparticles. J. Am. Ceram. Soc. 2006, 89, 2418−2425. (29) Johnson, P.; Christy, R. Optical Contstants of Noble Metals. Phys. Rev. B 1972, 6, 4370−4379.

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