CryoSEM Investigation of Latex Coatings Dried in Walled Substrates

Letter pubs.acs.org/Langmuir

CryoSEM Investigation of Latex Coatings Dried in Walled Substrates Kyle K. Price, Alon V. McCormick, and Lorraine F. Francis* Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455, United States S Supporting Information *

ABSTRACT: Nonuniformities, such as heavy edges or “coffee rings”, frequently develop as particulate coatings dry. One idea for avoiding these nonuniformities is to engineer the substrate edges. In this work, monodisperse latex coatings were deposited on substrates with photoresist walls around their edges. Cryogenic scanning electron microscopy (cryoSEM) results show particle accumulation near the walls and at the free surface. The contact line, pinned at the wall, generates lateral transport of water and particles, leading to a nonuniform coating thickness. Still, coatings on substrates with walls were shown to have a higher degree of thickness uniformity after drying than those without walls.

I. INTRODUCTION The ability to control the particle distribution in a colloidal suspension during drying is important in many technologies, including printed electronics,1,2 3-D nanoparticle assembly,3 and nanowire fabrication.4 For example, inkjet printing is an attractive method for producing low-cost, large-area organic light-emitting devices;1 however, this application requires the printing of pixels as small discrete areas of uniform thickness. Precise control of the particle distribution, and hence coating uniformity, is especially important for small printed features or small substrates where nonuniformities near the edges of the coating may dominate because of lateral drying or the coffee ring effect.5−12 Various remedies have been suggested to suppress nonuniform edge effects during drying. Previous attempts to achieve uniformity in drying drops have focused on the manipulation of lateral flow by inducing Marangoni effects.2,13,14 Another method suggested for improving uniformity is to include raised walls at the edges of the coating or printed feature.15−18 When a printed droplet is deposited into a shallow well, the presence of the wall changes the shape of the wet coating, altering the coating deposition. Some studies have been conducted describing the thickness profile of polymer coatings dried within “bank” structures.17,18 These researchers note that flow toward the walls concentrates polymer there, leading to nonuniformity. Recent particle image velocimetry (PIV) measurements of polymer solutions near a vertical wall also come to this conclusion.19 However, the use of walled substrates for particulate suspensions has not received the same attention, and there has been no direct visualization of microstructure development in particulate coatings dried in a walled substrate. In this research, we use cryogenic scanning electron microscopy (cryoSEM) to visualize the effects of walled substrates on the coating microstructure during drying. © 2012 American Chemical Society

CryoSEM has been employed in the past to image particle distributions and microstructure development directly during the drying of latex and other particulate coatings.20−23 This technique complements methods, such as magnetic resonance imaging10,11 (MRI) and inverse micro-Raman spectroscopy12 (IMRS), that have been used to track the water content in drying latex coatings. The current research focuses on the microstructure development in a latex coating as it dries in a walled substrate, a topic that has not received much attention. The cryoSEM studies show the origin of particle distributions in the final dried coatings. Understanding the relationship between the presence of a wall or border and the development of the coating structure is an important step toward controlling uniformity.

II. MATERIALS AND EQUIPMENT Standard photolithographic techniques were used to pattern an array of walls onto a 4 in. silicon wafer. The photoresist, SU-8 2100 (MicroChem Corp.), was processed according to the manufacturer's recommendations. The wafer was then diced into 5 × 7 mm2 pieces by cutting along the center of the patterned walls. The patterned walls were approximately 500 μm wide and 280 μm high, forming shallow wells (6 × 4 mm2) where the latex suspension was deposited. A schematic diagram of a walled substrate and a cross section of the wall are shown in Figure 1. A slight overhang near the top of this wall is noted in this case, but this feature was not universally observed. For comparison, samples were also prepared on 5 × 7 mm2 pieces without walls. A monodisperse poly(methyl methacrylate-co-n-butyl) acrylate (PMMA-co-PBA) latex was synthesized at Arkema Inc. (Cary, NC). The latex was produced via surfactant-free emulsion polymerization in a batch reaction scheme similar to the method reported by Grunlan et al.24 The number-average particle size (Dn) was 737 nm (consistent Received: June 7, 2012 Published: June 28, 2012 10329

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Figure 1. SEM micrograph (left) showing a cross section of the SU-8 wall on a silicon substrate; in this image, the edge of the substrate is on the right side. A schematic diagram (right) shows dimensions for the walled substrate geometry. with cryoSEM measurements) with a polydispersity (Dv/Dn) of 1.1. The MMA/BA ratio of the latex was 48.4:51.6, and the suspension contained 17.9 wt % solids. CryoSEM was used to image the microstructure of latex coatings during different stages of drying. A micropipet was used to deposit a controlled volume of the latex suspension onto the substrate. The coatings were dried under ambient conditions for various times before plunge freezing into liquid ethane. Rapid freezing arrests particle movement in the wet coating and preserves the developing microstructure.20−23 Frozen samples can survive the high-vacuum conditions in the cryoSEM chamber during imaging because of the low vapor pressure of the solid medium. After being frozen, the samples were transferred to an Emitech K1250 (Emitech, Kent, U.K.) preparation chamber. Samples were fractured along their midpoints to expose the cross section and sublimed at −96 °C for 5 min to remove some of the frozen matrix and improve the topographical contrast for imaging. To reduce charging, a few nanometers of platinum were sputtered onto the frozen fracture surface. The samples were imaged at approximately −150 °C on a stage cooled by liquid nitrogen in a Hitachi S-4700 field emission scanning electron microscope (Hitachi, Pleasanton, CA). Cross sections of fully dried coatings were also imaged using conventional SEM.

III. RESULTS AND DISCUSSION In this work, particle distributions within drying latex coatings were visualized using cryoSEM. As an example, in Figure 2 a cryoSEM cross-section shows the coating microstructure for a sample dried for 12 min on a substrate without walls. The sublimation of ice from the frozen fracture surface exposes embedded latex particles, making them appear bright in the secondary electron image. In the image, two distinct regions are present: a consolidated region of closely packed particles and a suspended region with lower particle concentration. The boundary between these regions (dashed line) marks the packing front. Ice, formed by condensation on the specimen surface, and debris, formed during sample preparation, are also present on the fracture surface. The debris and ice contamination are not indicative of the microstructure development. For coatings dried on substrates without walls, particles accumulate both at the free surface and at the edge. Far from the edge, the free surface descends because of evaporation, and particles accumulate there if diffusion is not fast enough to redistribute them. This accumulation can be predicted using the Peclet number, Pe, which is defined as23,25

Figure 2. CryoSEM cross sections of a latex coating prepared on a flat substrate frozen after 12 min of drying. The lower-magnification image a shows the shape of the free surface. The higher-magnification image b shows the coating microstructure and the packing front.

Pe =

6πηR oHE ̇ HE ̇ = Do kT

In this equation, H is the wet coating thickness, Ė is the evaporation rate (∼5 μm/min by weight loss experiments26), η is the water viscosity (∼1 mPa·s), Ro is the particle radius (∼368 nm), Do is the diffusion coefficient, (∼5.2 × 10−13 m2/ s), and kT is the thermal energy (∼3.8 × 10−21 J). The calculated Peclet number for the latex is 39, which is much greater than 1, predicting that particles will accumulate at the free surface. Accumulation near the edges of drying drops and coatings has been observed and studied by many researchers.3−14 Particles in the suspension pin the contact line of a drying drop; therefore, volume lost to evaporation must be 10330

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Figure 3. CryoSEM cross sections of coatings dried in walled substrates. The white dashed line marks the particle packing front. Images a and b show the same coating, dried for 1 min, at different magnifications. Images c and d show another coating, dried for 12 min, at different magnifications. For the sample in images a and b, the wall (to the right of the coating) broke away from the substrate during cryofracture. The wall (to the left) appears in images c and d, but during the specimen preparation process, the coating separated from the wall and fractured some distance from it. A schematic diagram of the microstructure development for filled, walled substrates is shown in e.

Figure 4. CryoSEM cross sections of coatings dried in overfilled walled substrates. The white dashed line marks the particle packing front. Images a and b show the same coating, dried for 12 min at different magnifications (wall to the left of the coating). Images c and d show another coating, dried for 25 min, at different magnifications (wall to the right of the coating). A schematic diagram of the microstructure development for overfilled walled substrates is shown in e.

replaced by a flow of water from the center to the edge. This flux of water carries particles to the edge where they accumulate, forming the observed packing. The packing front moves inward as evaporation proceeds. Further details on microstructure development for drying on substrates without walls may be found in the Supporting Information. The lateral flow of particles within the coating generates a nonuniform coating thickness near the edges. When the coating is fully dried, only 55% of the coating crosssectional length has a uniform thickness. Figure 3 shows how the particle distribution during drying is affected by the walls. After 1 min of drying (Figure 3a), the coating surface has descended in the center, but the contact line is pinned to the top of the wall. A high-magnification micrograph (Figure 3b) reveals that particles have accumulated at the free surface and near the wall. The packed particle region at the free surface has a uniform thickness of approximately 8 μm, except near the wall where it is thicker. After 12 min of

drying (Figure 3c), the coating remains pinned at the top of the wall, creating a more pronounced concave shape of the free surface. Approximately 340 μm from the wall, the packing front, which formed because of accumulation at the free surface, has reached the substrate. Near the wall, a region of dispersion remains. Figure 3d shows the changes in thickness of the packed region. Next to the wall, the packing is approximately 145 μm thick. Far from the wall, the packing, 41 μm thick, has already reached the substrate. In walled substrates, particles accumulate at the free surface and at the walled edge, as described schematically in Figure 3e. Far from the wall, particles accumulate at the free surface, as expected on the basis of the high Peclet number; however, at the wall, the contact line is pinned. Contact line pinning restricts the ability of the coating to shrink in response to evaporative losses. Consequently, for the contact line to remain pinned, water must flow from the center to replace any volume lost to evaporation near the wall. The flow of water toward the 10331

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Langmuir wall transports particles, causing the packing near the wall to grow. This behavior is similar to the coffee ring effect described for substrates without walls. Flow toward the wall is consistent with results obtained for polymer solutions dried within similar confined geometries.17−19 As drying continues, the accumulation at the free surface forms a packing of uniform thickness over the central region of the substrate. This consolidated packing eventually reaches the substrate, forming a region of uniform thickness far from the wall. This structural development sequence and the uniform central region are linked to the high Pe. At the end of drying, the region of uniform coating thickness comprises approximately 84% of the coating crosssectional length. Therefore, uniformity is improved by the use of walled substrates. For comparison, a walled substrate was overfilled with the coating suspension, forming a convex free surface above the walls. Figure 4 shows cryoSEM cross sections of coatings prepared in overfilled, walled substrates at two times during drying. Early in the drying process (Figure 4a,b), the convex free surface resembles a coating on a flat substrate. Again, the contact line is pinned at the top of the wall throughout drying. As water evaporates, particles accumulate both at the free surface and near the edges. With further evaporation (Figure 4c,d), the volume of the coating decreases, and the free surface drops below the height of the wall. At this stage, the shape of the coating resembles the filled, walled substrate geometry. As drying continues, particles continue to accumulate near the wall because of the pinned contact line. For the overfilled case, the region of uniform coating thickness covers approximately 76% of the coating cross-sectional length. Therefore, the uniformity is not improved by overfilling.

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REFERENCES

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ASSOCIATED CONTENT

* Supporting Information S

Video of the drying sequence for flat and walled substrates, comprising still photographs taken on a digital microscope every 20 s during the drying process. Additional cryoSEM micrographs for the substrates without walls and conventional SEM cross sections for the fully dried coatings. This material is available free of charge via the Internet at http://pubs.acs.org.

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ACKNOWLEDGMENTS

We gratefully acknowledge support from the National Science Foundation (NSF) under award no. CBET 0967348 and the industrial supporters of the Coating Process Fundamentals Program. We also thank Kurt Wood and Wenjun Wu of Arkema, Inc. Acknowledgements go to Kathleen Crawford, Bin Huang, Chris Frethem, and Wieslaw Suszynski for their contributions to this project. Parts of this work were carried out at the Characterization Facility, University of Minnesota, which receives partial support from the NSF through the MRSEC program. Portions of this work were also conducted in the Nanofabrication Center of the University of Minnesota, which is part of the National Nanotechnology Infrastructure Network (NNIN) and supported by the NSF.

IV. CONCLUSIONS CryoSEM was used to characterize the effect of walled edges on the development of a latex coating microstructure. The images reveal the existence of competing particle accumulation fronts near the edges or walls and at the free surface during drying. In all cases, pinning of the contact line generates the lateral transport of water and particles. The principles of the coffee ring effect for substrates without walls also explain how pinning at the top of a wall generates nonuniformities in the final coating. However, for the particular coatings and drying conditions studied here, the final dried coatings in walled substrates are more uniform than coatings dried on substrates without walls. These results also point to possible directions for improving uniformity, including modifying wall surfaces to prevent pinning and increasing suspension loading to accelerate the accumulation at the free surface.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest. 10332

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