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Ceramic Nanoparticle/Monodisperse Latex Coatings Hui Luo,† Christine M. Cardinal, L. E. Scriven,‡ and Lorraine F. Francis* Department of Chemical Engineering & Materials Science, UniVersity of Minnesota, 421 Washington AVe. SE, Minneapolis, Minnesota 55455, USA ReceiVed January 9, 2008. ReVised Manuscript ReceiVed February 19, 2008 Ceramic nanoparticle/monodisperse latex coatings with a nanoparticle-rich surface and a latex-rich body were created by depositing aqueous dispersions of monodisperse latex, ∼550 nm in diameter, and nanosized ceramic particles onto substrates and drying. On the top surface of the dried coating, the latex particles are closely packed with nanoparticles uniformly occupying the interstitial spaces, and along the cross section, nanoparticles fill the spaces between the latex particles in the near surface region; a compacted latex structure, nearly devoid of nanoparticles, lies beneath. Cryogenic scanning electron microscopy images of partially dried coatings at successive drying stages reveal two important steps in forming this structure: top-down consolidation of latex particles and accumulation of nanoparticles in interstitial spaces among latex particles near the surface. A systematic study of the effect of processing conditions, including nanoparticle concentration, nanoparticle size, latex glass transition temperature, and drying conditions, on the final microstructure was carried out. The unique microstructure described above forms when the monodisperse latex is large enough to create pore channels for the transport of nanosized particles and the drying conditions favor “top-down” as opposed to “edge-in” drying.
Introduction Particulate coatings prepared from aqueous dispersions of functional ceramic particles and polymer latex are used in the paint, adhesive, and paper industries and are finding increased interest for advanced applications in flexible electronics. The distribution of functional ceramic particles in the coatings is crucial to coating properties.1–3 For example, Sun and coworkers2,3 created composite coatings from dispersions of semiconducting ceramic nanoparticles (∼20 nm) and polydisperse latex (50-600 nm); the final dried microstructures of these conducting coatings consisted of a uniform distribution of ceramic nanoparticles. Cryogenic SEM (cryo-SEM) studies revealed that the ceramic nanoparticles were trapped in the interstitial spaces between the latex particles during drying, resulting in an interconnected network of ceramic particles in the coating after latex consolidation and compaction.4 In the present study, coatings were prepared from dispersions of ceramic nanoparticles and monodisperse latex, and the result is a much different microstructure with a nonuniform distribution of nanoparticles. The microstructure development during drying was characterized by cryo-SEM, and the role of process parameters on microstructure was explored. The distribution of particles and binders in drying coatings has been studied for many years. Much of the activity and interest has been in the field of paper coatings, which are ordinarily prepared from dispersions of pigment particles, such as calcium carbonate and clay, and binder (either soluble polymer or * To whom correspondence should be addressed. Tel:1-612-625-1313. Fax: 1-612-626-7246. Email:
[email protected]. † Present address: Brady Corporation, 2230 West Florist Avenue, Milwaukee, WI 53209. ‡ Deceased, 2 August 2007. (1) Tiarks, F.; Frechen, T.; Kirsch, S.; Leuninger, J.; Melan, M.; Pfau, A.; Richter, F.; Schuler, B.; Zhao, C. Macromol. Symp. 2002, 187, 739–751. (2) Sun, J.; Gerberich, W. W.; Francis, L. F. J. Polym. Sci., Part B: Polym. Phys. 2003, 41, 1744–1761. (3) Sun, J.; Velamakanni, B. V.; Gerberich, W. W.; Francis, L. F. J. Colloid Interface Sci. 2004, 280, 387–399. (4) Luo, H.; Scriven, L. E.; Francis, L. F. J. Colloid Interface Sci. 2007, 316, 500–509.
particulate latex polymer). After drying, the concentration of polymer binder at the coating surface is sometimes higher than in the bulk of the coating.5–13 Since the as-deposited dispersions are homogeneous, the concentration gradient develops during drying; this phenomenon is called binder migration. For soluble polymer binders, such as starch, binder migration is most pronounced when coatings are dried with hot air impingement and is absent with room temperature drying in still air.5 Binder migration is linked to the competing effects of drying, which concentrates the binder at the evaporating surface; diffusion, which acts to erase the concentration gradient as it develops; and capillary-driven flow, which brings liquid to curved liquid–vapor menisci that develop in the coating as particles consolidate or liquid intrudes into a porous base.12,14,15 These competing effects are influenced by the composition of the coating, the drying conditions, and often the porous paper substrate. Migration of latex binders is particularly sensitive to these factors. For example, Bitla et al.13 found that latex migration was pronounced when the ratio of pigment particle size to latex particle size was greater than 3, and Al-Turaif and Bousfield16 noted that latex migration was suppressed when the pigment had a broad particle size distribution. In addition, latex migration can be slowed in the presence of a soluble polymer10 and is sensitive to the sorption properties of the base paper.8 (5) Dappen, J. W. Tappi J. 1951, 34, 325–335. (6) Krishnagopalan, A.; Simard, G. L. Tappi J. 1976, 59, 96–99. (7) Tomimasu, H.; Ogawa, S.; Sakai, Y.; Yamasaki, T.; Ogura, T. ESCA to analyze surface binder concentration of coated paper. In Proceedings of the 1986 TAPPI Coating Conference, Washington, DC, 4–7 May 1986; TAPPI Press: Atlanta, 1986; pp 35–43.. (8) Engstroem, G.; Rigdahl, M.; Kline, J.; Ahlroos, J. Tappi J. 1991, 74, 171– 179. (9) Kline, J. E. Tappi J. 1991, 74, 177–182. (10) Bushhouse, S. G. Tappi J. 1992, 75, 231–237. (11) Yamazaki, K.; Nishioka, T.; Hattori, Y.; Fujita, K. Tappi J. 1993, 76, 79–84. (12) Binder Migration in Paper and Paperboard Coatings; Whalen-Shaw, M. J.; Ed.; TAPPI Press: Atlanta, GA, 1993. (13) Bitla, S.; Tripp, C. P.; Bousfield, D. W. J. Pulp Paper Sci. 2003, 29, 382–385. (14) Hagen, K. G. Tappi J. 1986, 69, 93–96. (15) Pan, S. X.; Davis, H. T.; Scriven, L. E. Tappi J. 1995, 78, 127–143. (16) Al-Turaif, H. A.; Bousfield, D. W. Nord. Pulp Pap. Res. J. 2005, 20, 335–339.
10.1021/la800050u CCC: $40.75 2008 American Chemical Society Published on Web 04/17/2008
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Nonuniform particle distributions have also been reported in coatings made from latex blends. Eckersley et al.17 reported a higher concentration of small latex particles on the top surface for coatings prepared from a bimodal blend with a large to small particle size ratio of 4. Ma et al.18 used cryo-SEM to study the stages of latex film formation in monodisperse and bimodal latex blends. In a bimodal system with small, soft latex particles and large, hard latex particles, they found a higher concentration of small latex particles near the coating surface in cryo-SEM images of partially dried coatings. They proposed that the smaller particles, which occupied the spaces between the larger particles, were drawn by convection to the evaporating surface and left there because Brownian diffusion was slow relative to evaporation. Recently, researchers have uncovered interesting microstructures in materials made by convective assembly or drying of bimodal dispersions of particles. Tessier and co-workers19 used convective assembly to create 2D latex colloidal crystals with gold nanoparticles packed in the interstices selectively near the base of the coating. They postulated that the latex consolidated and ordered first and the gold particles were concentrated at the drying front, but the details of the evolution in structure were not given. Kitaev and Ozin20 performed a similar study with convective assembly of bimodal dispersions. Harris and coworkers21 prepared coatings from a mixture of large silica microspheres and nanoparticles and dried them beneath a mask that controlled the spatial pattern of evaporation. Particles were drawn to the unmasked regions where evaporation rate was highest; they proposed that the larger particles packed first and the smaller particles were pulled through the pore space to the evaporating surface by a capillary pressure gradient. Burkert and co-workers22 printed hemispherical droplets of bimodal polystyrene and found regions of order in the latex particles and interstitial spaces filled with the small particles. They did not speculate on the microstructure development. Qi and Birnie23 prepared coatings with monodisperse latex and titania nanoparticles and then removed the latex by pyrolysis to create a porous titania. Likewise, the microstructure development during drying of the original composite was not discussed. Although the phenomena responsible for redistribution of particles in drying coatings have been laid out, there have been no systematic studies on the effects of process variables on microstructure development in bimodal systems and little direct evidence for microstructure development during drying. In this research, we use cryo-SEM to follow the steps of microstructure evolution during the drying of coatings made from dispersions of ceramic nanoparticles and larger, monodisperse latex and explore the effects of processing variables on microstructure. A microstructure formation mechanism is proposed, building on the research reviewed above. With the proper choice of processing variables, composite coatings with a nanoparticle-rich surface and a compacted and coalesced latex base were prepared.
Experimental Section Synthesis and Characterization of Surfactant-Free Latex Poly(methyl methacrylate-cobutyl acrylate) copolymer (PMMAco-BA) latex was prepared by a batch, surfactant-free emulsion (17) Eckersley, S. T.; Helmer, B. J. J. Coat. Technol. 1997, 69, 97–107. (18) Ma, Y.; Davis, H. T.; Scriven, L. E. Prog. Org. Coat. 2005, 52, 46–62. (19) (a) Tessier, P. M.; Velev, O. D.; Kalambur, A. T.; Rabolt, J. F.; Lenhoff, A. M.; Kaler, E. W. J. Am. Chem. Soc. 2000, 122, 9554–9555. (b) Tessier, P. M.; Velev, O. D.; Kalambur, A. T.; Lenhoff, A. M.; Rabolt, J. F.; Kaler, E. W AdV. Mater. 2001, 13, 396–400. (20) Kitaev, V.; Ozin, G. A. AdV. Mater. 2003, 15, 75–78. (21) Harris, D. J.; Hu, H.; Conrad, J. C.; Lewis, J. A Phys. ReV. Lett. 2007, 98148301/1–148301/4. (22) Burkert, K.; Neumann, T.; Wang, J.; Jonas, U.; Knoll, W.; Ottleben, H. Langmuir 2007, 23, 3478–3484. (23) Qi, L.; Birnie, D. P. Mater. Lett. 2007, 61, 2191–2194.
Langmuir, Vol. 24, No. 10, 2008 5553 polymerization. Methyl methacrylate (MMA) and butyl acrylate (BA) monomers (Aldrich, St. Louis, MO) were washed with sodium hydroxide and distilled water three times to remove the monomethyl ether hydroquinone inhibitor. Sodium persulfate was used as the initiator; the ionic groups from the initiator on surfaces of latex particles provided electrostatic repulsion to stabilize the latex. Sodium bicarbonate was used as a pH buffer. The synthesis was carried out in a 1-L three-neck flask equipped with a reflux condenser, nitrogen gas inlet, and a mechanical stirrer. Detailed synthesis procedures can be found elsewhere.24 The glass transition temperature (Tg) of latex increases as the ratio of MMA to BA increases. Two latex compositions were prepared: high Tg latex with the MMA to BA ratio (w/w) of 62.5:37.5 and low Tg latex with the MMA to BA ratio (w/w) of 30:70. The Tg’s measured by a differential scanning calorimeter Q1000 (TA Instruments, New Castle, DE) were 48 and -35 °C. Particle size, measured with a Coulter LS particle size analyzer (Coulter Corp., Miami, FL), was 550 nm with a polydispersity index (PDI) of 1.05 for both low Tg and high Tg latex. The ζ potential of the high Tg latex in 10-3 M potassium chloride solution at pH ) 8.5 was found to be -80 mV, using a ζ potential analyzer (Brookhaven Instruments Corporation, Holtsville, NY). Ceramic Nanoparticle Dispersions. Most coatings in this research were prepared with a commercial, aqueous silica nanoparticle dispersion, Cabot PG-002 (Cabot, Tuscola, IL). The Cabot silica consists of open aggregates of 20 nm silica particles with a median aggregate diameter of about 150 nm, as specified by the manufacturer’s light-scattering results. The as-received dispersion (20 wt % solids) is stabilized at a pH of 9.2 such that the silica has a negative ζ potential. Throughout the paper these particles are referred to as “silica”. In addition, the ceramic particles are referred to as nanoparticles. Several other commercial nanoparticles were also used in the exploration of processing variables. Ludox SM-30 (Aldrich, St. Louis, MO) is an aqueous silica dispersion of 7 nm particles that are not aggregated. The as-received dispersion (30 wt % solids) is stabilized at pH 10 such that the silica has a negative ζ potential. This dispersion will be referred to as “Ludox silica” to distinguish it from the Cabot silica, which was used for the bulk of the research. Alumina and antimony doped tin oxide (ATO) nanoparticles were dry powders as received (Nanophase Technologies, Romeoville, IL). Alumina and ATO particles have average particle sizes of 45 and 30 nm, respectively. Both particles were first dispersed in deionized water with a solid loading of 0.3 wt %. The ATO dispersion had a pH of 3.5. Darvan 7 (R.T. Vanderbilt, Norwalk, CT), sodium polymethacrylate solution, with an amount of 5 wt % of dry alumina powder was then added to the alumina dispersion, and the dispersion had a pH of 8. Under these conditions both the ATO3 and the alumina25 have negative ζ potentials and are stable. Preparation of Silica Nanoparticle/Latex Dispersions and Coatings To prepare a silica nanoparticle/latex dispersion, the required amount of the Cabot silica dispersion was added to deionized water and then sonicated in a Branson 2200 sonicator (Branson Ultrasonics Corp., Danbury, CT) for 30 min. Next, the required amount of PMMA-co-BA latex was added to the diluted silica dispersion during magnetic stirring. The silica/latex dispersion was further stirred for 2–3 h. The pH of the silica/latex dispersion was 8.5. The total solids content (silica and latex) of the dispersion was 5 wt %, and the content of the silica in the dried coating was 6 wt % (i.e., 94 wt % polymer) in the standard case (see Table 1). To prepare the composite coating, a controlled volume (i.e., 10 µL) of the freshly prepared silica/latex dispersion was transferred by a micropipette and spread with the pipet tip onto a 5 × 7 mm silicon substrate. In the standard case, the coating was dried at a mechanical convection oven (Blue M, Blue Island, IL) at 60 °C. A small substrate (i.e., 5 mm × 7 mm) was used to facilitate the cryo(24) Grunlan, J. C.; Ma, Y.; Grunlan, M. A.; Gerberich, W. W.; Francis, L. F. Polymer 2001, 42, 6913–6921. (25) Pettersson, A.; Marino, G.; Pursiheimo, A.; Rosenholm, J. B. J. Colloid Interface Sci. 2000, 228, 73–81.
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Table 1. Conditions for Exploration of the Effects of Processing Variables on Microstructure variable
nanoparticle
latex
nanoparticle content (wt % dry)
standard nanoparticle content drying temperature air flow latex Tg nanoparticle size nanoparticle type
Cabot silica Cabot silica Cabot silica Cabot silica Cabot silica Ludox silica ATO, alumina, Ludox silica
high Tg high Tg high Tg high Tg low Tg high Tg high Tg
6 28 6 6 6 6 6
SEM study. The dispersion was also deposited in a similar manner onto a 2.5 cm × 7.5 cm glass slide, and the resultant coating was dried under the same conditions. Cryo-SEM of Silica Nanoparticle/Latex Dispersion and Partially Dried Coatings Cryogenic scanning electron microscopy (cryo-SEM) was used to image the microstructures of the Cabot silica/latex dispersions and coatings at different stages of drying. The first step of cryo-SEM preparation, rapid freezing, preserves and immobilizes the microstructure of the wet sample. In addition, because of the low vapor pressure of the resulting solid, the frozen sample can survive in the high-vacuum chamber of the SEM. High-pressure freezing and plunge freezing were two fast freezing methods used in this work. High-pressure freezing at 2.1 kbar was used to fast freeze thick dispersion samples (i.e., 300 µm). Dispersion samples were sealed between two freezing planchettes (type A, Ted Pella Inc., Redding, CA) with 3 mm diameter and loaded into a high-pressure freezing machine (Bal-Tec AG, Balzers, Liechtenstein). Plunge freezing was used to fast freeze thin wet coatings (e.g., 100 µm), which were applied to small silicon substrates and dried at 60 °C in the convection oven. Identically prepared coatings were dried for different times, quickly removed from the oven, and hand-plunged into liquid ethane at its freezing temperature (i.e., -183 °C). After samples were fast frozen, they were fractured to expose internal structures, sublimed at -90 to -110 °C for 5–18 min to remove some frozen water and enhance topographical contrast, and sputter-coated with a few nanometers of platinum to minimize charging of the frozen samples. All freeze-fracture, sublimation, and metal coating steps were carried out in high-vacuum preparation chambers. Finally, the frozen samples were imaged with field emission scanning electron microscopes (FESEMs) equipped with cold stages at around -160 °C. A Hitachi S-900 FESEM (Nissei Sangyo America, Rolling Meadows, IL) and a Hitachi S-4700 FESEM were used for dispersions and coatings, respectively. In the case of frozen coatings, cross-sectional images were taken approximately the same location, near the specimen center. Detailed sample preparation procedures can be found elsewhere.4 Exploration of Processing Variables. Table 1 shows the different processing variables changed to investigate the microstructure formation mechanism. The effects of nanoparticle content, nanoparticle size, latex Tg, drying temperature, and air flow on the final, dried microstructure of the coating were determined. In most cases only one processing variable was changed and all other conditions were identical to those described in detail above for the standard case. Characterization of Final Coating Microstructures. SEM and atomic force microscopy (AFM) were used to image microstructures of the dried nanoparticle/latex coatings. SEM images of both top surfaces and cross sections were taken on a Hitachi S-4700 FESEM. The cross sections of the coatings were prepared by fracturing the coatings in liquid nitrogen to minimize the plastic deformation during the fracture. AFM images of the top surfaces were taken in a tapping mode atomic force microscope (Digital Instruments, Nanoscope III, Woodbury, NY).
Results Microstructure of Dried Silica Nanoparticle/Latex Coating The microstructure of a dried silica nanoparticle/latex coating is shown in Figure 1. In the high magnification SEM image (Figure 1a), dark circles of uniform size (diameter, 320 nm) and
drying conditions 60 60 22 20 60 60 60
°C, °C, °C, °C, °C, °C, °C,
convection convection convection still air convection convection convection
oven oven oven; 40 °C, convection oven oven oven oven
Figure 1. SEM images of the top surface of a dried silica/latex coating prepared using standard conditions: (a) high magnification and (b) low magnification.
hexagonal packing are the latex particles, and the bright, textured regions between the latex particles are silica nanoparticles. The high contrast originates from the difference in secondary electron (SE) emission from the two regions: the smooth, flat latex surfaces emit far fewer SE’s than the porous nanoparticle silica regions. The dark circles have a smaller diameter than the latex particles, as measured by light scattering, but the center-to-center distance between two circles is close to the diameter of the latex particle, indicating the particles are closely packed. Silica nanoparticles, which are open aggregates of ∼ 150 nm in diameter, fill the spaces between the latex particles. The low magnification image (Figure 1b) shows ordered domains with a size of about 10 µm and bright, silica-rich boundaries between the domains. The cross-sectional microstructure of the same coating is shown in Figure 2. The high magnification image (Figure 2a) reveals a silica-rich zone near the surface. The low magnification image (Figure 2b) shows a silica-rich zone and a latex-rich structure located beneath. The silica-rich zone has a thickness of ∼2.5 µm, which is 20% of the dried coating thickness. Assuming the monodisperse latex particles are close-packed and occupy 72 vol % of the coating and the nanoparticles pack to 50% dense as they fill the spaces between latex from the top down, the
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Figure 3. Cryo-SEM image of a frozen silica/latex dispersion. Large spherical latex particles and small silica aggregates (Cabot) are distributed uniformly in the frozen water matrix
Figure 2. SEM images of the cross section of a dried silica/latex coating prepared using standard conditions: (a) high magnification and (b) low magnification.
nanoparticle-rich zone is estimated to be about 21% of the coating thickness. After coalescence and therefore shrinkage of the latexrich zone, this value rises to 27%. The images and these estimates indicate that the nanoparticles are mostly in the nanoparticlerich zone near the surface. Figure 2a also shows the packing of the latex particles near the surface. Latex particles elongated by fracture are present in the image along with spherical latex sockets, which are spaces left behind after the latex particles were plucked out to the complementary fracture surface. These round sockets indicate that the latex particles did not deform substantially during drying. Since the latex particles are closely packed, latex-latex contacts exist, but flattening of the contacts is restricted by the nanoparticles in the interstitial spaces. In contrast, below the nanoparticle-rich zone, latex particles deform and compact to polyhedra (Figure 2b). In this location the usual sequence of film formation26,27 proceeds unimpeded. Cryo-SEM of Silica/Latex Dispersion The distribution of latex and silica particles in the starting Cabot silica/latex dispersion is shown in Figure 3, a cryo-SEM image of a frozen dispersion. The frozen dispersion was sublimed at -110 °C for 5 min to remove a small amount of frozen water to enhance contrast. Latex particles are the larger, spherical particles and silica aggregates are the smaller clusters. The latex particles have a range of diameters because the fracture plane intersects them at different locations. The elongated stems on some of the particles (26) (a) Vanderhoff, J. W. Br. Polym. J. 1970, 2, 161. (b) Keddie, J. L. Mater. Sci. Eng., R 1997, R21, 101–170. (c) Winnik, M. A. Curr. Opin. Colloid Interface Sci. 1997, 2, 192–199. (27) Ma, Y. Ph. D. Thesis, University of Minnesota, MN, 2002.
result from plastic deformation during the fracture.28 Individual latex particles and silica aggregates distribute homogeneously in the frozen dispersion. Both latex and silica particles have negative ζ potentials at the dispersion pH of 8.5. The separation between latex particles and silica aggregates indicates electrostatic repulsion is sufficient to stabilize the dispersion. This result concurs with observations of the stability of the silica/latex dispersions. Cryo-SEM of Silica/Latex Coatings at Successive Drying Times. Figure 4 shows a sequence of low magnification cryoSEM images of identically prepared silica/latex coatings dried at 60 °C for 20, 35, 55, and 75 s. The coating thickness shrinks from 180 µm after 20 s of drying to 40 µm after 75 s of drying, corresponding to a shrinkage rate of ∼2.5 µm/s. A common feature in these images is a packing of latex particles at the coating surface and a dispersed structure beneath. The silica nanoparticles are not easily distinguished at this magnification. The vertical features in the dispersed structure are either from the fracture or freezing artifacts as these areas have high water concentration.4 The boundary separating the packed layer at the surface and the dispersed structure beneath is called the “topdown” consolidation front.18,27 In addition to this top-down consolidation process, there was an “edge-in” consolidation front. Under the standard drying conditions, the edge-in front was highly localized29 and did not impact the microstructure development sequence shown here. The top-down front progressed from a position 4 µm beneath the surface after 20 s of drying to 20 µm after 75 s of drying. From analysis of all the images, the consolidation front grows at an approximately constant rate of 0.28 µm/s. Additional information about microstructure development during drying was revealed by visual observation. Iridescent spots on the coating surface appeared during drying and then spread over the coating surface. These optical effects are due to the ordered packing of latex at the surface, which creates the opalescent scattering. Figures 5 and 6 are high magnification cryo-SEM images from coatings dried at 60 °C for 60 and 75 s, respectively. Both frozen coatings were sublimed at -90 °C for 20 min to expose nanoparticles embedded in the frozen water on the fractured (28) Ge, H.; Zhao, C.; Porzio, S.; Zhuo, L.; Davis, H. T.; Scriven, L. E. Macromolecules 2006, 39, 5531–5539. (29) Luo, H. Ph.D. Thesis, University of Minnesota, Minneapolis, MN, 2007.
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Figure 4. Sequence of cryo-SEM images of identically prepared silica/latex coatings dried at 60 °C for (a) 20, (b) 35, (c) 55, and (d) 75 s. Coating thicknesses are (a) 180, (b) 130, (c) 90, and (d) 40 µm. The position of the top-down consolidation front is noted on the images.
Figure 5. High magnification cryo-SEM images of a silica/latex coating dried for 60 s at 60 °C: (a) tilted to show top surface and (b) cross section. The silica nanoparticles partially occupy interstitial spaces among orderly packed latex particles on the surface.
Figure 6. High magnification cryo-SEM images of a silica/latex coating dried for 75 s at 60 °C: (a) tilted to show top surfaces and (b) cross section. The silica nanoparticles occupy all the interstitial spaces among the closely packed latex particles on the surface.
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Figure 7. SEM images of coatings prepared from dispersions with different silica concentrations and dried at 60 °C. A dried coating containing 6 wt % silica: (a) surface and (b) cross section. A dried coating containing 28 wt % silica: (c) surface and (d) cross section.
surface. On the top surface of the coating dried for 60 s (Figure 5a), silica nanoparticles partially occupy interstitial spaces among orderly packed latex particles. On the cross section of the same coating (Figure 5b), the latex particles pack in an ordered array. On the top surface of the coating dried for 75 s (Figure 6a), more silica nanoparticles appear and these particles occupy all interstitial spaces among the orderly packed latex particles. On the cross section of the same coating (Figure 6b), a higher concentration of silica is also apparent and latex particles are more closely packed. The low magnification image of the coating dried for 75 s (Figure 4d), reveals the consolidation front is still above the substrate. Therefore, nanoparticles begin to fill the interstitial spaces among the latex particles in the top layer before the coating completely consolidates. The cryo-SEM images of microstructure development reveal two critical microstructure formation steps: top-down consolidation of latex particles and an increase of nanoparticle population on and near the surface during drying. Processing Conditions Affecting Microstructure. Figure 7 compares the top surface and cross-sectional microstructures of coatings with 6 wt % silica and 28 wt % silica in the final dried coatings. The surface microstructures are nearly identical with silica particles distributed in interstitial spaces among ordered latex arrays. Image analysis results show the center-to-center distance between the neighboring latex particles is close to the diameter of individual latex particles in both cases. On the cross sections, the nanoparticles are preferentially distributed in the latex matrix near the surface in the case of low silica content (Figure 7b), whereas the nanoparticles are completely distributed in the latex matrix along the entire cross section in the case of high silica content (Figure 7d). With the latex packing to ∼72 vol %, the 28 wt % silica coating contains enough nanoparticles
to fill all the interstitial spaces with a 50% dense packing of aggregates. The effect of drying temperature on top surface microstructure is shown in Figure 8. The AFM images show that as the drying temperature increases, the quantity of nanoparticles on the surface (or surface area coverage) increases and the size of the exposed latex dome decreases. In each case, the center-to-center distance between neighboring latex particles is nearly the same as the diameter of the latex particle, indicating good packing. However, nanoparticles do not uniformly fill all of the interstitial spaces in the coating that is dried at 22 °C. Cross-sectional SEMs show that the nanoparticle-rich zone in coatings dried at 40 and 22 °C is not as distinct as that for the coatings dried at 60 °C; more nanoparticles are found deeper in these coatings and the latexrich zone is not compacted (see Supporting Information). Also, the coating dried at 22 °C had variations in coating thickness. The nonuniformities found with lower drying temperature were exacerbated when drying was carried out in still air at room temperature; see Figure 9. The center area (Figure 9a) is depleted of particles, indicating lateral flow to the edges or edge-in drying under these conditions. Nearer to the edge (Figure 9b), latex particles pack but the interstitial filling is irregular. Figure 10 compares the AFM images of the top surface of a coating prepared from Ludox silica (7 nm) and dried at 60 °C in a convection oven. Compared with the Cabot silica-containing coating prepared under identical conditions (Figure 8c), the coating containing the 7 nm particles has fewer nanoparticles on the surface and larger exposed latex dome size. The center-tocenter distance between neighboring latex particles is nearly the same as the diameter of the latex particle, indicating the latex packs, and the interstitial spaces are filled uniformly with nanoparticles. The cross section of the Ludox silica/latex coating
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Figure 9. SEM images of a silica/latex coating dried in still air at 20 °C: (a) center and (b) nearer to edge. Figure 8. AFM images of top surfaces of silica/latex coatings prepared from the same dispersion but dried in the convection oven at (a) 22, (b) 40, and (c) 60 °C. Coatings dried at a higher temperature have a higher amount of silica nanoparticles and smaller latex dome size on the surface.
contains a nanoparticle-rich zone, but this zone is thicker than that noted for the Cabot silica/latex coating and more silica nanoparticles are found deeper in the coating (see Supporting Information). Figure 11 compares the microstructures of the top surfaces of coatings prepared from low Tg, -35 °C, or high Tg, 48 °C, latex with all other parameters the same as the standard conditions. Both images show that nanoparticles uniformly fill interstitial spaces among the packed latex particles. In addition, both coatings have a similar amount of silica particles distributed among interstitial spaces and similar exposed dome size for the latex particles. The nanoparticle-rich zone near the surface is the same in cross-sectional images (see Supporting Information). The glass transition temperature of the latex does not influence the amount of nanoparticles on the surface or the nanoparticle distribution. This simple coating method was used with other types of nanoparticles. Antimony-doped tin oxide (ATO)/latex and alumina/latex coatings were prepared by the same method and dried at 60 °C in the convection oven. A set of SEM images of the top surfaces of ceramic/latex nanoparticle coatings shown in Figure 12 demonstrates that the same pattern of nanoparticles uniformly distributed among orderly packed latex particles. The latex particles in Figure 12b appear brighter than those in the other images due to the larger latex dome size (see Figure 10), which creates greater curvature and higher secondary electron emission. Likewise, a nanoparticle-rich zone near the surface is found for all in the cross-sectional analysis.
Figure 10. AFM image of the top surface of a coating prepared with Ludox silica and dried at 60 °C. Compared with Figure 8c, a lesser amount of silica is found on the surface and the latex domes are larger.
Discussion and Conclusion In this section, the microstructure formation captured by cryoSEM and the effect of process variables on the final dried coating microstructure are discussed in light of the transport phenomena elaborated in studies of binder migration12–16 and binary colloidal mixtures18,19,21 and the established physics of latex film formation.26 As drying starts, water evaporates and the free surface of the coating descends toward the substrate (i.e., the coating thickness decreases), creating a particle concentration gradient through the coating thickness. Brownian diffusion tends to erase this
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Figure 11. SEM images of top surfaces of silica/latex coatings prepared from latex with different Tg’s and dried at 60 °C: (a) low Tg, –35 °C; (b) high Tg, 48 °C. Both coatings have a similar amount of silica on the surface.
concentration gradient. The competition between evaporationinduced accumulation and diffusion-induced homogenization can be characterized by a Peclet number (Pe):30,31
Pe )
HE D
(1)
where H is the starting wet coating thickness, E is the evaporation rate expressed as the velocity of free surface descent, and D is the diffusion coefficient of the particles. For spherical particles, D can be calculated from the Stokes–Einstein equation:
D)
kBT 6πRη
(2)
where R is the particle radius, kB is Boltzman’s constant, T is the absolute temperature, and η is the viscosity of the surrounding liquid. When Pe . 1, evaporation dominates over diffusion, and the particles accumulate at the free surface. In the standard coating, E is estimated to be 2.5 µm/s from the cryo-SEM images (Figure 4), D is estimated to be 1.9 µm2/s for latex particles, and H is estimated to be 350 µm. Therefore, Pe is calculated to be 460 for latex particles. Assuming an equivalent spherical diameter of 150 nm for the silica aggregates, Pe for the nanoparticles is approximately 120; this value has greater error due to the particle morphology and range in size. Hence both types of particles should accumulate as the free surface descends. (30) Routh, A. F.; Russel, W. B. AIChE J. 1998, 44, 2088–2098. (31) Routh, A. F.; Zimmerman, W. B. Chem. Eng. Sci. 2004, 59, 2961–2968.
Latex particles, present in far greater volume concentration in coating dispersion, form a surface layer first. This surface layer likely begins as small clusters of particles, which then grow to a continuous layer of packed particles. Brownian motion or lateral capillary attraction due to nonuniform wetting of the particles creates clusters.32 Once a cluster with a few particles (i.e., three particles) nucleates, a meniscus between the particles curves with continued drying. Thus, the lowered pressure in the liquid beneath the curved meniscus initiates a convective flow, which transports other nearby particles laterally to the growing cluster. Order is achieved because the particles are monodisperse and have repulsion interactions, which allow them to rearrange.33,34 This order was observed visually as iridescence. Multiple clusters nucleate and grow, and eventually they impinge to form a continuous layer of orderly packed latex particles on the top surface. Because the impinged clusters cannot rotate to align to one orientation, ordered domains with different orientations appear with disordered domain boundaries (Figure 1b). The accumulation of latex particles at the surface creates a consolidation front above which the latex is in a closed packed structure and below which it is dispersed. Cryo-SEM results show that the front moves down (i.e., the consolidated layer thickness grows) at a constant rate and the coating thickness decreases at a constant rate as drying continues (Figure 4). A simple volume balance calculation was used to confirm that the consolidation front growth results primarily from the free surface descent with little if any effect of a convective flow. There is a pressure gradient across the consolidated layer from atmospheric pressure (in the liquid just beneath the consolidated layer) to subatmospheric pressure (in the liquid just beneath the curved menisci at the top surface)35 that causes convective flow within the consolidated layer, but this flow does not draw particles beneath to the layer itself. Particles, latex and nanoparticles alike, join the consolidated layer because the free surface is descending. The convective flow is important to the distribution of ceramic nanoparticles, as described below. Order among the monodisperse latex particles appears to degrade deeper in the coating, an effect that has also been observed by cryo-SEM in monodisperse latex coatings (with no nanoparticles present).18 One possible reason for this behavior is that evaporation drives up the ionic strength of the water, lessening the repulsion that allows the particles to rearrange. Nanoparticles accumulate at the surface, filling in the spaces between the latex particles in the consolidated layer from the top down (Figures 4–6). Due to their size, nanoparticles are trapped between latex particles in the consolidated layer and convective flow transports them to the evaporating menisci located between the packed latex particles on the surface. As drying continues and the consolidated layer thickens, the flow resistance within the layer increases and the menisci at the free surface must curve more to keep a constant drying rate,18 which was found in the analysis of cryo-SEM data (Figure 4). Hence, the pressure gradient increases and draws more and more nanoparticles caught up in the layer to the surface, resulting eventually in a nanoparticlerich zone at the surface. In addition to the cryo-SEM images, support for this sequence is found in the fact that the thickness of the nanoparticle-rich zone increased when the silica particle content in the dispersion increased, but the surface microstructure was not changed (Figure 7). (32) Stamou, D.; Duschl, C.; Johannsmann, D. Phys. ReV. E 2000, 62, 5263– 5272. (33) Pieranski, P. Phys. ReV. Lett. 1980, 45, 569–572. (34) Van Winkle, D. H.; Murray, C. A. Phys. ReV. A. 1986, 34, 562–573. (35) Arlinghaus, E. G. Ph.D. Thesis, University of Minnesota, Minneapolis, MN, 2004.
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Figure 12. SEM images of top surfaces of coatings prepared from a variety of nanoparticles: (a) Cabot silica, (b) Ludox silica, (c) antimony-doped tin oxide (ATO), and (d) alumina. All the coatings show the same pattern of nanoparticles uniformly filling interstitial spaces among ordered arrays of latex particles.
Figure 13. Schematic drawing of coating microstructure formation: (a) initial coating, (b) layer of latex forms on the surface, curved menisci form, (c) consolidated layer thickens, nanoparticles are pulled through consolidated layer to surface, (d) consolidated layer reaches the substrate, (e) menisci recede into coating, and (f) latex particles compact.
To be transported through the consolidated layer, the nanoparticle size must be smaller than the size of pore throat and particle–particle interactions should be repulsive. In a hexagonal packing of monodisperse particles, the ratio between the radius of the pore throat and the particle radius is about 0.15.35 For the latex in this study, the pore throat diameter is about 80 nm, big enough to allow the passage of the irregularly shaped silica aggregates as well as several other nanoparticles (Figure 12). The aggregates of 20 nm primary particles are in an open structure, which apparently has one dimension small enough for passage through the channels. This sort of sifting of nanoparticles to the drying surface has also been observed and attributed to convective flow in other studies of bimodal particle coatings.21 Past research on coatings prepared with polydisperse latex and ceramic nanoparticles2,4 showed no nanoparticle-rich zones because the polydispersity restricted the size of the pore throats.
The accumulation of nanoparticles on the coating surface is sensitive to the processing conditions. In general, nanoparticle accumulation results from a competition between evaporation, convective flow, and diffusion. Evaporation and convective flow lead to an accumulation of nanoparticles at the air–water interface, whereas diffusion tends to equalize the concentration gradient. The convective flow is proportional to the pressure difference, which increases with the curvature of the liquid/vapor menisci. The menisci curve more at a high evaporation rate to compensate for water loss. Hence increasing the drying rate promotes stronger convective flow and more accumulation of nanoparticles. The conditions used for most of this research (60 °C drying in a convective air flow oven) result in a high evaporation rate and strong convective flows. When the drying temperature was lowered such that the evaporation rate dropped and the convective flow weakened, fewer nanoparticles were found on the surface
Ceramic Nanoparticle/Monodisperse Latex Coatings
(Figure 8). It should be noted that the diffusion coefficient drops with drying temperature, but this decrease is less than an order of magnitude and is likely overshadowed by a large change in the evaporation rate. The surfaces of coatings prepared with smaller nanoparticles (7 nm spherical particles) had fewer nanoparticles (Figure 10), presumably because their higher diffusion coefficient was more effective at equalizing the concentration gradient. With further drying, the consolidation front reaches the substrate and the menisci retreat into the particle packing. The final surface coverage of nanoparticles observed in AFM and SEM images is likely linked to the concentration of nanoparticles in the water occupying the uppermost interstitial spaces at this time point. Additionally, the continued drying results in the expected progression of latex film formation.26 In the upper portion of coating where nanoparticles occupy in interstitial spaces, the latex-latex contacts flatten to a certain degree and the interstitial spaces shrink, but latex particles still keep their spherical shape because flattening is restricted by nanoparticles (see Figure 2). Below the nanoparticle-rich zone, latex particles deform and compact to polyhedra. Nanoparticles apparently fill the interstitial spaces in the consolidated latex layer before the next stage of latex coalescence begins, because the nanoparticles are trapped in the near surface region for both low Tg and high Tg latex (see Figure 11); that is, flattening at latex-latex contacts occurs after the nanoparticles have migrated to the upper surface of the coating. In conclusion, the schematic drawing in Figure 13 summarizes the coating microstructure formation sequence. First, the descending free surface captures individual latex particles, which form clusters, grow, and link into a well-packed latex layer on the surface. With further drying, latex particles and nanoparticles
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are captured into a consolidated layer. By virtue of their monodispersity, the latex particles pack orderly in the consolidated layer. At the free surface, liquid/vapor menisci between latex particles curve to accommodate continued evaporation, driving a vertical convective flow through the layer. By virtue of their size, the nanoparticles are transported through channels among the latex in the consolidated layer to the evaporating menisci at the surface. As drying continues, the coating is completely consolidated, and latex compaction and coalescence follows to form a coherent coating. To create the well-ordered microstructure and the nanoparticle-rich zone at the surface, high drying rates are required so that the top-down consolidation is favored, and a large latex to nanoparticle size ratio is necessary for the easy transport of the nanoparticles through the latex packing. The final microstructure with a nanoparticle-rich surface potentially has advantages for applications requiring the chemical or physical properties of the nanoparticles, and the structural integrity and adhesion of a latex-based coating. Acknowledgment. This paper is dedicated to memory Prof. L. E. (Skip) Scriven. The authors thank the industrial supporters of the Coating Process Fundamentals Program of the Industrial Partnership for Interfacial and Materials Engineering (IPRIME) for supporting this research. Parts of this work were carried out in the University of Minnesota I.T. Characterization Facility, which receives partial support from NSF through the NNIN program. Supporting Information Available: SEM images of cross sections of coatings shown in Figures 8b, 9b, 11a and 12b. This material is available free of charge via the Internet at http://pubs.acs.org. LA800050U
