Langmuir 2001, 17, 5671-5677
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Meso- and Microscopic Behavior of Spherical Polymer Particles Assembling at the Air-Water Interface Elizabeth Wolert, Stefan M. Setz, Royale S. Underhill, and Randolph S. Duran* Butler Polymer Laboratory, Department of Chemistry, University of Florida, Gainesville, Florida 32611
Michel Schappacher and Alan Deffieux Laboratoire de Chimie des Polymeres Organiques, ENSCPB-CNRS-Universite Bordeaux 1 (UMR) 5629, Avenue Pey Berland, BP 108, 33 402 Talence Cedex, France
Matthias Ho¨lderle and Rolf Mu¨lhaupt Institut fu¨ r Makromolekulare Chemie, Stefan-Meier-Strasse 31, D-79104 Freiburg i.Br., Germany Received February 5, 2001. In Final Form: June 4, 2001 Using the Langmuir Blodgett (LB) technique, monolayers of spherical polymer particles were investigated at the air-water interface. In this study, LB methods were used to examine particle interactions, packing, morphology, and viscosity of polymer films. Films composed of polymer microgels (diameters of 113-427 nm) having a block-copolymer dispersant were investigated through isotherm, atomic force microscopy imaging, and two-dimensional (2D) viscosity studies. During monolayer formation, the effect of the dispersant and particle size on the behavior of the isotherms was examined. The contact cross-sectional area of the isotherms for these spheres was compared to the ideal hexagonal close packing (hcp) model. Initial investigations concerning the flow properties of the particles in 2D slurries were also performed. In addition, spherical hyperbranched polymer particles (≈60 nm) with and without a modified hydrophilic surface were also studied using isotherm and hysteresis experiments and the hcp model. Both of these studies may show the influence of surface composition and particle size on the interaction of polymer particles at the airwater interface.
Introduction Nanoparticles have become of great significance due to their interesting optoelectric and chemical properties.1-4 Many practical applications require the assembly of the colloidal particles into thin films for commercial exploitation of their unique properties.3 Two-dimensional (2D) arrays of particulate species show potential for application in photovoltaics5 as well as microelectronics and data storage.6 Wang et al. were able to create 2D ordered arrays of silica nanoparticles through spin-coating and solvent evaporation.7 Fendler and co-workers have demonstrated the utility of using the air-water interface of a LangmuirBlodgett (LB) trough to produce such monoparticulate layers. Forming monolayers from particulates can be considered analogous to the formation of monolayers from small molecule surfactants.8 Monoparticulate layers have been formed from a variety of colloidal particles, mostly inorganic particles made hydrophobic by the adsorption * To whom correspondence should be addressed. Phone: 1-352392-2011. Fax: 1-392-352-9741. E-mail:
[email protected]. (1) Sastry, M.; Gole, A.; Sainkar, S. R. Langmuir 2000, 16, 3553. (2) Collier, C. P.; Saykally, R. J.; Shiang, J. J.; Henrichs, S. E.; Heath, J. R. Science 1997, 277, 1978. (3) Damle, C.; Cole, S.; Sastry, M. J. Mater. Chem. 2000, 10, 1389. (4) Berry, C. R. Phys. Rev. 1967, 161, 848. (5) Yablonovich, E.; Cody, G. IEEE Trans. Electron Devices 1988, ED-9, 300. (6) Hayashi, S.; Kumamoto, Y.; Suzuki, T.; Hirai, T. J. Colloid Interface Sci. 1991, 144, 538. (7) Wang, C.; Zhang, Y.; Dong, L.; Fu, L.; Bai, Y.; Li, T.; Xu, J.; Wei, Y. Chem. Mater. 2000, 12, 3662. (8) For a review, see: Fendler, J. H.; Meldrum, F. C. Adv. Mater. 1995, 7, 607 and the references therein.
of organic oligomers or polymers. Some examples of such monoparticulate layers include polystyrene microspheres,9 silylated glass beads,10 colloidal silver particles,2 and organo-clay complexes.11 As well as using the LB technique to form the monolayers, it is also possible to use the method to examine lateral interactions, packing, and morphology in monolayers of insoluble surfactant molecules. In this study, LB methods will be used to investigate polymer particles at the air-water interface. Both the composition and chemical environment of the system influence the 2D packing and isothermal behavior of particles. Particle size is an important parameter when investigating the formation and stability of particulate monolayers. Studies have shown that the effects of the size of surfactant-coated nanoparticles on the appearance of their isotherms were due to particle interactions.12 Greater interaction energies and horizontal forces between spherical particles are seen with increasing radii.13 Particles of micron size can be considered “macroscopic”, and the primary order and stabilization of the film is achieved through steric interactions and hydrodynamic forces. Capillary forces dominate these particles, and the (9) For example: (a) Pieranski, P. Phys. Rev. Lett. 1980, 45, 569. (b) Robinson, D. J.; Earnshaw, J. C. Phys. Rev. A. 1992, 46, 2045. (c) Onoda, G. Phys. Rev. Lett. 1985, 55, 226. (10) Ho´rvo¨lgyi, Z.; Ne´meth, S.; Fendler, J. H. Colloids Surf., A 1993, 71, 327. (11) Kotov, N. A.; Meldrum, F. C.; Wu, C.; Fendler, J. H. J. Phys. Chem. 1994, 98, 2735. (12) Lefebure, S.; Me´nager, C.; Cabuil, V.; Assenheimer, M.; Gallet, F.; Flamet, C. J. Phys. Chem. B 1998, 102, 2733. (13) Chan, D. Y. C.; Henry, J. D.; White, L. R. J. Colloid Interface Sci. 1981, 79, 410.
10.1021/la0101853 CCC: $20.00 © 2001 American Chemical Society Published on Web 08/04/2001
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Wolert et al.
Figure 1. The definition of the two types of empty space seen in the ordering of dark gray spheres on the air-water interface. The light gray indicates empty space between spheres. The white area is the confined empty space(s) between ordered particles.
contact angle of the particles to the water subphase can be determined from the pressure-area (Π-A) isotherm collapse pressure.14 Nanoparticulates, with “mesoscopic” sizes between those of standard particles and molecular species, may show a combination of the above macroscopic forces and molecular forces such as van der Waals interactions. When the particles are assumed to be spheres of identical size, they pack with each sphere in contact with six neighbors within the plane. In a three-dimensional (3D) system, placing spheres in the cavities of the first layer forms a second layer. If the spheres of the third layer are positioned to coincide with those of the first layer, the resultant ABAB... pattern of layers gives rise to a hexagonally close-packed (hcp) system.15 This packing is “hexagonal” because each sphere in each layer has six nearest neighbors within the same plane. It is also the closest identically sized spheres can be packed, with the minimum amount of empty space. In a 2D system, the spheres still pack according to the hcp model because each sphere still has six nearest neighbors. There are two different types of empty spacing between hexagonal ordered spheres (Figure 1). The first is defined as the amount of empty space between the spheres during hcp (Figure 1, light gray areas) and is included in the hcp model. The second is confined empty space, defined as the area difference between the contact cross-sectional area (CCSA) of the isotherm and a monolayer of the spheres with ideal hexagonal order (Figure 1, white areas). Since approximately 9.3% of the hcp area is occupied by the empty spaces between the spheres, the maximum amount of area that the particles may occupy is 90.7%. Such areas are therefore considered in analyzing the assembly of these particles in the following. Using the Langmuir-Blodgett technique, canal viscometry, and atomic force microscopy (AFM) imaging investigations, this paper will show how polymer particles of spherical shape can assemble on a water surface during compression. Further information about the stability, flow, and reversibility of the resulting monoparticulate films is sought. The particle size will be reflected in the monoparticulate layer behavior. Two types of spherical polymer particles will be examined, the first being oxazoline-functionalized poly(methyl methacrylate) (PMMA) microgels and the second being hyperbranched polymers of poly(vinyl ether) and polystyrene. Film formation of these materials was examined to discover the optimal conditions for ordering monolayers having controllable surface properties. Char(14) Aveyard, R.; Binks, B. P.; Fletcher, P. D. I.; Rutherford, C. E. Colloids Surf., A 1994, 83, 89. (15) Shriver, D. F.; Atkins, P. W.; Langford, C. H. Inorganic Chemistry; W. H. Freeman: New York, 1990; Chapter 4.
Figure 2. The composition and structure of the spherical hyperbranched polymers. The modified spheres contained an additional fifth generation of tri(ethylene glycol)vinyl ether. (A) First generation, poly(vinyl ether). (B) Second generation, poly(styrene). (C) Third generation, poly(vinyl ether). (D) Fourth generation, poly(styrene). (E) Tri(ethylene glycol)vinyl ether on periphery.
acterization of the particles can be achieved by examining their behavior during monolayer formation and arrangement on solid supports, showing the influence of superstructure and composition on film formation. Monolayer characterization achieved through these investigations will indicate factors concerning organization and behavior of the particles at the air-water interface. These studies will also provide an understanding of the factors that control stability. The microgels and hyperbranched particles studied here were chosen because they should show size-dependent properties. The effects of the dispersant and particle size on monolayer formation were determined by isotherm measurements, AFM imaging, and viscosity investigations using oxazoline-functionalized PMMA microgel particles with a hydrophobic block copolymer dispersant. Particles of poly(vinyl ether)-co-polystyrene with and without a hydrophilic tri(ethylene glycol)vinyl ether periphery were also investigated using both isotherm and hysteresis experiments. Experimental Section Particle Synthesis. Synthesis of the hyperbranched materials (Figure 2) has been discussed in previous publications.16 Viscosity and light scattering measurements (not discussed here) indicated that these particles were highly compact spheres with radii of gyration (Rg) of 28.5 ( 2.3 nm. The hyperbranched polymer was further modified with methyl triethylene glycol vinyl ether (MTGVE) (Figure 2) surface units (Rg ) 32.3 ( 2.0 nm). The functionalized PMMA microgels were prepared by radical copolymerization of the compounds as described previously by Ho¨lderle et al.17 A cross-linking agent of 5 mol % ethyleneglycoldimethacrylate was incorporated. Different diameters were obtained by varying the dispersant concentration in the reaction mixture. Diameters were determined from transmission electron microscopy, discussed elsewhere.17 Particle Dispersion. To be spread on the water subphase, the particles need to be suspended. For these experiments, all (16) (a) Deffieux, A.; Schappacher, M. Macromol. Symp. 1998, 132, 45. (b) Deffieux, A.; Schappacher, M. Macromolecules 1999, 32, 1797. (c) Schappacher, M.; Billaud, C.; Paulo, C.; Deffieux, A. Macromol. Chem. Phys. 1999, 200, 2377. (d) Schappacher, M.; Deffieux, A. Macromolecules 2000, 33, 7371.
Polymer Particles at the Air-Water Interface Scheme 1
Langmuir, Vol. 17, No. 18, 2001 5673 2D Viscometry. In addition to isotherm investigations, the microgel particulate was studied to determine the 2D shear viscosity coefficient of the system using a flow-through canal.19,20 To calculate the film’s 2D in-plane shear viscosity coefficient, a hydrodynamic theory developed by Harkins and Kirkwood20 was used to relate the in-plane steady shear viscosity (ηs) to observable quantities,
ηs )
hyperbranched particulates were in chloroform without the use of a dispersant. The PMMA microgel dispersions were achieved in three manners. The first involved mixtures of a block copolymer dispersant and particles in chloroform. Chloroform removes the copolymer from the particles during the spreading of the monolayer on the subphase. The second method of forming microgel dispersions involved suspension of the particles with the copolymer dispersant in chloroform followed by removal of the copolymer to leave suspended particles. The copolymer dispersant was removed utilizing sonication and centrifugation in tetrahydrofuran. Sonication was performed using an E/CM Corp. model 450 sonication bath, and a Sorvall RC5B Plus with SS 34 Rotor centrifuge rotated at 12 000 rpm for 30 min was used. This procedure was repeated three times. Using the weight percent of copolymer dispersant for each particle diameter, the disappearance of copolymer from the surface of the microgel was verified for small aliquots by evaporating the solvents and checking the weight ratios against the original formulation. The resulting dispersion was stable over the time of the experiment but might be unstable over longer times. The third suspension involved the block copolymer dispersant attached to the microgels (Scheme 1). Heptane was used as it did not separate the dispersant from the spheres because the dispersant is insoluble in heptane. Thus, the dispersant remains after spreading of the monolayer. Pressure-Area Isotherms. All of the dispersions went from cloudy to clear in appearance after 4 h of sonication and remained nonturbid throughout the experimental time frame. Likewise, the CSSAs of the isotherms did not change over time. Isotherms were recorded at 23 °C on a KSV LB5000 Teflon trough with Millipore (18 MΩ/cm) water. After the suspending solvent volatilized, the surface film was compressed continuously at a speed of 1500 mm2/min using Teflon barriers. Pressurearea (π-A) isotherms of the samples were obtained by monitoring the surface pressure using a Wilhemy balance that was kept parallel to the barrier. The CCSA was measured in a manner analogous to that used for onset areas with simple insoluble amphiphiles describing the zero pressure intercept determined by drawing a tangent at the steep part of the isotherm.18 All of the isotherms were run a minimum of four times. Atomic Force Microscopy. The surface layers were transferred onto freshly cleaved mica by vertical dipping at a surface pressure of 5 mN/m and an upstroke speed of 1 mm/min at 23 °C. The particulate films were observed with a Nanoscope III AFM (Digital Instruments, Inc., Santa Barbara, CA) in tapping mode using silicon tips on cantilevers with a nominal spring constant of 56 N/m. (17) (a) Ho¨lderle, M.; Bar, G.; Mu¨lhaupt, R. J. Polym. Sci., Part A 1997, 35, 2539. (b) Ho¨lderle, M.; Mu¨lhaupt, R. Acta Polym. 1996, 46, 226. (c) Ho¨lderle, M.; Bruch, M.; Mu¨lhaupt, R. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1997, 38 (1), 479. (d) Scha¨fer, M.; Ho¨lderle, M.; Mu¨lhaupt, R. Polymer 1998, 39, 1259. (e) Ho¨lderle, M. Synthese, Charakterisierung, Filmbildung und Compoundierung von, Polymernanopartikeln mit Kern, Schale-Architektur/vorgelegt von, Matthias Ho¨lderle, 1996. - 190 S.: graph. Darst.; (dt.), Freiburg i. Br., Univ., Diss., 1997. (18) Ho´rvo¨lgyi, Z.; Ne´meth, S.; Fendler, J. H. Langmuir 1996, 12, 997.
8∆Πω3 ωη π π4LQ
(1)
where ∆Π is the (positive) surface pressure gradient across the canal, ω is the canal width, L is the canal length, Q is the area flow rate, and η is the subphase steady shear viscosity. The assumptions are that ηs is
