Langmuir 2005, 21, 43-49
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Surface Heterogeneity of Polystyrene Latex Particles Determined by Dynamic Force Microscopy Susheng Tan,* Robert L. Sherman, Jr., Dongqi Qin, and Warren T. Ford* Department of Chemistry, Oklahoma State University, Stillwater, Oklahoma 74078 Received August 31, 2004. In Final Form: October 20, 2004
Atomic force microscopy (AFM) was employed to characterize the surface chemistry distribution on individual polystyrene latex particles. The particles were obtained by surfactant-free emulsion polymerization and contained hydrophilic quaternary ammonium chloride, sodium sulfonate, or hydroxyethyl groups. The phase shift in dynamic force mode AFM is sensitive to charge/chemical interactions between an oscillating atomic force microscope tip and a sample surface. In this work, the phase imaging technique distinguished phase domains of 50-100 nm on the surfaces of dried latex particles in ambient air. The domains are attributed to the separation of ion-rich and ion-poor components of the polymer on the particle surface.
Introduction In early work with ionomers, Eisenberg and co-workers1,2 proposed a morphological model for random ionomers. That model says that ionic multiplets/clusters limit the motion of surrounding molecular chains and increase the glass transition temperature (Tg) and Young’s modulus of ionomers relative to the parent polymer that lacks ionic groups. The thickness of the restricted mobility layer surrounding each multiplet is postulated to be of the order of the persistence length of the polymer. As the ion content is increased, the regions of the restricted mobility surrounding each multiplet overlap to form larger contiguous regions. When these regions become sufficiently large, they are termed clusters. The clusters behave as if they were phase-separated from the regions of more mobile segmental motion in that they exhibit their own Tg, which is significantly higher than the Tg value of the unclustered component. The model is in good agreement with a very wide range of experimentally observed phenomena, especially those based on dynamic mechanical and X-ray scattering techniques.1,3-5 Charge-stabilized polymer colloids are related to ionomers, but in each particle, most of the ionic units stay on the surface during emulsion polymerization. Previously, we reported that the surface compressive modulus of polystyrene latex particles dried under ambient conditions increased with an increase of the content of quaternary ammonium chloride units.6 Understanding the properties of polymer colloids both in dispersions and as dry films requires an understanding of their surface structures. There is currently intense interest in nanoscale structured materials, including materials built from latexes. The * Authors to whom correspondence should be addressed. Email:
[email protected] (S.T.);
[email protected] (W.T.F.). (1) Eisenberg, A.; Hird, B.; Moore, R. B. Macromolecules 1990, 23, 4098-4107. (2) Moore, R. B.; Gauthier, M.; Williams, C. E.; Eisenberg, A. Macromolecules 1992, 25, 5769-5773. (3) Kim, J. S.; Nah, Y. H.; Jarng, S. S.; Kim, W.; Lee, Y.; Kim, Y. W. Polymer 2000, 41, 3099-3102. (4) Eisenberg, A.; Navratil, M. Macromolecules 1973, 6, 604-612. (5) Ma, X.; Sauer, J. A.; Hara, M. Macromolecules 1995, 28, 39533962. (6) Tan, S.; Sherman, R. L., Jr.; Ford, W. T. Langmuir 2004, 20, 7015-7020.
structures assembled from latexes will be controlled by their surface and interface properties rather than by their bulk properties. The surface organization of latexes presents particular challenges to both experimental studies and theoretical modeling.7 The surfaces of colloidal particles also control their interactions in liquid dispersions. Different parts of the interaction potential and the environment govern very different properties in these systems.8 Electrostatic and chemical forces mediate the interactions in colloid systems. Many techniques exist for determining the average charged state of soluble macromolecules, such as electrophoretic, titration, electrokinetic, and redox potential measurements. However, relatively few experimental methods can determine the spatial distribution of charge on surfaces, such as membranes9 and polymer colloids.10,11 Theoretical models assume uniform surface charge (chemistry) distribution on the colloid surfaces.12-16 A nonuniform distribution of surface charge has been proposed to explain rotational electrophoresis10,11 and zonal centrifugation experiments.17,18 Galembeck has also reported charge nonuniformity on the basis of distorted particle shapes in scanning electron microscopy (SEM), transmission electron microscopy (TEM), and scanning electronic potential microscopy (SEPM) images of colloidal crystals (7) Goldbeck-Wood, G.; Bliznyuk, V. N.; Burlakov, V.; Assender, H. E.; Briggs, G. A. D.; Tsukahara, Y.; Anderson, K. L.; Windle, A. H. Macromolecules 2002, 35, 5283-5289. (8) Israelachvili, J. N. Intermolecular & Surface Forces, 2nd ed.; Academic Press: New York, 1991. (9) Mclaughlin, S. Annu. Rev. Biophys. Biophys. Chem. 1989, 18, 113-136. (10) Feick, J. D.; Chukwumah, N.; Noel, A. E.; Velegol, D. Langmuir 2004, 20, 3090-3095. (11) Feick, J. D.; Velegol, D. Langmuir 2002, 18, 3454-3458. (12) Lowen, H.; Kramposthuber, G. Europhys. Lett. 1993, 23, 673678. (13) Bowen, W. R.; Sharif, A. O. Nature 1998, 393, 663-665. (14) Adamczyk, Z.; Weronski, P. Adv. Colloid Interface Sci. 1999, 83, 137-226. (15) Lyklema, J.; van Leeuwen, H. P.; Minor, M. Adv. Colloid Interface Sci. 1999, 83, 33-69. (16) Miyahara, M.; Watanabe, S.; Gotoh, Y.; Higashitani, K. J. Chem. Phys. 2004, 120, 1524-1534. (17) Neto, J. M. M.; Cardoso, A. L. H.; Testa, A. P.; Galembeck, F. Langmuir 1994, 10, 2095-2099. (18) Neto, J. M. M.; Monteiro, V. A. D.; Galembeck, F. Colloids Surf., A 1996, 108, 83-89.
10.1021/la047821s CCC: $30.25 © 2005 American Chemical Society Published on Web 12/07/2004
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of polymer colloids.19,20 The chemical (charge) heterogeneity of surfaces plays a key role in the transport and fate of colloidal particles in subsurface environments. Therefore, the direct observation of local charge (chemistry) distributions on the surface of colloid particles would provide the most direct means to evaluate models of surface morphology under a range of conditions. Atomic force microscopy (AFM) is a powerful tool for determination of the topology and physical properties of the surfaces of polymeric materials.21 This has stimulated a reconsideration of many physical phenomena that occur at the surface and is causing critical review of some basic principles of the structural organization of polymers. An atomic force microscope measures forces between its probe and the surface of a sample. The forces can be attractive or repulsive and can be electrostatic, van der Waals, or chemically specific. AFM can provide both qualitative and quantitative information about surface interaction with the probe with a spatial resolution that is limited by the size and shape of the probe. In one example, AFM has been used to map the spatial distribution of surface charge using arrays of force-distance curves or using isoforce images based on a repulsive double-layer force.22 Phase imaging in dynamic force mode AFM is another important protocol for gaining high-resolution distribution of surface chemistry. The difference of phase angle between the cantilever oscillation and its response is associated with tip-sample interactions that involve energy dissipation such as adhesion energy hysteresis and viscoelasticity. Several authors have proposed recording the difference between the phase angle of the excitation signal and the phase angle of the cantilever oscillation as a way of obtaining compositional maps of heterogeneous samples and for imaging material properties.23-25 In this paper, we report the heterogeneity of the surfaces of model polystyrene latex particles detected by phase-sensitive dynamic mode AFM. Experimental Section Materials. All reagents were purchased from Aldrich and Fisher. Monomers were purified by passing them down basic alumina columns. Water was purified on a Barnstead 3 column e-pure system to a conductivity of
