DOI: 10.1021/la9041937 3815 Langmuir 2010, 26(6), 3815–3820 Published on Web 02/10/2010 pubs.acs.org/Langmuir © 2010 American Chemical Society Dilational Lateral Stress in Drying Latex Films Alexander M. K€ onig, † Elodie Bourgeat-Lami, ‡ V eronique Mellon, ‡ Kerstin von der Ehe, † Alexander F. Routh, § and Diethelm Johannsmann* ,† † Institute of Physical Chemistry, Clausthal University of Technology, Arnold-Sommerfeld-Str. 4, D-38678 Clausthal-Zellerfeld, Germany, ‡ Universit e de Lyon, Univ. Lyon 1, CPE Lyon, CNRS UMR 5265, Laboratoire de Chimie, Catalyse, Polym eres et Proc ed es (C2P2), LCPP group, 43 Bd du 11 Novembre 1918, F-69616, Villeurbanne, France, and § Department of Chemical Engineering and Biotechnology, BP Institute, University of Cambridge, Madingley Road, Cambridge CB3 0EZ, U.K. Received January 6, 2009. Revised Manuscript Received January 20, 2010 Drying latex films usually experience tensile stress due to the reduction in volume. While an unconstrained film would shrink affinely in all three dimensions, a coating can only shrink along the vertical and therefore exerts tensile stress onto the substrate. Using an instrument capable of producing maps of the stress distribution, we found that dilational stress sometimes develops as well. The in-plane stress was monitored by spreading the latex dispersion on a flexible membrane. Usually, the membrane bends upward under the tensile stress exerted by the film, but it may also bend downward. Dilational stress was only found with samples showing a strong coffee stain effect, that is, samples in which there is a significant lateral flow from the center to the edge while the film dries. During drying, particles consolidate first at the edge because of the lower height in this region. Continued evaporation from the consolidated region results in a water flow toward the edge, exerting a force onto the latex particles. At the time, when the network is formed, any single sphere must be in a force-balance condition: the network must exert an elastic force onto the sphere which just compensates the viscous drag. Pictorially speaking, a spring (an elastic network) is created while an external force acts onto it. Once the flow stops, the drag force vanishes and the internal stress, which previously compensated the drag, expands the film laterally. This phenomenon can lead to buckling. Given that lateral flow of liquid while films dry is a rather common occurrence, this mode of structure formation should be widespread. It requires lateral flow in conjunction with elastic recovery of the particle network. Introduction The process of film formation from aqueous polymer disper- sions (latexes) is of much practical relevance in the coatings industry because waterborne paints release little or no volatile organic compounds (VOCs) upon drying. 1-3 Environmental regulations have led to a steady increase in market volume of such coatings. Polymer dispersions offer numerous options to improve and adjust coating properties, which do not exist for conventional, solvent-based coatings. For instance, it is rather easy to produce nanostructured composites by employing either latex blends 4,5 or core-shell particles. 6 On the other hand, film formation poses challenges as well. During drying, the material experiences a sequence of transformations, involving water loss, particle deformation, and interparticle coalescence. Typical problems are skin formation, 7-10 the development of an “orange peel” like surface, 11 cracking, 12-14 and insufficient cohesion. 15 Of particular importance in the context of the study reported here are stresses developing during shrinkage. Stress is the source of one of the classical failure modes of film formation, which is cracking. In many (but not all) cases, shrinkage would occur affinely (both in-plane and along the vertical) if there were no external constraints. The substrate imposes such constraints, producing a tensile in-plane stress. If the substrate is slightly flexible, it bends upward, 16 which is easily detected. Tensile stress in drying films has been quantitatively deter- mined by numerous researchers employing the beam bending technique. 17-19 While tensile lateral stress certainly is the rule, one also finds the opposite behavior. In some cases, the film laterally expands. The phenomenon usually is localized and transient. Since the classical beam bending instruments measure the average stress, they easily miss such phenomena. We have recently developed an instrument capable of determining spatially re- solved maps of the stress distribution. 20 The measurement is based on the deformation of a flexible membrane (Figure 1A). Using automated image analysis, one generates a map of film stress as a function of location and time. *Corresponding author. E-mail: johannsmann@pc.tu-clausthal.de. (1) Keddie, J. L. Mater. Sci. Eng., R 1997, 21, 101. (2) Steward, P. A.; Hearn, J.; Wilkinson, M. C. Adv. Colloid Interface Sci. 2000, 86, 195. (3) Winnik, M. A. Curr. Opin. Colloid Interface Sci. 1997, 2, 192. (4) Tzitzinou, A.; Keddie, J. L.; Geurts, J. M.; Peters, A.; Satguru, R. Macro- molecules 2000, 33, 2695. (5) Colombini, D.; Hassander, H.; Karlsson, O. J.; Maurer, F. H. J. Macro- molecules 2004, 37, 6865. (6) Juhue, D.; Lang, J. Macromolecules 1995, 28, 1306. (7) Eckersley, S. T.; Rudin, A. Prog. Org. Coat. 1994, 23, 387. (8) Routh, A. F.; Russel, W. B. Ind. Eng. Chem. Res. 2001, 40, 4302. (9) Erkselius, S.; Wadso, L.; Karlsson, O. J. J. Colloid Interface Sci. 2008, 317, 83. (10) Koenig, A. M.; Weerakkody, T. G.; Keddie, J. L.; Johannsmann, D. Langmuir 2008, 24, 7580. (11) Abbasian, A.; Ghaffarian, S. R.; Mohammadi, N.; Khosroshahi, M. R.; Fathollahi, M. Prog. Org. Coat. 2004, 49, 229. (12) Lee, W. P.; Routh, A. F. Langmuir 2004, 20, 9885. (13) Russel, W. B.; Wu, N.; Man, W. Langmuir 2008, 24, 1721. (14) Singh, K. B.; Tirumkudulu, M. S. Phys. Rev. Lett. 2007, 98, 218302. (15) Voyutskii, S. S. J. Polym. Sci. 1958, 32, 528. (16) Stoney, G. Proc. R. Soc. London 1909, A82, 172. (17) Francis, L. F.; McCormick, A. V.; Vaessen, D. M.; Payne, J. A. J. Mater. Sci. 2002, 37, 4897. (18) Martinez, C. J.; Lewis, J. A. Langmuir 2002, 18, 4689. (19) Petersen, C.; Heldmann, C.; Johannsmann, D. Langmuir 1999, 15, 7745. (20) von der Ehe, K.; Johannsmann, D. Rev. Sci. Instrum. 2007, 78, 113904.