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Wetting−Dewetting Transition Line in Thin Polymer Films K Subscriber access provided by NATL INST STANDARDS & TECH Research Article Wetting−Dewetting Transition Line in Thin Polymer Films K. M. Ashley, D. Raghavan, J. F. Douglas, and A. Karim Langmuir, 2005, 21 (21), 9518-9523• DOI: 10.1021/la050482y • Publication Date (Web): 09 September 2005 Downloaded from http://pubs.acs.org on February 10, 2009 More About This Article Additional resources and features associated with this article are available within the HTML version: • Supporting Information • Links to the 9 articles that cite this article, as of the time of this article download • Access to high resolution figures • Links to articles and content related to this article • Copyright permission to reproduce figures and/or text from this article Langmuir is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 9518 Langmuir 2005, 21, 9518-9523 Wetting-Dewetting Transition Line in Thin Polymer Films K. M. Ashley,† D. Raghavan,*,† J. F. Douglas,*,‡ and A. Karim*,‡ Polymer Program, Department of Chemistry, Howard University, Washington, DC 20059, and Polymers Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 Received February 23, 2005. In Final Form: June 14, 2005 Thin polymeric films are increasingly being utilized in diverse technological applications, and it is crucial to have a reliable method to characterize the stability of these films against dewetting. The parameter space that influences the dewetting of thin polymer films is wide (molecular mass, temperature, film thickness, substrate interaction) and a combinatorial method of investigation is suitable. We thus construct a combinatorial library of observations for polystyrene (PS) films cast on substrates having orthogonal temperature and surface energy gradients and perform a series of measurements for a range of molecular masses (1800 g/mol < M < 35 000 g/mol) and film thicknesses h (30 nm < h < 40 nm) to explore these primary parameter axes. We were able to obtain a near-universal scaling curve describing a wetting- dewetting transition line for polystyrene films of fixed thickness by introducing reduced temperature and surface energy variables dependent on M. Our observations also indicate that the apparent polymer surface tension γp becomes appreciably modified in thin polymer films from its bulk counterpart for films thinner than about 100-200 nm, so that bulk γp measurements cannot be used to estimate the stability of ultrathin films. Both of these observations are potentially fundamental for the control of thin film stability in applications where film dewetting can compromise film function. Introduction These changes in film properties with film thickness h are natural since h is becoming comparable to molecular Contemporary coating technologies increasingly require dimensions and to the somewhat larger scales over which uniform and stable thin polymeric films for use in paints, glass-forming liquids become dynamically heterogeneous. adhesives, electronic packaging, and biotechnology ap- Moreover, the van der Waals interactions with the plications. A number of recent studies have shown that substrate can change the chain conformations in the film the film structure and properties of bulk polymers can be relative to the bulk since the chains are in effect subjected different from that of “ultrathin” films having a thickness - to an applied field. (Film preparation effects could also be h less than a thickness on the order of 100-200 nm.1 5 a factor in influencing these chain conformations if the [We define ultrathin films below by the thickness h range polymer chains become trapped into nonequilibrium in which the film properties deviate from their bulk states.) Changes in chain packing relative to the bulk in counterparts, but where h is larger than the molecularly turn suggest corresponding changes in thermodynamic thin range in which van der Waals interactions and the film properties.8 These effects are significant for the presence of a boundary strongly perturb the fluid struc- current study since the surface tension of fluids in the ture. In polymer fluids, this thickness regime typically γp bulk depends on the ratio of the isothermal compressibility ranges from scales on the order of 100-200 nm (regardless (κ ) to the density F of the fluid [i.e., ∼ (F/κ )1/2].9 In of polymer molecular mass) to a few nm in oligomeric T γp T particular, a decrease in the Debye-Waller factor points fluids.] For example, there have been numerous reports to a decrease in κ , and we might then expect a general of changes in the glass transition temperature T of thin T g tendency for to increase in thin films if the density is polymer films in this thickness range.6 Soles et al.7a have γp taken to be largely unchanged.10 A change in with film shown that the Debye-Waller factor, describing the γp thickness h of this kind would have significant ramifica- amplitude of thermally induced molecular motions, also tions for the stability of ultrathin films, the stability of tends to decrease in ultrathin films.7a,7b imprinted patterns in glassy films,11 the rate of film dewetting, etc. As a practical matter, this variation would * To whom correspondence should be addressed. † Howard University. imply that bulk γp measurements and probably even bulk ‡ National Institute of Standards and Technology. fluid polymer-surface substrate energy measurements (1) Reiter, G. Phy. Rev. Lett. 1992, 68 (1), 75. could not be used to estimate the stability of ultrathin (2) Bischoff, J.; Scherer, D.; Herminghaus, S.; Leiderer, P. Phys. Rev. films, and we indeed find evidence suggesting that this Lett. 1996, 77 (8), 1536. (3) Xie, R.; Karim, A.; Douglas, J. F.; Han C. C.; Weiss, R. A. Phys. is the case. Rev Lett. 1998, 81 (6), 1251. Substrate libraries with gradients in water contact angle (4) Jacobs, K.; Herminghaus, S.; Mecke, K. R. Langmuir 1998, 14, (θ ) ranging from 40° to 95° were obtained by the ozonolysis 965. of a self-assembled monolayer (SAM) by exposing the (5) Hall D. B.; Hooker, J. C.; Torkelson, J. M. Macromolecules 1997, 30, 667. substrate to ultra violet light for different lengths of time. (6) Soles, C. L.; Douglas, J. F.; Jones, R. L.; Wu, W. L. Macromolecules 2004, 37, 2901. See list of references in this article regarding earlier (8) Dee, G. T.; Sauer, B. B. Adv. Phys. 1998, 47 (2), 161. efforts to characterize the influence of film thickness on the Tg in thin (9) Sanchez, I. C. J. Chem. Phys. 1983, 79 (1), 405. Bhatia, Q. S.; polymer films. Chen, J.-K.; Koberstein, J. T.; Sohn J. E.; Emerson, J. A. J. Colloid (7) (a) Soles, C. L.; Douglas, J. F.; Wu, W. L.; Dimeo, R. M. Phys. Rev. Interface Sci. 1985, 106, 353. Lett. 2002, 88 (3), 037401. Soles, C. L.; Douglas, J. F.; Wu, W. L J. (10) Wallace, W. E.; Beck Tan, N. C.; Wu, W. L.; Satija, S. J. Chem. Polym. Sci. B: Polym. Phys. 2004, 42 (17), 3218. (b) Inuoue, R.; Kanaya, Phys. 1998, 108, 3978. T.; Nishida, K.; Tsukushi, I.; Shibata, K. Phys. Rev. Lett. 2005, 95, (11) Chou, S. Y.; Krauss, P. R.; Renstrom, P. J. Science 1996, 272 056102. (5258), 85. 10.1021/la050482y CCC: $30.25 © 2005 American Chemical Society Published on Web 09/09/2005 Wetting-Dewetting Transition Line Langmuir, Vol. 21, No. 21, 2005 9519 Libraries of thin films of PS coatings were placed on a Table 1. Tg and Tknee of PS Samples temperature (T)-gradient stage orthogonal to the surface Mw Tknee Tg Tknee/Tg energy gradient of the substrate. The T-gradient stage 1800 96.8 62.7 1.54 exposes the film to a linear temperature gradient. In this 9000 131.2 90.9 1.44 study, we present experimental observations of morpho- 15 200 143.3 91.7 1.56 logical “stability-map” for 30 nm PS film equilibrated on 22 000 148.9 90.8 1.64 substrates having orthogonal temperature and surface 35 000 150.5 102.6 1.47 energy gradients. Using optical and atomic force micros- copy (AFM), we study the dewetted morphology and scanning probe microscope from Digital Instruments was oper- contact angle of the dewetted PS droplets, respectively. ated in tapping mode under ambient conditions (24 ( 2 °C) using Owing to the lack of information on the role of T on the commercial silicon microcantilever probes. The manufacturer’s interfacial free energy between the polymer and the values for the probe tip radius and probe spring constant are in the ranges of 5-10 nm and 20-100 N/m, respectively. Topo- underlying substrate (γsp) and surface energy of polymer graphic images were obtained simultaneously using a resonance (γp) for thin films, it is only possible to present qualitative frequency of approximately 300 kHz for the probe oscillation, a trends in the morphological stability map. scan rate of (1 Hz), and a free-oscillation amplitude, A0,of145 ( 10 nm. The AFM scans were stored as (512 × 512) pixel images. Experimental Methodology A planefit filter was applied to all images prior to further analysis. Macros program in combination with IGOR analysis was used Certain commercial materials and equipment are identified to automatically identify droplet(s), take vertical sections of the in this article to adequately specify the experimental procedure. droplet along the major and minor axis. About 80 droplets were In no case does such identification imply the recommendation processed to generate slopes of the section profiles and the height or endorsement by NIST, nor does it imply that these are so that the PS contact angle data can be generated. necessarily the best available for the purpose. The glass transition temperatures of PS of various molecular A silicon wafer (orientation 100 from Wafer World) was placed masses are listed in Table 1, as determined by differential in a UVO cleaner from Jelight Company, Inc (model number scanning calorimetry on a Perkin-Elmer DSC 7 with a mass of 342) for 15 min to remove organics and oxide buildup.
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