Chapter 9 Disentanglement in Ultrathin Polymer Films, Surface Strain, and Auto-Dewetting 1
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R. M. Overney , C. Buenviaje , S. Ge , M. Rafailovich , and J. Sokolov 1
Department of Chemical Engineering, University of Washington, Seattle, WA 98195 Department of Materials Science, State University of New York at Stony Brook, Stony Brook, NY 11794-2275
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In this paper, we emphasize the importance of the film preparation on the mechanical properties of polymer thin films. With shear and adhesion measurements, conducted with scanning force microscopy (SFM), we discuss the effect of spin casting on the degree of disentanglement of polymer chains in the boundary regime to the substrate surface. The thickness of the mechanically strained boundary layer is measured, and its unexpected far-field effect discussed with a two-fluid model. The surface interaction and mechanical properties, which were obtained from unannealed polyethylene copropylene (PEP) films, are compared to annealed PEP films and to auto-dewetting occurrences in strained systems. The measurements show that spin coating together with the interaction strength of the substrate surface can cause autophobicity. The mechanical properties of a polymer film depends, in a complex manner, on interfacial interactions (1-9). Interfacial interactions determine polymer chain conformation, dynamics, and reactivity. Designing polymer films with well-defined specific shear mechanical properties and surface properties requires, not only a detailed understanding of the physics, but also the ability to control interfacial interaction parameters. This paper will discuss, based on lateral force and force modulated scanning force microscopy (SFM) measurements, the effect of the sample preparation in conjunction with the interaction strength of the interface on the mechanical properties of thin polymer films. We will introduce the lateral disentanglement strength, P, of the unstressed bulk elastomer, measure the effect of interfacial stresses on P, relate Ρ to the lateral strain induced during the spin-coating process, and introduce a two-fluid model to describe mechanical properties of the interfacial boundary layer in order to justify the far-field effect of the interface as previsously reported (1). Finally, we will discuss the outcome of interfacial constraints on the dewetting processs of binary systems. 3
Corresponding author.
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© 2000 American Chemical Society
Tsukruk and Wahl; Microstructure and Microtribology of Polymer Surfaces ACS Symposium Series; American Chemical Society: Washington, DC, 1999.
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Equilibrium Theories and the Extent of Interfacial Confinement Based on thermodynamic bulk theories (e.g., free volume theory (10-13)) or flow theories of melts (e.g., Rouse Theory (14)) material properties, such as visco-elastic material constants or transition temperatures are related to molecular parameters. Bulk properties are theoretically described by structural models, for instance, the basic statistical coil model or the meander model (15). Such scaling theories work well because interaction lengths that have to be considered are rather short shortrange in nature (16-18). Existing classical mean-field theories or molecular dynamic simulations assume that, for instance, interfacial interactions are completely screened within a distance corresponding to the persistence length of the polymer (16,19). Thus, confinement effects because of interfacial interaction, like pinning of polymers at substrate surfaces, are only considered up to 0.6 to 1 nm distance (19). Effects of Film Coating Proceedures. Not considered in such scaling theories are "pre-historical" effects that occurred during the film coating process, for instance, in the case of fast solvent evaporation. In technologically highly relevant polymer coating procedures, such as spin casting, film preparation effects may cause longterm changes in the polymer viscoelastic properties that do not anneal within the characteristic diffusion times used for scaling theories (1,3). Hence, interfacial effects may be noticeable up to a distance that exceeds the polymer's persistence length by orders of magnitude. Long-term polymer viscoelastic frustration has been recently observed in a dewetting study of a binary homopolymer system (1,9). Shear Mechanical Measurements by SFM Recent research efforts are strongly focusing on static (mechanical) and kinematic (diffusion) properties of polymeric materials in the vicinity to interfaces (1-9,20-26). Particularly one technique, the scanning probe microscopy (SPM), has been found to be very suitable in determining surface shear and viscoelastic properties on the submicrometer scale (20,27-41). In previous studies on polyethylene-copropylene (PEP) and polystyrene (PS) films annealed above their respective T , we found that the properties of the films change in the vicinity of interactive substrate such as silicon (l) For PS, the tracer diffusion coefficients, which are inversely proportional to the zero shear viscosity, were found to gradually decrease by more than two orders of magnitude with decreasing film thickness (3). Simple surface pinning was ruled out since the onset of the effect appeared at approximately 200 nm, or at least 10-Rg-distancefromthe interface where the probability of direct surface contact was nearly zero. A similar effect was observed for PEP in dewetting experiments where the velocity of the dewetting film was inversely proportional to the viscosity (1). Surface mechanical measurements of PEP films showed a similar trend (1). As the film thickness decreasedfromapproximately 200 nm, the film surface became more elastic and less viscous, approaching the characteristics of a completely solid surface for films of 200 nm. The present g
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Tsukruk and Wahl; Microstructure and Microtribology of Polymer Surfaces ACS Symposium Series; American Chemical Society: Washington, DC, 1999.
156 rheological data (as outlined in detail below) is consistent with thesefindingsand provide an explanation for the long-range effects of the interactive surface.
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Experimental Materials used in this Study. The effect of spin casting on mechanical properties of polymers in the boundary regime to interactive interfaces was studied with the monodisperse homopolymer, polyethylene-copropylene (PEP) (Mw=374,000, Mw/Mn
