Langmuir 1993,9,475-480
476
Continuity of Very Thin Polymer Films Charles W.Extrandt Collage de France, Physique de la Matiare Condensbe, 11, Place Marcelin-Berthelot, 75231 Paris Cbdex 05, France Received February 20,1992. In Final Form: November 10,1992
Thin f h (0.5-100nm) of natural rubber, polystyrene, and poly(methy1methacrylate) were deposited on silicon wafers by spin-castingfrom dilute solution. The average thickness of the layers was determined by ellipsometry. Contact angle measurements, along with atomic force microscopy,were used to investigate the homogeneity of the layers. The critical average film thickness e, required to completely cover the surface differed greatly (between 1.6 and 30 nm), depending on the surface treatment of the wafers and the polymer used. 1. Introduction
First Langmuir'J and then later Zisman and his co-workers3-5showed that a single monolayer is sufficient to completely alter the wetting properties of a surface. For smooth homogeneous surfaces, a coating of about 1 nm is often enough to screen out the underlying substrate.6J But in practice, thin films may be heterogeneous at the microscopic scale. Thus, what thickness is required to completely cover a surface? And how does wettability vary? Wetting properties of inhomogeneoussurfaceshave been studied previously.e1s But, there are varied reports in the literature as to how the contact angle changes with coverage, as will be discussed later. In this preliminary study, thin films of natural rubber (NR),polystyrene (PS),and poly(methy1 methacrylate) (PMMA)of different thickness e have been spun cast on smooth silicon wafers. Some of them so thin, that they were not continuous. An example is shown in Figure 1for NR on ozone-treated silicon wafers. For filmsof less than about 30nm, contact angles 6 of ethylene glycol and water varied between that of bare wafer and their respective bulk values for NR. For layers thicker than 30 nm, the t
Present addreaa: Induetrial Products Research Institute, 1-1-4
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