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An Intrinsic Relationship Between Molecular Structure In Self-Assembled N-Alkylsiloxane Monolayers And Deposition Temperature

A. Parikh, D. Allara, I. B. Azouz, F. Rondelez
Published 1994 · Chemistry

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We have studied the effect of preparation temperature, in the range 5-65 OC, on the structures of n-octadecylsiloxane monolayers prepared by self-assembly from dilute solution of n-octadecyltrichlorosilane onto the surface of freshly hydrated, oxidized silicon substrates. Structural features of the films were characterized using a combination of liquid drop contact angle measurements, null ellipsometry, and infrared transmission spectroscopy. The contact angle data confirm a previously reported observation of a critical temperature, Tc - 28 f 5 OC, below which the surface energy is constant at a near-limiting value of a pure CH3 surface and above which the surface energy monotonically increases with increasing temperature. Coverages and chain organization, as measured by ellipsometry and vibrational spectroscopic features (peak positions and integrated intensities of methylene C-H stretching modes), respectively, show changes in the same temperature region as the wetting behavior. We conclude that when prepared below T,, the films exhibit a heterogeneous structure with closely spaced islands of densely packed, nearly all-trans alkyl chains arranged nearly vertical to the surface. In contrast, when prepared above T,, the films exhibit monotonically diminishing coverage with increasing preparation temperature and the alkyl chains increasingly assume higher contents of conformational disorder. Further, the infrared data indicate that these higher temperature films are heterogeneous with coexisting domains of high and low chain conformational ordering. All the data, taken together, are in good conformity with a film formation mechanism which involves, prior to siloxy group cross-linking, the intervention of intermediate structural phases of mobile alkylsiloxy species adsorbed on a water layer adjacent to the solid substrate surface. In support of this mechanism, a strong parallel is apparent between Tc and the triple point temperature at which gas (G), liquid-expanded (LE), and liquid-condensed (LC) phases coexist for chain Langmuir monolayers at the air/bulk water interface. Below Tc the self-assembled film structure is similar to that of the nearly pure LC Langmuir phase while above Tc the film structure is similar to that of coexisting LE and LC phases. Deviations of the self-assembled film structures for the analogous equilibrium Langmuir phase structures occur at higher preparation temperatures and are rationalized in terms of both the known occurrence of nonequilibrium phases in Langmuir films above the triple point temperature and the relative acceleration of the Si-O-Si cross-linking reaction in the self-assembled film to form structures with frozen-in defects.
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