Determination of structure-property relationships for 3-aminopropyltriethoxysilane films using x-ray reflectivity
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rence L. Kirley Department of Pharmacology and Cell Biophysics, University of Cincinnati, Cincinnati, Ohio 45267-0575
Dale W. Schaefera) School of Energy, Environmental, Biological and Medical Engineering, University of Cincinnati, Cincinnati, Ohio 45221-0012 (Received 4 December 2012; accepted 28 February 2013)
Films of 3-aminopropyltriethoxysilane films (APTES) deposited from nonpolar solvents show unusual hardness and tribological properties. The morphological origin of this behavior is determined using x-ray reflectivity. The deposited APTES films are smooth, evolving from a sparse structure when less than two-molecule-thick (,1 g/cm3) to a dense structure (1.26 g/cm3) when thicker. Previously reported improvements in wear resistance and hardness are due to the unusually dense nature of the APTES film. The density of multilayered APTES film has implications for its use as an interface-coupling agent because the film density limits the reactivity of embedded amine groups. A high-temperature cure (120 °C) does not affect film density but does significantly improve hydrolytic stability. Given their high density, predictable reactivity, stability and resistance to wear, multilayered APTES films are well suited for interfacial modification designed to improve mechanical properties, provided the films are properly cured. I. INTRODUCTION
3-aminopropyltriethoxysilane (APTES) is a widely used silane that allows facile amine functionalization of hydroxylated inorganic substrates. The amine functionality of APTES has been used to immobilize polymers1 and proteins,2 as well as to promote cell adhesion.3,4 APTES is also used to strengthen the interface between polymer and inorganic phases in composites.5,6 Although the effect of deposition and curing conditions on the surface morphology of APTES films has been reported,2,7–11 the methods used do no not probe the internal structure of the films, and accordingly they provide no insight into the origin of improved hardness and wear resistance of APTES films. APTES films deposited from toluene and heptane, for example, are over two orders of magnitude harder than those prepared from polar or hydrated solvent systems.12 It also appears that multilayered APTES films are more wear resistant,13 hinting that multilayered APTES films prepared from nonpolar solvents may be structurally unique. In this article, we explore the relationship between reported performance improvements and the internal film structure. The internal structure and surface morphology a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2013.54 1118
J. Mater. Res., Vol. 28, No. 8, Apr 28, 2013
http://journals.cambridge.org
Downloaded: 15 Mar 2015
of APTES films were measured using specular x-ray reflectivity (XRR), complemented by atomic force microscopy (AFM) and dye-binding analysis of the surface-active amine functionality. This is the first study of evolution and degradation of the internal structure of multilayer APTES films. XRR provides structural information perpendicular to the sub
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