Swadener et. al. (1999) have shown via X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) that mixed-mode fracture near a glass/epoxy interface left a 3 nm layer of epoxy on the glass surface. Such a cohesive crack path helped explain why the high intrinsic toughness of the interface was 20 times higher than the thermodynamic work of adhesion of the glass/epoxy interface. Other studies of glass/epoxy interfaces (Drzal, 1986; Winter and Houston, 1998) have indicated the presence of an interphase region where polymer properties are affected by the substrate so that there is a transition from the substrate to the bulk behavior of the polymer. Presumably, the subinterfacial fracture noted by Swadener et. al. (1999) took place in such an interphase region. The traction-separation law of this region was extracted in an iterative manner via a combination of crack opening interferometry and finite element analysis. The objective of this work is to make more direct assessments of traction-separation laws of interphase regions. One promising instrument for these purposes is the interfacial force microscope (IFM). The IFM is unique in that it employs a self-balancing, force feedback sensor which allows force profiles (load vs. displacement data) to be obtained while the sensor remains rigidly fixed in position throughout the measurement (Joyce and Houston, 1991; Houston and Michalske, 1992). In this study, the (IFM) was first used to probe the nanomechanical properties of hydrolyzed g-aminopropyltriethoxysilane (g -APS) films fabricated by spin coating on soda glass and SiO2 surfaces. The g-APS is widely used as a coupling agent for fiberglass-reinforced composites and as a primer for adhesive joints with thicknesses ranging from a few nanometers to hundreds of nanometers. It was found that the modulus of 50 nm thick g-APS films on glass were a quarter of those on SiO2. XPS analysis revealed that leaching of Na+ ions from the glass into the g-APS destroyed cross-links, making it softer. In order to have more control on polymer film thickness, a self-assembled octadecyltrichlorosilane (OTS) monolayer adsorbed on SiO2 substrates is being used as a model fracture surface. Its nanomechanical properties are now being probed using the IFM so that comparisons can be made with the g-APS results. Once this is understood, epoxy on glass fracture surfaces will be examined in a similar way.
Key words: Interfacial fracture, interphases, nano-indentation, spectroscopy
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Winter, R. M., and Houston, J. E., (1998), Proc. of the SEM Spring Conf. on Exp. and Applied Mechanics, Houston, TX, June 1-3, 1998.
Joyce, S. A., and Houston, J. E., (1991), Rev. Sci. Instr., 62 (3), 710 - 715.
Houston, J. E., and Michalske, T. A., (1992), Nature, 356, pp. 266 - 267.