Interfacial layer visualization

Interfacial Layer Visualization. One of the key results of extensive spinning drop experiments between aqueous mixed emulsifier solutions and styrene was visual evidence for the formation of mixed emulsifier interfacial films. This interfacial layer is depicted in Figure 1 by the formation of "tails" as a function of time on the rotating styrene drop in an aqueous solution of 1 1 SLS/CA, based on lOmM SLS. 

Bond failure may occur at any of the locations indicated in Fig. 1. Visual determination of the locus of failure is possible only if failure has occurred in the relatively thick polymer layer, leaving continuous layers of material on both sides of the fracture. The appearance of a metallic-appearing fracture surface is not definite proof of interfacial failure since the coupling agent, polymer films, or oxide layers may be so thin that they are not detectable visually. Surface-sensitive techniques such as X-ray photoelectron spectroscopy (XPS) and contact angle measurements are appropriate to determine the nature of the failure surfaces scanning electron microscopy (SEM) may also be helpful if the failed surface can be identified. 

During the last few years, the latter view has received more supporting evidence. Already the early experimental work of Giraultand Schiffrin [71], who determined the surface excess of water at the interface with 1,2-dichloroethane, had indicated the existence of a mixed boundary layer. Recent X-ray scattering experiments [72] indicate an average interfacial width of the order of 3 to 6 A. These experiments are in line both with model calculations based on the density functional formalism [73] and with computer simulations [74, 75]. Accordingly, the interface is best visualized as rough on a molecular scale as indicated in Fig. 13. 

Oh et al. studied PA composite membranes prepared by the conventional interfacial polymerization of PA active layers on the surface of various microporous polyacrylonitrile (PAN) supports [80]. The PAN supports were prepared by using PAN/NMP solution with various compositions (10/90,15/85, and 20/80 wt.%). The PAN supports were further modified with NaOH solutions of different concentrations for 1 h at40 °Cto form -COOH groups on their surfaces [81]. Figure 4.50 shows AFM photographs of PAN membranes treated with different NaOH concentrations after their formation from a 15 wt.% PAN solution. Figure 4.50 indicates visually the difference in the surface morphology between those membranes. The surface became smoother as the concentration of NaOH increased. 

An interfacial instability causes distortion of the streamline where two layers of a coextruded film meet (Fig. 5.4). This can affect both the performance and visual properties of the film. Defects such as thickness inconsistencies, reduction in clarity, and even delamination of layers can result. 

As an illustration, consider Fig. 1, which shows four different failure modes for aluminum adherends bonded with a model epoxy adhesive. Simply by altering the loading mode, one can obtain cohesive failures, apparent (visually) interfacial failures, failures that oscillate within the adhesive layer, and alternating failures that jump back and forth from one interface to the other. Predicted theoretically, this dramatic variation in locus of failure was experimentally achieved not by altering the substrates or the surface preparation but simply by changing the loading conditions for these beam-type fracture specimens. 

The first step in failure analysis is a visual inspection. In a few cases, such as a failure through the middle of an adhesive, this examination is sufficient to identify the locus of failure. However, in most situations, it is not that easy. For example, a failure may visually appear interfacial or adhesive, but crack propagation may have occurred within one of the bond components close to the interface. The eye is not usually able to detect a thin (< 100 nm) layer on a sample. Similarly, unless it is gross, contamination cannot generally be detected visually. 

The above comments are seen to be reinforced by observations on the failure path in joints before and after environmental attack. The locus of joint failure of adhesive joints when initially prepared is usually by cohesive fracture in the adhesive layer, or possibly in the substrate materials. However, a classic symptom of environmental attack is that, after such attack, the joints exhibit some degree of apparently interfacial failure between the substrate (or primer) and the substrate. The extent of such apparently interfacial attack increases with time of exposure to the hostile environment. In many instances environmental attack is not accompanied by gross corrosion and the substrates appear clean and in a pristine condition, whilst in other instances the substrates may be heavily corroded. However, as will be shown later, first appearances may be deceptive. For example, to determine whether the failure path is truly at the interface, or whether it is in the oxide layer, or in a boundary layer of the adhesive or primer (if present), requires the use of modern surface analytical methods one cannot rely simply upon a visual assessment. Also, the presence of corrosion on the failed surfaces does not necessarily imply that it was a key aspect in the mechanism of environmental attack. In many instances, corrosion only occurs once the intrinsic adhesion forces at the adhesive/substrate interface, or the oxide layer itself, have failed due the ingressing liquid the substrate surface is now exposed and a liquid electrolyte is present so that post-failure corrosion of the substrate may now result. 

Typically when adhesive Joints undergo fracture in a relatively dry environment they do not exhibit failure via a true interfacial failure between the adhesive and substrate. Instead, although visually interfacial failure may have appeared to have occurred, detailed examination of the fracture surfaces reveals the presence of a thin layer of adhesive retained on the substrate. This observation is particularly relevant to the structural adhesives where the adhesive is relatively strong and tough. However, in the presence of an aggressive environment, then true interfacial failure may indeed occur, and may often result at a very low applied load. The obvious question is why an interface, which can withstand comparatively high stresses when initially prepared, should be so unstable in the presence of liquids, such as water. 

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