The present work was undertaken to develop a composite osmotic distillation (OD) membrane consisting of a thin hydrogel coating on a microporous hydrophobic substrate for the concentration of aqueous feeds containing surface-active agents. The range of OD applications using the hydrophobic membrane alone have been severely limited by the propensity for membrane wet-out when contacted by amphiphilic agents such as oils, fats and detergents. Wet-out allows the feed solution to track freely through the membrane pores with a resulting loss of solutes and a decrease in selectivity. The rationale for the approach taken was based on the hypothesis that the high water selectivity of the hydrophilic coating would maintain good water mass transfer to the underlying hydrophobic substrate but exclude other components including surface-active agents. The first stage of this work involved the identification of potential coating materials and the fabrication and structural characterization of films of these materials to determine their suitability. The second stage involved the development of techniques to facilitate adhesion of the hydrophilic coatings to the hydrophobic substrate, and the testing of the resulting composite membranes for OD performance and resistance to wet-out by surface-active agents. Sodium alginate was selected as the major coating component on the basis of its non-toxicity and its potential for stable hydrogel formation. Structural characterization of noncrosslinked films and films crosslinked using a water-soluble carbodiimide (WSC) was carried out using differential scanning calorimetry (DSC), Fourier Transform infrared spectroscopy (FT-IR) and swelling measurements. Maximum crosslinking through esterification of hydroxyl and carboxylic acid groups on adjacent polymer strands using the film immersion method was achieved with a non-solvent (ethanol) concentration of 60 vol % and a WSC concentration of 100 mM at pH 4. These conditions resulted in a hydrogel with an equilibrium water content of 60 wt %. DSC measurements of noncrosslinked and crosslinked alginate films showed an increase in crystallinity and hence rigidity on crosslinking. Therefore, several coatings were prepared as blends of sodium alginate and amorphous highly flexible carrageenan gum in order to meet the flexibility requirements of a membrane subjected to varying operating pressures in an industrial OD plant. Structural characterization with respect to polymer blend ratio was carried out using scanning electron microscopy (SEM), DSC, X-ray diffraction (XRD). The optimisation for crosslinking conditions was undertaken as for sodium alginate alone. Optimum conditions for film preparation were 20 wt % carrageenan content and a crosslinking medium containing 60 vol % non-solvent (ethanol) and 120 mM WSC at pH 4. These conditions produced a hydrogel with an equilibrium water content of 85 wt %. Two different techniques were employed to anchor the coatings on substrate PTFE membranes. For membranes with a nominal diameter of 0.2 µm, the technique involved surface tension adjustment of the coating solution by ethanol addition in order to enhance penetration of the coating solution meniscus into the substrate pores. This was followed by polymer precipitation by the selective removal of water using OD to provide structural interlocking. T-peel strength measurements showed that this technique resulted in a ten-fold increase in adhesion strength when compared with a coating cast without surface tension adjustments. For membranes with a nominal diameter of 0.1µm, an interfacial bonding agent, myristyltrimethylammonium bromide (MTMA), was used. This technique gave a three-fold increase in adhesion strength relative to that of coating cast without the use of MTMA. The composite membranes were tested in extended OD trials using pure water and feeds containing limonene, the major surface-active components of orange oil. The sodium alginate-carrageenan blend membrane, which was the preferred membrane based on flexibility and water sorption considerations, was also tested against full-cream milk and an industrial detergent, sodium dodecylbenzene sulfonate (DBS). The results indicated that the coatings offered little resistance to water transport and were effective in providing protection against membrane wet-out. Durability trials showed that the composite membranes retained their integrity in water for a minimum of 30 days. Overall, this study has expanded the potential applications of OD to include many important industrial concentration steps that are currently being undertaken by conventional processes with unsatisfactory results. These include the concentration of citrus juices, full-cream milk and nuclear power plant liquid waste. These feeds contain limonene, fats and detergents respectively, all of which wet out unprotected hydrophobic membranes.

When bonded joints are subjected to harsh environmental conditions, the interphase, the three-dimensional region surrounding the adhesive/substrate interface, becomes critically important. Frequently, failure occurs in this region after adhesively bonded systems are subjected to elevated temperature oxidative aging. In a previous study, this was found to be the case with a polyimide adhesive bonded to chromic acid anodized (CAA) Ti-6Al-4V. The objective of the current research has been twofold: 1) to investigate the effect of thermal aging on the interphase region of polyimide/titanium adhesive joints, and 2) to evaluate the method used in the current study for durability characterization of other adhesive/substrate systems. The method used in this research has been to characterize the effect of elevated temperature aging on the following systems: 1) Notched coating adhesion (NCA) specimens and 2) bulk samples of dispersed substrate particles in an adhesive matrix. The NCA test has the advantages of an accelerated aging geometry and a mode mix that leads to failure through the interphase, the region of interest. The bulk samples have the advantage of an increased interphase volume and allow for the application of bulk analysis techniques to the interphase, a region that is traditionally limited to surface analysis techniques. The adhesive systems studied consisted of one of two polyimide adhesives, LaRC© PETI-5 or Cytec Fiberite© FM-5, bonded to CAA Ti-6Al-4V. The model filled system consisted of a PETI-5 matrix with amorphous titanium dioxide filler. Through the use of the NCA test, it was determined that bonded specimens made with FM-5 lose approximately 50% of their original fracture energy when aged in air at 177°C for 30 days. This aging temperature is well below the glass transition temperature of the adhesive, 250°C. At the same time, the failure location moves from the anodized oxide layer to the adhesive that is directly adjacent to the substrate surface, the interphase region. Through surface analysis of this region, it is determined that the adhesive penetrates the pores of the CAA surface to a depth of 70 to 100 nm, promoting adhesion at the interface. With aging, the adhesive in the interphase region appears to be weakening, although analysis of the bulk adhesive after aging shows little change. This indicates that adhesive degradation is enhanced in the interphase compared to the bulk. Analysis of the model filled system gave similar information. Specimens containing titanium dioxide filler had glass transition temperatures that were approximately 20°C lower than the neat polyimide samples. In addition, the filled samples contained a significant portion of low molecular weight extractable material that was not present in the neat specimens. The tan delta spectra from dynamic mechanical thermal analysis of the filled specimens exhibited a shoulder on the high-temperature side of the glass transition peak. This shoulder is attributed to the glass transition of the interphase, a distinct phase of the polyimide which is constrained by adsorption onto the filler particle surfaces. As a function of aging time at 177° or 204°C, the shoulder decreases substantially in magnitude, which may relate to loss of adhesive strength between the polyimide and the filler particles. From this research, it has been illustrated that information relating to the durability of adhesively bonded systems is gained using an interfacially debonding adhesive test and a model system of substrate particles dispersed in an adhesive matrix / Ph. D.