SynopsisA thermodynamic analysis of polymer particle morphology highlights the role of interfacial tensions in controlling particle structure. The influence of the surfactant and the nature of the incompatible polymers is seen through their individual and collective effects upon these interfacial tensions. It has been found that by simply changing the type of surfactant used in the emulsion the particle morphology can change from core-shell to hemispherical, in agreement with thermodynamic predictions. Several apparently different morphologies (hemispherical, sandwich, multiple lobes) have been found to coexist a t the same time within a single emulsion, suggesting that they may be simply different states of phase separation and not thermodynamically stable, unique morphologies. The thermodynamic analyses are independent of particle size and method of emulsion processing. Experimental evidence shows that the morphology of particles formed via in situ polymerization ( a s in a synthetic latex) is controlled by interfacial tensions in the same manner as those particles formed via solvent evaporation from a solution of an incompatible polymer pair (as in a n artificial latex or microencapsulation).

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Two different series of mixed Langmuir-Blodgett (LB) films with a controllable degree of polarity, deposited on mica, have been studied by wetting and surface force techniques. Both series contain 50% eicosylamine (EA). Films of one series consist of EA, arachidic acid, and docosandioic acid, while those of the other consist of EA, 1-eicosanol, and 1,22-docosandiol. Carboxylic acid groups give lower contact angles than hydroxy groups. Concerning the stability of the LB films in aqueous solutions, repeated exposure to a three-phase line and high salt solutions were found to cause breakdown. Surface force measurements on carboxylic acid-containing films show that films with a 0% (contact angle = 1 1 3 O ) and 25% (contact angle = 90°) content of diacid interact with a long-range (hydrophobic) attraction across water. No similar long-range attraction is observed for the 50% case (contact angle 65"). Surface force measurements also detected instabilities and imperfections of the films.

Further studies of the hydrophobic attraction between fluorocarbon surfaces in water have been carried out, including (i) a comparison between surfaces prepared by Langmuir-Blodgett deposition and by adsorption from solution using the same surfactant and (ii) the effect of added electrolyte. In general, the surfaces prepared by deposition show the longest range of the attraction-measurable at 80-90 nm with an exponential decay length of about 15 nm at separations above 25 nm. In the case of adsorbed monolayers incomplete adsorption or additional adsorption to the hydrophobic monolayer surfaces often leads to an effective attraction that is of considerably shorter range. Under optimal conditions, however, adsorbed monolayers give an attraction of a similar range. In both cases the interaction becomes more attractive at small separations and appears to decay exponentially with a decay length of 2-3 nm below about 15 nm. The deposited fluorocarbon monolayers are unstable in sodium or potassium salt solutions but remain intact in tetrapentylammonium bromide. With increasing concentration of this salt the range of the hydrophobic interaction decreases as more and more ions adsorb to the surfaces. The short-range interaction seems to be less sensitive to the presence of electrolyte-the adhesion at contact shows a much less dramatic decrease with increasing electrolyte concentration. Nevertheless, the measured electrolyte dependence of the hydrophobic interaction is at least partly due to the surfaces becoming less hydrophobic themselves, as reflected by their cavitation behavior.

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