The formation of thermalized positronium atoms is greatly reduced if increasing amounts of water become solubilized in reversed micelles formed by sodium bis(2-ethylhexyl) sulfosuccinate in apolar solvents. Similar observations have been made if the surfactant is Triton X-100. The application of the positron annihilation technique to the study of microemulsions consisting of potassium oleate-alcohol-oil-water mixtures indicates, consistent with previous results, that microemulsion formation requires a certain water/oil ratio if the oil is a long-chain aliphatic hydrocarbon such as hexadecane. This ratio is 0.4 in the case of a 1-pentanol-and 0.2 for a 1-hexanol-containing mixture. This minimum water content is strongly reduced if the oil is an aromatic hydrocarbon. The positron annihilation data also sensitively reflect structural rearrangements in these solutions occurring upon further addition of water, such as the transition of spherical aggregates to a disk-like lamellae structure.
Rate constants ( q / E values) for laser pulse initiated photopolymerization have been determined for 1200-2700-&diameter bilayer surfactant vesicles prepared from dioctadecylmethyl(2-[ (4-vinylbenzoy1)-oxylethy1)ammonium bromide (31, from mixtures of dioctadecyldimethylammonium bromide (DODAB) and vinylbenzoic acid, from mixtures of 3 and DODAB, and from mixtures of 3 and vinylbenzoic acid. q / E values were 1.35 J-' for 1650-A-diameter 3 vesicles, were 0.059 J-' for mixed vesicles prepared from vinylbenzoic acid and DODAB (0.25:l mole fraction), varied between 0.123 and 1.46 J-' for vesicles prepared from different mixtures of DODAB and 3, and varied between 0.16 and 0.314 J-l for different mixtures of vinylbenzoic acid and DODAB. Plots of polymerization rate constants against the mole fraction of 3 in vesicles prepared by cosonicating 3 + DODAB were found to increase curvilinearly to a plateau value, indicating domain formation. Subsequent to vesicle polymerizations, cumulants, weight-and number-average molecular weights of the separated poly(viny1benzoates) and, hence, the average chain lengths were determined by gel exclusion chromatography. The average chain lengths varied from 10 in laser-polymerized 3 vesicles, through 20 in laser-polymerized vesicles prepared from mixtures of DODAB and vinylbenzoic acid, to 40 in laser-polymerized vesicles prepared from mixtures of 3 and vinylbenzoic acid. These relatively small chain lengths were discussed in terms of the restricted geometries prevailing a t intravesicular surface polymerizations. Fluorescence measurements and a Monte Carlo based computer simulation of the photopolymerization were used to substantiate the proposed mechanism of vesicle polymerization.
Positron lifetime measurements were performed for two different kinds of polymers (low density polyethylene and a polyimide 6FDA-TMPD) during sorption of various vapors (hexane, cyclohexane, benzene, acrylic acid, methyl acrylate, water, and oxygen). The vapor sorption affected the long-lived component (ortho-positronium component) in a systematic way regardless of the kind of the vapor molecules, i.e. for the polyethylene both the lifetime and the intensity of the long-lived component were enhanced, while for the polyimide they were decreased significantly. These different effects are interpreted in terms of different states of sorbed molecules in rubbery (the polyethylene) and in glassy (the polyimide) polymers.
SYNOPSISPositron annihilation lifetime (PAL) spectra were measured for two different kinds of polymers: polypropylene (PP) and polytetrafluoroethylene (PTFE), during sorption of vapors of the following molecules: n-hexane, cyclohexane, benzene, methyl methacrylate, acrylic acid, and water. The behavior of ortho-positronium (0-Ps) annihilation parameters: lifetime, T,, and intensity, 13, can be explained by considering different sorption mechanisms in rubbery and glassy polymers. The mean size of the intermolecular-space holes and the relative free-volume fraction of PP and PTFE were estimated before and during the sorption process from the 0-Ps parameters obtained. Sorption of n-hexane, cyclohexane, and benzene in both polymers affect the 0-Ps component in a systematic way, for PP T~ and I3 tend to decrease, while in PTFE, 7, stay almost constant and I3 diminishes. These effects are interpreted in relation to the different states of the polymers studied rubbery for PP and glassy for PTFE. In case of methyl methacrylate molecules, the behavior of 0-Ps parameters was more complicated. For acrylic acid and water, their associated polarity provokes a diminution in I,, which is explained from the viewpoint of the spur model of positronium formation. Large changes in the mean size and the fraction of free volume, as well as their constant behavior in some cases, were interpreted to be due to positive or negative interactions, respectively, between vapor molecules and PP and PTFE, which could be correlated with the solvent-interaction parameters calculated. 0 1995 John Wiley & Sons, Inc.
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