J . Phys. Chem. 1989, 93, 4861-4813
the gain in entropy is less significant than in the normal cases. However, the origin of these two interpretations is the same; that is, the predominant conformation of the surfactant chains is the stretched form. Acknowledgment. The financial supports from the Swedish Natural Science Research Council and the Swedish Board of Technical Development are gratefully acknowledged. Masski
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Yasuda is thanked for valuable assistance in the synthesis of C,Et6. Thanks are also due to Eva Hansson and Krister Fontell for performing the X-ray scattering experiment and for valuable discussions concerning this experiment. T.C.W. thanks the University of Missouri for granting a research leave. Registry No. CzoEt6,105476-67-3; 11-bromoundecanoic acid, 283405-1; 1,20-dibromoeicosane,14296-16-3.
Percolation and Particle Nucleation in Inverse Microemulslon Polymerization M. T. Carver,+ E. Hirsch,t J. C. Wittmann? R. M. Fit&,$ and F. Candau*yt Institut Charles Sadron (CRM-EAHP), 6 , rue Boussingault. 67083 Strasbourg Cedex, France, Laboratoire de Spectrometrie et d'lmagerie Ultrasonores, Universite Louis Pasteur, 4, rue Blaise Pascal, 67070 Strasbourg Cedex, France, and Corporate Research, S . C . Johnson and Son,Inc.. Racine, Wisconsin 53403 (Received: August 3, 1988; In Final Form: January 19, 1989)
The electrical conductivity of toluene/AOT/(acrylamide + H20) water-in-oil microemulsions and of polymer latexes formed from these microemulsions was investigated. For microemulsions containing acrylamide, a sharp rise in conductivity with increasing acrylamide concentration centered around an inflection point was noted and was attributed to a percolation process. The conductivity of percolating microemulsions fell rapidly as the microemulsion was polymerized. Quasielastic light scattering (QELS), transmission electron microscopy, and viscometry were used to monitor changes in particle size as a function of the extent of polymerization. Microscopy and viscometry experiments were combined to show that particle nucleation occurs continuously throughout the polymerization. QELS measurements show differing behaviors depending upon whether the initial microemulsions were percolating or nonpercolating. Mechanisms are proposed to explain both types of behavior.
Introduction In recent years a number of works involving the polymerization of water-soluble monomers in water-in-oil microemulsions have been published.14 These reports have dealt mainly with the formulation of the initial microemulsions (anionic or nonionic surfactants; toluene, heptane, or isopar M as the oil phase; etc.) and with the latexes obtained after complete polymerization of the microemulsions. For toluene/aerosol OT/(H20 acrylamide) microemulsions, it has been shown that the initial solutions contain swollen micelles with diameters in the range of 50-65 8, and that, upon polymerization, final latex particles reach diameters between 300 and 500 A, on average.' In addition, kinetic studies have established that, for these microemulsions, the termination mechanism of the polymerization is monoradical, rather than biradial, due to a radical transfer from the growing polymer chain to solvent, and that both the rate of polymerization and the molecular weight of the polymers are first order in monomer c ~ n c e n t r a t i o n . ~Nevertheless, .~ much remains to be understood about the processes occurring within these systems, both before and during the polymerization. In the present work, we demonstrate the presence of percolation phenomena in these systems and examine the particle nucleation process as a function of the extent of conversion of monomer to polymer.
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Experimental Section Materials. Toluene (Aldrich Gold Label) was used as received, water was deionized and distilled. Sodium bis(2-ethylhexyl) sulfosuccinate (AOT) was dissolved in dry methanol to precipitate saline impurities and then filtered and shaken with petroleum ether to extract organics. Acrylamide (AM) was recrystallized twice from chloroform, and azobis[isobutyronitrile] (AIBN), used as a photoinitiator, was recrystallized twice from ethanol. Volume fractions were calculated assuming ideal mixing behavior using densities of water (1 .OO g/cm3), toluene (0.87 g/cm3), AOT (1.13 *To whom correspondence should be addressed. 'funiversite lnstitut Charles Sadron (CRM-EAHP). Louis Pasteur. IS.C. Johnson and Son, Inc.
g/cm3), and AM (1.12 g/cm3). Photopolymerization of toluene/AOT/(H20 AM) microemulsions has been described in detail e l ~ e w h e r e .For ~ a given microemulsion polymerization, a series of polymerizations, carried to a predetermined percent of conversion, was made. The start and duration of the polymerizations were followed by means of a thermocouple and recorder; the polymerizations were stopped by extinguishing the light and then adding 4-methoxyphenol (
