Chapter 13
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Inverse Emulsion Polymerization of Acrylamide in Near-Critical and Supercritical Continuous Phases Eric J. Beckman, John L. Fulton, Dean W. Matson, and Richard D. Smith Chemical Methods and Separations Group, Chemical Sciences Department, Pacific Northwest Laboratory, Richland, WA 99352
The inverse microemulsion polymerization of watersoluble acrylamide monomers within near-critical and supercritical alkane continuous phase provides a potential route for production of polymers with novel physical properties and at high reaction rates. In order to define conditions for a model polymerization process, the phase behavior of a nonionic surfactant/ acrylamide/water system in near- and supercritical mixtures of ethane and propane was examined. Results show that in mixtures of ethane and propane the continuous-phase density determines the phase behavior. Results also show that acrylamide acts as a co-surfactant with C E /C E (Brij) surfactant blend used for these experiments. Surprisingly, increasing the total dispersed-phase volume fraction lowers the density (and consequently pressure) required to form a stable microemulsion. Dynamic light scattering results suggest the presence of strong micelle-micelle interactions, or clustering, the extent of which increases rapidly as the phase boundary is approached. Initial polymerization results indicate possible dependencies of both the polymerization rate and the molecular weight on continuous-phase density and/or the degree of micelle-micelle clustering, suggesting that the monomer may not be as accessible to a growing chain as in a classical emulsion polymerization. 16
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Emulsion polymerization i s an important commercial process because, i n contrast to the same f r e e - r a d i c a l polymerization performed i n the bulk, molecular weight and reaction rate can be increased simultaneously (1-3). Furthermore, the lower v i s c o s i t y of an emulsion system compared with that of the corresponding bulk process provides better control over heat t r a n s f e r . Commercial emulsion processes usually use a surfactant/water/monomer system 0097-6156/89/0406-0184$06.75/0 o 1989 American Chemical Society
In Supercritical Fluid Science and Technology; Johnston, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
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BECKMANEXAL»
Inverse Emulsion Polymerization ofAcrylamide 185
that i s s t a b i l i z e d by vigorous s t i r r i n g . The dispersed phase contains micelles, approximately 10 t o 50 nm i n diameter, as well as monomer droplets, which can be 10 to 100 times larger than the m i c e l l e s . In the absence of a g i t a t i o n , these monomer droplets w i l l coagulate and separate as a second phase. I f , as i s the usual p r a c t i c e (1), a continuous-phase soluble i n i t i a t o r i s used, polymerization commences at the m i c e l l e i n t e r f a c e and proceeds within the m i c e l l e s . During the reaction, monomer d i f f u s e s from the large droplets into the m i c e l l e s . Exhaustion of these monomer reservoirs signals the end of the polymerization. In contrast t o the emulsion system described above, a microemulsion i s thermodynamically stable, and thus one-phase and o p t i c a l l y c l e a r i n the absence of a g i t a t i o n . Microemulsion polymerization has been used to produce stable l a t i c e s with a very f i n e (approx. 50 nm) p a r t i c l e s i z e (J.). Microemulsion polymerization i n a s u p e r c r i t i c a l f l u i d may provide some s i g n i f i c a n t advantages compared with the same reaction i n a conventional J i q u i d . Removal of the continuous phase following polymerization would c e r t a i n l y be f a s t e r and easier than removal following a s i m i l a r reaction c a r r i e d out i n a conventional l i q u i d . The a b i l i t y to remove the continuous phase without the formation of a liquid-vapor meniscus and i t s accompanying strong surface forces could allow production of polymer with a very f i n e p a r t i c l e s i z e . In t h i s a r t i c l e we describe the phase behavior of a microemulsion system chosen f o r the free r a d i c a l polymerization of acrylamide within n e a r - c r i t i c a l and s u p e r c r i t i c a l alkane continuous phases. The e f f e c t s of pressure, temperature, and composition on the phase behavior a l l influence the choice of operating parameters f o r the polymerization. These r e s u l t s not only provide a basis f o r subsequent polymerization studies, but also provide data on the properties of reverse m i c e l l e s formed i n s u p e r c r i t i c a l f l u i d s from nonionic surfactants. In addition, we present some i n i t i a l r e s u l t s on the e f f e c t of pressure, temperature, and the various composition variables on the rate of polymerization and the molecular weight of the polymer formed.
Experimental Materials. Nonionic surfactants B r i j 52 (B52) and B r i j 30 (B30) were obtained from the Sigma Chemical Company and used as received. These surfactants are ethoxylated alcohols with the nominal structures Ci6 2 and C 1 2 E 4 , respectively, where Ε represents the number of ethylene oxide u n i t s . Acrylamide was obtained from the A l d r i c h Chemical Company (Gold Label 99+%) and r e c r y s t a l l i z e d twice from chloroform. Azo b i s ( i s o b u t y r n i t r i l e ) (AIBN), obtained from the A l f a Products D i v i s i o n of Morton Thiokol, was r e c r y s t a l l i z e d from methanol. Water was doubly deionized. Propane obtained from Union Carbide Linde D i v i s i o n (CP Grade) and ethane from A i r Products (CP Grade) were used without further p u r i f i c a t i o n . E
In Supercritical Fluid Science and Technology; Johnston, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
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SUPERCRITICAL FLUID S C I E N C E A N D T E C H N O L O G Y
Phase Behavior. Phase t r a n s i t i o n s were observed v i s u a l l y using a high-pressure view c e l l (volume - 47 cm ), capable of pressures t o 600 bar, whose design has been previously described (5). Material was introduced to the magnetically s t i r r e d c e l l , which was then sealed and pressurized with the f l u i d of choice using a Varian 8500 syringe pump. Gas mixtures were prepared by weight (composition ±0.25%) i n a 400 cm lecture b o t t l e , s t i r r e d f o r 15 minutes, then t r a n s f e r r e d t o the syringe pump. Temperature i n the c e l l was c o n t r o l l e d t o within 0.1°C using an Omega thermocouple/ temperature programmer. Pressure was measured using a Precise Sensor 0- to 10,000-psi transducer, and readout was c a l i b r a t e d to within ±10 p s i using a Heise Bourdon-tube gauge. 3
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Quasi-Elastic Light Scattering. Dynamic l i g h t s c a t t e r i n g measurements were made using a Malvern PCS-100 spectrophotometer modified f o r high-pressure work. The conventional s c a t t e r i n g c e l l was replaced by a high-precision sapphire tube, described elsewhere (£), which allowed measurements at pressures up t o 500 bar. Temperature i n the c e l l , whose volume i s approximately 1.5 cm , i s maintained v i a a thermostated (±0.1 C) toluene bath. The spectrophotometer used a 5-W argon laser (488 nm), and the s i g n a l from the photomultiplier was processed on a 128-channel, real-time d i g i t a l c o r r e l a t o r (K7032-OS) with a 50-ns sample time. The instrument alignment was checked using 58-nm polystyrene latex spheres dispersed i n water; the measured s i z e was within 5% of the reported value. The photon a u t o c o r r e l a t i o n f u n c t i o n was analyzed by the method of cumulants (2)t i n which the logarithm of the normalized a u t o c o r r e l a t i o n function, G (q, t ) , i s f i t to a polynomial using a nonlinear, least squares routine, 3
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