ISSN 00405795, Theoretical Foundations of Chemical Engineering, 2013, Vol. 47, No. 5, pp. 600–603. © Pleiades Publishing, Ltd., 2013. Original Russian Text © A.A. Oganesyan, M. Khaddazh, I.A. Gritskova, S.P. Gubin, G.K. Grigoryan, G.M. Muradyan, A.G. Nadaryan, 2013, published in Teoreticheskie Osnovy Khimicheskoi Tekhnologii, 2013, Vol. 47, No. 5, pp. 580–583.

Polymerization in the Static Heterogeneous System Styrene–Water in the Presence of Methanol A. A. Oganesyana, M. Khaddazhb, I. A. Gritskovac, S. P. Gubinb, G. K. Grigoryana, G. M. Muradyana, and A. G. Nadaryana a

Scientific Technological Center of Organic and Pharmaceutical Chemistry, National Academy of Science of Armenia, Azatutian ave. 26, Yerevan 0014, Republic of Armenia b Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Leninskii pr. 31, Moscow, 119991 Russia c Moscow State University of Fine Chemical Technologies, pr. Vernadskogo 86, Moscow, 119571 Russia email: [email protected] Received February 19, 2013

Abstract—Because of the density gradient generated by methanol diffusion into the static styrene–aqueous potassium persulfate solution system, the dispersed particles nucleating during polymerization localize in a narrow zone of the liquid phase, allowing their density to be estimated by pycnometry. These measurements have corroborated the hypothesis that the dispersed particle nucleate under the action of polymerization tak ing place at the monomer/water interface and are monomer microdrops containing a certain number of poly mer molecules. DOI: 10.1134/S0040579513050230

INTRODUCTION There is extensive literature on the properties of microemulsions and their formation conditions [1–4]. Researchers’ attention has mainly been focused on thermodynamically equilibrated lyophilic systems that form at high surfactant concentrations and under vigor ous stirring of the system. As distinct from true spontaneous emulsification [5, 6], dispersion of a heterogeneous liquid–liquid system can occur owing to the thermodynamic instability of the interfacial layer arising from the mass transfer of one phase into the other [7, 8]. Mass transfer across the interface has been the sub ject of many works (see, e.g., [9–11]). However, in the case of the aforesaid dispersion of a heterogeneous liq uid–liquid system, this process has notable specific fea tures. It is commonly believed that the microemulsions resulting from interfacial turbulence or from the inter facial transfer of components of a system are only kinet ically stable and differ significantly from conventional, thermodynamically stable microemulsions. The size of monomer microdrops is larger than the characteristic size of emulsifier micelles but is much smaller than the size of drops than can be produced by mechanic disper sion of a monomer at a certain surface tension. It was hypothesized that monomer microdrops are involved in the formation of polymer–monomer parti cles, and the conditions under which these microdrops become the main source of polymer–monomer parti cles were determined [12].

The size of latex particles and their size distribution are of exceptional significance for their subsequent use in various applications, including microspheres onto which various nanoparticles (e.g., nanoparticles of sil ver, other noble metals, metal oxides, and semiconduc tor compounds) can be deposited [13, 14]. The first data demonstrating the possibility of pre paring aqueous dispersions of polymers without employing an emulsifier in a static monomer–water polymerization system were reported in [15–20]. The static heterophase polymerization of styrene is carried out in a very simple way. Styrene is carefully lay ered onto an aqueous solution of potassium persulfate in a temperaturecontrolled test tube. This initiates polymerization in the immediate vicinity of the mono mer/water interface. After a certain time interval (~2 h at 60°С), the aqueous phase becomes turbid, indicating the formation of dispersed particles in the system. The long induction between the onset of polymerization and the observation of turbidity is a consequence of the slow diffusion of the particles into the bulk of the aqueous phase [14]. The mechanism of dispersion of the static mono mer–water system containing no emulsifier remains obscure in many respects. It was reported [17, 21, 22] that the dispersion process (formation of a new inter face) takes place owing to the increase in free energy due to the heat of polymerization. It was hypothesized that the polymerization reactions occur in the immedi ate vicinity of the monomer–water interface and dis persed particles nucleate as monomer microdrops [17]. It was also assumed that the density of the particles

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increases during polymerization and they pass into the aqueous phase. In our experimental investigation of the topological mechanism of the formation of dispersed particles under experimental conditions, we noticed that, after the monomer was layered onto the aqueous phase, dif fusion began and the density distribution over the height of the aqueous became nonuniform. Because of this and because the diffusion rate of the dispersed particles is much lower than that of nonpolymeric molecules, it was expected that the introduction of methanol, a com pound soluble in both phases, would increase the solu bility of the monomer in water and, accordingly, the intensity of mass transfer between the phases and would make it possible to observe the buildup of particles in the immediate vicinity of the place of their generation.

T, % 75 4 50

3 25

2

1

40

60 t, min

0 20

EXPERIMENTAL, RESULTS AND DISCUSSION Styrene (Aldrich) was purified from the stabilizer by vacuum distillation. Potassium persulfate (Sigma Ald rich, reagent grade) contained 99.9% active compound and was used as received. Methanol (brand A) was used without being distilled. Water was twice distilled. Polymerization in the static styrene–water system was performed in temperaturecontrolled test tubes at 60°С. Methanol was introduced into the system in two ways. In the first series of experiments, methanol was initially dissolved in the aqueous phase; in the second, methanol and styrene were carefully layered onto the surface of the aqueous phase. The formation of dis persed particles was monitored as the variation of the turbidity of the aqueous phase. (These measurements were carried out on an SF24 spectrophotometer at a wavelength of 540 nm.) Density was determined using a pycnometer. Experiments were simultaneously per formed in four test tubes, and a single sample was taken from each one. In all experimental series, the styrene volume was 2 mL and the volume of the aqueous phase was 30 mL. The kinetics of turbidization of the aqueous phase in the case of methanol initially dissolved in water is illus trated in Fig. 1. As can be seen from Fig. 1, the induction period of water turbidization is shorter when the aqueous phase is initially saturated with styrene. This is due to the sooner onset of styrene polymerization in water. However, irre spective of whether the aqueous phase was initially sat urated with styrene or the monomer entered the aque ous phase via diffusion, the introduction of methanol into the system significantly shortened the induction period of the turbidization of the aqueous phase. Sam ples were taken from the middle part of the aqueous phase. The dramatic shortening of the induction period of the turbidization of the aqueous phase was also observed with ethanol in place of methanol. This suggests that the observed kinetic effect is largely due to enhanced mass

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Fig. 1.Effect of methanol on the induction period of the turbidization of the middle part of the aqueous phase in the static styrene–aqueous potassium persulfate solution sys tem. Aqueous phase composition: (1) 30 mL of 0.4% aque ous potassium persulfate solution, (2) 30 mL of 0.4% aqueous potassium persulfate solution saturated with styrene, (3) 15 mL of 0.4% aqueous potassium persulfate solution + 15 mL of methanol, and (4) 15 mL of 0.4% aqueous potassium persulfate solution saturated with sty rene + 15 mL of methanol.

transfer rather than the possible reaction between potassium persulfate and methanol. (Potassium persul fate decomposition in the presence of methanol is known to proceed via a chain mechanism and is greatly accelerated by the alcohol [23].) In order to produce a density gradient along the height of the aqueous phase, in the second experimental series the static system was formed by alternately layer ing methanol and styrene onto the surface of a 0.4% aqueous potassium persulfate solution. The height of the aqueous phase was 10 cm, its density was 1.0082 g/cm3, and the volume of styrene and methanol was 2 and 5 mL, respectively. In these experiments, a specific turbidity pattern was observed for the aqueous phase along with a shortening of the induction period. Photographs of the polymerization systems made in different time intervals are presented in Fig. 2. In the presence of methanol, a turbid zone appears in the middle of the aqueous phase, and, acquiring a conical shape, it moves to the interface (Fig. 2). Later, the dispersed particles sink slowly into the bulk of the aqueous phase, turning it into latex. At the instant tur bidity appears (after an approximately 10minlong thermostating of the system), samples were taken from simultaneous experiments and density was measured in the upper, middle, and lower zones of the aqueous phase. The results of these measurements are presented in the table. For comparison, we list the densities of sty rene, 0.4% potassium persulfate solution in water, and a

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Fig. 2. Dynamics of latex formation in the static styrene–aqueous potassium persulfate solution system in the presence of methanol.

Density of different zones of the aqueous phase at the in stant turbidity appears in the system En try 1 2 3 4 5

Place of measurement

ρ, g/cm3

Upper zone of the liquid phase Middle zone of the liquid phase Lower zone of the liquid phase 0.4 aqueous solution of potassium persulfate Methanol + 0.4% aqueous solution of po tassium persulfate (1 : 6 vol/vol)

0.9428 0.9522 0.9620 1.0082 0.9444

mixture of methanol and 0.4% potassium persulfate solution in a 1 : 6 volume ratio, which would be reached upon the uniform distribution of methanol over the height of the aqueous phase. The density measurements in these experiments were performed at 25°C. A comparison of the densities in the different zones of the system and the styrene and polystyrene densities (0.906 and 1.06 g/cm3) suggests that, at the initial stage of the formation of the dispersed particles, their density is much lower than the density of polystyrene and, apparently, they are monomer microdrops containing a certain number of polymer molecules. CONCLUSIONS Potassium persulfate–initiated styrene polymeriza tion was investigated in the static heterogeneous system styrene–water without an emulsifier in the presence of methanol. It was proved that there is a density gradient in the system, which results from methanol diffusion into the static styrene–aqueous potassium persulfate solution system. The dispersed particles nucleating in the polymerization process localize in a narrow zone of the liquid phase. Polymer–monomer particles form from monomer microdrops. The observed diffusion phenomena and the appearance of a conical turbid zone in the system are obviously interesting and original

experimental finding, whose physical nature needs to be further studied. NOTATION T —turbidity, %; t —time, min; ρ —density, g/cm3. REFERENCES 1. Micellization, Solubilization, and Microemulsions, Mittal, K.L., Ed., New York: Plenum, 1977. 2. Entov, V.M., Kaminskii, V.A., and Lapiga, E.Ya., Calcu lation of the rate of emulsion coalescence in turbulent flow, Izv. Akad. Nauk SSSR, Mekh. Zhidk. Gaza, 1976, no. 3, p. 47. 3. Hicke, H.F. and Rehark, J., On the formation of water/oil microemulsion, Helv. Chim. Acta, 1976, vol. 59, p. 2883. 4. Rebinder, P.A. and Taubman, A.B., Notes on the aggre gative stability of dispersions, Kolloidn. Zh., 1961, vol. 23, p. 359. 5. Pravednikov, A.N., Gritskova, I.A., Muradyan, et al., New method of study of latex dispersion, Kolloidn. Zh., 1981, vol. 41, p. 595. 6. ElAasser, M.S., Lack, C.D., Chou, Y.T., et al., Interfa cial aspects of miniemulsions and miniemulsion poly mers, Colloids Surf., 1984, vol. 12, p. 79. 7. Chalykh, A.E., Diffuziya v polimernykh sistemakh (Diffu sion in Polymer Systems), Moscow: Khimiya, 1987. 8. Gritskova, I.A. and Kaminskii, V.A., Interphase phe nomena and the formation of particles in emulsion poly merization, Russ. J. Phys. Chem. A, 1996, vol. 70, p. 1413. 9. Serdyukov, S.I., Higher order heat and mass transfer equations and their justification in extended irreversible thermodynamics, Theor. Found. Chem. Eng., 2013, vol. 47, p. 89. 10. Dilman, V.V., Kashirskaya, O.A., and Lotkhov, V.A., Specific features of multicomponent diffusion, Theor. Found. Chem. Eng., 2010, vol. 44, p. 379.

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POLYMERIZATION IN THE STATIC HETEROGENEOUS SYSTEM STYRENE–WATER 11. Kuzmin, A.O., Parmon, V.N., Pravdina, M.Kh., et al., Mass transfer in a medium with a rapidly renewed inter face, Theor. Found. Chem. Eng., 2006, vol. 40, p. 225. 12. Strnek, F., Peremeshivanie i apparaty s meshalkami (Stir ring and Stirred Apparatuses), Leningrad: Khimiya, 1975. 13. Munro, D., Goodall, A.R., Wilkinson, M.C., et al., Study of particle nucleation, flocculation, and growth in the emulsifierfree polymerization of styrene in water by total intensity light scattering and photon correlation spectroscopy, J. Colloid Interface Sci., 1979, vol. 68, p. 1. 14. Gubin, S.P., Yurkov, G.Yu., and Kataeva, N.A., Nano chastitsy blagorodnykh metallov i materialy na ikh osnove, (Noble Metal Nanoparticles and Related Materials), Moscow: Azbuka, 2006. 15. Goodall, A.R., Randle, K.J., and Wilkinson, M.C., A study of the emulsifierfree polymerization of styrene by laser lightscattering techniques, J. Colloid Interface Sci., 1980, vol. 75, p. 493. 16. Oganesyan, A.A., Gukasyan, A.V., Matsoyan, S.G., et al., Diffusion and polymerization of styrene in an aqueous solution of potassium persulfate under static conditions Dokl. Phys. Chem., 1985, vol. 281, nos. 4–6, p. 377. 17. Oganesyan, A.A., Freeradical polymerization and phase formation in heterogeneous monomer/water sys

18.

19. 20.

21.

22.

23.

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tems, Doctoral (Chem.) Dissertation, Moscow: Moscow Inst. of Fine Chemical Technology, 1986. Prokopov, N.I., Gritskova, I.A., Cherkasov, V.R., and Chalykh, A.E., Synthesis of monodisperse functional polymeric microspheres for immunoassay, Russ. Chem. Rev., 1996, vol. 65, p. 167. Tauer, K., Kozempel, S., and Rother, G., The interface engine: experimental consequences, J. Colloid Interface Sci., 2007, vol. 312, p. 432. Oganesyan, A.A., Grigoryan, G.K., and Atane syan, A.K., Growth of profiled single crystals of lithium iodate using a shaper from polymer nanoparticles, Izv. Nats. Akad. Nauk Arm., Fiz., 2008, vol. 43, p. 383. Oganesyan, A.A., Grigoryan, G.K., and Mura dyan, G.M., mechanism of the formation of a dispersed phase at the micellar stage of emulsion polymerization, Arm. Khim. Zh., 2006, vol. 59, no. 3, p. 130. Prokopov, N.I., Gritskova, I.A., Kiryutina, O.P., et al., The mechanism of surfactantfree emulsion polymeriza tion of styrene, Polym. Sci., Ser. B: Polym. Chem., 2010, vol. 52, p. 339. Bartlett, P.D. and Cotman, J.D., The kinetics of the decomposition of potassium persulfate in aqueous solu tions of methanol, J. Am. Chem. Soc., 1949, vol. 71, p. 1419.

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