ISSN 15600904, Polymer Science, Ser. B, 2010, Vol. 52, Nos. 9–10, pp. 542–548. © Pleiades Publishing, Ltd., 2010. Original Russian Text © I.A. Gritskova, V.M. Kopylov, G.A. Simakova, S.A. Gusev, I.Yu. Markuze, E.N. Levshenko, 2010, published in Russian in Vysokomolekulyarnye Soedineniya, Ser. B, 2010, Vol. 52, No. 9, pp. 1689–1695.
POLYMERIZATION
Polymerization of Styrene in the Presence of Organosilicon Surfactants of Various Natures I. A. Gritskovaa, V. M. Kopylovb, G. A. Simakovaa, S. A. Gusevc, I. Yu. Markuzeb, and E. N. Levshenkoa a
Lomonosov State Academy of Fine Chemical Technology, pr. Vernadskogo 86, Moscow, 119571 Russia b State Research Institute of Chemistry and Technology of Organoelement Compounds, sh. Entuziastov 38, Moscow, 111123 Russia c Research Institute of Physicochemical Medicine, Malaya Pirogovskaya ul. 1a, Moscow, 119861 Russia email:
[email protected] Received September 21, 2009; Revised Manuscript Received January 19, 2010
Abstract—The properties of polymer suspensions obtained with the use of organosilicon surfactants of vari ous structures are compared. The polymer suspensions are characterized by a narrow particle size distribution and contain functional groups on their surface. DOI: 10.1134/S1560090410090046
INTRODUCTION The problem of synthesis of polymer suspensions with particles of a controlled diameter and a narrow size distribution is very topical because their applica tion area is extremely broad. Especially interesting are polymer suspensions for biology and medicine [1], where suspension particles can be used as protein carriers. In this case, polymer particles should have a narrow size distribution, should be stable in physiological solutions in which immunological reactions occur, and should contain functional groups on their surface. (These groups can be covalently linked with the corresponding groups of protein molecules [2].) It is known [3] that these suspensions can be pre pared in the presence of surfactants that are insoluble in water but ensure formation of a strong interfacial layer on the surface of polymer–monomer particles. α(Carboxyethyl)ω(trimethylsiloxy)polydimethyl siloxane (PDMS) was used as a surfactant. The parti cles of polymer suspensions obtained in the presence of this polymer contained functional groups on its sur face that are needed for covalent bonding with func tional groups of protein molecules. However, the syn thesis of PDMS is quite difficult to perform in large amounts; therefore, PDMS is synthesized only under laboratory conditions. In this study, we examined whether α,ωbis[10 carboxydecyl]polydimethylsiloxane HOOC–(CH2)10–(Me2SiO)30–SiMe2(CH2)10–COOH
can be used as a carboxylcontaining organosilicon surfactant (OS). EXPERIMENTAL Reagents Technicalgrade styrene was treated with a 5% aqueous solution of sodium hydroxide to remove the stabilizer, was washed with water to neutrality, was dried over calcined calcium chloride, and was distilled twice in vacuum; d420 = 0.906 g/cm3 and nD20 = 1.5450. Reagentgrade potassium persulfate was used as received; the content of the active substance was 99.9%. The organosilicon compound had Мn = 2.6 × 103, Мw = 5.8 × 103, d420 = 1.3905, nD20 = 0.906 g/cm3, and η25 = 139 cSt; the acidic number was 43.6 mg KOH/g (calcd., 41.2). Bidistilled water was used as a dispersion medium. Analytical Procedures The rate of polymerization was determined by dilatometry. The emulsion was formed via rotation of a magnetic stirrer in a wide part of a dilatometer at a speed of 600 rpm. Degree of polymerization Р was cal culated from the equation P = ( Δ H / Δ H max ) × 100%
542
POLYMERIZATION OF STYRENE IN THE PRESENCE
543
σ, mJ/m2 36
32 1
28
2 24
0
5
10
15
20
25 c, mol/m3
Fig. 1. Interfacialtension isotherms for (1) OS and (2) PDMS solutions in toluene at the interface with water.
Here ΔH is the current change in the level in the cap illary of a dilatometer (cm); ΔHmax is a change in the level in the capillary of a dilatometer corresponding to 100% conversion (cm): H max = Vm × (ρ p − ρm) / (S × ρ p) where Vm is the volume of the monomer (cm3), ρm is the density of the monomer (g/cm3), ρp is the density of the polymer (g/cm3), and S is the crosssectional area of the capillary (cm2). The particle size of polymer suspensions was deter mined via scanning electron microscopy on a Hitachi S570 microscope. Weightaverage diameters Dw and numberaverage diameters Dn of particles were calcu lated through the relationships
It was shown that OS is well soluble in styrene and toluene and is practically insoluble in water. The inter facialtension isotherm at the interface between the surfactant–toluene solution and water is shown in Fig. 1. It is seen that the minimum interfacial tension at this interface is ~28 mJ/m2. The isotherm illustrat ing a change in the interfacial tension of PDMS on this interface is shown for the sake of comparison.
(∑ N D /∑ N ) = ( ∑ N D / ( ∑ N D ))
The limiting values of adsorption are 3.3 × 10 ⎯6 mol/m2 for OS and 5.9 × 10–6 mol/m2 for PDMS. The minimum area occupied by these surfactants in the saturated adsorption layer of particles is 155 Å2 for PDMS; for OS, this value is somewhat smaller, ~75 Å2. It is believed that the difference in the values of S0 is explained by the surfactant structure, namely, by the presence of two polar groups at the ends of oligo mer chains.
Dn = Dw
1/3
3
i
i
i
6
i
i
3
i
1/3
i
The stability of polymersuspension particles in electrolyte solutions was determined by titration. The electrolyte concentration was varied in the range 0.1– 0.25 mol/l. RESULTS AND DISCUSSION The experiments began with the study of colloidal and chemical properties of OS.
On the basis of the interfacialtension isotherm, the colloidal and chemical properties of the surfactant were calculated; the adsorption characteristics of the surfactant are listed in Table 1. The value of surface activity G for OS is 5.6 mN m2/mol; for PDMS, this parameter is almost half.
In the presence of OS, as in the presence of PDMS, a direct emulsion of styrene is formed (of the oilin water type). However, this emulsion is unstable and quickly decomposes after the end of stirring.
Table 1. Colloidal and chemical properties of surfactants σ1,2 , mJ/m2
G, (mN m2)/mol
Гmax × 106, mol/m2
S0 , Å2
δ × 109, m
PDMS
25.2
3.6
5.9
155
4.6
OS
28.1
5.6
3.3
75
9.5
Surfactant
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GRITSKOVA et al. Conversion, % 100 1
2
70
3 40
10 0
200
400
600
800 Time, min
Fig. 2. Conversion–time curves obtained for styrene polymerization in the presence of surfactants of various natures: (1) OS, (2) PDMS, (3) bulk polymerization.
Formation of the polymer as a result of polymer ization initiation on the surface of monomer droplets will facilitate strengthening of the adsorption layer and, as a result, will lead to an increase in the stability of suspension particles, beginning from the initial steps of polymerization. This statement is confirmed by the experimental data described below. Styrene was polymerized under conditions [4] usual for the synthesis of polymer suspensions intended for immunochemical investigations: The sty renetowater volume ratio was 1 : 9, the concentra tion of stabilizer was 1 wt % of the amount of styrene, the concentration of potassium persulfate initiator was 1 wt % of the amount of styrene, and the temperature was T = 80 ± 0.5°C. It was shown that polymerization proceeds almost to full conversion of the monomer over 4 h and the reaction system maintains its stability.
The conversion–time kinetic curve is presented in Fig. 2 (curve 1) together with the corresponding curves obtained under the same conditions but in the pres ence of PDMS (1 wt % of the monomer) (curve 2) and in the bulk polymerization of styrene (curve 3). It is seen that the dependence has the Sshaped pattern that is characteristic for suspension and emul sion polymerizations; the characters of these depen dences are the same, but they differ in the rates of polymerization. The rate of polymerization of styrene in the presence of OS is higher than that in the pres ence of PDMS and is substantially higher than the rate of bulk polymerization. Polystyrene suspensions obtained in the presence of PDMS and OS differ in stability during polymerization and in the narrow particle size distribution (Table 2). Microphotographs and particlesizedistribution his tograms are presented in Fig. 3. As is seen from
Table 2. Characteristics of polystyrene suspensions stabilized by various surfactants Numberaverage particle diameter, μm
Polydispersity Dw /Dn
Stability, mol/l (NaCl)
Content of coagulum in suspension
PDMS
0.43
1.008
0.20
–
OS
0.55
1.014
0.20
–
Surfactant
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Content of particles, %
(а)
(b)
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90 60 30 0 0.44
0.50
0.56
0.62
0.68
0.40 0.44 0.48 Diameter of particles, μm
0.52
90 60 30 0 0.36
Fig. 3. Microphotographs of polystyrene particles and histograms of the particlesize distribution obtained in the presence of (a) OS and (b) PDMS.
Table 3, when OS is used, the formed particles are larger than those obtained when PDMS is used. Polymerization yields polystyrene suspensions with an average particle diameter of 0.55 μm and a narrow size distribution. The suspensions thus prepared are stable in physiological solutions and during storage. To gain insight into the mechanism governing for mation of polymer–monomer particles in the pres ence of OS, it was necessary to study a change in the diameter of particles during polymerization. Particles have spherical shape and narrow size dis tribution from the onset of conversion of the monomer until its full transformation into the polymer. This cir cumstance makes possible to state that the interfacial
layer forms on the surface of polymer–monomer par ticles at the initial step of the process during initiation of styrene polymerization by potassium persulfate. At early steps, stability factors are formed (structural– mechanical and electrostatic) that determine the sta bility of polymer–monomer particles [5]. It was of interest to assess the limiting concentra tion of the monomer in emulsion, at which the stabil ity of suspensions and the narrow particle size distri bution retain under the selected conditions of poly merization. To evaluate the influence of monomer concentra tion on the diameter of particles and their size distri bution in the presence of OS, the polymerization of
Table 3. Characteristics of polystyrene suspensions stabilized by OS at various phase ratios* Monomertowater volume ratio
Numberaverage particle diameter, μm
Polydispersity Dw /Dn
Stability, mol/l (NaCl)
1:9
0.55/0.43
1.014/1.010
0.20/0.20
–/–
1:6
0.73/0.55
1.020/1.020
0.20/0.15
–/–
1:4
0.80/0.60
1.041/1.050
0.15/0.15
<1%/–
1:2
0.92/0.70
1.471/1.100
0/0.10
* The value in the numerator is for OS; the value in the denominator, for PDMS.
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Coagulum/<1%
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(a) 1 2 3
60
20
50
150
Conversion, % 100
250 (b)
350 Time, min
1 2
4 3
60
20
100
300
500
700 Time, min
Fig. 4. Conversion–time curves obtained for styrene polymerization in the presence of (a) OS and (b) PDMS at monomerto water ratios of (1) 1 : 9, (2) 1 : 6, (3) 1 : 4, and (4) 1 : 2.
styrene was conducted under the same conditions (1% surfactant and 1% potassium persulfate per monomer, 80°С) in the range of monomertowater volume ratios from 1 : 2 to 1 : 9. Figure 4 shows conversion–time kinetic curves. It is seen that the character of kinetic dependence did not change. However, when PDMS is used, the complete conversion of styrene is achieved within 12– 14 h, while when OS is used, the complete conversion is achieved within 4–5 h. This fact suggests the a large number of polymer–monomer particles in the pres ence of OS forms probably owing to their higher sta bility than that in the case of PDMS. However, this suggestion calls for experimental verification. In the case of OS, as the volume content of the monomer in the emulsion was increased from 1 : 9 to 1 : 6, while the concentrations of initiator and stabi lizer remained unchanged, the stability of the reaction system was maintained high, as evidenced by the
absence of coagulum (Table 3), and the polymer sus pensions formed were characterized by a narrow par ticle size distribution (Fig. 5a–5c). The polymer sus pension obtained at a volume phase ratio of 1 : 2 was found to be unstable. Similar results were obtained in the case of PDMS (Fig. 5d–5e). Table 3 illustrates how the monomer concentration affects the characteristics of polystyrene suspensions formed in the presence of OS and (for comparison) PDMS. Our data confirm that, in both cases, electrostatic and structural–mechanical factors of stabilization form on the surfaces of polymer–monomer particles at early conversions. The structural–mechanical fac tor of stabilization forms during orientation of the sur factant in the interfacial layer and formation of the polymer in interfacial layers of particles, while the electrostatic factor appears owing to the orientation of
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Content of particles, %
(b)
Content of particles, %
(c)
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0.52
90 60 30
0
90 60 30
0
90 60 30
0
90 60 30
0 0.6
0.7 0.8 0.9 Diameter of particles, μm
1.0
Fig. 5. Microphotographs of polystyrene particles and histograms of the particlesize distribution obtained with the use of (a–c) OS and (d, e) PDMS. Volume ratios of phases are (a, d) 1 : 9, (b) 1 : 6, (c) 1 : 4, and (e) 1 : 2.
ionogenic groups of terminal polymer chains at inter faces. The maximum diameter of particles of the stable polymer suspension achieved with the use of OS is 0.73 μm at a monomertowater ratio of 1 : 6 over ~4 h. In POLYMER SCIENCE
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the case of PDMS, this value is 0.55 μm during a ~10 h usage. As a result, polystyrene suspensions satisfying all necessary requirements were obtained: They have a narrow particle size distribution, show stability in 2010
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physiological solutions, and contain functional groups (carboxylic) on the surface. The organosilicon sub stance, α,ωbis[10carboxydecyl]polydimethylsilox ane, can be used to manufacture suspensions with desired properties suitable for immunochemical assays. REFERENCES 1. N. I. Prokopov, I. A. Gritskova, V. R. Cherkasov, and F. E. Chalykh, Usp. Khim. 65, 178 (1996).
2. I. A. Gritskova, A. N. Lobanov, Ya. M. Stanishevskii, et al., in Biotechnology (Akademiya Biotekhnologii, Moscow, 2003), No. 2, p. 81. 3. I. A. Gritskova, A. A. Zhdanov, O. V. Chirikova, and O. I. Shchegolikhina, Dokl. Akad. Nauk 334, 57 (1994). 4. O. V. Chirikova, Extended Abstract of Candidate’s Dis sertation in Chemistry (Moscow, 1994). 5. A. Yu. Men’shikova, T. G. Evseeva, M. V. Peretolchin, et al., Polymer Science, Ser. A 43, 366 (2001) [Vysoko mol. Soedin., Ser. A 43, 607 (2001)].
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