ISSN 15600904, Polymer Science, Ser. B, 2015, Vol. 57, No. 6, pp. 560–566. © Pleiades Publishing, Ltd., 2015. Original Russian Text © I.A. Gritskova, Yu.N. Malakhova, V.M. Kopylov, D.I. Shragin, E.V. Milushkova, A.I. Buzin, A.A. Ezhova, A.D. Lukashevich, S.M. Levachev, N.I. Prokopov, 2015, published in Russian in Vysokomolekulyarnye Soedineniya, Ser. B, 2015, Vol. 57, No. 6, pp. 396–403.
POLYMERIZATION
Organosilicon Surfactants: Effects of Structure on the Kinetics of Heterophase Polymerization of Methyl Methacrylate and Behavior in Langmuir Films on the Surface of Water I. A. Gritskovaa, Yu. N. Malakhovab,c, V. M. Kopylovb, D. I. Shraginb, E. V. Milushkovaa, A. I. Buzinb, A. A. Ezhovaa,*, A. D. Lukashevicha, S. M. Levachevd, and N. I. Prokopova a Lomonosov
b
State University of Fine Chemical Technology, pr. Vernadskogo 86, Moscow, 119571 Russia Enikolopov Institute of Synthetic Polymer Materials, Russian Academy of Sciences, ul. Profsoyuznaya 70, Moscow, 117393 Russia c National Research Centre “Kurchatov Institute,” pl. Akademika Kurchatova 1, Moscow, 123182 Russia d Faculty of Chemistry, Moscow State University, Moscow, 119991 Russia *email:
[email protected] Received April 27, 2015; Revised Manuscript Received July 29, 2015
Abstract—The kinetics of methyl methacrylate polymerization in the presence of α,ωfunctional polydim ethylsiloxanes is investigated. The formation of polymer suspensions with a narrow particlesize distribution is explained by the ability of the surfactants to form strong adsorption layers in the interfacial layers of poly mer–monomer particles. This assumption is confirmed by data obtained via the Langmuir method with the use of model systems: thin films of the mentioned organosilicon compounds. DOI: 10.1134/S1560090415060068
INTRODUCTION α,ωFunctional polydimethylsiloxanes are water insoluble surfactants. As was shown in [1–8], polymer suspensions, which have found use as carriers of bioli gands in immunochemical reactions, are formed dur ing the polymerization of styrene in the presence of these polymers. The formation of polymer suspensions with narrow particlesize distributions is a specific feature of poly merization performed in the presence of waterinsol uble surfactants and is related to a common mecha nism controlling formation of polymer–monomer particles from monomer microdroplets and strong interfacial adsorption layers on their surfaces [9–12]. This statement was confirmed by the rheological stud ies of interfacial adsorption layers at the (polystyrene solution/surfactant)/water interface, which showed that, in the presence of organosilicon surfactants, ulti mate shear stress Ps at the interface is an order of mag nitude higher than that in the presence of ionic surfac tants [13, 14]. Furthermore, in the case of Langmuir monolayers formed from mixtures of ionic and orga nosilicon surfactants at the water/air interface, the stability of polymer suspensions becomes high at cer tain mass ratios of surfactants [15, 16]. It may be expected that the observed effect is more pronounced if a new class of organosilicon surfactants containing substituents of various kinds at the ends of the oligomeric polydimethylsiloxane chain is used as
stabilizers of heterophase polymerization. The col loid–chemical properties of these surfactants (namely, a low interfacial tension and the presence of liquid crystalline phases) provide the formation of the strong interfacial adsorption layer on the surfaces of mono mer droplets and polymer–monomer particles (PMPs) and ensure their high stability even at a high concentration of the surfactant. The behavior of sur factants at the interface is determined by both the properties of the main chain and the kinds of func tional substituents at silicon atoms [17]. During compression, the Langmuir films of PDMS molecules transfer from the planar transzigzag con formation to the maximally dense helical packing [17–26]. The second step, corresponding to the detachment of hydrophilic end groups from the sur face of water, appears on the π–A isotherms of such oligomers [17, 18]. This circumstance may be respon sible for a decrease in interfacial tension and for stabi lization of polymer dispersions at the boundary with the water phase. The purpose of this study was to investigate the influence of the kind of end polar PDMS groups on the kinetic features of heterophase polymerization of methyl methacrylate and the behavior of organosili con surfactants at the water/air interface with the use of the Langmuir method.
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EXPERIMENTAL MMA was purified via the standard technique, and a fraction boiling at Т = 33°С (7.3 kPa), d420 = 0.944 g/cm3, and nD20 = 1.413 was used. Potassium
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persulfate (SigmaAldrich, 99.9%) was used without further purification. Organosilicon surfactants with the same lengths of siloxane chains, n ≈ 30, and end groups of various kinds, namely, α,ωbis(10carboxydecyl)polydimethylsiloxane,
CH3 CH3 CH3 O O – Si Si Si C (CH2)10 C O O (CH2)10 C OH , HO CH3 CH3 CH3 n
I
α,ωbis(3glycidyloxypropyl)polydimethylsiloxane CH3 CH3 CH3 – – Si Si Si CH2 CH CH2 O O (CH2)3 O O (CH2)3 O CH2 CH CH2 , O O CH3 CH3 CH3 n
II
and α,ωbis(3aminopropyl)polydimethylsiloxane H2N (CH2)3
CH3 CH3 CH3 Si O Si O Si (CH2)3 NH2 CH3 CH3 CH3 n
III
were synthesized and studied as described in [27]. Their characteristics are listed in Table 1. The kinetics of polymerization was studied via dilatometry [28]. The synthesis of polymer suspensions was per formed as described in [30]. The sizes of particles in polymer suspensions were determined via electrophoretic light scattering on a Zetasizer Nano ZS particle analyzer (Malvern, United Kingdom) and light microscopy with the use of an XSZG light microscope (COIC, China). For light microscopic studies, an aqueous 0.1% suspension of polymer particles was prepared. The sample was placed on a glass and photographed with the help of the light microscope, equipped with a system for photo and video recording. The diameters of particles were estimated from the obtained micrographs with the use of the program ImageProPlus 6.0 (Media Cybernetics Inc.). The intrinsic viscosities of polymer solutions were measured in toluene in the concentration range 0.002–0.01 g/dL at 30°С. In the case of the PMMA– toluene system, the values of constants К = 3.105 × 10–4 and α = 0.58 were used to calculate viscosity average molecular masses of polymers [29]. The molecularmass characteristics of organosili con surfactants were investigated via GPC relative to polystyrene standards. According to the GPC data, the content of lowmolecularmass admixtures in the sur factants did not exceed 3 wt %. The content of func POLYMER SCIENCE
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tional groups was determined via 1H NMR spectros copy and acid–base titration with a 0.1 N HCl solution for surfactant III, a 0.1 N NaOH solution for surfactant I, and a 0.1 N HBr solution for surfactant II. The properties of Langmuir films were investigated with a Minitrough Extended Langmuir trough (KSV, Finland) during compression and expansion between movable barriers; the surface area was changed at a rate of 15 cm2/min. The surface pressure was mea sured with a precision of 0.1 mN/m via the Wilhelmy method with the use of a rough platinum plate. The surface area per molecule was estimated with a preci sion of 3%. The surface potential was measured via the vibratingelectrode method with the aid of a SPOT sensor (KSV, Finland). The precision of measure ments was 1 mV. The morphology of Langmuir films directly on the surface of water was visualized with the use of a BAM300 Brewster angle microscope (KSV, Finland). Micrographs presented in this study were geometrically corrected with allowance for observa Table 1. Characteristics of organosilicon surfactants Surfac Mn × 10–3 Mw/Mn tant I II III
3.2 3.6 3.1
2.0 1.9 1.8
Content of functional groups, wt % titration
NMR 1Н
3.18 3.26 1.28
3.25 3.30 1.26
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K, % 100
(a) 3
1 2
80
(b)
2
4 60
40
40
20
20
10
20
30
4
1
80
60
0
3
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K, % 100
0
10
20
30
40
50 Time, min
(c)
80 1 2
3
60 40 20 0
5
10
15
20
25
30
35
40
45 50 Time, min
Fig. 1. Time dependences of conversion during the polymerization of MMA in the presence of surfactants (a) II, (b) I, and (c) III. The MMAtowater phase volume ratios are (a, b) (1) 1 : 9, (2) 1 : 6, (3) 1 : 4, and (4) 1 : 2 and (c) (1) 1 : 20, (2) 1 : 9, and (3) 1 : 6.
tion under a Brewster angle of 53.1° and corresponded to an interfacial surface area of 200 × 200 μm. In order to protect the surface subphase from external effects leading to its oscillation, the Langmuir trough with sensors and the Brewster angle microscope were installed under a protective cover on a base with active vibration protection (Accurion, Germany). The dem ineralized water, which was purified with a MilliQ Integral Water Purification System (Millipore, United States), served as a subphase. The subphase was ther mostatted at 20°С. Toluene was used as a solvent for organosilicon surfactants. RESULTS AND DISCUSSION The polymerizations of MMA in the presence of functional organosilicon surfactants I–III were stud ied under the same conditions: concentrations of sur factant and potassium persulfate of 1 wt % with respect to the monomer weight, Т = 80 ± 0.5°С, and pH 7. Figure 1 shows the corresponding time dependences of conversion. The patterns of kinetic curves are iden tical, and the character of their change corresponds to that usually observed for the heterophase polymeriza tion of vinyl monomers.
Table 2 illustrates change in the sizes of PMPs dur ing the polymerization of MMA. The sizes of PMPs slightly increase with an increase in monomer conver sion. PMPs are formed at the initial stage of polymer ization at monomer conversions not above ~20%. The diameters of particles are determined by the combina tion of following factors: interfacial tension, rate of stirring, and time of formation of interfacial adsorp tion layers controlling the coalescence stability of PMPs. It is impossible to determine the effect of each of these parameters in a complex heterophase system, and only the sum of their effects can be estimated. For all the studied surfactants, PMPs are formed from monomer droplets and the interfacial layer is formed on their surfaces apparently in the same way. Then, the times of PMPs formation in different systems should be close. The polymer suspensions are stable and fea ture narrow particlesize distributions at all stages of synthesis. In the presence of all organosilicon surfac tants, the total MMA conversion is achieved over ~2 h. The diameters of polymer particles obtained in the presence of surfactants I and II are lower than those obtained in the presence of surfactant III, and the sta bility of all polymer dispersions is high, as evidenced POLYMER SCIENCE
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Table 2. Characteristics of PMMA suspensions stabilized by surfactants at various monomer conversions (a 1 : 9 MMAtowater phase ratio) Surfactant
Monomer conversion, %
Average diameter of particles, d, μm
Dw/Dn
ζ potential, mV
In the absence of surfactant
10 20 30 50 100 10 20 30 50 100 10 20 30 50 100 100
0.30 0.32 0.35 0.40 0.40 0.30 0.35 0.44 0.44 0.44 0.60 0.70 0.75 0.78 0.78 0.20
1.04 1.04 1.03 1.03 1.02 1.03 1.03 1.02 1.02 1.02 1.02 1.02 1.02 1.02 1.02 3.10
–42.7 –38.3 –35.9 –31.4 –27.9 –57.4 –53.2 –50.9 –48.8 –47.4 –45.0 –43.2 –38.6 –33.4 –30.9 –80.0
I
II
III
Table 3. Characteristics of PMMA suspensions stabilized by organosilicon surfactants at various phase ratios Surfactant I
II
III
MMA : water (vol/vol)
d, μm
Dw /Dn
ζ, mV
Mη × 10–5
vP × 105, mol/(L s)
1:9 1:6 1:4 1:2 1:9 1:6 1:4 1:2 1 : 20 1:9 1:6
0.40 0.48 0.51 0.65 0.44 0.47 0.65 0.94 0.65 0.78 0.80
1.02 1.02 1.01 1.03 1.02 1.02 1.02 1.03 1.02 1.02 1.03
–27.9 –26.5 –25.4 –23.9 –47.4 –46.3 –46.1 –36.6 –30.2 –30.9 –31.9
10.1 8.8 7.1 6.2 9.5 8.6 6.2 6.1 12.0 10.0 8.6
3.60 3.40 3.11 2.34 6.43 6.40 5.11 4.28 2.73 2.07 1.56
by the absence of coagulum from the system. This result may be explained by the formation of strong interfacial adsorption layers on the surfaces of PMPs [27]. The polymer formed on the surfaces of PMPs as a result of polymerization initiation and precipitation by water, which serves as a precipitant, and the surfac tant that is forcibly displaced by the polymer from the volume of PMPs on the surface because of their incompatibility are involved in the formation of inter facial adsorption layers. The latter was confirmed by independent experiments [31]. The influences of MMA concentration on the rate of polymerization, vp, in the presence of surfactants I–III and the diameters of the formed PMMA parti cles are shown in Fig. 2 and Table 3. The rate of poly merization decreases and the diameters of PMPs POLYMER SCIENCE
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increase with an increase in MMA content. The value of the ζ potential depends on the degree of dissocia tion of functional PDMS groups on condition that the amounts of end functional fragments of initiating mol ecules are the same and equal to –27, –47, and ⎯30 mV for dicarboxy, diglycidyloxy, and diamino PDMS derivatives, respectively. The value of the ζ potential is almost independent of the monomerto water phase volume ratio. For surfactant I, the ζ potential changes within the limits of experimental error and is almost constant for surfactants II and III. All polymer suspensions are characterized by narrow particlesize distributions (Table 3, Fig. 2). Moreover, in the case of surfactant III, systems with a monomertowater ratio not above 1 : 6 were stable, whereas for surfactants I and II, this result
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(a)
2 μm
(b)
2 μm
(c)
2 μm
(d)
2 μm
(e)
2 μm
(f)
2 μm
Fig. 2. Micrographs of PMMA particles obtained in the presence of (a, b) surfactant II, (c, d) surfactant I, and (e, f) surfactant III. The MMAtowater volume ratios are (a, c) 1 : 9, (b, d) 1 : 2, (e) 1 : 20, and (f) 1 : 6.
could be achieved even at a monomertowater ratio of 1 : 2. It may be assumed that organosilicon surfactants I–III can provide the formation of stable and narrowly dispersed suspensions during the heterophase poly merization of MMA owing to the formation of strong interfacial adsorption layers on the surfaces of mono mer droplets and PMPs. These considerations were confirmed by the study of model systems: thin films of organosilicon surfactants formed on the surface of the water subphase. The presence of polar groups located at both ends of oligomeric chains of PDMS molecules should affect the colloid–chemical properties of sur factants, in particular, the parameters of surface pres sure (π–A) and surface potential (ΔU–A) isotherms. For the three organosilicon surfactants, the shapes of π–A isotherms of Langmuir films (Fig. 3, curves 1, 3, 5) and their surface morphologies are similar.
Micrographs of the surface film measured under the Brewster angle for surfactant I are presented in Fig. 4. During compression of the films based on surfac tants I–III, the start of the rise in surface potential ΔU (Fig. 3, curves 2, 4, 6) is observed already before the increase in π. A sharp jump of ΔU for surfactant II demonstrates the concordant intramolecular orienta tion of dipoles. ΔU–A isotherms achieve a plateau in the region of the onset of rise on the π–A isotherms. The magnitudes of the ΔU jump are as high as 115, 180, and 170 mV for surfactants I, II, and III, respec tively. As follows from the micrograph measured under the Brewster angle in the region before the rise in π (Fig. 4a), the subphase surface is incompletely covered by the monolayer of the organosilicon surfactant. The film completely covers the interface surface in the region of the π rise at the first stage of the compression isotherms (Fig. 4b). Extrapolation of the compression portion for the Langmuir films of organosilicon surfactants to the zero value of π yields the same values of the interfacial surface area for all three types of surfactants, which are found to be approximately 800 Å2 per molecule. A smooth attainment of the π–A isotherm of the plateau was observed at approximately 600 Å2 per molecule. A change in the length of a hydrophobic spacer (the number of methylene groups) and the replacement of functional groups lead to insignificant changes in the shapes of π–A and ΔU–A isotherms. The transition from ten methylene groups in sur factant I to three groups in surfactant II and surfactant III is accompanied by an increase in the value of π on the plateau from 7.5 mN/m (surfactant I) to 10.5 ± 0.5 mN/m (surfactants II and III). It appears that the transition of PDMS chains from the extended confor mation to the helical conformation occurs in this region [23]. The value of ΔU remains constant for sur factant I or slightly increases for surfactants II and III in the surface area range from 800 to 300 Å2 per mole cule. A weakly defined inflection associated with attain ment of the second plateau typical for PDMS [26] was observed on the π–A isotherms of surfactants I–III as the interfacial surface area was decreased. The layer contrast on the micrograph increased upon subse quent compression of the film (Fig. 4c), a result that indirectly indicated an increase in the layer thickness. The third stage with the inflection point (shown by open circles on curves 1, 3, 5 in Fig. 3), which is not typical for PDMS without hydrophilic end groups, was observed on the π–A isotherms of surfactants I– III. The third stage appeared on the π–A isotherms during the hydrolysis of siloxanes [20] and for PDMS with functional groups [17, 18]. The kind of end groups shows a strong effect on the π value of this transition. For surfactants I and II, the transition is observed at a surface area of А = 145 Å2 POLYMER SCIENCE
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ORGANOSILICON SURFACTANTS: EFFECTS OF STRUCTURE ON THE KINETICS π, mN/m 50
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ΔU, mV 6
4
250
40 200
5 30
150
2 20
1 100
3 c 10
50 a
b 0
200
600 1000 Area per molecule A, Å2
1400
Fig. 3. (1, 3, 5) π–A and (2, 4, 6) ΔU–A compression isotherms of Langmuir films for organosilicon surfactants (1, 2) I, (3, 4) II, and (5, 6) III on the surface of the water subphase. T = 20°C. The letters correspond to the micrographs shown in Fig. 4.
50 μm
(a)
50 μm
(b)
(c)
50 μm
Fig. 4. Micrographs under the Brewster angle for Langmuir films of surfactant I formed on the surface of the water subphase at π (a) 0, (b) 7.5, and (c) 12.8 mN/m.
per molecule and π = 20 ± 1 mN/m; for surfactant III, the transition is observed at А = 95 Å2 and π = 40 mN/m. Thus, surfactant III possess a greater ability to decrease surface and interfacial tensions than sur factants I and II. The π–A and ΔU–A expansion isotherms coincide with the corresponding isotherms of compression obtained for surfactants I–III, and the film morphol ogy observed under the Brewster angle during expan sion corresponded to the same stages as those during compression. Thus, in the compression–expansion cycle, hysteresis was absent for all these surfactants. The investigation of Langmuir films revealed that siloxane chains play an important role in the formation of the interfacial layer owing to their high conforma tional mobility. An increase in the number of methylene groups in the alkyl spacers connecting polar functional POLYMER SCIENCE
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groups with siloxane chains leads to a reduction in the surface pressure of transition from the extended confor mation to the helical conformation. In contrast, the replacement of carboxyl or glycidyloxy groups with amino groups considerably increases the surface pres sure of transition related to the kind of functional groups and, accordingly, the stability of layers. CONCLUSIONS Our results have demonstrated that polymer sus pensions with narrow particlesize distributions are formed in the presence of all investigated organosili con surfactants and that the suspensions remain stable at drymatter contents below 33 wt %. The study of model systems—Langmuir films derived from func tional organosilicon surfactants—has shown that sur
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factants form stable layers at the water/air interface and undergo conformational transformations. The suspension with the maximum sizes of particles is formed in the presence of the surfactant withstanding the highest surface pressure at the water/air interface. ACKNOWLEDGMENTS This work was supported in part by the Russian Foundation for Basic Research (project number 1G 61352) and by the Council for Grants of the President of the Russian Federation for Support of Young Rus sian Scientists—Candidates of Sciences (MK 5779.2015.3) REFERENCES 1. I. A. Gritskova, I. G. Krasheninnikova, A. M. Evtush enko, and A. I. Kadantseva, Polym. Sci., Ser. B 48 (11– 12), 339 (2006). 2. I. A. Gritskova, O. V. Chirikova, O. I. Shchegolikhina, and A. A. Zhdanov, Dokl. Ross. Akad. Nauk 334 (1), 57 (1994). 3. I. A. Gritskova, N. I. Prokopov, A. N. Lobanov, Ya. M. Stanishevskii, and A. Ozhekhovski, Polym. Sci., Ser. A 44 (11), 1107 (2002). 4. I. A. Gritzkova, G. B. Adebayo, I. G. Krasheninnikova, and V. A. Kaminsky, Colloid Polym. Sci. 276 (12), 1068 (1998). 5. I. A. Gritskova, V. S. Papkov, I. G. Krasheninnikova, and A. M. Evtushenko, Polym. Sci., Ser. A 49 (3), 235 (2007). 6. M. Khaddazh, G. I. Litvinenko, and I. A. Gritskova, Polym. Sci., Ser. B 53 (5–6), 283 (2011). 7. I. A. Gritskova and V. A. Kaminskii, Russ. J. Phys. Chem. A 70 (8), 1413 (1996). 8. I. A. Gritskova, P. N. Chadaev, D. I. Shragin, E. N. Levshenko, L. A. Zlydneva, E. V. Volkova, and N. V. Rassokha, Izv. Vyssh. Uchebn. Zaved., Khim. Khim. Tekhnol. 54 (9), 70 (2011). 9. A. A. Oganesyan, G. K. Grigoryan, G. M. Muradyan, A. G. Nadaryan, M. Khaddazh, S. P. Gubin, and I. A. Gritskova, Theor. Found. Chem. Eng. 47 (5), 600 (2013). 10. I. A. Gritskova, V. M. Kopylov, G. A. Simakova, S. A. Gusev, I. Yu. Markuze, and E. N. Levshenko, Polym. Sci., Ser. B 52 (9–10), 542 (2010). 11. I. D. SimonovEmelianov, A. V. Markov, N. I. Prokopov, I. A. Gritskova, N. I. Munkin, and L. Hexel, Polimery 58 (9), 703 (2013).
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Translated by E. Bushina
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