Langmuir 1991, 7, 1775-1778
1775
Polymerization of Styrene Adsolubilized in Polymerizable Surfactant Bilayer on Alumina Kunio Esumi,* Nobuaki Watanabe, and Kenjiro Meguro Department of Applied Chemistry, Institute of Colloid and Interface Science, Science University of Tokyo, Kagurazaka, Shinjuku-ku, Tokyo 162, Japan Received September 24,1990. I n Final Form: March 15, 1991 When styrene is adsolubilized into a bilayer of sodium 10-undecenoate on alumina, the adsolubilized amount of styrene increases with equilibriumconcentrationof styrene and reaches a plateau. Subsequent polymerization of such a system with UV irradiation in the presence of an initiator enhances dispersion stability of the alumina compared with the polymerization of bilayer of sodium 10-undecenoatealone on the alumina. The dispersion stability is well correlated with the [ potential of the alumina. The physicochemical properties of alumina before and after the polymerization have been characterized by using a probe technique and thermal analysis.
Introduction
crystalline media.20 Further, by using the polymerizable surfactant, the present authors21have reported that a biWhen surfactants are adsorbed on particles having a layer of sodium 10-undecenoate on alumina is polymercharge opposite to that of the surfactants, it is well k n ~ w n l - ~ ized by UV irradiation; dispersion stability of the alumina that a bilayer of the surfactant is formed on the particles. with the polymerized film is enhanced. In this bilayer, the interaction between the first and second In this paper, we report adsolubilization of styrene in layers is attributed to hydrophobic forces between the a bilayer of sodium 10-undecenoate on alumina and its surfactant, so that water-insoluble substances are easily subsequent polymerization by UV irradiation. The surface incorporated in the bilayer, which is called adsolubilizaproperties of alumina before and after polymerization have tion.' There are some studies of adsolubilization by albeen characterized by means of fluorescence, ESR, and coholsF*6dyes: and mon0mers.~*8*~ The present authors8 thermal analysis measurements. Further, the effect of and Wu et al.49 have reported the polymerization of styrene polymerization on the dispersion stability of alumina has adsolubilized in a sodium dodecyl sulfate bilayer on been studied. particles and a thin polystyrene film is obtained on their surfaces. Lando et al.l0J1have performed a seriesof studies Experimental Section on the polymerization of Langmuir-Blodgett monolayer Materials. 10-Undecenoicacidwas obtained from Wako Pure and multilayer structures of vinyl stearate a t an air-water Chemicals Ltd., and its sodium salt was prepared by addition of interface and at a solid-liquid interface. sodium hydroxide. Recently, there has been considerable interest in poa-Alumina of 99.995% purity was supplied by Showa Denkou lymerizable surfactants.12-16 Polymerization of sodium K. K. The average particle size and specific surface area of the Wundecenoate micellar solutions either by irradiation17J8 alumina were 200 nm and 31.4 m2 g1,respectively. or free radical initiatorsl9 has been shown to yield low Pyrene obtained from Tokyo Kasei Industries was purified molecular weight oligomers that are capable of dissolving four times by recrystallizationwithethanol. Styrenewas obtained in water. Also, these have been polymerized to high mofrom Wako Pure Chemicals Ltd.,and was distilled before use. 2,2,6,6-Tetramethylpiperidmyl-l-oxy (TEMPO)was purchased lecular weight species with a free radical initiator in liquidfrom Aldrich Chemical Co. and used as received. Sodium persulfate was obtained from Wako Pure Chemicals Ltd. (1) Meguro, K. Kogyo Kagaku Zasshi, 1966,68,906. Water used was purified by passing it through a Milli-Qsystem (2)Somasundamn, P.;Fuersbnau,D. W. J. Phys. Chem. 1966,70,90. until the conductivity fell below 0.1 pS cm-I. (3)Ebumi, K.; Ono, Y.; Ishizuka, M.;Meguro, K. Colloids Surf. 1988, 32,139. Procedure. Alumina (0.15 g) was added to some alkaline (4)Wu,J.; Harwell, J. H.; ORear, E.A. Langmuir, 1987,3,631. solution, and then 10 mmol dm9 sodium nitrate solution was (6)Barton, J. W.;Fitzgerald, T. P.; Lee, C.; ORear, E. A.; Harwell, J. added. Then, various concentrations of sodium 10-undecenoate H. Sep. Sci. Technol. 1988,23,637. in aqueoussolutionwere added. Finally,some volume of styrene(6) Eeumi, K.; Shihyama, M.; Meguro, K. hngmuir 1990,6,826. saturated aqueous solution was added to make up a total volume (7)Ebumi, K.; Sakamoto, Y.; Nagahama,T.;Meguro, K. Bull. Chem. of 0.03 dma, and the suspension was shaken to reach an equiSOC.J n. 1989,62,2602. (8) keguro, K.; Yabe, T.;Ishioka, S.;Kato, K.;Eeumi, K. Bull. Chem. librium condition for 12 h. This suspension was transferred to SOC.Jpn. 1988,69,3019. a quartz beaker with a cover, and 0.48 g of sodium persulfate was (9)Wu,J.; Harwell, J. H.; ORear,E.A. J. Phys. Chem. 1987,91,623. to it. After purging with nitrogen gas, polymerization of added (10)Pubman, M.; Fort, T.,Jr.; Lando, J. B. J. Colloid Interface Sci. the suspension was performed under irradiation by a UV lamp 1974,47,906. (40 W). The suspension was stirred during the polymerization. (11)Ehkelmann.. V.:. Lando. J. B. J . Polvm. Sci.,. Polvm. Chem. Ed. 1977,16,1843. (12)Finkel", H.; Rehage, G. Adu. Polym. Sci. 1984,60,99. (13)Durair ' B.; Blum, F. D. Langmuir 1989,6,370. (14)Regen,%. L.;Singh, A.; Oehme, G.;Singh, M. J. Am. Chem. SOC. iwa. 101.791. (15) Grw,L.; Ringadorf,H.;Schupp, H. Angew. Chem.,Int. Ed. Engl. 1981.20.306. (16) Fendler, J. H. Acc. Chem. Res. 1980,13,7. (17)Larrabee, C.E.,Jr.; Sprague, E.D. J. Polym. Sci., Polym. Lett. Ed. 1919. 17. 749. (18) Pd&, C. M.;Stasrinopoulou,C. I.; Maillaris, A. J. Phys. Chem. 1983,87,261. (19)Durairqi, B.;Blum, F. D. Polym. Prepr. 1986,26,239.
0743-7463/91/2407-1775$02.50/0
Measurements. Samples before and after the polymerization were examined by means of IR spectroscopy (215 type, Hi-
tachi Co.) to determine whether polymerization had occurred. Steady-state emission spectra of pyrene and ESR spectra of TEMPO in the euspension before and after irradiation were obtained by using a fluorescence spectrophotometer (650-10S, Hitachi Co.) and a JEOL JES FE 3-Xspectrometer,respectively. (20)Thundathil,R.;Stoffer, J. 0.;Friberg, 5.E.J. Polym. Sci., Polym. Chem. Ed. 1980,18,2629. (21)Eeumi, K.; Watanabe, N.;Meguro,K. Langmuir 1989,6,1420.
63 1991 American Chemical Society
Esumi et al.
Langmuir, Vol. 7, No. 8, 1991
ro I
'O
0
20
40
60
Equilibrium concn. of sodium IO-~ndecenoatelmmoEdfi~
Figure 1. Adsorbed amount of sodium 10-undecenoate( 0 )and {potential of alumina ( 0 )as a function of equilibrium concentration of sodium 10-undecenoate.
The concentration of pyrene was from 1 X 1V to 1 X le7mol dm-8, while that of TEMPOwas 2 X l W mol dm-8. The excitation wavelength for pyrene was 335 nm. The suspensions containing the probes were prepared as follows. A known volume of the probe ethanol solution was placed into a flask and the solvent was removed by vacuum pumping; the suspensions before and after the polymerization were added in the flask. Then, the suspensions were shaken for 2 h. The adsorption of sodium 10-undecenoateon the aluminawas obtained by measuring the concentration in the solution before and after adsorption. The aqueous solution of sodium 10-undecenoate was placed in contact with alumina in a test tube at 25 O C . The test tubes were shaken for 12 h, after which the suspension was centrifuged and analyzed. The concentration of sodium 10-undecenoate,which has an absorption band at 227 nm, was analyzed by using a UV-vis spectrophotometer (200A, Hitachi Co.). Similarly, the adsolubilized amount of styrene, which has an absorption band at 247 nm, was determined by use of the UV-vis spectrophotometer. The molar absorptivities of sodium 10-undecenoateand styrene were 6.2 and 6148 mol-' dm3 cm-1, respectively. In order to estimate an effect of polymerization on the dispersion stability of alumina, the apparent absorbance of the aqueous suspension, which had stood for 1 day after the polymerization,was measured at 600 nm by use of the UV-vis spectrophotometer. { potentials of the suspensionswere measured by means of an electrophoretic apparatus (Laser Zee 500, Pen Kem, Inc.). Molecular weights of polystyrene and poly(sodium 10-undecenoate) extracted by THF from the alumina surface were determined by means of gel-permeation chromatography (510 type, Japan Waters Ltd.). Here, standard polystyrene samples with various molecular weights were used. Particle size was determined by means of a particle analyzer that is based on photon correlation spectroscopy (Autosizer Model 700, Malvern Co., Ltd.). Thermal properties of alumina after the polymerization were measuredwith a thermal analysisinstrument (DTA-40,Shimadzu CO.).
Experiments of adsorption, adsolubilization, and polymerization were performed at pH 8.0 and at 25 O C .
Results and Discussion This experiment was performed a t pH 8.0, since sodium 10-undecenoate dissociates fully as an anionic surfactant and the alumina shows an appropriate positive {potential, Le. +30 mV. This experimental condition suggests that a t a first adsorption layer, adsorption of sodium 10-undecenoate occurs on positively charged alumina because of an electric attraction force. Figure 1shows the adsorbed amount of sodium 10-un-
Equilibrium concn of styrene I umol.dfi3
Figure 2. Adsolubilizedamount of styrene in a bilayer of sodium
10-undecenoateon alumina as a function of equilibrium concentration of styrene in aqueous solution: initial concentration of sodium 10-undecenoate. 60 mmol dm-*. decenoate and the {potential of alumina as a function of equilibrium concentration of sodium 10-undecenoate. The results show that the 5 potential of alumina decreases, reaches zero, and finally becomes negative with an increase of the concentration of sodium 10-undecenoate,while the adsorbed amount of sodium 10-undecenoate increases gradually and reaches a plateau. Although not shown in this figure, the mean particle size of alumina also increases and then decreases with an increase of the concentration of sodium 10-undecenoate: the change in the {potential of alumina corresponds to that in the mean particle size of alumina. Thus, alumina is flocculated and then redispersed, depending on the equilibrium concentration of sodium 10-undecenoate. These processes can be interpreted as follows. When sodium 10-undecenoate is adsorbed on positively charged alumina, the alumina flocculates because the hydrophobic chain of sodium 10undecenoate is oriented outward. On further addition of sodium 10-undecenoate, the adsorption of sodium 10-undecenoate occurs by hydrophobic chain to hydrophobic chain interaction, resulting in a formation of bilayer with the hydrophilic groups of sodium 10-undecenoate oriented toward the liquid phase. The above results support the idea that a complete bilayer is formed above 40 mmol dm-3 concentrations of sodium 10-undecenoate. For 40 mmol dm" sodium 10-undecenoate, the occupied area of 10-undecenoate is about 8.7 A*, which is smaller than the expected area of carboxyl groups for the bilayer (12.5 A2). This also suggests the formation of a complete bilayer. In an adsolubilization study, 60 mmol dm" sodium 10undecenoate was used, which concentration was sufficient to form a bilayer. Figure 2 shows the adsolubilized amount of styrene in the bilayer of sodium 10-undecenoateon the alumina as a function of equilibrium concentration of styrene in the aqueous solution. The adsolubiliied amount of styrene increases with concentration of styrene and reaches a plateau. At the plateau region, the molar ratio of styrene adsolubilized to sodium 10-undecenoate adsorbed is about 1:2. A similar ratio has been obtained on the styrene-sodium dodecyl sulfate-alumina systemf Such an adsolubilizedstateof styrene has been estimated by using fluorescence spectra of pyrene. Pyrene probe exhibita five fluorescenceabsorption bands, where the ratio of the first vibronic and the third vibronic bands (11/Zs) is very sensitive to the polarity of the environment where the pyrene is located. As shown in Figure 3,ZI/Zs of the alumina suspension, by addition of sodium 10-undecenoate, decreases from 1.8 to 1.0, while the incorporation of styrene into the layer of sodium 10-undecenoaterenders
Langmuir, Vol. 7, No. 8, 1991 1777
Polymerization of Styrene in Polymerizable Surfactant
10
1 \I
LL
A LI
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20
40
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0
Equilibrium concn. d sodiuml0-undecenoateImmoldn?
A A
I
I
I
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4
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UV irradiation time/ h
Figure 3. Change of Illla value of alumina suspension as functionof equilibrium concentration of sodium 10-undecenoate: (0) in absence of styrene; (A)in presence of 3 X 108 mol dme3 styrene.
Figure Change of rotational correlation time of TEMPO for sodium 10-undecenoate-aluminasuspension (0) without and (A) with 3 X lo-' mol gl of styrene adsolubilized as a function of UV
the 11/13value much lower. This indicates that the incorporation of styrene into the bilayer renders the micropolarity of the bilayer much lower,where styrene resides in the hydrocarbon chain of sodium 10-undecenoate.This micropolarity means the polarity where pyrene resides in the microenvironment. The suspensions containing styrene adsolubilized into the bilayer of sodium 10-undecenoateon the alumina were irradiated by UV lamp, where the sample having adsolubilized styrene of 3 X 10" mol gl was used. After the irradiation, the absorption bands at 3100 (=CH stretch) and 1640 cm-l (C=C stretch) due to double bands of styrene and sodium 10-undecenoate were confirmed to diappear by IR measurements, supporting that such a system is polymerized by UV irradiation. Although the 11/13values after the polymerization are not given in the figure, the values are almost the same before and after the polymerization, indicating that the micropolarity estimated from the pyrene spectra is not changed by the polymerization. The results obtained with TEMPO, which is added to the suspension after the polymerization, are given in Figure 4. The relative anisotropy observed in an ESR spectrum of TEMPO is directly related to the rotational mobility of TEMPO, which is correlated with the microviscosityof media where TEMPO is located. The rotational correlation timeszz are obtained from
layer of the polystyrene layer on the alumina, so that the rotational correlation time of TEMPO is low. The properties of sodium 10-undecenoateand of styrene adsolubilized in its layer after polymerization (polymerization time, 6 h) were studied by means of gel permeation chromatography and thermal analysis. The molecular weights of poly(sodium 10-undecenoate) extracted from the sodium 10-undecenoate-alumina system after the polymerization were found to be about 120 OOO, 60 OOO, and 800, where the poly(sodium 10-undecenoate) with molecular weight 800 was predominant. However, it should be pointed out that these molecular weights do not represent the true weights of the sample because the hydrodynamicvolume of standard polystyrene is different from that of poly(sodium 10-undecenoate). This discussion is available for the distribution of molecular weights. It has been reported13 that in aqueous solution polymerization of sodium 10-undecenoate in micelles provides a molecular weight of about 2000. In the case of polymerization of styrene adsolubilized in the sodium 10-undecenoate layer on the alumina, the molecular weights extracted were also found to be about 120 OOO, 60 OOO, and 1000, where the polymers with molecular weight of 1OOO were also predominant. However, whether the polymers were polystyrene, or poly(sodium 10-undecenoate, or mixtures of these polymers was not confirmed. These low molecular weights may indicate that a bilayer of sodium 10-undecenoate on the alumina is a patchlike layer where styrene is adsolubilized, and such a layer is polymerized. Very recently, from the polymerization of ((methacry1oyloxy)ethyl)ethyldidodecylammonium chloride adsorbed on a Laponite surface,the polymer correspondingto seven monomer units has been obtained.24 This study also shows a patchlike adsorbed layer. The DTA and TGA results show that a distinct weight loss and an exothermic peak are observed at about 480 "C for the system in the absence of styrene, which corresponds to loss of poly(sodium 10undecenoate), while in the styrene-adsolubilized system a main change occurs at about 510 "C, which might be attributed to the decomposition of poly(sodium 10-undecenoate) and polystyrene. However, a clear difference in the thermal decomposition temperature between poly(sodium 10-undecenoate) and polystyrene formed on the alumina was not observed. Further, an effect of polymerization on dispersion stability of alumina has been studied. Here, the dispersion
T,
= (6.6 X 10-10)AH[(h-l/h+l)1'2- 11
where AH is the peak-to-peak line width of the low-field line (in gauss) and h-1 and h+l are peak-to-peak heights of the low- and high-field lines, respectively. Figure 4 shows that the rotational correlation time of TEMPO for the sodium 10-undecenoate-alumina system increases rapidly with UV irradiation time, reaches a maximum at about 30 min, and then becomes constant, while that for the styrene adsolubilized in the bilayer of sodium 10-undecenoate does not change with UV irradiation time and is considerably smaller compared with the sodium 10undecenoate-alumina system. Since it is knownz3that most TEMPO moleculesare adsolubilized near the surface region of micelles and reside in a relatively highly polar environment, the results obtained with TEMPO provide information on the microviscosity near the outer layer of polymers on the alumina. In this study, it seems likely that TEMPO molecules do not easily penetrate the outer (22) Martinie, J.; Michon, J.; h a t , A. J. Am. Chem. SOC.1976,97, 1818. (23) Ramachadran,C.; Pyter, R. A.; Mukerjee,P. J.Phys. Chem. 1982, 86, 3198.
irradiation time.
(24) Kunyima, B.; Viaene, K.; H m a n Khalil, M. M.; Schoonheydt,R.
A.; Crutzen, M.; De Schryver, F. C. Langmuir 1990, 6, 482.
Esumi et al.
1778 Langmuir, Vol. 7,No. 8, 1991 stability of alumina is evaluated from the magnitude of apparent absorbance; i.e., a higher apparent absorbance indicates a higher dispersion stability, whereas a lower value shows a poorer stability because alumina flocculates and settles down. The apparent absorbances of the suspensions for both systems increase with UV irradiation time and become constant above 6 h, where the apparent absorbancefor the styrene adsolubilized system is greater than that for th system in the absence of styrene over the entire UV irradiation time (Figure 5a). This result correspondsto the change in the {potential of the suspension (Figure 5b): the larger the absolute tpotential, the greater the apparent absorbance of the suspension is. Although the reason for these differences is not clearly understood, a fixation of counterions to the ionized groups of the polymer layer on the alumina may be inhibited by the polymerized styrene incorporated in the layer, so that the { potential of the alumina for the styrene-adsolubilized system becomes more negative. Accordingly, it can be said that an electric repulsion between the alumina with polymer layer plays an important role for the dispersion stability of alumina.
Conclusions Styrene is adsolubilized into a bilayer of sodium 10undecenoate on alumina, resulting in a decrease of micropolarity of the bilayer, which is estimated by pyrene fluorescence. Polymerization of the above system with UV irradiation occurs, which has been confirmed by IR. The molecular weights of extractive polymers by THF were predominantly below lo00 for the samples with and without polystyrene. Further, the dispersion stability of
1.0 . (a)
I
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I
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UV irradiation time I h
Figure 5. Change of (a) absorbanceat 600 nm and (b) {potential of alumina suspension shown in Figure 4 as a function of W irradiation time.
alumina having a polystyrene layer is increased with UV irradiation time and somewhat enhanced compared with that of alumina without the polystyrene layer, probably due to an increase of electric repulsion force between the alumina particles.
