Langmuir 1998, 14, 5691-5694
5691
Microemulsion Polymerization of Methyl Methacrylate Photoinitiated by Symmetrical Azocompounds of Different Hydrophobicity M. V. Encinas,* E. A. Lissi, A. M. Rufs, and J. Alvarez Departamento de Quı´mica, Facultad de Quı´mica y Biologı´a, Universidad de Santiago de Chile, Casilla 40, Correo 33, Santiago, Chile Received September 27, 1996. In Final Form: May 1, 1998 Symmetrical azo compounds of different charge were used as photoinitiators of methyl methacrylate (MMA) polymerization in homogeneous solution and in microemulsions supported by cationic and anionic surfactants. The measured constants for association of initiators to the aggregates are those expected from the micellar surface potential, which is reduced by the presence of monomer. Polymerization rates and photocleavage yields of azo compounds were measured in different media. The results are interpreted in terms of a fast interchange of the counterions between micelles that reduces the intra-Stern layer primary radical combination.
Introduction Photoinitiation efficiencies in homogeneous free radical polymerizations are determined by the free radical quantum yield of the photochemical process and the reactivity of the primary radicals.1 In emulsion polymerization, the efficiency of a photoinitiator is also determined by its localization. In particular, it is recognized that photoinitiators localized in the dispersed medium are more efficient than those incorporated to the microphases that are the locus of the polymerization process.2-6 This apparently contradictory result is associated to the high intraparticle primary termination when two radicals are initially confined in the same particle.3,7 This type of comparison has usually been carried out employing initiators that are excluded from the microemulsion particles (such as persulfate ions in SDS-containing microphases) and lipid-soluble initiators that are mainly incorporated to the polymerization locus.4-6,8 Moreover, the properties of the employed watersoluble and lipid-soluble photoinitiators are frequently widely different, rendering difficult a quantitative evaluation of the effect of the initial location of free radicals. Furthermore, in microemulsions, at least three distinct localizations can be recognized for the photoinitiator and hence for the initial pair of geminate radicals: the micellar core, the Stern layer, and the outer dispersed phase. In principle, in these localizations will mainly reside hydrophobic noncharged solutes, counterions, and co-ions, respectively. The efficiency of the initiation by free radical pairs localized in the Stern layer has only been addressed by (1) Lissi, E. A.; Encinas, M. V., Photochemistry and Photophysics; Rabek, J. F., Ed.; CRC Press: Boca Raton, FL, 1991; Vol. IV, Chapter 4. (2) Backley, D. C. Emulsion Polymerization; John Wiley: New York, 1975. (3) Al-Shabile, W. A. G.; Dunn, A. S. Polymer 1980, 21, 429. (4) Barton, J.; Ka´rpa´tyova´, A. Makromol. Chem. 1987, 188, 693. (5) Feng, L.; Ng, K. Y. Macromolecules 1990, 23, 1048. (6) Gan, L. M.; Chew, C. H.; Lye, I. Makromol. Chem. 1992, 193, 1249. (7) Turro, N. J.; Chow, M. F.; Chung, C. J.; Tung, C. H. J. Am. Chem. Soc. 1983, 105, 1572. (8) Puig, J. E.; Pe´rez-Luna, V. H.; Pe´rez-Gonza´lez, M.; Rodrı´guez, B. E.; Kaler, E. W. Colloid Polym. Sci. 1993, 271, 114.
Fouassier and Lougnot,9 employing cationic and anionic benzophenones in the presence of amines as coinitiators. However, in this system the initiation mechanism is complex and the free radical yield is strongly dependent on the medium,10,11 rendering difficult an evaluation of the importance of primary radical recombination. Furthermore, in these systems only one of the initially formed radicals is ionic and hence geminate recombination can be avoided by escape of the ion and/or the neutral species. Symmetrical azo compounds, due to the simplicity of their primary processes and the fact that the photocleavage produces two identical free radicals, constitute a family of photoinitiators very suitable to evaluate the relevance of primary recombination following radical pair formation inside, in the border, or outside the microemulsion particles. In this work we present results bearing on the photopolymerization efficiencies of lipid-soluble (2,2′azobis(isobutyronitrile), AIBN), positively charged (2,2′azobis(2-amidinopropane), ABAP), and negatively charged (4,4′-azobis(4-cyanovaleric acid), ABCV) azo compounds in homogeneous solution and in microemulsions supported by cationic (cetyltrimethylammonium chloride, CTAC) and anionic (sodium dodecyl sulfate, SDS) surfactants. These comparisons allow a quantitative evaluation of the effect of the initial free radical pair characteristics upon the efficiency of primary free radical recombination. Experimental Section Cetyltrimethylammonium chloride (CTAC) from Fluka was recrystallized from an ethanol-acetone mixture (1:3 v/v). Sodium lauryl sulfate (SDS, Aldrich) was purified by recrystallization from boiling ethanol. AIBN (Fluka) was recrystallized from ethanol. ABAP (Wako) and ABCV (Aldrich) were used as supplied. MMA monomer (Fluka) was purified by shaking with 10% aqueous NaOH solution, washing with distilled water, and drying with CaCl2. After this treatment the monomer was purified by vacuum distillation. 2-Hydroxyethyl methacrylate (HEMA), from Aldrich, was vacuum-distilled prior use. Microemulsions were prepared by adding 0.35 M methyl methacrylate to an aqueous solution containing 0.35 M SDS or (9) Fouassier, J. P.; Lougnot, D. J. J. Appl. Polym. Sci. 1986, 32, 6209. (10) Mattay, J.; Vondenhof, M. Top. Curr. Chem. 1991, 159, 219. (11) Devadoss, C.; Fessenden, R. W. J. Phys. Chem. 1991, 95, 7253.
S0743-7463(96)00941-9 CCC: $15.00 © 1998 American Chemical Society Published on Web 09/03/1998
5692 Langmuir, Vol. 14, No. 20, 1998
Encinas et al.
CTAC. This monomer concentration is below the saturation limit.12 When ABCV was employed, NaOH was added up to pH 10. Polymerization of MMA in microemulsions was carried out in a glass dilatometer at 20 °C. After degassing at 0.01 Torr, the microemulsion was introduced directly into the dilatometer. The sample was irradiated with light from a medium-pressure mercury lamp with a glass filter to isolate the 366 nm band. The change of the liquid level in the capillary of the dilatometer was monitored as a function of time by a cathetoscope. Photoinitiators were used under matched conditions. Photoconsumption of the azo compounds was evaluated from the change in their near UV absorption band in a Shimatzu-160 spectrophotometer. All these experiments were carried out under matched conditions. Bleaching quantum yields were determined using potassium ferrioxalate as actinometer. Fluorescence experiments were carried out using a Spex-Fluorolog spectrofluorometer. Microemulsion conductivities were measured with a Jenway Modelo 4070 conductimeter and a calibrated immersion cell.
Results and Discussion Characteristics of the Microaggregates. Polymerizations were carried out in systems with equal concentrations of monomer and surfactant. Under these conditions, more than 75% of the monomer is incorporated into the microaggregates and the system comprises only the dispersed medium and monomer-swollen micelles.12 The free concentration of MMA is nearly 0.1 M. The presence of the monomer can alter the properties of the micellar aggregates. Of particular relevance is the aggregation number, which indicates the concentration of microdomains present in the solution. Fluorescence quenching experiments allow an evaluation of the aggregation number, according to the procedure developed by Turro and Yekta13 that employs Ru(bipy)32+ and 9-methylanthracene as donor and quencher, respectively. This procedure was applied to SDS micelles and MMAswollen SDS micelles. In both cases, the data were well represented by considering static quenching, allowing the evaluation of the aggregation number. A value of 110 was obtained for SDS micelles at 0.35 M detergent concentration, indicating an increase in the size of the micelles at this high surfactant concentration. However, the presence of MMA reduces the aggregation number to approximately 82. This result implies that, although the size of the aggregates remains almost unchanged due to the MMA incorporation, the number of micelles increases in the presence of the monomer. Localization of the Photoinitiators. The association of the initiators to the micelles was determined by ultracentrifugation techniques. The percentage of azo compound bound to the micelles was evaluated by eq 1
(%)bound ) 100([azo]total - [azo]U)/[azo]total
(1)
where [azo]total and [azo]U are the total concentration and the concentration of the azo compound in the ultrafiltrate, respectively. From these data, a pseudoassociation constant can be obtained by
Kass ) ([azo]bound/[azo]free)/[surf]
(2)
where [surf] can be taken as the analytical surfactant concentration, due to the high surfactant concentrations employed. The data obtained for the three azo compounds (12) Lissi, E. A.; Ca´ceres, T.; Veliz, C. Bol. Soc. Chil. Quim. 1983, 28, 13. (13) Turro, N. J.; Yekta, A. J. Am. Chem. Soc. 1978, 100, 5951.
Table 1. Distribution of Azo Compounds between the Dispersed Medium and the Micellar Aggregatesa azo compound
system
Kass (M-1)
% bound
ABAP
SDS SDS/MMA (1:1) CTAC CTAC/MMA (1:1) SDS SDS/MMA (1:1) CTAC CTAC/MMA (1:1) SDS SDS/MMA (1:1) CTAC CTAC/MMA (1:1)
34 56 0.45 0.75 0.2 0.4 >100 >100 >100 >100 >100 >100
92 95 14 20 6 12 >99 >99 >99 >99 >99 >99
ABCV
AIBN
a The value of K ass is defined in eq 2, and the percent bound corresponds to that measured in 0.35 M solutions in the corresponding surfactant.
considered in the present work in SDS and CTAC micelles and in the surfactant/MMA aggregates are given in Table 1. These values indicate that, under the present conditions, AIBN is totally incorporated to the microaggregates. The same conclusion can be obtained for ABCV in CTAC aggregates, a result that can be understood in terms of the surface potential of these micelles. It is interesting to note that this potential is fully operative also in the presence of the monomer. The data of Table 1 show that ABAP is also almost totally associated to SDS aggregates. Our results are similar to those reported by Barclay et al.,14 who, by measuring diffusion coefficients, concluded that 90% of the ABAP is bound to SDS micelles at 0.1 M surfactant concentration. This large extent of binding was also explained in terms of the surface potential of the aggregates. The presence of this micellar potential also explains the rather low percentage of co-ions (ABAP in CTAC and ABCV in SDS) located in the Stern layer of the aggregates. Interestingly, in both systems the presence of the monomer increases the percentage of co-ion association, a result that can be interpreted in terms of a reduced surface potential15 and/or the larger volume of the lipidic pseudophase. Evidence of the diminution of the micellar potential by the presence of monomer is provided by the increased conductivity of the micellar solution in the presence of MMA. Indeed, we found that the conductivity of the SDS and CTAC increases by approximately 6 and 12% by the addition of the monomer. The partition data conclusively show that the surface potential is able to discriminate the azo compounds by their charge. The average localization of the charged azo compounds will then be completely different when they bear the same or the opposite charge than the surfactant. Photoinitiation Efficiencies. Photopolymerization rates were measured by dilatometry, and relative polymerization rates were obtained by comparison of the contraction rates in the steady-state region. The relative rate values obtained in a homogeneous medium, monomer (HEMA)-polar cosolvent mixture, are given in Table 2. HEMA was employed as monomer in order to allow the evaluation of photoinitiation efficiencies in polar solvent that mimics the dispersed medium and the interfacial region of the emulsions. These data show a slightly higher initiation efficiency for the cationic azo compound. (14) Barclay, L. R. C.; Baskin, K. A.; Locke, S. J.; Schaefer, T. D. Can. J. Chem. 1987, 65, 2529. (15) Karaman, M. E.; Meagher, L.; Pashley, R. M. Langmuir 1993, 9, 1220.
Microemulsion Polymerization of Methyl Methacrylate
Langmuir, Vol. 14, No. 20, 1998 5693
Table 2. Initiation Efficiencies in Homogeneous Mediaa AZO ABAP ABCV AIBN
Rpb 1 0.91 0.88
φcleavc
Ri 1 0.83 0.77
(0.48)d
0.46 0.52 (0.47)e 0.48
φescapef 2.17 1.56 1.61
a Rate values given for AIBN and ABCV are relative to those obtained employing ABAP as photoinitiator. b HEMA in monomermethanol (1:2, v/v). c In methanol. d In water. e In methanolwater (1:1). f Relative values, see text.
Homogeneous polymerizations have a rate law given by16,17
RP ) kRi0.5
(3)
and the initiation rate Ri can be expressed as
Ri ) Iabsφcleavφescape
(4)
where φcleav is the cleavage quantum yield and φescape is the fraction of free radical pairs that avoid primary recombination. To obtain the efficiency of escape, it is necessary to evaluate φcleav values. Since aliphatic azo compounds photodecompose quantitatively to yield nitrogen,18-20 their photoconsumption yield can be considered equal to their photocleavage yield. Photocleavage quantum yields were determined from the consumption of the azo compounds in methanolic and aqueous solution. The values obtained are given in Table 2 and show that the photocleavage yields for the three azo compounds studied are similar and that they are independent of the media. Relative values of Ri and φescape in homogeneous solution can be evaluated from relative polymerization rates and photocleavage quantum yields. These data are included in Table 2. These results show markedly higher escape yields for the cationic compound. This fact can be interpreted in terms of the repulsion between the positive charges that, in this compound, are very close to the reactive center. The repulsion will increase the rate of escape, relative to that of the in-cage recombination of the alkyl radicals. The polymerization carried out in surfactant/MMA microemulsions follows a similar pattern to that observed in homogeneous solutions. Representative conversiontime plots are shown in Figure 1. In all cases the conversion was lower than 20%. It is observed that after an induction time, that can be ascribed to both the presence of oxygen traces and the delay to reach the stage of the micellar entry nucleation mechanism, a steady-state polymerization is reached. This induction time is related to the initiation rate, being shortest for ABAP and longest for AIBN. Since the polymerization rate is proportional to the volume diminution, relative polymerization rates can be obtained from the comparison of the slopes. The polymerization rates relative to ABAP are given in Table 3. By changing the light intensity, it was concluded that the steady-state polymerization follows a kinetic law such as that given in eq 3. The results are then similar to those (16) Encinas, M. V.; Lissi, E. A.; Quiroz, J. Eur. Polym. J. 1992, 28, 471. (17) Encinas, M. V.; Lissi, E. A.; Martı´nez, C. Eur. Polym. J. 1996, 32, 1151. (18) Hammond, G. S.; Sen, J. N.; Boozer, C. E. J. Am. Chem. Soc. 1955, 77, 3244. (19) Winterle, J. S.; Mill, T. J. Am. Chem. Soc. 1980, 102, 6336. (20) Barclay, L. R. C.; Locke, S. J.; MacNeil, J. M.; VanKessel, J.; Burton, G. W.; Ingold, K. U. J. Am. Chem. Soc. 1984, 106, 2479.
Figure 1. Conversion-time plots of the microemulsion polymerization of MMA (0.35 M monomer, 0.35 M SDS) using different photoinitiators: (9) ABAP; (b) ABCV; (2) AIBN. Table 3. Kinetic Data in the Emulsion Polymerizationa azo
micelleb
ABAP
SDS SDS/ester CTAC CTAC/ester SDS SDS/ester CTAC CTAC/ester SDS SDS/ester CTAC CTAC/ester
ABCV
AIBN
Rp
Ri
1
1
1
1
0.72
0.52
0.74
0.55
0.45
0.20
0.45
0.20
φcleav
φescapec
0.44 0.44 0.53 0.54 0.53 0.54 0.50 0.51 0.44 0.40 0.40 0.38
2.27 1.89 0.98 1.1 0.46 0.51
a Rate values for AIBN and ABCV are given relative to those obtained employing ABAP as photoinitiator. b Micelle, 0.35 M; ester, 0.35 M methyl butyrate. c Relative to ABAP.
obtained when ketones were employed as photoinitiators,21 where an order 1/2 in light intensity was also reported. The relative initiation rates obtained by considering this rate law are also included in Table 3. To obtain the efficiency of escape, photocleavage quantum yields were measured from photoconsumption of the initiator. The values obtained in micelles and micelles swollen with methyl butyrate are given in Table 3. They are similar to that obtained in homogeneous solutions and independent of the ester addition. Escape efficiencies relative to ABAP were evaluated from the initiation rate and photocleavage quantum yields. These values are included in Table 3 and show that the lowest values of φescape are those obtained employing AIBN. This is explained in terms of the micellar aggregate acting as a “supracage” favoring primary termination.7,22 The value obtained for AIBN relative to that obtained when the photoinitiator is mainly in the aqueous media (ca. 0.23 in both micelles) is considerably higher than that obtained employing tert-butyl ketone as photoinitiator, where a value of approximately 7% has been reported.21 The difference can be ascribed to the faster exit rate of the cyano-isopropyl radicals than that of the tert-butyl (21) Encinas, M. V.; Lissi, E. A.; Olea, A. E. J. Polym. Sci.: Polym. Chem. Ed. 1983, 21, 2157. (22) Turro, N. J. Pure Appl. Chem. 1995, 67, 199.
5694 Langmuir, Vol. 14, No. 20, 1998
radicals. In fact, the higher hydrophobicity of the latter radicals must significantly decrease its exit rate.23 The data obtained employing ABAP and ABCV in cationic and anionic micelles allow an evaluation of the effect of the location of the initial free radical pair in the Stern layer. The analysis of the data given in Table 3 indicates that the behavior as a photoinitiator is almost independent of its localization, being the same when it is located mainly in the dispersed medium or in the Stern layer. This rather surprising result indicates that interchange of the counterions between micelles is faster than the intra-Stern layer primary radical combination. In a cell model for the micelles this implies that exchange of radicals between the micellar cells is faster than primary recombination in a micellar cell. The exchange rate of counterions between cells increases when the surface potential decreases and when the number of micelles (23) Abraham, M. H. Chem. Soc. Rev. 1993, 73.
Encinas et al.
increases. The very high surfactant concentration employed in the present work, together with the presence of the monomer that can reduce the surface potential,15 could explain then the low importance of the intra-Stern layer primary radical recombination. The rather low aggregation number of the micelles implies a high micellar concentration (ca. 4.4 mM for SDS/MMA) that could also contribute to a fast intramicellar exchange of counterions. However, it must be taken into account that the evaluated concentration of micelles is that present prior to the polymerization process and that it must decrease with conversion. Acknowledgment. We are grateful to FONDECYT (grant nos. 1941068 and 2950078) and DICYT (Universidad de Santiago de Chile) for financial support of this work. LA960941I
