J. Phys. Chem. 1992,96, 1505-1511
ls0S
Copolymerization of Acrylamide and a Hydrophobic Monomer in an Aqueous Micellar Medium: Effect of the Surfactant on the Copolymer Microstructure S. Biggs, A. Hill, J. Selb, and F. Candau* Institut Charles Sadron (CRM-EAHP), 6 Rue Boussingault. 67083 Strasbourg Cedex, France (Received: June 17, 1991; In Final Form: September 12, 1991)
The effects of surfactant on the radical copolymerization of acrylamide with small amounts of a hydrophobic comonomer, in an aqueous micellar medium, have been investigated. At all the surfactant and hydrophobe concentrations used here, monomers conversion-time data can be fitted by a theoretical curve calculated for the homopolymerization of acrylamide in pure water solution. Classical light scattering measurements have led to copolymer molecular weights, as a function of monomer conversion, which are in the range 1.5 X 106-3 X 106. The molecular weight is seen to depend on both the surfactant and the hydrophobe concentrations. The copolymer compositions, also as a function of conversion, have been determined by UV spectrophotometry. Comparison with a copolymerization performed in a homogeneous water/formamide mixture has shown clearly that the presence of micelles in the reaction medium increases the initial rate of hydrophobic monomer incorporation into the copolymers. As a result, the average hydrophobe incorporation in the copolymers was seen to decrease toward the feed composition as a function of conversion, the larger the decrease the higher the hydrophobe to surfactant ratio. Analysis of all the above data has led to the proposal of a mechanism of copolymerization in an aqueous micellar medium.
Introduction
Water soluble polymers modified with relatively low amounts of a hydrophobic comonomer (1-5 mol %) have recently become the subject of extensive research.lv2 In particular, the use of copolymers of polyacrylamide with various hydrophobic functionalities has proved to be of great Many uses for polymers of this type have been suggested, including microencapsulation and catalysis, although the main area of interest remains their use as aqueous viscosity modifiers in tertiary oil recovery and latex paint systems.l,* Polyacrylamide is usually prepared by a free-radical polymerization process in an aqueous environment. However, by definition, hydrophobic monomers are insoluble in such a reaction medium. A variety of possibilities have been suggested to overcome this problem: (i) the use of a solvent mixture in which both monomers are soluble? (ii) the solubilization of the hydrophobe into micelles dispersed in the water continuous m e d i ~ m , ~and -'~ (iii) the use of a fine suspension of the insoluble hydrophobe monomer?-" It was found, however, that only the micellar copolymerizationroute gave products with both a high molecular weight and adequate hydrophobe incorporation for use as aqueous viscosifiers. In this paper we describe the results of a study on the use of the micellar polymerization route for the preparation of polyacrylamide modified with low amounts of N-(Cethylpheny1)acrylamide. This hydrophobe was chosen in order to obtain copolymers which give lower aqueous solution viscosities than are found with the more commonly employed CBor C12aliphatic N-alkylacrylamide monomer^.^ A more complete discussion of this point may be found elsewherel' in a previous publication from this laboratory concerning the rheological behavior of such copolymers in aqueous solution. The kinetics of the free-radical polymerization of acrylamide in homogeneous aqueous solution have been extensively studied.'2-1s The chain-transfer constants to waterI2 and polymerI6 were shown to be very small or even zero at temperatures of up to 50 O C . Transfers to initiator and to monomer are such that the molecular weight of polyacrylamide, prepared with a persulphate initiator at moderate temperatures (25-50 "C),is always on the order of several millions. The effects of cationic, anionic, and neutral surfactants, on this polymerization, both above and below the critical micelle concentration (cmc) have also been previously reported.I6 The results indicated that only cationic surfactants above their cmc values had any effect, lowering both the reaction rate and the molecular weight of the resultant To whom correspondence should be. addressed.
0022-3654/92/2096- 1505$03.00/0
polymers. It was postulated that these effects were caused by an attraction of the persulphate anion for the positively charged micelles, which lowers the rate of decomposition to a radical initiating species. To date, however, the effects of low amounts of a hydrophobic monomer on the kinetics of the aqueous solution polymerization of acrylamide have not been reported. Valint and co-workers have, in a recent series of publicati0ns,4*~J~ described the preparation of copolymers of polyacrylamide with a variety of UV-active hydrophobic monomers in an aqueous micellar reaction medium. As a umsequence of this UV activity, they have been able to accurately determine the molar percentage incorporation of hydrophobe into any copolymer. An initial study of the incorporati~n,'~ as a function of monomer to copolymer amversion, at various levels of surfactant and a constant hydrophobe level, indicated a high initial incorporation which decreased with conversion to a level equal to that of the initial molar percentage of hydrophobe in the reaction mixture. The use of the UV-active hydrophobe iV-(4ethylphenyl)acrylamide, in this study, has enabled us to perform a complementary study over a wider range of surfactant and hydrophobe concentrations. Furthermore, the kinetics of the copolymerization and of the hydrophobe incorporation for the case of a copolymer prepared in a homogeneous solvent medium have been determined. Com(1) McCorraick, C. L.; Bock, J.; Schulz, D. N. In Encyclopedia of Polymer Science and Engineering, 2nd cd; Mark, H.F.,Bilcales, N.M., Overberger, Wiky-Interscience: New York, 1989; Vol. 17, p 772. C. G.,Mmges, G.,a.; (2) Polymers in Aqueous Media; Glass,J. E., Ed.; Advances in Chemistry Series 223; American Chemical Society: Washington, DC, 1989. (3) (a) Peer, W. J. Polym. Mater. Sci. Eng. 1987, 57,492. (b) Peer, W. J. In ref 2, p 381. (4) (a) Valint, P. L., Jr.; Bock, J.; Schulz, D. N. Polym. Mater. Sci. Eng. 1987,57,482. (b) Valint, P. L., Jr.; Bock, J.; Schulz, D. N. In ref 2, p 399. (5) (a) Bock, J.; Siano, D. B.; Valint, P. L., Jr.; Pace, S.J. Polym. Mater. Sci. Eng. 1987, 57, 487. (b) Bock, J.; Siano, D. B.; Valint, P. L., Jr.; Pace, S.J. In-ref 2, p 411. (6) (a) Siano, D. B.; Bock, J.; Myer, P.; Valint, P. L., Jr. Polym. Mater. Sci. Ens. 1987.57. 609. (b) . , Siano. D. B.: Bock.. J.:. Mver. . . P.: Valint.. P. L... Jr. In ref 2, p 425.' (7) Dowling, K. C.; Thomas, J. K. Macromolecules 1990, 23, 1059. (8) Evani, S.; Rose, G. D. Polym. Mater. Sci. Eng. 1987, 57, 471. (9) Bock, J.; Siano, D. B.; Schulz, D. N.;Turner, S.R.;Valiit, P.L.,Jr.; Pace, S.J. Polym. Mater. Sci. Eng. 1986, 55, 355. (IO) Evani, S., U.S. Patent 4432881. (1 1) Hill, A,; Candau, F.; Selb, J. b o g . Colloid Polym. Sci. 1991,81, 61. (12) Dainton, F. S. J . Chem. Soc. 1952, 1533. (13) Kulicke, W. M.; Kniewske, R.;Klein, J. Prog. Polym. Sci. 1982,8, 373. (14) (1 5) (16) (17)
Riggs, J. P.; Rodriguez, F. J. Polym. Sci., Part A-1 1967, 5, 3 15 1. Shawki, S. M.; Hamielm, A. E . J. Appl. Polym. Sci. 1979,23,3341. Friend, J. P.; Alexander, A. E. J . Polym. Sci., Part A-1 1968,6, 1833. Valint, P. L., Jr.; Bock, J.; Ogletree, J.; Zushma, S.;Pace,S . J. Polym. Prepr. (Am. Chem. Soc., Diu. Polym. Chem.) 1990, 31, 61.
0 1992 American Chemical Society
Biggs et al.
1506 The Journal of Physical Chemistry, Vol. 96, No. 3, 1992
TABLE I sample code MAM 1 MAM10 MAM 9 MAM 2 MAM 5 MAM 8 MAM 3 MAM 7
2
SDS,
hydrophobe,
wt %" 3 0 7.5 3 3 3 3 3
mol %b 0 1 1 1 1 1
3 3
no. hydrophobes per micelleC 0.0 0.0 1 .o 2.6 2.6 2.6 1.4 1.4
w/w in water. Percentage of total moles of monomer. Using cmc = 9.2 X IO-' mol L-'and an aggregation number = 60 for SDS at 50 OC.
parison of these results with those of the reaction in a micellar medium has allowed the proposition of a possible reaction mechanism.
Experimental Section Reagents. Acrylamide (Merck) was twice recrystallized from chloroform and was stored in the dark at 4 "C until required. Sodium dodecylsulphate (SDS)(Touzart et Matignon, >99.5% purity) was used as supplied. Its critical micelle concentration at 25 "Cwas estimated by fluorescence spectrosoopy, using pyrene as a microenvironment sensitive probe, and was found to be 8.2 X mol L-*. This value is in good agreement with known literature values confirming the purity of the SDS used here.l* Potassium persulphate (Aldrich) was used as supplied without further purification. N-(4-Ethylphenyl)acrylamide was prepared and purified according to the technique of Valint et al.4 All water used here was deionized and then distilled from an all-glass apparatus. Polymerization. A number of polymers and copolymers were prepared during this study (Table I). In general, for all polymerizations the initial concentration of monomers in water (total of acrylamide and hydrophobe) was constant at 3% (w/w) with the initiator concentration at 0.3% (w/w) relative to the monomer feed. The following description for a copolymer with 1 mol % hydrophobe, based on the total monomers feed, prepared in a solution of 3% (w/w) SDS is typical of all polymerizations performed for this report. A solution of 17.56 g of acrylamide in 600 mL of water was thoroughly deoxygenated before being added to 18 g of SDS and 0.44 g of hydrophobe in a 1-L reaction vessel. This reaction vessel was equipped with a water condensor, inert-gas inlet, thermometer, mechanical stirrer, and thermostated water jacket. The continuously stirred solution was maintained at 50 "Cunder a continuous flow of nitrogen while the hydrophobe was solubilized by the SDS micelles (usually 30-60 min). When dissolution was complete, the homogeneous reaction solution was transferred, by a double-needle technique under nitrogen gas, in 75-mL aliquots into seven separate preweighed 100-mL bottles. Each of these bottles, previously purged with nitrogen, was equipped with a magnetic stirring bar and was sealed with a rubber septum cap. They were then placed in a thermostated water bath at 50 "C,and the reaction solutions were agitated magnetically. Polymerization was initiated in each by the injection of 2.5 mL of a K2S20sstock solution (0.054 g in 20 mL of deoxygenated water). The seven separate reactions were terminated at various times after initiation, ranging from 15 min to 7 h, by addition of an excess of hydroquinone and cooling in an ice-water bath. After the bottle was weighed, to accurately determine the weight of reaction solution in each bottle, the polymers were precipitated by slowly pouring into a constantly stirred 6 times excess of methanol. After filtration, each polymer was washed repeatedly in methanol, to remove all traces of water, surfactant, and residual monomer, before being filtered and dried under reduced pressure, at 50 "C, for 4 days. Each polymer fraction was then accurately weighed to give percentage conversion versus time. (18) Flockhart, B. D. J . Colloid Sci. 1%1, 26, 484.
(D
0
* \ 0 *
1.40
d
H
.a10
Y
.215
1.1 1.46
2. 18
2.92
4.
38
c*104
Figure 1. Zimm plot obtained, in formamide solution, for copolymer sample MAM 5 at 40% monomer conversion.
Mdecular Weight Determination The molecular weight of each polymer sample in formamide solution was determined from classical light scattering using a multiangle spectrometer (Amtec Model MM1). Values of the refractive index increment, dnldc, for copolymers ranging in hydrophobic monomer content from 2% to 12% (wt/wt) were determined. The values were seen to have insignificant differences and, within the bounds of experimental error, to be equal to that of polyacrylamide in formamide solution (dnldc = 0.1 11). The measured values of the molecular weight, M,,were therefore considered to be real and not apparent values. A typical example of a Zimm plot obtained is given in Figure 1. UV Spectral Analysis. A calibration curve of the absorbance as a function of hydrophobic homologue concentration (mol/L) was performed initially in a water/2% ethanol (v/v) mixture according to the technique of Valint et al.4 The hydrophobic homologue in this case was N-(4-ethylphenyl)propionamide. The UV spectra were examined in the range 200-300 nm, the hydrophobic monomer used here giving an absorbance maximum between 245 and 247 nm. If the homologue is assumed to give the same concentration response as the copolymer samples, the weight fraction, W, and hence mole fraction of hydrophobe in each polymer sample may be calculated from their absorbance maxima, Do,using eqs 1 and 2. Do 175.23 w=12400 Y (W/175.23) mol % = x 100 (2) [(W/175.23) ((100 - W)/71.08)]
+
where y is the weight of polymer in g/1000 cm3and 12400 is the value of the molar absorption coefficent measured for the model compound19in units of L mol-' cm-'. The values 71.08 and 175.23 are the acrylamide and hydrophobe unit molecular weights, respectively. Partition Coefficient. The partition coefficient of the hydrophobic monomer was determined from measurements of its solubility limit in aqueous solutions of SDS at different concentrations. At each concentration of SDS,an excess of ethylphenylacrylamide was added to the solution and then the solution was agitated continuously for approximately 1 day. After agitation, any unsolubilized hydrophobic monomer was removed by centrifugation. The supernatant, after an appropriate dilution, was then analyzed by UV spectroscopy to determine the amount of monomer solubilized. The UV analysis was similar to that described above for the copolymers except that the molar absorption (19) Hill, A. Thesis, Universite Louis Pasteur, Strasbourg, France, 1991.
Copolymerization of Acrylamide and a Hydrophobic Monomer
t i m e (mins) Figure 2. Conversion-time data for the polymerization of acrylamide (MAM 1) in aqueous SDS solution (3%w/w): T = 50 '(2 [MIo = 0,422mol L-I; [I], = 3.33 X lo-' mol L-I; [SDS] = 0.104mol L-?(-, theoretical curve predicted in absence of surfactant).
The Journal of Physical Chemistry, Vol. 96, No. 3, 1992 1507
Figure 4. Conversion-time data for the polymerization of acrylamide/ N-(4-ethylphenyl)acrylamide in aqueous SDS solution (3% w/w) at a hydrophobe concentration (monomer/monomers) of 3 mol %: T = 50 O C ; [MI,, = 0.393 mol L-I; [MI,,,, = 1.181 X mol L-'; [I], = 3.33 X lo4 mol L-I; [SDS] = 0.104 mol L-I. MAM 3, 0; MAM 7, 0 (-, thwretical curve predicted for acrylamide in absence of surfactant
and hydrophobe).
0.8 C
:.
0.6
0.8
I Q)
>
E 0.4 0
0.2 100 ' 200 ' 300 ' 460 t i m e (mins) Figure 3. Conversion-time data for the polymerization of acrylamide/ N-(4-ethylphenyl)acrylamide in aqueous SDS solution (3% w/w) at a hydrophobe concentration (monomer/monomers) of 1 mol % T = 50 OC; [MI, = 0.412mol L-I; [MIObd= 4.09 X lW3mol L-I; [I], = 3.33 X lo-" mol L-I; [SDS] = 0.104 mol L-I. MAM 2,A; MAM 5,O; MAM 8, (-, theoretical curve predicted for acrylamide in absence of surfactant and hydrophobe). '
coefficient was determined for the hydrophobic monomer in water/ethanol and not for a saturated homologue (10 300 L mol-' = 270-272 nm). cm-l, A,,
Results The experimental conversion/time data curves are given in Figures 2-5. Molecular weight data (M,)as a function of conversion are given in Figure 6. In each case theoretical curves of conversion versus time are also given. These curves were calculated from expressions for the conversion-time behavior of acrylamide polymerized in water with a persulphate initiator as derived by Shawki and Hamielec.I5 Essentially the conversion as a function of time may be computed from the following expression:
3.5 -
9 3.0 0 X
1.5 1.0L' ' ' ' " 0.0 0.2 0.4 0.6 0.8 1.0 conversion Figure 6. Evolution of the weight-average molecular weight as a function of monomer conversions: (a) polyacrylamide, MAM 1,O;(b) 1 mol 96 hydrophobe/3% SDS, MAM 5 , 0 ; (c) 3 mol % hydrophobe/3%SDS, MAM 7, A; (d) 1 mol % hydrophobe/7.5%SDS, MAM 9, 0. I
where [I0] and [Mol are the initial concentrations of initiator and monomer, respectively, and [MI is the monomer concentration at time 1. K125 is the empirical rate constant determined by Riggs and Rodriguezi4 and is given as K 1 2 5= 1.7 X 10" exp(-l6900/1.99T) (4) where T i s the temperature in K. Figures 7 and 8 show the variation of the mole percentage of hydrophobe in the copolymers as a function of monomer conversion. In Figure 7, data are presented for those copolymers
'
I
'
prepared at 1 mol % hydrophobe in the following reaction media: (a) 3% SDS, (b) 7.5% SDS, (c) homogeneous water/formamide mixture. It is clear that for the copolymers prepared in a homogeneous solvent mixture of water and fonnamide, in the absence of SDS, there is no variation of the hydrophobe incorporation with time. For those copolymers prepared in an aqueous micellar
1508 The Journal of Physical Chemistry, Vol. 96, No. 3, 1992
Biggs et al. 1 .o
2.0 0)
-a
21.8 n 41.6
0.8 S
0.6
x
r Q
L
1.4
Q,
$0.4
0)
21.2 c
V
0)
El.0 n
0.8' ' I 0.0 0.2 0.4 0.6 0.8 1.0 "
"
"
"
conversion Figure 7. Mole percentage incorporation of hydrophobe as a function of conversion for differing concentrations of surfactant at a constant monomer mole ratio of l%: (a) MAM 2, o; (b) MAM 5, D; (c) MAM 8, A (all 3% SDS w/w);(d) MAM 9, (7.5% SDS w/w);(e) MAM 10, X (0% SDS w/w).
+
2.0
0.0 0.2 0.4 0.6 0.8 1.0 conversion
Figure 8. Mole percentage incorporation of hydrophobe as a function of conversion for a surfactant concentration of 3% (w/w)and a hydrophobe mole ratio (monomer/monomer)of 3%: (a) MAM 3, 0;(b) MAM 7, 0.
medium, however, it can be seen that the polymer samples are richer in their average hydrophobe incorporation at the beginning of the polymerization, this value decreasing toward the feed composition at higher levels of conversion. A comparable result was obtained for the copolymers prepared at 3 mol % hydrophobe (Figure 8) in a 3% SDS micellar medium although the initial deviation from the feed composition was further exaggerated. A knowledge of both the monomer conversion and the percentage incorporation of the hydrophobe at any time t allows a calculation of the rate of hydrophobe consumption. In Figure 9, curves for the hydrophobe conversion versus time as a percentage of the total hydrophobe in the monomer feed are given. Also shown is the theoretical acrylamide conversion curve derived above. It is clear that at high ratios of hydrophobe to surfactant there is a rapid incorporation of this monomer into the copolymers, resulting in complete consumption when only 40% of the total monomer feed has been used. As the ratio of hydrophobe to surfactant decreases, there is a corresponding decrease in the difference between the rates of conversion of the hydrophobe and of the total monomer feed. Discussion Kinetics. It is evident from the data presented here that at all the different molar concentrations of hydrophobe and weight concentrations of SDS,used in this study, there is no noticeable change in the rate of total monomer conversion with time. The very good fit of the experimental data by the theoretical curve calculated for pure polyacrylamide in water solution, in the absence of surfactants, suggests that neither the hydrophobe nor the surfactant exert any influence on the rate of polymerization. This
O*OA
'
100
'
200
'
300
4AO
time (mins)
Figure 9. HydroDhobic monomer conversion as a function of time for three hydropho&/SDS ratios: (a) MAM 7, 0; (b) MAM 5, 0; (c) MAM 9, A (- - -, theoretical curve predicted for acrylamide polymerization in the absence of surfactant and hydrophobe).
lack of surfactant effect is in agreement with the fmdings of Friend and Alexander,I6 who reported, in a study of the effects of surfactants on the rate of acrylamide polymerization, that anionic surfactants (i.e. SDS) have no effect at all concentrations. It should be noted that these authors studied these effects at concentrations up to 5 times the cmc whereas our results are for SDS concentrations of up to approximately 30 times the cmc. The fact that the hydrophobic monomer has no noticeable effect on the rate may simply be due to its being always at very low molar percentages. The similarity in the structure of the hydrophobe and the acrylamide may also lead to very similar values of their reactivity ratios. Theoretical calculations of copolymer compositions with a variety of different monomer reactivity ratios, rl and r2,show that the constant composition observed as a function of conversion for the copolymer prepared in homogeneous solution (Figure 7) can only be achieved for values of rl and r2 which are both relatively close to a value of one. However, no data are available on these ratios and the necessity of keeping the amount of hydrophobe low in order to maintain solubility both before and after the polymerization makes their determination here impossible. Molecular Weights. The results of the molecular weight variation as a function of conversion (Figure 6) are more complicated to diclcuss. In all cases,including pure polyacrylamide, lower values were obtained experimentally, at all levels of conversion, than might be expected in the absence of surfactants. Theoretical predictions for the number average molecular weight (M,,), after the theory of Shawki and Hamielec,Is for polyacrylamide in homogeneous aqueous solution, suggest values which range from 2.3 X 106 to 1.7 X 106 as a function of increasing conversion. These values were calculated for initiator and acrylamide concentrations which correspond to those used in this study. Since the molecular weight distribution, M,/M,,, for a polyacrylamide prepared by free-radical polymerization is at least 2, these theoretical values are obviously much greater than those recorded experimentally here. By contrast however, the experimental results of the same authors, for polyacrylamide in the absence of surfactant, were in excellent agreement with those of their theory. Comparison of the data obtained at 7.5% SDS with that for 3% SDS,at a constant hydrophobe level of 1 mol %, demonstrates clearly that increased surfactant concentrations lead to an increased lowering of the observed average molecular weight. Both these results indicate that the surfactant causes a lowering of the molecular weight by an increase in transfer reactions, an effect which may also be attributed to the presence of impurities, particularly alcohols, in the SDS. In Figure 10, the molecular weight data as a function of hydrophobe content at three levels of monomer conversion and a constant surfactant concentration of 3% are presented, the values of the hydrophobe content being taken from Figures 7 and 8. It is apparent that increased amounts of hydrophobe cause a lowering of the observed molecular weight. One possible explanation of
Copolymerization of Acrylamide and a Hydrophobic Monomer
4.0
7
1
3.5
$\
\
p 3.01
1.0’
’
’
’
’
’
’
’
’
2.0 4.0 6.0 8.0 percentage hydrophobe
0.0
Figme 10. Variation of the molecular weight as a function of hydrophobc of monomer conversion and a constant surfactant content at three deconcenttation (i.e. 3%): (a) 25-3096 conversion, 0; (b) 75% conversion, 0 ; (c) 90435% conversion, A.
this may be that of radical transfer to the hydrophobic monomer. It was previously reported by Carver et al.*O that, for the polymerization of acrylamide in toluene continuous microemulsions, degradative transfer to toluene forming a stable benzyl radical may occur. Since the hydrophobic monomer used here has a pendant ethylphenyl group, a similar process may be reasonably considered. For the copolymer prepared at 3 mol 7%hydrophobe and 3% SDS,a rise in the molecular weight is observed initially (Figure 6)before a slight decrease at high conversion levels. From Figure 9, it is apparent that the copolymers are very rich in hydrophobe initially, explaining their correspondingly low molecular weights. However, at approximately 40% total monomer conversion, the hydrophobe is exhausted and only pure polyacrylamide is prepared, thus the average molecular weight rises. The lowering of molecular weight with increased hydrophobe content of the copolymers (Figure 10) is in direct contrast to the reported findings of Siano et a1.t who observed, for copolymer samples ranging from 0%to 1% hydrophobe, a rise in molecular weight from 3 X lo6 to 10 X lo6. Their measurements were performed in water solution, however, which may have led to the formation of aggregates between the hydrophobic blocks of the copolymers. This aggregation will obviously be greater at higher levels of hydrophobe, which explains their observed rise in molecular weight. In this study, the use of formamide, a good solvent for both the monomers of the copolymer, should mean we have no aggregate structures and hence measure only the molecular weight of isolated coils. Hydroplwbe Content. Before discussing the results of the hydrophobe incorporation into the copolymers, as a function of the monomers conversion, it is necessary to emphasize that this socalled “micellar polymerization” is in fact very different from either emulsion or microemulsion polymerization processes. In the case examined here, the vast majority of the copolymerization will occur in the continuous aqueous phase containing the acrylamide monomer. This copolymerization is, therefore, a dilute aqueous solution polymerization of acrylamide which is modified by the presence of surfactant micelles and low amounts of a hydrophobic comonomer. The following discussion attempts to explain the observed data for the hydrophobe incorporation and to define where, within the system, the hydrophobic monomer is polymerized and how the two monomers may come into contact. For those copolymers prepared in a micellar medium, the initial deviation of hydrophobe content from the feed composition was seen to be dependent upon the initial ratio of hydrophobe to surfactant, the larger the ratio the larger the deviation. In Table I, indicative values of the initial average number of hydrophobe molecules per SDS micelle are presented, these values being based upon an aggregation number of 60 and a cmc of 9.2 X M (20) Carver, M. T.; Candau, F.; Fitch, R. M. J . Polym. Sci., Parr A-1
1989, 27, 2179.
The Journal of Physical Chemistry, Vol. 96, No. 3, 1992 1509 for SDS in aqueous solution at 50 OC.’* This result supports the findings of a recent report by Valint et al.,” who examined the effects of altering the concentration of surfactant at a constant initial level of hydrophobe. Their results showed an increased initial deviation at lower levels of surfactant. These results, together with those of the molecular weight variations, suggest compositional heterogeneity at all levels of conversion, which increases with increasing conversion levels. Clearly, in the case where we have an average of one hydrophobe per micelle (Figure 7) (7.5% SDS),the variation as a function of conversion is considerably diminished although a small decrease is still recorded. The initially rapid incorporation of hydrophobe leads us to believe that the hydrophobe is relatively accessible to attack from growing radicals in the bulk solution. The availability of the double bond to attack may be explained if we consider the micellar interfacial region as a mixed monolayer of SDS,the acrylamide, and the hydrophobe, thus promoting contact between the monomers. It was previously reported by this laboratory21q22 that acrylamide does in fact act as a sort of cosurfactant when in micellar solution, interfacial tension measurements showing weakly surfactant-like behavior.23 The penetration of the interfacial layer by acrylamide has also been suggested from fluorescence measurement^.'^ In an aqueous solution saturated with pyrene, acrylamide acts as a quencher, lowering the fluorescence intensity. It is well known that in a micellar solution pyrene is preferentially located within the micellar palisade layer.% The presence of acrylamide in a pyrene-containing micellar solution is seen to cause a lowering of fluorescence intensity from which it has been concluded that the pyrene and acrylamide must be in intimate contact. If the acrylamide is indeed considered to be a weak surfactant, then we may reasonably expect a surface excess concentration at the micellar interface. The micelle may then be considered as a region of higher acrylamide concentration in close proximity to the solubilized hydrophobe. This may facilitate reactive encounters between the different monomer species. Thus, any polymer chain with a radical head group, in the bulk solution, which encounters a micelle may polymerize some of the acrylamide at the interface. This radical may then either attack the hydrophobic monomer or further attack acrylamide. The over concentration of monomers in the interfacial region, relative to the bulk solution, will lead to further polymerization of both types of monomer in this region, a point which will be considered in more detail below. This process will, therefore, give regions in the copolymer with a high hydrophobe content but never, as such, blocks of pure hydrophobe. The kinetics will remain unaltered, however, since at any instant the number of reactions occuring at these interface regions is very small compared to that in the bulk solution. From our results,it is difficult to amve at a clear understanding of the reaction mechanism. Comparison of the hydrophobe incorporation data (Figure 7) for the copolymer sample MAM 10 prepared in a homogeneous water/formamide solution in the absence of surfactant with that of MAM 9 prepared in a micellar solution of SDS such that we should have an average of only one hydrophobe per micelle shows clear differences. These differences are apparent even though we should have a statistically even distribution of hydrophobe throughout the reaction solution. It is clear that, in all cases, the presence of micelles accelerates the initial incorporation of hydrophobic monomer into the copolymer. It is interesting to ask, therefore, how this might be achieved. In any aqueous micellar solution, it is well known that there is a collstant exchange of solubilized material between the micelles, by transfer via the continuous bulk medium.25 The dynamics (21) Candau, F.; Leon& Y. S.;Pouyet, G.; Candau, S.J. Colloid Inretface (22) Holtzscherer, C.; Candau, F. J. Colloid Interface Sci. 1988,125, 97. (23) Graillat. C.: Lemis. M.: Pichot, C. J . Disuersion Sci. Technol. 1990, I I ,‘453. (24) Ganesh, K.; Mitna, P.; Balasubramanian,D. J . Phys. Chem. 1982, 86, 4291. (25) a n a , R. In Surfactanis in Solution; Mittal, K. L., Bothorel, P., Eds.; Plenum Press: New York, 1986; Vol. 4, p 115.
Sci: 1 h 4 , 101, 167.
1510 The Journal of Physical Chemistry, Vol. 96, No. 3, 1992
of this transfer process have been studied previously by fluorescence quenching techniques,26the rate of transfer beiig dependent upon the relative rates of entry and exit (k+ and k-) of the solubilizate into and out of the micelle. The following expression relating these entry and exit rate constants to the partition coefficient of the solubilizate between the micelle and the bulk water phase has been derived by Almgren et aLZ6 k- = 55.5k+/Ks where K, is the partition coefficient. The entry rate constant, k+, has been found to be approximately constant for all possible solubilizates between lo9 and 1Olo M-’ s-I. Thus from a measurement of the partition coefficient and using an average valuez6for k+ of 7 X lo9 M-‘ s-l ,wecanobtain a value of the exit rate constant. The higher this value is the more rapid the transfer to another micelle. The partition coefficient for N-4-(ethylphenyl)acrylamidein the aqueous SDS solution was experimentally determined from solubility measurements to be approximately 4.5 X lo3. Using this value, we calculate a value of 9 X IO7 s-’ for the exit rate constant. This means that the exchange time of one monomer s. Since, for acrylamide between two micelles is less than polymerization, the time required for a monomer to add onto a s, we may expect fast growing radicalz7is on the order of transfer of the hydrophobic monomer from other micelles during the residence time of a radical in the interfacial region of any one micelle. Also,since only free, nonpolymerized, hydrophobes may participate in this transfer process, there will be a net flow of this monomer into a micelle where there is a growing radical. This situation is, in some ways, analogous to that of emulsion polymerization in which a constant monomer feed is set up from unnucleated dispersed droplets toward those with an initiating radical. There are major differences, however, in that in our case the radical head group is not isolated from the bulk solution but rather it is relatively free to exit the micellar interfacial region due to the extensive contact between the hydrophobic and hydrophilic monomers discussed above. It should also be emphasized that any radical head group which is found at the interface of a micelle will have a “tail”of polyacrylamide, or of copolymer, which remains in the water phase and never enters the micelle. Thus, although a net hydrophobic monomer feed toward any micelle which has a radical at its interface will be set up, causing the hydrophobe regions in the polymer to be richer than if only the hydrophobe content of one micelle were polymerized, this situation will not endure until all the hydrophobe is consumed. Instead, the growing polymer radical head group may leave the vicinity of the micelle and polymerize some of the acrylamide monomer in the bulk solution before encountering another micelle. Evidence for the transfer of hydrophobic monomer between micelles during polymerization has also been reported by Peere3 He described a series of experiments in which the polymerization system consisted of a hydrophobic monomer solubilized in an aqueous SDS micellar solution in the absence of the acrylamide monomer. In its place were substituted propionamide and a chain-transfer agent (CTA) to mimic the effects of the water soluble monomer. The hydrophobic monomer was initiated by a lipophilic initiator and the molecular weight of the resultant polymers recorded as a function of a number of reaction variables. It was observed that at concentrationsof initiator equal to or greater than that required for one initiator molecule per micelle the molecular weight was dependent upon the concentration of CTA. With no CTA, the molecular weight was seen to be considerably higher than if the contents of each micelle were polymerized separately, from which it was concluded that the transfer of monomer between micelles must have occurred. A further explanation of the increased initial monomer incorporation may be that it is simply due to an increased probability of a growing polymer radical in the bulk solution encountering (26) Almgren, M.; Grieser, F.; Thomas, J. K. J . Am. Chem. SOC.1979,
101, 279.
(27) Currie, D. J.; Dainton, F. S.;Watt, W. S.Polymer 1965, 6, 451.
Biggs et al. a hydrophobic monomer. This increased probability is caused by the effective volume differences between the hydrophobe in homogeneous solution where it is molecularly dispersed and the hydrophobe in micellar solution where it is solubilized in a micelle. Obviously, the micelle presents a greatly enlarged effective size for the hydrophobe when viewed from the radical in solution. It should also be remembered that the micellar interfacial region represents an acrylamide-rich region of the reaction medium in which the monomers are in intimate contact facilitating reactive encounters. Both of the above arguments may be used to explain the differences between the homogeneous solution polymerization and that performed at one hydrophobe per micelle. However, when we consider the difference in the initial rate of hydrophobe incorporation between the copolymers prepared at 1% hydrophobe and 3% SDS and those prepared at 1% hydrophobe and 7.5% SDS, the simple probability case is no longer valid as a sole explanation. While at higher levels of surfactant there are lower numbers of hydrophobe units per micelle, there are proportionally more micelles, thus we might expect equivalent results for the two systems, a result never achieved. In fact the difference may be explained by the residence time of a radical head group within the vicinity of a micelle; in those systems with higher numbers of hydrophobe units per micelle, the radical obviously rests a disproportionally longer time. If we consider the relative rates of reaction within the micelle, the rate will be greater when the hydrophobe concentration is higher but, since there is proportionally more monomer, the polymerization times should be equivalent. However, the concentration gradient which is set up, with the other micelles, as the monomer is consumed, is obviously greater in those systems with more hydrophobes per micelle. Thus the rate of diffusion of monomer is greater; hence the residence time rises and a larger proportion of the hydrophobe feed may be consumed initially. This is shown clearly in Figure 9, where for a sample prepared at 3% hydrophobe and 3% SDS (i.e. 7.4 hydrophobes/micelle) practically all of the hydrophobic monomer is consumed after 60 min, which corresponds to 40% conversion of the total monomer feed. A similar result was reported by HarkinsZsfor the polymerization of styrene solubilized within an aqueous micellar system. It was found that, at two initial monomer/surfactant ratios, the degree of polymerization was always greater than the number of styrene molecules per micelle. This increase was disproportionally greater at the higher styrene/ surfactant ratio, a result which was also attributed to a difference in diffusion rates between the two systems. The nature of the decrease of hydrophobe incorporation with time may be explained in one of two ways. (i) If each micelle stays associated with the hydrophobic block after polymerization, the decrease can be explained by the lowering of the micelle concentration. Thus, the number of micelles encountered by each growing polymer chain will decrease as a function of time. So, although each block remains the same length, the average number of blocks incorporated into each copolymer chain decreases with time. (ii) If some or all of the surfactant does not stay associated with the hydrophobic region of the chain, then the micelle concentration will remain approximately constant while the hydrophobe concentration decreases. Thus, a lowering of the average number of hydrophobe units per micelle will occur as a function of time due to redistributionof the monomer between the micelles. Therefore subsequent chains may have the same number of blocks but at an ever decreasing number of hydrophobe units. This process will continue just until the number of hydrophobe units is less than the total number of micelles, at which point the number of “empty” micelles begins to increase according to the same pattern and the number of “blocks” per chain decreases. Obviously, when the number of hydrophobe units per micelle is initially one, only the final stage can occur and so the variation is much decreased (MAM 9). From the curves presented in Figure 9, it is evident that at high levels of hydrophobe/SDS the incorporation is essentially very rapid due to a high rate of monomer diffusion (28) Harkins, W. D. J . Polym. Sci. 1950, 5 , 217,
J. Phys. Chem. 1992,96, 1511-1514 toward micelles associated with a radical. In such casea,the effects of redistribution are probably not very important. However, at lower hydrophobe/SDS ratios where the hydrophobe is never fully consumed before full conversion of the acrylamide, these processes will play an increasingly important role. Considering all of the points discussed here, it is clear that any copolymer sample taken at high conversion levels will be polydisperse, both in composition and in molecular weight distribution. Indeed, those copolymers prepared at high ratios of hydrophobe/SDS will also contain a high proportion of acrylamide homopolymer since the hydrophobe is exhausted before the acrylamide monomer (Figure 9).
Conclusions The results reported here show that despite the presence of high levels of surfactant (35cmc) and low levels of a hydrophobic comonomer (
