Micellization of Model Macromolecular Surfactants as Studied by

Micellization of ionically end-capped polystyrene-block-polyisoprene copolymers (PS-b-PI) in N,N-dimethylacetamide (DMAc) is investigated by static li...
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J. Phys. Chem. B 2000, 104, 5049-5052

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Micellization of Model Macromolecular Surfactants as Studied by Static Light Scattering V. Scha1 dler, C. Nardin, U. Wiesner,† and E. Mendes* Laboratoire de Dynamique des Fluides Complexes,UMR 7506, CNRS-UniVersite´ Louis Pasteur, 4, rue Blaise Pascal, F-67070 Strasbourg, France ReceiVed: October 19, 1999; In Final Form: April 10, 2000

Micellization of ionically end-capped polystyrene-block-polyisoprene copolymers (PS-b-PI) in N,N-dimethylacetamide (DMAc) is investigated by static light scattering. DMAc is a polar selective solvent for PS which ensures for sufficient dissociation of ion pairs in solution. The use of polyisoprene as the nonsoluble block provides micelles in thermodynamic equilibrium since PI is a liquid at ambient temperature. Two different types of ionic end functionality are considered: (i) a monofunctional diblock with a lithium sulfonate group at the PI chain end and (ii) an R,ω-macrozwitterionic species with an additional ammonium group at the PS chain end. They are both compared to a neutral diblock of same mass and block sizes (12 kg/mol per block). It is shown that micelles formed by block copolymers containing a sulfonate group at the PI end (core of the micelle) exhibit an aggregation number (Nagg) which is only about 30% of Nagg of the neutral block copolymer. The critical micelle concentration (cmc) in this case is increased by 1 order of magnitude. The macrozwitterionic samples behave qualitatively as the monofunctionalized polymer, however, presenting a larger aggregation number and a lower cmc with respect to the monofunctionalized sample. This effect is attributed to a selfscreening mechanism in micelles of the R,ω-macrozwitterions, where counterions are bound through chains of the soluble block.

When block copolymers are dispersed in a selective solvent, which acts thermodynamically as a good solvent for one block and a precipitant for the other, micelles are formed.1,2 In recent years, this phenomenon has been studied extensively for both aqueous and nonaqueous systems. Within many respects, the self-assembly of these polymeric materials can be viewed in strong analogy to low molecular weight surfactants in water.3 The similarity is obvious, e.g., by the occurrence of a critical micelle concentration (cmc) for surfactants4 and block copolymer (bc) micelle solutions,5 or by comparing the rich phase behavior of, e.g., poly(ethylene oxide-propyleneoxide-ethylene oxide) triblock copolymers in water6 with that of surfactant/ water mixtures.7 Since a block copolymer chain may consist of more than 100 monomeric units, however, the free energy per micelle is much larger for bc’s than for surfactants in water. As a consequence, the resulting bc colloids usually exhibit a very low cmc and a narrow micellar size distribution, which, as an example, makes them attractive for templating well-defined nanoparticles by solubilization, such as noble metal colloids.8,9 The discrepancy between phases formed by bc’s in a selective solvent and classical lyotropic phases becomes severe when phase transformations are kinetically restricted due to the presence of glassy cores,10 such that the structure which is finally observed significantly depends on sample preparation.11,12 In order to acquire a more general understanding of the selfassembly phenomenon, it is desirable to bridge the gap between surfactants in water and bc’s in a selective solvent. Usually, in surfactant systems, an ionic headgroup ensures for the partial solubility of the molecule. The structure and the dynamics of the self-assemblies are then governed by the competition between electrostatic interactions and hydrophobicity. In order * To whom all correspondence should be addressed. † Max Planck-Institut fu ¨ r Polymerforschung, Postfach 3148, D-55021 Mainz, Germany.

to introduce the electrostatic interaction in bc assemblies, we decided to combine a highly nonpolar block copolymer structure with a surfactant-like topology. To this end, we synthesized ionically end-functionalized block copolymers of polystyrene (PS) and polyisoprene (PI)13 which were then dispersed in the solvent N,N-dimethylacetamide (DMAc). DMAc has two major advantages for studying micellization of the present model systems: (i) it is a selective solvent for PS,14,15 thus the micellar core is formed by the PI block, which ensures for thermodynamic equilibrium due to the low Tg of PI; (ii) DMAc is polar enough to promote dissociation of the ionic moieties of the copolymers in solution ( ) 39).16 Micellization of the present model macromolecular surfactants is studied by static light scattering. The electrostatic interaction is introduced either (i) by the presence of a sulfonate group at chain end of the nonsoluble PI block or (ii) by the presence of both a sulfonate group at the PI end and a quaternary ammonium group at the chain end of the soluble PS block, that is, an R,ωmacrozwitterionic block copolymer. By comparison with uncharged bc’s of same mass, we can systematically investigate how electrostatics influences micellization and the critical micelle concentration, cmc. PS-b-PI diblock copolymers were obtained by sequential anionic polymerization using [2-N,N-dimethylaminomethylphenyl]lithium as initiator and 1,3-propane sultone as terminating reagent. The synthesis method and the characterization techniques employed are described in detail in ref 13. Chain end functionality of the telechelic species was characterized by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) as well as more conventional methods such as dye extraction and titration (see ref 13 for details). They are shown to be both higher than 95%. Solutions of PS-b-PI diblock copolymers in dimethylacetamide (DMAc) were prepared using dry HPLC grade (>99.9%) DMAc as

10.1021/jp9937205 CCC: $19.00 © 2000 American Chemical Society Published on Web 05/09/2000

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Figure 1. Schematic representation of the three block copolymer species investigated and the nomenclature employed.

purchased from Sigma-Aldrich. Although the polymers dissolved within a few minutes, all solutions were allowed to stand for >12 h before measurement, in order to ensure complete equilibrium. All diblock copolymer solutions were purified by filtering through solvent inert 0.45 µm PTFE/PP filters (Schleicher & Schuell, Germany) prior to use. All experiments were performed at T ) 293 K using a laboratory-built light scattering setup capable of time-averaged scattering intensity in the angular range of 10°-140°. The light source employed was an ion argon laser from Spectra Physics operating at 488 nm. A photomultiplier of EMI type was used coupled to an ALV-5000 correlator. Values for dn/dc were determined by a high-accuracy scanning Michelson interferometer as described in detail in ref 17. Six concentrations were measured for each refractive index increment determination, and a linear fit was used for obtaining the dn/dc value. It should be noted that since the polymers are heterogeneous in composition only apparent values rather than true values of Mw are obtained by static light scattering. The difference between Mw and (Mw)app as measured by static light scattering (SLS), however, should be rather small since the refractive index of PS and PI are both significantly larger than the solvent refractive index (compare nPS ) 1583; nPI ) 1521 with nDMAc ) 1438 at 293 K).18 The experimentally determined dn/dc values were (120 ( 2) × 10-6dm3/g for all block copolymers, irrespective of the chain end functionality. A schematic representation of the end-functionalized diblock copolymers is given in Figure 1. In the present study, constant molecular weight materials (Mw ) 24 kg/mol, 12 kg/mol per block) are compared. Two cases of different ionically endcapped chains are considered. The ω-functionalized diblock, where a lithium sulfonate group is introduced at the end of the PI block (S24), and the R,ω-macrozwitterionic diblock copolymer, bearing a lithium sulfonate end group at the PI block and an ammonium bromide end group at the PS block (Z24). A neutral diblock with a tertiary amino group at the end of the PS block (H24) is used as the reference nonionic sample. Since the macrozwitterionic samples were precipitated in methanol after quaternization, the counterion content (Li+Br-) is of the order of 25%, as evidenced by neutron activation analysis. All end-capped species were derived from a single polymerization batch. They therefore exhibit identical Mw distribution and PI microstructure. The impact of different ionic chain ends can thus be directly studied by comparison. For end-capped polymers, the fraction of actual end groups is >95%. In order to obtain information about the association behavior of the end-functionalized block copolymers in DMAc, the angular and concentration dependence of the time-averaged light scattering intensity was measured. Since static light scattering (SLS) represents an absolute method for determining the apparent weight-average molar mass of the solute, (Mw)app, the aggregation number (Nagg) of block copolymer micelles (bcm’s) can be calculated from (Mw)app as measured by SLS, and Mw

Figure 2. Reduced light scattering intensity, Kc/R (θ ) 0), extrapolated to zero scattering angle as a function of concentration, c, for species H24, S24, and Z24. The solvent is DMAc at T ) 293 K. Lines through the data correspond to the fits based on eq 7 employing parameters as listed in Table 1.

of the block copolymer single chain, as determined by a combination of GPC and NMR.13 In Figure 2, the reduced light scattering intensity Kc/R (θ ) 0), i.e., extrapolated to zero scattering angle θ, is plotted vs concentration, c, for the species H24, S24, and Z24 in DMAc at 293 K. The concentration regime is c e 5 g/dm3. Therein, R(θ) is the Rayleigh ratio of the solute, as determined from the known Rayleigh ratio of toluene. K is the optical constant given by19

K ) 4π2n2(dn/dc)2/λ4NA

(1)

with n the refractive index of the solvent, dn/dc the refractive index increment, λ the wavelength of the light in a vacuum, and NA Avogadro’s constant. Within this concentration domain, the scattered intensity showed no significant angular dependence. For higher concentrations, however, the onset of a structure peak at high q vectors was observed, such that for c > 5 g/dm3, an extrapolation to θ ) 0 would become meaningless for a molar mass determination. In Figure 2, a strong upturn in the reduced light-scattering intensity Kc/R(0) is observed for all three polymers below a characteristic polymer concentration. The onset of the upturn is ca. 0.5 g/dm3 for the samples S24 and Z24 and ca. 0.1 g/dm3 for H24 (dashed line). This indicates that at sufficiently low polymer concentration, the weight-average molar mass decreases strongly, which is a characteristic feature of a bcm solution close to the cmc. Usually, the cmc, i.e., the concentration below which only unimers exist, is very low for bcm solutions and cannot be directly measured by SLS. Because of the presence of a pronounced transition region, however, where unimers and micelles coexist, the cmc can be estimated on the basis of the closed association model:20 Following this model, it is assumed that in the transition region only unimers and micelles contribute to the total scattering intensity, i.e.

R(θ) ) Ru(θ) + Rm(θ)

(2)

where Ru(θ) and Rm(θ) represent the Rayleigh ratio for the unimers and the micelles, respectively. The scattering from a micellar solution can be described by

1 + 2A2(Mw,app)mcm Kc ) Rm(0) (Mw,app)m

(3)

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J. Phys. Chem. B, Vol. 104, No. 21, 2000 5051

TABLE 1: Micellar Properties of Ionic End-Capped PS-b-PI Diblock Copolymers in Hydrogenated DMAc Obtained from Light Scatteringa light scattering

SANS

A2 (10-9 (Mw)app cmc sample (g/dm3) dm3 mol/g2) (106 g/mol) H24 S24 Z24

0.0177 0.110 0.065

1.62 7.01 5.44

3.17 0.950 1.12

Nagg

(Mw)app (106 g/mol) Nagg

132 39.6 46.5

2.59 1.25 1.25

108 52 52

a Similar quantities obtained from small-angle neutron scattering on the same samples in deuterated DMAc are recalled for comparison.

where A2 is the second virial coefficient and cm is the concentration of micelles. Writing down an analogous equation for the unimers and combining eq 2 and eq 3 yields for the total scattering at θ ) 0:

R(0) )

Kcm(Mw,app)m 1 + 2A2cm(Mw,app)m

+

Kcu(Mw,app)u 1 + 2A2cu(Mw,app)u

(4)

For (Mw, app)m . (Mw, app)u and small values of A2 and cu (as given in the present case14) we can drop the virial expansion for the unimers in eq 4 to a good approximation. Thus, eq 4 becomes

R(0) )

Kcm(Mw,app)m 1 + 2A2cm(Mw,app)m

+ Kcu(Mw,app)u

(5)

Since c ) cm + cu, we can write for cu ) cmc:

R(0) )

K(c - cmc)(Mw,app)m 1 + 2A2(c - cmc)(Mw,app)m

+ Kcmc(Mw,app)u

(6)

or, by straightforward arrangement of this equation

Kc ) R(0)

(Mw,app)u +

c (Mw,app)m(c - cmc)

(7)

1 + 2A2(Mw,app)m(c - cmc)

Using eq 7, the experimental data points shown in Figure 2 were fitted. Initial values of A2 and (Mw, app)m were obtained from data points at intermediate concentrations where the slope and intercept of an apparent linear fit of the data were used. Because of the experimental uncertainties at very low polymer concentration, the confidence of experimental data was weighted for the fitting with a larger error bar for small values of c. The result of the final fitting with the adjusted parameters A2, (Mw, app)m and cmc is shown by the full lines in Figure 2. As illustrated by the good agreement between the fitting and the experimental data, eq 7 properly describes the transition regime between block copolymer unimers and micelles within the experimental error. On the basis of the fitted data we will now discuss the characteristic differences between the block copolymer species of distinct end functionality. In Table 1, the fitting parameters obtained for H24, S24, and Z24 are listed. The average aggregation number Nagg was calculated using Mw of the precursor block copolymer as determined by GPC and NMR The data reveal that the presence of ionic groups on the PI end, as given for S24 and Z24, decreases Nagg of the block copolymer micelles by a factor of ca. 3: It is 132 for the nonionic sample, H24, but only 39.6 and 46.5 for S24 and Z24, respectively. Furthermore, the cmc increases by almost 1 order of magnitude for S24 (0.110 g/dm3), compared to the nonfunctionalized

species, H24 (0.0177 g/dm3). A strong increase in the cmc is also observed in the case of the macrozwitterion, Z24 (0.065 g/dm3), although it is somewhat smaller. Hence, the main factor shifting the unimer-micelle equilibrium toward unimers is the presence of the sulfonate group at the chain end of the PI block. The presence of the opposite charge at the extremity of the PS block seems to play a less important role then the one attached to the PI less soluble block. As suggested before,21,22 this feature can be attributed to a high preference of the sulfonate group for the polar solvent, DMAc. Since there is always a fraction of solvent present in the PI core (which is liquid at 293 K) due to the finite value of the Flory parameter between PI and DMAc, it is expected that electrostatic repulsion causes the ionized sulfonate groups to be preferentially placed at the PI core surface. This probably forces part of the PI chain ends to loop toward the outer shell. The two contributions to the micelle free energy, (i) electrostatic repulsion between sulfonate groups and (ii) looping of the PI chain toward the outer shell, cost extra energy to the ionic S24 micelle which decreases the aggregation number with respect to the neutral micelle. The free energy calculation for both cases, H and S, is performed in ref 21 where the change in the aggregation number due to this energy penalty is evaluated. Previous results of refs 21and 22 on similar samples were obtained using small-angle neutron scattering. Values for micelle core radii (and hence aggregation numbers) were obtained from a fitting procedure of the scattering curves that contains some approximations: (i) the micelle form factor used in the fitting procedure contained the approximation of corona Gaussian chains that are artificially displaced outward the micelle and (ii) only the high q values of the scattering curve were used in the fitting, assuming that the influence of the structure factor present in SANS spectra was weak enough at that q range. Values of the aggregation numbers obtained in the SANS experiments22 are displayed in Table 1, together with the obtained micelle mass. Although the solvent used in SANS experiments was deuterated, and the concentrations studied much higher than that in the present case, the present LS results are in very good agreement with the previous SANS ones. This confirms the validity of the fitting procedure of refs 21 and 22. The difference between the cmc of both H24 and S24 species can be understood following similar arguments. When in sufficiently low concentration, block copolymer molecules in a selective solvent have the tendency to form a tadpole (macrounimer), with the collapsed block forming the head of the tadpole and the swollen block acting as a tail. The presence of the ionic group placed at PI collapsed block considerably increases the solubility of the macro-unimer S24 with respect to that of H24, leading to a strong increase in the cmc of sample S24. In order to consider the less pronounced differences between the monofunctional diblock copolymer S24 and the R,ωmacrozwitterionic species Z24, the electrostatics of the outer shell of the micelles has to be taken into account. The ammonium group present at the chain end of the PS block in the case of Z24 is probably responsible for a higher ionic strength in the outer shell of Z24 micelles than that of the S24 sample. Some of the (positive) counterions of the monofunctional species, S24, can leave the micelle toward the bulk solution while the (positive) ammonium groups on R,ωmacrozwitterionic species Z24 that are linked to the micelle core through a PS chain cannot. The ammonium group can, therefore, be considered as a counterion which is bound to the micellar core via the PS chain. Using the micellar radii as found

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in ref 22, it is possible to estimate the concentration of the quaternary ammonium ions inside the outer shell of the Z24 micelles:

cammonium ≈

3Nagg 4NA((R

core

+ RgPS)3 - (Rcore)3)

(8)

where Rcore and RgPS are the radius of the micellar core and the radius of gyration of the PS chain in the corona, respectively. With the value of Rcore ) 6.5 nm as obtained from neutron scattering on similar samples22 and using RgPS ) 3.0 nm as an approximation for the corona thickness, one obtains cammonium ) 0.08 mol/dm3 for the concentration of the bound counterions in the corona. The R,ω-macrozwitterionic species Z24 releases, when in solution, two different free ions, namely, Li+ and Br -. For a 1 g/dm3 solution, a dilute solution above cmc, the estimated cammonium concentration in the corona of Z24 micelles is almost 104 times higher than that of the free ions Li+ and Br -, considering that they are completely free in the bulk solution. The bound counterion might govern, therefore, intramicellar electrostatic screening. Micelles formed from the monofunctional species, S24, can lose part of their counterions to the bulk solution. Electrostatic screening of the sulfonate groups at the core of micelles is, therefore, less effective in the case of the monofunctional species than in the case of the zwitterionic species, leading to a higher aggregation number for the last system. Differences in cmc between the R,ω-macrozwitterionic species Z24 and the monofunctional S24 species are also probably due to electrostatic self-screening present in the Z24 macrounimer. In this case, the quaternary ammonium at the end of the PS chain (macro-unimer tail) cannot leave to the bulk solution and screens partially the charge at the collapsed PI block (macro-unimer head). This mechanism decreases the solubility of the, R,ω-macrozwitterionic macro-unimer, Z24, with respect to that of the monofunctional S24, but it is not strong enough to screen completely the electrostatic interaction. The cmc of the R,ω-macrozwitterionic species is, therefore, observed in a intermediate concentration between the cmc of the neutral, H24 and that of the monofunctional, S24, species. The influence of electrostatics upon micellization of ionically end-capped block copolymers in a polar, selective solvent has been explored. As a conclusion, we can stress that for systems of well-defined ionic macromolecular surfactants, the presence of a single ionic group at the chain end of the insoluble block significantly decreases the aggregation number of the micelles, while the critical micelle concentration, cmc, is increased by 1 order of magnitude. When a second, oppositely charged ionic group is attached to the chain end of the block at the outer shell, the increase in cmc and the decrease in Nagg with respect to the uncharged sample are less pronounced. As suggested in the text, this is probably caused by a self-screening mechanism due the ions which are chemically attached to the corona chains.

Acknowledgment. The authors acknowledge Prof. H. W. Spiess and Dr. S. J. Candau for the encouraging support of this work and Prof. M. Schmidt (Mainz University) for helpful discussions. We furthermore thank A. Franck (Mainz) for his help during the polymer synthesis, B. Mu¨ller (Mainz) for the dn/dc measurements, and Dr. F. Schosseler, Prof. J. P. Munch, and Dr. G. Nisato for technical assistance during the LS experiments in Strasbourg. Financial support from the Deutsche Forschungsgemeinschaft (DFG-Schwerpunkt Polyelektrolyte) and the Studienstiftung des deutschen Volkes/DAAD for a scholar (V.S.) is gratefully acknowledged. References and Notes (1) Sadron, C. Angew. Chem. 1963, 75, 472; Angew. Chem., Int. Ed. Engl. 1963, 2, 248. (2) Tuzar, Z.; Kratochvil, P. In Surface and Colloid Science; Matijevic, E., Ed.; Plenum Press: New York, 1993; Vol. 15, Chapter 1, pp 1-83. (3) Furukawa, J. Colloid Polym. Sci. 1993, 271, 852. (4) Laughlin, R. G. The Aqueous Phase BehaViour of Surfactants; Academic Press: San Diego, CA, 1994. (5) see e.g. Tuzar, Z.; Stepanek, P.; Konak, C.; Kratochvil, P. Colloid Interface Sci. 1985, 105, 372. (6) Alexandridis, P.; Olsson, U.; Lindman, B. Macromolecules 1995, 28, 7700. (7) See e.g.: Fontell, K. Colloid Polym. Sci. 1990, 268, 264. (8) Sankaran, V.; Cummins, C. C.; Schrock, R. R.; Cohen, R. E.; Silbey, R. J. J. Am. Chem. Soc. 1990, 112, 6858. Spatz, J. P.; Roescher, A.; Moeller, M. AdV. Mater. 1996, 8, 337. Spatz, J. P.; Sheiko, S.; Moeller, M. AdV. Mater. 1996, 8, 513. (9) Moffit, M.; McMahon, L.; Pessel, V.; Eisenberg, A. Chem. Mater. 1995, 7, 1185. Antonietti, M.; Fo¨rster, S.; Hartmann, J.; Oestreich, S. Macromolecules 1996, 29, 3800. (10) Rager. T.; Meyer, W. H.; Wegner, G.; Winnik, M. A. Macromolecules 1997, 30, 4911. (11) Zhang, L.; Eisenberg, A. Science 1995, 268, 1728. (12) Zhang, L.; Yu, K. Eisenberg, A. Science 1996, 272, 1777. (13) Scha¨dler, V.; Spickermann, J.; Ra¨der, H.-J.; Wiesner, U. Macromolecules 1996, 29, 4865. (14) Booth, C.;. Naylor, T. D.; Price, C.; Rajab, N. S.; Stubbersfiled, R. B. J. Chem. Soc., Faraday Trans. 1 1978, 74, 2352. (15) Price, C.; Canham, P. A.; Duggleby, M. C.; Naylor, T.deV.; Rajab, N. S.; Stubbersfiled, R. B. Polymer 1979, 20, 615. Price, C.; Chan, E. K. M.; Hudd, A. L.; Stubbersfiled, R. B. Polymer Commun. 1986, 27, 196. (16) cf. ionomer solutions in DMF: Hara, M.; Wu, J.; Je´roˆme, R.; Granville, M. Macromolecules 1988, 21, 3330. Antonietti, M.; Heyne, J.; Sillescu, H. Makromol. Chem. 1991, 192, 3021. (17) Becker, A.; Ko¨hler, W.; Mu¨ller, B. Ber. Bunsen-Ges. Phys. Chem. 1995, 99, 9/4, 600. (18) Following ref 19: Mw,app/Mw ) 1 + (Q/Mw)(νA - νB/ν)2, where ν and νA,B are the refractive index increments of the block copolymer and the corresponding homopolymers A and B, respectively. Q/Mw represents a heterogenity parameter. Using νPS/DMAc = 0.152 dm3/g, νPI/DMAc = 0.082 dm3/g, νPS-b-PI/DMAc ) 0.113 dm3/g, and Q/Mw < 0, 1 for block copolymers we obtain: Mw,app/Mw < 1.04. Brandrup, J., Immergut, E. H., Eds.; Polymer Handbook, 2nd ed.; Wiley: New York, 1975; p 19. (19) Light Scattering from Polymer Solutions; Huglin, M. B., Ed.; Academic: New York, 1972. (20) Elias, H.-G. Chapter 9 in ref 19. See also: Zhong, X. F.; Eisenberg, A. Macromolecules 1994, 27, 1751. (21) Mendes, E.; Scha¨dler, V.; Marques, C. M.; Lindner, P.; Wiesner, U. Europhys. Lett. 1997, 40 (5), 521. (22) Scha¨dler, V.; Lindner, P.; Mendes, E.; Wiesner, U. J. Phys. Chem. B 1998, 102 (38), 7316-7318.