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Characterization of Nanoparticles Based on Block Copolymer Micelles J. Plesˇtil,*,† H. Pospı´sˇil,† J. Krˇ´ızˇ,† P. Kadlec,† Z. Tuzar,† and R. Cubitt‡ Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Heyrovsky Square 2, 162 06 Prague 6, Czech Republic, and Institut Laue-Langevin, 38042 Grenoble, France Received April 19, 2001. In Final Form: July 26, 2001 Three-layer nanoparticles were prepared by γ-radiation-induced polymerization of methyl methacrylate (MMA) in polystyrene-block-poly(methacrylic acid) aqueous micellar solution, and their structure was studied by small-angle neutron scattering (SANS). The scattering data were fitted using the bare-core approximation for a two-component core to elucidate the distribution of MMA before polymerization and the structure of nanoparticles with polymerized MMA. The newly formed polymer (poly(methyl methacrylate), PMMA) is deposited on the surface of polystyrene cores of the original micelles. The thickness of the PMMA layer (8-139 Å for the presented series of samples) depends on the monomer concentration. The SANS results indicate that the PMMA layer is penetrated by the poly(methacrylic acid) corona chains forming channels accessible to water.
Introduction One of the most prominent properties of amphiphilic block copolymers is their ability to form micelles (for recent reviews and books on the topic, see refs 1-4). The micelles consist of a dense core and a protective corona. The core formed by insoluble blocks can serve as a host for insoluble low-molecular-weight compounds, such as drugs. Numerous realized or potential applications are based on this property. A possibility of controlling the transport into or from the micelle core and its solubilization capacity is of great importance for applications of micelles. Three-layer (onion-type) micelles rank among promising candidates for the controlled uptake/release processes. It has been shown quantitatively5 that the rate of the uptake of cyclohexane into the poly(2-ethylhexyl acrylate) cores of micelles with poly(acrylic acid) shells can be slowed by the presence of a poly(methyl methacrylate) (PMMA) middle layer in the micelles. The three-layer micelles have been prepared using a pair of diblock copolymers of types AB and BC,6-8 a heteroarm star copolymer (AnBn) with a BC copolymer,9 or an ABC block copolymer.5,10 The common drawback of these methods of preparing onion* Corresponding author fax: +420(2)3535 7981; e-mail: plestil@ imc.cas.cz. † Institute of Macromolecular Chemistry. ‡ Institut Laue-Langevin. (1) Tuzar, Z.; Kratochvı´l, P. In Surface and Colloid Science; Matijevic, E., Ed.; Plenum Press: New York, 1993; Vol. 15. (2) Webber, S. E., Munk, P., Tuzar, Z., Eds. Solvents and SelfOrganization of Polymers; Kluwer Academic Publishers: Dordrecht, 1996. (3) Mortensen, K. Curr. Opin. Colloid Interface Sci. 1998, 3, 12-19. (4) Alexandridis, P., Lindman, B., Eds. Amphiphilic Block Copolymers: Self-Assembly and Applications; Elsevier Science: Amsterdam, 2000. (5) Krˇ´ızˇ, J.; Masarˇ, B.; Plesˇtil, J.; Tuzar, Z.; Pospı´sˇil, H.; Doskocˇilova´, D. Macromolecules 1998, 31, 41-51. (6) Procha´zka, K.; Martin, T. J.; Webber, S. E.; Munk, P. Macromolecules 1996, 29, 6526-6530. (7) Talingting, M. R.; Munk, P.; Webber, S. E.; Tuzar, Z. Macromolecules 1999, 32, 1593-1601. (8) Plesˇtil, J.; Krˇ´ızˇ, J.; Tuzar, Z.; Procha´zka, K.; Melnichenko, Yu. B.; Wignall, G. D.; Talingting, R.; Munk, P.; Webber, S. E. Macromol. Chem. Phys. 2001, 202, 553-563. (9) Tsitsilianis, C.; Voulgaris, D.; Sˇ teˇpa´nek, M.; Podha´jecka´, K.; Procha´zka, K.; Tuzar, Z.; Brown, W. Langmuir 2000, 16, 6868-6876.
type micelles is that the parameters of the resulting micelles are to a great extent predetermined by the properties of the copolymers used and cannot be easily varied. Another possible technique of preparing multilayer particles is polymerization of a suitable monomer dispersed in a micellar solution. Liu et al.11 studied polymerization of methyl methacrylate (MMA) and butyl acrylate in aqueous micellar solutions of poly(methyl methacrylate)-block-poly(methacrylic acid) (PMMA-bPMA). Hirzinger et al.12 prepared PMMA particles using micelles of the copolymer polystyrene-block-poly(etheneco-propene) in n-decane. In refs 11 and 12, light-scattering techniques were used for the structure characterization. These methods could not provide direct information on the inner structure of the particles. In our previous paper,13 we reported the results of a small-angle neutron scattering (SANS) study of the nanoparticles prepared by polymerization of MMA in an aqueous micellar solution of polystyrene-block-poly(methacrylic acid) (PS-b-PMA). Contrast-variation SANS experiments revealed that the newly formed polymer forms a layer on the surface of the polystyrene (PS) core of the original micelles. NMR studies on similar systems14,15 were aimed at the elucidation of the distribution of MMA monomer inside and outside micelles. Here, we report SANS observations made on a new series of samples with varying MMA amounts. Extension of the q range of the SANS experiment to small values made it possible to extract information on both the distribution of monomer before polymerization and the structure of final particles. (10) Plesˇtil, J.; Pospı´sˇil, H.; Masarˇ, B.; Kiselev, M. A. Annual Report 1998, FLNP; Joint Institute for Nuclear Research: Dubna, Russia, 1998; pp 63-66. (11) Liu, T.; Schuch, H.; Gerst, M.; Chu, B. Macromolecules 1999, 32, 6031-6042. (12) Hirzinger, B.; Helmstedt, M.; Stejskal, J. Polymer 2000, 41, 2883-2991. (13) Plesˇtil, J.; Pospı´sˇil, J.; Kadlec, P.; Tuzar, Z.; Krˇ´ızˇ, J.; Gordeliy, V. I. Polymer 2001, 42, 2941-2946. (14) Krˇ´ızˇ, J.; Kurkova´, D.; Kadlec, P.; Tuzar, Z.; Plesˇtil, J. Macromolecules 2000, 33, 1978-1985. (15) Krˇ´ızˇ, J. Langmuir 2000, 16, 9770-9774.
10.1021/la0105782 CCC: $20.00 © 2001 American Chemical Society Published on Web 09/18/2001
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Experimental Section Samples. A micellar solution of PS-b-PMA (Mw, 44 × 103 g/mol; weight fraction of PS, 0.68) was prepared by transferring the micellar solution in a 1,4-dioxane/20 vol % H2O mixture into 0.1 M borax in D2O by dialysis.16 The copolymer concentration was 1.1 g/L. MMA monomer was added to the micellar solution at various concentrations below the MMA solubility limit. The solutions were stored for 1 day, and then the MMA was polymerized by γ-radiation (60Co, dose 5 kGy) at 295 K. SANS Measurements. The experiments were performed using the D11 spectrometer at the ILL Grenoble17,18 with the neutron wavelength of 6 Å (wavelength spread, ∆λ/λ ) 9%). Using the ILL standard procedures, the data were corrected for instrumental backgrounds and detector efficiency on a cell-bycell basis prior to radial averaging to give a q range of 0.0028 Å-1 < q (4πλ-1 sin Θ) < 0.1 Å-1, where 2Θ is the scattering angle. Incoherent contribution to the scattering was determined by measuring the scattering intensities from the buffer and was subtracted from the data. The net intensities were converted to an absolute differential cross section per unit sample volume [dΣ/dΩ(q) in units of cm-1] by comparison with scattering from water.19 Two sample-detector distances were used, and the data overlapped in the region 0.01 Å-1 < q < 0.02 Å-1 within experimental errors without any arbitrary scaling factors. The absolute cross section (in cm-1) will be referred to as the scattering intensity or I(q). Dilution (1:4) of the sample with the highest weight concentration of the particles (7 g/L) did not lead to any significant change in the shape of the SANS curves. Hence, the concentration effects are neglected throughout this work. Analysis of SANS Data. The theoretical scattering function of spherical particles was fitted to experimental SANS data within a properly chosen q range (bare-core approximation,20 hereinafter BCA). Only the middle part of the SANS curve is taken into consideration in this approach. The innermost part, where the scattering from the whole micelle is observed, and the high q part, where the scattering from the corona-forming single chains predominates, were disregarded in the fitting process. Unlike in the previous papers,13,20 a two-component spherical core will be assumed here. The scattering intensity can be written as
∫V
I(q) ) n
∞
2
0
(R)[(F1 - F2)Φ(qR) + γ3F2Φ(qγR)]2N(R) dR (1)
where n is the number density of particles; V(R) ) 4πR3/3; and R and γR are the inner and outer core radii, respectively (γ g 1). F1 and F2 are the excess scattering length densities of the inner and outer component, respectively. The scattering amplitude of a sphere of radius R is
Φ(qR) ) 3
sin(qR) - qR cos(qR)
(2)
(qR)3
For the number distribution of the radii, we used a two-parameter Schulz-Zimm function:
N(R) )
( ) Z+1 R0
(
Z+1 RZ exp R R0 Γ(Z + 1)
Z+1
)
Figure 1. SANS curve of PS-b-PMA micelles in 0.1 M borax in D2O (c ) 1.1 g/L): experimental points (O); scattering curve of the micelle core fitted using the BCA (solid line). calculated using the nth moment of the Schulz-Zimm distribution:
〈Rn〉 )
( )
R0 nΓ(Z + n + 1) Z+1 Γ(Z + 1)
(4)
Results and Discussion PS-b-PMA Micelles. In D2O with 0.1 M borax, the micelles have a PS core and a poly(methacrylic acid) (PMA) corona. The SANS curve (Figure 1) indicates narrow size distribution of the micelles. Using the BCA approach, we obtained the following values of the fitting parameters: the mean radius of the core, R0 ) 121 Å; the polydispersity, σ/R0 ) 0.15 (Z ) 42); and the number density of micelles (n) at the concentration 1.1 g/L, 0.72 × 1014 cm-3. In the fitting procedure, F1 was fixed at 4.72 × 1010 cm-2, the excess scattering length density of PS in the buffer used, and the ratio of the radii (γ) was kept equal to 1 for the one-component core. Variation of the starting point of the fit within the interval qmin ) 0.016-0.030 Å-1 (see Figure 1) did not lead to any significant change in the fitting parameters. The core volume can be calculated as
Vcore ≡
〈V2〉 4π 〈R6〉 ) 3 〈R3〉 〈V〉
(5)
Molar mass of the core is then (3)
where Γ(x) is the gamma function, R0 is the mean radius, and Z (Z > -1) is the width parameter. The fit provides the mean inner radius of the core (R0), the relative standard deviation (σ/R0 ) 1/x(Z + 1), the ratio (γ), the number density of particles (n), and also, in principle, the excess scattering densities of the core components (F1 and F2). Other characteristics of the particle core (volume and mass) can be (16) Tuzar, Z.; Webber, S. E.; Ramireddy, C.; Munk, P. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1991, 32 (1), 525-526. (17) Ibel, K. J. Appl. Crystallogr. 1976, 9, 296-309. (18) Lindner, P.; May, R. P.; Timmins, P. A. Physica B 1992, 180181, 967-972. (19) Lindner, P. J. Appl. Crystallogr. 2000, 33, 807-811. (20) Plesˇtil, J. J. Appl. Crystallogr. 2000, 33, 600-604.
Mcore ) VcoredNA
(6)
where d is the density of the PS core (1.05 g/cm3, ref 21) and NA is Avogadro’s constant. From the above given parameters, we obtained Vcore ) 9.8 × 106 Å3 and Mcore ) 6.3 × 106 g/mol. By dividing the latter result by the weight fraction of PS (0.68), one arrives at the micellar molar mass Mmic ) 9.3 × 106 g/mol. Molar mass can also be determined from the absolute scattering intensity extrapolated to vanishing q. The procedure does not use any assumption on the internal structure of the micelle. The relevant relation reads (21) Van Krevelen, D. W. Properties of Polymers; Elsevier Science: Amsterdam, 1997; p 82.
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Mmic ) I(0)NA/c∆b2
(7)
where ∆b and c are, respectively, the excess scattering amplitude (in cm/g) and the concentration (in g/cm3) of the copolymer. The extrapolated intensity, I(0), was determined using the Guinier approximation of the experimental SANS data. Using ∆b ) -3.45 × 1010 cm/g, we obtained for the present system Mmic ) 9.9 × 106 g/mol. Good agreement of this result with that based on the core parameters justifies the use of the BCA approach for characterization of the micelles studied. The BCA approach is appropriate here because of two circumstances: For the copolymer used, the ratio of the excess scattering amplitudes, ∆Bcorona/∆Bcore, is very low (0.16-0.25, depending on the degree of ionization of PMA) and the corona is much larger than the core. For this purpose, the corona size was identified with the hydrodynamic radius RH ) 500 Å determined using dynamic light scattering. As demonstrated in ref 20 (Figure 3), the fitted values of the core parameters are very close to the true ones in cases such as this. For the nanoparticles based on these micelles, the BCA approach is expected to work even better than for the original micelles because the relative scattering power of the core is higher in the former case. Location of MMA Monomers in PS-b-PMA Micellar Solution. To get information on the structure of the systems before polymerization, the micelles with various amounts of MMA were studied. The monomer concentration ranged from 0 to 14 g/L. The highest concentration is close to the solubility limit of MMA in water (15.6 g/L at 25 °C).11 With an addition of MMA monomer to the PS-b-PMA micellar solution, a slight increase in the SANS intensities was observed (Figure 2). This suggests that no change in the aggregation number of micelles was induced by the addition of MMA monomer. This finding differs from that reported by Liu et al.11 for the PMMA-b-PMA block copolymers with a shorter hydrophobic block. The observed changes in the SANS curves indicate that the monomer concentration inside the micelles is higher than that in the aqueous phase. Quantitative information on the distribution of monomers was obtained by comparison of the zero-angle scattering intensities for the neat micelles and for the micelles with the monomer added. This can be done for the total scattering intensities or for the scattering intensities of the particle core. The former procedure leads to the total amount of the monomer inside the micelle, while the latter provides information about accumulation of the monomers inside the micelle core or near the core/corona interface. The present experiment cannot distinguish between the hydrogenous (nondeuterated) MMA monomers inside the core and the hydrogenous MMA monomers at the interface. The results provided by contrast-variation SANS13 and NMR14,15 techniques indicate that the MMA molecules are located near the surface of the PS core. For the micelle with a solubilized monomer, the zeroangle scattering intensity can be written as
Ip(0) )
(
) (
cmic∆b2micMmic cm∆bm 1+ NA cmic∆bmic Imic(0)
∆b2cop(cmw)
∆b2mic(cmw)0)
2
)
1+
)
cm∆bm cmic∆bmic
2
(8)
where subscripts mic and m denote micelle and monomer, respectively. The fraction before the parentheses on the right-hand side of eq 8 accounts for a slight change in the
Figure 2. SANS curve of PS-b-PMA micelles with varying concentrations of MMA monomer.
Figure 3. Dependence of the excess concentration of MMA accumulated in PS-b-PMA micelles on the total concentration of MMA calculated from eq 8 using the scattering intensity of the cores (4) and the whole particle (O), respectively. The slopes are equal to the fraction of MMA monomers located inside the micelles.
micelle contrast corresponding to the presence of monomer outside micelles with the concentration cmw. The excess concentration of the MMA monomer in the micellar phase (excess over cmw), cm, calculated using eq 8 is shown in Figure 3 as a function of the total concentration of the monomer. The values calculated from the total SANS intensity as well as those based on the scattering from the cores exhibit linear dependence on cMMA. The slope of this dependence is the fraction of the MMA located inside the micelles (about 0.01). This means that 99% of the MMA is distributed within the aqueous phase. This result is in contradiction with the finding published recently by Krˇ´ızˇ.15 Using NMR technique, the author found that in a system similar to that studied in this work, the MMA monomers reside almost exclusively in micellar coronas. However, NMR measurements of the present system indicate a much less exclusive distribution
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Figure 4. SANS curves of nanoparticles prepared by radiationinduced polymerization of MMA monomer in PS-b-PMA micellar solutions (c ) 1.1 g/L).
of MMA. So far, we have no plausible explanation for the different behavior of the present system. The total scattering intensity should lead to a higher value of cm than the intensity of scattering from the cores. However, our results shown in Figure 3 do not confirm this expectation. A possible reason for this discrepancy may consist of including inner parts of the corona chains (in a collapsed state or swollen with MMA) in the scattering from the cores. This leads to an overestimated cm value, while the result based on the total SANS intensity is not affected. PS-b-PMA/PMMA Nanoparticles. Irradiation of the micellar solutions with added MMA leads to dramatic changes in the scattering curves (Figure 4). Their analysis indicates that the MMA polymerized and the polymer was accumulated inside the original micelles to form threelayer particles. The middle part of experimental SANS curves was fitted to the theoretical scattering function of a two-component spherical particle (eqs 1-3). The extreme inner and outer parts were not used in fitting because other scattering contributions, coming from polymer chains, may be important in these regions.22,23 The SANS experiments indicated neither disintegration nor aggregation of the micelles induced by the addition of MMA. Hence, we assumed that the cores of the nanoparticles consist of the PS core of the original micelles and a PMMA layer on its surface. The mean radius of the PS core (R0 ) 121 Å), its excess scattering length density (F1 ) 4.72 × 1010 cm-2), and the number density of particles (n ) 0.72 × 1014 cm-3) were kept fixed during the fitting process. The adjustable parameters were the ratio of the core radii (γ), the width parameter (Z), and the excess scattering length density of the PMMA layer (F2). For dry, unswollen PMMA in the D2O buffer used, the excess scattering length density is F2,dry ) 5.1 × 1010 cm-2. In the swollen state, this value is reduced to
F2 ) F2,dry/Q
(9)
where Q is the degree of swelling (V/Vdry). (22) Pedersen, J. S.; Gerstenberg, M. C. Macromolecules 1996, 29, 1363-1365.
Figure 5. Example of the SANS curve of PS/PMMA/PMA nanoparticle (PS-b-PMA micelle concentration ) 1.1 g/L, MMA concentration ) 10 g/L): experimental data (O); a twocomponent BCA fit (solid line). Table 1. Structure Parameters of the Nanoparticles Prepared by Polymerization of MMA Monomer in PS-b-PMA Micellar Solutiona cMMAb (g/L)
Mwc (106 g/mol)
Rcored (Å)
TPMMAe (Å)
MMAf conversion
QPMMAg
0 0.94 1.87 3.74 5.62 9.36 14.04
9.9 10.6 12.4 14.9 21.8 34.3 54.4
121 129 136 147 192 223 260
8 15 26 71 102 139
0.08 (0.04) 0.15 (0.03) 0.15 (0.02) 0.24 (0.02) 0.29 (0.02) 0.35 (0.02)
1.00 (0.05) 1.02 (0.07) 1.16 (0.03) 1.36 (0.04) 1.40 (0.02) 1.33 (0.03)
a Copolymer concentration ) 1.1 g/L. b Monomer concentration. Particle molar mass. d Mean radius of the core (PS + PMMA). e Thickness of the PMMA layer. f MMA conversion calculated from the total extrapolated intensity I(0). g Degree of swelling of the PMMA layer estimated from the fitted excess scattering length density. The estimated standard deviations are given in parentheses. c
Figure 5 shows an example of the fit for the particles obtained with the monomer concentration 10 g/L. In contrast to neat micelles (Figure 1) or particles with small amounts of MMA, the BCA fit for this sample (as well as for the 6 and 15 g/L) follows the experimental SANS curve at smallest angles. This is due to the enhanced relative scattering amplitude of the particle core with increasing MMA content. On the other hand, the BCA fits for high MMA contents go below the tails of the experimental curves (see Figures 4 and 5). The excess scattering (excess over the BCA fit) decays like q-2. This behavior is typical of chainlike particles. The origin of this scattering contribution will be discussed later. The structure parameters of the nanoparticles are given in Table 1. Our results indicate that the parameters can be controlled by variation of the MMA concentration. For the present series, the molar mass of particles ranges from 10 to 54 × 106 g/mol, and the core radius ranges from 121 to 260 Å. The relative standard deviation of the core radii, σ/Rcore, was for all of the samples close to that of the original micelles (0.15 ( 0.01). The decrease in polydispersity after polymerization reported by Liu et al.11 was not observed (23) Pedersen, J. S. J. Appl. Crystallogr. 2000, 33, 637-640.
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Figure 6. Dependence of MMA conversion on the MMA concentration: from the total scattering intensity (O); from the scattering intensity of the core (0); from the core volume (4).
for our samples. The thickness of the PMMA layer on the surface of the PS inner core varies over more than 1 order of magnitude. The excess scattering length density of this layer obtained from the fit was in most cases lower than the value expected for dry PMMA. This suggests that the PMMA layer is swollen with water. It is not likely that the layer is swollen homogeneously like, for instance, a gel. Rather, hydrophilic corona chains penetrate through the layer thus forming channels accessible to water molecules. The degree of swelling was calculated using eq 9. The volume fraction of water at low MMA contents is less than 15%, while at higher MMA concentrations it reaches a value of 30%. Although the estimated standard deviations (Table 1) suggest that the determined values of the swelling degree are reliable, direct evidence of swelling of the PMMA layer is still missing because our arguments are not based on model-free calculations. Concentration of the newly formed PMMA can be estimated from comparison of the characteristics of nanoparticles (subscript p) with those of original micelles (subscript mic). From the zero-angle scattering intensity:
(( ) )
cPMMA ) cmic
Ip(0)
Imic(0)
1/2
-1
∆bmic ∆bPMMA
(10)
Conclusions
From the core volume:
(Vp,core - Vmic,core) dPMMA cPMMA ) cmic,core Vmic,coreQPMMA dmic,core
lower than the solubility limit. These oligomers are not taken into account if the conversion is estimated from the particle characteristics. If this limit is exceeded, the oligomers are transferred into the particles. This results in a higher conversion. Even at high MMA concentrations, the conversions are rather low (25-35%). To check whether the radiation dose (5 kGy) was sufficient for full polymerization, a series of samples of one MMA concentration using different radiation doses (1.5, 3.0, 4.5, 6.0, and 7.5 kGy) were prepared and examined. The SANS intensities of these samples were equal within experimental error to those of the sample with the radiation dose used throughout this study (5 kGy). Thus, an insufficient radiation dose as a reason for the low conversion can be rejected. It is more likely that the low conversion is due to the presence of PMMA oligomers. The presence of chainlike scatterers is indicated by a q-2 scattering contribution observed for the samples with cMMA g6 g/L. Such behavior can also be observed in the tails of SANS curves of micellar particles21,22 if the scattering from the corona chains is predominating. Because the q-2 excess scattering is not observed for the neat micelles, this contribution does not come from the corona chains but rather from chains not incorporated in the nanoparticles. Assuming that the chains are PMMA oligomers and taking into account the maximum observable excess scattering intensity and the concentration of MMA units not incorporated in the nanoparticles, we obtained, using eq 7, a lower estimate of the chain molar mass as 4000 g/mol. It is not likely that such PMMA chains would be soluble in water. Therefore, the possible presence of MMA oligomers can hardly explain the observed scattering behavior. NMR measurements indicate that some mobile methacrylic acid (MA) units are present in the solutions studied. As their signal is not detectable for the neat PS-b-PMA micelles, they cannot be identified with those of the corona chains because these chains are expected to be stiff and immobile at high pH. Rather, the MA units appeared as a result of MMA hydrolysis. Our NMR experiments24 showed slow hydrolysis of MMA in 0.1 M borax in both H2O and D2O. Thus, we believe that under the conditions of our experiments the PMMA chains can contain some hydrolyzed units. These chains can be soluble enough to remain in the aqueous phase even with rather high contents of the MMA units.
(11)
In eqs 10 and 11, c, I, ∆b, V, Q, and d, respectively, denote concentration, scattering intensity, excess scattering amplitude, volume, degree of swelling, and density. Equation 10 can be used for both the total experimental intensity and the fitted scattering intensity of the cores. Figure 6 shows the MMA conversion (cPMMA/cMMA) as a function of the initial monomer concentration, cMMA. Although the results of the calculations based on different characteristics are not in full agreement, they show a clear common trend: At low MMA concentrations, the MMA conversion is lower than at high MMA concentrations. The same trend is indicated by the results published by Liu et al.11 The authors explain this by formation of PMMA oligomers with a limited solubility. The oligomers can remain in the aqueous medium if their concentration is
We have investigated the structure of PS-b-PMA micelles, the accumulation of MMA monomer in these micelles, and the structure of the nanoparticles formed in polymerization of the MMA. Cores of the nanoparticles consist of the PS core of the original micelle and the PMMA shell. It was demonstrated that the thickness of the PMMA shell can be easily controlled by variation of the monomer concentration. The PMA corona chains penetrating through the PMMA layer form channels that are probably accessible to water. The conversion of MMA is rather low: 10-15% at low MMA concentrations and 25-35% at high MMA concentrations. Analysis of the SANS data suggests that, besides the nanoparticles chains, random copolymers of MA and MMA exist in solution. Both of these observations can be a result of partial hydrolysis of MMA, revealed by NMR measurements for the solution of pH 9.3. Before polymerization, only a small fraction of MMA (≈1%) is ac(24) Krˇ´ızˇ, J. Unpublished results.
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cumulated inside the PS-b-PMA micelles, and the rest is distributed within the aqueous phase. The simple procedure consisting of polymerization of the monomer in the presence of micelles can be employed to prepare particles of a finely tunable size or with a layer of controllable thickness on the surface of the micelle core.
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Acknowledgment. The authors thank the Academy of Sciences of the Czech Republic (Grant K 4050111/12) and the Grant Agency of the Czech Republic (Grants 203/ 00/1317 and 203/01/0536) for financial support. LA0105782
