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Langmuir 2006, 22, 4766-4776
Rheology and Phase Behavior of Poly(n-butyl acrylate)-block-poly(acrylic acid) in Aqueous Solution Elham Eghbali,† Olivier Colombani,‡ Markus Drechsler,‡ Axel H. E. Mu¨ller,†,‡ and Heinz Hoffmann*,† BZKG, UniVersita¨t Bayreuth, Gottlieb-Keim-Strasse 60, 95448 Bayreuth, Germany, and Makromolekulare Chemie II, UniVersita¨t Bayreuth, 95440 Bayreuth, Germany ReceiVed December 3, 2005. In Final Form: March 9, 2006 The surface activity and the rheological properties of aqueous solutions of the amphiphilic block copolymer poly(n-butyl acrylate)-block-poly(acrylic acid) (PnBA-b-PAA) were studied as a function of the degree of neutralization, R, of the poly(acrylic acid) block. Although the block copolymer spontaneously forms spherical micelles having a stretched PAA corona and a collapsed PnBA core in water for R > 0.1, the solutions do not exhibit any surface activity at this degree of neutralization. Cryo-TEM micrographs show that the radii of the hydrophobic core of the largest micelles are as long as the length of the hydrophobic chain. The micelles, however, have a broad size distribution, and on average, as shown by SANS, the micelles are only about half as long. At concentrations as low as 1 wt %, the solutions exhibit highly viscoelastic behavior and have a yield stress value depending on R. The globular micelles are highly ordered in the bulk phase, and the viscoelastic properties are a result of the dense packing of the micelles. The addition of salt or cationic surfactants dramatically decreases the viscosity of the solution. The observed properties seem to be due to electrostatic interactions between the PAA chains of the micelles.
Introduction In recent years, we have seen increasing interest in the aggregation behavior of different block copolymers, the morphology of aggregates, and the properties of different phases.1-4 Block copolymers can form all of the micellar structures that are known from the aggregation behavior of surfactants.2,5-7 Although most of these studies have been carried out to gain a better theoretical understanding of the compounds, these compounds have a great potential in various applications. Block copolymers can be used to stabilize dispersions and emulsions, act as compatibilizers for blends, control the rheology of complex fluids in various formulations, and act as templates for the synthesis of fine particles or new materials in nanotechnology. In most cases, the compounds consist of a hydrophobic and a hydrophilic block, where the hydrophilic block can be either neutral or ionic, and their micellization behavior has been studied by several research groups.1,2 A crucial parameter for the properties of block copolymer micelles is the glass-transition temperature, Tg, of the hydrophobic block. For systems with T < Tg, the core of the micelles can be in a frozen state, and the micelles can no longer exchange monomers and can be in a metastable state2. Micelles with T > Tg are expected to be dynamic species that exchange their monomers with the monomers in the bulk phase or with monomers from other micelles, given that the hydrophobic block is not too long. As a consequence of the exchange, the micelles may reach * To whom correspondence should be addressed.
[email protected]. Fax: +49-921-50736139. † BZKG, Universita ¨ t Bayreuth. ‡ Makromolekulare Chemie II, Universita ¨ t Bayreuth.
E-mail:
(1) Hamley I. W. The Physics of Block Copolymers; Oxford University Press: New York, 1998. (2) Riess G. Prog. Polym. Sci. 2003, 28, 1107-1170. (3) Tuzar, Z.; Kratochvil, P. AdV. Colloid Interface Sci. 1976, 6, 201-232. (4) Selb, J.; Gallot, Y. In DeVelopments in Block Copolymers 2nd ed.; Goodman, I., Ed.; Elsevier: Amsterdam, 1985. (5) Fo¨rster, S.; Zisenis, M.; Wenz, E.; Antonietti, M. J. Chem. Phys. 1996, 104, 9956-9970. (6) Yu, Y.; Zhang, L.; Eisenberg, A. Macromolecules 1998, 31, 1144-1154. (7) Yu, Y.; Eisenberg, A. J. Am. Chem. Soc. 1997, 119, 8383-8384.
a thermodynamically stable state rather quickly. Block copolymers with a polyelectrolyte block have also been the subject of theoretical and experimental studies by several groups in the last two decades.1,2,8-10 The first experimental systematic investigation of polyelectrolyte block copolymer micellization in water goes back to Selb and Gallot.4,11 The experimental aspects of block polyelectrolyte micellization and the influence of ionic strength,7 pH,12,13 added salt,11,12,14-16 block length,14,16 temperature,13,17,18 solvent composition,6,7,17,19 added homopolymer,17,19 and interaction with oppositely charged surfactants and polymers20-25 have been investigated. Whereas most investigations on block copolymer systems are concerned with the size of the micelles, their aggregation number, (8) Dan, N.; Tirrel, M. Macromolecules 1993, 26, 4310-4315. (9) Marko, J. F.; Rabin, Y. Macromolecule 1992, 25, 1503-1509. (10) Ronis, D. Macromolecules 1993, 26, 2016-2024. (11) Fo¨rster, S.; Hermsdorf, N.; Bo¨ttcher, C.; Lindner, P. Macromolecules 2002, 35, 4096-4105. (12) Matsuoka, H.; Matsutani, M.; Mouri, E.; Matsumoto, K. Macromolecules 2003, 36, 5321-5330. (13) Schilli, C. M.; Zhang, M.; Rizzardo, E.; Thang, S.; Chong, Y. K.; Edwards, K.; Karlsson, G.; Mu¨ller. A. H. Macromolecules 2004, 37, 7867-7866. (14) (a) Astafieva, I.; Khougaz, K.; Eisenberg, A. Macromolecules 1995, 28, 7127-7134. (b) Rager, T.; Meyer, W. H.; Wegner, G.; Mathauer, K.; Ma¨chtle, W.; Schrof, W.; Urban, D. Macromol. Chem. Phys. 1999, 200, 1681-1691. (15) Matsuoka, H.; Maeda, S.; Kaewsaiha, P.; Matsumoto, K. Langmuir 2004, 20, 7412-7421. (16) (a) Matsumoto, K.; Ishizuka, T.; Harada, T.; Matsuoka, H. Langmuir 2004, 20, 7270-7282. (b) Kaewsaiha, P.; Matsumoto, K.; Matsuoka, H. Langmuir 2005, 21, 9938-9945. (17) Shen, H.; Zhang, L.; Eisenberg, A. J. Phys. Chem. B 1997, 101, 46974708. (18) Astafieva, I.; Zhong, X. F.; Eisenberg, A. Macromolecules 1993, 26, 7339-7352. (19) Zhang, L.; Eisenberg, A. J. Polym. Sci., Part B: Polym. Phys. 1999, 37, 1469-1684. (20) Kabanov, A. V.; Bronich, T. K.; Kabanov, V. A.; Yu, K.; Eisenberg, A. J. Am. Chem. Soc. 1998, 120, 9941-9942. (21) Berret, J. F.; Herve, P.; Aguerre-Chariol, O.; Oberdisse, J. J. Phys. Chem. B 2003, 107, 8111-8118. (22) Li, Y.; Nakashima, K. Langmuir 2003, 19, 548-553. (23) Wang, C.; Tam, K. C.; Tan, C. B. Langmuir 2004, 20, 7933-7939. (24) Pergushov, D.; Remizova, E.; Gradzielski, M.; Lindner, P.; Feldthusen, J.; Zezin, A.; Mu¨ller, A. H. E.; Kabanov, V. A. Polymer 2004, 445, 367-378. (25) Pergushov, D.; Gradzielski, M.; Burkhardt, M.; Remizova, E.; Zezin, A.; Kabanov, V. A.; Mu¨ller, A. H. E. Polym. Prepr. 2004, 45, 236-237.
10.1021/la053272u CCC: $33.50 © 2006 American Chemical Society Published on Web 04/14/2006
Rheology of PnBA-b-PAA in Aqueous Solution
and their detailed shape at neutral pH, we would also like to study the surface tension of micellar solutions as a function of the degree of neutralization R. In principle, polyelectrolyte block copolymers are amphiphilic molecules and should therefore be surface-active. This behavior has been considered in only a few investigations. We assumed that the failure of the surface activity of the molecules that have been studied by Matsuoka is a result of the high charge density on the polyelectrolyte chain of the block copolymer.15 For this reason, we wanted to be able to control the charge density in the chain and choose a compound with a poly(acrylic acid) chain, namely, the system poly(n-butyl acrylate)-block-poly(acrylic acid), PnBA-b-PAA. In comparison to other systems, this system has the advantage that at room temperature the hydrophobic block is in the liquid and not the glassy state. The micelles and the adsorbed molecules on the surface of aqueous phases are therefore expected to reach equilibrium rather quickly. Block copolymers, particularly in combination with normal surfactants, are used to stabilize colloidal systems. For this reason, it is also of interest to study the interaction of the compounds with surfactants. For some block copolymers, such studies have already been carried out.21,22,26-30 For some combinations, the micelles of the block copolymers dissolve, and the hydrophobic parts of the block copolymers are saturated with surfactants.26,27,29,30 The block copolymer micelles seem to persist in other cases,21,22,28 but the surfactants bind to the micelles of the compounds. This seems to depend on the state of the hydrophobic core of the micelles, whether it is in the liquid or glassy state. Combinations of single-chain polyelectrolytes with surfactants have often been studied.31-34 Such systems usually form water-insoluble complexes. Detailed measurements have shown, however, that the insoluble precipitates are liquidcrystalline phases in which the ionic surfactants form micelles and the polyelectrolytes are wrapped around them to neutralize the charge, acting as counterions. Experimental Section Synthesis of Block Copolymer PnBA100-b-PAA150. Poly(n-butyl acrylate)-block-poly(acrylic acid) was synthesized by the acidcatalyzed elimination of poly(n-butyl acrylate)-block-poly(tertbutylacrylate), which was synthesized by atom-transfer radical polymerization (ATRP). Details will be given in another publication.35 In short, the ATRP of n-butyl acrylate (nBA) was initiated by ethyl2-bromoisobutyrate using CuBr and N,N,N′,N′′,N′′-pentamethyldiethylenetriamine as the catalyst system in a 25% acetone solution at 77 °C for up to 70% conversion. The polymer was purified and analyzed by size-exclusion chromatography (SEC) and MALDITOF mass spectrometry (Mn ) 12 500 g/mol, Mw/Mn ) 1.07). The PnBA was then used as a macroinitiator for the polymerization of the second block of poly(tert-butyl acrylate) (PtBA). The polymerization was performed in a 75% acetone solution using the above catalyst system at 60 °C for up to 60% conversion. After purification, the tert-butyl groups were removed by isobutylene elimination catalyzed by CF3COOH in dichloromethane. Unreacted homo(PnBA) (26) Hecht, E.; Mortensen, K.; Gradzielski, M.; Hoffmann, H. J. Phys. Chem. 1995, 99, 4866-4874. (27) Hecht, E.; Hoffman, H. Langmuir 1994, 10, 86-91. (28) Bronstein, L. M.; Chernyshov, D. M.; Vorontsov, E.; Timofeeva, G. I.; Dubrovina, L. V.; Valetsky, P. M.; Kazakov, S.; Khokhlov, A. R. J. Phys. Chem. B 2001, 105, 9077-9082. (29) Pispas, S.; Hadjichristidis, N. Langmuir 2003, 19, 48-54. (30) Zheng, Y.; Davis, H. T. Langmuir 2000, 16, 6453-6459. (31) Thalberg, K.; Lindman, B.; Bergfeldt, K. Langmuir 1991, 7, 2893-2898. (32) Bronstein, L. M.; Platonova, O. A.; Yakunin, A. N.; Yanovskaya, I. M.; Valetsky, P. M.; Dembo, A. T.; Makhaeva, E. E.; Mironov, A. V.; Khokhlov, A. R. Langmuir 1998, 14, 252-259. (33) Ilekti, P.; Martin, T.; Cabane, B.; Piculell, L. J. Phys. Chem. B 1999, 103, 9831-9840. (34) Hansson, P.; Almgren, M. Langmuir 1994, 10, 2115-2124.
Langmuir, Vol. 22, No. 10, 2006 4767 was removed by Soxhlet extraction with cyclohexane. The selective removal of the tert-butyl groups was confirmed by 1H and 13C NMR, and the absence of residual homopolymer in the final diblock copolymer was checked by SEC. Sample Preparation. A 5 wt % stock solution of the block copolymer was prepared by dispersing the polymer in water and stirring the solution overnight with a magnetic stirrer at 50 °C. This solution was diluted to the required concentrations immediately after homogenization. Different degrees of neutralization, R, were achieved upon the addition of a 100 mM NaOH aqueous solution and stirring for a few hours at room temperature. All measurements were performed within 2 days after sample preparation. The measurements, unless otherwise specified, were performed at 25 °C. pH Measurements. The pH of the samples was measured in a thermostated bath at 25 °C with a WTW pH 530 instrument. The glass electrode was calibrated with buffer solutions of pH 2 and pH 7. Conductivity Measurements. The conductivity was measured in a thermostated bath at 25 °C with a WTW LF 521 connected to a platinum electrode. The electrode was calibrated with a 100 mM KCl solution. Surface Tension Measurements. Surface tension was measured by the Du Nuoy ring method using a Lauda TE1C instrument with a Pt-Ir ring. The ring was cleaned in a flame before each measurement. Rheological Measurements. The rheological measurements for the low-viscosity solutions were performed with a Haake Rheostress 300 instrument with Couette geometry, connected to a thermostat. The rheometer is equipped with a double-gap sensor; the rotating hollow cylinder is 55 mm long, and its wall is 3.4 mm thick. The outer gap is 0.3 mm wide, and the inner one is 0.25 mm wide. The outer radius of the rotating cylinder is 21.55 mm. For highly viscous solutions, the rheological measurements were performed with a Haake Rheostress RS600 instrument equipped with a cone-plate sensor with a radius of 30 mm and a cone angle of 1°; temperature was controlled by a Haake Peltier TC81 temperature controller. Cryo-TEM. The cryogenic preparations were performed by depositing a drop of the sample onto a copper TEM grid (600 mesh, Science Services, Mu¨nchen, Germany), blotting most of the liquid and leaving a thin film stretched over the grid holes, and then plunging the grid rapidly into liquid ethane cooled to ca. 90 K by liquid nitrogen in a temperature-controlled chamber (Zeiss Cryobox, Zeiss NTS GmbH, Oberkochen, Germany). The grid was then transferred via a cryotransfer holder (CT 3500, Gatan, Mu¨nchen, Germany) to a Zeiss EM 922 EFTEM (Zeiss NTS GmbH, Oberkochen, Germany). The TEM was operated at a voltage of 200 kV. Zero-loss filtered images (DE ) 0 eV) were taken under reduced dose conditions (100-1000 e/nm2). All images were registered digitally by a bottommounted CCD camera system (Ultrascan 1000, Gatan, Mu¨nchen, Germany) combined and processed with a digital imaging processing system (Digital Micrograph 3.9 for GMS 1.4, Gatan, Mu¨nchen, Germany). Small-Angle Neutron Scattering (SANS). The solutions were prepared by dissolving the polymer in D2O (99.98%, Aldrich) in the absence of salt. The degree of neutralization was controlled by the addition of a D2O solution of NaOH. Measurements were performed at the D11 beamline at the Institut Laue-Langevin (ILL) in Grenoble, France. Scattering intensities were recorded with a 2D positionsensitive 3He detector. Four different instrument settings were used: sample-detector distances of 1.1, 4, and 16 m with a neutron wavelength of 6 Å, and a sample-detector distance of 34 m with a neutron wavelength of 10 Å. This corresponds to a momentum transfer range of 0.0012 < q < 0.33 Å-1. All samples were measured in 2 mm Hellma cells at room temperature. H2O, serving as a calibration standard, was put into a 1 mm Hellma cell. After the determination of the central detector coordinates for each sampledetector distance, the 2D raw data were radially averaged. Averaged data were normalized by use of the known wavelength-dependent effective differential cross section of H2O.
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of the hydrophobic core, Vh, and the volume of the hydrophobic chain Vs
4π 3 l 3 h
(2)
MWphob FNA
(3)
4πlh3FNA 3MWphob
(4)
Vh ) Vs )
n)
Figure 1. Schematic presentation of a block copolyelectrolyte micelle in water. The hydrophobic block (PnBA in our case) builds the core of the micelle, where a few chains are stretched but most of them are coiled and the stretched polyelectrolyte chains (PAA in our case) form the corona of the micelle. The dimensions are adopted from the contour length of the hydrophobic and polyelectrolyte blocks according to eq 1.
Simple Model for the Estimation of the Size of Block Copolymer Polyelectrolyte Micelles All available theoretical models for the micelles of the block copolymer polyelectrolytes in polar solvents assume that the hydrophobic blocks of the molecules form a hydrophobic core that is surrounded by a corona from the polyelectrolyte chains.8-11 It is generally argued that the hydrophobic chains do not want to be in the stretched configuration for entropic reasons. The polyelectrolyte chains protrude from the core, and because of their electrostatic interaction, they try to get as far away from each other as possible. By doing so, they pull on the hydrophobic chains and try to stretch them. As a consequence, the hydrophobic chains are stretched until the pulling force is compensated by the entropic force for coiling. The exact radius of the core is therefore the result of two opposing forces. A micelle according to this assumption is presented in Figure 1. For the calculation of various parameters such as the aggregation number n, the mean distance d between the micelles, and the total volume fraction of the micelles for a given block copolymer concentration, we assume for simplicity’s sake that the radius of the core is simply given by the contour length of the hydrophobic chain in the all-trans configuration, that is,
lh ) 2(0.126 nm)(DP)
(1)
where DP is the degree of polymerization (i.e., the number of monomers in the hydrophobic block) and nm is the distance in nanometers (nm) between two carbons of the hydrocarbon chain in the direction of the chain. With these assumptions, the model gives us an upper limit for the size of the core, but the real value can be smaller. The obtained results will show that the size of the largest observed micelles is not much smaller than that predicted by this model. However, because the micelles have a size distribution, on average the micelles are considerably smaller. From the model, it is clear that only a few molecules have to be present in the stretched configuration whereas most of the molecules can be coiled and fill out the available space of the sphere. The situation is thus very similar to that for normal surfactant micelles. The aggregation number n is now simply given by the volume
where F is the density of the core and MWphob is the molecular mass of the hydrophobic blocks that form the core. With the aggregation number n it is now possible to calculate the number density 1c of the micelles from the polymer concentration cg (mass-related concentration in units of weight per volume) and the density of the core F: 1
c)
cgNA MWpolymern
(5)
1c
has the dimension of a number of particles per volume unit. Replacing n from eq 4 results in
c)
3cgMWphob
1
4πlh3FMWpolymer
(6)
Without making a large error, we can simply use a value of 1.09 g/mL for the density F. From the number density, we can calculate the mean distance d between the micelles from 1
c)
1 d3
(7)
To have a reasonable view of the space-filling situation of the micelles, we need an assumption for the thickness of the corona. It is known that polyelectrolytes in dilute solution have a rather long persistence length. Theories for the present situation assume that they are stretched at the core region and more and more coiled toward the outside. For simplicity, we assume that they are completely stretched. The volume of a micelle is then VM ) 4π(lh + lc)3/3 when lc is the length of the polyelectrolyte chain. Because it is now assumed that the water in the corona is part of the micelles, we obtain for the volume fraction Φ of the micelles
Φ)
4π(lh + lc)3 3d3
(8)
The average area bi2 covered by one hydrophilic chain at the core/shell interface is given by the surface of the core and the aggregation number, that is,
4πlh2 bi ) n 2
(9)
and the corresponding bo2 value on the outside of the corona is
bo2 )
4π(lh + lc)2 n
(10)
The calculated values for our studied block copolymer according to eqs 1-10 are presented in Table 1.
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Table 1. Characteristic Parameters of the Micelles in the Solution radius of the hydrophobic core, lh thickness of the hydrophilic shell, lc Rmicelle ) lc + lh aggregation number, n area per chain at the core/shell interface area per chain at the outer surface of the shell cross section, a2, at the air-water interface (R ) 0)3 intermicellar distance, d, at 0.5 wt % intermicellar distance, d, at 1 wt % effective volume fraction, Φ, of micelles at 0.5 wt % effective volume fraction, Φ, of micelles at 1 wt %
theory1
SANS2
TEM
25 nm 38 nm 63 nm 2760 2.84 nm2 18.00 nm2 0.56 nm2 276 nm 219 nm 4.75% 9.5%
11.4 nm
ca. 15 nm
296 5.5 nm2 102 nm2 100 nm 104 nm 44.3%
a
Based on the assumption that the core radius RC equals the contour length LC of the hydrophobic block. b Based on intermicellar distance of 104 nm from SANS for 1% concentration. c From surface tension measurements.
Figure 2. Neutralization curves for a 0.5% solution of PnBA100PAA150 with NaOH. Some changes in the visual appearance of the samples are also mentioned.
The model could now be further used to calculate interaction energies between the micelles. It is obvious that not all of the counterions of the polyelectrolyte will remain inside the corona. The entropic force will cause some counterions to dissociate into the water surrounding the micelle until the increasing negative charge of the micelle stops this process for enthalpic reasons. A good estimate of the concentration of the counterions outside of the corona can be gained from the conductivity of the samples. If we assume further that the free ions are the main contributors to the conductivity, then we can calculate their concentration. This allows the calculation of the number of excess ionic charges per micelle and hence the charge density of the micellar surface from which it is possible to calculate a surface potential.
Results and Discussion Potentiometric Titration and Phase Behavior. Aqueous solutions of the block copolymers in the acidic form are turbid. In a few days, they separate into two phases. The samples are therefore thermodynamically unstable and are already present in a microscopically demixed state. This assumption was also proved through cryo-TEM micrographs (Figure 7a). On titration with NaOH, the samples become completely transparent at a degree of neutralization of R ) 0.1 and remain transparent until R ) 1. The titration curve in Figure 2 is consistent with the titration of a weak acid with pKa ) 6.5. Titration of the 0.5% solution in Figure 2 leads to a concentration of 30 mM acid, and this value is consistent with the assumed composition of the block copolymer. If the samples are overtitrated with NaOH, then the solutions became bluish and opaque. This can be the result of shielding of the ionic charges of the particles by excess salt, in this case, NaOH.
Figure 3. Visual appearance of partially neutralized block copolymer solutions with varying extent of neutralization (R). The different R values are given on the samples. (a) PnBA100-PAA150 block copolymer (0.5%) + NaOH with (b) PnBA100-PAA150 block copolymer (1.0%) + NaOH; (c) PnBA100-PAA150 polymer (1.0%) in 100 mM NaCl + NaOH; and (d) PnBA100-PAA150 polymer (1.0%) at R ) 0.5 + NaCl.
Some samples with different degrees of neutralization are shown in Figure 3. As expected from theoretical considerations, the turbidity of the samples depends very much on the concentration of excess salt.35-38 In salt-free solutions, the aggregates are expected to be highly ordered because of their high charge density. With increasing salt concentration and shielding of charges, the order is lost, and the particles can assume a random distribution in the solution. This expected behavior is indeed reflected in the TEM micrographs that will be shown later (Figure 9). The effects are very large for the investigated systems, and it is interesting to view and to compare the samples under different conditions. Some samples are shown in Figure 3. (35) Colombani, O.; Mu¨ller, A. H. E., et al. To be submitted for publication. (36) Goddard, E. D.; Hannan, R. B. J. Colloid Interface Sci. 1976, 55, 73-74. (37) Pergushov, D. V.; Remizova, E.; Feldthusen, J.; Zezin, A. B.; Mu¨ller, A. H. E.; Kabanov, V. A. J. Phys. Chem. B 2003, 107, 8093-8096. (38) Hayter, J. B.; Penfold, J. Mol. Phys. 1981, 42, 109-118.
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Figure 6. SANS intensity as a function of the scattering vector for a 1% solution of PnBA100-PAA150 and different R values. For R ) 0.2, the original values are shown; for each increase in R, the curves are shifted upward by a factor of 4 for readability.
Figure 4. Surface tension measurements for completely protonated and partially protonated solutions of PnBA100-PAA150. (a) For the protonated polymer, surface tension is measured as a function of polymer concentration. (b) For the 0.5% block copolymer solution, surface tension is measured as a function of the degree of neutralization, R.
Figure 5. Surface tension of a 0.5% solution of PnBA100-PAA150, neutralized to R ) 0.5 as a function of added salt concentration.
It is noteworthy that the samples in Figure 2 in the first row (a) at a concentration of 0.5% scatter more than the samples at a concentration of 1% in the second row. This decrease in turbidity is probably due to the denser packing of the micelles. A consequence of the denser packing is also visible in another feature of the samples. The samples in row b contain small bubbles that do not rise. These bubbles are trapped by the rheological yield stress of the samples that is due to the dense packing of the micelles. The samples in row a do not have a yield stress. Row c demonstrates that the samples in the presence of salt are more turbid than the samples without salt and that the yield stress value has disappeared. Finally, row d shows that the turbidity is increasing with salt concentration. The molar concentration of NaOH and the neutralization grid of the polyacrylate, R, are shown on the lower and upper x axes, respectively.
The change in scattering intensity for different physicochemical conditions could in principle be evaluated in a quantitative way, and information on the size and shape of the aggregates could be obtained by light-scattering measurements. However, this is not necessary because this information will be available from the cryo-TEM micrographs. We also note that a partially ionized sample can be transformed back into the neutral form by adding HCl. The solution is then transparent for some time. This indicates that the originally observed turbid phase that reacts when the polymer is first dispersed in water and the state that is reached by reneutralization do not represent thermodynamic equilibrium states. One is obliged to think that the differences in optical appearance are due to the different sizes of the particles in the different samples or even to multiphase samples. However, this may not be the case, and the difference could be due to the different structure factor S (q ) 0) of the particles that can vary from close to 1 in the presence of salt to 0.01 without salt.38 The micelles are large enough that for shielded conditions the solution can scatter strongly whereas for the same concentration for unshielded conditions the solution can be transparent. Surface Tension of the Block Copolymer Solutions. Amphiphilic block copolymers are expected to be surface-active. Surface tension measurements of the acidic compounds are given in Figure 4a. The nonneutralized polymer shows behavior that is typical of surfactants with a linear range in the semilogarithmic plot of surface tension, σ, versus concentration with a sharp break that can be understood to be the critical micelle concentration (cmc) of the polymer at ca. 0.1 wt % or 0.044 mM. The surface tension at the cmc is 49 mN/m. The plot demonstrates that the compound adsorbs reversibly at the water surface. The Gibb’s adsorption equation is
Γ)
-1 dσ RT d ln cg
(11)
Τhe excess surface concentration can be calculated, and from one gets a2, the cross-sectional area per adsorbed molecule as nm2/molecule through eq 12
a2 )
1014 NAΓ
(12)
The experiment yields a surprisingly low value of 0.56 nm2. A Langmuir adsorption film at the cmc is usually densely packed. The area a2 should therefore have a similar value to the area b2, the area for one polyelectrolyte chain at the core/shell
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Figure 7. Cryo-TEM micrographs of solutions of PnBA100-PAA150 with different R values: (a) 0.5% polymer, R ) 0; (b) 1% polymer, R ) 0.25; (c) 0.5% polymer, R ) 0.4; (d) 0.5% polymer, R ) 1.0.
Figure 8. Cryo-TEM micrograph of a 0.5% PnBA100-PAA150 solution neutralized to R ) 0.5.
Figure 9. Cryo-TEM micrograph of a 0.5% PnBA100-PAA150 solution neutralized to R ) 0.5 with 2.5 mM NaCl.
interface. However, a2 can be determined only for the neutral block copolymers from surface tension measurements whereas b2 can be determined for the charged micelles from the size of the core and the aggregation number of the micelles. This situation will become clear after a few sentences. Because of the different physicochemical conditions of the block copolymer solutions, it is not surprising that a2 and b2 differ very much. The area b2 is very much larger than a2, as expected from the influence of the charge. The low value of a2 would require that the block
copolymer does not lie flat on the water/air interface but that both chains are in a rather stretched conformation with the hydrophilic chain in the water and the hydrophobic chain in the air and both of them perpendicular to the surface. We also determined the surface tension as a function of the degree of neutralization, R, at c ) 0.5 wt % (i.e., above the cmc of the block copolymer) (Figure 4). The results are very surprising. At 10% neutralization, the surface tension already shows an abrupt transition from 47 to 72 mN/m (corresponding to the
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surface tension of pure water) and remains constant for R values up to 1. This obviously shows that the ionic block copolymers with R > 0.1 do not adsorb at the aqueous surface although they form micelles in the bulk phase. Similar behavior was observed for the system PS-b-PSS by Matsuoka et al. and for the system PIp-h2-b-PSSNa by Kaewsaiha et al.16 The authors tried to attribute the unexpected and unusual behavior to the high charge density of the compound. However, a quantitative theory does not seem to be available at present. The addition of salt has no effect on the surface tension at R ) 0.5 (Figure 5). The surface tension measurements were carried out with the turbid solutions. The above results indicate that block copolymers with a polyelectrolyte chain do not adsorb at the water/air interface even for the case when the charge density along the chain is 10 times lower than for normal polyvinyl-type polyelectrolytes where every second carbon atom carries one net charge. The transition from adsorption at the air/water interface to nonadsorption as a function of the charge density is quite sharp, taking place within R ) 0-0.1. More experimental results are necessary to resolve this interesting phenomenon. SANS Measurements. In Figure 6, SANS scattering curves of a 1% solution in D2O are shown as a function of the degree of neutralization. The solutions contain no excess salt. The scattering intensity is given as a function of the scattering vector q ) 4π/λ sin(Θ/2) where Θ is the scattering angle and λ the wavelength of the neutrons. We use only part of the information from the curves to obtain aggregation numbers of the micelles. To obtain exact values of n, one must first fit the whole scattering function to the product of the structure factor, S(q), and the form factor, P(q), and then determine n from the q value at the maximum of S(q). The scattering curves look very complicated. In contrast to scattering curves from micellar solutions from ionic surfactants and also from block polymer-polyelectrolyte systems, the scattering curves show five maxima, three at low q between q ) 5 × 10-3 Å-1 and two at high q between q ) 5 × 10-2 and 1 × 10-1. The scattering curves are obviously generated from a highly ordered phase. In the analysis, we assume that the first three peaks come from the structure factor S(q) and that the second two peaks at high q come from the form factor P(a). In the first approximation, the q value at the maximum of S(q) is about the same as the q value where I(q) has a maximum. The mean distance between the micelles is then given by d ) 2π/ qmax, and the number density of the micelles is 1c ) d - 3. The aggregation number n of the micelles is then simply given by
cgNA n)1 cMWpolymer
(13)
Many SANS data have been evaluated in this way, and the difference between the exact method of evaluation and the approximate method was usually very small, on the order of a few percent. Table 1 contains the corresponding parameters. The intermicellar distance for R ) 0.2 is 104 nm, leading to a weight-average aggregation number of n ) 300. It is interesting that the scattering data can be compared to the cryo-TEM data that are discussed in the next paragraph. The intermicellar distance d ) 104 nm is directly visible in the TEM micrographs. See Figure 7c. Assuming that the core consists of PnBA with a bulk density of 1.09 g/cm3, we can calculate the radius of the core from the aggregation number (cf. eq 4), leading to a weightaverage radius of 11.4 nm. This value of the hydrophobic core of the micelles seems to be reflected in the scattering function at high q coming from the form factor P(q). Again, this value is directly seen from the cryo-TEM micrographs in Figure 7. The
scattering data and the TEM data are completely consistent. When increasing R to 0.5, the qmax values shift to slightly larger values. This is an indication that the aggregation number decreases, a result that is to be expected intuitively. However, no further increase is seen when the degree of neutralization is further increased. Finally, we point out that because of the high order in the system and, in particular, because of the visibility of the higher-order scattering peaks, the 1% solutions may actually be a liquid-crystalline cubic phase. The answer to this question must wait until a detailed analysis of the scattering data has been carried out. Cryo-TEM Microscopy. Of particular interest in the investigation was the question of whether the size and shape of the micelles in the samples change with increasing degree of neutralization, R. A series of micrographs with varying R and concentration are shown in Figure 7. The micrograph in Figure 7a was obtained from the turbid solutions of the acidic form of the compound. One clearly sees branched wormlike micelles that have a diameter of about 40 nm. The wormlike micelles are not evenly distributed over the whole area but are clustered in domains with a high concentration of micelles while most of the volume is free of micelles. This is a clear sign of the attractive interaction between the micelles and evidence for the beginning of phase separation in the samples due to the low hydrophilicity of protonated PAA. These results thus clearly explain the turbidity of the phases. Phases with evenly distributed wormlike micelles would probably be clear. The details of the micrograph show that some of the end caps of the wormlike micelles are enlarged with respect to the diameter of the worms, a result that has been observed for wormlike micelles from surfactants, which is theoretically to be expected. The micrographs in Figure 7b-d with R values from 0.25 to 1 show spherical micelles. (Note that for Figure 7b the polymer concentration is 1%.) The spherical micelles are polydisperse, and the number density of the micelles seems to be the same for all degrees of neutralization. The micrographs clearly show that there is no further transition of the shape of the micelles for R > 0.1, even though the viscosity changes markedly with R at this concentration (see below). When micrographs are prepared from a single preparation, we should keep in mind that the micrographs might not look exactly the same because the film thickness in different micrographs might vary and hence the number of micelles for the same area can differ considerably. It is obvious that the film thickness in Figure 7b is somewhat higher than in the other three shown micrographs because some micelle pairs that are located on top of each other can be distinguished. These micelles might not be in contact with each other because the micrograph shows a 2D projection of a 3D situation. At first glance, one notices that the micelles in the micrographs from Figure 7b-d do not seem to have a random distribution but are quite ordered as expected from the mutual repulsion of their charges. The order of the micelles seems to increase with increasing R. However, on closer inspection one sees not only that the order is increasing with R but also that the arrangement of the micelles seems to change with R. One can identify strings with at least 6-8 globular micelles in Figure 7b. This means that there seems to be some attraction between the micelles even though they are charged. It is conceivable that the charges on the multimers are not evenly distributed but are more concentrated at both ends of the strings. The large number of micelles in Figure 7b demonstrates that the micrograph was taken from a rather thick film. However, it is not clear why this necklace-type arrangement of micelles is formed. The multimers do not seem to be responsible
Rheology of PnBA-b-PAA in Aqueous Solution
for the increased viscosity of the samples, and the viscosity has its highest value at R ) 0.5. As will be shown, the high order of the system is fully developed, and the stringlike aggregates have disappeared completely. In some of the micrographs, the micelles are extremely well ordered whereas in others the order is not as well expressed. We shall discuss such a highly ordered micrograph (Figure 8). In Figure 8, a cryo-TEM micrograph from a 0.5% block copolymer solution is shown with R ) 0.5. The solution contains no excess salt. The micrograph shows highly ordered micelles that have a large polydispersity (diameter of core ) 20-45 nm). Obviously, only the dense core of the micelles is clearly visible whereas the polyacrylate corona of the micelles is only faintly visible. The reason for this is mainly the low volume fraction of the polyelectrolyte in the shell (straightforward, simple calculations assuming fully stretched chains yield values no higher than 17%); therefore, the contrast between the surrounding water and the shell is very small. With this assumption, the largest diameter of the micelles that are shown is not much smaller than the diameter that is expected from the length of the fully stretched chain of the hydrophobic block. A vast majority of the micelles, however, are considerably smaller than this value. The discussed model therefore yields approximative values only for the size, aggregation number, and number density of the micelles. The predicted intermicellar distance is 276 nm for 0.5 wt % solutions, which is much higher than the values from SANS (see below) and, much more directly, from cryo-TEM. The micrographs show that the mean distance between the micelles is about 100 nm. Within experimental error, this dimension is the same as the mean distance that is determined from the correlation peak from SANS experiments. This good agreement between results from the bulk solution with results from the thin film makes it likely that the film from which the micrograph is shown has a thickness that corresponds to the diameter of a whole micelle including its corona, that is, about 100 nm. The high order that is visible is an indication that there are particles only in a single monolayer. These results and conclusions from the micrograph support the experimental results for thin films from micellar solutions that have shown that the thickness of such films decreases in a stepwise manner with time in which each step corresponds to a monolayer of a micellar solution. Intermediate thicknesses are not stable for longer times.39 The high order in the film is a result of the electrostatic repulsion between the micelles. Including the corona of the micelles with a thickness of about 35 nm for the stretched acrylate layer leads to a full diameter for the whole micelle of about 120 nm according to our model, where the core radius equals the contour length of the hydrophobic block. Again, we see the limited applicability of the model when looking at the volume fraction of the micelles and comparing the results from our model to cryo-TEM results. Micrographs show that the intermicellar distance is very similar to the micellar diameter as predicted by the model. Indeed, the model predicts diameters that are larger than the measured distances. This would imply a dense packing of spheres, which contrasts strongly with the calculated volume fraction of 4.75%, whereas a dense packing has a volume fraction of ca. 74%. Because the viscosity of a 0.5% solution is 35 mPas, the volume fraction must be higher than 4.75%. The high order of the micelles in the thin aqueous film is lost as soon as even small amounts of excess salt are added to the micellar solution (Figure 9). The solution from which the micrograph in Figure 9 was obtained had the same concentration as the solution from which the micrograph in Figure 8 was (39) Nikolov, A. D.; Wasan, D. T. J. Colloid Interface Sci. 1989, 133, 1-12.
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Figure 10. Zero-shear viscosity of a 0.5% solution of PnBA100PAA150 as a function of R.
Figure 11. Rheogram of a 1% solution of PnBA100-PAA150 deprotonated to an R value of 0.5.
obtained, but with 2.5 mM NaCl added. The size of the micelles is about the same, but the high order of the micelles is lost. The disappearance of the order is again due to the fact that the thickness of the palisade layer is reduced by the beginning of the coiling of the acrylate but also by the shielding of the total charge of the micelles by excess salt. It is interesting that these systems are doubly influenced by excess salt, and for this reason, the viscosity of the system is very sensitive to excess salt. Rheological Measurements. The viscosity of the block polyelectrolyte solutions depends on the polymer concentration and the degree of neutralization. Surprisingly, the zero-shear solution viscosity passes through a maximum at R ) 0.5 (Figure 10). This effect starts being visible at 0.3 wt %. For polymer concentrations higher than 1%, the half-neutralized solution forms a viscoelastic gel. In Figure 11, a rheogram of a viscoelastic sample with c ) 1 wt % and R ) 0.5 is shown. Both the storage and the loss moduli are frequency-independent over four orders of frequency, and G′ is about 1 order of magnitude larger than G′′. These properties are typical features of gels with a yield stress value. Actually, small bubbles that are dispersed in the system do not rise to the top of the sample. These samples must have a yield stress value (Figure 3b). A quantitative value of the yield stress can be obtained by measuring the deformation of samples as a function of the shear stress. Rheological experiments show that up to 6 Pa the samples are only elastically deformed whereas for stresses >6 Pa the samples begin to flow; the deformation becomes time-dependent (data not shown). These few results demonstrate clearly that the block copolymers can be used as thickeners in aqueous formulations. Even for concentration as low as 1% the block copolymers form weak gels. It is noteworthy to mention that the gel-like properties are not produced by a 3D network
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Figure 12. Zero-shear viscosity for a 0.5% solution of PnBA100PAA150 deprotonated to an R value of 0.5 as a function of added salt.
that is chemically or physically cross linked but simply by the packing of repulsive spheres. If one wishes to increase the efficiency of the system, that is, to lower the concentration at which gels are formed, one has to increase the length of polyelectrolyte chains. Preliminary data from our group indeed show that PnBA90-PAA100 remains fluidic at 50 or 100% neutralization without added salt and at 1 wt % concentration. However, PnBA90-PAA300 is clearly more viscous than a solution of PnBA100-PAA150 under the same conditions. Even if these observations are qualitative, they support the above conclusions. One should note, however, that the systems can be used only as thickeners in salt-free systems. In the presence of salt, the thickness of the electrical double layer becomes smaller. The extended polyelectrolyte chains coil, and the dense packing is lost. The consequence is a decrease in the viscosity and a breakdown of the viscoelastic properties. (Figure 12). The increase in viscosity in Figure 10 could be a sphere-to-rod transition of the micelles, as often observed for ionic surfactants. However, this is not the case because we do not find any rodlike micelles in the cryo-TEM images. Moreover, for such a situation one would expect a monotonic increase in viscosity up to R ) 1. However, the shape of the micelles does not change with the degree of neutralization. The maximum in the viscosity seems to be related to the morphological changes observed by cryo-TEM and appears to be completely electrostatic in nature. The phenomenon could perhaps be attributed to the hypothesis that latex particles with line charges attract each other. This assumption is in clear contradiction to the obtained TEM micrographs. They show no signs of existing attractions between the micelles. They keep a well-ordered state between R ) 0.5 and 1, and no voids are visible. Even though we do not have a quantitative theory for the maximum, we propose the following explanation. Theoretical considerations and the data from SANS suggest that the polyacrylate chains on the micelles become more and more stretched with increasing degree of neutralization. The conductivity data show that simultaneously with increasing R more and more counterions are released from the corona to the outside bulk phase and increase the ionic strength in the bulk phase. We assume therefore that the polyacrylate chains have already reached their most stretched configuration at R ) 0.5. For R > 0.5, the ionic strength is increasing, and the thickness of the electrical double layer is therefore decreasing. This means that the diameter of the micelles, including the thickness of the double layer, must therefore become smaller again. As for any densely packed systems, the viscosity must therefore decrease.
Figure 13. Schematic presentation of the dense packing of block copolyelectrolyte micelles.
Figure 14. Conductivity vs R for a 0.5% solution of PnBA100PAA150.
The previous arguments have shown that a 1% solution is in the range of a densely packed system of charged spheres(Figure 13). Conductivity Measurements. In Figure 14, the conductivity of a 0.5% block copolymer solution is plotted against the degree of neutralization R. The conductivity increases linearly with R. This is an interesting result. One could have imagined that the conductivity first increases linearly with R as long as the charge density along the polyelectrolyte chain is low enough for counterion condensation not to occur. At some critical R value, counterion condensation should theoretically set in, and a moderate break in the conductivity versus c(NaOH) curve should be seen that separates a linear region with a somewhat higher slope at lower concentration from another linear region with a somewhat smaller slope at higher concentration. Maybe the slight bulge in Figure 14 at around 15 mM NaOH could be interpreted in that manner. It should also be stated that the conductivity for the titration of an equimolar acrylic acid solution increases much more quickly than for the titration of the block copolymer.
Rheology of PnBA-b-PAA in Aqueous Solution
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Figure 15. Surface tension for (1) 0.5% PnBA100-PAA150, R ) 0.5 with surfactant; (2) TTAB without block copolymer in water; and (3) 0.5% PnBA100-PAA150, R ) 0.5, 10 mM NaCl with surfactant as a function of added surfactant concentration.
This indicates that the block copolymer micelles entrap more counterions than the polyacrylate on its own. In any case, a vast majority of the counterions are confined in the corona and are restricted in mobility, but they do not seem to condense.40,41 A polyelectrolyte chain at the core/corona interface has an available area of about 6 nm2, assuming that the values in Table 1 describe the system reasonably well. This area is more than 10 times the area of an alkylcarboxylate/surfactant on a normal micelle. The conductivity of a 0.5% block copolymer solution at R ) 1 is about 4 mS/cm (Figure 14), which is only about 1/8 of the conductivity of an equimolar sodium acrylate solution. If we assume that in both cases the contribution of the negatively charged ions contributes about the same fraction to the total conductivity, then we find that about 13% of the counterions of the micelles are outside of the corona and can diffuse freely. The area per net charge of the micelle calculated at the interface of the corona and the surrounding water is 5 nm2, which is not far away from the charge density in a “normal” surfactant micelle. This is not surprising because electrostatic interactions in aqueous solutions make electrostatic potentials (which are connected to the surface charge density) of more than kBT/eo ≈ 25 mV quite impossible. This also means that the potential of the block copolymer micelles would be the same as for normal micelles. Interaction of Cationic Surfactants with Block Copolymer Micelles. Because the nonionized hydrophilic block copolymers are surface-active and the partially ionized ones are not, it was of interest to study the interaction with low-molecular-weight cationic surfactants. For this purpose, tetradecyltrimethylammonium bromide (TTAB) was chosen. In Figure 15, surface tension measurements of a solution of 0.5% block copolymer at R ) 0.5 are shown as a function of TTAB concentration. The original solution was clear and viscous. With increasing TTAB concentration, the surface tension first remains constant (i.e., close to that of water), which is much higher than the value of a solution of TTAB at the same concentration, but without the polymer. Only when the TTAB concentration reaches ca. 0.3 mM. (i.e., ca. 1 mol % of the concentration of AA monomer units) does the surface tension decrease. After the addition of >5 mM of the surfactant (ca. 15% of AA units or 30% of the neutralized units), the solutions become turbid and separate into two phases. These measurements clearly demonstrate that TTAB binds to the ionic corona of the block copolymer micelles. (40) Jusufi, A.; Likos, C. N.; Lo¨wen, H. J. Chem. Phys. 2002, 116, 11011. (41) Plamper, F.; Becker, H.; Lanzendo¨rfer, M.; Pate, M.; Wittemann, A.; Ballauff, M.; Mu¨ller, A. H. E. Macromol. Chem. Phys. 2005, 206, 1813-1825.
Figure 16. Cryo-TEM micrograph of a 1% polymer solution neutralized to R ) 0.5 + 15 mM TTABr
The surfactant molecules do not seem to bind in a statistical manner to the ionic block copolymers but rather in a cooperative way. Small amounts of TTAB bind to individual compounds or micelles and completely compensate for the ionic charges on these molecules or micelles while the rest of the compounds are still charged. This can be concluded from the fact that the solution becomes turbid much below the point of zero charge of the whole system (i.e., 15 mM). It is already known that for polyelectrolytes and oppositely charged surfactants phase separation occurs only after the ratio of positive to negative charges, Z+/Z-, exceeds a critical value31,33,34, that is much smaller than 1. The same is also observed for the interaction of block copolyelectrolytes with oppositely charged polyelectrolytes.25,37 For ionic surfactants and oppositely charged low-molecular-weight surfactants, some systems aggregate to vesicles near the electroneutrality point, and no phase separation occurs. In our system, for 0.5 wt % polymer and for R values between 0.2 and 0.5, phase separation begins at Z+/Z) 0.3, in contrast to the results obtained by Thalberg.31 In case of a polyelectrolyte, the addition of salt up to 200 mM does not suppress precipitation. (In Thalberg’s studied system of sodium polyacrylate and alkyltrimethylammonium bromide, the addition of salt has suppressed the phase separation.) A very sharp decrease in viscosity is observed after the addition of only a 0.1 mM concentration (Z+/Z-) 0,007) of the cationic surfactant to the 50% neutralized polymer solution. This decrease does not seem to influence the bound surfactants but rather the release of excess salt by the binding of the surfactant. The micelles formed in the presence of TTAB look very different from those seen before (Figure 16). It seems that the core of these micelles is covered with smaller particles, presumably TTAB micelles that are decorated with polyacrylate. These micelles are considerably smaller than the core of the large PnBA-b-PAA-micelles.
Conclusions Solutions of the block copolymer PnBA-b-PAA with a degree of neutralization R > 0.1 are soluble in water and form globular micelles with a hydrophobic core and a tethered corona from the polyelectrolyte block. The core of the micelles is in the liquid state at room temperature. (Tg was measured in the bulk by DSC for the PnBA block: Tg ) -55 °C.) As a consequence of their high charge density, the micelles are strongly ordered in the bulk phase and show a correlation peak in SANS measurements. In thin films with a thickness that
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is similar to the total diameter of the micelles, a 2D arrangement is observed with the same average intermicellar distance as in the bulk phase. As a consequence of the stretched polyelectrolyte chains, the total volume of the micelles is about 40 times larger than the real volume of the block copolymer in the micelle, and on increasing the concentration, the micelles become densely packed at about 1% w/w. For this reason, the solutions become highly viscoelastic and have a yield stress value. At lower concentrations, they are viscous only with a maximum of the zero-shear viscosity at R ) 0.5. The viscosity maximum can be explained by the changing conformation of the polyelectrolyte chain and the electrical double layer of the micelles. With increasing R, the chains begin to stretch and have reached their most extended, fully stretched conformation at R ≈ 0.5. On further neutralization, the counterion concentration in the bulk phase outside of the corona increases, and this lowers the thickness of the diffuse double layer. The strong repulsion between the micelles is therefore reduced, and the viscosity decreases. Conductivity data show that only a fraction of the total counterions are in the free state and contribute to the conductivity. Most of the counterions are trapped in the corona of the micelles and do not contribute to the conductivity. Surprisingly, the block copolymers are not surface-active and do not lower the surface tension even though they form micelles
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in the bulk phase under the same concentration. The solution in this sense behaves the same as solutions of PS-b-PSS that have been studied by Matsuoka et al. On increasing R, the solution shows an abrupt transition in surface activity from surface-active to non-surface-active at R ≈ 0.1. The block copolymers bind cationic surfactants and form mixed micelles. In cryo-TEM micrographs, these micelles have a raspberry-like shape. We conclude that these micelles have a hydrophobic core of n-BA with small micelles from the surfactant attached to the surface. The polyelectrolyte chains are wrapped around the surface of the small surfactant micelles. Acknowledgment. This work was financially supported by a grant from Bayer Material Science. We express our gratitude to the company for its support. H.H. thanks Professor Hideki Matsuoka for a valuable and stimulating discussion on block copolymers with a polyelectrolyte block. Supporting Information Available: Synthesis details of the PnBA100-Br macroinitiator (OC05) and PnBA100-PtBA150 diblock (OC35) and hydrolysis of the PnBA100-PtBA150 diblock to PnBA100-PAA150 (OC37). This material is available free of charge via the Internet at http://pubs.acs.org. LA053272U
