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Langmuir 1999, 15, 4026-4032
Novel Modified Polymers with Permanent Cationic Groups† W. Jaeger,* U. Wendler, A. Lieske, and J. Bohrisch Fraunhofer-Institut fu¨ r Angewandte Polymerforschung, Kantstrasse 55, 14513 Teltow, Germany Received August 28, 1998. In Final Form: December 10, 1998 Several new polyelectrolytes with regular structure are introduced: block copolymers containing poly(ethylene glycol) and cationic polysoap blocks, amphiphilic block copolymers containing cationic and strong hydrophobic blocks, and polycarboxybetaines with narrow molecular weight distribution. Determination of the molecular weight of the polycarboxybetaines could be performed by SEC. As revealed by viscometry and potentiometric titration, the solution properties of the polyzwitterions depend to a high extent on the steric hindrance of the N+ moiety. Viscometry as well as measurement of the micelle polarity and of the solubilization capacity proved both intra- and intermolecular aggregation to take place with the polysoap block copolymers. Investigations of the micellization of the block copolymers containing strong hydrophobic blocks were followed by application of these polymers as stabilizers in the emulsion polymerization.
Introduction Polyelectrolytes are of continuously growing interest due to their many applications in both industrial processes and daily life. The limited number of cationic or anionic groups can be attached to practically any known polymer backbone. Additionally, the polymer structure can be further varied by the synthesis of copolymers containing different amounts of ionic and nonionic monomeric units. Therefore, a large number of different polyelectrolyte structures with broad variability of their properties are known.1 The great majority of these polymers is characterized by a statistical distribution of molecular and electrochemical parameters. However, polymers with welldefined molecular parameters and architecture are required2 for investigations of the interaction of polyelectrolytes with oppositely charged macroions, colloidal particles, and macroscopic solids. These investigations serve as model experiments for biological as well as technical processes. Polymers of interest are ionically charged macromolecules with regular structure, e.g. cationic block copolymers and polycarboxybetaines, due to their formation of supramolecular structures and their emulsifying and surface stabilizing properties (block copolymers) and due to their relationship to proteins and living matter (polycarboxybetaines). Cationic block copolymers are much less described than the corresponding anionic structures. Only a few papers deal with their synthesis and properties.3-9 Polymeric betaines containing an equal number of positively and negatively charged sites † Presented at Polyelectrolytes ’98, Inuyama, Japan, May 31June 3, 1998.
(1) Dautzenberg, H.; Jaeger, W.; Ko¨tz, J.; Philipp, B.; Seidel, Ch.; Stscherbina, D. Polyelectrolytes-Formation, Characterization, Application; Carl Hanser Publ.: Munich, Germany, 1994, pp 11-65. (2) Reference 1, pp 65-66, 166-167, and 328-332. (3) Selb, J.; Gallot, Y. Macromol. Chem. 1981, 182, 1513. (4) Oh, J. M.; Lee, H. J.; Shim, H. K.; Choi, S. K. Polym. Bull. 1994, 32, 149. (5) Lilt, M. H.; Lin, S. J. Polym. Sci., Part A: Polym. Chem. 1992, 30, 779. (6) Lieske, A.; Jaeger, W. Macromol. Chem. Phys. 1998, 199, 255. (7) Wendler, U.; Bohrisch, J.; Jaeger, W.; Rother, G.; Dautzenberg, H. Macromol. Rapid Commun. 1998, 19, 185. (8) Creutz, S.; et al. Macromolecules 1997, 30, 1-5, 6-9, 5596-5601. (9) Baines, F. L., et al. Macromolecules 1996, 29, 3096-3102, 34163420, 8151-8159.
in strictly equal concentration on each monomer unit are often discussed,10 but the great majority of these polyzwitterions are polymeric sulfobetaines. The number of publications referring to polycarboxybetaines is considerably lower.10-12 In this paper we introduce some new amphiphilic cationic block copolymers as well as several polycarboxybetaines. The synthesis of the latter was carried out via controlled free radical polymerization of reactive monomers. The resulting reactive precursor polymers with narrow molecular weight distribution could readily be modified leading to polycarboxybetaines with a narrow molecular weight distribution. Block copolymers containing cationic and uncharged hydrophobic blocks can also be obtained by this technique. Additionally, block copolymers containing uncharged hydrophilic and cationic polysoap blocks representing a new group of micellar polymers were prepared for the first time. Some typical properties of these different polymers will be discussed. Experimental Section Materials. Vinylbenzyl chloride (dow chemical) was purified by washing with aqueous NaOH and water and drying. Styrene (Merck) and 4-vinylpyridine (Aldrich) were distilled under reduced pressure prior to use. Dibenzoyl peroxide (BPO) was purified by reprecipitation from chloroform/methanol. V50 (2,2′azobis(2-methylpropanamidine)dihydrochloride, Wako), poly(ethylene glycol) (Fluka), diallyldimethylammonium chloride (5) (Aldrich), and all other products were used without further purification. Synthesis. Controlled free radical polymerization of vinylbenzyl chloride7 and 4-vinylpyridine14 resulted in reactive polymers with narrow molecular weight distribution, which were further modified. Syntheses of polycarboxybetaines 1 and 2 were performed as described elsewhere (1 in ref 7 and 2 in ref 14; Scheme 1). The (10) Galin, J. C. Polyzwitterions. In Polymeric Materials Encyclopedia; Ed. Salomone, J. C., Ed.; CRC Press: Boca Raton, FL, 1996; Vol. 9, p 7189. (11) Liaw, D.-J.; Huang, C.-C.; Lee, W.-F.; Borbely, J.; Kang, E.-T. J. Polym. Sci., Polym. Chem. 1997, 35, 3527. (12) Liaw, D.-J.; Huang, C.-C. Polymer 1997, 38, 6355. (13) Kathmann, E. E. L.; White, L. A.; McCormick, Ch. L. Macromolecules 1997, 30, 5297. (14) Bohrisch, J.; Wendler, U.; Jaeger, W. Rapid Commun. 1997, 18, 975.
10.1021/la981118k CCC: $18.00 © 1999 American Chemical Society Published on Web 03/31/1999
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Scheme 1. Synthesized Polycarboxybetaines from Poly(4-vinylpyridine) and Poly(vinylbenzyl chloride)
Scheme 2. Macroinitiated Polymerization of Reactive Surfactants: Synthesis of Block Copolymers Containing Polysoap Blocks
degree of functionalization was almost 100% (1H NMR analysis). Polycarboxybetaine 3 was obtained using narrowly distributed polyvinylbenzyl chloride.7 A total of 1.5 g of the polymer was stirred with 7.2 g (10 equiv) of diethylamine in 30 mL of dry acetone for 40 h at room temperature. Evaporation of the liquid phase yielded a slightly yellow resin. This was dissolved in 30 mL of dry methanol and stirred for 90 h with 16.7 g (10 equiv) of bromoethyl acetate at room temperature. Evaporation of the solvent, adding 50 mL of water, extraction of the bromoethyl acetate with 3 × 10 mL of chloroform, ultrafiltration (Omega open channel membrane, 1 kD), and freeze-drying yielded intermediate polyelectrolytes. Ester hydrolysis in aqueous NaOH (pH 10), ultrafiltration, and freeze-drying afforded 3 in a total yield of 85%. Polymers with molecular weights (Mn, calculated from precursor polymers for complete conversion) of 9700 (3a), 18 500 (3b), 29 000 (3c), and 58 000 (3d) were obtained. Synthesis of polymerizable surfactants 4a,b was performed as described by Yang et al.15 Block copolymers containing poly(ethylene glycol) and polysoap blocks [PEG-poly(4a-co-5) and PEG-poly(4b-co-5)] were synthesized by macroinitiated copolymerization of 4a or 4b and 5 according to Scheme 2. Experimental conditions followed the data given for the polymerization of 5.6 Polysoaps [poly(4a-co-5) and (poly(4b-co-5)] were prepared by copolymerization of 4a or 4b and 5 in water solution using V50 as initiator following the procedure described for the homopolymerization of 5.16 (15) Yang, Y. J.; Engberts, J. B. F. N. J. Org. Chem. 1991, 56, 4300. (16) Dautzenberg, H.; Go¨rnitz, E.; Jaeger, W. Macromol. Chem. Phys., in press.
Scheme 3: Synthesis of Amphiphilic Block Copolymers by Controlled Free Radical Polymerization Followed by Polymer Analogous Reaction
Cationic amphiphilic block copolymers 6 were obtained by functionalization of poly[(vinylbenzyl chloride)-block-styrene], the latter being synthesized by controlled free radical polymerization7 (Scheme 3). Again 100% conversion was observed. Emulsion Polymerization. A 10.4 g amount of freshly distilled styrene, 90.0 g of water, and the chosen amount of the cationic block copolymer 6 as stabilizer (concentration mol/L corresponds to the number average of the molecular mass of the block copolymer) were placed in a batch reactor, purged with argon for 16 h, and then thermostated at 70 °C. The stirrer speed was maintained at 400 rpm. A 0.05 mol/L concentration of H2O2 was added to the mixture. The polymerization time was 6 h. The resulting latices with a solid content of approximately 10 wt % were purified by dialysis against pure water (Serva membrane) and by ultrafiltration (Berghof cell, membrane UF BM 10009) until a constant low conductivity of the filtrate was obtained. Surface Charge Density. Investigations of the surface charge density of the diluted latices (concentration 0.1-10 g/L, pH 6-7, salt free) were performed by measuring the streaming potential in a particle charge detector (PCD-03, Mu¨tek) during titration with a 10-4 M aqueous solution of an anionic titrant (potassium poly(vinyl sulfonate), sodium dodecyl sulfate) as described by Paulke et al.17
4028 Langmuir, Vol. 15, No. 12, 1999 Particle Size of Latices and Swelling Behavior of Micelles. The particle size of the polystyrene latices was determined by photon correlation spectroscopy (Nicomp C370 particle sizer). The swelling behavior of the micelles was recorded with the same equipment after stirring for 16 h at room temperature of a mixture of stabilizer and styrene according to the emulsion polymerization recipe. Static Light Scattering. Static light scattering measurements (Sofica; Wippler & Scheibling (France)) of the amphiphilic cationic block copolymers were performed in 0.1 M NaCl (dn/dc ) 0.200). UV-Vis Spectra of Methyl Orange. UV-vis spectra of methyl orange (2.5 × 10-5 mol/L) in a sodium borate buffer (pH 9.55) in the presence of 0.5 wt % of the polymers were recorded on an Uvikon 930 spectrophotometer. Before measurement the polymer containing solutions were shaken overnight at 25 °C. Turbidity Measurement. Aqueous solutions containing 0.5 wt % of the polymers were shaken overnight. A 100 mL volume of the solution was titrated stepwise with n-decanol. Each step was followed by intensive mixing (5 min of ultrasound). The transmission of the solution was recorded (Monitek CT4, λ ) 624 nm). The solubilization limit is supposed to be the point where the transmission sharply decreases. The value is corrected by the solubility of n-decanol in water (3.6 mg/L). SEC. Molecular weight and polydispersity of the polycarboxybetaines 1 were determined by SEC (Spectra Physics chromatograph equipped with multiangle light scattering (Wyatt), refractive index detection (Viscotek) on a column combination (Tskgel 6, 17µm, 6000, 5000, 3000 PW, Pssgel Hemabio 40, 10 µm; eluent, 10 g/L acetic acid, 0.2 M sodium sulfate in water). Viscometry. Viscosity measurements were performed using a viscosimeter with automatic dilution (Viscoboy 2; Lauda) at 30 °C. Titration of Carboxylic Groups. The acid-base titrations were carried out with a Titroprocessor 670 and a Dosimat 665 (automatic mode). Solutions containing 0.1 g of polymer in 50 mL of deionized water or 0.5 M NaCl were equilibrated at pH ) 2.5 (0.1 M HCl) and then titrated at 25 or 50 °C with 0.1 M NaOH.
Results and Discussion 1. Polycarboxybetaines. It is known from the literature that solution properties of polycarboxybetaines are essentially determined by the distance between, and the chemical environment of, the opposite charges. In recent publications solution properties of acrylatebased polycarboxybetaines were investigated. Viscosity measurements for “diluted” (statistical copolymer)13,18 and “pure” 11,13 polycarboxybetaines with 2 or 3 methylene moieties between N+ and COO- were carried out in different salt solutions (different anions and monovalent or divalent cations) or at different pH. In general additional anions show stronger effects than cations. As expected, an “anti polyelectrolyte effect” was found above pH 3 caused by increasing deprotonation of the carboxylic groups (pKa ≈ 3-4). We aimed at investigating the differences in solution behavior of three polycarboxybetaines with one methylene group between the charges differing in their steric hindrance of the N+ moiety (Scheme 1). The polymers were synthesized in a two-step procedure. At first reactive precursor polymers were obtained by controlled free radical polymerization of 4-vinylpyridine14 or vinylbenzyl chloride.7 According to 1H NMR the subsequent modification reactions were run with 85-100% conversion, leading to polyzwitterions 1-3 with comparatively narrow molecular weight distribution resulting from starting reactive polymers. The intramolecular dipolar interaction in solu(17) Paulke, B. R.; Mo¨glich, P. M.; Knippel, E.; Budde, A.; Nitzsche, R.; Mu¨ller, R. H. Langmuir 1995, 11, 70. (18) Vamvakaki, M.; Billingham, N. C.; Armes, S. P. Polymer 1998, 39, 2331.
Jaeger et al.
Figure 1. Viscometry of polycarboxybetaines 1-3 in water. Table 1. Titration of the Carboxylic Groups of the Polycarboxybetaines titration of COOH groups in % sample
N
water
0.5 mol/L NaCl
0.5 mol/L NaCl at 50 °C
1a 1b 1c 2 3a 3b
82 146 202 135 38 72
7 6 6 18 60 47
14 12 11 25 74 59
15 15 14 27 66 47
Table 2. SEC Measurement of Polycarboxybetaines 1 polycarboxybetaine sample 1a 1b 1c
precursor N Mw/Mn 82 146 202
1.50 1.38 1.40
Mn calc (g/mol)
Mn found (g/mol)
Mw found (g/mol)
Mw/Mn
13 300 23 800 32 900
11 900 15 200 28 800
16 500 21 300 41 400
1.38 1.40 1.44
tion should decrease with increasing size of substituents in the order pyridinium < -N+(CH3)2 < -N+(C2H5)2. Measurement of the access of the carboxylic group by protons (equilibration with 0.1 mol/L HCl, titration with 0.1 mol/L NaOH) shows significant differences due to chemical structure (Scheme 1). In pure water only 6% of the carboxylic groups of carboxybetaine 1 (pyridinium structure) were found. Addition of NaCl resulted in an increase up to only 15%. No significant changes occurred at 50 °C. Measurement over several days showed no time dependence of the percentage of titratable carboxylic groups. This property is probably caused by the very strong interactions between anionic and the freely accessible cationic groups. Polycarboxybetaine 3 shows a completely different behavior. The larger ethyl groups are responsible for decreasing intramolecular interaction; a much higher proportion of acid groups is titratable (up to 75% in 0,5 mol/L NaCl) (Table 1). These results are in good agreement with the viscosity measurements. While polycarboxybetaine 3 shows a typical polyelectrolyte effect in water (increase of the reduced viscosity with decreasing polyelectrolyte concentration), polycarboxybetaine 1 as well as polymer 2 showed a nearly linear slope (Figure 1). Because of internal salt formation of the opposite charges, the sum of free charges of the polycarboxybetaine 1 and 2 is close to zero in water. In contrast polycarboxybetaine 3 has a noticeable number of free charges because of the hindered formation of an internal salt. Polycarboxybetaines as strongly bipolar structures show some difficulties in direct molecular weight determination
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Table 3. Intrinsic Viscosities [η] and Huggins and Schulz-Blaschke Constants kH and kSB of the Block Polymers Containing Polysoap Blocks composn (mol %)a
a
Huggins
Schulz-Blaschke
polymer
EG:4:5
4:5
[η]H (cm3‚g-1)
kH
[η]SB(cm3‚g-1)
kSB
poly(4a-co-5)c poly(4b-co-5)c PEG-poly(4a-co-5) 2c PEG-poly(4a-co-5) 3b PEG-poly(4a-co-5) 4b PEG-poly(4a-co-5) 5c PEG-poly(4b-co-5) 2b
0:41:59 0:12:88 60:16:24 62:5:33 58:11:31 51:19:30 64:7:29
41:59 12:88 40:60 13:87 26:74 39:61 19:81
17.2
2.04
45.8 57.7 48.5 40.5
0.81 0.44 0.67 1.14
17.9 27.8 46.5 58.9 49.5 42.12 25.4
1.32 3.04 0.56 0.31 0.47 0.70 1.49
Determined with 1H NMR. b In 1 M aqueous NaCl. c In 0.5 M aqueous NaCl.
via SEC. Recently Lowe et al.19 described SEC of poly(sulfopropyl)betaines using PEG/PEO standards in concentrated salt solution. Because of our strategy of synthesis (SEC characterization of the precursors), we are able to control the results from molecular weight measurement by SEC. As eluent, we chose aqueous salt solution (0.2 M Na2SO4) with a small amount of acetic acid (1 wt %). In our experience this organic solvent restricts the interaction between the charged polymer and the organic column material. Table 2 shows the results of the SEC measurement and allows the comparison with the predicted values, calculated for 100% conversion. There is a fairly good agreement between measured and calculated values. 2. Block Copolymers. Both water-soluble amphiphilic block copolymers20 as well as polysoaps21 are micelleforming polymers but with very different properties. The block copolymers form micelles above the critical micelle concentration (cmc). In contrast, polysoaps form micelles at any concentration due to intramolecular hydrophobic aggregation; a cmc is generally missed. Block copolymers containing polysoap blocks represent a novel type of micellar polymers. These structures can be synthesized by free radical polymerization of surface active monomers by means of a macro-azoinitiator containing poly(ethylene glycol) (PEG). Thus PEG is the second component of the block copolymer (Scheme 2). Polymerization of pure monomers 4a,b leads to water-insoluble polymers due to their hydrophobic conformation22 in water caused by the head structure. This finding is independent of whether a macroinitiator or a low molecular weight azoinitiator is used. Polymers with sufficient water solubility are available by copolymerization of 4a or 4b with the similarly structured hydrophilic monomer 5, which proceeds as an ideal copolymerization. The content of 5 must be at least 60 mol % for poly(4a-co-5) and 86 mol % for poly(4b-co-5). Viscosity measurement of the block copolymers in aqueous NaCl solution hint at their solution properties. Intrinsic viscosities [η] and the corresponding constants kH and kSB were taken from the linear Huggins and Schulz-Blaschke plots and are listed in Table 3. The composition of the block copolymers has a pronounced influence on the results. With block copolymers containing nearly the same amount of PEG kH as well as kSB increases and [η] decreases with increasing content of 4a. A decrease of the PEG content leads to increasing values of kH and decreasing values of [η] if the amount of 4a remains constant. Block copolymers containing 4b have smaller [η] and higher kSB than those containing 4a. The pure (19) Lowe, A. B.; Billingham, N. C.; Armes, S. P. Chem. Commun. 1996, 1555. (20) Lundsted, L. G.; Schmolka, I. R. In Block and Graft Copolymerisation; Ceresa, R. J., Ed.; Wiley: London, 1976; Vol. 2, p 113. (21) Laschewsky, A. Adv. Polym. Sci. 1995, 124, 1. (22) Ko¨berle, P.; Laschewsky, A.; van den Boogard, D. Polymer 1992, 33, 4029.
Figure 2. Solubilization capacity and maximum absorption of block copolymers and polysoaps. Table 4. Absorption Maxima of Methyl Orange in Aqueous Block Copolymer Solutions (pH 9.55) and Solubilization Capacity of the Block Copolymers for n-Decanol (25 °C)
polymer poly(4a-co-5) poly(4b-co-5) PEG-poly(4a-co-5) 2 PEG-poly(4a-co-5) 5 PEG-poly(4b-co-5) 1 PEG-poly(4b-co-5) 3 PEG-poly(5) a
composn (mol %)a EG:4:5 4:5 0:41:59 0:12:88 60:16:24 51:19:30 70:4:26 59:4:37 44:0:66
41:59 12:88 40:60 39:61 13:87 10:90
solubilized λmax amt of n-decanol (nm) (mol/mol of alkyl) 459 441 462 434 430 438 469
0.062 0.94 0.081 0.317 1.67 1.02 0
Determined with 1H NMR.
polysoaps containing no PEG fit well into this picture. Probably, the intramolecular aggregation of the polysoap block increases with decreasing content of PEG as well as rising content and length of the alkyl chain of monomer 4, leading to an increase of the coil density. Additionally some intermolecular aggregation may occur, caused by interaction of hydrophobic areas on the surface of the intramolecular micelles as a result of inadequate shielding of the alkyl chains within the micelles.23 The maximum absorption of the hydrophobic dye methyl orange at 465 nm in aqueous solution is shifted depending on the polarity of its environment. Binding of hydrophobic domains present in the solution leads to a hypsochromic shift of the absorption maximum. Polyelectrolytes forming no micelles result in a bathochromic shift due to electrostatic interaction of the charged parts of the molecules. Typical results of the UV-vis measurement depending on the composition of the block copolymers are summarized in (23) Strauss, U. P.; Gershfeld, N. L.; Crook, E. H. J. Phys. Chem. 1956, 60, 577.
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Table 5. Analytical Data for the Amphiphilic Cationic Block Copolymers 6 in 0.1 M NaCl before and after Tempering at 130 °C for 5 h micelles single molecule
before tempering
after tempering
sample
10-3Mw (g/mol)a
Mw/Mn
Nhydrophil
Nhydrophob
10-6Mw (g/mol)b
Z
10-6Mw (g/mol)b
Z
6a 6b 6c 6d 6e
11.0 14.1 16.3 23.3 25.2
1.25 1.23 1.22 1.29 1.31
27 27 27 63 63
31 57 75 47 57
1.65 6.45 21.33
150 460 1300
1.00 2.40 6.14
110 210 460
12.04
480
4.92
260
a
Calculated from SEC measurement7 of the precursor polymers for 100% conversion as revealed by 1H NMR. b Determined from static light scattering.
Table 4. With block copolymers containing 4a an amount of 40 mol % or more is necessary to observe a small hypsochromic shift, if the molar ratio of ethylene glycol (EG) to (4a + 5) is higher than 1:1. This shift corresponds to the value of the pure polysoap, indicating in both cases that an octyl chain15 normally does not establish a pronounced hydrophobic environment within the micelles. If the molar ratio EG to (4a + 5) equals 1:1 while the content of 4a in the polysoap block remains at about 40 mol % a strong hypsochromic shift is observed. Eventually, an integration of the PEG block into the micelles occurs. Due to the longer alkyl chain of 4b block copolymers containing this monomer in the polysoap block as well as the pure polysoaps have a more pronounced shift of the absorption maximum. As expected the polyelectrolyte PEG-poly(5) leads to a bathochromic shift. Measurement of the solubilization capacity of the block copolymers for n-decanol were carried out by means of determining the turbidity after stepwise addition of the alcohol to 0.5 wt % aqueous polymer solutions (Table 4). As expected the solubilization capacity increases with decreasing micelle polarity. Block copolymers containing 4a generally have low capacity. An exception is again the polymer with the 1:1 ratio of EG and cationic monomers corresponding to its lower micelle polarity. Polymers containing 4b solubilize much higher quantities, but this capacity depends markedly on their composition. Increasing content of PEG leads to clearly higher solubilization capacities. This finding cannot be solely explained by the limited change of the micelle polarity. We suppose that the block copolymers form aggregates with a core of intraand intermolecularly aggregated polysoap blocks bearing a PEG shell. The shell density increases with the length of the PEG blocks and with the length of the alkyl chain, due to the higher aggregation of the core in the latter case. Depending on its density, the shell contributes to the solubilization, as discussed for PEG-containing nonionic surfactants.24 An overview on the relationship between chemical structure of the copolymers and both absorption maxima and solubilization capacity is given in Figure 2. Controlled free radical polymerization using TEMPO as terminator is a simple and efficient method to synthesize polymers with narrow polydispersity as well as block copolymers with different block lengths and block length ratios.25 Using this technique, AB type block copolymers consisting of strongly hydrophilic quaternary ammonium groups and strongly hydrophobic polystyrene blocks with adjustable lengths and ratios of the two blocks are available.7 The reaction sequence using a poly(vinylbenzyl (24) Shinoda; Nakagawa; Tamamushi; Isemura. In Colloidal Surfactants; Academic Press: New York, 1963. (25) Controlled Radical Polymerization; Matyjaszewski, K., Ed.; ACS Symposium Series 685; American Chemical Society: Washington, DC, 1998; p 170.
Figure 3. Aggregation number depending on the length of the hydrophobic block of 6: influence of tempering.
Figure 4. Huggins plots of the block copolymer 6b in different solvents.
chloride) (p-VBC) macromonomer and styrene and the subsequent quantitative quaternization of the reactive p-VBC block leading to the desired amphiphilic cationic block copolymers is described in Scheme 3. The synthesized polymers are listed in Table 5. As discussed in a previous paper,7 these polymers are micellarly soluble in water and in organic solvents, where in 0.1 M NaCl solution the aggregation number Z, the sedimentation coefficient, and the radius of gyration increase with increasing length of the hydrophobic block. Usually the dependence of Z on the degree of polymerization N of the insoluble block of block copolymer micelles follows the equation Z ) kNinsolR with R e 2, as discussed by Fo¨rster et al.26 The linear plot in Figure 3 results in an exponent of R ) 2.3. Due to the
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Langmuir, Vol. 15, No. 12, 1999 4031
Table 6. Emulsion Polymerization of Styrene with Amphiphilic Cationic Block Copolymers 6 as Stabilizer surface charge density (µC/cm2) stabilizer
Nhydrophil
Nhydrophob
6a 6a 6b 6c 6c 6d 6e 6e CTMAC
27 27 27 27 27 63 63 63 1
31 31 57 75 75 47 57 57
cstabilizer (mol/L)
particle size (nm)
potassium poly(vinyl sulfonate)
sodium dodecyl sulfate
0.0010 0.0003 0.0003 0.0003 0.0010 0.0003 0.0003 0.0010 0.0010
86 130 171 222 103 116 121 102 151
27.0 18.6 33.8 30.9 39.3 29.2 30.2 99.0 4.2
29.6 20.4 38.7 40.7 50.9 33.9 40.8 161.0 4.0
comparatively high TG value of the water-insoluble polystyrene block (TG ∼ 100 °C for polystyrene) the micellization process normally is not in a thermodynamical equilibrium, resulting in so-called “frozen micelles” and leading to the mentioned exponent clearly above the suggested limit of 2.0. By tempering of the micellar system for 5 h at 130 °C and subsequent rapid cooling to room temperature a “frozen equilibrium” was established. Determination of the size of the micelles by static light scattering now leads to an R value of 1.5 (Figure 3) which is within the expected range of published data.27 Results of viscosity measurements of the block copolymer 6b are recorded in Figure 4. Measurement in pure water indicates a strong polyelectrolyte effect. Using the Fuoss equation 1/ηred ) 1/[η] + Kxcpolymer a value of 65.2 cm3/g for [η] (R ) 0.986) was determined. With 1 M aqueous NaCl as solvent [η] ) 7.6 cm3/g (R ) 0.999) results from a Huggins plot. We suggest these pronounced differences of the intrinsic viscosities are not exclusively caused by the extension of the comparatively low molecular ionic block (N ) 27) in pure water. Intermolecular interaction of the charged parts as discussed by Antonietti et al.28 may have a contribution. The block copolymers 6 are efficient stabilizers in the emulsion polymerization of styrene resulting in cationic dispersions extremely stable against dilution and at high ionic strength. Prior to the polymerization process the swelling behavior of the block copolymers in the presence of the monomeric styrene under conditions similar to the polymerization, but without initiator, was investigated. Despite the high TG of the micelle core a significant quantity of the monomer was able to diffuse into the micelle as measured by PCS. For example with block copolymer 6b the size of the micelle increases from 29 to 41 nm. The subsequent emulsion polymerization resulted in monomodal latices with standard deviations of the diameter of about 0.2 µm. H2O2 was used as free radical initiator, thus avoiding the incorporation of additional ionic charges into the latices by initiator fragments. As usual the particle size decreases with increasing amount of stabilizer (Table 6, Figure 5). Furthermore, an increase of the particle size with a rising number of styrene units in the block copolymer was observed. The surface charge density of latices stabilized by the block copolymers is much higher than the corresponding values obtained with the cationic low molecular surfactant cetyltrimethylammonium chloride (CTAC) (Table 6). In general, the charge density increases with increasing stabilizer concentration but different numerical values were obtained with different titrants. Titration (26) Fo¨rster, S.; Zisenis, M.; Wenz, E.; Antonietti M. J. Chem. Phys. 1996, 104, 9956. (27) Antonietti, M.; Fo¨rster, S.; Oesterreich, S. Macromol. Symp. 1997, 121, 75. (28) Antonietti, M.; Briel, A.; Fo¨rster, S. J. Chem. Phys. 1996, 105, 7795.
Figure 5. Emulsion polymerization of styrene with block copolymers 6 as stabilizer and dependence of the particle size on the length of the hydrophobic block. Chart 1. Model of Highly Charged Lattices Stabilized with Amphiphilic Cationic Block Copolymers, “Corona Latex”
with the polyelectrolyte potassium poly(vinyl sulfonate) indicates an apparently lower charge density than titration with the low molecular surfactant sodium dodecyl sulfate. This holds only in the case of block copolymer stabilized latices. Latices stabilized with CTAC gave similar charge densities with both titrants. Therefore we suggest the structure given in Chart 1 for the block copolymer stabilized latices. The styrene blocks of the stabilizer are incorporated into the hard polystyrene ball, and the watersoluble cationic blocks form a corona in the dispersion medium. Titration with an anionic polyelectrolyte leads to insoluble polyelectrolyte complexes with the outer part of the cationic blocks. The access to charges closer to the core is blocked. Due to this shielding a lower value of the surface charge density is determined. In contrast, the low molecular weight anionic surfactant is more mobile and able to interact with all countercharges, thus resulting in higher and more realistic values.
4032 Langmuir, Vol. 15, No. 12, 1999
Conclusions Cationic block copolymers containing strong hydrophobic blocks as well as polycarboxybetaines are available by functionalization of reactive precursor polymers from controlled radical polymerization processes. The application of the amphiphilic block copolymers as stabilizers in the emulsion polymerization leads to latices with high cationic surface charges. Solution properties of the polycarboxybetaines are determined by the steric hindrance of the N+ moiety. The possibility of SEC measurement of these polymers was demonstrated due to the good agreement of the data of a precursor polymer and of the polyzwitterion. Block copolymers containing poly(ethylene
Jaeger et al.
glycol) and cationic polysoap blocks were synthesized by macroinitiated polymerization. This new type of micellar polymer forms aggregates by both intra- and intermolecular aggregation. A strong influence of the chemical structure on the micellization behavior was observed. The new polymers can be the basis for the development of materials with high solubilization capacity. Acknowledgment. Financial support of the BASF AG as well as of the Deutsche Forschungsgemeinschaft (Grant JA 555/5-1) and of the Fonds der chemischen Industrie is gratefully acknowledged. LA981118K
