ARTICLE pubs.acs.org/IECR
Synthesis of Spherical Polyelectrolyte Brushes by Photoemulsion Polymerization with Different Photoinitiators Xiang Wang,† Shuang Wu,† Li Li,† Rui Zhang,† Yan Zhu,† Matthias Ballauff,‡ Yan Lu,‡ and Xuhong Guo*,† †
State-Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai, 200237, People's Republic of China ‡ Soft Matter and Functional Materials, Helmholtz-Zentrum Berlin f€ur Materialien und Energie GmbH, Berlin 14109, Germany ABSTRACT: Nanosized spherical polyelectrolyte brushes (SPBs) with tunable size and grafting density were synthesized successfully by photoemulsion polymerization using three photoinitiators. Due to their difference in structure, UV absorption, and mechanism of photoinitiation, the kinetics of photoemulsion polymerization using these respective photoinitiators are different. The grafting densities of poly(acrylic acid) (PAA) brushes varied from 0.012 to 0.081 nm-2 by using different photoinitiators, as determined by cutting off the PAA chains from the core particles. All of the spherical PAA brushes prepared by these three photoinitiators are pH and ionic strength sensitive as demonstrated by DLS measurements. Well-defined spherical morphology with narrow size distribution for all of the brushes prepared has been observed by FESEM. This work opens a new way to control the synthesis of SPBs with tunable structure for potential applications in catalysis, wastewater treatment, disease diagnosis, and protein immobilization.
’ INTRODUCTION If long linear polyelectrolyte chains are grafted densely to a solid surface, then polyelectrolytes brushes will be formed.1,2 The structure of polyelectrolyte brushes brings them unique surface properties, which has been well documented in several review articles.3,4 There are two main ways to prepare polyelectrolyte brushes on an interface or a surface: physical adsorption and chemical grafting.5 Chemical grafting can be divided into two techniques: (a) “Grafting to”, where the end-functionalized polymers were synthesized and reacted with appropriate groups immobilized on the substrate; (b) “Grafting from”, where the polymer layers are formed by in situ polymerization initiated by the immobilized initiators on the surface.2 The “grafting from” technique can prepare brushes with high grafting density and thus has been widely used.4 Ruehe and co-workers used a surface-attached monolayer of an azo initiator to start the free radical polymerization of a variety of monomers and obtained polymer brushes with high grafting densitities.6-11 Controlled radical polymerization was employed by several authors to synthesize spherical polymer brushes on the surface of polymer or inorganic particles.12-16 Recently, we reported a new method of photoemulsion polymerization to prepare spherical polyelectrolyte brushes (SPBs) with a solid polystyrene (PS) core and a dense PAA brush shell (Figure 1).17-19 As a “grafting from” technique, photoemulsion polymerization shows several advantages: (1) the synthesis can be easily controlled by turning on/off the UV light and adjusting the light power; (2) the obtained SPBs show a well-defined brush structure with high grafting density; and (3) the SPB size and grafting density can be modulated by the amount of monomer and photoinitiator, respectively. But our understanding of photoemulsion polymerization is still limited. In particular, the effect of photoinitiators on the photoemulsion polymerization and on the structure and properties of the SPBs remains unclear. r 2011 American Chemical Society
Figure 1. Schematic representation of the synthesis route of spherical polyelectrolyte brushes via photoemulsion polymerization.
In this work, two novel photoinitiators, 4-acryloxybenzophenone (ABP) and benzoin acrylate (BA) were synthesized. Their performances in preparation of SPBs by photoemulsion polymerization were compared with those of the previously reported photoinitiator 2-[p-(2-hydroxy-2-methyl-propio-phenone)]ethylene glycol-methacrylate (HMEM).12-14 The properties of the SPBs prepared by these three photoinitiators were compared and discussed in detail.
’ EXPERIMENTAL SECTION Materials. Styrene and acrylic acid (AA) were purchased from Sigma-Aldrich, and distilled under reduced pressure to remove the inhibitor before use. Potassium persulfate (KPS) (from Merck), sodium dodecyl sulfate (SDS, from Merck), 4-hydroxybenzophenone (from Sigma-Aldrich), acryloyl chloride (from TCI), triethylamine (from Fluka), benzoin (from Received: August 23, 2010 Accepted: January 14, 2011 Revised: January 4, 2011 Published: February 18, 2011 3564
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Figure 2. Synthesis of ABP and BA.
Sigma-Aldrich), dimethylaniline (from Fluka), 2-hydroxy-40 -hydroxyethoxy-2-methylpropiophenone (from Ciba with a commercial name of IRGACURE 2959), methacryloyl chloride (from TCI), pyridine (from Fluka) were used without further purification. The water used in this work was purified by reverse osmosis (Millipore). Synthesis of Photoinitiators. The synthetic procedure of HMEM was reported in our previous publications.10-12 Figure 2 shows the synthesis routes of ABP and BA. ABP was synthesized by a Schotten-Baumann reaction. In a typical run, 11.7 g (0.125 mol) acryloyl chloride was added to the mixture of 15 g (0.075 mol) 4-hydroxybenzophenone and 14.5 g triethylamine in 125 mL anhydrous diethyl ether solution. The addition rate was controlled to maintain a gentle reflux of the solvent. The reaction mixture was refluxed for 1 h, and then hydrolyzed with 0.1 M aqueous HCl after cooling. The organic layer was washed alternatively with 5% aqueous NaOH and water at least 5 times to remove the impurities. After being dried by anhydrous Na2SO4, the solvent was removed under vacuum to give a red crude solid. This crude product was dissolved in boiling ethanol and then filtered after treatment with activated charcoal. A white powder of ABP was obtained by recrystallizing the solution from ethanol at 0 °C. The total yield was about 70%. ABP 1H NMR (CD3COCD3, δ in ppm): 5.36 (t, 2 H, =CH2), 5.48 (m, 2 H, dCH2), 6.08, 6.56 (s, 1H, CH), 7.06 (d, 1H, C6H5), 7.30 (m, 2H, C6H4), 7.52 (m, 2H, C6H4), 7.73 (m, 2H, C6H5). IR (drift): 3061, 1734, 1649, 1597, 1499, 1450, 1407, 1305, 1281, 1251, 1202, 1163, 1024, 972, 925, 846,789, 738, 705, 655 cm-1. UV (chloroform): λmax = 258 nm. BA was also prepared by the Schotten-Baumann reaction. Under vigorous stirring, 57 g (0.63 mol) acryloyl chloride (AC) was dropped into the mixture of 22.2 g (0.102 mol) benzoin, 54 mL dimethylaniline, and 90 mL chloroform below 10 °C. After the addition of AC, the reaction kept for 1 h over a gentle reflux of the solvent. The remaining AC and the solvent were removed under vacuum. The solid residue was hydrolyzed with 6 M aqueous H2SO4, and then washed with water for ten times. The final product was recrystallized from ethanol twice and a white acicular crystal was obtained. The overall yield was 90%. BA 1H NMR (CD3COCD3, δ in ppm): 5.99, 6.01(d, 2H, dCH2), 6.28-6.32 (q, 2H, dCH2), 6.43, 6.47 (s, 1H, CH), 7.11 (s, 1H, C6H5), 7.41 (m, 2H, C6H5), 7.51 (t, 2H, C6H5), 7.61 (m, 2H, C6H5), 8.07 (d, 1H, C6H5). FTIR (drift): 3066, 2360, 1970, 1717, 1693, 1616, 1597, 1499, 1449, 1408, 1367, 1285, 1255,
Figure 3. Growth of spherical polyelectrolyte brushes with different photoinitiators as monitored by DLS. Symbols denote: (0) BA, (O) HMEM, (r)ABP with tert-butanol, and (Δ) ABP without tert-butanol.
Figure 4. Mechanism for photoinitiation. (a) The cleavage mechanism (R is an alkyl group) (BA and HMEM). (b) The hydrogen abstraction mechanism (ABP).
Figure 5. Chemical structure of photoinitiators and their analogues. 3565
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Figure 6. UV absorption spectra of photoinitiators and emission spectrum of UV lamp.
1231, 1196, 1061, 989, 963, 936, 861, 810, 760, 696 cm-1. UV (chloroform): λmax = 250 nm. Synthesis of Core Latex. The polystyrene (PS) core latex was synthesized in a 3000-mL double-wall reactor via conventional emulsion polymerization. The reactor consisted of a thermometer, a reflux condenser, a paddle stirrer system (IKA), and a thermostatic system (Lauda R400). In a typical run, 3.14 g sodium dodecyl sulfate (SDS) was dissolved in 1240 mL H2O under stirring. After addition of 312 g styrene, the reactor was degassed by repeated evacuation and subsequent addition of nitrogen at least 5 times. The emulsion polymerization was started by adding 0.66 g potassium persulfate (KPS), which was dissolved in 20 mL H2O, at the temperature of 80 °C. The reaction lasted for 1 h at 80 °C. Then, the temperature was reduced to 70 °C in order to add photoinitiator. To achieve a well-distributed photoinitiator layer on the particle surface of core, a defined amount of photoinitiator dissolved in acetone was added by a doser motor under starved conditions (0.05 mL/min) at 70 °C. Ten minutes after the total amount of photoinitiators were added, the latex was cooled down to room temperature and purified by extensive serum replacement against pure water until the conductivity of outer water became constant. Preparation of SPB. The core latex modified by a thin layer of photoinitiator was charged in a UV-reactor (Heraeus TQ 150, volume: 650 cm3, range of wavelengths: 200-600 nm) and diluted to 2.5 wt % with water. The total weight was adjusted to 700 g. Then a defined amount of monomers were added under stirring. The photoreactor was degassed by repeated evacuation and subsequent addition of nitrogen at least 5 times. Photoemulsion polymerization was initiated by UV-vis-radiation at room
temperature. Vigorous stirring ensured homogeneous conditions. To remove possible coagulum the latex was filtered over glass fiber. The SPB latex was purified by serum replacement against pure water until the conductivity of outer water became constant. Characterization. 1H NMR spectra were recorded at room temperature in acetone-d6 by a Bruker Avance 500 NMR spectrometer. The transmission FTIR spectroscopy (KBr pellet or film) was performed by a Bruker IFS28 spectrometer. UV-spectra were recorded on a Lamda-3 UV-spectrometer. Dynamic light scattering (DLS) was performed by a Peters ALV 4000 light scattering goniometry with the angle of 90°. FESEM was carried out by a Zeiss 1530 Gemini instrument equipped with a field-emission cathode whose lateral resolution was approximately 2 nm.
’ RESULTS AND DISCUSSION The kinetics of photoemulsion polymerization were determined by monitoring the growth of brushes using DLS (Figure 3). The PS cores with the same amount of photoinitiator (1 mol % BA, HMEM, or ABP to styrene) and monomer AA (50 mol % to styrene) were employed in these experiments. As shown in Figure 3, the thickness of brushes using BA increased rapidly with time and reached a plateau of ∼300 nm after 150 min. For HMEM, the brush layer increased more slowly and the final thickness was smaller (∼130 nm after 3 h) compared to BA. In the case of ABP, an induction time of 60 min was observed and the brush thickness was only 60 nm after 3 h. The variation in kinetics among these three photoinitiators results from their difference in mechanism of photoinitiation, in molecular structures and in UV absorption spectra. These three 3566
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Table 1. Parameters of Spherical Polyelectrolyte Brushes sample
a d
initiator
Ia (%)
AAb (%)
Rc (nm)
Lcd (nm)
Mη (g/mol)
Rg (nm)
σ (nm-2)
D (nm)
A1
ABP
0.5
10
55
47
18500
4.05
0.054
4.86
A2
ABP
0.5
30
55
63
23000
4.57
0.061
4.57
A3
ABP
0.5
50
56
76
29000
5.18
0.062
4.53
A4
ABP
1.0
10
54
42
16000
3.75
0.076
4.09
A5
ABP
1.0
30
55
61
23000
4.57
0.078
4.04
A6
ABP
1.0
50
55
78
30000
5.28
0.081
3.96
A7
ABP
3.0
10
63
38
15000
3.62
0.081
3.96
A8 A9
ABP ABP
3.0 3.0
30 50
62 61
55 63
19500 24000
4.17 4.68
0.083 0.084
3.92 3.89
B1
BA
0.5
10
57
105
39500
6.14
0.016
8.92
B2
BA
0.5
30
57
155
58500
7.61
0.017
8.65
B3
BA
0.5
50
57
189
70500
8.43
0.026
7.00
B4
BA
1.0
10
57
102
38000
6.01
0.012
10.30
B5
BA
1.0
30
57
134
50500
7.02
0.023
7.44
B6
BA
1.0
50
58
175
66500
8.16
0.023
7.44
B7 B8
BA BA
3.0 3.0
10 30
63 62
77 121
30500 46800
5.33 6.74
0.039 0.040
5.71 5.64
B9
BA
3.0
50
63
201
78500
8.94
0.045
5.32
H1
HMEM
0.5
10
54
90
33500
5.61
0.021
7.79
H2
HMEM
0.5
30
54
117
44600
6.57
0.026
7.00
H3
HMEM
0.5
50
55
176
67200
8.21
0.035
6.03
H4
HMEM
1.0
10
61
81
33750
5.63
0.041
5.57
H5
HMEM
1.0
30
59
106
37500
5.97
0.042
4.57
H6 H7
HMEM HMEM
1.0 3.0
50 10
56 58
161 80
58200 31100
7.59 5.39
0.052 0.044
4.95 5.38
H8
HMEM
3.0
30
64
91
35600
5.80
0.065
4.43
H9
HMEM
3.0
50
60
132
50000
6.98
0.069
4.30
Amount of photoinitiator (molar percent of styrene). b Amount of monomer AA (molar percent of styrene). c Radius of SPB core determined by DLS. Contour length of PAA chains.
photoinitiators follow two mechanisms.20 Under irradiation, BA and HMEM split into two initiating radicals following a cleavage mechanism as shown in Figure 4a. However, under UV light exposure, ABP gave rise to free radical species with a hydrogen abstraction mechanism by forming a ketone singlet excited state first, following by the evolution to a triplet state by an intersystem crossing process (ISC) and final photoreduction by a hydrogen donor (RH) (Figure 4b).20 In our experiment, tertbutanol was used as a hydrogen donor. Without tert-butanol, no brush can be determined on the core surface after three hours (Figure 3). The induction period observed for the photoemulsion polymerization using ABP as initiator is probably due to the transition of the intermediate state of photoinitiation in a hydrogen abstraction mechanism. The other two photoinitiators formed radicals more rapidly according to the direct cleavage mechanism, which resulted in the fast growth of brushes in the early stage.20 The kinetics in Figure 3 also reflected the different reactivities of these three photoinitiators. The reactivity depends on the quantum yield of initiation, which is defined as the number of starting chains per absorbed photon.20 The quantum yield mainly depends on the structure of the photoinitiator. As reported by Fouassier et al.,20,21 photoinitiators 1, 2, and 3 in Figure 5 with similar structures of BA, HMEM, and ABP have the quantum yields of 36, 8, and 4, respectively. The experimental
results shown in Figure 3 agree well with the sequence of the quantum yields (BA > HMEM > ABP). It is worth noting that the matching between the absorption spectrum of photoinitiator and the emission spectrum of the UV reactor lamp will influence the quantum yield as well. As shown in Figure 6, the maximum absorption peaks of BA and ABP locate at around 250 and 258 nm, respectively, which match better than that of HMEM (λmax = 277 nm) with the emission peaks of the UV reactor (TQ 150) lamp (250-270 nm). Considering the quantum yields of initiation is mainly dependent on the initiator’s structure, the small difference in the absorption spectra of initiators will not influence the yield significantly. Even if we consider the absorption difference, the sequence of the quantum yields of photoinitiator will still be BA > HMEM > ABP. To form a brush structure, the average distance (D) between neighboring grafted points of polymer chains should be smaller than two times of the gyration radius (Rg) of a free polymer chain, which can be estimated by eq 1:22 3Mη ½η 1=3 Rg ¼ ð1Þ 10πNA Since the ester bond of the photoinitiator can be cleaved by strong alkali, the PAA chains were removed from the surface of the particles by alkali hydrolysis. After being heated at 120 °C in 2 M aqueous NaOH for 8 days, the latex was coagulated due to the 3567
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Figure 7. Thickness as a function of pH and ionic strength for SPBs prepared by using photoinitiator (a) BA (B5 in Table 1), (b) HMEM (H5), and (c) ABP (A5). Symbols represent the ionic strength of (0) 0.001M, (O) 0.01 M, and (r) 0.1M.
Figure 8. FESEM images of spherical polyelectrolyte brushes prepared by photoemulsion polymerization using photoinitiator of (a) ABP (A5, see Table 1), (b) BA (B5), and (c) HMEM (H5).
loss of the steric stabilization by PAA chains. The amount of PAA chain in the supernatant serum was determined by UV spectroscopy. The intrinsic viscosity [η] was measured by using an Ubblohde viscometer in 2.0 M NaOH at 25 °C. The viscosity average molecular weight (Mη) of the PAA chains can be calculated according to the Mark-Houwink relation (eq 2) (K = 4.22 10-2 mL/g, R = 0.64):23 ½η ¼ KM R ð2Þ When the molecular weight Mη and the amount of PAA on the surface have been known, the number of chains per particle and hence the grafting density (σ) can be calculated. Table 1 lists the determined parameters of SPBs synthesized with different photoinitiators, various amount of photoinitiators, and doses of monomer AA. As shown in Table 1, all of the samples prepared by three photoinitiators meet the criteria for brush structure (D < 2Rg). In all cases, the molecular weight of PAA brush chains enhanced monotonically upon increasing the amount of monomer. The contour length Lc is defined as the maximal length that a polymer chain can stretch, which can be calculated directly from the dividing of molecular weight by the effective bond length 0.252 nm per repeating unit of PAA (assuming the bonding angle between two neighboring bonds is 109.5°). Therefore, the contour length is proportional to molecular weight, and should follow the same tendency with molecular weight of PAA chains in
brushes. The brush thickness should be roughly proportional to the contour length or the molecular weight of PAA at the same pH value and ionic strength. As shown in Table 1, the thickness of brush can be controlled efficiently by the dose of monomer. Table 1 also confirms that almost all of the grafting density increased upon increasing the dose of photoinitiator. In addition, it is interesting that the grafting density of SPBs can be tuned within a wide range by using different photoinitiators with the same dose (Table 1). SPBs prepared by ABP have the highest grafting density (0.081 nm-2 with the dose of 1 mol % of styrene) while those by BA are the lowest (0.012 nm-2 with the same dose). The grafting density sequence is opposite to the reactivity one during photoemulsion polymerization (BA > HMEM > ABP) (Figure 3). Considering the grafted PAA amounts on the PS core surface are identical for three photoinitiators, the longer SPBs (which grows faster such as those prepared by using BA as photoinitiator) should be sparser. Therefore, the grafting density of SPBs can be tuned by not only the amount but also the kind of photoinitiators. The SPBs prepared by using BA, HMEM, and ABP as photoinitiators are sensitive to pH value and ionic strength because of the change of dissociation degree of carboxylic groups and thus the electrostatic repulsion between PAA chains.24,25 As shown in Figure 7, all of the SPBs displayed a significant swelling when the pH value increased from 4 to 9. The brush thickness increased significantly upon decreasing the ionic strength from 3568
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Industrial & Engineering Chemistry Research 0.1 to 0.001 M especially at high pH due to the reduced screen effect of counterions.17-19 The different behaviors of BA, HMEM, and ABP upon pH changing in Figure 7 result mainly from their difference in grafting density. The grafting density sequence is BA < HMEM < ABP, as shown in Table 1. Being stretched in higher extension, denser brushes show smaller changes in size upon varying pH value or ionic strength. Thus, the variation of the brush thickness with pH is much less for the brushes using ABP as initiator compared to the other two brushes, as seen in Figure 7. All of the SPBs synthesized by photoemulsion polymerization using BA, HMEM and ABP as photoinitiators exhibited welldefined spherical structure with narrow size distribution as observed by FESEM (Figure 8). Their diameters are all around 100 nm, which is close to the core size as determined by DLS because the PAA brush chains lie down onto the core surface during drying of the sample and can hardly be determined in this case. From Figure 8, narrow size distribution of core particles can be observed. The polydispersity index of PAA chains cleaved from the brush surface prepared by HMEM (H4 in Table 1) is 2.0 determined by GPC. This result is acceptable and reasonable because the PAA chains are generated by a free radical polymerization.
’ CONCLUSIONS Three photoinitiators, ABP, BA, and HMEM, were synthesized in this work. The spherical polyelectrolyte brushes (SPBs) were successfully prepared by photoemulsion polymerization using these three different photoinitiators. Their difference in structure, UV absorption, and mechanism of photoinitiation resulted in significant differences in kinetics during photoemulsion polymerization. The reactivity sequence is BA > HMEM > ABP, while the grafting density sequence of SPBs prepared by them is the opposite. The brush structures for all SPBs were confirmed by determining the grafting density of PAA chains on the core surface. The grafting densities of SPBs were tunable ranging from 0.012 to 0.084 nm-2 by employing different photoinitiators. The SPB thickness is sensitive to pH and ionic strength as determined by DLS. Brushes with higher grafting density show smaller changes in size upon varying pH value or ionic strength. FESEM results indicated the well-defined spherical structure and narrow size distribution of core for all SPBs prepared by three photoinitiators. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT We gratefully acknowledge the National Natural Science Foundation of China (Grant Nos. 20644003, 20774028), Shanghai Pujiang Talent Project (Grant No. 08PJ14036), 111 Project Grant B08021, the Key Basic Research Project of Shanghai Science and Technology Commission (Grant No. 10JC1403800), and the Fundamental Research Funds for the Central Universities for support of this work. ’ REFERENCES (1) Brittain, W. J.; Minko, S. A structure definition of polymer brushes. J. Polym. Sci., Part A 2007, 45, 3505. (2) Ruehe, J.; Ballauff, M.; Biesalski, M.; Dziezok, P.; Groehn, F.; Johannsmann, D.; Houbenov, N.; Hugenberg, N.; Konradi, R.; Minko,
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