Block copolymer preparation by atom transfer radical polymerization under emulsion conditions using a nanoprecipitation technique

Block Copolymer Preparation by Atom Transfer Radical Polymerization under Emulsion Conditions Using a Nanoprecipitation Technique DELPHINE CHAN-SENG,1 DAVID A. RIDER,2 GE´RALD GUE´RIN,2 MICHAEL K. GEORGES1 1

Department of Chemical and Physical Sciences, University of Toronto at Mississauga, 3359 Mississauga Road N., Mississauga, Ontario, L5L 1C6, Canada 2

Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario M5S 3H6, Canada

Received 23 March 2007; accepted 23 September 2007 DOI: 10.1002/pola.22410 Published online in Wiley InterScience (www.interscience.wiley.com).

Living-radical polymerization of acrylates were performed under emulsion atom transfer radical polymerization (ATRP) conditions using latexes prepared by a nanoprecipitation technique previously employed and optimized for the polymerization of styrene. A macroinitiator of poly(n-butyl acrylate) prepared under bulk ATRP was dissolved in acetone and precipitated in an aqueous solution of Brij 98 to preform latex particles, which were then swollen with monomer and heated. Various monomers (i.e. n-butyl acrylate, styrene, and tert-butyl acrylate) were used to swell the particles to prepare homo- and block copolymers from the poly(n-butyl acrylate) macroinitiator. Under these conditions latexes with a relatively good colloidal stability were obtained. Furthermore, amphiphilic block copolymers were prepared by hydrolysis of the tertbutyl groups and the resulting block copolymers were characterized by dynamic light scattering (DLS) and transmission electron microscopy (TEM). The bulk morphologies of the polystyrene-b-poly(n-butyl acrylate) and poly(n-butyl acrylate)-b-poly(acrylic acid) copolymers were investigated by atomic force microscopy (AFM) and small angle C 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: X-ray scattering (SAXS). V ABSTRACT:

625–635, 2008

Keywords: atom transfer radical polymerization (ATRP); block copolymer; emulsion polymerization; living polymerization; nanoprecipitation technique

INTRODUCTION The desire for advanced materials with new or improved physical, chemical or mechanical properties has led to a growing interest in synthesizing advanced polymers. Historically living anionic polymerization has been used to control the molecular weight and molecular weight distribution of polymers, enabling the preparation of well-defined Correspondence to: M. K. Georges (E-mail: mgeorges@ utm.utoronto.ca) Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 46, 625–635 (2008) C 2007 Wiley Periodicals, Inc. V

homopolymers, random copolymers, block copolymers, and end-functionalized polymers.1,2 However, living anionic polymerizations are sensitive to impurities and various functional groups, limitations which can be overcome by using ‘‘living’’/controlled radical polymerization. Various types of living-radical polymerizations have been reported and include stable free radical polymerization (SFRP),3–6 atom transfer radical polymerization (ATRP),7–10 reversible addition-fragmentation chain transfer (RAFT)11 and degenerative transfer polymerization using iodine exchange.12 These techniques are applicable to various monomers with a large advantage being their tolerance to 625

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aqueous systems, such as emulsion, miniemulsion, suspension, and dispersion systems. Initial attempts at living-radical polymerization under emulsion conditions were difficult. Poor colloidal stability was reported for SFRP,13,14 ATRP,15–17 and RAFT.18,19 To overcome the problem of poor colloidal stability, a nanoprecipitation technique was developed for preparing a latex that could be used to successfully perform an emulsion polymerization under SFRP conditions.14 Emulsion particles were prepared by precipitating an oligomer prepared in bulk, dissolved in acetone, into an aqueous solution of poly(vinyl alcohol). Subsequent particle swelling and heating of the emulsion mixture resulted in a stable latex containing a polymer that increased in molecular weight with time. The distribution of the polymer chain lengths remained narrow throughout the course of the polymerization. The nanoprecipitation technique was subsequently successfully extended to the ATRP process.20 For this system, the surfactant type, hydrophobic ligand structure, nature of the metal catalyst, molecular weight of the macroinitiator and reaction temperature, as they related to the system livingness, molecular weight control and latex stability, were studied. The best results were obtained using the nonionic surfactant Brij 98 in combination with the hydrophobic ligand N,N-bis(2-pyridylmethyl)octadecylamine (BPMODA) and a ligand to CuBr ratio of 1.5. The polymer samples prepared with this technique had narrow molecular weight distributions and the latexes obtained had good colloidal stability with average particle diameters of 200 nm. Previous studies have reported on the synthesis of block copolymers under ATRP conditions under suspension,21 miniemulsion,22 and seeded emulsion23,24 conditions. Okubo et al. reported the preparation of a poly(i-butyl methacrylate) (Pi-BMA) macroinitiator under miniemulsion conditions which they subsequently used in a seeded emulsion to prepare poly(i-butyl methacrylate)-b-polystyrene. Although the synthesis of pure block copolymer was proven by thin layer chromatography, the GPC traces overlapped making it difficult to determine the purity of the prepared block copolymer.23 Peng et al. prepared a poly(n-butyl methacrylate) (PBMA) macroinitiator under emulsion conditions, but the latexes were reported to be slightly unstable, at least 5% coagulum was observed.17 The PBMA macroinitiator was then used for a chain extension with styrene but it was difficult to know if pure block

copolymers were obtained since GPC chromatographs were not provided and the molecular weight distributions were around 1.4.24 Moreover, in the previous reference the authors did not provide any comments on the colloidal stability of the block copolymer latexes prepared. We report here the successful preparation of block copolymers under ATRP emulsion polymerization conditions using the nanoprecipitation technique. The block copolymers are unique in that one of the blocks is short relative to the second block, a structural feature imposed by the nanoprecipitation procedure. Block copolymers of styrene, tert-butyl acrylate and acrylic acid consisting of a short poly(n-butyl acrylate) block were prepared. Amphiphilic poly(n-butyl acrylate)-b-poly(acrylic acid) copolymers were obtained by hydrolyzing the tert-butyl acrylate groups of a poly(n-butyl acrylate)-b-poly(tert-butyl acrylate) copolymer and the resulting block copolymers were investigated for their ability to form micellar aggregates in water. The bulk morphologies of the different block copolymers prepared were also studied.

EXPERIMENTAL Materials All reagents were purchased from Aldrich. nButyl acrylate (n-BA, 99þ%), ethyl 2-bromopropionate (EBP, 99%), N,N,N0 ,N@,N@-pentamethyldiethylenetriamine (PMDETA, 99%), 2-butanone (99þ%), Brij 98 (polyoxyethylene(20) oleyl ether, average Mn ca. 1150), styrene (99%), tert-butyl acrylate (98%), trifluoroacetic acid (99%) were used without further purification. Copper(I) bromide (98%), CuBr, (98%) was purified by stirring in acetic acid, washing with methanol and then drying under vacuum at room temperature. N,Nbis(2-pyridylmethyl)octadecylamine (BPMODA) was synthesized as described previously.20

Synthesis of the Macroinitiator Using PMDETA as Ligand The poly(n-butyl acrylate) macroinitiators syntheses were performed using the following conditions: [EBP]:[CuBr]:[PMDETA] ¼ 1:0.2:0.2 and nbutyl acrylate/2-butanone ¼ 70/30 by volume. The ratio [n-BA]:[EBP] used was calculated to yield the molecular weight targeted. n-Butyl Journal of Polymer Science: Part A: Polymer Chemistry DOI 10.1002/pola

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acrylate (50 mL, 350 mmol), 2-butanone (20 mL), PMDETA and CuBr were introduced in a pre-degassed three-necked round-bottom flask equipped with an argon inlet and a condenser. After the reaction mixture was purged with argon for 1 h, ethyl 2-bromopropionate, EBP, was added and the flask was placed in an oil bath maintained at 70 8C for 24 h. After polymerization, the reaction mixture was passed through an alumina column using CH2Cl2 as eluent to remove the copper salts. The remaining monomer was removed from the oligo(n-butyl acrylate)s by leaving them under vacuum. The polystyrene macroinitiators were prepared as described previously.20 Latex Formation and Emulsion Polymerization The purified oligo(n-butyl acrylate) or oligostyrene (0.30 mmol) prepared previously, CuBr (43 mg, 0.30 mmol), BPMODA (206 mg, 0.46 mmol) were dissolved in acetone (50 mL) and the resulting solution was added dropwise to a solution of Brij 98 (0.58 g) in deionized water (100 mL). The acetone was removed by rotary evaporation and deionized water was added to replace the water lost during evaporation. Fresh monomer (10 mL), n-butyl acrylate for homopolymer preparation, or styrene, tert-butyl acrylate, for block copolymer synthesis, was added to the resulting latex and the mixture was stirred overnight to allow the particles to swell. The latex was placed in a modified Parr bomb reactor equipped with a pressure gauge, four-bladed propeller mixer, thermocouple, argon inlet, and sampler tube. The system was degassed under stirring by pressurizing the reactor to 6.9 bar with argon gas for 1 min and then depressurizing the reactor. This procedure was repeated 10 times. The reactor was placed under a 6.9 bar atmosphere of argon and heated at 85 8C. Samples, taken periodically during the reaction, were dried in air until the weight remained constant and then they were passed through a small alumina column (CH2Cl2 as eluent) to remove the copper salts. Separation of the Block Copolymer from the Latexes A mixture of 50 mL of latex and 50 mL of dichloromethane were placed in an erlenmeyer. Sodium chloride was added and the mixture was stirred until the emulsion broke. The dichloromethane phase was separated and dried over anhydrous sodium sulfate. The copper salts were Journal of Polymer Science: Part A: Polymer Chemistry DOI 10.1002/pola

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removed by passing the resulting solution through an alumina column (CH2Cl2 as eluent). Dichloromethane was removed by rotary evaporation and the remaining thick oil was left under house vacuum (
20 mm Hg) overnight.

Preparation of Poly(n-butyl acrylate)-b-poly(acrylic acid) Copolymers The block copolymers from Table 4 (0.5 g) were dissolved in dichloromethane (5 mL) and about five-fold molar excess of trifluoroacetic acid with respect to the ester groups was added. The solutions were stirred at room temperature for 24 h. When hydrolyzed, the polymers precipitated in dichloromethane. The precipitates were separated by filtration under vacuum, washed with dichloromethane repeatedly and then left to dry under vacuum. The hydrolyzed polymers, PnBAb-PAA, were characterized by FT-IR and 1H NMR using as solvent a mixture of CDCl3 and DMSOd6 in a ratio 1 to 1 by volume. The micellar solutions were prepared by dissolving the block copolymer in water at a concentration of 0.05 mg mL1. These solutions were then studied by dynamic light scattering (DLS) and by scanning transmission electron microscopy (STEM).

Film Preparation Thin films were prepared by dip coating from 10 mg mL1 polymer solutions. For poly(acrylic acid) containing block copolymers methanol was used as the solvent, for polystyrene containing block copolymers dichloromethane was used for the casting solutions. For dip coating, single-side polished silicon substrates that were pre-cleaned by repeated rinsing with solvent were immersed in the casting solutions and gently withdrawn at
10 cm s1. Once removed, excess solution was removed by gently touching the corner of the substrate with a Kimwipe. Samples were allowed to dry at room temperature for a minimum of 6 h prior to analysis or further processing. For spin coating casting solutions were filtered with a Whatman 13 mm GD/X nylon syringe filter (0.45 lm pore size) immediately prior to use. The about 1 cm2 Si wafers, cleaned as above, were coated with the polymer solution and immediately accelerated to 2000 rpm for 60 s. Again, samples were allowed to dry at room temperature for a minimum of 6 h prior to analysis or further process-

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ing. Thermal annealing was done at 130 8C under dynamic vacuum (103 mmHg) for 5 days. Characterization Polymer molecular weights and polydispersity indexes (Mw/Mn) were estimated by gel permeation chromatography (GPC) using a Waters/Millipore liquid chromatograph equipped with a Waters model 510 pump, UltrastyragelÓ columns HR1, HR2, and HR4, and a Waters model 410 differential refractometer (RI). Polystyrene standards were used for calibration. THF was used as the eluent at a flow rate of 0.35 mL min1. GPC was performed on samples after removal of the remaining monomer by evaporation with a stream of air and removal of the copper salts by passing the samples through an alumina column using CH2Cl2 as eluent. Percentage conversions were determined gravimetrically. Particles sizes were determined using a Malvern Mastersizer 2000. For each sample, the value of the weight mean diameter, dw, and the polydispersity, dn/dw, were calculated as an average of 10 measurements. 500 MHz 1H NMR spectra were recorded on a Varian Unity INOVA 500 spectrometer at 20 8C in CDCl3/DMSO-d6 solutions with tetramethylsilane as an internal standard. Infrared measurements, performed on KBr pellets, were made on a Nicolet Avatar 360 FT-IR. Dynamic light scattering (DLS) measurements were performed using a wide angle light scattering photometer from ALV. The light source was a JDS Uniphase He-Ne laser (k ¼ 632.8 nm, 35 mW) emitting vertically polarized light. The cells were placed into the ALV/DLS/SLS-5000 Compact Goniometer System and sat in a vat of thermostated cis-decahydronaphthalene, which matched the index of refraction of the glass cells. The TEM samples were prepared on a precoated copper grid by placing a drop of the block copolymer solution in water on it and removing the excess of water. The STEM images were obtained using a Hitachi HD-2000 in the SE mode. Atomic force microscopy (AFM), operated in tapping mode, was performed on a Multimedia Nanoscope IIIa AFM (Digital instrument/Veeco-Metrology Group). The AFM tips had resonant frequencies close to 170 kHz. Small-angle X-ray scattering (SAXS) data were obtained using a Molecular Metrology instrument operated with a two-dimensional gasfilled multiwire detector and 0.030 kW microsource X-ray tube with a confocal multilayer optic to produce a focused monochromatic X-ray beam

(k ¼ 0.154 nm). The q spacing was calibrated using silver behenate, and the sample-to-detector distance was 1.5 m.

RESULTS AND DISCUSSION Block Copolymer Synthesis under Emulsion Conditions using the Nanoprecipitation Technique From our successful homopolymerization of styrene under emulsion conditions using the nanoprecipitation technique reported previously,20 we decided to extent this work to the preparation of block copolymers. Our first attempt was performed using a bromo-terminated polystyrene macroinitiator prepared previously.20 The macroinitiator was precipitated from a solution of acetone into an aqueous solution of Brij 98, followed by evaporation of the acetone to give emulsion particles that were swollen with n-butyl acrylate. The chain extension was performed under emulsion polymerization conditions in a modified Parr reactor at 85 8C and the GPC results are shown in Figure 1. The number average molecular weights, molecular weight distributions and conversions for each sample are reported in Table 1. The system shows some livingness with narrow molecular weight distributions. However, it can be seen that some macroinitiator remained, as was the case for the homopolymerization of styrene.20 This may be due to some termination reactions occurring in the early portion of the reinitiation stage. To overcome this problem, it was decided to prepare the block copolymers by inversing the monomer addition sequence. To that end, a series of poly(n-butyl acrylate) macroinitiators with different molecular weights were synthesized. All the macroinitiators showed a narrow molecular weight distribution (Mw/Mn between 1.11 and 1.09) and the number-average molecular

Figure 1. GPC chromatograph of the emulsion polymerization of n-butyl acrylate from a polystyrene macroinitiator (see Table 1, entry 1). Journal of Polymer Science: Part A: Polymer Chemistry DOI 10.1002/pola

ATRP BLOCK COPOLYMERS PREPARATION

629

Table 1. Synthesis of Polystyrene-b-Poly(n-Butyl Acrylate) Block Copolymers Initiated with Polystyrene Macroinitiators under ATRP Emulsion Polymerizationa Using the Nanoprecipitation Technique Entry

Macroinitiator

Time (h)

Mn

Mw/Mn

Conv. (%)

1 PS21-b-PnBA179

Mn ¼ 2200 Mw/Mn ¼ 1.09

2 PS31-b-PnBA199

Mn ¼ 3200 Mw/Mn ¼ 1.09

3 PS41-b-PnBA191

Mn ¼ 4300 Mw/Mn ¼ 1.08

0.5 1 2 0.5 1 2 0.5 1 2

22,100 24,000 25,100 21,400 23,400 28,700 18,800 24,400 28,800

1.13 1.11 1.14 1.12 1.11 1.13 1.08 1.08 1.11

30 40 50 26 30 37 22 29 37

dw (nm)

dw/dn

250

4

330

5

1160b

18

a 0.30 mmol of macroinitiator, 0.30 mmol of CuBr, and 0.46 mmol BPMODA in a solution of 0.58 g of Brij 98 in 100 mL of deionized water. b Bimodal.

weights (Mn,GPC ¼ 2200, 3600, and 5200) agreed with the calculated molecular weight targeted (Mn,target ¼ 2000, 3400, and 5200, respectively). Emulsion particles were prepared using the nanoprecipitation technique. To verify the efficiency of the poly(n-butyl acrylate) macroinitiator as an initiator, the homopolymerization of n-butyl acrylate was performed (Table 2). The GPC distributions in Figure 2 show no macroinitiator remained after initiation confirming a higher efficiency of the poly(n-butyl acrylate) macroinitiator to reinitiate. From this encouraging result, block copolymers were prepared using these poly(n-butyl acrylate) macroinitiators by chain extending them with styrene or tert-butyl acrylate. In the case of styrene, the system was living (see the results reported in Table 3) and the latexes exhibited good colloidal stability. In the case of block copolymers using tert-butyl acrylate, while the latexes were colloidally stable, the polymerizations reached high molecular weight within the first hour and then the polymerizations stopped, giving conversions of less than 30% (see Table 4). We suspect this is due to the hydrolysis of the tert-butyl group under the conditions to acrylic acid. It is known that acrylic acid will not polymerize under these atom transfer radical polymerization conditions.9 The livingness of a polymerization system is usually evaluated by analyzing a plot of the dependence of the molecular weight on the conversion and the first order kinetic plot, which ‘‘quantify’’ the chain transfer and termination reactions, respectively. However, Penczek et al. demonstrated that the two criteria of livingness could be combined on one plot by plotting ln(1Journal of Polymer Science: Part A: Polymer Chemistry DOI 10.1002/pola

DPn[I]0/[M]0) versus time, where DPn is the number–average degree of polymerization, [I]0 the initial concentration of initiator and [M]0 the initial concentration of monomer.26 The corresponding plots for some of the previously described homo- and copolymers are shown in Figure 3. The poly(n-butyl acrylate) homopolymer is almost linear, although it seems to slightly deviate from linearity when reaching higher conversion. In the case of the copolymers of poly(nbutyl acrylate) and polystyrene, the behavior regarding the livingness is dramatically different depending on the nature of the macroinitiator used. When a polystyrene macroinitiator is used, the evolution of ln(1-DPn[I]0/[M]0) versus time shows a significant deviation from linearity and shows an exponential evolution instead, whereas reinitiation of the poly(n-butyl acrylate) macroinitiator with styrene leads to a perfect straight line. We conclude from this result that a poly(n-butyl acrylate) is a better macroinitiator than a polystyrene macroinitiator to reinitate polymerization.

Figure 2. GPC chromatographs for the chain extension polymerization of a poly(n-butyl acrylate) macroinitiator with n-butyl acrylate (Table 2, entry 1).

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CHAN-SENG ET AL.

Table 2. Preparation of Poly(n-Butyl Acrylate) from a Poly(n-Butyl Acrylate) Macroinitiator under ATRP Emulsion Polymerizationa Using the Nanoprecipitation Technique Entry

Macroinitiator

Time (h)

Mn

Mw/Mn

Conv. (%)

1 PnBA247

Mn ¼ 2200 Mw/Mn ¼ 1.11

2 PnBA218

Mn ¼ 3600 Mw/Mn ¼ 1.10

3 PnBA220

Mn ¼ 5200 Mw/Mn ¼ 1.09

0.5 1 2 0.5 1 2 0.5 1 2

24,000 26,500 31,600 17,900 21,800 27,900 23,300 27,300 28,200

1.09 1.09 1.08 1.15 1.13 1.11 1.10 1.08 1.12

35 48 59 38 48 62 32 43 54

dw (nm)

dw/dn

490b

8

610b

10

4,000c

57

Styrene: 87 mmol; n-butyl acrylate: 69 mmol: tert-butyl acrylate:69 mmol. a 0.30 mmol of macroinitiator, 0.30 mmol CuBr and 0.46 mmol BPMODA in a solution of 0.58 g of Brij 98 in 100 mL of deionized water. b Shoulder. c bimodal.

Amphiphilic Block Copolymers Preparation and Characterization By selective hydrolysis of the poly(tert-butyl acrylate) block of the copolymers, amphiphilic block copolymers with one short block of poly(n-butyl acrylate) and a relatively long block of poly (acrylic acid) were obtained. The hydrolysis procedure using trifluoroacetic acid was simple and effective. As shown in Figure 4, the disappearance of the characteristic strong peak at 1.4 ppm corresponding to the methyl protons of the tertbutyl groups and the appearance of the characteristic peak of the carboxylic groups at 12.1 ppm

demonstrated the success of the hydrolysis of the poly(tert-butyl acrylate) block. The hydrolysis reaction was further confirmed by FT-IR showing the characteristic carboxylic acid absorbance at 2800–3600 cm1 (Fig. 5). The different amphiphilic poly(n-butyl acrylate)-b-poly(acrylic acid), PnBA-b-PAA, prepared were expected to form micellar aggregates in water based on work reported by Eghbali et al.27 on the micellization behavior in aqueous solution of a PnBA100-b-PAA150 polymer prepared by ATRP in an acetone solution. These authors demonstrated that a certain degree of neutralization

Table 3. Synthesis of Poly(n-Butyl Acrylate)-b-Polystyrene Block Copolymers from Poly(n-Butyl Acrylate) Macroinitiators under ATRP Emulsion Polymerization Conditionsa Using the Nanoprecipitation Technique Entry

Macroinitiator

Time (h)

Mn

Mw/Mn

Conv. (%)

1 PnBA17-b-PS223

Mn ¼ 2200 Mw/Mn ¼ 1.11

2 PnBA28-b-PS309

Mn ¼ 3600 Mw/Mn ¼ 1.10

3 PnBA41-b-PS382

Mn ¼ 5200 Mw/Mn ¼ 1.09

1 2 4 6 1 2 4 6 0.5 1 2 4 6

10,800 15,400 21,600 25,400 13,100 19,900 32,700 35,700 13,700 16,600 24,000 35,900 44,900

1.08 1.10 1.11 1.13 1.10 1.08 1.07 1.09 1.09 1.10 1.09 1.14 1.11

25 38 55 58 32 37 58 65 18 25 38 50 40

dw (nm)

dw/dn

230

3.7

360b

5.8

390c

6.2

a 0.30 mmol of macroinitiator, 0.30 mmol CuBr and 0.46 mmol BPMODA in a solution of 0.58 g of Brij 98 in 100 mL of deionized water. b Shoulder. c Fouling.

Journal of Polymer Science: Part A: Polymer Chemistry DOI 10.1002/pola

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631

Table 4. Synthesis of Poly(n-Butyl Acrylate)-b-Poly(tert-Butyl Acrylate) Block Copolymer from a Poly(n-Butyl Acrylate) Macroinitiator under ATRP Emulsion Polymerization Conditionsa Using the Nanoprecipitation Technique Entry

Macroinitiator

Time (h)

Mn

Mw/Mn

Conv. (%)

dw (nm)

dw/dn

1 PnBA17-b-PtBA120 2 PnBA28-b-PtBA100 3 PnBA41-b-PtBA77

Mn ¼ 2200 Mw/Mn ¼ 1.11 Mn ¼ 3600 Mw/Mn ¼ 1.10 Mn ¼ 5200 Mw/Mn ¼ 1.09

1

17,600

1.19

27

360

6

1

16,400

1.09

24

2,000b

32

1

15,000

1.11

21

3,570b

55

a 0.30 mmol of macroinitiator, 0.30 mmol of CuBr and 0.46 mmol BPMODA in a solution of 0.58 g of Brij 98 in 100 mL of deionized water. b Bimodal.

is necessary to obtain stable solutions of block copolymers in water and globular micelles. However, their SANS measurements showed the formation of larger aggregates than one would expect from the block copolymer micellization. We were interested in investigating the PnBA-bPAA block copolymers that we had prepared under emulsion polymerization using this nanoprecipitation technique to see if we would obtain comparable results. However, it should be noted that the block copolymers that we are able to prepare can only have a short first block, here poly(n-butyl acrylate), due to the limit imposed by the nanoprecipitation process.20 The presence and size distributions of micellar aggregates were investigated by dynamic light scattering (DLS). Since the hydrophobic poly(n-butyl acrylate) blocks are short compared to the poly(acrylic

acid) block, the formation of star-shaped micelles was expected. The DLS measurements clearly showed the formation of two distributions of colloidal objects in water (see Fig. 6) when the concentration of PnBA-b-PAA block copolymer was above the critical micellar concentration (cmc). This observation was confirmed by scanning transmission electron microscopy (STEM). As seen in Figure 7, micelles with a wide range of sizes were obtained along with some aggregates, which corresponded to the two populations observed in DLS, that is micelles with an average hydrodynamic radius of 30 nm and aggregates of 150 nm. The abundance of colloidal objects observed in STEM and DLS increased with increasing length of the poly(nbutyl acrylate) block due probably to the fact that the short poly(n-butyl acrylate) blocks do not con-

Figure 3. Dependence of ln(1-DPn[I]0/[M]0) versus time for poly(n-butyl acrylate) homopolymer (u, Table 2, entry 1), polystyrene-b-poly(n-butyl acrylate) (~, Table 1, entry 1) and poly(n-butyl acrylate)-b-polystyrene (*, Table 3, entry 1) copolymers.

Figure 4. 1H NMR of poly(n-butyl acrylate)-b-poly (tert-butyl acrylate) (Table 4, entry 1) in a mixture of CDCl3:DMSO-d6 (1:1 by volume) (a) before hydrolysis and (b) after hydrolysis to yield poly(n-butyl acrylate)b-poly(acrylic acid) (PnBA-b-PAA).

Journal of Polymer Science: Part A: Polymer Chemistry DOI 10.1002/pola

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(acrylic acid) exhibits considerably less hydrogen bonding when it is in its ionized form at high pH. Self-Assembly Behavior of Block Copolymers

Figure 5. FT-IR spectra of poly(n-butyl acrylate)-bpoly(tert-butyl acrylate) (Table 4, entry 1) after hydrolysis to yield poly(n-butyl acrylate)-b-poly(acrylic acid) (PnBA-b-PAA).

Block copolymers have been largely explored as precursors for the preparation of nanoscopic materials.28–30 The nanostructures are the result of microseparation due to the incompatibility of the different blocks constituting the copolymer. A very common example is polystyrene-b-poly(alkyl methacrylate), which can adopt different morphologies, such as lamella,31–33 cylinder31,34 or sphere.33 However, to the best of our knowledge the self-assembly of block copolymers having a poly(n-butyl acrylate) block has had scant attention.35–38 The small number of studies of block copolymers containing a poly(n-butyl acrylate) block may be due to the fact that poly(n-butyl acrylate) can not be directly prepared by anionic polymerization, requiring first the polymerization

tain enough hydrophobic component to self-assemble in water. The effect of pH was also investigated on selfassembly of the block copolymers (Fig. 6). The addition of ammonium hydroxide to give a pH of 12 resulted in significant micelle formation, whereas a pH of 3, obtained by the addition of acetic acid, favored the formation of aggregates. The poly(acrylic acid), in its acid form at low pH, undergoes significant hydrogen bonding leading to a large amount of aggregates, whereas poly

Figure 6. The intensity distribution of the hydrodynamic radii of micellar aggregates of a PnBA41-bPAA77 (hydrolyzed product of Table 4, entry 3) solution in water at a concentration of 1 mg mL1 at 908 at various pH.

Figure 7. STEM images of the micellar aggregates solution in water from a poly(n-butyl acrylate)-b-poly (acrylic acid), PnBA41-b-PAA77 (hydrolyzed product from Table 4, entry 3). Journal of Polymer Science: Part A: Polymer Chemistry DOI 10.1002/pola

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633

of tert-butyl acrylate by anionic polymerization followed by the transalcoholysis of the tert-butyl groups with n-butanol. Jerome et al.35,36,39 investigated the properties of triblock copolymers consisting of a middle poly(n-butyl acrylate) block and poly(methyl methacrylate) outer blocks prepared by both anionic polymerization and atom transfer radical polymerization (ATRP). Whereas, the typical behavior of thermoplastic elastomers was observed for the triblock copolymers prepared by anionic polymerization, the ATRP triblock copolymers having broader molecular weight distributions showed only partial characteristics of thermoplastic elastomers. Another example was reported by Matyjaszewski where a triblock copolymer, polacrylonitrile-b-poly(n-butyl acrylate)-b-polyacrylonitrile PAN45-b-PBA530-bPAN45, was prepared by ATRP and used as a precursor for nanostructured carbon arrays.37 The rigid polyacrylonitrile domains having a spherical morphology were readily observed in the poly(nbutyl acrylate) matrix by AFM.

Figure 9. (a) AFM phase image and (b) SAXS measurement results of PnBA41-b-PAA77 (hydrolyzed product of Table 4, entry 3). The arrows indicate the position of the expected minima in the case of a cylindrical morphology.

Figure 8. (a) AFM phase image and (b) SAXS measurement results for PnBA41-b-PS382 (Table 3, entry 3). Journal of Polymer Science: Part A: Polymer Chemistry DOI 10.1002/pola

When the block copolymers with a short polystyrene block and a long poly(n-butyl acrylate) block (see Table 1) we prepared were studied by AFM, only a smooth and homogeneous surface was observed. This result suggests that these block copolymers with a short polystyrene block are in a disordered state. While AFM images of poly(n-butyl acrylate)-b-polystyrene copolymers with a short poly(n-butyl acrylate) block (see Table 3) showed some structure which could have suggested some phase segregation, the smallangle X-ray scattering (SAXS) measurement showed no order (see Fig. 8). On the other hand, two poly(n-butyl acrylate)-b-poly(acrylic acid) copolymers (hydrolyzed products from Table 4, entries 2 and 3) showed order. Whereas, it was

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strated. A low molecular weight polymer of nbutyl acrylate prepared by ATRP in solution was used as a macroinitiator to polymerize various monomers (styrene, tert-butyl acrylate) under emulsion conditions to provide well-defined homopolymers and block copolymers with narrow molecular weight distributions. The poly(n-butyl acrylate) oligomer was shown to be a better macroinitiator than the polystyrene macroinitiator, probably due to a higher activation rate constant, for the preparation of block copolymers. The poly(n-butyl acrylate) oligomer gave ‘‘pure’’ block copolymers not contaminated by remaining macroinitiator. Amphiphilic block copolymers produced from the hydrolysis of the poly(tert-butyl acrylate) to form the poly(acrylic acid) were investigated and shown to form micelles in water and nanostructures in bulk due to the incompatibility of their constituent blocks. The authors thank Andrea Liskova and Michael F. Cunningham, Queen’s University (Kingston, Ontario), for the particle size measurements, Ilya Gourevich for the STEM measurements and Jiayu Wang (University of Massachussetts at Amherst) for the SAXS measurements. Funding for this project was provided by NSERC through a Strategic Grant.

REFERENCES AND NOTES

Figure 10. (a) AFM phase image and (b) SAXS measurement results of PnBA28-b-PAA100 (hydrolyzed product of Table 4, entry 2), the arrows indicate the position of the expected minima in the case of a cylindrical morphology.40

relatively easy to determine that PnBA41-bPAA77 as a thin film showed a cylindrical microphase segregation as seen by the AFM image and the results from the SAXS measurement (Fig. 9), the assignment of the bulk morphology adopted by PnBA28-b-PAA100 was more arduous. While the AFM image [Fig. 10(a)] suggested the formation of spheres or cylinders standing on the surface, the SAXS measurement [Fig. 10(b)] was not conclusive enough, due to the weakness of the higher ordered peaks, to confirm this conclusion.

CONCLUSIONS The preparation of block copolymers under atom transfer radical emulsion polymerization using the nanoprecipitation technique was demon-

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