Switchable Block Copolymer Surfactants for Preparation of Reversibly

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Switchable Block Copolymer Surfactants for Preparation of Reversibly Coagulatable and Redispersible Poly(methyl methacrylate) Latexes Qi Zhang,†,‡ Guoqiang Yu,† Wen-Jun Wang,†,* Haomiao Yuan,§,⊥ Bo-Geng Li,† and Shiping Zhu∥,* †

State Key Lab of Chemical Engineering, Key Laboratory of Biomass Chemical Engineering of Ministry of Education, Department of Chemical and Biological Engineering, Zhejiang University, Hangzhou, Zhejiang, P R China 310027 § Department of Polymer Science and Engineering, Zhejiang University, Hangzhou, Zhejiang, P R China 310027 ∥ Department of Chemical Engineering, McMaster University, Hamilton, Ontario, Canada L8S 4L7 ABSTRACT: Poly(2-(dimethylamino)ethyl methacrylate)block-poly(methyl methacrylate) (PDMAEMA-b-PMMA) has been used as a surfactant in the preparation of PMMA latexes that can be coagulated and redispersed reversibly. In this work, we investigated in detail the effects of the block copolymer composition on the coagulation/redispersion performance of the latexes. A series of well-defined PDMAEMA-b-PMMA samples that have 10−30 DMAEMA units and 5−45 MMA units were synthesized through a two-step solution RAFT polymerization process and were used as the surfactant in the emulsion polymerization of MMA. PMMA latexes obtained with surfactants having MMA weight fraction (FMMA) < 58.5% were stable. The particle size of the resulting PMMA latexes decreased gradually with the increase of FMMA. The latex particles initially coagulated with a small amount of caustic soda could be repeatedly redispersed into fresh water through CO2 bubbling with ultrasonication and coagulated through N2 bubbling with some heating. The coagulation/redispersion process was repeatable through the CO2/N2 bubbling. The latexes showed excellent redispersibility with the surfactants of FMMA < 46%. With FMMA > 46%, the latex particle sizes could increase over 20% after the coagulation/redispersion process.

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INTRODUCTION Latex is a type of important polymer products with variety applications in plastic, rubber, paper, coating, leather, textile, and construction industries,1 as well as in biomedical and pharmaceutical areas.2 In an emulsion polymerization system, water accounts for about half of the product in volume, serving as nonsolvent. Since production and application sites of latex materials are often not in the same location, it means 50% costs in transportation and storage wasted in moving water around. Although much effort has been made to reduce water content in high solid content emulsion polymerization,3 an ideal scenario would be to coagulate latex particles on the production site and ship the product to an application site in a paste or dried form, redisperse the particles and prepare the latexes on site prior to their applications. However, coagulation and redispersion of the latexes prepared using traditional surfactants are challenging, if not impossible. Coagulation is usually implemented by adding a large amount of salt, acid, or alkali,4 generating large amount of wastewater. Surfactant loses its ability of restabilizing coagulated particles for a stable latex due to desorption.5 Recent advances in switchable surfactants provide a great opportunity in the preparation of redispersible latexes.6 Several switchable surfactants have been reported, with few suitable for emulsion polymerization, except for amidine compounds.6f,7 © 2013 American Chemical Society

The amidine surfactants, whose surface activities could be turned on and off by bubbling CO2 and N2 (or Ar) gases,6f were first developed by Jessop’s group. They reported the synthesis of redispersible polystyrene (PS) latexes using CO2switchable initiator 2,2-azobis[2-(2-imidazolin-2-yl)propane] bicarbonate (VA-061) either in combination with a C12amidine surfactant8 or VA-061 alone in soap-free emulsion polymerization.9 The latex particles were coagulated by bubbling N2 with some heating and redispersed by CO2 bubbling coupled with ultrasonication.8,9 Combining aryl amidine and tertiary amine as cosurfactants, they also prepared poly(methyl methacrylate) (PMMA) latexes that could be coagulated more rapidly by nitrogen.10 However, these latexes could not survive in multiple coagulation/redispersion cycles, probably attributed to surfactant desorption.5 Survivability in multicycle processes would give latex products a further advantage in waste saving and management. In order to improve latex stabilities in reversible coagulation and redispersion, we proposed a comonomer-surfactant approach that can incorporate functional amidine and/or amine groups onto latex particle surface by covalent bonds. Hence, we Received: December 5, 2012 Revised: January 30, 2013 Published: February 14, 2013 1261

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Scheme 1. Sketch of Reversibly Coagulatable/Redispersible PMMA Latexes Prepared through Emulsion Polymerization of MMA Using PDM Surfactant

Scheme 2. Synthesis Route of PDMAEMA-b-PMMA Block Copolymers

developed a reversibly coagulatable/redispersible PS latex system using a cyclic amidine comonomer in a soap-free emulsion polymerization of styrene (St).11 The latex particles were easily coagulated with addition of a small amount of caustic soda. The coagulated particles, no matter in the forms of washed paste and dried powder, could be easily redispersed to fresh water through CO2 bubbling without additional stabilizer.11 We also introduced (N-amidino)dodecyl acrylamide as a switchable reactive surfactant and prepared N2/CO2switchable PS latexes.12 Although switchability of the latexes was satisfactory, a partial hydrolysis of the amidine surfactant was experienced during emulsion polymerization.12 High costs incurred from the amidine synthesis was also a concern in commercial practices. Zhao and co-workers13 recently reported that the lower critical solution temperature (LCST) of poly(2(dimethylamino)ethyl methacrylate) (PDMAEMA) can be tuned reversibly by controlled protonation of the amine group with CO2. Hydrophicility and solubility of PDMAEMA were significantly improved after treating with CO2 and could also be switched back by N2 bubbling.13 Inspired by this discovery, we developed a polymeric surfactant, PDMAEMA10b-PMMA14 and prepared PMMA latexes.14 The PDMAEMA10b-PMMA14 was protonated with hydrochloride before emulsion polymerization of MMA. The PMMA latexes could be easily coagulated by a small amount caustic soda. After washing, fresh water was added to redisperse the coagulated particles and stable latexes were obtained with CO2 bubbling and ultrasonication. The recovered latexes were coagulated again with N2 bubbling and some heat. This coagulation/redispersion cycle was repeatable with alternative CO2/N2 bubbling. The system is quite promising with good potential in commercial exploitations. Very recently, the Queen’s group reported the preparation of N2-coagulatable and CO2-redispersible PS latexes through emulsion polymerization of St and 2-(diethyl)aminoethyl methacrylate.15

The feasibility study of exploring switchable block copolymer surfactants for preparation of reversibly coagulatable/redispersible latexes was published as a rapid communication.14 In this work, we provide a detailed study on one of the key parameters, that is, the composition of the block copolymer surfactant, and its effects on the performance of emulsion polymerization, latex stability and switchability. For this purpose, we synthesize a series of PDMAEMA-b-PMMA block copolymer (PDM) samples and apply them to the emulsion polymerization of MMA. The influences of the numbers of DMAEMA and MMA units on the emulsion polymerization and latex coagulation and redispersion are systematically investigated. The chain microstructure of the copolymer surfactant suitable for stable and reversibly switchable PMMA latexes is elucidated. Scheme 1 schematically represents the preparation of reversibly coagulatable/redispersible PMMA latexes through the emulsion polymerization of MMA with the switchable block copolymer surfactant.

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EXPERIMENTAL SECTION

It should be clearly pointed out here that the experimental procedures used in this work for the synthesis of block copolymer surfactants, the emulsion polymerization, and the coagulation/dispersion tests of polymer latexes are the same as those in the feasibility study.14 The descriptions of the detailed procedures are repeated here for the convenience of readers. Materials. 2-(Dimethylamino)ethyl methacrylate (DMAEMA, 98%) and MMA were supplied by Acros and were distilled under vacuum before use. 2,2′-Azobis(isobutyronitrile) (AIBN) was recrystallized with ethanol. 2,2′-azobis(2-methylpropionamidine) dihydrochloride (V-50) was purchased from Qingdao Runxing Photoelectric Material Co. and was used as received. Other analytical grade chemicals were used without further purification. 2-cyanopropan-2-yl dodecyl trithiocarbonate (CPDTTC, 99% purity) was donated by Professor Yingwu Luo of Zhejiang University.16 Nitrogen (99.999%) and carbon dioxide (dry ice grade) were supplied by Jingong Air Co. 1262

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Table 1. Experimental Results of the Synthesized PDMAEMA-b-PMMAs (PDM)a run

designed PDM

xDMAEMA [%]c

xMMA [%]d

synthesized PDM structure

FMMA [wt %]e

Mn,NMRf

Mn,GPCg

PDIg

S1 S2 S3 S4 S5 S6 S7 S8 S9 S10

PDMAEMA10-b-PMMA5b

96 95 93 98 95 94 96 94 90 90

94 93 93 96 90 86 86 88 94 96

PDMAEMA9.6-b-P(MMA4.7-co-DMAEMA0.4) PDMAEMA9.5-b-P(MMA9.3-co-DMAEMA0.5) PDMAEMA9.3-b-P(MMA14.0-co-DMAEMA0.7) PDMAEMA9.8-b-P(MMA19.2-co-DMAEMA0.2) PDMAEMA9.5-b-P(MMA27.0-co-DMAEMA0.5) PDMAEMA18.8-b-P(MMA8.6-co-DMAEMA1.2) PDMAEMA19.2-b-P(MMA17.1-co-DMAEMA0.8) PDMAEMA18.8-b-P(MMA35.0-co-DMAEMA1.2) PDMAEMA27.0-b-P(MMA9.4-co-DMAEMA3.0) PDMAEMA27.0-b-P(MMA43.0-co-DMAEMA3.0)

19.7 32.7 42.2 50.2 58.5 19.8 32.9 50.1 15.7 45.9

2390 2850 3320 3850 4620 4350 5200 6990 6000 9370

1280 1570 2090 2900 3930 3760 4870 6970 5640 7620

1.17 1.16 1.14 1.18 1.18 1.19 1.18 1.18 1.17 1.16

PDMAEMA10-b-PMMA10 PDMAEMA10-b-PMMA15 PDMAEMA10-b-PMMA20 PDMAEMA10-b-PMMA30 PDMAEMA20-b-PMMA10 PDMAEMA20-b-PMMA20 PDMAEMA20-b-PMMA40 PDMAEMA30-b-PMMA10 PDMAEMA30-b-PMMA45

a All PDMAEMA-b-PMMA block copolymers (PDM) were synthesized by a two-step solution RAFT polymerization. Polymerization for each step was carried out at 70 °C for 9 h using dioxane as the solvent. bDesigned PDM block structure. cConversion of DMAEMA in the first step of polymerization. dConversion of MMA. eWeight fraction of MMA in the PDM. fNumber-average molecule weight (Mn) estimated from 1H NMR spectrum. gDetermined by GPC.

Coagulation and Redispersion of Latexes. In the first coagulation, 0.5 mL of 0.1 M NaOH solution was added to 2 g of the PMMA latex at room temperature (0.05 mmol base versus 0.023 mmol amine groups). The coagulated latex particles were separated from the solution by centrifugation. The particles were subsequently washed with fresh deionized water by two to three times until pH value around 7. In the following two cycles of coagulation, the redispersed latexes were heated to 40 °C through bubbling N2. The coagulation occurred within 10 min for each cycle. For the first cycle of redispersion, additional fresh deionized water was added to the coagulated latex particles to maintain the same solid content as the original latex. The mixture was treated through several minutes’ CO2 bubbling and ultrasonication until a stable latex was obtained. For the next two cycles of redispersion, no additional water was added as little water was lost during the coagulation. CO2 was bubbled through the mixture directly for several minutes, followed by ultrasonication to form a stable latex.

Synthesis of PDMAEMAm-b-PMMAn Copolymers. The PDMs with varying NDMAEMA and NMMA were synthesized through a two-step solution RAFT polymerization (Scheme 2). First, RAFT chain transfer agent CPDTTC (0.691g (2 mmol) for runs S1−S5, 0.346 g (1 mmol) for runs S6−S8 and 0.230 g (0.67 mmol) for runs S9−S10), AIBN (32.8 mg (0.2 mmol) for runs S1−S5, 16.4 mg (0.1 mmol) for runs S6−S8 and 10.9 mg (0.067) mmol for runs S9−S10), DMAEMA (3.144 g (20 mmol)) and dioxane (5.8 g) were charged into a flask. The reaction system was purged by N2 for 0.5 h and polymerized at 70 °C. After 9 h of polymerization, over 90% of DMAEMA conversion was achieved, which was determined by 1H NMR. A deoxygenated solution of MMA (based on the molar ratio of MMA to DMAEMA), AIBN (the same amount of AIBN as that used in the previous step), and dioxane (4.5 g) was then added into the reaction system. The polymerization continued at 70 °C for additional 9 h. The PDM products were collected by precipitation of the reactants in a cold nhexane two times and were dried at room temperature under vacuum. The NDMAEMA and NMMA in the PDM were determined from the 1H NMR spectra and were further confirmed with the DMAEMA and MMA conversion data. The PDMs were acidified with excess 1 M HCl solution followed by freeze-drying prior to use for emulsion polymerization. Synthesis of PMMA Latexes by Emulsion Polymerization. For each emulsion polymerization run, a certain amount of the acidified PDM, comprising 0.29 mmol of DMAEMA, was used. At the beginning, 19 g deionized water was introduced to dissolve the PDM surfactant in a 50 mL three-necked flask. MMA (4.55 g (45.4 mmol)) was then added. After the flask was purged with N2 for 0.5 h at 500 rpm, the V-50 (44 mg, 1.0 wt % of monomer) water solution (dissolved in 1 g of deionized water) was charged and the system was quickly raised to 70 °C. The polymerization was maintained at 70 °C for 4 h under 300 rpm. The monomer conversion was determined gravimetrically. Characterizations. 1H NMR spectra of the PDMs were acquired in a Bruker Advance 2B 400 MHz spectrometer. Particle sizes and distributions of PMMA latexes were measured using a Malvern Zetasizer 3000HSA. ζ-Potentials of the latexes were determined in a Malvern Zetasizer Nano ZS using a capillary cuvette. During the ζ potential measurement for the original latex, 50 μL of the latex was diluted in 2 mL of deionized water. When the measurement was conducted for the recovered latex samples, CO2-saturated deionized water was used. The sample was diluted to the same concentration as that of the original latex. Molecular weights and distributions of the PDMs were determined at 35 °C with a flow rate of THF at 1.0 mL/ min using a Waters GPC. The GPC comprised a Waters 717 Autosampler, a Waters 1525 HPLC Pump, a set of two PLgel 10 μm MIXED-BLS columns and one PLgel 5 μm 100 Å column, and a Waters 2414 Refractive Index Detector. The calibration was conducted using narrow PMMA standards.

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RESULTS AND DISCUSSION Synthesis of PDMAEMA-b-PMMAs. Using the two-step solution RAFT polymerization, we synthesized ten PDM samples having NDMAEMA varying from 10 to 30 and NMMA from 5 to 45. The polymerization of DMAEMA in the first step of each run reached a conversion over 90% in 9 h. In the second step of the MMA polymerization, 86−96% MMA conversions were reacted. The residual DMAEMA monomers in the first step were completely copolymerized with MMA because DMAEMA has a higher reactivity ratio than MMA.17 The block structures of the PDM copolymers were thus obtained. The copolymer compositions of the PDMs were further confirmed from the 1H NMR spectra after purification. The weight fractions of MMA (FMMA) in the PDMs were estimated from 1H NMR results and listed in Table 1. The molecular weight of the RAFT chain transfer agent CPDTTC (MWCPDTTC = 346) was taken into account in the calculation. The FMMA values were between 15.7 and 58.5%. The GPC measurements showed that the PDMs had low polydispersity indexes (PDI) between 1.14 and 1.19 as calibrated by the PMMA standards. The number-average molecular weights (Mn) obtained from the GPC measurements were between 1280 and 7620 g/mol, in comparison with 2390−9370 g/mol from the 1H NMR measurements. Emulsion Polymerization of MMA with PDMAEMA-bPMMA as Surfactant. In our previous work,14 we produced stable PMMA latexes using HCl-acidified PDMAEMA10-bPMMA14 as surfactant. We also found that the polymerization runs with CO2-treated PDM as surfactant was not successful. 1263

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Table 2. Experimental Conditions and Results of the Emulsion Polymerization of MMAa [PDM] run

PDM

[wt % vs MMA]

[10−4 mol/L]b

x [wt %]c

E1 E2 E3 E4 E5 E6 E7 E8 E9 E10

PDMAEMA10-b-PMMA5 PDMAEMA10-b-PMMA10 PDMAEMA10-b-PMMA15 PDMAEMA10-b-PMMA20 PDMAEMA10-b-PMMA30 PDMAEMA20-b-PMMA10 PDMAEMA20-b-PMMA20 PDMAEMA20-b-PMMA40 PDMAEMA30-b-PMMA10 PDMAEMA30-b-PMMA45

1.5 1.8 2.1 2.5 3.0 1.4 1.7 2.2 1.3 2.0

11.7 11.7 11.7 11.7 11.7 5.9 5.9 5.9 3.9 3.9

95.9 96.2 98.2 95.8 geli 99.6 97.4 99.9 97.9 99.9

Dz [nm]d

DV/ D Nd

Np [1017/L]e

ζ-pot. [mV]f

N+ × 10−4 g

CD [nm−2]h

126.9 114.1 111.5 82.0

1.28 1.42 1.37 1.38

1.49 2.05 2.06 5.52

47.1 44.7 44.5 44.2

4.69 3.41 3.18 1.26

0.93 0.83 0.81 0.60

120.3 111.6 88.2 143.6 99.5

1.36 1.49 1.28 1.42 1.34

1.75 2.23 4.44 1.03 3.09

47.1 45.1 44.7 48.2 44.5

3.99 3.19 1.57 6.79 2.26

0.88 0.82 0.64 1.05 0.73

a

All runs comprised of 4.55 g MMA, 44 mg V-50, 20 g deionized water and PDM (0.64 mol % of DMEMA with respect to monomer MMA in emulsion polymerization). The polymerization was carried out at 70 °C and 300 rpm for 4 h. bWith respect to per liter of the latex. cMonomer MMA conversion determined gravimetrically. dDz, Dv, and Dn are the respective z-, volume-, and number-average particle diameters determined by Malvern Zetasizer 3000HSA. eNp is the number of PMMA particles per liter of the latex. fζ-potential determined by Malvern Zetasizer Nano ZS. gNumber of DMAEMA cationic charges on each PMMA latex particle. hCD is the charge density per nm2 of PMMA latex particle surface. iThe emulsion turned into a creamy gelation in 15 min of polymerization.

particle sizes increased with NDMAEMA at both NMMA levels. It was reported that a longer hydrophilic block with the same hydrophobic block length led to the formation of larger size micelles in polystyrene-block-poly[poly(ethylene glycol) methyl ether methacrylate] (PS-b-PPEGMA) 18c and poly(butyl acrylate)-block-poly(acrylic acid) (PBA-b-PAA) systems.18d The longer PDMAEMA block with the same PMMA block in the PDMs yielded the larger PMMA latex particles in this work. Similar observations were made with the latex particles synthesized by emulsion polymerization of St using PS-bPPEGMA having a fixed PS block but varying PPEGMA block18c and that of BMA using PBMA-b-PMAA having the same PBMA length but varying PMAA length.18b The effects of hydrophilic and hydrophobic balance on latex particle sizes were further examined. The latexes for runs E1 and E6, runs E2 and E7, runs E3 and E10, and runs E4 and E8 were prepared with PDMs having the same NDMAEMA/NMMA ratios (2/1, 1/1, 2/3, 1/2) but varying the total molecular weights. It was found that the NDMAEMA/NMMA ratio had a major influence on the latex particle size, while the copolymer molecule weight played a minor role. All the Dz data of the latexes were plotted versus the weight fraction of MMA (FMMA) in the PDMs in Figure 1. The Dz decreased gradually with the increase of FMMA. The particle size was a strong function of the surfactant structure, that is,

Therefore, all the PDMs in this work were acidified with 1 M HCl solution, followed by freeze-drying to remove water and excess acid prior to use. V-50 was used to initiate the polymerization, which was carried out at 70 °C for 4 h under N2 atmosphere. The monomer conversion, particle size, and ζpotential data with their corresponding experimental conditions are summarized in Table 2. Stable PMMA latexes with MMA conversion over 95% and little coagulum were obtained except for run E5. When PDMAEMA10-b-PMMA30 was used, the polymerization system turned into a creamy gel within 15 min. The FMMA in PDMAEMA10-b-PMMA30 was 58.5%. The latex particles in runs E1−E4 and runs E6−E10 had the Z-average diameters (Dz) of 82.0−143.6 nm and the size distributions Dv/Dn of 1.28−1.49. There were (1.03−5.52) × 1017 PMMA particles per liter of latex. The latexes had the ζpotentials of 44.2−48.2 mV. The effect of PDM hydrophobic block length on latex particle size was examined. Runs E1-E4 latexes were prepared with PDMs of NDMAEMA = 10 and varying NMMA = 5, 10, 15, and 20. Runs E6−E8 latexes were prepared with PDMs of NDMAEMA = 20 and NMMA =10, 20, and 40. E9−E10 latexes were prepared with PDMs of NDMAEMA = 30 and NMMA =10 and 45. It can be seen that, at all the three levels of NDMAEMA, the Dz values of the latex particles decreased with the increase of hydrophobic PMMA block. The same phenomena were observed with PS latexes synthesized by the emulsion polymerization of St with poly(acrylic acid)-block-polystyrene (PAA-b-PS) having a fixed hydrophilic PAA block but varying hydrophobic PS block18a and poly(butyl methacrylate (BMA)) latexes by the emulsion polymerization in the presence of poly(butyl methacrylate)-block-poly(methacrylic acid) (PBMAb-PMAA) having the same PMAA length but varying PBMA length.18b Further increase of NMMA in the PDMs (i.e., NMMA = 30 in run E5) led to the formation of very small PMMA latex particles, resulting in insufficient coverage of PDM on the particle surfaces and thus the latex gelation. The latexes synthesized with the PDMs having different hydrophilic block lengths were also studied. Runs E2, E6, and E9 latexes were prepared with PDMs having NMMA = 10 and varying NDMAEMA = 10, 20, and 30. Runs E4 and E7 were prepared with NMMA = 20 and NDMAEMA = 20 and 30. The

Figure 1. Variations of Z-average particle size and ζ-potential of the PMMA latexes with the weight percentage of MMA in the PDMs. 1264

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NDMAEMA and NMMA of the PDMs. It thus offers an opportunity to tailor the latex particle size through control of the block composition. The ζ-potentials of the latexes were determined and the data were also plotted against FMMA in Figure 1. The ζ-potential initially decreased with FMMA and then leveled off at FMMA ∼ 33%. Assuming no entanglement of PDM chains and all PDM chains resided on particle surfaces, we estimated the maximum average number of DMAEMA cationic charges per particle (Nmax+) using the following equation, + Nmax = (nDMAEMANA )/(NpV )

(1)

where nDMAEMA is the mole of DMAEMA units in PDM used in emulsion polymerization and it is the same value of 0.29 mmol in all the runs, NA is Avogadro’s constant, Np is the number of PMMA particles per liter of latex, and V is the volume of the PMMA latex. The maximum numbers of DMAEMA cationic charge per particle were between 1.26 × 104 and 6.79 × 104 for all the latexes, corresponding to 750−4690 PDM chains per particle. Considering the low initiator dosage, the charges on latex particle surfaces were mainly from the ammonium cations of PDM. The maximum charge density (CD) on the PMMA particle surface could thus be estimated from CD = (nDMAEMANA )/(πDz 2NpV )

(2)

The maximum CD values were between 0.60 and 1.05 per nm2, equivalent to a maximum of 0.024−0.093 PDM chains per nm2 of particle surface. Reversible Coagulation and Redispersion. Because HCl-protonated PDMs was employed, it was difficult to directly coagulate the latex particles by N2. Instead, a small amount of caustic soda was needed. The coagulated particles were separated centrifugally and were washed with fresh deionized water twice or three times until pH ∼ 7. In the following coagulation and redispersion cycles, additional fresh deionized water was added to the coagulated particles to ensure the same solid content as the latexes before coagulation. The obtained latexes were used for further N2-coagulatable and CO2-redispersible switchability studies. As an example, Figure 2 shows the images of run E7 sample in three cycles of coagulation and redispersion. In the first redispersion process, the E7−S1 mixture was bubbled with CO2 and ultrasonicated for a few minutes. This CO2 bubbling/ultrasonication process was repeated for 3−4 times to achieve a stable latex sample (E7−R1) that had a bright bluish appearance. In the second cycle of coagulation, the recovered latex sample (E7−R1) was heated to 40 °C while bubbling with N2. The coagulation occurred in 5−10 min. The coagulated mixture (E7−D2) was then applied for the second cycle of redispersion following the same procedure as the first cycle. Stable bluish latex sample (E7−R2) was also obtained, as shown in Figure 2. The switchability and repeatability of the latexes was further confirmed by another cycle of N2-coagulation and CO2redispersion. The Malvern Zetasizer 3000HSA was used to determine the particle sizes of the recovered latexes in each cycle (E7−R1 ∼ E7−R3). The Z-average particle sizes increased less than 8 nm after three cycles of coagulation and redispersion, while the size distribution curves remained almost the same, as shown in Figure 3. The ζ-potential of the original latex (E7), determined by Malvern Zetasizer Nano ZS, was 45.1 mV. The ζ-potentials for the recovered latexes were 41.9−42.3 mV after redispersion through CO2-bubbling and ultra-

Figure 2. Coagulation and redispersion of run E7 latex sample: the first cycle of coagulation (E7−D1) was conducted with caustic soda and the following two cycles of coagulation (E7−D2 and E7−D3) were carried out by N2 bubbling with gentle heating; redispersions (E7−R1 ∼ E7−R3) of the coagulated particles were conducted by CO2 bubbling with ultrasonication.

Figure 3. Particle size distribution curves of the latex E7 before and after each cycle of coagulation and redispersion as determined by Malvern Particle Sizer.

sonication. These values were close to that of the original latex with an approximately 3 mV decrease. This might attribute to the removal of residual initiator in water after washing during the first cycle of coagulation. All the other latex samples prepared using different PDMs in this work were thoroughly investigated for their coagulatability and redispersibility. Figure 4 summarized the data of Dz for each latex sample, both original and recovered ones. For most prepared latexes, the light scattering measurement results were 1265

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CONCLUSIONS

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AUTHOR INFORMATION

Article

In summary, we extended our previous feasibility study on the preparation of reversibly coagulatable/redispersible polymer latexes using switch block copolymer surfactants. Our objective is to investigate the relationship between the copolymer block composition and the latex performance. A series of well-defined block copolymers of 2-(dimethylamino)ethyl methacrylate (DMAEMA) and methyl methacrylate (MMA) were synthesized via a two-step RAFT polymerization process. The block copolymers contained 10−30 DMAEMA units and 5−45 MMA units, with the weight fraction of MMA (FMMA) ranging from 15.7−58.5%. After acidification, the copolymers were employed as surfactant in the emulsion polymerization of MMA. The resulted PMMA latexes were stable when the FMMA of the surfactant was lower than 58.5%. The particle size of the PMMA latexes decreased gradually with the increase of FMMA. The initial latex particles could be easily coagulated with addition of a small amount of caustic soda. The coagulated latex particles, even after washing, could still be redispersed into fresh water to form stable latexes through CO2 bubbling and ultrasonication. The recovered latexes were coagulated again through N2 bubbling and some heating. The obtained pastes could still be redispersed into fresh water to form stable latexes. The latexes showed excellent redispersibility with the surfactants of FMMA < 46%. However, the coagulation/ redispersion process could result in >20% increase in the latex particle size if a surfactant having the MMA weight fraction >46% was employed. The charge density on particle surface was found to be a key parameter in determining the latex redispersibility. This work elucidated the effects of the block composition of the copolymer surfactant on the preparation of reversibly coagulatable and redispersible PMMA latexes. It provided useful guidance for designning effective polymer surfactants.

Figure 4. Variations of the Z-average diameter for all the PMMA latex samples and their redispersed forms with three cycles of coagulation and redispersion. For each run from left to right: original (black), R1 (red), R2 (blue), and R3 (green).

as good as run E7. The Dz increases in run E4, E8, and E10 were significant in the first cycle, about 20−30 nm, but changed little in the successive cycles. The ζ-potentials were not less than 41 mV for all the recovered latex samples, suggesting good stability as the original ones. The average changes in Dz and ζ-potential after three cycles of coagulation and redisperison were a good indicator for redispersibility of the latex particles and for assessing performance of the PDM surfactants. It is interesting to observe that the change in Dz was closely related to the charge density (CD). As shown in Figure 5, the changes in Dz were all

Corresponding Author

*(W.-J.W.) Telephone: +86-571-8795-2772. Fax: +86-5718795-2772. E-mail: [email protected]. (S.Z.) Telephone: +1-905-525-9140, ext 24962. Fax: +1-905-521-1350. E-mail: [email protected].

Figure 5. Variations of the average changes in Dz and ζ potential after three cycles of coagulation and redispersion versus the original latex charge density per nm2 of PMMA particle surface.

Present Addresses ‡

Global Research Center GE - SABIC Innovative Plastics Program. ⊥ Polymer Science and Engineering, University of Massachusetts Amherst, Amherst, MA.

−2

0.42 nm . However, the Dz value increased >20% when CD was