CO2 Triggered Reversibly Coagulatable and

Article pubs.acs.org/Langmuir

Preparation of N2/CO2 Triggered Reversibly Coagulatable and Redispersible Latexes by Emulsion Polymerization of Styrene with a Reactive Switchable Surfactant Qi Zhang,† Guoqiang Yu,† Wen-Jun Wang,*,† Haomiao Yuan,‡ Bo-Geng Li,† and Shiping Zhu*,§ †

State Key Lab of Chemical Engineering, Institute of Polymerization and Polymer Engineering, Department of Chemical and Biological Engineering, and ‡Department of Polymer Science and Engineering, Zhejiang University, Hangzhou, Zhejiang 310027, People’s Republic of China § Department of Chemical Engineering, McMaster University, Hamilton, Ontario L8S 4L7, Canada S Supporting Information *

ABSTRACT: This work reports the development of a reversibly coagulatable and redispersible polystyrene latex system that can be triggered by N2/CO2. The coagulatability and redispersibility of the latexes were achieved by employing 0.9− 5.6 wt % (N-amidino)dodecyl acrylamide (DAm), a reactive switchable surfactant, in an emulsion polymerization of styrene under CO2 atmosphere. The resulted latex particles were readily coagulated by N2 bubbling at 60 °C and redispersed by CO2 bubbling and ultrasonication, which switched amidine moieties between neutral and ionic states. The coagulation/redispersion processes were repeatable. The prepared latexes showed good stabilities against electrolytes, especially with higher charges.

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amount of protective stabilizer6a or by introduction of functional groups on polymer particles,6b−d these practices have not reached a level of industrial satisfaction, as high costs in energy consumption and operation occurred. Type and amount of used surfactant critically influence latex coagulation and redispersion. Generally speaking, the better the surfactant works in preserving stability, the more difficult the latex is to be coagulated. Coagulation is often implemented by adding a large amount of salt, acid, or alkali, to help the particles to overcome energy barriers. Upon over the barrier, a primary minimum occurs, and, in most cases, this will become irreversible.7 Therefore, it is very challenging, if not impossible, for coagulated particles to be redispersed into water to form a stable latex product. However, the recent development of switchable surfactants offers an alternative control of latex stability and the potential to achieve reversible coagulation and redispersion.8−13 These surfactant molecules bear functional stimuli-responsive groups that can be switched alternatively between surface-active or surface-inactive forms by some triggers. Among the reported switchable surfactants, only amidine surfactants, first developed by Jessop’s group, were applied in the preparation of polymer latexes,14,15 whose surface activities could be turned on and off by bubbling CO2 and N2 (or Ar) gases.13

INTRODUCTION As an important type of product in the polymer industry, polymer latexes have been widely used in many areas, such as coatings, adhesives, binders, etc.1 The latexes have also been applied in biomedical and pharmaceutical fields lately through preparation of particles with tailored functionality, specific microstructure, or morphology.2 In many applications, dry polymer powders are required. The latex particles, therefore, must be coagulated, filtrated, washed, and dried from their emulsions.3 In the industry, coagulation or destabilization of latexes is often accomplished by adding salts, acids for anionically stabilized latexes, or alkali for cationically charged latexes.1,4 A washing process must be followed to remove residual inorganic additives, which generates substantial amount of wastewater. Hence, it is highly desired for sustainability of the industry to develop less energy intensive and more environmentally benign coagulation processes without using inorganic additives for latex particle separation.5 In such applications as waterborne coating, the desired product form is latex itself. However, production and application of emulsion products are often not in the same location, about one-half of transportation and storage costs are wasted in moving water from one place to another, not to mention its inconvenience in liquid handling. Therefore, the development of redispersible latex products is indeed of significant contribution to meet sustainability challenges of the emulsion polymerization industry.6 Although latex redispersibility could be achieved either by adding an adequate © 2012 American Chemical Society

Received: January 4, 2012 Revised: March 13, 2012 Published: March 15, 2012 5940

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We developed a reversible coagulatable and redispersible polystyrene (PS) latex system by incorporating amidine functional groups into polymer chains through a soap-free emulsion copolymerization.16 The latex particles could be easily coagulated by adding a small amount of caustic soda, and the coagulated particles could be redispersed into water by bubbling CO2 to prepare a stable latex without adding extra stabilizer. The coagulation and redispersion processes were repeatable. Although the latex products were not N2coagulatable, the work clearly showed that switchable latexes could be achieved if amidine moieties could be covalently bounded to particle surfaces. However, salt accumulation from the caustic soda addition would limit the number of coagulation/redispersion cycles. Recently, Jessop’s group prepared polystyrene latex that could be aggregated using nitrogen with gentle heating and be redispersed using CO2 with sonication.15 The switchability was achieved by employing C12-amidine surfactant in combination with 2,2-azobis[2-(2-imidazolin-2-yl)propane] (VA-061) bicarbonate initiator. The long-chain alkyl amidine surfactant was physically adsorbed onto particle surface and could be readily washed off, making redispersion not feasible. The switchability was also reported with polystyrene latex synthesized by soapfree emulsion polymerization with VA-061 as initiator lately.17 Our previous work proved that the amidine-containing comonomer approach is effective in tackling the problem caused by the desorption of physically absorbed surfactant molecules from particle surface.18 It is also known that acyclic amidine moieties on latex surface are preferred for effective coagulation by N2. Therefore, we hypothesized that a reversibly coagulatable and redispersible latex product be achievable if a N2/CO2-switchable acyclic amidine-containing comonomer is employed as surfactant in emulsion polymerization. This approach could offer a good control of latex stability without additional additive. Because of micellar nucleation, it also gives narrower polymer molecular weight distribution than in homogeneous nucleation. Yuan et al. timely reported an acrylamide-derivative amidine,19 which could serve as a candidate for the reactive surfactant. The dual functionalities, that is, amidine and vinyl groups, were connected by an alkyl chain C12H24. We incorporated a small amount of this reactive switchable surfactant in the emulsion polymerization of styrene (St). Reported in this Article are the synthesis of this surfactant (N-amidino)dodecyl acrylamide (DAm), emulsion polymerization of St with DAm, and demonstration of reversible coagulation and redispersion of the PS latex product. Latex coagulation was triggered by bubbling N2 with heat, while latex redispersion was through bubbling CO2. Scheme 1 represents the N2/CO2-triggered reversibly coagulatable and redispersible polystyrene latex prepared by the switchable reactive surfactant DAm.

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Scheme 1. Sketch of N2/CO2 Triggered Reversibly Coagulatable and Redispersible Polystyrene Latex Prepared by the Switchable Reactive Surfactant DAm

(0.875 g, 3.5 mmol) in water (20 g) until the aqueous solution became transparent. a. Synthesis of N-Boc-1,12-diaminododecane (1). 1,12-Diaminododecane (10.02 g, 50 mmol) was dissolved in CHCl3 (200 mL). Boc anhydride (2.18 g, 10 mmol) was dissolved separately in CHCl3 (50 mL), and this solution was then added dropwise to the diamine solution. The resulting mixture was stirred overnight. After filtration, the filtrate was concentrated, adsorbed onto SiO2, and purified by flash column chromatography (SiO2) eluting with 6% MeOH/dichloromethane to obtain pure mono-Boc-protected diamine 1 after drying in vacuo (2.34 g, 78%). b. Synthesis of (N-Boc-amido)dodecyl Acrylamide (2). N-Boc1,12-diaminododecane (1.44 g, 4.8 mmol) and Et3N (1 mL, 7 mmol) were dissolved in CHCl3 (20 mL). Acrylyl chloride (0.5 mL, 5.8 mmol) was added dropwise, while temperature was kept at 0−5 °C. After being stirred for 4 h at room temperature, the reaction was stopped. The mixture was washed with 1 M HCl solution, 1 M NaOH solution, and DI water successively. The organic layer was collected and was dried over anhydrous Na2SO4. After removal of Na2SO4, the solvent was evaporated to obtain 2 (1.67 g, 98%) as a white solid. c. Synthesis of (N-Amido)dodecyl Acrylamide·TFA (3). (N-Bocamido)dodecyl acrylamide (1.67 g, 4.7 mmol) was dissolved in ethanol (10 mL). After TFA addition (10 equiv), the mixture was left to stir overnight at room temperature. The solvent and excess TFA were removed by a rotation evaporator; crude product was then purified by recrystallization in ether to obtain white solid 3 (1.56 g, 90%). d. Synthesis of (N-Amidino)dodecyl Acrylamide (DAm). The amine TFA salt 3 (1.36 g, 3.7 mmol) was dispersed in CHCl3 (10 mL), and washed with 1 M NaOH solution (5 mL) and DI water (5 mL), successively. The organic layer was collected and was dried over anhydrous Na2SO4. After removal of Na2SO4, the solvent was evaporated. 2 M dimethyl amine (8 mL) and dimethylacetamide dimethyl acetal (0.53 g, 4 mmol) were added, and the reaction mixture was stirred for 18 h in dark. The product was obtained by drying under vacuum. Determined gravimetrically, the amidine yield was nearly quantitative. A higher purity sample was obtained by bubbling CO2 for 60 min through a wet acetonitrile mixture solution to collect the white solid. The solid was again dissolved in a small quantity of acetonitrile and then bubbled with N2 to recover the deionized structure. Finally, the solvent was removed, and the end-product was dried (1.06 g, 89%). 1 H NMR (400 MHz, CDCl3, δ): 6.21 (d, J = 17.0 Hz, 1H; CH2 CH−), 6.10 (q, 1H; CH2CH−), 5.56 (d, J = 10.2 Hz, 1H; CH2 CH−), 3.28 (t, J = 7.1 Hz, 2H; OC−NH−CH2), 3.17 (t, J = 7.5 Hz,

EXPERIMENTAL SECTION

Materials. 1,12-Diaminododecane, N,N-dimethylacetamide dimethyl acetal (98%), and 2,2-azobis[2-(2-imidazolin-2-yl)propane] (VA-061) were supplied by Acros and were used without further purification. Boc anhydride, acryloyl chloride, and trifluoroacetic acid (TFA) were supplied by Aladdin-Reagent Co. Other chemicals were analytical-grade reagents and were used as received. All aqueous solutions were prepared with deionized (DI) water. Carbon dioxide (dry ice grade) and nitrogen (99.999%) were purchased from Jingong Air Co. Styrene was distilled under vacuum prior to use. The initiator solution was prepared by bubbling carbon dioxide through VA-061 5941

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2H; CH2−NC−), 2.89 (s, 6H; −N(CH3)2), 1.49 (s, 3H; −NC− CH3), 1.23−1.27 (m, 20H; −(CH2)10−). MS (ESI, m/z): [M + H]+ calcd for C19H38N3O, 324.52; found, 324.1. e. Synthesis of (N-Amidino)dodecyl Acrylamide Hydrochloride Salt (DAm·HCl). The hydrochloride salt of DAm was obtained by recrystallization in 1 M HCl solution. The white solid was dried in vacuum to constant weight. Scheme 2 summarizes the synthesis of the switchable amidine surfactant.

Scheme 2. Synthesis Route of Switchable Reactive Surfactant DAma Figure 1. Surface tension versus DAm·HCl concentration in aqueous solutions at 25.5 ± 0.1 °C.

per surfactant molecule (as) at the air/water surface were estimated according to ref 20. Table 1 lists the results of CMC value and as of DAm·HCl. As compared to the other two surfactants, the synthesized

a (a) (Boc)2O/CHCl3, room temperature overnight; (b) CH2 CHCOCl, Et3N/CHCl3; (c) CF3COOH/EtOH, room tempertaure 8 h; (d) N,N-dimethyl acetamide dimethyl acetal, ET3N/CHCl3, 65 °C, 0.5 h.

Table 1. Comparison of CMC and as Values of Three Surfactants

Preparation of Polystyrene Latexes. St (4.55 g) and DAm (0.075−0.25 g) were added to a 50 mL three-necked flask that contained 18 g of DI water. The mixture was agitated at 500 rpm and was purged with CO2 for 30 min. The initiator solution was prepared by bubbling CO2 through an aqueous solution of VA-061 (87 mg in 2 g of water) until it was well dissolved. The initiator solution was added to the reactor by a syringe. The mixture was quickly heated to 55 °C at 300 rpm. The polymerization was conducted for 6 h. The monomer conversion was determined gravimetrically. The freeze-dried latex samples were prepared for GPC and NMR measurements. Latex Coagulation and Redispersion. The latex (2 g) was heated to 60 °C with bubbling N2. Once gel formation was observed, additional water (1 mL) was added to facilitate separation, yielding suspension. The suspension was further treated by bubbling N2 with heating for 1 h. The polymer particles were left to settle under gravity. Centrifugation was used to accelerate the separation of coagulated particles from the solution. To redisperse the coagulated PS particles into the same solution, the mixture was bubbled with CO2 for several minutes, followed by ultrasonication. The bubbling and ultrasonication processes were repeated three to four times until a stable latex was obtained. Characterizations. 1H and 13C NMR spectra were acquired in a Bruker Advance 2B 400 MHz spectrometer. ESI/MS measurement was conducted using a Finnigan LCQ DECA XPplus instrument (150−2000 m/z). The critical micelle concentration (CMC) of DAm·HCl was determined at 25.5 ± 0.1 °C by surface tension measurement on aqueous solution (OCA20 Video-based Contaot Measuring Device). The conductivity of DAm in DMSO was measured using a Lei-ci conductivity meter DDSJ-308A at 25 ± 0.2 °C. Prior to the conductivity measurement, the solution was bubbled with N2 for 1 h until a stable conductivity was reached. The particle size and distribution were determined by a Malvern Zetasizer 3000HSA (670 nm, 3 mW, 90° scattering angle) at 25 °C. The particle morphology was visualized using a JEM-1200EX TEM.

a

The surface tension at the CMC. bReference 12.

amdinum appeared to have a higher CMC value and a smaller molecule area. For an ionic surfactant, smaller as means higher charge density on latex particle surface, leading to better emulsion stability. The switchability of DAm was monitored by conductivity tests. The conductivity of 10 mM of DAm in dimethyl sulfoxide (DMSO) was determined at 25 ± 0.2 °C for three cycles of alternated bubbling CO2 or N2. Figure 2 shows the results. The conductivity of the solution increased from 37.4 to 77.6 μS/cm in 15 min and leveled off when CO2 was bubbled. It reduced to its initial value in 65 min upon bubbling with N2. The

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RESULTS AND DISCUSSION Characterization of Switchable Reactive Surfactant DAm. As a cationic surfactant, we first investigated its surface activity. The CMC value of DAm·HCl was determined from the breakpoint of the plot of surface tension versus concentration of their aqueous solutions at 25.5 °C, as shown in Figure 1. The surface excess concentration (Γ) and the area

Figure 2. Variation of the conductivity of DAm in DMSO solution (10 mM) at 25 °C versus time during three cycles of alternatively bubbling CO2 and N2. 5942

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Table 2. Experimental Conditions and Results from the Emulsion Polymerization of Styrene with Switchable Reactive Surfactant DAma run

[DAm]/[St] × 102

x [%]b

coag. [%]

hydrolysis [%]c

Fad × 102d

Fad × 102e

Np [1017/L]

Dz [nm]

E1 E2 E3 E4 E5 E6

0.3 0.6 0.9 1.2 1.5 1.8

89.0 90.1 93.1 93.3 94.3 95.1

3.7 0.0 4.9 0.8 2.8 2.2

18 14 19 11 11 15

0.28 0.57 0.78 1.14 1.42 1.61

0.42 0.59 0.65 1.13 1.43 1.51

0.92 1.75 2.68 3.60 4.75 6.66

157.5 126.6 110.8 100.0 90.5 78.9

a All runs contained 4.55 g of St and 20 g of water, and were carried out at 55 °C and 300 rpm for 6 h under CO2 atmosphere; initiator is VA-061 (0.8 mol % of St for each run). bTotal monomer conversion determined gravimetrically. cPercentage of amidine hydrolysis during emulsion polymerization as measured by 1H NMR. dThe molar ratio of switchable amidine incorporated with PS as estimated from the conversion results. e The molar ratio of switchable amidine incorporated with PS as determined by 1H NMR.

successive cycles gave similar conductivity data, which showed good reversibility and repeatability of the amidine. Good switchability of DAm by CO2 and N2 bubbling is clearly demonstrated. Polystyrene Latexes by Emulsion Polymerization of Styrene with DAm. After the surface activity and switchability of the surfactant were confirmed, the cationic groups of DAm were introduced onto particle surfaces to stabilize the emulsion. DAm acted as surfactant for micellar nucleation, and also acted as comonomer for unwashable stabilizer, as shown in Scheme 1. DAm 0.3−1.8 mol % (equivalent to 0.9−5.6 wt %) was used in the emulsion polymerization experiments, which were initiated by VA-061 and carried out under CO2 atmosphere at 55 °C for 6 h. Table 2 summarizes the experimental conditions, as well as the data of monomer conversion, amidine hydrolysis, polymer composition, and particle size. The amidinium surfactant effectively stabilized the emulsion in all runs. Stable PS latexes were obtained with little coagulum. The samples were stored under CO2 atmosphere at room temperature for more than 3 months without any observable separation. The overall monomer conversions were between 89% and 95%. 1H NMR measurements of the resulted latex samples were used to estimate the percentage of amidine hydrolysis and molar ratio of amidine copolymerized with St. The samples were prepared by freeze-drying. As DAm has a high boiling point,19 only unreacted St and water contents were removed, but the unreacted DAm should remain, if any. There were no residual vinyl groups determined at the chemical shifts of 6.21, 6.10, and 5.56 ppm, suggesting complete reaction of DAm during the polymerization. It has been reported that amidine groups were stable under acidic conditions and their hydrolysis occurred under basic conditions.21 The NMR measurement was used to examine the hydrolysis of DAm during polymerization. Figure 3 shows a representative 1H NMR spectrum from the run E3 polymer sample. While the integration of the peaks at 3.43 ppm was calibrated to 4 to represent the f and g protons, the integration of the e protons at 3.24 ppm was found to be 4.86, which was supposed to be 6 if there was no amidine hydrolysis. The percentage of hydrolyzed amidine during emulsion polymerization was 19% (1−4.86/6 = 0.19) in run E3. The amidine hydrolysis percentages in other runs were between 11% and 19%. An analysis of 1H NMR spectra also revealed that the molar fractions of the switchable amidine in the polymer samples after freeze-drying were between 0.42 and 1.51 mol %, agreeing with the data estimated from the conversion data. Run E4 was further treated by three cycles of tetrahydrofuran dissolution and methanol precipitation to remove water-soluble

Figure 3. 1H NMR spectrum of run E3 polymer sample.

oligomer formed by DAm in the latex. The 1H NMR measurement showed the DAm amount in the treated polymer sample was 17% less than the freeze-dried sample, suggesting that 17% DAm was homopolymerized while 83% was incorporated into the copolymer with styrene during emulsion polymerization. The polymer particles had a Z-average diameter (Dz) between 78.9 and 157.5 nm, and there were (0.85−6.74) × 1017 PS particles per liter of latex. The effects of surfactant concentration on monomer conversion, particle size, and number of particles (Np) were examined. With an increase in the DAm concentration, the total monomer conversion increased from 89% to 95% and the particle size decreased from 157.5 to 78.9 nm, while the Np increased from 0.85 × 1017 to 6.74 × 1017 L−1. The surfactant concentration dependence of monomer conversion, particle size, and Np is characteristic of batch emulsion polymerization with micellar nucleation.22 Reversible Coagulation and Redispersion of Latexes. It has been demonstrated in our previous work that reversibly coagulatable and redispersible latex product is achievable if amidine moieties can be covalently bounded to particle surface.15 The vinyl-containing DAm in this work copolymerized with St during emulsion polymerization, and thus the amidine functional groups were incorporated onto the latex particles. We also used VA-061 as a switchable initiator to provide an additional source of switchability, following Jessop’s recent work.14 The initiator amount was 0.8 mol % of St. Because of its low initiator efficiency (typically 2−10%), there was less than 0.08 mol % VA-061 attached to polymer chains, which was far lower than the molar ratio of DAm in the polymer (0.42−1.51 mol %). The latex switchability was mainly contributed by the reactive switchable surfactant DAm. Because DAm could be readily switched between charge and neutral 5943

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Figure 4. Three cycles of latex coagulation by bubbling N2 with heating (E3-D1−E3-D3) and redispersion of the coagulated particles with CO2 bubbling (E3-R1−E3-R3) for run E3 sample.

Figure 5. Particle size distributions for PS latex samples and their redispersed forms with three cycles of coagulation and redispersion. For each run: original (black), RS1 (red), RS2 (blue), and RS3 (green). (a) Run E3; (b) run E4; (c) run E5; and (d) run E6.

forms by purging with CO2 and N2, the switchability of the PS latexes was also examined with N2 and CO2. The reaction of converting amidinum bicarbonate to amidine was endothermic.14 Elevating temperature helped particle encounters, thus prompting particle aggregation.14 Therefore, in our system, the coagulation process was performed by heating 2.0 g of the latex at 60 °C with continuously bubbling N2. In destabilizing run E3−E6 samples, the latexes experienced gel formation, which occurred in 30−60 min. The higher was the amidine concentration, the shorter was the time of gel formation. For run E5 and E6 samples that had the highest amidine contents, the gel formation occurred so fast that the systems could not be stirred easily. Once gel formation was observed, an dditional 1 mL of water was added to facilitate separation, giving a suspension. The suspension was further

treated by bubbling N2 with heating for another hour. The latex particles were left to settle under gravity. Use of centrifugation could accelerate the separation of coagulated particles from solution. Figure 4 shows a typical example of run E3 latex sample. The other samples (runs E4−6) had similar performances in coagulation. However, the samples having lower amidine concentrations (runs E1 and E2) did not have gel formation even by extending heating to 2 h or elevating temperature to 70 °C. The ease of latex coagulation depended on the amount of amidine functionality.13 Because of their incomplete destabilization, run E1 and E2 samples were not chosen for redispersion studies. The coagulated particles were redispersed to the same solution by bubbling CO2 followed by ultrasonication three to four times. The coagulated PS particles were well dispersed and 5944

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Table 3. Z-Average Particle Size, Number of Particles per Liter of Latex, and Particle Volume Ratio of Redispersed Latex to Its Original Sample before and after Each Cycle of Coagulation and Redispersion Np [1017/L]

Dz [nm] run E3 E4 E5 E6

original 110.8 100.0 90.5 78.9

± ± ± ±

3.6 3.2 4.0 3.7

RS1 131.7 104.4 108.3 104.2

± ± ± ±

RS2 2.4 1.0 3.8 5.3

132.4 107.7 113.7 104.8

± ± ± ±

RS3 6.7 0.5 9.8 0.7

131.4 110.3 111.6 109.7

± ± ± ±

(Dz/Dz0)3

original

RS1

RS2

RS3

RS1

RS2

RS3

2.68 3.60 4.75 6.66

1.82 2.98 3.21 4.49

1.76 2.75 3.06 3.43

1.81 2.48 2.95 2.76

1.68 1.14 1.71 2.30

1.71 1.25 1.98 2.34

1.67 1.34 1.88 2.69

4.0 1.6 0.5 1.0

Table 4. Latex Stability against Electrolytes: Changes of Dz (nm)a

restabilized. Stable latex emulsions had a slightly bluish appearance after redispersion. The coagulation and redispersion processes were repeated for three cycles with the run E3 sample. All of the repeats had the same observation as the first cycle. Figure 4 also shows the images in the three cycles of coagulation and redispersion processes. Their particle size distributions were determined by Malvern Particle Sizer as shown in Figure 5. Although it increased about 20 nm after the first cycle, the average particle size changed little in the successive cycles. The particle size measurement gave similar monomodal distributions for E3-R1 to E3-R3, without large aggregates observed. As compared to the original latex, the particle size distribution experienced a small shift toward the larger size end and also appeared a little wider. The included TEM images strongly supported the DLS results. Almost all of the particles in the original latex sample were nicely isolated from each other with little aggregation. However, the redispersed latex samples had a few small clusters of particles, although no microscale large aggregates were present. Run E4−6 latex samples were also thoroughly investigated for their coagulatability and redispersibility, and all of the measurements gave consistently good results as run E3. Their particle size distributions are shown in Figure 5. Table 3 summarizes the Z-average particle size and the number of particles per liter of latex for runs E3−E6 and their recovered latex samples. The Np values were calculated from the latex particle size distributions. The particle volume ratios of redispersed latex to its original sample, (Dz/Dz0)3, are also presented in the table. All of the runs had Np between 1.8 and 6.7 × 1017 particles per liter of latex. After three-cycle coagulation and redispersion, Dz increased by 10−39% and Np decreased 31−59%, while the (Dz/Dz0)3 were 1.14−2.69. These changes in Dz and Np were caused by a few small clusters, which was also shown as (Dz/Dz0)3 > 1. Latex Stability against Electrolytes. The latex stability against electrolyte was studied by adding 0.5 mL of electrolyte solution to 0.5 g of latex. Three types of salts at various concentration levels were used. After 24 h, the particle sizes were measured again with the DLS. Table 4 shows the results. By adding 0.2 M NaCl solution, some coagulation was observed for run E1 and E2 samples. All of the latexes experienced a total coagulation with addition of 0.5 M NaCl solution. The increase in particle size seemed to slow from CaCl2 to AlCl3. The prepared latex had good stability against electrolytes, especially those with higher charges. The higher was the amidine concentration, the better was the stability against electrolyte.

run

original

NaCl (0.2 M)

E1 E2 E3 E4 E5 E6

157.5 126.6 111.5 100.0 90.5 78.9

× × × × 114.6 98.5

NaCl (0.5 M)

CaCl2 (0.1M)

CaCl2 (0.2 M)

AlCl3 (0.02 M)

AlCl3 (0.05 M)

×× ×× ×× ×× ×× ××

251.6 142.1 117.2 118.2 96.6 83.8

× 160.6 136.1 147.8 130.2 104.6

159.3 129.6 114.3 100.1 92.9 83.1

161.7 134.7 122.4 108.1 95.2 91.8

a

Determined by adding 0.5 mL of an electrolyte solution to 0.5 g of latex. After 24 h, the particle size was measured again with DLS. ××, total coagulation on salt addition; ×, some coagulation on salt addition, but total coagulation after 24 h.

surfactant bearing both vinyl and amidine functions. The surface activity of the surfactant was reversibly switchable, as the amidine function could be shifted between ionic and neutral states by alternatively bubbling CO2 and N2. The surfactant molecules acted as an effective stabilizer in the emulsion polymerization and became unwashable upon incorporation into polystyrene chains. The latex particles thus formed could be easily coagulated by bubbling N2 with slight heating, without salt, acid, or base addition as often required. The coagulated particles could be redispersed into water by bubbling CO2 and ultrasonication to form a stable latex, without extra stabilizer addition. The coagulation and redispersion processes were repeatable. The redispersed latexes showed good stability against electrolytes. Such a reversibly coagulatable and redispersible latex system directly prepared from an emulsion polymerization is believed to have a commercial potential in cost savings in the areas of separation, storage, and transportation that benefit economy and environment.

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ASSOCIATED CONTENT

S Supporting Information *

1

H NMR spectrum and mass spectrum of DAm. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-571-8795-2772 (W.-J.W.); (905) 525-9140, ext 24962 (S.Z.). Fax: +86-571-8795-2772 (W.-J.W.); (905) 5211350 (S.Z.). E-mail: [email protected] (W.-J.W.). Notes

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The authors declare no competing financial interest.

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CONCLUSIONS We successfully developed a N2/CO2 triggered reversibly coagulatable and redispersible polystyrene latex system. The latexes were prepared through emulsion polymerization of styrene employing a small amount of reactive switchable

ACKNOWLEDGMENTS This work is financially supported by the National Natural Science Foundation of China (Grant No. 20976153), the Chinese State Key Laboratory of Chemical Engineering at 5945

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Zhejiang University (Grant No. SKL-ChE-12T05), and the Program for Changjiang Scholars and Innovative Research Team in University in China (IRT0942).

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dx.doi.org/10.1021/la300051w | Langmuir 2012, 28, 5940−5946