Unusual Soluble–Insoluble–Soluble Phase Transition in Two-Phase

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Unusual Soluble−Insoluble−Soluble Phase Transition in Two-Phase Copolymerization of Acrylamide and an Anionic Comonomer in a Poly(ethylene glycol) Aqueous Solution Kuanxiang Shang, Guorong Shan,* and Pengju Pan State Key Laboratory of Chemical Engineering, Department of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, China ABSTRACT: The two-phase copolymerization of acrylamide (AM) and anionic comonomer, sodium 2-(acrylamido)-2methylpropanesulfonate (NaAMPS), in a poly(ethylene glycol) (PEG) aqueous solution was investigated. An unusual soluble− insoluble−soluble phase transition was found. The effects of the monomer ratio, PEG concentration, initiator concentration, salt concentration, and pH value of the reaction media on the phase transition behavior were studied. As the NaAMPS molar fraction increased or the PEG concentration decreased, the insoluble−soluble phase transition first appeared and then disappeared. However, all of the aqueous two-phase polymerization systems underwent a soluble−insoluble−soluble phase transition with increasing initiator concentration. The results of critical conversion and copolymer composition indicated that there were two competitive effects governing the phase separation, that is, the solubility enhancement of a polyelectrolyte and Debye−Hückel screening effect of an anionic monomer. The insoluble−soluble phase transition disappeared at higher salt concentration. A mechanism for the unique soluble−insoluble−soluble phase transition was proposed, which was ascribed to the synergistic effects of the polymer concentration, solubility enhancement of a polyelectrolyte, and screening effect of an anionic comonomer. On the basis of this mechanism, the phase transition was successfully tuned by varying the pH value of reaction media. been developed in recent years.27−34 Before phase separation, all of the components are soluble in water and the mixture is transparent. After phase separation occurs at a critical conversion, the polymer-rich dispersed phase is isolated from the reaction mixture, and then polymerization proceeds in two phases.30 Compared to homogeneous solution polymerization, two-phase polymerization has a faster polymerization rate, a higher molecular weight of the product, and a lower viscosity of the reaction mixture. Therefore, phase separation is an essential process for the ATPP system. Understanding the underlying mechanism of phase separation in ATPP is of fundamental importance for controlling the polymerization process and performance of the resulting products. There are many factors governing the phase behavior of the ATPP system. Besides the reaction temperature, the molecular weight and concentration of the dispersant and conversion of the monomer influence the phase behavior significantly. Moreover, when an ionic monomer is present in the reaction mixture, the as-prepared polymer is a polyelectrolyte. The phase behavior will be synergistically influenced by the solubility enhancement of the polyelectrolyte and the screening effect of salt, which will lead to a more complex phase behavior. Anionic polyacrylamide (APAM) is a typical water-soluble polymer that is widely employed in wastewater treatment.35−39 However, little work has been conducted on the ATPP of APAM, and the phase behavior of such systems remains unclear.38 The objective of this work is to study the phase

1. INTRODUCTION Phase separation is a well-known phenomenon in mixed polymer solutions. Control of phase separation is highly important for the preparation and separation of polymers, such as the aqueous two-phase partitioning of biomacromolecules.1−3 Experiments and model calculations have been carried out on the phase behavior of different polymer mixtures, such as the aqueous mixtures of nonionic polymers,1 nonionic polymers and polyelectrolytes,4,5 oppositely charged macromolecules,6−10 and mixtures of polyelectrolytes and surfactants.11−16 It has been found that the introduction of a charged group in a polymer chain can significantly improve its solubility in aqueous solution,17−19 which is entropy-driven. When a nonionic polymer and a polyelectrolyte were mixed in water, the mixing entropy of the dissociated counterions is significant. It is entropically unfavorable to isolate the counterions in one of the phases, and they will disperse in both phases, which results in an increased miscibility. However, this effect of solubility enhancement becomes less essential when salt is present in the aqueous mixtures because of the Debye−Hückel screening effect.19 Polymerization-induced phase separation (PIPS), in which an initially homogeneous solution of monomer and solvent becomes phase-separated with the progression of polymerization, has been observed in many polymerization systems.20−22 Through control of the phase separation and polymerization processes, PIPS has been widely employed to prepare various polymeric materials such as the polymer suspension, porous thermosetting polymers,23 macroporous materials for chromatography,24 and hydrogels.25,26 On the basis of PIPS, a novel green approach to prepare water-soluble polymers, i.e., aqueous two-phase polymerization (ATPP), has © XXXX American Chemical Society

Received: April 20, 2014 Revised: June 10, 2014 Accepted: June 12, 2014

A

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behavior in ATPP of acrylamide (AM) and an anionic comonomer in a poly(ethylene glycol) (PEG) aqueous solution. Sodium 2-(acrylamido)-2-methylpropanesulfonic acid (NaAMPS) was selected as a model anionic comonomer. The effects of the monomer ratio, PEG concentration, salt concentration, and pH value of the reaction media on the phase behavior were investigated. An unusual soluble−insoluble− soluble phase transition was found. The change of the copolymer composition in the polymerization process was also examined. On the basis of these results, a mechanism for the unique phase separation behavior in the ATPP of AM and an anionic comonomer was proposed.

ωi =

msfi Ai mfs A s

× 100% (1)

where ωi is the mass fraction of component i, ms and m are the masses of the standard substrate and sample, respectively, f i and fs are the relative mass correction factors of component i and the standard substrate, respectively, and Ai and As are the peak areas of component i and the standard substrate, respectively.

3. RESULTS AND DISCUSSION 3.1. Phase Behavior of Aqueous Two-Phase Homopolymerization Systems. The aqueous two-phase homopolymerization systems of AM and NaAMPS were investigated. The transmittance changes in the polymerization process are shown in Figure 1. Before polymerization, the systems are

2. EXPERIMENTAL SECTION 2.1. Materials. Acrylamide (AM; 99.9%, Acros Organics) was dried in vacuo at 45 °C before use. 2-(Acrylamido)-2methylpropanesulfonate (AMPS; 98%, J&K Chemical), 2,2′azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride (VA044), and poly(ethylene glycol) (PEG) with a molecular weight of 20000 (PEG20000; Acros Organics) were used as received. Deionized H2O was used throughout the work. Other reagents were of analytical grade and were used as received. 2.2. General Procedure of ATPP. ATPP of AM and NaAMPS in a PEG aqueous solution was performed in a 500 mL glass-jacketed reactor. PEG, AM, AMPS, NaOH, and H2O were added to the reactor. The mixture was heated to 45 °C and purged with argon for 30 min. Then, the aqueous solution of initiator was injected to start the copolymerization. The stirring speed was maintained at 150 rpm, and the temperature was kept constant. The monomer conversion, which was defined as the total conversion of double bonds, was determined by a modified bromination method.27 2.3. Measurement of Transmittance and Conversion. In the copolymerization process, transmittance of the reaction mixture was determined using a UV−visible spectrophotometer (Shimadzu UV-1800) in situ at a wavelength of 500 nm. PEG, AM, AMPS, and NaOH were dissolved in a glass beaker, and the aqueous initiator was injected into the mixture. After purging with argon for 30 min in a water−ice bath, 5 mL of the mixture was removed to a glass cell and sealed with a cover. Then the cell was placed in the sample chamber of the UV− visible spectrophotometer, in which the temperature had been set to 45 °C. Then, polymerization was started, and variation of transmittance in the polymerization process was detected. Before the start of polymerization, the reaction mixture is transparent and its transmittance was around 100%. The critical conversion was defined as the conversion at which transmittance began to decrease. Three parallel measurements were carried out for each sample. The average value of these three measurements was taken as the critical conversion. 2.4. Copolymer Composition. The copolymer composition was investigated by gas chromatography (GC; Agilent 7820A). In the copolymerization process, 1.0 g of the reaction mixture was removed by a syringe and then diluted with 4.0 mL of H2O. A total of 0.1 g of methanol was added to the mixture as the internal standard. A total of 0.1 μL of the diluted solution was injected into the GC instrument by a microsyringe. The temperatures of the vaporizing chamber and detector were 250 and 230 °C, respectively. On the basis of the GC elution curve, the contents of residual monomers were calculated as

Figure 1. Transmittance change in the two-phase homopolymerization of AM and NaAMPS in a PEG aqueous solution (monomer = 10 g, PEG20000 = 20 g, H2O = 70 g, VA-044 = 0.02 g, T = 45 °C, pH = 7).

transparent and transmittance is around 100%. A decrease of transmittance indicates the occurrence of liquid−liquid phase separation. As shown in Figure 1, transmittance decreased suddenly in the ATPP of AM after conversion reached 6.2%, indicating the occurrence of phase separation. However, in the case of NaAMPS, the reaction mixture was transparent and no phase separation took place. It is known that the solubility of the polymer in aqueous media can be significantly improved by ionization.19,40 From the thermodynamic point of view, when the polymer is partially charged (f N per chain, f < 1), the effect of the charge on the polymer is equivalent to the renormalization19 N → N eff ≡

N 1 + fN

(2)

where N is the real polymerization degree of the polymer and Neff is the effective polymerization degree. Neff is always smaller than N, and so the charged copolymer has a better solubility than the uncharged one. In this work, NaAMPS is anionic and its polymer is a polyelectrolyte. Therefore, poly(NaAMPS) has good solubility, and no phase separation took place in ATPP. 3.2. Phase Behavior for ATPP of AM and NaAMPS. NaAMPS is often copolymerized with AM to prepare APAM. Figure 2a shows the effect of the NaAMPS molar fraction ( f NaAMPS) on phase separation in ATPP of AM and NaAMPS. When f NaAMPS was less than 0.10, the phase behavior was similar to that of ATPP of AM, and only a soluble−insoluble transition was observed. When f NaAMPS was higher than 0.30, the phase behavior was the same as that of ATPP of NaAMPS B

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and TH decreased from 100% to 0, but the TH−TL value first increased and then decreased, with the presence of a maximum of 70% at PEG/H2O = 20/70. Upon comparison of Figures 2 and 3, it can be seen that the increase of the NaAMPS molar fraction and the decrease of the PEG concentration have similar effects on the phase transitions of the ATPP system. Decreasing the PEG concentration increased the hydrophilicity of reaction media, while the increase of the NaAMPS units in copolymers resulted in a dramatic enhancement of the solubility. Both of these effects led to better solubility of the polymer. The effects of the initiator concentration on the phase transitions were also investigated, and the transmittance curves are shown in Figure 4a. All of the ATPP systems underwent a

Figure 2. Effects of the NaAMPS molar fraction on transmittance curves (a) and critical transmittances (b) in ATPP of AM and NaAMPS (AM + NaAMPS = 10 g, PEG20000 = 20 g, H2O = 70 g, VA-044 = 0.02 g, T = 45 °C, pH = 7).

and no phase separation was detected. However, when f NaAMPS was between 0.10 and 0.30, not only the soluble−insoluble phase separation but also an additional insoluble−soluble transition was observed. This unusual phase behavior is much different from that of the aqueous two-phase homopolymerization systems. The effects of the NaAMPS molar fraction on the critical transmittances are shown in Figure 2b. To discuss the transmittance change in polymerization, TL is defined as the lowest transmittance in the soluble−insoluble transition and TH is defined as the highest transmittance in the insoluble−soluble transition. It can be seen that, as f NaAMPS increased, both TL and TH gradually increased from 0 to 100%, but TH−TL value first increased to 75% and then decreased to 0 again. The concentration of PEG is a key factor deciding the hydrophilicity of reaction media, and therefore it greatly affects the phase behavior of the ATPP system. The transmittance changes for two-phase copolymerization of AM and NaAMPS in the solution with different PEG concentrations are shown in Figure 3a. The critical transmittances are shown in Figure 3b.

Figure 4. Effects of the initiator concentration on transmittance curves (a) and critical transmittances (b) in ATPP of AM and NaAMPS (AM = 6.6 g, NaAMPS = 3.4 g, PEG20000 = 20 g, H2O = 70 g, T = 45 °C, pH = 7). The ratio VA-044/(AM + NaAMPS) represents the molar ratio.

soluble−insoluble−soluble phase transition, and the phase transitions were sped up with an increase of the initiator concentration, ascribed to the increase of the polymerization rate. Critical transmittances are also illustrated in Figure 4b. With an increase of the initiator concentration, TL, TH, and TH−TL all slightly increased first and then decreased. This indicates that, compared to the effects of the molar fraction of NaAMPS and the PEG concentration, the effect of the initiator on the phase transitions is so much weaker that it can nearly be neglected. This is because of its really low concentration (0.33− 1.32 mM). 3.3. Critical Conversion. The concentration of the resulting copolymer at the critical point of phase separation was defined as the critical conversion, which is a critical parameter for phase transitions. Figure 5 illustrates the critical conversion for the ATPP system of AM and NaAMPS with different NaAMPS molar fractions. A minimum was observed at f NaAMPS = 0.10. When f NaAMPS was lower than 0.10, the critical conversion decreased with an increase of f NaAMPS. However, when f NaAMPS was larger than 0.10, the critical conversion increased with an increase of f NaAMPS. On the basis of the NaAMPS/AM reactivity ratio (r1 = 0.50 ± 0.01 and r2 = 1.02 ± 0.01),41 the copolymer composition (FNaAMPS) at the critical point of phase separation was calculated by the recurrence method.42 As seen in Figure 5, FNaAMPS increased linearly with an increase of f NaAMPS. Considering the effect of the ionic group on the solubility, an increase of FNaAMPS would enhance the solubility of the asprepared copolymer, which leads to an increase of the critical conversion. This was consistent with the experimental results at

Figure 3. Influences of the PEG concentration on transmittance curves (a) and critical transmittances (b) in ATPP of AM and NaAMPS (AM = 6.6 g, NaAMPS = 3.4 g, PEG20000 + H2O = 90 g, VA-044 = 0.02 g, T = 45 °C, pH = 7).

At low PEG concentration (PEG/H2O < 15/75), the reaction mixture was transparent in the whole polymerization process. At high PEG concentration (PEG/H2O > 25/65), no obvious insoluble−soluble transition was observed in the later stage of polymerization, in which TL and TH were equal to 0. However, at medium PEG concentration (15/75 < PEG/H2O < 25/65), a soluble−insoluble−soluble transition was observed. As the PEG/H2O mass ratio increased from 15/75 to 25/65, both TL C

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Figure 7. Changes of the transmittance, concentration of unreacted NaAMPS (a), and molar fraction of the NaAMPS unit in the copolymer (b) in ATPP of AM and NaAMPS with f NaAMPS of 0.15 (AM = 6.6 g, NaAMPS = 3.4 g, PEG20000 = 20 g, H2O = 70 g, VA044 = 0.02 g, T = 45 °C, pH = 7).

Figure 5. Critical conversion and copolymer composition (FNaAMPS) for the ATPP of AM and NaAMPS with different NaAMPS molar fractions (AM + NaAMPS = 10 g, PEG20000 = 20 g, H2O = 70 g, VA044 = 0.02 g, T = 45 °C, pH = 7).

monomer concentration (cNaAMPS) was more prominent at higher conversion (>60%) than that at lower conversion ( 5. On the basis of the above-mentioned mechanism, it is reasonable to predict that the phase behavior can be tuned by varying the pH value of the solution. Figure 11 illustrates the effect of the pH value on the phase behavior in ATPP of AM and NaAMPS. When the pH value of the solution was decreased from 7 to 2 [in other words, the extent of deprotonated AMPS increased from about 0 to 90.9% (as shown in Figure 10)], TL increased from 11.6% to 100% and TH increased from 86.5% to 100%. When the pH value of the solution was decreased to about 3, which was the pKa value of poly(AMPS), the molar fraction of neutralized AMPS reached a critical value (about 50.0%), and nearly all of the copolymers solubilized in the solution. As a result, the insoluble−soluble transition disappeared. This indicated that the solubility of the copolymer was enhanced with increasing extent of protonated AMPS. In addition, as a hydrochloride salt, the protonated extent of VA-044 was also influenced by the pH value of the system, with the same trend as that of AMPS. It may also contribute partially to the effect of the pH value on the phase transitions. However, because of its very low concentration, the contribution of VA-044 on the phase transitions was so little that it can nearly be neglected. Two-phase copolymerization of AM and AMPS in the PEG aqueous solution was also investigated. The changes of

(3) E

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increased. When the salt concentration was increased, the insoluble−soluble phase transition gradually disappeared. With the progression of polymerization, the concentration of unreacted NaAMPS decreased, but the content of the NaAMPS unit in the copolymer first decreased and then increased. On basis of these results, a mechanism for the unusual soluble− insoluble−soluble phase transition in ATPP of AM and an anionic monomer was proposed. This study not only is essential for the preparation of APAM suspension but also provides a new method to control the phase behavior in the ATPP process.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

Figure 11. Influence of the pH value on the phase behavior in ATPP of AM and NaAMPS (AM = 6.6 g, NaAMPS = 3.4 g, PEG20000 = 20 g, H2O = 70 g, VA-044 = 0.02 g, T = 45 °C).

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the National Natural Scientific Foundation of China (Grant 21176210) and Outstanding Youth Foundation of Zhejiang Province (Grant R4110199) is gratefully acknowledged.

transmittance in the polymerization process are shown in Figure 12. At fAMPS = 0.05, the soluble−insoluble phase

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REFERENCES

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Figure 12. Effect of the AMPS molar fraction on the phase behavior in ATPP of AM and AMPS (AM + AMPS = 10 g, PEG20000 = 20 g, H2O = 70 g, VA044 = 0.02 g, T = 45 °C).

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4. CONCLUSIONS The phase behavior in ATPP of AM and an anionic comonomer was investigated, and an unusual soluble− insoluble−soluble phase transition was found. When the NaAMPS molar fraction was increased or the PEG concentration decreased, the insoluble−soluble phase transition first appeared and then disappeared. However, all of the ATPP systems underwent a soluble−insoluble−soluble phase transition with increasing initiator concentration. As the NaAMPS molar fraction increased, critical conversion for the soluble− insoluble phase transition was first decreased and then F

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