Synthesis, characterization and pH sensitivity of polyampholyte containing aromatic rings

Abstract

This work describes the synthesis and pH sensitivity of polyampholytes containing aromatic rings which were prepared from a poly(acrylamide-co-vinylamine) oligomer and aryl sulfonate derivatives. The structures of the products were characterized by Fourier transform infrared spectroscopy, liquid chromatography-mass spectrometry and nuclear magnetic resonance spectroscopy analysis. The effects of degree of substitution, concentration of the polyampholyte solution, the amount of sulfonate groups in the aryl sulfonate derivatives, and the aromatic ring on the pH sensitivity of the polyampholytes were discussed in detail. The results exhibited that the pH sensitivity of these polyampholytes was mainly affected by the molar ratio of cationic to anionic groups in the polyampholytes which can be controlled by changing both the degree of substitution and the amount of sulfonate groups in the aryl sulfonate derivatives at the fixed degree of substitution.

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1 Introduction

pH sensitive polyampholytes have received much more attention in the past two decades due to their outstanding physical and solution properties which can be manipulated reversibly by changes in external pH.^1,2 They have applications in various technological fields, including adhesion, drug loading and release, protein separation, catalysis and oil industry.^3–5 In all kinds of the polyampholytes, the polyampholytes containing aromatic rings have specially drawn significant research interest recently owing to their unique physical–chemical properties.^6,7 Lin et al. prepared a novel polyampholyte containing carboxybenzyl groups based on chitosan and can be used as pH-sensitive carrier for drug delivery system.⁸ Meanwhile, these polyampholytes can be potentially applied in activated CO₂ sorption, microfluidic and sensor devices.^9,10 Unlike other polyampholytes, polyampholytes containing aromatic rings can interact with the material surface via the π electrons of the aromatic molecules. They can also be used as a nanoparticle stabilizer.¹¹ Li et al. reported a polyampholyte containing benzene rings, sulfonated cardo poly(arylene ether sulfone), and it was used to disperse the gold nanoparticles though interaction of the π electrons of the benzene rings with gold surfaces.¹² In our previous study, taking advantage of the aromatic moiety having strong π–π interaction with condensed aromatic rings of carbon material surfaces, a novel pH-responsive polyampholyte containing amine groups and aryl sulfonates was used for aqueous carbon black (CB) dispersion, and reversible dispersion and precipitation of the prepared CB dispersion can be achieved by pH changes.¹³

pH sensitivity of the polyampholyte is a key role in the application and related to its composition. For example, polyvinylamine–phenylboronic acid with 16% substitution, which is phase-separated from pH 7.5 to 9.5, has the greatest wet adhesion to cellulose hydrogel, whereas polyvinylamine–phenylboronic acid with 51% substitution is phase-separated over most of the pH range.^7,14 Although, the pH sensitivity of polyampholytes containing aromatic rings can be affected by changing the degree of substitution (DS), some applications require these polyampholytes having a certain DS in order to keep the special properties such as the strong π–π interaction, so it is also very important to tune the pH sensitivity of the polyampholyte at the fixed DS. However, so far, there is little information in the literature concerning this aspect.

Herein, we present a series of the polyampholytes containing aromatic rings (defined as PAV-ASD-x) prepared from poly(acrylamide-co-vinylamine) oligomer and the designed aryl sulfonate derivatives (ASD-x, Fig. 1). This study aims to investigate in detail the pH sensitivity of these polyampholytes and especially emphasize the control of the pH sensitivity of these polyampholytes at the fixed DS.

Fig. 1 Chemical structures of the aryl sulfonate derivatives, ASD-1 ∼ ASD-5.

2 Experimental

2.1 Materials and characterization

Poly(acrylamide-co-vinylamine) (PAV) oligomer (GPC, Mn = 603 g mol⁻¹, PDI = 1.69) with 75% amine group content was prepared using the Hofmann degradation of polyacrylamide according to a previous study.¹⁵ Cyanuric chloride (99%), aniline (99%) and 2-naphthylamine-1-sulfonic acid (98%) were obtained from Aladdin Co., Ltd (China). Metanilic acid (98%) and 2-amino-1,4-benzenedisulfonic acid (98%) were supplied by Shandong Weifang Arylchem Co., Ltd (China). All other chemicals were analytical grade quality and used as received.

IR spectra were registered with FTIR-Nicolet 5700 spectrophotometer (USA) in the scanning range of 4000–400 cm⁻¹ using a KBr pellet method. UV-visible spectra were obtained with samples contained in a 1 cm quartz cuvette on an Agilent 8453 UV-vis spectrometer. ¹H-NMR (D₂O + NaOD) was recorded on Varian INOVA 400 NMR spectrometer. Mass spectra were recorded at CID = 50–200 V with an HP 1100 HPLC/MS system from Hewlett Packard, USA.

2.2 Synthesis

2.2.1 Synthesis of the ASD-1. Cyanuric choride (0.0103 mol), ice, and water were stirred for 30 min in 250 mL beaker, and then solution of metanilic acid (0.01 mol) was added slowly with constant stirring. The reaction was performed at 0–5 °C and pH = 4–5 for 1 h until no reaction to the Ehrlich reagent (1% p-dimethylaminobenzaldehyde dissolved in equal volumes of alcohol and concentrated hydrochloric acid) was detectable. Afterward, aniline (0.01 mol) was added to reaction solution, and the temperature was increased to 30–40 °C for 2 h. Thin layer chromatography (TLC) was used to monitor the completion of the reaction (R_f = 0.69, Silica GF254, i-BuOH : n-PrOH : EtOAc : H₂O, 2 : 4 : 1 : 3, v/v). The product was isolated by adding potassium acetate, collected by filtration, washed with ethanol for several times to remove potassium acetate, and dried in a vacuum (yield: 95.6%). λ_max (H₂O) = 267 nm. ε_max = 3.52 × 10⁴ (mol⁻¹ dm³ cm⁻¹). FT-IT: 1621 cm⁻¹ (N–H stretching vibration); 1590 cm⁻¹ (keleton vibration of phenyl ring); 1508 cm⁻¹ (stretching vibration of thiotriazinone); 1171, 1033 cm⁻¹ (–SO₃⁻). HPLC-MS: purity: 99.6%; MS: 376.1 ([M − H]⁻).

2.2.2 Synthesis of the ASD-2. The ASD-2 was synthesized as nearly the same process as the ASD-1 except that the aniline was replaced by the metanilic acid. R_f = 0.53, yield: 94.7%. λ_max (H₂O) = 272 nm. ε_max = 3.17 × 10⁴ (mol⁻¹ dm³ cm⁻¹). FT-IR: 1621 cm⁻¹ (N–H stretching vibration); 1574 cm⁻¹ (keleton vibration of phenyl ring); 1482 cm⁻¹ (stretching vibration of thiotriazinone); 1187, 1040 cm⁻¹ (–SO₃⁻). HPLC-MS: purity: 99.9%; MS: 456.0 ([M − H]⁻), 227.4 ([M − 2H]²⁻/2).

2.2.3 Synthesis of the ASD-3. The ASD-3 was synthesized as nearly the same process as the ASD-1 except that the aniline was replaced by the 2-amino-1,4-benzenedisulfonic acid. R_f = 0.43, yield: 93.3%. λ_max (H₂O) = 273 nm. ε_max = 3.01 × 10⁴ (mol⁻¹ dm³ cm⁻¹). FT-IR: 1619 cm⁻¹ (N–H stretching vibration); 1574 cm⁻¹ (keleton vibration of phenyl ring); 1516 cm⁻¹ (stretching vibration of thiotriazinone); 1187, 1040 cm⁻¹ (–SO₃⁻). HPLC-MS: purity: 99.7%; MS: 535.9 ([M − H]⁻), 267.5 ([M − 2H]²⁻/2), 119.2 ([M − 3H]³⁻/3).

2.2.4 Synthesis of the ASD-4. The ASD-4 was synthesized as nearly the same process as the ASD-1 except that the metanilic acid and aniline were replaced by the 2-amino-1,4-benzenedisulfonic acid. R_f = 0.27, yield: 94.3%. λ_max (H₂O) = 272 nm. ε_max = 3.01 × 10⁴ (mol⁻¹ dm³ cm⁻¹). FT-IR: 1621 cm⁻¹ (N–H stretching vibration); 1572 cm⁻¹ (keleton vibration of phenyl ring); 1509 cm⁻¹ (stretching vibration of thiotriazinone); 1185, 1040 cm⁻¹ (–SO₃⁻). HPLC-MS: purity: 94.3%; MS: 615.8 ([M − H]⁻), 307.4 ([M − 2H]²⁻/2), 204.7 ([M − 3H]³⁻/3).

2.2.5 Synthesis of the ASD-5. The ASD-5 was synthesized as nearly the same process as the ASD-1 except that the aniline was replaced by the 2-naphthylamine-1-sulfonic acid. R_f = 0.44, yield: 95.5%. λ_max (H₂O) = 266 nm. ε_max = 4.14 × 10⁴ (mol⁻¹ dm³ cm⁻¹). FT-IR: 1613 cm⁻¹ (N–H stretching vibration); 1564 cm⁻¹ (keleton vibration of aromatic ring); 1500 cm⁻¹ (stretching vibration of thiotriazinone); 1213, 1040 cm⁻¹ (–SO₃⁻). HPLC-MS: purity: 97.7%; MS: 506.1 ([M − H] ⁻), 252.5 ([M − 2H]²⁻/2).

2.2.6 Synthesis of the polyampholytes containing aromatic rings (PAV-ASD-x). PAV (5.0 g, 0.048 mol primary amine groups) was dissolved in 100 mL water, and pH value was adjusted to 11.0 by 10% NaOH. Solution of aryl sulfonate derivative with desired amount was added slowly when the temperature was heated to 70 °C. The system was kept at 70 °C and pH 10.0–11.0 until the reaction was over, and TLC (Silica GF254, i-BuOH : n-PrOH : EtOAc : H₂O, 2 : 4 : 1 : 3, v/v) was used to monitor the completion of the reaction (R_f of the product was 0.0). The resultant solution was adjusted to pH = 2 by 6 mol L⁻¹ HCl and poured into 300 mL methanol. The precipitates were collected by filtration. After drying under vacuum at 40 °C for 24 hours, the solid product was obtained.

2.3 pH sensitivity of the PAV-ASD-x

The pH sensitivity of the prepared polyampholytes was conducted by visible light transmittance measurements at a wavelength of 600 nm at 25 °C as functions of DS, concentration of the polyampholyte solution, the amount of sulfonate groups in the aryl sulfonate derivatives and the aromatic ring in 0.01 N NaCl using an Agilent 8453 UV-visible spectrophotometer.

2.4 Zeta potential experiment

Zeta potential experiment of the prepared polyampholytes was carried out by ZETASIZER nano series Nano-ZS90 from Malvern Instruments Corp. All samples were measured at 25 °C and the reported values were based on five measurements with 20 cycles for each.

2.5 Potentiometric and conductometric titrations

Potentiometric titration was carried out with a PHS-3C precise acidimeter (LeiCi, Shanghai) on 1.0 g L⁻¹ aqueous solutions of the polyampholytes from pH 2 to 12 using 1 M NaOH titrant at 25 °C. Conductometric titration was performed with a DDSJ-308A conductivity meter (LeiCI, Shanghai) on 1.0 g L⁻¹ aqueous solutions of the polyampholytes from pH 12 to 2 using 1 M HCl titrant at 25 °C. A wait time of 60 s between injections was used to ensure full equilibration of polyampholyte solution.

3 Results and discussion

3.1 Preparation of the PAV-ASD-x

The synthetic scheme of the polyampholytes containing aromatic rings is exhibited in Fig. 2. As well known, primary amine has high activity as a nucleophilic reagent, and can easily react with monochlorotriazine group. In order to make the degree of neutralization of the poly(acrylamide-co-vinylamine) oligomer is nearly 100%, pH value of 11 is required.^16,17 Due to the presence of cationic and anionic groups in this system, the pH value largely influenced the success of the grafting reaction. In process of experiment, it can be found the gel-like polymer separated out from the system when the pH value below 10. On the contrary, the reaction can be conducted successfully and quickly at pH = 11. TLC was used to monitor the completion of the reaction (R_f of product was 0.0). Table 1 lists the results and data of the products.

Fig. 2 Synthesis of the PAV-ASD-x.

Table 1 The data of the polyampholytes containing aromatic rings

Sample	Amount of ASD-x (%) (calculated)	Amount of ASD-x (%) (measured by ^1H-NMR)	Ratio of cationic to anionic groups^a
a Determined by the molar content of –NH2 and –NH^− groups of the main chain divided by the molar content of –SO3^− groups.
PAV-ASD-1-20	20	19.3	3.89
PAV-ASD-2-20	20	19.5	1.92
PAV-ASD-3-5	5	4.8	5.21
PAV-ASD-3-10	10	9.6	2.60
PAV-ASD-3-13.3	13.3	13.1	1.91
PAV-ASD-3-15	15	14.6	1.71
PAV-ASD-3-20	20	19.6	1.28
PAV-ASD-3-25	25	24.9	1.00
PAV-ASD-3-30	30	29.2	0.86
PAV-ASD-4-10	10	9.8	1.91
PAV-ASD-4-20	20	19.5	0.96
PAV-ASD-5-20	20	19.5	1.92

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FT-IR (Fig. 3) and ¹H-NMR (Fig. 4) spectra were used to confirm the structure of the products. In Fig. 3, the absorption band at 3436 cm⁻¹ was assigned to the N–H stretching vibration of –NH₂. The wide absorption peak at 2916 cm⁻¹ was assigned to the C–H stretching vibration of –CH₂⁻ and –CH⁻. 1624 cm⁻¹ was assigned to the N–H deformation vibration of –NH₂. However, compared with the FT-IR spectrum of PAV HCl, a new absorption band at 1595 cm⁻¹ was assigned to the aromatic ring and absorption bands of 1180 cm⁻¹ and 1036 cm⁻¹ indicated the presence of the sulfonate group.

Fig. 3 FI-IR spectra of the PAV HCl (a) and PAV-ASD-x (b–f).

Fig. 4 ¹H-NMR spectra of the PAV and PAV-ASD-x.

Compared with ¹H-NMR spectrum of PAV, the obvious broad aromatic peaks of the products at 6–9 ppm can be observed (Fig. 4) besides the peaks of aliphatic methylene (–CH₂⁻) and methane (–CH⁻) protons, which provided the further evidence that the aryl sulfonate derivatives are successfully introduced into the PAV. In addition, the amount of aryl sulfonate derivatives in the products was determined from relative areas of the aliphatic and aromatic peaks in the proton NMR spectra.

3.2 pH sensitivity of the PAV-ASD-x

pH sensitivity of the polyampholyte can be investigated by the light transmittance of the polyampholyte solution.¹⁸ Fig. 5 exhibits the transmittance and zeta potential of the PAV-ASD-3-10 in 0.01 N NaCl. It can be seen that the PAV-ASD-3-10 solution is soluble at pH > 10.5 and pH < 5. Macroscopic precipitates were formed between pH value of 5.5–9.0. This can be explained as follows: sulfonate groups are negatively charged at pH > 1 and most of the cationic groups (–NH₃⁺ and –NH₂⁺–) at pH 10.5 are deprotonated (–NH₂ and –NH⁻).^16,19–21 Therefore, at pH > 10.5, the PAV-ASD-3-10 is soluble because of presence of deprotonated amine groups and negative charged sulfonate groups. At pH < 5, the amounts of protonated amine groups are more than the sulfonate groups so that the PAV-ASD-3-10 is positive (Fig. 5b) and soluble because of hydrophilicity of the ammonium groups. The macroscopic precipitates can be found near the isoelectric point (IEP = 8.9, Fig. 5b), where the charge of PAV-ASD-3-10 is neutral. Unlike ordinary polyampholytes, polyampholytes containing aromatic rings can make the aggregates still exist when pH value derivates from the IEP.⁶

Fig. 5 Transmittance (a) and zeta potential (b) of PAV-ASD-3-10 (1 g L⁻¹) in 0.01 N NaCl.

3.3 Effect of DS on the pH sensitivity of the PAV-ASD-x

By adjusting the amount of aryl sulfonate derivatives in the polyampholyte, the pH sensitivity of the PAV-ASD-x can be manipulated. Fig. 6 plots the transmittance of PAV-ASD-3 with 5–30% DS in 0.01 N NaCl. When the DS of PAV-ASD-3 is 5% and 10%, they have two transparent regions. However, with the increase of the DS, only one transparent region can be observed. When the DS reached 30%, the PAV-ASD-3 can be soluble in all the test pH value. It can be explained from the ratio of cationic to anionic groups in the PAV-ASD-3, which was calculated from the molar content of aliphatic amine groups of the main chain of the PAV divided by the molar content of sulfonate groups. When the DS of PAV-ASD-3 are 5%, 10%, 15%, 20%, 25% and 30%, the values of these ratios are 5.21, 2.60, 1.71, 1.28, 1.00 and 0.86, respectively. At 5% and 10% DS, the PAV-ASD-3 can be soluble not only in basic condition due to the ionic sulfonate groups (–SO₃⁻) and deprotonated amine groups (–NH₂ and –NH⁻) but also in acid condition owing to residual ammonium groups. With the increase of the DS, the amount of sulfonate groups in the PAV-ASD-3 increased, pH sensitivity of PAV-ASD-3 was gradually affected more by the sulfonate groups than amine groups. At the acid condition, the residual ammonium groups, beside the ammonium groups interacted with sulfonate groups, were not enough to make the PAV-ASD-3 soluble. Especially, at 30% DS, the amount of cationic groups was less than that of the anionic groups, and residual anionic groups which are insensitive to the changes of pH value make the PAV-ASD-3 soluble in all test pH value.

Fig. 6 Transmittance of the PAV-ASD-3 (1 g L⁻¹) with different DS in 0.01 N NaCl.

pH sensitivity of the polyampholyte is related to its IEP, and the IEP can be determined according to the dependence of the conductivity on pH by the conductometric titration.¹⁹ Fig. 7 exhibits the potentiometric and conductometric titrations of the PAV-ASD-3 (1 g L⁻¹) with different DS in 0.01 N NaCl at 25 °C. Two transition points (about pH 11.0 and 2.5) can be found from both the pH and conductivity titration curves in Fig. 7. The first transition (pH ∼ 11.0) corresponded to the onset of protonation of amine groups and a pH at ∼2.5 corresponded to the end of amine group protonation. For the conductometric titration curve, the point of the minimum value is regarded as the point of zero charge due to that the macroscopic precipitates of polyampholyte separated from the solution at the IEP so that the conductivity of the solution is the lowest. Therefore, the IEP can be obtained by the corresponding intersection point in the conductometric titration curves of the conductivity versus pH values. From Fig. 7, it can be calculated that the IEP of PAV-ASD-3-5 ∼ PAV-ASD-3-25 are 9.31, 8.79, 8.03, 6.64 and 3.35, respectively. However, for PAV-ASD-3-30, the IEP of the PAV-ASD-3-30 can't be calculated due to that no minimum value in the conductometric titration curve can be found. Meanwhile, it can be concluded that the aggregates of the obtained polyampholytes formed near the isoelectric point and still existed when pH value derivates from the IEP about 2.5–3.0 units.

Fig. 7 Potentiometric and conductometric titrations of the PAV-ASD-3 (1 g L⁻¹) with different DS in 0.01 N NaCl at 25 °C.

In addition, the relationship of the charge-pH behaviors between the PAV-ASD-3-x and its DS was investigated. Fig. 8a shows the plot of pH versus the degree of neutralization (α) of the PAV-ASD-3-x. As shown in the figure, the PAV-ASD-3-x curves are similar to that of PAV HCl which indicated that the DS has a very limited influence on the process of the removal of protons in the PAV HCl. This can be easily accepted because the anionic groups in the PAV-ASD-3-x are the sulfonate groups and they are negatively charged at pH > 1.²² Meanwhile, the effect of DS on the pK_a of the PAV-ASD-3-x was also conducted. As well known, dissociation constants of the polyelectrolytes can be obtained from the potentiometric titration and the modified Henderson–Hasselbach eqn (1) was usually used to calculate the pK_a of the polymer.^17,23

where α is the degree of neutralization of the functional groups of the polymer, m is a parameter related to electrostatic interaction of neighboring groups on the main chain of the polymer and K_a is the average dissociation constant. The value of m can be obtained from the slope of plots at α value range between 0.2 and 0.6, and pK_a can be obtained from the intercept of the plot. Fig. 8b depicts the effect of the DS on the pK_a of poly(acrylamide-co-vinylamine). The results showed that the average pK_a of PAV-ASD-3-x (pK_a = 8.3–8.5) is nearly the same as that of PAV (pK_a = 8.2).

Fig. 8 Charge-pH behaviors between the PAV-ASD-3-x and its DS: (a) pH vs. α; (b) pK_a vs. DS.

3.4 Effect of concentration on the pH sensitivity of the PAV-ASD-x

The increment of the concentration can make the intermolecular interaction of the polyampholyte more easily that reinforced the tendency of the aggregation.²⁴ Fig. 9 illustrates the effects of the concentration of the PAV-ASD-3-10 on its pH sensitivity. It can be found that with the increase of the concentration, the region of precipitates of the PAV-ASD-3-10 enlarged. At the concentration of 1.0 g L⁻¹, PAV-ASD-3-10 separated out from the solution between pH 5.5–9.0, while, the aggregation of the PAV-ASD-3-10 at 10 g L⁻¹ was formed between pH 5.0–10.0. In addition, the transmittance of the PAV-ASD-3-10 also decreased with the increment of the concentration. At the pH value of 11, the transmittance of PAV-ASD-3-10 at 1.0 g L⁻¹ was 99%, however, this value dropped to 90% when the concentration of the PAV-ASD-3-10 is 10 g L⁻¹.

Fig. 9 Transmittance of the PAV-ASD-3-10 of the different concentration in 0.01 N NaCl.

3.5 Effect of amounts of sulfonate groups in the aryl sulfonate derivatives on the pH sensitivity of the PAV-ASD-x

In general, the pH sensitivity of the polyampholyte was controlled by changing the DS that can be used to adjust its ratio of cationic to anionic groups. However, this ratio in the PAV-ASD-x can also be regulated by changing the amounts of sulfonate groups in the aryl sulfonate derivatives at the fixed DS. Therefore, four aryl sulfonate derivatives in which the amounts of sulfonate groups are 1, 2, 3 and 4, respectively, were designed and incorporated into the poly(acrylamide-co-vinylamine) oligomer with the fixed DS (PAV-ASD-1-20 ∼ PAV-ASD-4-20).

Fig. 10 shows the pH sensitivity of these polyampholyte. It is evident that PAV-ASD-1-20 has two transparent regions at pH > 10 and pH < 4.0, and PAV-ASD-2-20 ∼ PAV-ASD-4-20 have only one transparent region at pH above 7, 5 and 3, respectively. It is interesting to found the transparent region of PAV-ASD-2-20 ∼ PAV-ASD-4-20 increased about two pH units for each additional sulfonate group in the aryl sulfonate derivatives. The pH sensitivity of these polyampholyte can also be explained from their ratios of the cationic to anionic groups in which these values are 3.89, 1.92, 1.28 and 0.96, respectively. By varying the amounts of sulfonate groups in the aryl sulfonate derivatives, it is possible to change the IEP of the PAV-ASD-x, for example, the IEP of PAV-ASD-1-20 is 9.19 and that is 7.12 for PAV-ASD-2-20 (Fig. 11). Hence, at the fixed DS, the pH sensitivity of our prepared polyampholytes can be tuned by changing the amount of sulfonate groups in the aryl sulfonate derivatives.

Fig. 10 Transmittance of PAV-ASD-1-20 ∼ PAV-ASD-4-20 (1 g L⁻¹) in 0.01 N NaCl.

Fig. 11 Potentiometric and conductometric titrations of the PAV-ASD-1-20 and PAV-ASD-2-20 (1 g L⁻¹) in 0.01 N NaCl at 25 °C.

3.6 Effect of the aromatic ring on the pH sensitivity of the PAV-ASD-x

As mentioned before, the amount of the aromatic rings in the polyampholyte is intimate to its physical–chemical properties and application.¹⁴ Meanwhile, in our previous study, it was also found the dispersion ability of the polyampholyte on carbon black depended on the amount of the aromatic rings.¹³ Therefore, the effect of the amount of the aromatic rings on the pH sensitivity of the PAV-ASD-x was investigated firstly. At the fixed ratio of the cationic to anionic groups, pH sensitivities of the PAV-ASD-2-20, PAV-ASD-3-13.3 and PAV-ASD-4-10 were compared (Fig. 12). It can be found that the critical pH values of the sharp transition in the transmittance of the PAV-ASD-2-20, PAV-ASD-3-13.3 and PAV-ASD-4-10 solution are 7.4, 8.3 and 8.5, respectively. The PAV-ASD-x became more hydrophobic as the more amounts of the aromatic rings in the PAV-ASD-x, and the smaller critical pH value of the PAV-ASD-x was obtained. Meantime, the difference of the critical pH value between PAV-ASD-2-20 and PAV-ASD-3-13.3 (0.9 pH unit) is larger than that between PAV-ASD-3-13.3 and PAV-ASD-4-10 (0.2 pH unit). This may be caused by that the different amount of sulfonate substitution on benzene ring should strongly change the π–π interaction and hydrophobicity of benzene ring. In addition, the effect of structure of the aromatic ring on the pH sensitivity of the polyampholyte was also studied and the results were demonstrated in Fig. 12. The transmittance of the PAV-ASD-2-20 containing two benzene rings in the ASD-2 sharply dropped from 95% to 5% at about pH = 7.5 and the value is about 8.0 for the PAV-ASD-5-20 containing one benzene ring and one naphthalene ring in the ASD-5. The tiny different inflection point for the PAV-ASD-2-20 and PAV-ASD-5-20 may be caused by the difference of hydrophobicity between the benzene ring and naphthalene ring.

Fig. 12 Transmittance of PAV-ASD-2-x ∼ PAV-ASD-5-x (1 g L⁻¹) in 0.01 N NaCl.

As discussed before, the pH sensitivity of the prepared polyampholytes can be controlled by adjusting the molar ratio of the cationic to anionic groups in the polyampholytes. Therefore, the effect of this ratio on the phase behavior of the PAV-ASD-x in dilute solution (1 g L⁻¹) was also discussed (Fig. 13) in order to further understand the pH sensitivity of the PAV-ASD-x. The PAV-ASD-x can be fully soluble at pH above 11. When the ratio of the cationic to anionic groups is above 2.0, the soluble region of the PAV-ASD-x at acid condition enlarged with the increase of this ratio. The soluble pH value of the PAV-ASD-x was nearly linearly decreased with increasing the ratio from 1.0 to 2.0. However, solubility of the PAV-ASD-x is not pH sensitive when its ratio of the cationic to anionic groups is blow 1.0.

Fig. 13 Effect of the ratio of cationic to anionic groups on the phase behavior of the PAV-ASD-x (1 g L⁻¹) in 0.01 N NaCl.

4 Conclusions

A series of pH sensitive polyampholytes containing aromatic rings were prepared from poly(acrylamide-co-vinylamine) oligomer and aryl sulfonate derivatives. The ratio of cationic to anionic groups in these polyampholytes is the most important factor in tuning their pH sensitivity. By adjusting the DS of the polyampholytes or at the fixed DS, changing the number of sulfonate groups in the aryl sulfonate derivatives can be used to control this ratio. At the fixed molar ratio, the polyampholytes became more hydrophobic as the more amounts of the aromatic rings in the polyampholytes. The structure of aromatic ring in the polyampholyte has limited effect on the pH sensitivity.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21506102), and by Natural Science Foundation of the Higher Education Institutions of Jiangsu Province, China (14KJD150007).

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