A novel and controllable route for preparing high solid-content and low-viscosity poly(acrylamide-co-acrylic acid) aqueous latex dispersions

Abstract

High solid-content and low-viscosity poly(acrylamide-co-acrylic acid) aqueous latex dispersions have been successfully synthesized by copolymerization of acrylamide (AM) and acrylic acid (AA) in an aqueous solution of ammonium sulfate (AS) and lithium sulfate (LS) based on a distinctly novel strategy, the so called Swollen-Diffusion-In situ redox Polymerization (SDIP), which involves swelling followed by diffusion and redox initialized polymerization inside the seed particle, avoiding the high viscosity progress resulting from homogeneous nucleation in the continuous phase. Compared to the widely used one stage synthetic protocol, this process affords much more effective control over the viscosity of the dispersion and molecular weight of the resultant polymer by simply changing the concentration of inorganic salts and addition rate of the oxidant. The synthesized aqueous latex dispersions have been characterized using Fourier-transform infrared (FT-IR) spectroscopy, H nuclear magnetic resonance (¹HNMR) spectroscopy, and optical microscopy. The mechanism governing the formation of the latex dispersion is also extensively discussed.

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Introduction

Over the past ten years, there have been increasing research activities centered on acrylamide-based polymers and copolymers either in technological or industrial fields.^1–5 Functional copolymers with tailored structures have shown significant commercial applications, and they are widely used as flocculants in waste water treatment, drainage and retention aids in papermaking, and viscosity-control agents for enhanced oil recovery.^1,3–12

Various methods for preparing acrylamide-based polymers have been developed, such as aqueous solution polymerization,^13–16 inverse emulsion polymerization¹¹ and dispersion polymerization.^17–23 However, reaction heat is difficult to remove in aqueous solution polymerization and the viscosity of this system could be very high, often resulting in gels.^24,25 And therefore the resultant dry powder require intensive agitating and elevated temperature to dissolve. Although the inverse emulsion polymerization overcome this disadvantage, some surfactants and organic solvents used during the course of polymerization became pollutants to the environment in the end use application of the polymer.²⁶

In order to overcome these defects, aqueous dispersion polymerization were used to produce acrylamide-based polymers with high molecular weight and excellent solubility. Ray and Mandal carried out the dispersion polymerization of acrylamide in water/tent-butyl alcohol media.^18,27 Wang reported the effects of the structure and molecular weight of the polyelectrolyte stabilizer on the dispersion polymerization of water-soluble monomers in aqueous inorganic salt media.²³ However, there still existed some inherent disadvantages: (1) the monomer concentration is very low (not higher than 20%), making for inefficiency in the workplace and higher cost in the transportation; (2) the resultant polymer's molecular weight can not reach the requirement of practical application (not higher than 7 dL g⁻¹).^28–32 These above-mentioned defects inhibit the development of this technology and its application in industry. For in this type of dispersion polymerization, polymerization proceeded mostly in the continuous phase and the resultant polymers could not phase out in time from the continuous phase because the good solubility of the polymer. So the viscosity of the polymerization system would be very high with the most part of polymers dissolved in the continuous phase. When the total monomer concentration was higher than 20%, stable dispersion even could hardly been obtained by this method according to available reports.^28–32

In this article we proposed a distinctly novel strategy to fabricate poly(acrylamide-co-acrylic acid) aqueous latex dispersion. In our system, copolymers were formed through a diffusion and redox initialized polymerization restricted inside the seed particle. The continuous phase just acted as a warehouse for monomers rather than the location of polymerization, and continuously supplied monomers for polymerizing during the synthetic process. Thus polymerization could proceed steadily without or with low peak of viscosity at even higher monomers concentration (25%). Moreover, profiting from the unique “inside in situ redox polymerization” process, the final conversion of monomers increased (>99.5%) and the molecular weight (12.83 dL g⁻¹) of resultant polymers enhanced because of the gel effect resulting from much higher viscosity inside the particle. High molecular weight and high conversion had significant value for final application of this copolymer.

The final copolymers were characterized by Fourier-transform infrared (FT-IR) spectroscopy, H nuclear magnetic resonance (¹HNMR) spectroscopy and element analysis to confirm the participation of all monomers in such system. Optical microscope were used to investigate the particle morphology. The effects of concentration of inorganic salts and addition rate of oxidant on the viscosity of dispersion and molecular weight of resultant polymer are studied. The formation mechanism is also fully discussed.

Experimental section

Materials

Acrylamide (AM), acrylic acid(AA), Dimethylaminoethyl Methacrylate (DMAEMA), and 2,2′-azobis(2-methylpropionamidine) dihydrochloride (V50) were purchased from Aladdin Reagents (Shanghai) Co., Ltd. 2,2′-Azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride(VA044), 2-acrylamido-2-methylpropanesulfonic acid (AMPS), ammonium sulfate (AS), lithium sulfate (LS), potassium persulfate(KPS), potassium bromide (KBr), potassium bromate (KBrO₃), and potassium iodide (KI) were purchased from J&K Scientific Ltd. Ethanol (analytical grade) and acetic acid (analytical grade) were from Sinopharm Chemical Reagent Co., Ltd. All the reagents were used as received. Deionized water was used throughout all the experiments.

Preparation of stabilizer

Unless otherwise stated, all the following reaction mixtures were degassed by nitrogen bubbling before the adding of initiator solution. All the reactions were performed under nitrogen atmosphere.

The stabilizer PAMPSNa was prepared in our lab by free radical solution polymerization according to Wang's previous study.³² The AMPS was neutralized by NaOH at 1 : 1, then the 15 wt% aqueous solution of sodium AMPSNa was made and polymerized at 50 °C under a nitrogen atmosphere for 20 h using V-50 as initiator. After 20 h the solution was heated to 80 °C and maintained the temperature for 6 h. The designed molecular weight PAMPSNa was synthesized by varying initiator concentration. The intrinsic viscosity of the synthesized stabilizer PAMPSNa determined in a 1 mol L⁻¹ NaCl solution at 25 °C was 2.77.

Preparation of seed polymer dispersion

In a typical procedure, AM (55.77 g), AA (24.23 g), DMAEMA (8.80 g), PAMPSNa (133.33 g stabilizer solution, containing 20.00 g PAMPSNa), AS (150.00 g), LS (10.00 g), and deionized water (490.27 g) were added into a four-necked round-bottom flask with a mechanical stirrer, a nitrogen inlet tube, a nitrogen outlet tube and a rubber stopper. The mixture was mixed at 350 rpm, and the temperature was gradually raised to 35 °C. After all the species was dissolved, the initiator solution (2 g aqueous solution containing 40.0 mg VA044) was injected into the reaction mixture to start the polymerization. The reaction was kept at 35 °C for 24 h and then stopped. The other reactions were performed in the same procedure except the amount of reagents.

Preparation of polymer dispersion by swollen-diffusion-in situ redox-polymerization

In a typical procedure, AM (83.66 g), AA (36.35 g) and resultant seed polymer dispersion (870 g) were added into a four-necked round-bottom flask with a mechanical stirrer, a nitrogen inlet tube, a nitrogen outlet tube and a rubber stopper. The mixture was mixed at 350 rpm, and the temperature was gradually raised to 30 °C for nearly 2 hours. Then the oxidant solution (10 g aqueous solution containing 0.2 g KPS) was added into the reaction mixture, dropwise via syringe in 5 hours. The temperature was maintained at 30 °C for the first 5 hours of polymerization, after which polymerization was carried out for 10 hours at 35 °C. The other reactions were performed in the same procedure except the amount of reagents.

Preparation of polymer dispersion by one stage dispersion polymerization

In a typical procedure, AM (83.66 g), AA (36.35 g), PAMPSNa (80.00 g stabilizer solution, containing 12.00 g PAMPSNa), AS (120.00 g), LS (8.0 g), and deionized water (470.00 g) were added into a four-necked round-bottom flask with a mechanical stirrer, a nitrogen inlet tube, a nitrogen outlet tube and a rubber stopper. The mixture was mixed at 350 rpm, and the temperature was gradually raised to 35 °C. After all the species was dissolved, the initiator solution (2 g aqueous solution containing 60.0 mg VA044) was injected into the reaction mixture to start the polymerization. The reaction was kept at 35 °C for 24 h and then stopped. The other reactions were performed in the same procedure except the amount of reagents.

It should be noted that low monomer concentration was selected in one stage polymerization because stable dispersion could not be obtained if it was high.

Characterization

All the Fourier transform infrared (FT-IR) spectra were collected by a Nicolet Avatar 370 spectrometer. Nuclear magnetic resonance (¹HNMR) spectra were recorded in D₂O solutions on a Bruker AVANCE 500 MHz NMR spectrometer. The molar ratio of N and C of copolymer was obtained by Vario ELIII.

At different reaction stages, 5 mL of the reaction mixture was taken out by a syringe. Saturated aqueous solution of hydroquinone (25 μL) was added into the mixture immediately to terminate the polymerization. The apparent viscosity of the reaction mixture was measured by a Nirun Brookfield viscometer. One drop of the reaction mixture was dropped onto a thin glass slide and covered by another glass slide, in order to prevent solvent evaporation. The particle morphology was investigated by an optical microscope (Nikon Eclipse E400), particle size was measured with Nano-Measurer (Fudan University). The continuous phase and dispersed phase could be separated by centrifuging at 10 000 rpm for 5 min. The concentrations of unreacted monomers in the total reaction mixture, the continuous phase, and dispersed phase were measured by a modified-bromine titration method.

To measure the intrinsic viscosity of the polymers, the polymer should be purified. The polymer dispersion was dissolved in water and the copolymer (P(AM-co-AA)) was precipitated by pouring the polymer solution into a large quantity of ethanol and washed with acetone. Three precipitation/dissolution/washing cycles were repeated to remove all the inorganic salts and unreacted monomers. The final precipitation was dissolved in water again and the aqueous solution was dried at 40 °C under vacuum. The dried sample was used to determine the intrinsic viscosity [η] of the copolymer in a 1 mol L⁻¹ NaCl aqueous solution with an Ubbelohde capillary viscometer at 25 °C. In the following discussion, [η] was used to indicate the relative molecular weight of the yielded P(AM-co-AA).

Results and discussion

The schematic process of preparing the aqueous latex dispersion was displayed in Scheme 1. After initially forming AM/AA-swollen seed particles,^31,33–35 which was prepared beforehand by dispersion polymerization of AM, AA and DMAEMA in an aqueous solution of ammonium sulfate (AS) and lithium sulfate (LS), the oxidant potassium persulfate (KPS) was slowly added to the system, thereafter diffused into the seed particles and encountered the reductant, tertiary amine group plug-in the seed polymer chain. Following the free radical was initialized through a redox reaction between KPS and tertiary amine groups,³⁶ copolymerization of AM and AA took place inside the seed particles. If a delicate balance among the diffusion of the monomers, the diffusion of the radicals, and polymerization of the monomers could be achieved, the continuous phase could just acted as reservoirs of monomers that constantly released AM and AA to the seed particles to keep the processes of diffusion and polymerization going as the formation proceeded, avoiding the high viscosity progress produced by homogeneous nucleation in the continuous phase.

Scheme 1 Schematic representation of preparing poly(acrylamide-co-acrylic acid) aqueous latex dispersion by swollen-diffusion-in situ redox-polymerization method.

Fig. 1a represented ¹HNMR spectra of the seed copolymer poly(AM-co-AA-co-DMAEMA). Each proton could be easily distinguished from the resonance peaks of the copolymer.^30,31 We choose resonance peak at 2.88 (N–CH₃ of DMAEMA) to reduce overlap. We choose resonance peak at 2.15(–CH of AM and AA) and resonance peak at 1.57(–CH₂ of AM and AA) as a whole in order to minimize the effect of overlap. The percent of DMAEMA to the seed copolymer was calculated at 5.0% (molar ratio). The observed content of C, H, N and O of the seed copolymer was 51.00% (calculated: 50.92%), 6.76% (calculated: 6.73%), 13.67% (calculated: 13.55%) and 28.57% (calculated: 28.80%) respectively. Note that the content of O was obtained by equation (O% = 100% − C% − H% − N%). According to these data, molar ratio of AM : AA : DMAEMA could be calculated at 67.1 : 27.9 : 5.0, which was close to the dosage ratio 66.6 : 28.6 : 4.8. Fig. 1b represents ¹HNMR spectra of the resultant copolymer dispersion. Through the same method, molar ratio of AM : AA : DMAEMA could be calculated at 68.1 : 28.6 : 3.3, which was similar to the fed ratio 67.8 : 29.0 : 3.2.

Fig. 1 ¹HNMR spectrum of: (a) seed copolymer; (b) copolymer prepared by SDIP method. Seed synthesized procedure: monomer: 10 wt%, AM/AA: 70/30 (mol), (NH₄)₂SO₄: 17.2 wt%, Li₂SO₄: 1.1 wt%, PAMPSNa (η = 2.77) 20 wt%, initiator: 0.05 wt%, temperature: 35 °C. SDIP method: monomer: 20 wt%, AM/AA: 70/30 (mol), (NH₄)₂SO₄: 15 wt%, Li₂SO₄: 1 wt%, PAMPSNa (η = 2.77) 10 wt%, oxidant: 0.1 wt% added in 5 hours, temperature: 30 °C.

Fig. 2 showed the FTIR spectrum of the seed copolymer and the final formed copolymer dispersion prepared using different AM/AA molar ratios of 7 : 3 and 9 : 1. For all copolymer dispersion, the absorption bands appeared notably at 1718 cm⁻¹ due to the carbonyl group of the AA units, 1660 cm⁻¹ due to the carbonyl group of the AM units and 1120 cm⁻¹ due to tertiary amine group of the DMAEMA units.³² The amide carbonyl adsorption band gradually became stronger with the increase of AM/AA ratio, whereas the carboxylic acid adsorption band gradually became weaker.

Fig. 2 FT-IR spectrum of: (a) the seed copolymer; (b) the final formed copolymer with AM/AA molar ratio of 7 : 3, and (c) 9 : 1 monomer: 20 wt%, (NH₄)₂SO₄: 15 wt%, Li₂SO₄: 1 wt%, PAMPSNa (η = 2.77) 10 wt%, oxidant: 0.1 wt% added in 5 hours, temperature: 30 °C.

For this dispersion with AS and LS solution as media, H₂O would evaporate fast and inorganic salts would crystallize during TEM and SEM measurement course, causing particle morphology changing. Therefore optical microscope was introduced and shown in Fig. 3. In our system a good affinity of AM/AA with seed copolymer should lead them enriched inside the seed particles rather than the continuous phase. Swelling of the seed particles with AM/AA was confirmed by the gradual enlargement of particles (Fig. 3a and c). Fig. 3b and d showed the distribution of particle size before and after swelling procedure. The average diameter of seed particles ranged from 1.2 to 5.3 μm (average diameter 2.5 μm), and this is contrasted with that of particles swollen by monomer where the average diameters range from 1.9 to 5.6 μm (average diameter 3.3 μm).

Fig. 3 Optical micrographs and particle size distributions of copolymer particles before (a and b) and after swelling procedure (c and d) monomer: 20 wt%, AM/AA: 70/30 (mol), (NH₄)₂SO₄: 15 wt%, Li₂SO₄: 1 wt%, PAMPSNa (η = 2.77) 10 wt%, temperature: 30 °C.

More evidence was provided from the evolution of AM/AA monomer ratio in the seed particle versus total fed monomer monitored during the swelling procedure, as shown in Fig. 4. In the initial stage, monomer was almost totally dissolved in the continuous phase. After a certain period, AM/AA redistributed between the disperse phase and continuous phase to reach a new equilibrium. The pace of this equilibrium quickened measurably at 5 °C, but this temperature was not enough for the redox initiation reaction. At 20 °C, the absorption proceeded very slowly and the absorbed amount was not so high. At 30 °C it cost nearly 2 hours to reach the new equilibrium and this temperature is high enough for the redox initiation reaction.

Fig. 4 Evolution of monomer in the seed particles versus total fed monomer at varied temperature. Monomer: 20 wt%, AM/AA: 70/30 (mol), (NH₄)₂SO₄: 15 wt%, Li₂SO₄: 1 wt%, PAMPSNa (η = 2.77) 10 wt%.

Optical microscope of the resultant copolymer dispersion prepared by SDIP method was shown in Fig. 5a. These particles were nearly spherical and smooth, with the diameter from 2.20 to 9.50 μm (average diameter 4.5 μm). By contrast, the morphology of copolymer dispersion prepared by one stage aqueous dispersion polymerization (Fig. 5c) were less uniform. Some ellipsoidal, even dumbbell-like particles with larger diameter (average diameter 5.2 μm) and broader diameter distribution (from 0.97 to 16.50 μm, exhibiting a bimodal distribution) were observed.

Fig. 5 Optical micrographs and size distributions of copolymer particles prepare by SDIP method (a and b) and one stage method (c and d) SDIP method: monomer: 20 wt%, AM/AA: 70/30 (mol), (NH₄)₂SO₄: 15 wt%, Li₂SO₄: 1 wt%, PAMPSNa (η = 2.77) 10 wt%, oxidant: 0.1 wt% added in 5 hours, temperature: 30 °C. One stage method: monomer: 15 wt%, AM/AA: 70/30 (mol), (NH₄)₂SO₄: 15 wt%, Li₂SO₄: 1 wt%, PAMPSNa (η = 2.77) 10 wt%, initiator: 0.05 wt%, temperature: 35 °C.

For dispersion polymerization in organic media, the particle number would not change after particle forming stage and the particle morphology could be very uniform.^37,38 But for one stage aqueous dispersion polymerization of AM/AA in AS solution, it was difficult to obtain good uniformity and sophericity partially because the primary particles were not very stable at the beginning of polymerization, as well as the high concentration of monomer in the continuous phase, leading to increasing particle number during all the polymerization. Aggregation of particles could not be inevitable, so the particles became non-uniform and polydispersed. As for SDIP method, polymerization occurred almost entirely inside the seed particles, avoiding the homogeneous nucleation in the continuous phase. So the resultant particles possessed more uniform and regular-shaped morphology.

Fig. 6 showed the viscosity evolution during the polymerization of one stage method (Fig. 6a, 18% and Fig. 6b, 15%) and SDIP method (Fig. 6c, 25% and Fig. 6d, 20%) at different monomer concentration, providing more evidence about details of these two methods. At a relative low monomer concentration (Fig. 6b, 15%), one stage method attained stable dispersion with higher viscosity than SDIP method and appeared rather complex viscosity evolution behaviour. It could be generally described as four stages which was also observed by Shan:³⁹ the viscosity develops sharply in stage 1, then increase smoothly in stage 2 followed by an acute enhancement to the peak value in stage 3, and finally decrease slightly during stage 4. In conventional dispersion polymerization using organic medium, polymers could be separated from continuous phase instantly because of their poor affinity. However, in one stage dispersion polymerization, the polymers could not phase out in time from the continuous phase because the molecular weight had not exceeded the critical molecular length.⁴⁰ So the viscosity of the polymerization system would be very high with the polymers dissolved in the continuous phase, and that is the stage 1. When the molecular weight reached the critical value, polymers started to precipitate from medium to form the primary nucleus, so the viscosity increase gently even decrease, and that is the stage 2. Although many primary nucleus precipitate from reaction medium, secondary nucleation in the continuous phase could not be inevitable so the dispersion viscosity climb up quickly again and that is stage 3. Consequently the viscosity decrease again after the monomer almost exhausted and that is stage 4. In SDIP method, dispersion viscosity increasing gently with the development of polymerization (Fig. 6c, 25% and Fig. 6d, 20%). Despite final viscosity becoming larger as total monomer concentration were increasing, it could be controlled within an acceptable range in contrast with the one stage method where stable dispersion latex could even not be obtained in 18% monomer concentration (Fig. 6a, gelation occurs 3.5 h after reaction).

Fig. 6 Viscosity evolution during the polymerization of one stage method (a and b) and SDIP method (c and d) at varied monomer concentration. SDIP method: AM/AA: 70/30 (mol), (NH₄)₂SO₄: 15 wt%, Li₂SO₄: 1 wt%, PAMPSNa (η = 2.77) 10 wt%, oxidant: 0.1 wt% added in 5 hours, temperature: 30 °C. One stage method: AM/AA: 70/30 (mol), (NH₄)₂SO₄: 15 wt%, Li₂SO₄: 1 wt%, PAMPSNa (η = 2.77) 10 wt%, initiator: 0.05 wt%, temperature: 35 °C.

Polymer content in the continuous phase during polymerization reaction was presented in Fig. 7a, throwing great light on the primary and secondary nucleation process. Meanwhile, the viscosity of the SDIP method developed more gently, and polymer content in the continuous phase were much lower than that in one stage method and remained more or less unchanged over the whole process, as depicted in Fig. 7b. This phenomena indicated that there were little polymer dissolved in the continuous phase, suggesting that polymerization in SDIP method took place in the disperse phase, that is, inside the seed particle.

Fig. 7 Polymer content in the disperse phase and continuous phase during polymerization reaction of one stage method (a) and SDIP method (b) SDIP method: Monomer: 20 wt%, AM/AA: 70/30 (mol), (NH₄)₂SO₄: 15 wt%, Li₂SO₄: 1 wt%, PAMPSNa (η = 2.77) 10 wt%, oxidant: 0.1 wt% added in 5 hours, temperature: 30 °C. One stage method: monomer: 15 wt%, AM/AA: 70/30 (mol), (NH₄)₂SO₄: 15 wt%, Li₂SO₄: 1 wt%, PAMPSNa (η = 2.77) 10 wt%, initiator: 0.05 wt%, temperature: 35 °C.

Table 1 gave the properties of a serial of copolymer dispersion prepared by one stage method and SDIP method. As shown in Table 1, by one stage method, reaction could not perform smoothly at relative high monomers concentration because the problem concerned with peak viscosity was very difficult to overcome before stable dispersion formed while polymerization could proceed steadily without or with low peak of viscosity at even higher monomers concentration when SDIP method was involved. Moreover, profiting from the unique “inside in situ redox polymerization” process, the final conversion of monomers increased and the molecular weight of resultant polymers enhanced because of the gel effect resulting from much higher viscosity inside the particle. High molecular weight and high conversion had significant value for final application of this copolymer.

Table 1 Preparation of copolymer aqueous dispersion with different methods^a

Polymerization method	Total monomer concentration	Intrinsic viscosity (dL g^−1)	Dispersion viscosity (mPa S)	Particle diameter (μm)	Conversion
a SDIP method: AM/AA: 70/30 (mol), (NH4)2SO4: 15 wt%, Li2SO4: 1 wt%, PAMPSNa (η = 2.77) 10 wt%, oxidant: 0.1 wt% added in 5 hours, temperature: 30 °C. One stage method: AM/AA: 70/30 (mol), (NH4)2SO4: 15 wt%, Li2SO4: 1 wt%, PAMPSNa (η = 2.77) 10 wt%, initiator: 0.05 wt%, temperature: 35 °C.
One stage	10%	5.21	508	11.2	97.3%
	15%	6.87	1735	14.5	98.2%
	18%	Gelation 3.5 h after reaction
SDIP	15%	9.67	165	10.3	99.7%
	20%	10.45	561	12.1	99.8%
	25%	12.83	853	13.4	99.8%

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According to the formation mechanism of the resultant copolymer dispersion latex, an alternative way to drive the forming and diffusion of free radicals, and thus to control the properties of the dispersion, is to vary the addition rate of oxidant. As depicted in Fig. 8, it was corroborated that the addition rate of KPS solution had an important effect on the properties of the dispersion. When KPS added quickly, lower intrinsic viscosity, lower conversion, and high apparent viscosity dispersion was produced. On the one hand, too quick addition of oxidant resulted in more free radical inside the seed particle, causing the acceleration of bimolecular termination, and thus lower molecule weight polymers were produced. On the other hand, too much free radicals means there would be no time for them to be captured by monomers swollen inside the seed particle. Some of them diffused into the continuous phase and initiated the polymerization of monomers dissolve in the continuous medium. Polymers would be produced and dissolved in the continuous phase, resulting high viscosity of dispersion and lower conversion of monomer.

Fig. 8 Effect of addition rate of oxidant on swollen-diffusion-in situ redox-polymerization. Monomer: 20 wt%, AM/AA: 70/30 (mol), (NH₄)₂SO₄: 15 wt%, Li₂SO₄: 1 wt%, PAMPSNa (η = 2.77) 10 wt%, oxidant: 0.1 wt% added in varied time, temperature: 30 °C.

The polymer content in the continuous phase with different oxidant addition time was monitored during polymerization reaction and shown in Fig. 9. The steep rise of polymer content in the continuous phase when oxidant added in a short period of time proved that the addition rate of KPS was crucial to drive the outward diffusion of radical. From the viewpoint of match between the diffusion of free radical and polymerization of the monomer, the viscosity of dispersion and molecular weight of resultant polymer could be controlled.

Fig. 9 Polymer content in the continuous phase during polymerization reaction with different oxidant addition time. Monomer: 20 wt%, AM/AA: 70/30 (mol), (NH₄)₂SO₄: 15 wt%, Li₂SO₄: 1 wt%, PAMPSNa (η = 2.77) 10 wt%, oxidant: 0.1 wt% added in varied time, temperature: 30 °C.

A serial of experiments were carried out at varied ammonium sulfate concentration from 10% to 23%, while the total monomer concentration, molar fed ratio of AM/AA, stabilizer concentration were fixed at 25%, 70 : 30, and 10%, respectively. As shown in Fig. 10, monomer conversion fluctuated slightly with the varying of AS concentration. The viscosity of obtained dispersion decreased first, reach to a minimum at 17% AS concentration and then increased again while the intrinsic viscosity of the produced copolymer peaked at 15% AS concentration.

Fig. 10 Effect of ammonium sulfate concentration on swollen-diffusion-in situ redox-polymerization. Monomer: 20 wt%, AM/AA: 70/30 (mol), (NH₄)₂SO₄: varied, Li₂SO₄: 1 wt%, PAMPSNa (η = 2.77) 10 wt%, oxidant: 0.1 wt% added in 5 hours, temperature: 30 °C.

Varying the ammonium sulfate concentration affected the solubility of monomer in the continuous phase so the swelling equilibrium was disturbed, monomer rebalancing between the seed particle and continuous phase and, thus, to control the swelling dynamics of monomers. When the concentration of AS was lower than 12%, swelling of monomer to the seed particle was retarded because of the improvement of solubility in continuous medium, as depicted in Fig. 11. Consequently, supply of monomer failed to cope with the rate of generation of free radicals though it was controlled at a very low level by adjusting the addition rate of oxidant and radicals would diffuse into the continuous phase, resulting in high viscosity of dispersion and lower intrinsic viscosity of polymer. However, when the concentration of AS was higher than 20%, seed particles shrank too much because of strong salting out effect of AS on water soluble polymer through changing the water structure in the bulk or in the hydration shell around the polymer.⁴¹ As a result, both monomers and radicals could hardly pass through the solvent channel to sustain the further polymerization.^42,43 So polymerization would mostly occurred in the continuous phase, similar to the one stage polymerization.

Fig. 11 Evolution of monomer in the seed particles versus total fed monomer at varied ammonium sulfate concentration. Monomer: 20 wt%, AM/AA: 70/30 (mol), (NH₄)₂SO₄: varied, Li₂SO₄: 1 wt%, PAMPSNa (η = 2.77) 10 wt%, temperature: 30 °C.

Conclusions

In summary, high solid-content, high molecular weight and low-viscosity poly(acrylamide-co-acrylic acid) aqueous latex dispersion have been successfully synthesized through a novel process of swelling followed by diffusion and polymerization inside the particle. Detailed characterization demonstrated that the resultant product was copolymer of AM/AA, and, concretely the particles with micro-meter size were well dispersed in AS solution with the help of polyelectrolyte stabilizer. What is more, this proposed method afforded much more effective control over the viscosity of dispersion and molecular weight of resultant polymer by simply changing the concentration of inorganic salts and addition rate of oxidant. These factors also reveal the formation mechanism of this synthetic protocol.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (51408273), Natural Science Foundation of Jiangsu Province (BK20131013) and Natural Science Fund for Distinguished Youth Scholars of Jiangsu Province (BK20130048).

Notes and references

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