Self-assembly properties of a temperature- and salt-tolerant amphoteric hydrophobically associating polyacrylamide

Abstract

We prepared amphoteric hydrophobically associating polyacrylamides (AHAPAM) consisting mostly of hydrophilic polyacrylamide backbones, but also including the ionic hydrophobic monomer N,N-dimethyl octadeyl allyl ammonium chloride (DOAC) and the anionic monomer sodium 4-styrenesulfonate (SSS). The AHAPAM copolymer was prepared by carrying out aqueous solution polymerization. Macroscopic and microscopic self-assembly properties of AHAPAM in solution, as well as the effects of salt, temperature, and shearing on its association behavior were studied by carrying out viscosimetry, rheology, fluorescence spectroscopy (FS), and environmental scanning electron microscopy (ESEM) analyses. The results show that the association of the aqueous copolymer solutions were greatly affected by the concentration of the copolymer. The critical association concentration (CAC) of the AHAPAM solution was found to be 0.165 wt%, which was determined by carrying out viscometry and fluorescence spectroscopy experiments. Adding sodium chloride resulted in an increase in the apparent viscosity, which corresponded to the anti-polyelectrolyte solution behavior of AHAPAM. Meanwhile, intermolecular hydrophobic associations helped AHAPAM form a dynamic physically crosslinked network in its structure, conferring on AHAPAM strong heat- and shearing-resistance properties. The apparent viscosity of the 0.5 wt% copolymer solution was maintained at 92 mPa s at 140 °C and 170 s⁻¹ shearing for 1 h. FTIR and ¹H NMR spectra indicated the structure of the hydrophobically associating copolymers. And using the dilution extrapolation method, the intrinsic viscosity [η] of AHAPAM was shown to be 858.5 mL g⁻¹.

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1. Introduction

Hydrophobically associating polymers consist of hydrophilic polymer main chains to which are attached a relatively small number of hydrophobic side chains. These side chains form hydrophobic microdomains in solution because of hydrophobic association, which leads to intramolecular and intermolecular associations.¹ In dilute solution, macromolecules mainly associate intramolecularly, which reduces the hydrodynamic volume. At copolymer concentrations greater than the critical association concentration (CAC), macromolecules have an increased tendency to associate intermolecularly and can as a result form a supramolecular network. Hence, the hydrodynamic volume increases and the apparent viscosity increases significantly.^2–5 With the special functional groups, the polymer solution displays good salt tolerance, temperature resistance and the resistance to shear. Because of these excellent qualities, hydrophobically associating polymers have found practical applications in oilfield chemicals, dyeing, coating and other fields.^6–8

In order to tolerate reservoir conditions, which are typically severe, new polymers have been designed and synthesized to achieve improved properties; these new polymers have included hydrophobically associating polymers, hyperbranched polymers, amphoteric polymers, star polymers, etc.^9,10 The amphoteric hydrophobically associating polyacrylamides have both cationic and anionic groups, resulting in an anti-polyelectrolyte effect and an apparent viscosity increase, along with an increase in its salt concentration stemming from the anti-polyelectrolyte effect.^3,11–16 This feature allows amphoteric hydrophobically associating polyacrylamides to be applicable at high-salt concentrations. Furthermore, the thickening properties, temperature resistance and shearing resistance of the polymer can be improved by modifying the hydrophobic groups of the polymer.^17–20

In the current work, a new kind of amphoteric HMPAM (AHAPAM), consisting mostly of hydrophilic polyacrylamide backbones but also some ionic hydrophobic groups, specifically N,N-dimethyl octadeyl allyl ammonium chloride (DOAC) and anionic monomer sodium 4-styrenesulfonate (SSS), was synthesized. The effect of polymer concentration, temperature and salt on rheological properties were also evaluated. The results indicated that AHAPAM can be applied as a thickening agent at high temperature (140 °C) and high salt concentration (10%). And the proposed dosage is more than 0.3% mass concentration.

2. Experimental

2.1 Materials

Acrylamide (AM, AR), sodium chloride (NaCl, AR), calcium chloride (CaCl₂, AR), ammonium persulfate ((NH₄)₂S₂O₈, AR), sodium hydrogen sulfite (NaHSO₃, AR), and ethanol (>99.7%) were purchased from Chengdu Kelong Chemical Factory. Sodium p-styrenesulfonate (SSS, AR) was purchased from Shanghai Bangcheng Chemical Factory. N,N-Dimethyl octadeyl allyl ammonium chloride (DOAC, >99%) was purchased from Sigma-Aldrich.

2.2 Synthesis of AHAPAM

The route taken to synthesize the AHAPAM copolymer is illustrated in Scheme 1. First, AM dissolved in water was poured into a 150 mL three-necked flask, after which DOAC and SSS were added. The mole fraction of DOAC was 1.8% and the mole fraction of SSS was 0.6%. The concentration of the mixture was 25%. The flask was put into a water bath, and then N₂ was bubbled into the mixture for 30 min. Then (NH₄)₂S₂O₈/NaHSO₃ (0.2 wt% to monomer) were added as initiators under magnetic stirring until they were completely dissolved. Polymerization was carried out at 40 °C for several hours. Subsequently, the polymerization reaction was terminated and the crude product was precipitated after adding a large volume of ethanol. After filtration, the AHAPAM was collected, washed several times with ethanol, and dried in a vacuum environment at 60 °C for 24 h.

Scheme 1 Route used to synthesize AHAPAM.

2.3 Characterization and measurements

Structural characterization. The infrared spectrum of the copolymer was recorded in a KBr pellet using a Nicolet Nexus 170SX Fourier transform infrared spectrophotometer, and the ¹H NMR spectrum was acquired for the copolymer in deuteroxide (D₂O) with a Bruker ASCEND-400 NMR apparatus.

Characterizing the molecular weights of the copolymers. Using an aqueous solution of NaCl with a concentration of 1 mol L⁻¹ as solvent and at a constant temperature of 30.0 ± 0.1 °C, the viscosities of the aqueous solutions of the polymers were measured with a Ubbelohde viscometer, the intrinsic viscosities [η] of the polymers were captured by the dilution extrapolation method, and the molecular weights of the polymers were expressed relative to the intrinsic viscosities [η].²¹ Also, in order to reduce the effect of any intramolecular hydrophobic interaction on the determination of the viscosity-average molecular weight for AHAPAM, formamide had been added into the polymer solution with a 50% volume concentration.

Rheological analysis. The apparent viscosity values of the aqueous polymer solution at a variety of concentrations were measured by using an NDJ-8S with 3^# rotor at 20 °C. The CAC was determined from the resulting viscosity curve. The viscoelasticity analysis was carried out at 25 °C on a Malvern CVOR-200 rheometer equipped with a cone-plate geometry, and the stress was 1 Pa. The apparent viscosity at high temperature was measured by using an HAAK MARSIII rheometer.

Fluorescence analysis. FS measurements were taken by using a PerkinElmer fluorescence spectrometer LS 55 in the scanning range of 330–460 nm. Pyrene was used as the fluorescent probe.²² Solutions for FS measurements were prepared by first pipetting 0.04 mL of pyrene stock solutions (1 × 10⁻³ mol L⁻¹ in ethanol) into a 50 mL volumetric flask. After the ethanol dried, copolymer solutions with different concentrations were prepared in the flasks and subjected to ultrasound for 0.5 h before FS measurements were taken.

ESEM analysis. The microstructure of the copolymer solution was observed by using a Phenom Pro scanning electron microscope. A drop of solution was placed on the refrigerant table and then frozen. The structure was then observed and photographed in a vacuum environment.

3. Results and discussion

3.1 Characterization of AHAPAM

The FT-IR spectrum of AHAPAM is shown in Fig. 1. The absorption bands at 3407 and 1625 cm⁻¹ can be assigned to the N–H and C=O stretching vibration peaks for acrylamide, indicating the presence of PAAM units in the copolymer. The peaks at 2912 and 2847 cm⁻¹ were assigned to the C–H stretching vibrations of –CH₂– and –CH₃ in the polymer backbone and the hydrophobic carbon chain. The vibration bond at 3314 cm⁻¹ resulted from deformation of the methyl group of –N⁺(CH₃)₂–R, and confirmed the presence of the quaternary ammonium-based group. The characteristic benzene ring peak at 1402 cm⁻¹ as well as –SO₃⁻ peaks at 1167 and 1098 cm⁻¹ confirmed the presence of the benzenesulfonic group. And it can be deduced that there were no remaining monomers because of the absence of characteristic C=C absorption peaks from 3075 to 3090 cm⁻¹ and 900 to 1000 cm⁻¹. These results taken together indicated that the amphoteric hydrophobically associating polyacrylamide (AHAPAM) was synthesized successfully.

Fig. 1 The FT-IR spectrum of AHAPAM.

The ¹H NMR in Fig. 2 further verified the synthesis of AHAPAM. Chemical shifts appeared at 2.14 and 7.29 ppm, which could be attributed to the protons of hydrophilic polyacrylamide backbones marked as 1, 2 and 3. The chemical shift peak of the protons belonging to –CH₂– and –CH₃ bonded with the N atom of the quaternary ammonium was found at 2.14 ppm (marked as 6 and 7). The two peaks at 1.59 and 1.11 ppm (marked as 9 and 10) were assigned to the protons of the hydrophobic carbon chain. Chemical shifts also appeared at 3.56 and 7.70 ppm, which could be attributed to the protons of the benzene backbones marked as 13 and 14. The molecular structure of synthesized AHAPAM revealed by ¹H NMR is in good accord with the results of the FTIR spectrum. And the intrinsic viscosity [η] of AHAPAM was shown to be 858.5 mL g⁻¹ according to the dilution extrapolation method, as demonstrated in Fig. 3.

Fig. 2 The ¹H NMR (400 MHz, D₂O) spectrum of AHAPAM.

Fig. 3 Measuring the intrinsic viscosity [η] of AHAPAM with the dilution extrapolation method.

3.2 Self-assembly behaviors of AHAPAM in water at various concentrations

The long (18-C) carbon chain of the monomer DOAC provided the copolymer with a strong associative effect. This associative effect, which can form a dynamic physical cross-linking network and increase the hydrodynamic volume, led to the self-assembly of the copolymer in solution. But the assembly of the copolymer appeared to have followed different mechanisms at different concentrations. In a dilute solution, AHAPAM formed more intramolecular aggregates than intermolecular ones, curling the macromolecular chains and reducing the hydrodynamic volume. However, as the copolymer concentration surpassed the critical association concentration (CAC), intermolecular aggregates predominated and the hydrodynamics volume abruptly increased, as shown in Fig. 4. The CAC is an important parameter for hydrophobically associating polyacrylamides.

Fig. 4 Self-assembly of hydrophobically associating polyacrylamides.

Fig. 5 shows the results of the apparent viscosity measurements of the AHAPAM solutions at different concentrations from a macroscopic aspect. The apparent viscosity of the solution increased slowly with AHAPAM concentration at low concentrations, and increased abruptly thereafter. This growth spurt illustrated that DOAC was introduced into AHAPAM successfully. The abrupt increase in the apparent viscosity with concentration began at 0.19 wt% AHAPAM, indicating this concentration to be the CAC. At concentrations lower than 0.19 wt%, the intramolecular aggregates prevailed over the intermolecular ones, curling the macromolecular chains, and slowly increasing the apparent viscosity. When the concentration was higher than 0.19 wt%, intermolecular aggregates predominated and a three-dimensional network structure formed, leading to the sudden increase in apparent viscosity.

Fig. 5 The relationship between concentration and apparent viscosity of AHAPAM.

The apparent viscosity reflects the hydrophobic association effect on the macro level, whereas fluorometry can reveal the hydrophobic association properties of AHAPAM at a molecular level. Since nonpolar pyrene can solubilize into the hydrophobic region, it can be used as a fluorescence probe to reflect the intensity of the hydrophobic association effect. The fluorescence emission spectrum of the pyrene solution we prepared showed five peaks, approximately at 373, 379, 384, 394, and 480 nm. The solubility of pyrene can be deduced from the ratio of the fluorescence absorption intensity at 373 to that at 384 (I₁/I₃). The stronger the polarity of the microenvironment around the pyrene probe, the higher can be the value of I₁/I₃. The data showed that once intermolecular aggregation predominated, the value of I₁/I₃ showed an obvious decrease.

The fluorescence spectra of pyrene in AHAPAM solutions of various concentrations are shown in Fig. 6 (I₁ and I₃ are marked). Fig. 7 shows the values of I₁/I₃ at these AHAPAM concentrations. As shown in Fig. 7, the value of I₁/I₃ fell abruptly when the concentration surpassed 0.165 wt%, which was hence considered to be the CAC. This result indicated that when the concentration of AHAPAM solution was larger than 0.165 wt%, many hydrophobic regions were forming in the solution and the polarity of the microenvironment around the pyrene probe weakened. Due to the limited solubility of pyrene in the solutions, the value of I₁/I₃ leveled off after a certain concentration.

Fig. 6 The fluorescence spectra of AHAPAM at different concentrations.

Fig. 7 The ratio of I₁/I₃ at different AHAPAM concentrations.

The CAC determined via fluorescence spectroscopy, i.e., 0.165 wt%, differed somewhat from that determined via apparent viscosity, i.e., 0.19 wt%. The microscopic evidence may be considered to be more accurate, so the CAC of AHAPAM was considered to be 0.165 wt%.

Environmental scanning electron microscope (ESEM) images (magnified 4000 times) of AHAPAM solutions at concentrations of 0.12 wt% and 0.3 wt% are shown in Fig. 8(a) and (b), respectively. At the lower AHAPAM concentration, hardly any net structures and only some lines formed, by the twining of a few molecules. The sparse and scattered net structure resulted here because at a concentration lower than the CAC, the hydrophobic association was mostly manifested as intramolecular aggregates. It is also for this reason that the solution had low apparent viscosity at low AHAPAM concentrations. At the higher AHAPAM concentration, the net structure was much better defined, due to the predominantly intermolecular hydrophobic association. Correspondingly, the solution displayed high apparent viscosity values at high concentration.

Fig. 8 (a) An ESEM image of an aqueous 0.12 wt% AHAPAM solution magnified 4000 times. (b) An ESEM image of an aqueous 0.30 wt% AHAPAM solution magnified 4000 times.

3.3 The rheological properties of the AHAMAP solutions

Dynamic rheological tests are essential to study the visco-elastic behavior of polymers. The dependence of G′ and G′′ (the storage and the loss moduli) on frequency can reflect the relative motions of all molecules. When the storage moduli are larger than the loss moduli, the behavior of the solution is elastic. Conversely, the behavior of the solution is viscous.

As shown in Fig. 9, increasing the AHAPAM concentration clearly enhanced both moduli, indicating that the important elastic part and effective three-dimensional network structure formed in the polymer solution. And the storage moduli G′ were observed to be higher than the loss moduli G′′ throughout the tested range of frequencies, which confirmed the aqueous AHAPAM solutions to be elastic fluids.

Fig. 9 The dynamic moduli of AHAPAM at various concentrations.

The temperature and shear rate can markedly affect the apparent viscosity of hydrophobically associating polyacrylamides because of the physical crosslink. At high temperatures, intermolecular aggregation weakens and these aggregates break up, and a high shear rate occurs. Therefore, the temperature and shear rate are important factors to be taken into account when finding applications for these polyacrylamides. Fig. 10(a)–(c) show the effects of temperature from 20 °C to 140 °C on AHAPAM solution viscosity at 170 s⁻¹ shear rate for AHAPAM concentrations of 0.3 wt%, 0.4 wt%, and 0.5 wt%, and the duration of each test was one hour.

Fig. 10 (a) Effect of temperature on 0.3 wt% AHAPAM solution at 170 s⁻¹. (b) Effect of temperature on 0.4 wt% AHAPAM solution at 170 s⁻¹. (c) Effect of temperature on 0.5 wt% AHAPAM solution at 170 s⁻¹.

As shown in Fig. 10, the different tested AHAPAM concentrations yielded similar apparent viscosity versus temperature trends. The viscosity was reduced at first because the high shear rate (170 s⁻¹) broke part of the three-dimensional network structure. Then the apparent viscosity increased in a certain temperature range, and the endpoint temperture of this range increased as the AHAPAM concentration was increased. This behavior was attributed to the increasing temperature resulting in a longer hydrophilic spacer, which is more effective in promoting the formation of intermolecular hydrophobic aggregates.²³ The stretching polymer structure was favorable for intermolecular association, and the viscosity thus increased significantly. But as the temperature was increased further, the “iceberg structure” around the hydrophobic groups was destroyed, resulting in more rapid molecular motions, and hence weakening the hydrophobic association effect,²⁴ and causing the viscosity to decrease sharply but then finally level off. As shown in Fig. 10(c), the apparent viscosity of the 0.5 wt% AHAPAM solution remained at 92 mPa s at 140 °C and a 170 s⁻¹ shear rate. This result is indicative of the excellent temperature resistance of AHAPAM.

Adding the small-molecule electrolyte sodium chloride into the polymer solution increased the polarity of the medium, which affected the apparent viscosity of the solution. Fig. 11 shows the effect of sodium chloride on the apparent viscosity of the solution at different AHAPAM concentrations. As sodium chloride was added, the apparent viscosity of the AHAPAM solution decreased sharply at low salt concentration, then increased until the concentration of the sodium chloride was 10 wt%, at which point the apparent viscosity was over 75% of the initial apparent viscosity. These changes in apparent viscosity were due to two effects occurring in the copolymer–salt solution: a shielding effect and an anti-polyelectrolyte effect. The shielding effect involves the salt thinning the double electrode layers and hydration shells of the polymer chains and increasing the polarity of the medium, which allows the chain to fold up and induces a severe curl of the polymer molecules.^25,26 This change in shape can reduce the hydrodynamic volume and decrease the apparent viscosity. In the anti-polyelectrolyte effect, adding appropriate salt into a dilute or semi-dilute solution of amphoteric hydrophobically associating polyacrylamides can screen intra-backbone electrostatic attraction and hence stretch the chains, which promotes the intermolecular aggregation and increases apparent viscosity.

Fig. 11 The effect of sodium chloride on solution apparent viscosity at different AHAPAM concentrations.

Therefore, the shielding effect was apparently stronger than the anti-polyelectrolyte effect at low salt concentration (below 0.2 wt%) and high salt concentration (over 10.0 wt%), whereas the anti-polyelectrolyte effect was stronger than the shielding effect at the intermediate concentrations. The result is indicative of the excellent salt tolerance of the AHAPAM solution.

4. Conclusions

AHAPAM is a novel kind of amphoteric hydrophobically associating polyacrylamide prepared from AM, DOAC and SSS via an aqueous solution polymerization. AHAPAM contains hydrophobic groups, which can make the molecular chain aggregate and form a supramolecular structure by intermolecular hydrophobic associations, leading to an increase in the viscosity of the solution. The CAC was determined to be 0.165 wt% by viscosimetry and fluorescence spectroscopy (FS). The rheological test showed that the storage moduli G′ of AHAPAM were always higher than the loss moduli G′′, which indicated the aqueous AHAPAM solutions to be elastic fluids. The AHAPAM has both cationic and anionic groups. The addition of salt can screen the attractive electrostatic interaction between the cationic and anionic groups. This effect is called the anti-polyelectrolyte effect. The apparent viscosity of 0.3 wt% AHAPAM remained at 948 mPa s as the salt concentration was increased to 10.0 wt%. This result also reflects the excellent salt tolerance of AHAPAM. Also, the apparent viscosity of the 0.5 wt% copolymer in aqueous solution was 92 mPa s at 140 °C and 170 s⁻¹ shearing for 1 h. The excellent resistance to salt, high temperature, and shear makes this new amphoteric hydrophobically associating polyacrylamide AHAPAM applicable for fracturing fluids, flocculants, etc.

Acknowledgements

This work was supported financially by the Open Fund (contract grant number PLN1312) of the State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation (Southwest Petroleum University), scientific research starting project of SWPU (No. 2015QHZ015), and supported by the Opening Project of Oil & Gas Field Applied Chemistry Key Laboratory (YQKF201401).

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