Tunable Viscoelastic Properties of Sodium Polyacrylate Solution Via CO2-Responsive Switchable Water

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Article Tunable Viscoelastic Properties of Sodium Polyacrylate Solution via CO2-Responsive Switchable Water

Dianguo Wu 1,2, Yiwen Shi 3, Kun Lv 2, Bing Wei 1, Youyi Zhu 4, Hongyao Yin 2,* and Yujun Feng 2,*

1 State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu 610500, China; [email protected] (D.W.); [email protected] (B.W.) 2 Polymer Research Institute, Sichuan University, Chengdu 610065, China; [email protected] 3 Pittsburgh Institute, Sichuan University, Chengdu 610065, China; [email protected] 4 Research Institute of Petroleum Exploration and Development, CNPC, Beijing 100083, China; [email protected] * Correspondence: [email protected] (H.Y.); [email protected] (Y.F.)

Abstract: Upon stimulus by CO2, CO2-switchable viscoelastic ﬂuids experience a deliberate transition between non-viscous and highly viscous solution states. Despite attracting considerable recent attention, most such ﬂuids have not been applied at a large- scale due to their high costs and/or complex

synthesis processes. Here, we report the development of CO2-switchable viscoelastic ﬂuids using commercially available sodium polyacrylate (NaPAA) and N,N-dimethyl ethanol amine (DMEA)-

based switchable water. Upon bubbling CO2, into the solutions under study, DMEA molecules are protonated to generate quaternary ammonium salts, resulting in pronounced decreases in solutions viscosity and elasticity due to the influence of increased ionic strength on NaPAA molecular confor- mations. Upon removal of CO2 via introduction of N2, quaternary salts are deprotonated to tertiary amines, allowing recovery of fluid viscosity and elasticity to near the initial state. This work provides Citation: Wu, D.; Shi, Y.; Lv, K.; Wei, a simple approach to fabricating CO -switchable viscoelastic fluids, widening the potential use of B.; Zhu, Y.; Yin, H.; Feng, Y. Tunable 2 Viscoelastic Properties of Sodium CO2 in stimuli-responsive applications. Polyacrylate Solution via Keywords: N,N CO2-Responsive Switchable Water. viscoelastic fluids; CO2-switchable; sodium polyacrylate; -dimethyl ethanol amine Molecules 2021, 26, 3840. https:// doi.org/10.3390/molecules26133840

Academic Editor: 1. Introduction Apostolos Avgeropoulos As a non-Newtonian fluid, a viscoelastic solution usually exhibits unique rheological properties (i.e., both liquid-like fluidity under some circumstances and solid-like elasticity Received: 30 May 2021 under others) [1–4], which endow it with many distinct behaviors, such as the Weissenberg Accepted: 20 June 2021 effect [2,5], extrudate swell [3], and fading memory [6]. Those characteristics allow for great Published: 24 June 2021 potential applications in fields like food science [4], damping [7], tissue engineering [8], and oil development [9,10]. Over the past few decades, smart viscoelastic fluids that respond Publisher’s Note: MDPI stays neutral reversibly to environmental stimuli, e.g., temperature [11–15], pH [16,17], light [18], and with regard to jurisdictional claims in CO2 [19], to demonstrate tunable rheological properties have drawn extensive attention published maps and institutional affil- both from engineers in industry and fundamental theoretical scientists. Among above iations. triggers, CO2 has garnered considerable interests recently due to its nontoxicity, low cost, high availability, and good biocompatibility [19–22]. Generally, CO2-responsive viscoelastic systems can experience a purposeful alteration from non-viscous liquid to high- viscosity solution or gel, by the stimulus of CO2. As of yet, many different CO2-responsive Copyright: © 2021 by the authors. viscoelastic fluids have been reported, often utilizing a reversible reaction between CO2 Licensee MDPI, Basel, Switzerland. and guanidines, amidines, or amines [21]. This article is an open access article So far, CO2-responsive viscoelastic systems based on surfactants and/or polymers distributed under the terms and have been developed. Surfactant wormlike micelles (WLMs) are long flexible aggregates conditions of the Creative Commons that can entangle to form three-dimensional networks that impart remarkable viscoelastic Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ properties to the bulk solution. Thus, the reversible generation and destruction of wormlike 4.0/). micelles via external stimulations can realize switchable viscoelastic fluids [23]. Feng and

Molecules 2021, 26, 3840. https://doi.org/10.3390/molecules26133840 https://www.mdpi.com/journal/molecules Molecules 2021, 26, 3840 2 of 15

co-workers [23] pioneered CO2-switchable WLMs using sodium dodecyl sulfate (SDS) 0 0 and N,N,N ,N -tetramethyl-1,3-propanediamine (TMPDA). CO2 was shown to protonate tertiary amino groups of TMPDA, which then interacted with SDS to generate WLMs. Thus, the alternating introduction and removal of CO2 resulted in a reversible viscoelastic fluid. In addition to surfactant, water-soluble polymers, including synthetic and natural polymers, is another class of important material preparing viscoelastic fluids, which are widely used as rheology modifiers in the oil and gas industry. The strategy to construct CO2- switchable polymer viscoelastic fluid is usually to copolymerize a water-soluble species of monomer with a CO2-sensitive monomer, such as N,N-dimethylaminoethyl methacrylate (DMAEMA) [19,24] and N,N-diethylaminoethyl methacrylate (DEAEMA) [25]. The protonation and de-protonation of the tertiary amino group in these monomers in the presence and absence, respectively, of CO2 lead to a large change in hydrophilicity, which gives rise to variations in the fluid’s viscoelasticity. However, a complicated synthesis process as well as high cost of the CO2-sensitive monomer make large-scale industrial applications difficult to achieve. Therefore, it is highly desirable to find an inexpensive and simple system for the fabrication of CO2-switchable viscoelastic fluids. Sodium polyacrylate (NaPAA) is a typical polyelectrolyte, possessing many charged carboxylate groups along the main polymer chain upon its dissociation in water. Due to these groups, electrostatic repulsions arise between inter- and intramolecular chains, imparting high viscosity and elasticity to the aqueous solution. This strong thickening power as well as its low cost and easy availability facilitates wide use of NaPAA in industry. However, electrostatic repulsions between the charged groups of NaPAA are susceptible to interference from salts. Fujita et al. [26] revealed that an apparent decrease occurred in the viscosity of NaPAA solutions when sodium chloride (NaCl) was added, because the corresponding increase in ionic strength of the solution allowed PAA molecules to coil up more and more tightly. Klaus [27] found that the addition of Ca2+ to NaPAA solution similarly caused polymer coils to apparently shrink, resulting in dramatic viscosity decreases with incremental introduction of Ca2+. It has thus become clear that the rheological properties of NaPAA aqueous solution are highly sensitive to ionic strength. Therefore, a solution that incorporates switchable ionic strength is expected to impart the NaPAA fluid with smart, tunable rheological properties. Jessop and co-workers [28–30] pioneered the development of an aqueous solution of various amines, called “switchable water”, which demonstrated CO2-switchable ionic strength. In the absence of CO2, the ionic strength of switchable water was very low, but in the presence of CO2, the amines were converted into the bicarbonate salts, resulting in a dramatic and reversible rise in ionic strength. The aim of this work is to reversibly regulate the rheological properties of NaPAA- based solution via the incorporation of switchable water. Herein, N,N-dimethyl ethanol amine (DMEA) was chosen as an additive to prepare CO2-switchable water, and the rheological properties of NaPAA in the switchable water with and without CO2 were investigated. Both viscosity and elasticity of the solutions were found to decrease after CO2-treatment, and then were partially recovered once CO2 was removed by bubbling N2 at elevated temperature. The effects of NaPAA and DMEA concentrations on the variation of rheological properties were examined and the corresponding mechanism behind was also discussed.

2. Results and Discussion 2.1. Rheological Properties of NaPAA Aqueous Solutions and Their CO2-Responsive Behavior Owing to its strong thickening power and easy availability, NaPAA is widely used in various industries as rheology modiﬁer. The NaPAA used in this work was a commercial 6 6 −1 product with MW of 4 × 10 –5 × 10 g·mol , and its thickening ability was investigated. A series of aqueous solutions with different NaPAA concentrations were prepared, and the viscosity-shear rate curves (Figure S1) of each were measured to obtain the zero-shear viscosity (η0), which is the value when the shear rate tends to zero. Figure1a shows a plot of η0 at varying concentrations of NaPAA in pure water. One can ﬁnd that the Molecules 2021, 26, x FOR PEER REVIEW 3 of 15

Molecules 2021, 26, 3840 A series of aqueous solutions with different NaPAA concentrations were prepared,3 and of 15 the viscosity-shear rate curves (Figure S1) of each were measured to obtain the zero-shear viscosity (η0), which is the value when the shear rate tends to zero. Figure 1a shows a plot ofcurve η0 at hasvarying been concentrations divided into three of NaPAA concentration in pure regimeswater. One with can two find clear that breakpoints. the curve has At beenlow concentrations,divided into three the concentration NaPAA aqueous regimes solutions with exhibitedtwo clear Newtonianbreakpoints. fluids At low properties, concentrations,and the theη0 wasNaPAA found aqueous to be verysolutions close exhibited to that of Newtonian the solvent, fluids classifying properties, these and fluids the η as0 wasdilute found [31]. to Thebe very critical close overlap to that of concentrations the solvent, classifying (C*), at which these fluids the solution as dilute transitions [31]. The criticalfrom dilute overlap to semidiluteconcentrations behavior, (C*), at were which determined the solution from transitions the concentrations from dilute atto whichsemi- dilutethe measured behavior, solution were determined viscosity beganfrom the to concent abruptlyrations increase. at which Herein, the measured C* (≈ 0.0001 solution wt%) viscosityfalls in the began second to abruptly area shown increase. in Figure Herein,1a, inC* which (≈ 0.0001 the solutionwt%) falls viscosity in the second was found area shownto increase in Figure with 1a, the in increment which the of solution NaPAA viscosity concentration was found according to increase to a power with the law incre- with mentan exponent of NaPAA of 0.52,concentration approaching according a theoretically to a power predicted law with value an exponent (0.5) for of semidilute, 0.52, ap- proachingunentangled a theoretically polyelectrolyte predicted solutions value in (0.5) previous for semidilute, studies [32 unentangled–34]. In the polyelectrolyte third region of solutionsFigure1a, inη 0previousof the fluids studies increased [32–34]. more In the dramatically third region with of Figure concentration 1a, η0 of the than fluids it did inin creasedsemidilute, more unentangled dramatically regime, with concentration demonstrating than a powerit did in law semidilute, with an exponentunentangled of 1.77, regime,near thedemonstrating expected power a power index law (1.5) with of semidilute, an exponent entangled of 1.77, near polyelectrolyte the expected fluids power [33, 34in-]. dexBased (1.5) on of the semidilute, semidilute entangled unentangled polyelectrolyte and entangled fluids regime [33,34]. scaling, Based the on secondthe semidilute turning ≈ unentangledpoint was defined and entangled as the critical regime entanglement scaling, the concentration,second turningi.e. point, Ce was( 0.0007 defined wt%), as the as labeled in Figure1a. critical entanglement concentration, i.e., Ce (≈ 0.0007 wt%), as labeled in Figure 1a.

a b

FigureFigure 1. (a) Variation ofofη η00 withwith NaPAANaPAA concentration concentration in in pure pure water, water, and and (b )( theb) the viscosity viscosity alternation alternation of 0.65 of 0.65 and and 1.30 1.30 wt% −1 −1 2 2 ◦ wt%NaPAA NaPAA aqueous aqueous solutions solutions at shear at shear rate ofrate 10 of s 10upon s upon alternately alternately bubbling bubbling CO2 COand and N2 atN 25 at 25C. °C .

ToTo investigate investigate the the influence influence of of CO CO22 onon the the NaPAA NaPAA fluid, fluid, two two different different concentrations concentrations ofof NaPAA, NaPAA, 0.65 0.65 and and 1.30 1.30 wt%, wt%, representing representing solutions solutions with with high high viscosity viscosity and and elasticity elasticity in thein thesemidilute semidilute entangled entangled regime, regime, were were selected. selected. The NaPAA The NaPAA aqueous aqueous solutions solutions with withcon- centrationsconcentrations of 0.65 of 0.65wt% wt%and 1.30 and wt% 1.30 wt%were wereprepared, prepared, and their and rheological their rheological behaviors behaviors were examinedwere examined at 25 °C at 25under◦C under steady steady and d andynamic dynamic shear shear conditions. conditions. Figure Figure S2 shows S2 shows the vis- the cosityviscosity-shear-shear rate rate curves curves of ofthe the samples samples containing containing 0.65 0.65 wt% wt% and and 1.30 1.30 wt% wt% NaPAA NaPAA in in the the absenceabsence and and presence presence of of CO CO22. .It It can can be be found found that that the the fluids fluids showed showed Newtonian Newtonian behavior behavior withwith high, high, near near-constant-constant viscositie viscositiess at low at low shear shear rates, rates, then thenexhibited exhibited shear shearthinning thinning prop- ertiesproperties in which in which the viscosities the viscosities sharply sharply decreased decreased with the with increment the increment in shear in rate, shear which rate, waswhich consistent was consistent with typical with typicalrheological rheological properties properties of viscoelastic of viscoelastic fluids fluids[3,9]. After [3,9]. Afterbub- blingbubbling CO2 COfor 210for min 10 until min untilthe conductivity the conductivity of the of solutions the solutions stabilized, stabilized, the viscosity the viscosity of each of fluideach underwent fluid underwent an apparent an apparent decrease decrease over overthe entire the entire range range of tested of tested shear shear rates, rates, and andthe lowerthe lower the shear the shear rate, rate, the larger the larger the decrement the decrement in viscosity. in viscosity. ◦ N2 was subsequently introduced into the solutions at 60 C to remove CO2, and the viscosities of both the 0.65 wt% and 1.30 wt% NaPAA solution were partially recovered. Figure1b presents the variation in viscosity that arose for each fluid at a shear rate of

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N2 was subsequently introduced into the solutions at 60 °C to remove CO2, and the

Molecules 2021, 26, 3840 viscosities of both the 0.65 wt% and 1.30 wt% NaPAA solution were partially recovered.4 of 15 Figure 1b presents the variation in viscosity that arose for each fluid at a shear rate of 10 s−1 upon alternately bubbling CO2 and N2; both solutions exhibited a clear CO2-responsive −1 change10 s upon in viscosity. alternately For thebubbling 0.65 wt% CO 2 NaPAAand N 2 solution,; both solutions its viscosity exhibited was initially a clear CO 18212- mPa·s,responsive and changedropped in viscosity.sharply to For 1056 the mPa·s 0.65 wt% upon NaPAA CO2 treatment, solution, its then viscosity increased was initiallyto 1553 mPa·s1821 mPa when·s, andN2 was dropped bubbled sharply in. In contrast, to 1056 mPa the initial·s upon viscosity CO2 treatment, of the 1.30 then wt% increased NaPAA toin pure1553 water mPa·s decreased when N2 fromwas bubbled4314 mPa·s in. to In 2956 contrast, mPa·s the after initial treating viscosity with CO of the2, then 1.30 recov- wt% eredNaPAA to 3962 in pure mPa·s water when decreased bubbling from N2.4314 For the mPa 0.65·s to wt% 2956 and mPa 1.30·s after wt% treating NaPAA with aqueous CO2, −1 solutionsthen recovered at a shear to 3962 rate mPaof 10·s s when, the degrees bubbling of N viscosity2. For the recovery 0.65 wt% after and alternating 1.30 wt% NaPAACO2/N2 treatmentsaqueous solutions were 85% at a and shear 92%, rate respectively. of 10 s−1, the These degrees results of viscosity clearly recovery indicate afterthat alternating alternating additionCO2/N2 andtreatments removal were of CO 85%2 facilitated and 92%, a respectively. reversible change These in results the viscosities clearly indicate of the thatNa- PAAalternating aqueous addition solutions. and removal of CO2 facilitated a reversible change in the viscosities of the NaPAAThe effect aqueous of CO solutions.2 on the viscoelasticity of the NaPAA aqueous solutions was then investigatedThe effect via of oscillatory CO2 on the-shear viscoelasticity measurements. of the Before NaPAA measurements, aqueous solutions the linear was thenviscoe- in- lasticvestigated region via of oscillatory-shear the NaPAA aqueous measurements. solutions was Before confirmed measurements, through the a linearstrain viscoelasticsweep test. Asregion shown of the in NaPAAFigure S3, aqueous at strains solutions lower wasthan confirmed 40 % at 25 through °C, the afluids strain exhibited sweep test. linear As ◦ viscoelasticshown in Figure behavior, S3, atdefining strains the lower region than in 40which % at the 25 solutionsC, the fluids could exhibitedretain a stable linear struc- vis- turecoelastic without behavior, being defining destroyed the [35]. region Here, in which storage the modulus solutions could(G′) characterized retain a stable elastic structure be- 0 haviorwithout of being the NaPAA destroyed solutions, [35]. Here, while storage loss modulus modulus (G(G″)) characterizedrepresented its elastic viscous behavior proper- of ties.theNaPAA Under low solutions, strain, whileG′ > G″, loss for modulus both samples, (G”) represented indicating itsthat viscous they exhibited properties. solid Under gel- 0 likelow behaviors. strain, G > Contrarily, G”, for both in samples, the high indicating strain regime, that they G′ < exhibitedG″, demonstrating solid gel-like that behaviors. the both 0 samplesContrarily, showed in the fluid high- strainlike properties. regime, G Therefore,< G”, demonstrating frequency sweep that the curves both samplesfor the 0.65 showed wt% andfluid-like 1.30 wt% properties. NaPAA Therefore, aqueous solutions frequency were sweep obtained curves at for a strain the 0.65 of wt%10 %, and as shown 1.30 wt% in FigureNaPAA 2. aqueous solutions were obtained at a strain of 10 %, as shown in Figure2.

a b c

d e f

Figure 2. Storage modulus G′ G0 and loss modulus G″ G” as as a a function function of of frequency frequency for for ( (aa)) 0.65 0.65 wt% wt% and and ( (d)) 1.30 wt% NaPAA aqueous solutions in initial state, (b) 0.65 wt% and (e) 1.30 wt% NaPAA aqueous solutions after bubbling CO2, and (c) 0.65 aqueous solutions in initial state, (b) 0.65 wt% and (e) 1.30 wt% NaPAA aqueous solutions after bubbling CO2, and (c) wt% and (f) 1.30 wt% NaPAA aqueous solutions when treated by N2 to remove CO2. All tests were conducted at a shear 0.65 wt% and (f) 1.30 wt% NaPAA aqueous solutions when treated by N to remove CO . All tests were conducted at a strain of 10% at 25 °C. 2 2 shear strain of 10% at 25 ◦C.

Regardless of whether CO2 was present, the solutions displayed classic viscoelastic Regardless of whether CO2 was present, the solutions displayed classic viscoelastic character, i.e., i.e., they they showed showed a acrossover crossover of of G′ G and0 and G″ G” within within thethe tested tested range range of strain of strain frequencies. The maximum relaxation time (τR) for a viscoelastic fluid is generally defined as frequencies. The maximum relaxation time (τR) for a viscoelastic fluid is generally defined as the inverse of this intersectant frequency (ωc)[36]; thus, the fluid’s response can be 0 divided into two regimes based on τR. At low frequency (ω << ωc), G < G”, indicating 0 that samples possessed a viscous behavior; while at high frequency (ω >> ωc), G > G”, the Molecules 2021, 26, x FOR PEER REVIEW 5 of 15

the inverse of this intersectant frequency (ωc) [36]; thus, the fluid’s response can be divided Molecules 2021, 26, 3840 into two regimes based on τR. At low frequency (ω << ωc), G′ < G″, indicating that samples5 of 15 possessed a viscous behavior; while at high frequency (ω >> ωc), G′ > G″, the fluids behaved an elastic property. Additionally, for the 0.65 wt% NaPAA aqueous solution, the pointfluids of behaved intersection an elastic shifted property. to a higher Additionally, frequency for after the 0.65 CO wt%2 treatment, NaPAA corresponding aqueous solution, to athe decrease point of in intersection τR from ~25 shifted s to ~1.45 to a highers, then frequencyrecovered after to a COlower2 treatment, frequency corresponding upon bubbling to Na decrease2, i.e., τR increased in τR from to ~25 ~5.26 s tos. ~1.45In comparison, s, then recovered τR of the to 1.30 a lower wt% NaPAA frequency aqueous upon bubbling solution τ τ decreasedN2, i.e., R fromincreased ~50 s to to ~5.26 ~5.56 s. s Inafter comparison, bubbling COR of2, and the1.30 then wt% increased NaPAA to aqueous ~8.33 s with solution the decreased from ~50 s to ~5.56 s after bubbling CO , and then increased to ~8.33 s with the introduction of N2. These results clearly suggest that2 the viscoelastic character of NaPAA introduction of N . These results clearly suggest that the viscoelastic character of NaPAA aqueous solution can2 be reversibly tuned through alternating introduction of CO2 and N2. Moreover,aqueous solution both G′ can and be G″ reversibly were dependent tuned through on frequency alternating and introductionmuch lower in of the CO 2presenceand N2. Moreover, both G0 and G” were dependent on frequency and much lower in the presence of CO2, confirming that the introduction of CO2 weakened the strength of the fluids. of CO2, confirming that the introduction of CO2 weakened the strength of the fluids.

2.2. Effect of DMEA CO2-Switchable Water on Rheological Properties of NaPAA Solutions 2.2. Effect of DMEA CO2-Switchable Water on Rheological Properties of NaPAA Solutions DMEA contains a tertiary amino group that can be well dissolved in water and pro- DMEA contains a tertiary amino group that can be well dissolved in water and tonated by CO2 to produce ammonium bicarbonate salt, yielding a large increase in ionic protonated by CO2 to produce ammonium bicarbonate salt, yielding a large increase in strengthionic strength of the ofaqueous the aqueous solution. solution. The protonated The protonated product product can be canconverted be converted back into back to the original amine by bubbling N2, showing good reversibility [22,37]. For these reasons, into to the original amine by bubbling N2, showing good reversibility [22,37]. For these DMEA was chosen to prepare CO2-switchable water, and the effect of both DEMA con- reasons, DMEA was chosen to prepare CO2-switchable water, and the effect of both DEMA centration and the introduction of CO2 on the rheological behavior of NaPAA aqueous concentration and the introduction of CO2 on the rheological behavior of NaPAA aqueous solution was investigated. The inﬂuenceinfluence ofof differentdifferent DMEADMEA concentrations concentrations on on the the steady steady rheology rheology properties properties of of 0.65 wt% and 1.30 wt% NaPAA solutions in the presence and absence of CO2 is shown 0.65 wt% and 1.30 wt% NaPAA solutions in the presence and absence of CO2 is shown in inFigures Figures S4 S4 and and S5, S5, respectively. respectively. One One can can ﬁnd find that that the the initial initialsolution solution showedshowed thethe largest 2 viscositviscositiesies in low shear rate region, while thethe COCO2-treated-treated solutions exhibited the lowestlowest 0 viscosities at at the the same same shear shear rate. rate. Values Values of ofη wereη0 were obtained obtained from from Figures Figures S4 and S4 andS5, and S5, furtherand further plotted plotted against against DMEA DMEA concentrations. concentrations. As displayed As displayed in Figure in Figure 3a, η03 ofa, theη0 of0.65% the NaPAA0.65% NaPAA aqueous aqueous solution solution without without the addition the addition of DMEA of DMEA was initially was initially 303.1 303.1 Pa·s−1 Pa, then·s−1, −1 −1 −1 −1 decreasedthen decreased to 55.3 to Pa·s 55.3 Pa in· sthe inpresence the presence of CO of2, and CO2 ,recovered and recovered to 273.1 to 273.1Pa·s Pa after·s intro-after ducingintroducing N2. N2.

a b

Figure 3. (a) VariationVariation inin ηη00 of 0.65 wt% NaPAA solutionsolution with different DMEA concentrations before and after bubbling CO2 andand N 22,, and and ( (bb)) loss loss and and recovery recovery ratios ratios of of ηη00 forfor 0.65 0.65 wt% wt% NaPAA NaPAA solution solution with with different different DMEA DMEA concentration after treatment of CO2 and N2 at 25 °◦C. treatment of CO2 and N2 at 25 C.

Upon adding DMEA to the 0.65% NaPAA aqueous solution, the initial η0 of the 0.65wt% NaPAA solution decreased with the increment of DMEA, which can be interpreted as the introduction of DMEA increased the ionic strength of the mixed solution, weakening electrostatic repulsions between NaPAA chains. While bubbling CO2, tertiary amino groups in DMEA reacted with CO2 to produce ammonium bicarbonate salts, which further remarkably increased the ionic strength of the solution, resulting in a sharp decrease in Molecules 2021, 26, x FOR PEER REVIEW 6 of 15

Upon adding DMEA to the 0.65% NaPAA aqueous solution, the initial η0 of the 0.65wt% NaPAA solution decreased with the increment of DMEA, which can be interpreted as the introduction of DMEA increased the ionic strength of the mixed solution, weakening electrostatic repulsions between NaPAA chains. While bubbling CO2, tertiary Molecules 26 2021, , 3840 amino groups in DMEA reacted with CO2 to produce ammonium bicarbonate salts, which6 of 15 further remarkably increased the ionic strength of the solution, resulting in a sharp decrease in η0 of the 0.65 wt% NaPAA solutions. With respect to the CO2-treated solutions, η0 of the 0.65 wt% NaPAA solutions. With respect to the CO2-treated solutions, varying varying DMEA concentration from 0.6 wt% to 4.8 wt% led to decreases in η0 from− 12.61 DMEA concentration from 0.6 wt% to 4.8 wt% led to decreases in η0 from 12.6 Pa·s to Pa·s−1 to 2.6 Pa·s−1. Bubbling N2 to remove CO2 led to partial recovery of the initial solution 2.6 Pa·s−1. Bubbling N to remove CO led to partial recovery of the initial solution viscosity across all concentrations2 of DMEA,2 as shown in Figure 3a. viscosity across all concentrations of DMEA, as shown in Figure3a. To investigate the effect of CO2 on η0 in the presence of DMEA, a loss ratio index, i.e., To investigate the effect of CO2 on η0 in the presence of DMEA, a loss ratio index, i.e., the ratio of lost η0 in CO2-treated solution to its initial value, was introduced, as displayed the ratio of lost η0 in CO2-treated solution to its initial value, was introduced, as displayed in Figure 3b. The data indicates that the loss ratio of η0 upon introduction of CO2 increased in Figure3b. The data indicates that the loss ratio of η upon introduction of CO increased from 94.6 % to 97.9 % with the increment of DMEA, implying0 that the protonated2 DMEA from 94.6 % to 97.9 % with the increment of DMEA, implying that the protonated DMEA could greatly weaken the viscosity of NaPAA solution, demonstrating behavior similar to could greatly weaken the viscosity of NaPAA solution, demonstrating behavior similar to that of Na+ and Ca2+ in prior studies [26,27]. However, unlike Na+ and Ca2+, the loss of that of Na+ and Ca2+ in prior studies [26,27]. However, unlike Na+ and Ca2+, the loss of NaPAA solutionsolution viscosityviscosity induced induced by by protonated protonated DMEA DMEA could could be be reversed reversed to someto some extent, extent, as shown in Figure 3a. Therefore, to assess the degree of η0 that occurred after intro- as shown in Figure3a. Therefore, to assess the degree of η0 that occurred after introducing ducing N2, a recovery ratio, i.e., the ratio of η0 of the N2-treated solution to its initial value, N2, a recovery ratio, i.e., the ratio of η0 of the N2-treated solution to its initial value, is also ispresented also presented in Figure in 3Figureb. It can 3b. be It foundcan bethat found the that recovery the recovery ratio varied ratio betweenvaried between 8.3 % and 8.3 % and 18.2 %, indicating that the DMEA was not completely deprotonated by bubbling 18.2 %, indicating that the DMEA was not completely deprotonated by bubbling N2 at N602 ◦atC. 60 °C. The influence of different DMEA concentrations on η0 of the 1.30 wt% NaPAA solu- The influence of different DMEA concentrations on η0 of the 1.30 wt% NaPAA solution, tion, in the presence or absence of CO2, is displayed in Figure S6. The variation of η0 with in the presence or absence of CO2, is displayed in Figure S6. The variation of η0 with concentrations of of DMEA DMEA exhibited exhibited similar similar tendencies tendencies to tothat that of the of the 0.65 0.65 wt% wt% NaPAA NaPAA so- 0 lution.solution. However, However, the the loss loss ratio ratio in in ηη of0 of 1.30 1.30 wt% wt% NaPAA NaPAA solution solution was was lower lower than than that of 0.65 wt% solution, and the recovery ratio in η0 of 1.30 wt% NaPAA solution was higher than that of 0.65 0.65 wt% wt% solution, solution, indicat indicatinging that the inter inter-- and intramolecular entanglement of 1.30 wt% wt% NaPAA NaPAA solution is stronger than that of 0.65 wt% solution. These results prove that the viscosity of NaPAA solutions could be reversibly tuned viavia the protonation and deprotonation of DMEA. Then, the influence influence of DMEA concentration on the dynamic rheological behavior of NaPAA fluidsfluids uponupon COCO22/N22 treatmenttreatment was was also also investigated. investigated. Figure Figure 4 presents frequencyfrequency sweep curves of 0.65 0.65 wt% wt% NaPAA NaPAA solutions wit withh different DMEA contents. As mentioned previously, the GG′/G″0/G”-crossover-crossover point point on on the the frequency frequency sweep sweep curves curves provides provides the the fre- fre- quency of the viscous viscous-to-elastic-to-elastic transition corresponding to the maximum maximum relaxation relaxation time, time, τRR. .Measured Measured values values of of τRτ Rareare plotted plotted agai againstnst variations variations in inDMEA DMEA concentration concentration upon upon al- ternatealternate gas gas treatments treatments inin Figure Figure 5.5 .

a b c d

Figure 4.4. FrequencyFrequency sweepsweep curves curves of of 0.65 0.65 wt% wt% NaPAA NaPAA aqueous aqueous solutions solutions containing containing (a) 0.60(a) 0.60 wt%, wt%, (b) 1.20 (b) wt%,1.20 wt%,(c) 2.40 (c) wt%, 2.40 wt%, and (d) 4.80 wt% DMEA before and after treatment of CO2 and N2, measured at shear strain of 10 % ◦at 25 °C. and (d) 4.80 wt% DMEA before and after treatment of CO2 and N2, measured at shear strain of 10 % at 25 C.

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Figure 5a demonstrates that, for the 0.65 wt% NaPAA solutions, the initial τR gradually decreased from 25.00 s to 9.09 s with the DMEA content increased from 0 to 4.80 wt%. Thus, the strength of the NaPAA fluids was weakened by the addition of DMEA. Upon treatment of CO2 until conductivity stabilized, DMEA was protonated by CO2, resulting in the presence of quaternary ammonium salts in the solution [22,37]. Consequently, τR of the solutions further declined from 1.45 s to 0.32 s with increasing DMEA concentrations, suggesting that protonated DMEA enhanced CO2-induced weaking of the fluid. To remove CO2, N2 was introduced into the mixture, and τR was partially recovered to its initial value, again proving that the quaternary ammonium salts of DMEA were only partially deprotonated by N2 treatment. The variation of τR with different DMEA concentrations obtained for the 1.30 wt% NAPAA solution exhibited a similar variation tendency upon alternately bubbling CO2 and N2, as shown in Figure S7 and Figure 5b. These results illustrate that the viscoelasticity Molecules 2021, 26, 3840 of NaPAA solutions could be reversibly adjusted through the incorporation of DMEA7 of 15- based CO2-switchable water.

a b

Figure 5.5. VariationVariation in in maximum maximum relaxation relaxation time timeτR ofτR (ofa) 0.65(a) 0.65 wt%, wt%, and (andb) 1.30 (b) wt%1.30 NaPAAwt% NaPAA solutions solutions with different with different DMEA 2 ◦2 concentrationsDMEA concentration befores and before after and bubbling after bubbling CO2 and CO N2 andat 25 NC. at 25 °C.

2.3. MechanismFigure5a demonstrates of CO2-Switchable that, forViscoelasticity the 0.65 wt% NaPAA solutions, the initial τR gradually

decreasedRheological from 25.00characterizations s to 9.09 s with showed theDMEA that CO content2 treatment increased could fromdecrease 0 to the 4.80 viscos- wt%. Thus,ity and the weaken strength the of elasticity the NaPAA of NaPAA ﬂuids was fluids, weakened which bymight the additionbe ascribe of to DMEA. the reaction Upon treatment of CO until conductivity stabilized, DMEA was protonated by CO , resulting in between incorporated2 DMEA and CO2. To elucidate the corresponding mechanism,2 the thepH presenceand conductivity of quaternary of NaPAA ammonium solutions salts with in different the solution DMEA [22 ,concentrations37]. Consequently, wereτ con-R of the solutions further declined from 1.45 s to 0.32 s with increasing DMEA concentrations, tinuously monitored during cyclical CO2/N2 treatment at 25 °C. suggestingIn general, that protonatedthe basicity DMEA of functional enhanced groups CO2-induced can be judged weaking based of the on ﬂuid.the dissociation To remove CO2,N2 was introduced into the mixture, and τR was partially recovered to its initial constant (pKaH) of its conjugate acid; the larger the pKaH, the stronger the basicity [20]. The value, again proving that the quaternary ammonium salts of DMEA were only partially pH titration curves of aqueous solutions of DMEA and NaPAA against HCl solution were deprotonated by N2 treatment. obtained to determine the pKaH of DMEA and NaPAA. In a typical titration curve, the pH The variation of τR with different DMEA concentrations obtained for the 1.30 wt% corresponding to the half equivalence points is taken as the average pKaH [20]. The reaction NAPAA solution exhibited a similar variation tendency upon alternately bubbling CO and between DMEA and HCl is exhibited in Scheme 1, and the titration curve of the DMEA2 N2, as shown in Figure S7 and Figure5b. These results illustrate that the viscoelasticity of solution is shown in Figure 6a, in which the pKaH of DMEA was found to be 9.56. Similarly, NaPAA solutions could be reversibly adjusted through the incorporation of DMEA-based the pKaH of NaPAA used in this study was found to be 5.93, as shown in Figure 6b. CO2-switchable water.

2.3. Mechanism of CO2-Switchable Viscoelasticity

Rheological characterizations showed that CO2 treatment could decrease the viscosity and weaken the elasticity of NaPAA ﬂuids, which might be ascribe to the reaction between incorporated DMEA and CO2. To elucidate the corresponding mechanism, the pH and conductivity of NaPAA solutions with different DMEA concentrations were continuously ◦ monitored during cyclical CO2/N2 treatment at 25 C. In general, the basicity of functional groups can be judged based on the dissociation constant (pKaH) of its conjugate acid; the larger the pKaH, the stronger the basicity [20]. The pH titration curves of aqueous solutions of DMEA and NaPAA against HCl solution were obtained to determine the pKaH of DMEA and NaPAA. In a typical titration curve, the pH corresponding to the half equivalence points is taken as the average pKaH [20]. The reaction between DMEA and HCl is exhibited in Scheme1, and the titration curve of the DMEA solution is shown in Figure6a, in which the p KaH of DMEA was found to be 9.56. Similarly, the pKaH of NaPAA used in this study was found to be 5.93, as shown in Figure6b. Molecules 2021,, 26,, xx FORFOR PEERPEER REVIEWREVIEW 8 of 15 Molecules 2021, 26, 3840 8 of 15

Scheme 1. Protonation of DMEA with HCl or CO 2..

a b

FigureFigure 6. 6. pHpH titration titration curves curves of of ( (aa)) DMEA DMEA and and ( (bb)) NaPAA NaPAA aqueous solutions against 0.1 M HCl at 25 ◦°C.

InIn terms terms of of the the p pKKaHaH valuesvalues determined determined above, above, the the degree degree of of protonation protonation ( (δδ)),, i.e., i.e., the the proportionproportion of of protonated protonated organ organ-base,-base, can can be be calculated calculated using using Eq Equationsuations ( (1)1) and and ( (2)2) [20,38]. [20,38]. + [M][H++] Ka =[ M][H ] (1) KaKa= = + ((1)1) [MH[MH+]] 1 δ = 1 × 100% (2) δ =δ = pH-pKa ×× 100%100% ( (2)2) 1 +1 10+ 10pH−pKa [M][M] inin Eq Equationuation ( (1)1) represents the concentration of DMEA or NaPAA, whilewhile [MH[MH++]] denotesdenotes the the concentration concentration of of protonated protonated DMEA DMEA or NaPAA. UponUpon bubbling CO CO22,, the the pH pH of of the the 0.65 0.65 wt% wt% NaPAA NaPAA aqueous aqueous solution solution decreased decreased from from9.17 to9.17 5.82, to as5.82, displayed as displayed in Figure in Figure7a. According 7a. According to Equation to Eq (2),uation the initial(2), theδ initialof NaPAA δ of NaPAAwas ~0.06 was mol%. ~0.06 The mol%. change The in change pH that in occurredpH that occurred after sparging after withsparging CO2 withindicates CO2 thatindi-δ catesincreased that toδ increased ~56.30 mol%, to ~ and56.30 thus mol%, ~56.24 and mol% thus carboxylate ~56.24 mol% groups carboxylate were more groups protonated were morein the protonated presence of in CO the2. presence These results of CO coincide2. These withresults results coincide of the with rheology results experiments:of the rheol- ogyviscosities experiments:η and relaxationviscosities timesη andτ relaxationR of the 0.65 times wt% τR aqueous of the 0.65 solutions wt% aqueous dropped solutions sharply droppedafter bubbling sharply CO after2, revealing bubbling that CO protonation2, revealing of that carboxylate protonation side of groups carboxylate reduced side the groupscharge densityreduced and the corresponding charge density electrostatic and corresponding repulsions electrostatic between NaPAA repulsions chains between to allow NaPAAa smaller chains average to allow molecular a smaller size thanaverage in the molecular initial state. size than After in bubbling the initial N 2state., the pHAfter of bubbling0.65 wt% N NaPAA2, the pH aqueous of 0.65 wt% solution NaPAA recovered aqueous to 7.99,solution signifying recovered deprotonation to 7.99, signifying of the deprotonationcarboxylate groups. of the Thecarboxylate pH was groups. not totally The restored pH was to not its totally initial value,restored indicating to its initial that value,0.86 mol% indicating carboxylate that 0.86 groups mol% were carboxylate still protonated. groups Thiswere phenomenon still protonated. also This coincides phenom- with enonthe prior also rheologicalcoincides with results, the prior in which rheological the viscosities results,η inand which relaxation the viscosities times τ Rη andof NaPAA relax- aqueous solutions were not fully restored to their initial values. ation times τR of NaPAA aqueous solutions were not fully restored to their initial values.

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a b

FigureFigure 7. 7. VariationVariation of of pH pH for for ( (aa)) 0.65 0.65 wt% wt% and and ( (bb)) 1.30 1.30 wt% wt% NaPAA NaPAA solution solution at at different different DMEA DMEA concentration concentration while while alternatively bubbling CO2 and N2 at 25 °C.◦ alternatively bubbling CO2 and N2 at 25 C.

TheThe pH pH of of 0.65 0.65 wt% wt% NaPAA NaPAA solutions solutions containing containing different different DMEA DMEA concentrations concentrations waswas also also examined examined (Figure (Figure 7a).7a). It can be found that the pH of 0.65 wt% N NaPAAaPAA solutions increasedincreased apparently apparently with with the the increment increment of C ofDMEACDMEA, regardless, regardless of the of presence the presence of CO2 of(Table CO2 S1).(Table As S1).an example, As an example, the variation the variation in pH for in the pH 0.65 for wt% the 0.65NaPAA wt% solution NaPAA with solution 0.6 wt% with DMEA0.6 wt% upon DMEA the uponintroduction the introduction of CO2 and of CON22 isand discussed. N2 is discussed. As a typic Asal a tertiary typical tertiaryamine, DMEAamine, can DMEA be protonated can be protonated in water in in water the presence in the presence of CO2 ofto COgenerate2 to generate ammonium ammonium bicar- bonatebicarbonate salts (Scheme salts (Scheme 1) [39–142].)[ 39 The–42 initial]. The pH initial of 11.14 pH was of 11.14 reduced was to reduced 6.75 upon to 6.75bubbling upon CObubbling2, and COthen2, recovered and then recovered to 8.94 after to 8.94 introducing after introducing N2. Via N Eq2.uation Via Equation (2), the (2), initial the initial δ of DMEAδ of DMEA can be can calculated be calculated at ~2.56 at ~2.56 mol%, mol%, followed followed by an by increase an increase to ~99.85 to ~99.85 mol% mol% after afterthe introductionthe introduction of CO of2, COindicating2, indicating that the that tertiary the tertiary amino amino groups groups on DMEA on DMEA molecules molecules were almostwere almost completely completely protonated. protonated. After bubbling After bubbling N2, the pH N2, of the the pH sol ofution the solutiondid not recover did not torecover its initial to itsvalue; initial rather, value; 80.65 rather, mol% 80.65 of mol% the tertiary of the tertiarygroups were groups still were protonated, still protonated, which waswhich caused was causedby the larger by the the larger basicity, the basicity, the poorer the poorer the “switch” the “switch” capability capability [20]. [Besides,20]. Besides, the −4 −4× initialthe initial δ forδ NaPAAfor NaPAA of 6.17 of 6.17× 10 mol%10 mol% was incr waseased increased to 13.15 to 13.15mol% mol%after bubbling after bubbling CO2, × −2 thenCO2 again, then restored again restored to 9.76 to× 10 9.76−2 mol%.10 Thusmol%., one Thus, can find one that can even ﬁnd if that CO even2 is not if COavailable,2 is not thereavailable, was at there least was some at least small some amount small of amount protonated of protonated DMEA in DMEA the solution. in the solution. These results These properlyresults properly clarify why clarify viscoelasticity why viscoelasticity of the NaPAA of the solutions NaPAA containing solutions containing DMEA evidently DMEA evidently decreased before bubbling CO . By contrast, in the presence of CO , a large decreased before bubbling CO2. By contrast,2 in the presence of CO2, a large number2 of protonatednumber of DMEA protonated increased DMEA the increased charge density the charge of the density fluids, of together the ﬂuids, with together protonated with

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carboxylate groups reduced the charge density and electrostatic repulsions between mo- lecularprotonated chains, carboxylate then the synergy groups reducedof the two the effects charge diminished density and the electrostatic size of NaPAA repulsions mole- between molecular chains, then the synergy of the two effects diminished the size of NaPAA cules, resulting in the decrease of η and τR with the addition of DMEA. The change in pH molecules, resulting in the decrease of η and τ with the addition of DMEA. The change of 1.30 wt% NaPAA solution with different DMEAR concentrations demonstrated similar in pH of 1.30 wt% NaPAA solution with different DMEA concentrations demonstrated tendencies, as shown in Figure 7b and Table S2. similar tendencies, as shown in Figure7b and Table S2. As a typical polyelectrolyte, the conformation of NaPAA molecule chains is usually As a typical polyelectrolyte, the conformation of NaPAA molecule chains is usually affected by salts in solutions, which influences the rheological properties of the fluids. To affected by salts in solutions, which inﬂuences the rheological properties of the ﬂuids. To verify the changes in charge density in the NaPAA/DMEA solution at different states, verify the changes in charge density in the NaPAA/DMEA solution at different states, Figure 8a provides conductivity of 0.65 wt% solution with different DMEA concentrations Figure8a provides conductivity of 0.65 wt% solution with different DMEA concentrations in the presence or absence of CO2 at 25 °C. The◦ conductivity of the 0.65 wt% NaPAA aque- in the presence or absence of CO2 at 25 C. The conductivity of the 0.65 wt% NaPAA ous solution was initially 2.06 mS·cm−1, then−1 increased to 3.53 mS·cm−1 upon−1 CO2 treat- aqueous solution was initially 2.06 mS·cm , then increased to 3.53 mS·cm upon CO2 ment, suggesting that various inorganic ions (H+, HCO+ 3−, CO−32−) were2− produced. After treatment, suggesting that various inorganic ions (H , HCO3 , CO3 ) were produced. bubbling N2 to remove CO2, the conductivity of the 0.65 wt% aqueous solution returned After bubbling N2 to remove CO2, the conductivity of the 0.65 wt% aqueous solution −1 toreturned 2.54 mS·cm to 2.54, which mS·cm was−1, which very close was veryto its close initial to value. its initial Hence, value. the Hence, enhancement the enhancement of ionic strengthof ionic weakened strength weakened electrostatic electrostatic repulsions repulsions between inter between- and intramolec inter- andular intramolecular chains, resulting in the observed drop in η and τR. chains, resulting in the observed drop in η and τR.

a b

FiguFigurere 8. 8. VariationVariation in in conductivity conductivity of of ( (aa)) 0.65 0.65 wt% wt% and and ( (bb)) 1.30 1.30 wt% wt% NaPAA NaPAA solutions solutions at at different different DMEA DMEA concentrations, concentrations, while alternatively bubbling CO2 and N2 at 25 °◦C. while alternatively bubbling CO2 and N2 at 25 C.

Besides,Besides, the the protonated protonated DMEA, DMEA, i.e., i.e., DMEAH DMEAH++, ,further further increase increasedd the the ionic ionic strength strength of of thethe fluids. ﬂuids. Data Data shown shown in in Figure Figure 8a8a demonstrates that, when bubbling COCO22,, the the maximum maximum conductivityconductivity of of 0.65 0.65 wt% wt% NaPA NaPAAA solution solution increased increased with with the the addition addition of of DMEA, DMEA, implying implying

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that DMEAH+ was generated in those fluids. The charged DMEAH+ molecules shielded static repulsions among carboxylate groups (–COO−) of the NaPAA molecules, allowing the polymer chains to become coiled and thus reducing electrostatic repulsions between them. Thus, in the presence of CO2, the η and τR of 0.65 wt% NaPAA solution decreased with the increment of DMEA. After introducing N2, the conductivity of the fluids partially returned to their initial values, as exhibited in Figure8a and Table S3. These results properly explain why η and τR could not completely go back to their initial values when introducing N2 to remove CO2. Three repeated cycles indicated that DMEA possesses good CO2-induced switching behavior. In addition, the data in Figure8b and Table S4 also show that variation in conductivity of the 1.30 wt% NaPAA soution with different DMEA concentrations also obeyed the same rule. Consequently, the mechanism for CO2-switchable rheological behavior of NaPAA solutions, with or without the addition of DMEA, can be summarized in Scheme2. On Molecules 2021, 26, x FOR PEER REVIEW − 11 of 15 the one hand, for NaPAA aqueous solution, bubbling CO2 leads to –COO in side chains will be protonated to carboxylic acid (–COOH), resulting in a decrease in charge density on the molecular chains relative to that of the initial solution state, as shown in Scheme2a. Therefore, electrostaticthat DMEAH repulsions+ was generated between in intra- those and fluids. inter-molecules The charged due DMEAH to –COO+ −moleculesgroups shielded are weakened,static and repulsions the viscoelasticity among carboxylate of the NaPAA groups aqueous (–COO solution−) of the decreases NaPAA accordingly. molecules, allowing Treatmentthe with polymer N2 allows chains viscoelasticity to become coile of thed and solution thus toreducing partially electrostatic recover, due repulsions to the between partial deprotonationthem. Thus, of in –COOH the presence groups. of CO On2, thethe otherη and hand, τR of 0.65 for NaPAA wt% NaPAA solutions solution that decreased − contain DMEA,with the in additionincrement to of the DMEA. influence After ofintroducing protonation N2, of the –COO conductivity, the protonated of the fluids partially DMEA playsreturned a crucial to theirrole in initial determining values, as the exhibited viscoelasticity in Figure of the 8a solution and Table upon S3. the These results addition ofproperly CO2, as displayedexplain why in Schemeη and τ2Rb. could When not CO completely2 is bubbled go intoback the to their solutions, initial the values when DMEA moleculesintroducing are protonated N2 to remove to generate CO2. Three quaternary repeated ammonium cycles indicated salts, which that DMEA shield possesses charges remaininggood CO on2-induced the NaPAA switchi chains,ng behavior. allowing In them addition, to coil the more data tightly, in Figure resulting 8b and in Table a S4 also sharp decreaseshow in that viscoelasticity variation in of conductivity the fluids. Uponof the removal1.30 wt% of NaPAA CO2, the soution quaternary with different salts DMEA are deprotonated,concentrations causing also partial obeyed recovery the same of the rule. viscosity and elasticity of the solutions.

Scheme 2. Schematic illustration of the mechanism for CO2-switchable viscoelasticity of NaPAA in (a) pure water, and (b) Scheme 2. Schematic illustration of the mechanism for CO2-switchable viscoelasticity of NaPAA in DMEA solution.(a ) pure water, and (b) DMEA solution.

3. Materials andConsequently, Methods the mechanism for CO2-switchable rheological behavior of NaPAA 3.1. Materialssolutions, with or without the addition of DMEA, can be summarized in Scheme 2. On the one hand, for NaPAA6 aqueous6 solution,−1 bubbling CO2 leads to –COO− in side chains will NaPAA (MW = 4 × 10 –5 × 10 g·mol ) and DMEA (≥99%) were purchased from Sigma-Aldrichbe protonated and used as to received. carboxylic CO 2acid(≥99.998%) (–COOH), and resulting N2 (≥99.998%) in a decrease were supplied in charge by density on Xuyuan Chemicalthe molecular Industry chains Co., Ltd. relative (Chengdu, to that China) of the andinitial used solution without state, further as shown treatment. in Scheme 2a. Therefore, electrostatic repulsions between intra- and inter-molecules due to –COO− groups are weakened, and the viscoelasticity of the NaPAA aqueous solution decreases accordingly. Treatment with N2 allows viscoelasticity of the solution to partially recover, due to the partial deprotonation of –COOH groups. On the other hand, for NaPAA solutions that contain DMEA, in addition to the influence of protonation of –COO−, the protonated DMEA plays a crucial role in determining the viscoelasticity of the solution upon the addition of CO2, as displayed in Scheme 2b. When CO2 is bubbled into the solutions, the DMEA molecules are protonated to generate quaternary ammonium salts, which shield charges remaining on the NaPAA chains, allowing them to coil more tightly, resulting in a sharp decrease in viscoelasticity of the fluids. Upon removal of CO2, the quaternary salts are deprotonated, causing partial recovery of the viscosity and elasticity of the solutions.

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Ultrapure water with resistivity of 18.25 MΩ cm−1 was produced by an ultrapure water puriﬁcation system (Chengdu Ultrapure Technology Co., Ltd., Chengdu, China) and used throughout this study.

3.2. Sample Preparation A general method of preparing NaPAA-DMEA aqueous solutions is described as follows. First, a designated amount of NaPAA was added to a beaker with ultrapure water, and the mixture was stirred until NaPAA was completely dissolved. Second, a designated amount of DMEA was completely dissolved in ultrapure water. Then, the two solutions were mixed and stirred for 24 h at room temperature to produce the ﬁnal solution.

3.2.1. CO2 Treatment ◦ CO2 gas was bubbled into the ﬂuid via a syringe needle at 25 C, until the measured conductivity of the solution stabilized.

3.2.2. N2 Treatment

To remove CO2,N2 gas was bubbled into the previously CO2-treated ﬂuid via a syringe needle at 60 ◦C, until the measured conductivity of the solution remains stable.

3.3. Rheological Tests Both steady and dynamic rheological measurements were performed via a Physica MCR 302 rotational rheometer (Anton Paar, Rannachstrasse, Austria) equipped with con- centric cylinder geometry (CC27). All measurements were carried out in stress-controlled mode at 25 ◦C, and Cannon standard oil was used to calibrate the instrument before experimentation. All samples were centrifuged to eliminate the interior bubbles prior to measurements, then equilibrated at 25 ◦C for at least 10 min prior to experimentation. Dynamic frequency spectra were conducted in the linear viscoelastic region, as determined from dynamic stress sweep measurements.

3.4. pKaH Determination

The pKaH (pKa of the protonated species) values of DMEA and NaPAA were determined by titrating 20 mL of 0.1 M aqueous solutions with 0.1 M hydrochloric acid. The pH was continuously monitored at 25 ◦C with a S2-T Kit pH-meter (Mettler Toledo, ±0.01, Zurich, Switzerland) calibrated with standard buffer solution. The pKaH values were obtained by taking the pH readings at the mid-point between two pH jumps.

3.5. Conductivity Measurements The conductivity of NaPAA-DMEA solution was recorded with a S230-K conductome- ◦ ter (Mettler Toledo, Zurich, Switzerland) at 25 C while bubbling CO2 or N2 alternatively.

4. Conclusions In summary, the rheological properties of NaPAA aqueous solutions with different DMEA concentrations were investigated in the presence and absence of CO2. The effects of NaPAA and DMEA concentrations on measured solution properties were examined to elucidate the mechanism of CO2-responsive tunable rheological properties. It was found that NaPAA showed strong thickening power in pure water. The introduction/removal of CO2 imparted tunable viscoelasticity to the bulk solution, attributed to protonation/deprotonation of carboxylate groups and corresponding reduction/increase in charge density among molecular chains, allowing for reduction/increase in the size of solvated molecular chains. Moreover, the addition of DMEA gave rise to further reductions in viscoelasticity upon bubbling CO2, but initial values could be restored to some extent via N2 treatment. Besides, higher concentrations of DMEA corresponded to greater losses in viscoelasticity. The CO2-responsive switching behavior of DMEA-containing solutions was attributed to protonation of DMEA, which enabled NaPAA molecules to coil up more Molecules 2021, 26, 3840 13 of 15

tightly, thus lowering the viscoelasticity, and to deprotonation of DMEAH+ via bubbling N2, which enabled partial recuperation of the viscoelasticity. This work not only widens the utilization of CO2 in preparing smart viscoelastic ﬂuids but also demonstrates fabrication of such systems using a low-cost, industrial polymer and stimuli-responsive additives.

Supplementary Materials: The following are available online, Figure S1: viscosity as a function of shear rate for NaPAA aqueous solutions at 25 ◦C; Figure S2: viscosity-shear rate curves of 0.65 wt% ◦ and 1.30 wt% NaPAA solution while alternatively bubbling CO2 and N2 at 25 C; Figure S3: strain sweep curves of 0.65 wt% and 1.30 wt% NaPAA aqueous solution at 25 ◦C; Figure S4: viscosity-shear rate curves of 0.65 wt% NaPAA solution with different DMEA concentrations while alternatively ◦ bubbling CO2 and N2 at 25 C; Figure S5: viscosity-shear rate curves of 1.30 wt% NaPAA solution ◦ with different DMEA concentrations while alternatively bubbling CO2 and N2 at 25 C; Figure S6: η0 of 1.30 wt% NaPAA solution with different DMEA concentrations before and after bubbling CO2 or N2, and the loss and recovery ratio for 1.30 wt% NaPAA solution with different DMEA ◦ concentrations at 25 C after alternative treatment of CO2 and N2; Figure S7: frequency sweep curves of 1.30 wt% NaPAA solution with different DMEA concentrations while alternatively bubbling CO2 ◦ and N2 at 25 C; Table S1: pH of 0.65 wt% NaPAA solutions with different DMEA concentration at different conditions; Table S2: pH of 1.30 wt% NaPAA solutions with different DMEA concentration at different conditions; Table S3: conductivity of 0.65 wt% NaPAA solutions with different DMEA concentration at different conditions; Table S4: conductivity of 1.30 wt% NaPAA solutions with different DMEA concentration at different conditions. Author Contributions: Investigation, writing—original draft preparation, D.W.; investigation, data curation, Y.S.; investigation, data curation, K.L.; validation, B.W.; resources, Y.Z.; conceptualization, funding acquisition, writing—review and editing, H.Y.; supervision, Y.F. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by Open Fund (PLN201714) of State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation (Southwest Petroleum University), and PetroChina Innovation Foundation (2019D-5007-0217). Data Availability Statement: The data presented in this study are available on request from the corresponding author. Conﬂicts of Interest: The authors declare no conﬂict of interest. Sample Availability: Samples of the compounds, including NaPAA and DMEA, are available from the authors.

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