Next Article in Journal
Doubly Dynamic Hydrogel Formed by Combining Boronate Ester and Acylhydrazone Bonds
Previous Article in Journal
Structural Changes of Oak Wood Main Components Caused by Thermal Modification
 
 
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Temperature and pH-Dependent Response of Poly(Acrylic Acid) and Poly(Acrylic Acid-co-Methyl Acrylate) in Highly Concentrated Potassium Chloride Aqueous Solutions

1
Department of Chemical Engineering and Process Design, Faculty of Chemistry, Silesian University of Technology, 44-100 Gliwice, Poland
2
Department of Physical Chemistry and Technology of Polymers, Faculty of Chemistry, Silesian University of Technology, 44-100 Gliwice, Poland
3
Department of Applied Chemistry, Graduate School of Engineering, University of Hyogo, Hyogo 671-2280, Japan
*
Authors to whom correspondence should be addressed.
Polymers 2020, 12(2), 486; https://doi.org/10.3390/polym12020486
Received: 15 January 2020 / Revised: 13 February 2020 / Accepted: 17 February 2020 / Published: 21 February 2020
(This article belongs to the Section Polymer Processing and Engineering)

Abstract

:
In this study, the phase transition phenomena of linear poly(acrylic acid) (PAA) and linear or star-shaped poly(acrylic acid-co-methyl acrylate) (P(AA-co-MA)) in highly concentrated KCl solutions were investigated. The effects of polymer molecular weight, topology, and composition on their phase transition behavior in solution were investigated. The cloud point temperature (TCP) of polymers drastically increased as the KCl concentration (CKCl) and solution pH increased. CKCl strongly influenced the temperature range at which the phase transition of PAA occurred: CKCl of 1.0–2.2 M allowed the phase transition to occur between 30 and 75 °C. Unfortunately, at CKCl above 2.6 M, the TCP of PAA was too high to theoretically trigger the crystallization of KCl. The addition of hydrophobic methyl acrylate moieties decreased the TCP into a temperature region where KCl crystallization could occur. Additionally, the hydrodynamic diameters (Dh) and zeta potentials of commercial PAA samples were examined at room temperature and at their TCP using dynamic light scattering. The salt concentration (from 1 to 3 M) did not impact the hydrodynamic diameter of the molecules. Dh values were 1500 and 15 nm at room temperature and at TCP, respectively.

Graphical Abstract

1. Introduction

Stimuli-responsive polymers considerably change their properties, such as their hydrophilic/hydrophobic state and conformation due to small changes in environmental stimuli [1,2] in either a continuous or stepwise manner [3,4,5]. Different system properties may serve as stimuli, such as temperature, pH, redox systems, light, ionic strength, or host–guest interactions. Therefore, stimuli-responsive polymers have been used as drug release agents, flocculants or flotation agents, switchable surfaces, or in chromatography, gene therapy, tissue engineering, biosensors, bioimaging, and optical switching [6,7,8,9,10,11,12,13,14,15,16,17,18]. Only a few publications have been dedicated to polymers with an upper critical solution temperature (UCST), in which the phase transition occurs between 273 and 373 K. In contrast, many studies have been performed on their counterparts with a lower critical solution temperature (LCST), especially in the temperatures close to 309 K since they are meant as drug delivery systems in human body. Moreover, the specific mechanisms involved in phase transitions often restrict certain practical applications of polymers due to their strict requirements, such as salt-free solutions [10,19]. Interestingly, recent publications have reported polymers that exhibit both a UCST and LCST depending on their concentration, such as poly(trimethylene ether)glycol [20,21].
One of the key factors influencing the phase transition of ionic polymers is the presence of dissolved salts, which can be systemized according to the Hofmeister series [22,23]. These series are represented by anions: CO 3 2 > SO 4 2 > S 2 O 3 2 > H 2 PO 4 > F > Cl > Br   ~ NO 3   > I > ClO 4 > SCN as well as cations: Mg 2 + > Li + > Na + = K + > NH 4 + [24]. Generally, the ions to the left of Cl and K + in the above-mentioned series are called kosmotropes, whereas the ions to the right of Cl and K + are called chaotropes. These terms originally indicated the capacity of the ions to create or destroy water interactions, respectively [25]. The phase transition mechanism in the presence of these ions involves three main factors: direct interactions between ions and macromolecules, an alteration of surface tension between the macromolecules and the surrounding liquid, and dispersion forces [22]. These dependencies are comprehensively discussed by Kunz and co-workers [24]. The same paper presents other historical attempts to correlate Hofmeister effects with characteristics of ions and solvents, such as the application of salting-out coefficients, Gibbs free energy, entropy of hydration of ions, increasing surface tension, polarizability, partial molar volume, molar refractivities, Jones–Dole viscosity B-coefficients, entropy change of water, or varying the “empty volume of the solvent” [26,27,28,29,30,31].
In this paper, we focus on poly(acrylic acid) (PAA) and its copolymers with methyl acrylate in concentrated KCl solutions. PAA was chosen due to its solubility in water as well as its pH and thermoresponsiveness. KCl was used because the chloride anion defines the boundary between kosmotropes and chaotropes, so its mechanism of impact on macromolecules is not clear. Secondly, KCl has a strong dependence between solubility and temperature and a high exothermic effect on the nucleation process. The aim of this study was to assess the usability of thermoresponsive (co)polymers with a TCP in aqueous solution for later use as a trigger for the nucleation of KCl during isohydric crystallization upon cooling. For this purpose, we investigated PAA and its copolymers of methyl acrylate, which differed by molecular weight and topology in order to predict the cloud point temperatures of these polymers. This will allow us to design process conditions to allow a phase transition in the metastable zone of salt solutions at a desired temperature and salt concentration.

2. Materials and Methods

2.1. Materials

Methyl acrylate (MA, 99%, Sigma-Aldrich, Poznań, Poland), tert-butyl acrylate (tBuA, 99%, Alfa Aesar, Warsaw, Poland), and anisole (Alfa Aesar, 99%) were dried over molecular sieves and stored in a freezer under nitrogen. Copper(I) bromide (CuBr, 98%, Fluka, Steinheim, Germany) was purified by stirring in glacial acetic acid followed by filtration and washing with ethanol and diethyl ether. Then, solids were dried under vacuum. N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA, 99%, Sigma-Aldrich, Poznań, Poland), ethyl α-bromoisobutyrate (EBiB, 98%, Sigma-Aldrich, Poznań, Poland), acrylic acid (AA, 99%, Sigma-Aldrich, Poznań, Poland), azobisisobutyronitrile (AIBN, 98%, Sigma-Aldrich, Poznań, Poland), ethanol (EtOH, 98%, Fluka), sodium bicarbonate (NaHCO3, anhydrous, Avantor, Gliwice, Poland), and potassium chloride (KCl, VWR Chemicals, Gdańsk, Poland) were used as received without any purification. L1 (poly(acrylic acid), 240,000 Da, 25% w/w water solution) was purchased from Acros Organics. L2 (poly(acrylic acid)Mn = 25,000 Da) was purchased from Wako Pure Chemicals. Poly(acrylic acid) (PAA) 7100 g/mol and poly(acrylic acid-co-methyl acrylate) (P(AA-co-MA)) g/mol was kindly provided by Dr. Shin-ichi Yusa. Pentaerythritol tetrakis(2-bromoisobutyrate) (PTLBr) was obtained according to a previously reported procedure [6]. Samples of commercially available PAA were obtained in the form of a 25% w/w aqueous solution (Sigma Aldrich, analytical grade).

2.2. Synthetic Procedures

2.2.1. Synthesis of Linear Poly(Acrylic Acid-co-Methyl Acrylate) (P(AA-co-MA)) via Free Radical Polymerization

In a test tube, 1.3 g of acrylic acid and 0.17 g of methyl acrylate were dissolved in 1.5 mL of ethanol. Then, 3.91 mg of AIBN was added, and the solution was purged with argon for 30 min to remove oxygen. The reaction mixture was heated to 60 °C and held at this temperature for 17 h. After the reaction, the solution was added dropwise to diethyl ether to precipitate the product and remove unreacted monomers. After the product was dried under vacuum to a constant mass, 0.92 g of the polymer was obtained.

2.2.2. Synthesis of Star-Shaped Poly(Acrylic Acid) (PAA) and P(AA-co-MA) via Atom Transfer Radical Polymerization (ATRP) (Example S1, Scheme 1)

CuBr (24.5 mg; 0.17 mmol), tBuA (10 mL; 22.3 mmol), MA (0.04 mL; 0.46 mmol), PMDETA (36 µL; 0.17 mmol), and anisole (3 mL; 30 vol % of monomer) were placed in a Schlenk flask (25mL) and then degassed by three freeze–pump–thaw cycles. Then, pentaerythritol tetrakis(2-bromoisobutyrate) (PTLBr) (41.7 mg; 0.057 mmol/ 4 initiating sites) was added, and the reaction flask was immersed in an oil bath at 70 °C. The reaction was stopped by exposing the reaction mixture to air. Then, it was diluted with CH2Cl2 and passed through a neutral alumina column to remove the copper catalyst. The mixture was concentrated via rotary evaporation, and the remaining solution was purified by precipitation in cold toluene. The copolymer was isolated by decantation and dried under vacuum at room temperature to a constant mass (yield: 33%). The second step was acidolysis of the obtained copolymer using a previously reported procedure [2].

2.3. Instrumentation

Transmittance was measured using a specially designed apparatus comprised of an Agilent Cary 60 spectrometer (MS Spectrum, Warsaw, Poland) and a Quantum Northwest TC-1 temperature controller coupled with specialty software (MS Spectrum, Warsaw, Poland). All samples were freshly prepared prior to measurements. Before each analysis, a quartz cuvette and a Pasteur pipette were preheated in a laboratory dryer at least 10 K above the cloud point temperatures (TCP) (determined based on trial experiments). The Peltier holder for the cuvette was also preheated. All transmittance measurements were performed with a 1 K/min cooling rate. If the temperature was lower than 283.15 K, the cuvette was continuously washed with dry air to prevent steam condensation on the walls. The hydrodynamic size of macromolecules and their zeta potential were measured using a Malvern Nano-S90 and Malvern Nano-Z apparatuses (Malvern, UK). The measurements were performed at temperatures lower than TCP and at the TCP determined based on transmittance measurements. In case of atom transfer radical polymerization (ATRP), conversions of monomers were determined by gas chromatography. The chromatograph (6850 Network GC System, Agilent Technologies, Warsaw, Poland) was equipped with a flame ionization detector. Injector and detector temperatures were kept constant at 250 °C (conditions: anisole as internal standard, column initial temperature 40 °C; column final temperature 200 °C). The conversions were calculated by detecting the decrease of the monomer peak area relative to the standard peak area. 1H nuclear magnetic resonance (NMR) spectra were obtained using a Bruker DRX-500 500 MHz (Billerica, AM, USA) or Varian Inova 600 MHz spectrometer (Perlan Technologies, Warsaw, Poland) at 26 °C using tetramethylsilane (TMS) as an internal standard with a polymer concentration of 1 wt %, and CDCl3 or CD3OD (99.8%) as the solvents. The number-average molecular weights (Mn,SEC) and dispersity indices (Đ) were evaluated by a size-exclusion chromatography (SEC) system composed of an RI-8020 refractive index detector (Tosoh, Tokyo, Japan), SPD-10A UV detector (Shimadzu, Kyoto, Japan), DP-8020 pump (Tosoh, Tokyo, Japan), ERC-3215α degasser (Shodex, Tokyo, Japan), and CTO-10ASVP column oven (Shimadzu, Kyoto, Japan). A α-M column (Tosoh, Tokyo, Japan) was used with a phosphate buffer at pH 9.0 as the eluent and sodium poly(styrene sulfonate) samples as standards. In case of P(tBuA-co-MA) obtained via ATRP, molecular weights (Mn,SEC, Mw,SEC) and dispersity (Ð) indices were determined by size exclusion chromatograph (SEC, 1100 Agilent 1260 Infinity, Perlan Technologies, Warsaw, Poland) equipped with an isocratic pump, autosampler, degasser, thermostatic box for columns, and differential refractometer MDS RI Detector (Perlan Technologies, Warsaw, Poland). Addon Rev. B.01.02 data analysis software (1st edition, Agilent Technologies) was used for data collecting and processing. The SEC calculated molecular weight was based on calibration using linear polystyrene standards (580–300,000 g/mol). A pre-column guard 5 µm 50 mm × 7.5 mm and PLGel 5 µm MIXED-C 300 mm × 7.5 mm (Perlan Technologies, Warsaw, Poland) were used for separation. The measurements were carried out in tetrahydrofuran (THF) (high performance liquid chromatography (HPLC) grade) as the solvent at 40 °C with a flow rate of 0.8 mL/min. Scanning electron microscopy (SEM) analyses were carried out on a VE-9800 (Keyence, Osaka, Japan) with an accelerating voltage of 10 kV. Samples were prepared by dropping a sample solution onto conducting tape, sputtering with Pt, and completely drying at room temperature. The samples were sputtered with Pt using a Quick Coater SC-701MK (Sanyu electron, Tokyo, Japan).

3. Results and Discussion

3.1. Phase Transition of High Molecular Weight PAA in Aqueous KCl Solutions

First, we have studied the influence of KCl concentration (CKCl) from 1 to 2.4 M (pH between 2.97 and 3.18) and from 2.6 to 3 M (pH from 2.92 to 2.78) on the phase transition of PAA (L1, Mw = 240,000 g/mol, concentration 0.25% w/w in water). The results revealed that a higher CKCl decreased the pH of the obtained solution (Figure 1a). Moreover, as CKCl increased, the TCP of L1 increased. When CKCl was raised to 2.4 M, L1 would no longer dissolve. Surprisingly, the addition of HCl restored the solubility of L1 in highly CKCl solutions. Nevertheless, the addition of HCl decreased the pH of the solution, which may have also decreased the TCP of the L1 (Figure 1b).
Further, the behavior of selected 2.2 M and 3.0 M KCl solutions with different pH values showed that the TCP of 0.25% L1 decreased linearly with the quantitative addition of HCl (Figure 2).
Figure 3 shows that the presence of HCl and CKCl have a crucial influence on the phase transition. A higher CKCl broadened the pH range over which this phenomenon occurred. Moreover, this process could be designed to allow it to occur in the metastable zone, which opens up completely new possibilities to control the nucleation process.

3.2. Particle Size Distribution of High Molecular Weight PAA

As one should expect, the particle sizes of L1 in KCl solutions at 25 °C were larger than above the TCP (Figure 4). The hydrodynamic diameters were equal to Dh = 3557 ± 407 nm and Dh = 319 ± 4 nm, respectively. This is the result of macromolecules agglomeration due to the strong hydrophobic interactions. The presence of a hydrophobic surface restricts the natural structuring tendency of water, simply by imposing a barrier that prevents the growth of clusters in a given direction. Therefore, water confined in a gap between two such surfaces would not form clusters larger than a certain size. For a narrow gap, this could be a serious limitation and results in the increased free energy of the water in relation to bulk water. Therefore, there is an attraction between hydrophobic surfaces, which is a consequence of water molecules migrating from the gap to bulk water, where there are unrestricted hydrogen-bonding opportunities and lower free energy [32]. In contrast, the macromolecules shrank during their phase transition at the TCP from the loose coil into dense globules (just before the agglomeration process occurred). This process can be observed among the others in the work of Zhao and co-workers [19]. Further evidence comes from the analysis of the particle size distribution. At the TCP, the size distribution is very narrow and ranges between 9 and 30 nm (which represents the suspension of single, dense globules), while at 25 °C, the size distribution is very broad and ranges between 1000 and 6000 nm (which represents the system of multiple agglomerates comprised of macromolecule globules). However, it has to be emphasized that the KCl concentration had no impact on the particle size distribution. This fact proves that the interactions between macromolecules remain unchanged. The average zeta potential (ZP) values were around −1.1 mV for L1 in KCl solutions and −20 mV for L1 in pure water, indicating strong interactions between the carboxyl groups and salt ions.

3.3. Scanning Electron Microscopy (SEM) Analysis of High Molecular Weight PAA

The morphology of KCl and KCl with L1 (the starting solution was 0.25% L1 in 3 M KCl) are shown in Figure 5. The comparison of pure KCl and KCl in the presence of PAA showed that the crystal surfaces were covered with a polymeric matrix, which agglomerated them into larger entities. This fact may negatively impact the potential process of crystallization (which could be initialized by the generation of solution imbalance due to the appearance of hydrophobic particles [33]) if the investigated system would be situated below the solubility curve. Usually, the agglomeration of crystals is an undesired process during crystallization: it leads to a lower quality of the product. However, this issue will have to be investigated at significantly larger scales.

3.4. Influence of Molecular Weight and Composition of Polymers on TCP in KCl Solutions

The influence of the molecular weight of PAA on its TCP in 3M KCl was investigated. The first step was to check the solubility of (co)polymers L1–L4 in water and several KCl solutions (from 0.1 to 3 M). In the case of PAA homopolymers, the TCP was observed when molecular weight was equal to or higher than 7100 g/mol. The results in Table 1 show that the TCP values increased with an increasing molecular weight of PAA. According to the literature, the addition of a hydrophobic unit decreases the TCP of LCST-type polymers in aqueous solution and should increase the TCP pf UCST-type polymers [34,35]. However, the presence of salt ions in the solution changes the type of interactions between water and macromolecules as well as inter- and intramolecular interactions between macromolecules. Surprisingly, the introduction of hydrophobic methyl acrylate units resulted in a decrease of the TCP of a AA/MA copolymer L4, so that its phase transition occurred within the metastable zone of a salt solution (Table 1). Although the dispersity value for this copolymer was quite high (Đ = 7.0), we decided to perform the transmittance measurement based on previous results for commercial PAA, which showed no prominent evidence of the impact of broad molecular weight distribution on TCP. The first transmittance measurement at 0.2 M KCl showed that the TCP of the copolymer at pH = 3.1 was 26.2 °C. Moreover, although the L4 copolymer was soluble in water, its solubility decreased as the KCl concentration increased (Figure 6). This indicated that the amount of hydrophobic units was too high, and the composition of copolymer has to be changed. Thus, to gain control over the composition, other copolymers of AA and MA were obtained via ATRP and subsequent post-polymerization modification; however, the molar ratio of AA to MA was increased (fAA = 0.98 and fMA = 0.02). Additionally, four-arm star-shaped P(AA-co-MA) (S1) was obtained in order to investigate if the polymer topology influences the crystallization temperature of KCl.

3.5. Synthesis and Characterization of Well-Defined AA/MA Copolymers with Different Topology

First, tBuA/MA copolymers were synthesized by ATRP with low dispersity (Đ = 1.18–1.14). Next, post-polymerization modification of the tert-butyl groups into acidic groups was conducted (Table 2, Scheme 1), and successful acidolysis was confirmed via 1H NMR spectroscopy.
Figure 7 presents a representative 1H NMR spectra of star-shaped P(tBuA-co-MA) and its acidolysis product (S1). The ratios of the methoxy and methine signals’ integral values corresponding to the copolymer before and after acydolysis were constant and confirmed that the cleavage reaction of tert-butyl groups was selective and did not affect the ester linkages in MA units. The signal at 1.45 ppm corresponding to the tert-butyl protons disappeared after acidolysis, and the presence of the acidic functionality shifted the methine proton resonance signal from 2.23 to 2.43 ppm.
The investigation of TCP for AA/MA copolymers with 2% of MA units content showed that the copolymer L5 was completely soluble in 3M KCl. The measurements were performed for 1% and 0.25% L5 solutions. The pH value for 1% L5 in 3M KCl solution was equal to 1.72, which indicated that a dissociation constant of L5 was high despite the presence of salt. The increase of the molecular weight (copolymer L6) decreased the dissociation of protons, resulting in the appearance of the TCP.
The comparison of TCP of 3 M KCl solution containing P(AA-co-MA) with almost the same molecular weight and composition but different topology showed that the same cloud point temperature could be obtained in the presence of linear (L6) and 4-arm (S1) copolymers but at a different range of pH values depending on the copolymer concentration (Figure 8). A higher copolymer concentration and lower KCl solution pH were applied. Interestingly, the difference in the phase transition temperature triggered by the linear and star-shaped copolymers at the same pH decreased by 20 K with a decrease in copolymer concentration from 1% to 0.25%. Comparing L6 and S1 AA/MA copolymers with nearly the same composition showed that the star-shaped topology decreased TCP to 284 K.

4. Conclusions

In this paper, the phase transition phenomena of linear poly(acrylic acid) (PAA) and linear or star-shaped poly(acrylic acid-co-methyl acrylate) (P(AA-co-MA)) in the presence of highly concentrated KCl solutions were investigated. The most important conclusions are:
(1)
The TCP of PAA strongly depends on pH and salt concentration.
(2)
The TCP of PAA decreased upon the incorporation of hydrophobic repeating units, such as MA.
(3)
A comparison of linear and star-shaped AA/MA copolymers with nearly the same composition showed that the star-shaped topology decreased the TCP to 284 K.
(4)
It is possible to design process parameters to allow the phase transition to occur in the metastable zone of the salt solution.
The fact that the phase transition phenomenon may occur within the metastable zone implies some very interesting consequences. For example, the solution imbalance introduced by the appearance of hydrophobic particles may trigger the salt nucleation process. This phenomenon could be used as a convenient tool for crystallization process control. Moreover, the presence of macromolecules may influence the solubility of the salt, crystal size distribution, or crystal habit. This research will be continued in order to verify these possibilities.

Author Contributions

Funding acquisition, M.L.; Investigation, A.S., M.K., A.M., M.L. and S.-i.Y.; Methodology, A.M.; Project administration, M.L.; Supervision, A.M., M.L., and S.-i.Y.; Writing—original draft, A.M. and M.L.; Writing—review and editing, A.M., M.L., S.-i.Y., D.N., and A.G. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Science Centre (POLAND), grant number 2017/26/D/ST8/00050

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Lemanowicz, M.; Gierczycki, A.; Kuźnik, W. Review of stimuli-responsive polymers application as stabilization agents in solid-liquid dispersion systems. Polimery 2016, 61, 92–97. [Google Scholar] [CrossRef]
  2. Mielańczyk, A.; Neugebauer, D. Synthesis and characterization of D-(−)-salicine-based star copolymers containing pendant carboxyl groups with fluorophore dyes. J. Polym. Sci. Part A Polym. Chem. 2014, 52, 2399–2411. [Google Scholar] [CrossRef]
  3. Strandman, S.; Zhu, X.X. Thermo-responsive block copolymers with multiple phase transition temperatures in aqueous solutions. Prog. Polym. Sci. 2015, 42, 154–176. [Google Scholar] [CrossRef]
  4. Weiss, J.; Böttcher, C.; Laschewsky, A. Self-assembly of double thermoresponsive block copolymers end-capped with complementary trimethylsilyl groups. Soft Matter 2011, 7, 483–492. [Google Scholar] [CrossRef]
  5. Kotsuchibashi, Y.; Kuboshima, Y.; Yamamoto, K.; Aoyagi, T. Synthesis and characterization of double thermo-responsive block copolymer consisting N-isopropylacrylamide by atom transfer radical polymerization. J. Polym. Sci. Part A Polym. Chem. 2008, 46, 6142–6150. [Google Scholar] [CrossRef]
  6. Neugebauer, D.; Odrobińska, J.; Bielas, R.; Mielańczyk, A. Design of systems based on 4-armed star-shaped polyacids for indomethacin delivery. New J. Chem. 2016, 40, 10002–10011. [Google Scholar] [CrossRef]
  7. Maji, T.; Banerjee, S.; Biswas, Y.; Mandal, T.K. Dual-Stimuli-Responsive l -Serine-Based Zwitterionic UCST-Type Polymer with Tunable Thermosensitivity. Macromolecules 2015, 48, 4957–4966. [Google Scholar] [CrossRef]
  8. Kelley, E.G.; Albert, J.N.L.; Sullivan, M.O.; Epps, T.H. Stimuli-responsive copolymer solution and surface assemblies for biomedical applications. Chem. Soc. Rev. 2013, 42, 7057–7071. [Google Scholar] [CrossRef] [PubMed]
  9. Stuart, M.A.C.; Huck, W.T.S.; Genzer, J.; Müller, M.; Ober, C.; Stamm, M.; Sukhorukov, G.B.; Szleifer, I.; Tsukruk, V.V.; Urban, M.; et al. Emerging applications of stimuli-responsive polymer materials. Nat. Mater. 2010, 9, 101–113. [Google Scholar] [CrossRef] [PubMed]
  10. Joglekar, M.; Trewyn, B.G. Polymer-based stimuli-responsive nanosystems for biomedical applications. Biotechnol. J. 2013, 8, 931–945. [Google Scholar] [CrossRef] [PubMed]
  11. Seuring, J.; Agarwal, S. First example of a universal and cost-effective approach: Polymers with tunable upper critical solution temperature in water and electrolyte solution. Macromolecules 2012, 45, 3910–3918. [Google Scholar] [CrossRef]
  12. Yamada, N.; Okano, T.; Sakai, H.; Karikusa, F.; Sawasaki, Y.; Sakurai, Y. Thermo-responsive polymeric surfaces; control of attachment and detachment of cultured cells. Die Makromol. Chem. Rapid Commun. 1990, 11, 571–576. [Google Scholar] [CrossRef]
  13. Kanazawa, H.; Yamamoto, K.; Matsushima, Y.; Takai, N.; Kikuchi, A.; Sakurai, Y.; Okano, T. Temperature-responsive chromatography using poly(N-isopropylacrylamide)-modified silica. Anal. Chem. 1996, 68, 100–105. [Google Scholar] [CrossRef] [PubMed]
  14. Bae, Y.H.; Okano, T.; Hsu, R.; Kim, S.W. Thermo-sensitive polymers as on-off switches for drug release. Die Makromol. Chem. Rapid Commun. 1987, 8, 481–485. [Google Scholar] [CrossRef]
  15. Sanford, A.A.; Weissman, J.M.; Sunkara, H.B. Thermally Switchable Optical Devices. U.S. Patent US 6,165,389, 26 December 2000. [Google Scholar]
  16. Chen, J.P.; Huffman, A.S. Polymer-protein conjugates. II. Affinity precipitation separation of human immunogammaglobulin by a poly(N-isopropylacrylamide)-protein A conjugate. Biomaterials 1990, 11, 631–634. [Google Scholar] [CrossRef]
  17. Burdukova, E.; Li, H.; Bradshaw, D.J.; Franks, G.V. Poly (N-isopropylacrylamide) (PNIPAM) as a flotation collector: Effect of temperature and molecular weight. Miner. Eng. 2010, 23, 921–927. [Google Scholar] [CrossRef]
  18. Lemanowicz, M.; Gierczycki, A.; Kuźnik, W.; Sancewicz, R.; Imiela, P. Determination of Lower Critical Solution Temperature of thermosensitive flocculants. Miner. Eng. 2014, 69, 170–176. [Google Scholar] [CrossRef]
  19. Zhao, C.; Dolmans, L.; Zhu, X.X. Thermoresponsive Behavior of Poly(acrylic acid-co-acrylonitrile) with a UCST. Macromolecules 2019, 52, 4441–4446. [Google Scholar] [CrossRef]
  20. Käfer, F.; Liu, F.; Stahlschmidt, U.; Jérôme, V.; Freitag, R.; Karg, M.; Agarwal, S. LCST and UCST in One: Double Thermoresponsive Behavior of Block Copolymers of Poly(ethylene glycol) and Poly(acrylamide-co-acrylonitrile). Langmuir 2015, 31, 8940–8946. [Google Scholar] [CrossRef]
  21. Lee, H.N.; Rosen, B.M.; Fenyvesi, G.; Sunkara, H.B. UCST and LCST phase behavior of poly(trimethylene ether) glycol in water. J. Polym. Sci. Part A Polym. Chem. 2012, 50, 4311–4315. [Google Scholar] [CrossRef]
  22. Cho, Y.; Zhang, Y.; Christensen, T.; Sagle, L.B.; Chilkoti, A.; Cremer, P.S. Effects of hofmeister anions on the phase transition temperature of elastin-like polypeptides. J. Phys. Chem. B 2008, 112, 13765–13771. [Google Scholar] [CrossRef] [PubMed][Green Version]
  23. Deyerle, B.A.; Zhang, Y. Effects of hofmeister anions on the aggregation behavior of PEO-PPO-PEO triblock copolymers. Langmuir 2011, 27, 9203–9210. [Google Scholar] [CrossRef] [PubMed]
  24. Kunz, W.; Lo Nostro, P.; Ninham, B.W. The present state of affairs with Hofmeister effects. Curr. Opin. Colloid Interface Sci. 2004, 9, 1–18. [Google Scholar] [CrossRef]
  25. Zhang, Y.; Furyk, S.; Bergbreiter, D.E.; Cremer, P.S. Specific ion effects on the water solubility of macromolecules: PNIPAM and the Hofmeister series. J. Am. Chem. Soc. 2005, 127, 14505–14510. [Google Scholar] [CrossRef] [PubMed]
  26. Clarke, R.J.; Lüpfert, C. Influence of anions and cations on the dipole potential of phosphatidylcholine vesicles: A basis for the Hofmeister effect. Biophys. J. 1999, 76, 2614–2624. [Google Scholar] [CrossRef][Green Version]
  27. Baldwin, R.L. How Hofmeister ion interactions affect protein stability. Biophys. J. 1996, 71, 2056–2063. [Google Scholar] [CrossRef][Green Version]
  28. Conway, B.E.; Ayranci, E. Effective ionic radii and hydration volumes for evaluation of solution properties and ionic adsorption. J. Solut. Chem. 1999, 28, 163–192. [Google Scholar] [CrossRef]
  29. Karlström, G. On the effective interaction between an ion and a hydrophobic particle in polar solvents. A step towards an understanding of the Hofmeister effect? Phys. Chem. Chem. Phys. 2003, 5, 3238–3246. [Google Scholar] [CrossRef]
  30. Donald, H.; Jenkins, B.; Marcus, Y. Viscosity S-Coefficients of Ions in Solution. Chem. Rev. 1995, 95, 2695–2724. [Google Scholar]
  31. Hey, M.J.; Clough, J.M.; Taylor, D.J. Ion effects on macromolecules in aqueous solution. Nature 1976, 262, 807–809. [Google Scholar] [CrossRef]
  32. Gregory, J. Processing of Solid–Liquid Suspensions. In Processing of Solid-Liquid Suspensions; Shamlou, P.A., Ed.; Elsevier: Oxford, UK, 1993; pp. 59–92. ISBN 9780750611343. [Google Scholar]
  33. Bogacz, W.; Gierczycki, A.; Kuźnik, W.; Lemanowicz, M. Application of Stimuli Sensitive Polymers in Conducting the Crystallization Process. Polish patent P.411522, 9 March 2015. [Google Scholar]
  34. Jones, J.A.; Novo, N.; Flagler, K.; Pagnucco, C.D.; Carew, S.; Cheong, C.; Kong, X.Z.; Burke, N.A.D.; Stöver, H.D.H. Thermoresponsive copolymers of methacrylic acid and poly(ethylene glycol) methyl ether methacrylate. J. Polym. Sci. Part A Polym. Chem. 2005, 43, 6095–6104. [Google Scholar] [CrossRef]
  35. Shibanuma, T.; Aoki, T.; Sanui, K.; Ogata, N.; Kikuchi, A.; Sakurai, Y.; Okano, T. Thermosensitive Phase-Separation Behavior of Poly(acrylic acid)-graft-poly(N,N-dimethylacrylamide) Aqueous Solution. Macromolecules 2000, 33, 444–450. [Google Scholar] [CrossRef]
Figure 1. The cloud point temperature (TCP) curves for 0.25% L1 in KCl solutions with different concentrations without HCl (a) and after the addition of 140 μL HCl (b).
Figure 1. The cloud point temperature (TCP) curves for 0.25% L1 in KCl solutions with different concentrations without HCl (a) and after the addition of 140 μL HCl (b).
Polymers 12 00486 g001
Figure 2. TCP curves for 0.25% L1 in a 3 M KCl solution (a) and 2.2 M KCl (b); the linear decrease of TCP of 0.25% L1 as the volume of 0.01 M HCl increased in 3.0 M KCl (c) and 2.2 M KCl solutions (d).
Figure 2. TCP curves for 0.25% L1 in a 3 M KCl solution (a) and 2.2 M KCl (b); the linear decrease of TCP of 0.25% L1 as the volume of 0.01 M HCl increased in 3.0 M KCl (c) and 2.2 M KCl solutions (d).
Polymers 12 00486 g002
Figure 3. Comparison between the KCl solubility curve and TCP of L1 at different pH values.
Figure 3. Comparison between the KCl solubility curve and TCP of L1 at different pH values.
Polymers 12 00486 g003
Figure 4. Hydrodynamic diameters (Dh) of 0.25% L1 for various KCl concentrations at 25 °C (dots) and at TCP (triangles) (a); particle size distribution of 0.25% L1 in 3 M KCl solution at different temperatures (b); zeta potential (ZP) of 1% L1 in water at 25 °C (c).
Figure 4. Hydrodynamic diameters (Dh) of 0.25% L1 for various KCl concentrations at 25 °C (dots) and at TCP (triangles) (a); particle size distribution of 0.25% L1 in 3 M KCl solution at different temperatures (b); zeta potential (ZP) of 1% L1 in water at 25 °C (c).
Polymers 12 00486 g004
Figure 5. Scanning electron microscopy (SEM) micrographs of pure KCl (A,B) and L1 in 3 M KCl (C,D).
Figure 5. Scanning electron microscopy (SEM) micrographs of pure KCl (A,B) and L1 in 3 M KCl (C,D).
Polymers 12 00486 g005
Figure 6. The influence of KCl concentration on the transmittance (%T) of the L4 solution (0.25% w/w).
Figure 6. The influence of KCl concentration on the transmittance (%T) of the L4 solution (0.25% w/w).
Polymers 12 00486 g006
Scheme 1. The two-step procedure used to obtain linear and star-shaped acrylic acid (AA)/MA copolymers.
Scheme 1. The two-step procedure used to obtain linear and star-shaped acrylic acid (AA)/MA copolymers.
Polymers 12 00486 sch001
Figure 7. 1H nuclear magnetic resonance (NMR) (600 MHz) spectra of star-shaped P(tBuA-co-MA) in CDCl3 (a) and the product of its acidolysis S1 in CD3OD (b).
Figure 7. 1H nuclear magnetic resonance (NMR) (600 MHz) spectra of star-shaped P(tBuA-co-MA) in CDCl3 (a) and the product of its acidolysis S1 in CD3OD (b).
Polymers 12 00486 g007
Figure 8. TCP values of poly(acrylic acid) (PAA) (L1), linear poly(acrylic acid-co-methyl acrylate) (P(AA-co-MA)) (L6), and star-shaped P(AA-co-MA) (S1) with different concentrations as a function of pH.
Figure 8. TCP values of poly(acrylic acid) (PAA) (L1), linear poly(acrylic acid-co-methyl acrylate) (P(AA-co-MA)) (L6), and star-shaped P(AA-co-MA) (S1) with different concentrations as a function of pH.
Polymers 12 00486 g008
Table 1. Characterization of linear poly(acrylic acid) (PAA) and poly(acrylic acid-co-methyl acrylate) (P(AA-co-MA)) polymers.
Table 1. Characterization of linear poly(acrylic acid) (PAA) and poly(acrylic acid-co-methyl acrylate) (P(AA-co-MA)) polymers.
NameDPAA/DPMAMn,theo [g/mol]Mn,SEC a [g/mol]ĐaTCPbpH
L1 *--65,60013.356.32.8
L2326/-25,00023,5002.449.52.8
L399/-710071001.440.52.8
L496/8-45,3007.026.23.1
* Commercially available; a phosphate buffer (pH 9.0) as the eluent and sodium poly(styrene sulfonate) standards b KCl concentration = 3 M, polymer concentration = 0.25%.
Table 2. Characterization of linear and star-shaped P(AA-co-MA) samples obtained via atom transfer radical polymerization (ATRP) and subsequent acidolysis.
Table 2. Characterization of linear and star-shaped P(AA-co-MA) samples obtained via atom transfer radical polymerization (ATRP) and subsequent acidolysis.
NametBuA/MAAA/MA
DPtBuA/DPMAMn,theo [g/mol]Mn,SEC [g/mol]ĐDPAA/DPMAMn,theo [g/mol]Mn,SEC [g/mol]Đ
L596/212,50013,5001.1896/27100n.d.n.d.
L6184/323,80038,8001.16184/313,500n.d.n.d.
S1188/424,44067,1001.14188/413,900n.d.n.d.
Where n.d. indicates that although polymers were soluble in THF and N,N-dimethylformamide (DMF), the retention times of samples were too high for detection.

Share and Cite

MDPI and ACS Style

Sinek, A.; Kupczak, M.; Mielańczyk, A.; Lemanowicz, M.; Yusa, S.-i.; Neugebauer, D.; Gierczycki, A. Temperature and pH-Dependent Response of Poly(Acrylic Acid) and Poly(Acrylic Acid-co-Methyl Acrylate) in Highly Concentrated Potassium Chloride Aqueous Solutions. Polymers 2020, 12, 486. https://doi.org/10.3390/polym12020486

AMA Style

Sinek A, Kupczak M, Mielańczyk A, Lemanowicz M, Yusa S-i, Neugebauer D, Gierczycki A. Temperature and pH-Dependent Response of Poly(Acrylic Acid) and Poly(Acrylic Acid-co-Methyl Acrylate) in Highly Concentrated Potassium Chloride Aqueous Solutions. Polymers. 2020; 12(2):486. https://doi.org/10.3390/polym12020486

Chicago/Turabian Style

Sinek, Aleksander, Maria Kupczak, Anna Mielańczyk, Marcin Lemanowicz, Shin-ichi Yusa, Dorota Neugebauer, and Andrzej Gierczycki. 2020. "Temperature and pH-Dependent Response of Poly(Acrylic Acid) and Poly(Acrylic Acid-co-Methyl Acrylate) in Highly Concentrated Potassium Chloride Aqueous Solutions" Polymers 12, no. 2: 486. https://doi.org/10.3390/polym12020486

Note that from the first issue of 2016, MDPI journals use article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop