Translate this page into:
Stability of poly(N-isopropylacrylamide-co-acrylic acid) polymer microgels under various conditions of temperature, pH and salt concentration
⁎Corresponding author. Tel.: +92 5190642147. m_sidiq12@yahoo.com (Mohammad Siddiq)
-
Received: ,
Accepted: ,
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.
Peer review under responsibility of King Saud University.
Abstract
This research article describes the colloidal stability of poly(N-isopropylacrylamide-co-acrylic acid) [P(NIPAM-co-AAc)] polymer microgels with different acrylic acid contents in aqueous medium under various conditions of temperature, pH and sodium chloride concentrations. Three samples of multi-responsive P(NIPAM-co-AAc) polymer microgels were synthesized using different amounts of acrylic acid by free radical emulsion polymerization. Dynamic laser light scattering was used to investigate the responsive behavior and stability of the prepared microgels under various conditions of pH, temperature and ionic strength. The microgels were found to be stable at all pH values above the pKa value of acrylic acid moiety in the temperature range from 15 to 60 °C in the presence and absence of sodium chloride. Increase in temperature, salt concentration and decrease in pH causes aggregation and decreases the stability of microgels due to the decrease in hydrophilicity.
Keywords
Microgels
pH sensitivity
Stability
Temperature sensitivity
1 Introduction
Microgels have rapidly gained importance in field of materials science owing to their potential applications in drug delivery (Murthy et al., 2003), pollution control (Morris et al., 1997), enhanced oil recovery (Sun et al., 2011), sensors (Retama et al., 2003), catalysis (Lee et al., 2008), fabrication of photonic crystals (Jones and Lyon, 2000), template-based synthesis of inorganic nanoparticles (Zhang et al., 2004) and chemical separation (Nilsson and Hansson, 2005). A large number of applications of microgels arises from their ability to undergo reversible volume phase transitions in response to external stimuli such as a change in pH (Nisato et al., 1999), temperature (Dowding et al., 2000) and ionic strength of the surrounding medium (López-León et al., 2007). Poly(N-isopropylacrylamide) [P(NIPAM)] is a widely studied temperature responsive polymer microgel (Jones and Lyon, 2000). From application point of view, microgels would be much favorable if they could respond to several stimuli simultaneously. Therefore N-isopropylacrylamide (NIPAM) is copolymerized with ionic monomers to get multiresponsive microgels. For example copolymerization of NIPAM with ionic monomer containing carboxylic acid groups form a pH sensitive microgel in addition to the temperature responsive particles. The combination of acrylic acid (AAc) (Naeem et al., 2012) and NIPAM is largely used to prepare pH responsive polymer microgels, because AAc is hydrophilic and can increase the volume phase transition temperature (VPTT). Microgels with tunable VPTT have a potential to be used for biomedical applications (Farooqi et al., 2011a,b). They assigned a photoresponsive drug delivery system based upon gold (Au) nanorods and P(NIPAM-co-AAc) hybrid material (Gorelikov et al., 2004). Jones and his coworkers prepared poly(N-isopropylacrylamide-co-acrylic acid) [P(NIPAM-co-AAc)] core–shell microgel and studied its functioning for physiological applications. They studied its working at pH 3.5 and 6.5 under 0–0.8 M saline solutions (Jones and Lyon, 2003). Kim also synthesized microspheres of NIPAM and AAc and investigated its thermosensitivity within 25–48 °C range at pH 3.0 and 6.5 (Kim et al., 2004). They synthesized P(NIPAM-co-AAc) microgels and investigated their pH and temperature responsive bahaviour in aqueous medium (Debord and Lyon, 2003). Zhang et al. designed a glucose sensor by the chemical reaction of P(NIPAM-co-AAc) microgels with phenylboronic acid in the presence of N-(3-dimethylaminopropyl)-N-ethylcarbodiimide hydrochloride (EDC) catalyst (Zhang et al., 2006). Mohan et al. used P(NIPAM-co-AAc) microgels as microreactor to synthesize Ag nanoparticles. They were able to tune optical properties of nanoparticles by the variation of pH and temperature of the surrounding medium (Mohan et al., 2010).
It is important to investigate the range of stability of P(NIPAM-co-AAc) microgels under various conditions of temperature, pH and salt concentrations for all the above applications. To the best of our knowledge, no one has reported stability of P(NIPAM-co-AAc) microgels under such conditions in a systematic way in the previous studies.
Herein we prepared P(NIPAM-co-AAc) microgels with different feeding contents of AAc and NIPAM, and investigated their responsive behavior in the presence of various stimuli like pH, temperature and NaCl concentration using Dynamic Light Scattering (DLS). We reported the stability of microgels under various conditions and determined the range of temperature, pH and ionic strength in which microgel is stable. The aggregation behavior of the microgels has been explained on the basis of proposed theories.
2 Experimental
2.1 Materials
NIPAM and AAc were purchased from Sigma–Aldrich while N,N-methylenebis(acrylamide) (BIS) was obtained from ACROS. NIPAM was purified by recrystallization from n-hexane and dried in an oven, prior to use. AAc was purified by distillation under reduced pressure. Ammonium persulfate (APS), sodium dodecylsulfate (SDS) and BIS were used as received. Water used for all reactions, solutions preparation and polymer purification was distilled and deionized. The pH values were measured on a WTW Inolab pH 720 pH meter.
2.2 Synthesis of P(NIPAM-co-AAc) microgels
For synthesis of sample G1, 0.01237 mol of NIPAM, 0.000214 mol of BIS, 0.000197 mol of SDS and 0.000721 mol of AAc were dissolved in 100 mL of deionized water in a three necked round bottom flask which was equipped with a condenser and nitrogen inlet. The reaction mixture was heated at 70 °C under a gentle stream of nitrogen. After 1 h, polymerization was started with the addition of 5 mL APS solution (0.06 M). The resulting microgels were purified by dialysis for 1–2 weeks using Spectra/Por molecular porous membrane tubing (cutoff 12,000–14,000) against very frequently changed water at room temperature, and followed by centrifugation. Samples G2 and G3 were prepared and purified using the same method. The compositions of all the microgel samples are given in the Table 1.
| Sample code | NIPAM (g) | AAc (g) | BIS (g) | APS (M) | SDS (g) | Water (mL) |
|---|---|---|---|---|---|---|
| G1 | 1.4 | 0.052 | 0.033 | 0.06 | 0.057 | 100 |
| G2 | 1.4 | 0.079 | 0.033 | 0.06 | 0.057 | 100 |
| G3 | 1.4 | 0.136 | 0.033 | 0.06 | 0.057 | 100 |
2.3 Characterization
DLS was used to investigate the responsive behavior of microgels. Before taking measurement, all the solutions of microgel were filtered through a Millipore millex filter (0.45 μm) into the scattering cell (cuvette) whose outer surface was cleaned with a special type of cleaning paper. The dynamic light scattering experiment was carried out on a commercial LLS spectrometer BI-200SM motor-driven goniometer equipped with BI-9000AT digital autocorrelator and a cylindrical 22 mW uni-phase He–Ne laser (wavelength = 637 nm) at different temperatures (15–60 °C) and pH (2–9) under different concentrations of NaCl at a scattering angle of 90°.
3 Results
3.1 Fourier transforms infrared (FTIR) spectroscopy
The absorption peaks which were recorded in the spectra of three microgels samples are given in the Table 2. FTIR spectrum of G1 in Fig. 1 lacked peak of the carbon–carbon double bond, which confirmed that polymerization occurred at this point since NIPAM and AAc, both monomers have C⚌C groups. The peaks of all the remaining functional groups of monomers are recovered in the spectrum of microgels, confirming that polymerization has successfully occurred.
| Functional groups | Observed peaks for G-1 (cm−1) | Observed peaks for G-2 (cm−1) | Observed peaks for G-3 (cm−1) |
|---|---|---|---|
| C–N | 1148.00 | 1151.78 | 1154.56 |
| CH2 (b) | 1549.67 | 1535.53 | 1535.53 |
| C⚌O (amide) | 1615.64 | 1621.83 | 1625.78 |
| C–H (s) sp3 | 2971.35 | 2966.32 | 2954.43 |
| N–H (s) | 3307.00 | 3311.50 | 3308.47 |
3.2 Effect of temperature
The hydrodynamic radius (Rh) of dilute dispersion of sample G3 as a function of temperature at various pH values is shown in Fig. 2. Microgel dispersion was found to be stable in the pH range of 3.80–8.65 at 15–60 °C but it is unstable at pH = 2.25 at a temperature greater than the volume phase transition of microgels at this pH value. The increase in temperature and decrease in pH cause hydrophobicity in the system. Under such conditions, microgel particles combine with each other and aggregation takes place. That is why the Rh value suddenly increases at pH 2.25 and temperature of 32 °C. Increase in pH increases the stability of microgel dispersion but eliminates the temperature sensitivity. Microgel particles exist in a swollen state at pH greater than pKa value of AAc moiety and do not show any temperature responsive behavior but at pH < pKa, microgel is temperature sensitive. The reasoning behind the swelling of microgel particle with increase in pH is the deprotonation of carboxylic groups present in the network of microgel particles, as described previously (Farooqi et al., 2011a,b). A similar behavior was observed for sample G1 and G2.
3.3 Effect of pH
The plot of Rh of microgel particle of dispersion G3 as a function of pH of the medium at four different temperatures is given in Fig. 3. It is clear from this figure that size of microgels increases as pH increases due to the deprotonation of the AAc (Snowden et al., 1996). Hence electrostatic repulsion between the negatively charged carboxylate residues leads to the swelling of microgel. Under these conditions, the hydrophilicity of the network increases and the microgels are capable of uptaking large amounts of solvent, this also contributes to increase in the hydrodynamic radius of microgel (Fernández-Nieves et al., 2000).
The microgel dispersion is stable in the pH range of 2–9 at temperature ranging from 15 to 25 °C. The dispersion is unstable at pH 2.25 at 35 and 45 °C. The increase in Rh value at pH 2.25 at high temperature is a clear indication of aggregation of the dispersed particles (Naeem et al., 2012). The microgels at such a low pH and high temperature are not suitable candidates for different applications.
This aggregation behavior can be explained on the basis of the increase of polymer–polymer interaction at high temperature in aqueous medium, and protonation of carboxylic groups. These factors are main contributors to the aggregation of microgels.
3.4 Effect of salt concentration
Fig. 4 shows the plots of Rh as a function of temperature under different concentrations of NaCl salt at pH 9.02. Variation in the Rh of microgels is studied in 0.0, 0.05 and 0.1 M concentrations of NaCl of the surrounding medium. It is observed that Rh of microgel particles is greater in the absence of NaCl salt than in its presence. Size of the microgel particle is also a function of the NaCl concentration and it decreases with the increase of salt concentration.
The decrease in size of microgel particles is because of the poor quality of the given solvent and reduction in the electrostatic repulsion. The sodium ions of salt have opposite charges than those of the polymer chains and enter into the network. So they diminish the electrostatic repulsion. Then the particles collapse according to the extent of reduction of repulsion forces among the polymer chains. The value of Rh in the absence of NaCl at 20 °C is 430.23 nm and that in 0.05 and 0.1 M NaCl solutions at the same temperature is 156.12 and 133.56 nm, respectively. The decrease in Rh value with the addition of salt is due to the Debye screening as reported previously. The most important feature of Fig. 4 is that the microgel dispersion is stable within the experimental window of temperature at pH 9.02. These microgels are suitable candidates for biomedical, nanotechnological and catalytic applications in high ionic strength at a temperature range of 15–65 °C. The microgel dispersion is not temperature sensitive in the presence and absence of the salt at pH 9.02.
At low pH, the effect of salt on the microgel size and colloidal stability is shown in Fig. 5. NaCl concentration was varied between 0.05 and 0.1 M. Below VPTT, on increasing temperature the change in Rh reveals a minor dependence on salt concentration but after VPTT, Rh increases exponentially which is attributed to the formation of aggregates. The size of aggregates increases with increase in salt concentration as shown in Fig. 5. AAc are not in ionized form at low pH. There is no electrostatic interparticle repulsion due to AAc. The coagulation of the colloidal particles is due to the hydrophobic interaction of the NIPAM in polymer of microgel. As for lower salt concentration, this can be attributed to the well-known ‘salting-out’ effect (Park and Hoffman, 1993). With increase in salt concentration, the hydrophobic solvent quality increases which lowers the VPTT as well as favors the aggregation. So at temperature greater than VPTT, the interparticle interaction dominates and hydrogen bonding is formed between the carboxyl group of the AAc side chains and the amide groups of NIPAM which seems to favor the formation of larger aggregates. Fig. 5 also shows that the size of aggregates increases remarkably for P(NIPAM-co-AAc) copolymer microgels with increasing ionic strength indicating that electrostatic repulsion has a strong influence on the aggregation process.
3.5 Effect of acrylic acid moiety
Figs. 6–8 show the plots of Rh of all the three microgels samples as a function of temperature at different pH values. Observed VPTT of all the prepared P(NIPAM-co-AAc) microgel samples is higher than the VPTT of P(NIPAM) microgel (∼32 °C) in water (Dhara et al., 2001). This increase in transition temperature is observed because of the incorporation of hydrophilic group AAc in polymer which increases the VPTT and makes its graph less steep (Fernández-Nieves et al., 2000; Bradley et al., 2005). From the comparative analysis study, we also observed that the Rh of sample G3 is the greatest of all samples at all pH, because it contains the highest content of AAc than others. As AAc is strongly hydrophilic, so the water content of hydrogel increases more than the others, consequently the size of this microgel increases (Varga et al., 2001).
The swelling/deswelling behavior of the prepared P(NIPAM-co-AAc) hydrogels as a function of temperature having different mole percentages of the acrylic acid at pH 3.01, 4.72 and 9.02 is shown in Figs. 6–8, respectively. The Rh of all the prepared microgels having different amounts of acrylic acid is enlisted in Table 3. Figs. 6 and 7 show that all the three samples of microgel undergo phase transition on changing temperature from 15 to 60 °C at acidic pH and pka of AAc. Under basic conditions, no remarkable phase transition is visible.
| Microgel sample | Rh(swollen) at 20 °C | Rh(shrunken) at 55 °C | ||||
|---|---|---|---|---|---|---|
| pH = 3.01 | pH = 4.72 | pH = 9.02 | pH = 3.01 | pH = 4.72 | pH = 9.02 | |
| G-1 | 132 | 204.32 | 445 | 45 | 70.65 | 427.45 |
| G2 | 141 | 237.95 | 496 | 61.56 | 106.23 | 504 |
| G3 | 165.32 | 345.42 | 632 | 91.53 | 119 | 627.8 |
Below the pKa value the acidic groups are in the protonated form and have little or no effect on the volume phase transition temperature. Above the pKa, the AAc moiety is deprotonated and has a significant effect on the size and VPTT of microgel due to the hydrophilic nature of the anion, columbic repulsion and osmotic swelling (Fernández-Nieves et al., 2000; Ito et al., 1999).
At high pH, no phase transition occurs however, size increases with the increase in AAc amount because at higher pH the microgel is less temperature responsive and the pH effect is dominant in increasing its size (Kawasaki et al., 1997). The comparison of VPTT of all the microgel samples at different pH is illustrated in Fig. 9 which is truly in accordance to the above mentioned results and gives a proof of the stability of microgels at different pH and temperatures.
4 Conclusions
The colloidal stability of P(NIPAM-co-AAc) microgels with different AAc contents has been investigated in aqueous medium under various conditions of temperature, pH and NaCl concentrations to find the range of application of these microgels in the field of nanotechnology, biomedical technology and catalysis. High temperature and salt concentration, and low pH decrease the stability of microgel system. P(NIPAM-co-AAc) microgels are unstable below 2.25 pH and above 45 °C temperature. They cannot be used for aforementioned applications outside boundary of these conditions. Stimuli sensitive reversible swelling/deswelling behavior was not observed in microgels above pH 7. The microgel particles were in the swollen state at low temperature and in the shrunken state at high temperature. With increasing NaCl concentration, hydrodynamic radius of microgel particles decreases and VPTT shifts to lower temperature. The size of hydrogel particles increases with the increase of AAc contents and VPTT is shifted to higher temperature because of its hydrophilic nature.
Acknowledgments
Then Higher Education Commission Pakistan is gratefully acknowledged for the financial support under the US-Pakistan Science and Technology Cooperative Program.
References
- Equilibrium and kinetic aspects of the uptake of poly(ethylene oxide) by copolymer microgel particles of N-isopropylacrylamide and acrylic acid. Langmuir. 2005;21:1209-1215.
- [Google Scholar]
- Influence of particle volume fraction on packing in responsive hydrogel colloidal crystals. J. Phys. Chem. B. 2003;107:2927-2932.
- [Google Scholar]
- Interpenetrating networks of poly(N-isopropyl-acrylamide) with anionic and cationic polymers. Macromol. Chem. Phys.. 2001;202:3617-3623.
- [Google Scholar]
- Preparation and swelling properties of poly(NIPAM) “Minigel” particles prepared by inverse suspension polymerization. J. Colloid Interface Sci.. 2000;221:268-272.
- [Google Scholar]
- Temperature-induced volume change and glucose sensitivity of poly[(N-isopropylacry-lamide)-co-acrylamide-co-(phenylboronic acid)] microgels. Polym. Int.. 2011;60:1481-1486.
- [Google Scholar]
- Engineering of phenylboronic acid based glucose-sensitive microgels with 4-vinylpyridine for working at physiological pH and temperature. Macromol. Chem. Phys.. 2011;212:1510-1514.
- [Google Scholar]
- Charge controlled swelling of microgel particles. Macromolecules. 2000;33:2114-2118.
- [Google Scholar]
- Hybrid microgels photoresponsive in the near-infrared spectral range. J. Am. Chem. Soc.. 2004;126:15938-15939.
- [Google Scholar]
- Preparation of thermosensitive submicrometer gel particles with anionic and cationic charges. Langmuir. 1999;15:4289-4294.
- [Google Scholar]
- Synthesis and characterization of multiresponsive core–shell microgels. Macromolecules. 2000;33:8301-8306.
- [Google Scholar]
- Dependence of shell thickness on core compression in acrylic acid modified poly(N-isopropylacrylamide) core/shell microgels. Langmuir. 2003;19:4544-4547.
- [Google Scholar]
- Effect of pH on the volume phase transition of copolymer gels of N-isopropylacrylamide and sodium acrylate. J. Phys. Chem. B. 1997;101:5089-5093.
- [Google Scholar]
- Hydrogel microparticles as dynamically tunable microlenses. J. Am. Chem. Soc.. 2004;126:9512-9513.
- [Google Scholar]
- A nanoreactor framework of a Au@SiO2 yolk/shell structure for catalytic reduction of p-nitrophenol. Adv. Mater.. 2008;20:1523-1528.
- [Google Scholar]
- Hofmeister effects on poly(NIPAM) microgel particles: macroscopic evidence of ion adsorption and changes in water structure. Chem. Phys. Chem.. 2007;8:148-156.
- [Google Scholar]
- Controlling of silver nanoparticles structure by hydrogel networks. J. Colloid Interface Sci.. 2010;342:73-82.
- [Google Scholar]
- Adsorption of lead ions onto N-isopropylacrylamide and acrylic acid copolymer microgels. J. Colloid Interface Sci.. 1997;190:198-205.
- [Google Scholar]
- A macromolecular delivery vehicle for protein-based vaccines: acid-degradable protein-loaded microgels. Proc. Natl. Acad. Sci.. 2003;100:4995-5000.
- [Google Scholar]
- Synthesis and characterization of p(NIPAM-AA-AAm) microgels for tuning of optical properties of silver nanoparticles. J. Polym. Res.. 2012;19:1-10.
- [Google Scholar]
- Ion-exchange controls the kinetics of deswelling of polyelectrolyte microgels in solutions of oppositely charged surfactant. J. Phys. Chem. B. 2005;109:23843-23856.
- [Google Scholar]
- Swelling, structure, and elasticity of polyampholyte hydrogels. Langmuir. 1999;15:4236-4244.
- [Google Scholar]
- Sodium chloride-induced phase transition in nonionic poly(N-isopropylacrylamide) gel. Macromolecules. 1993;26:5045-5048.
- [Google Scholar]
- Microstructural modifications induced by the entrapped glucose oxidase in cross-linked polyacrylamide microgels used as glucose sensors. Biomaterials. 2003;24:2965-2973.
- [Google Scholar]
- Colloidal copolymer microgels of N-isopropylacrylamide and acrylic acid: pH, ionic strength and temperature effects. J. Chem. Soc. Faraday Trans.. 1996;92:5013-5016.
- [Google Scholar]
- Interfacial dilational rheology related to enhance oil recovery. Soft Matter. 2011;7:7601-7611.
- [Google Scholar]
- Effect of cross-link density on the internal structure of poly(N-isopropylacrylamide) microgels. J. Phys. Chem. B. 2001;105:9071-9076.
- [Google Scholar]
- Polymer microgels: reactors for semiconductor, metal, and magnetic nanoparticles. J. Am. Chem. Soc.. 2004;126:7908-7914.
- [Google Scholar]
- Synthesis and volume phase transitions of glucose-sensitive microgels. Biomacromolecules. 2006;7:3196-3201.
- [Google Scholar]