Reusable Self-Healing Hydrogels Realized via in Situ Polymerization

Article pubs.acs.org/JPCB

Reusable Self-Healing Hydrogels Realized via in Situ Polymerization Balachandran Vivek and Edamana Prasad* Department of Chemistry, Indian Institute of Technology Madras (IIT M), Chennai 600 036, India S Supporting Information *

ABSTRACT: In this work, a self-healing hydrogel has been prepared using in situ polymerization of acrylic acid and acrylamide in the presence of glycogen. The hydrogel was characterized using NMR, SEM, FT-IR, rheology, and dynamic light scattering (DLS) studies. The developed hydrogel exhibits self-healing properties at neutral pH, high swelling ability, high elasticity, and excellent mechanical strength. The hydrogel exhibits modulus values (G′, G″) as high as 106 Pa and shows an exceptionally high degree of swelling ratio (∼3.5 × 103). Further, the polymer based hydrogel adsorbs toxic metal ions (Cd2+, Pb2+, and Hg2+) and organic dyes (methylene blue and methyl orange) from contaminated water with remarkable efficiency (90−98%). The mechanistic analysis indicated the presence of pseudo-second-order reaction kinetics. The reusability of the hydrogel has been demonstrated by repeating the adsorption−desorption process over five cycles with identical results in the adsorption efficiency.

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INTRODUCTION Developing gel1−4 based smart materials5−9 is a major current research interest due to their potential role in areas such as catalysis,10,11 adsorption,12,13 tissue engineering,14−16 sensors,17 and drug delivery.18−21 Among the various types of smart materials, hydrogel based smart materials22−27 have emerged as promising candidates for biomedical applications due to their biocompatible nature.28 Moreover, self-healing29−32 properties of the hydrogels guarantee long-term use of such materials. One of the challenges in developing hydrogels with selfhealing properties is to provide sufficient mechanical strength to the system. The mechanical strength33 of hydrogels, in general, depends on how strongly the intermolecular interactions hold the three-dimensional molecular network which prevents the free-flow of the solvent. Another important desired functionality of self-healing hydrogel is its ability to exhibit self-healing properties at neutral pH. Many of the reported hydrogels show self-healing properties only at low pH which limits their in vivo applications.33−35 It would be of great advantage if hydrogels with self-healing properties could also possess additional desired properties such as high mechanical strength, enhanced swelling ability,36 and high viscoelasticity at neutral pH range. Herein we describe a hydrogel with robust mechanical strength, high swelling capacity, high elasticity, and superadsorption ability, along with self-healing properties at neutral pH. We have used glycogen, acrylic acid, and acrylamide, and developed a hydrogel through in situ polymerization. The hydrogel formed was characterized by NMR, SEM, FT-IR, rheology, and dynamic light scattering (DLS) studies. The novel three-component hydrogel described herein contains commonly available chemicals such as glycogen, poly(acrylic © 2015 American Chemical Society

acid), and poly(acrylamide). The self-assembly of the in situ formed polymers results in a gel with exceptional swelling ability and mechanical strength. We have utilized the gel to remove contaminants such as heavy metal ions (Pb2+, Hg2+, and Cd2+) and organic dyes (methylene blue and methyl orange) from water under ambient conditions. Further, we have determined the rate constants for the adsorption of the metal ions on the hydrogel. The performance of the gel for water purification37−43 has been evaluated, and the regeneration as well as reusability properties of the hydrogel through repeated adsorption−desorption cycles have also been investigated.

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EXPERIMENTAL SECTION Materials and Methods. Glycogen (extra pure) was purchased from Sigma-Aldrich Chemical Co. (USA). Acrylamide, acrylic acid, and N,N,N′,N′-tetramethylethylenediamine (TEMED) were purchased from Alfa Aesar (USA) and used without further purification. Ammonium persulfate (APS) and organic dyes (methylene blue and methyl orange) were obtained from Spectrochem Pvt. Ltd. (India). Metal salts (CdSO4, HgCl2, PbSO4) used for adsorption studies were purchased from Sisco Research Laboratories Pvt. Ltd. (India). FT-NMR spectra of the products at different stages were recorded with a Bruker 400 MHz spectrometer. FT-IR spectra of the hydrogel were recorded by a Jasco 4100 instrument. SEM images of the hydrogel were obtained by an FEI Quanta FEG 400 High Resolution Scanning Electron Microscope. Dynamic light scattering studies of the hydrogel in water were Received: November 25, 2014 Revised: March 12, 2015 Published: March 16, 2015 4881

DOI: 10.1021/jp511781e J. Phys. Chem. B 2015, 119, 4881−4887

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The Journal of Physical Chemistry B Scheme 1. Preparation of Glycogen Based Three-Component Hydrogel

carried out using a Malvern zetasizer nanoseries. Rheological measurements were performed using an Anton Paar rheometer. ICP-OES analysis was carried out by a PerkinElmer Optima 5300 DV. Uptake of organic dyes was monitored by a JASCO V-660 spectrophotometer. Preparation of Glycogen−Poly(acrylic acid)−Poly(acrylamide) Hydrogel and Characterization. The hydrogel preparation was accomplished by the polymerization of acrylic acid and acrylamide, in the presence of glycogen. The polymerization of acrylic acid was based on the radical polymerization using ammonium persulfate (APS) as an initiator and N,N,N′,N′-tetramethylethylenediamine (TEMED) as an accelerator. In a typical procedure, glycogen (1 g) was dissolved in doubly distilled water by heating at 80 °C and the mixture was cooled to room temperature and purged with nitrogen for 20 min. Ammonium persulfate (1 g, 4.38 × 10−3 mol) and tetraethylethylenediamine (1 mL, 6.68 × 10−3 mol) were added to the mixture and stirred for 20 min under a nitrogen atmosphere to generate free radicals. To the above stirred solution, acrylic acid (8 g, 0.11 mol) was added and stirred for 2 h under a nitrogen atmosphere. To the above reaction mixture, acrylamide (2 g, 0.028 mol) was added and stirred for 10 min to form the gel. Then, the mixture was kept for 24 h without stirring. The obtained product was washed with doubly distilled water five times and dried in a vacuum desiccator for 1 day. The morphology of the product was analyzed through SEM, FT-IR, and DLS studies. Details of the swelling degree and adsorption experiment are given in the Supporting Information.

Figure 1. (A) Photograph of the gel and (B) its capacity to withstand high elongation.

(COOH)−CH2−)n} appear at 2.15 and 2.52 ppm. In GLY− PAA, the chemical shift values are changed from 2.15 and 2.52 ppm to 3.79 and 2.767 ppm, respectively, indicating the binding between PAA and GLY. Similarly, the peak of the −CH2 group in the poly(acrylamide) {−(CH(CONH2)−CH2−)n} chain is also shifted from 3.47 to 2.16 ppm upon addition of the GLY− PAA mixture to the poly(acrylamide). This suggests that poly(acrylamide) might act as a cross-linker in the gel system. Furthermore, the NMR peaks in GLY−PAA−PAM get broadened, indicating the in situ polymerization in the system (Figures S1 and S2, Supporting Information). The FT-IR spectrum of the gel GLY−PAA−PAM was recorded at room temperature. Figure 2 contains FT-IR spectra of the gel, glycogen (GLY), and glycogen−poly(acrylic acid) (GLY−PAA) mixture. The FT-IR spectrum of glycogen exhibits the characteristic peaks at 3384 cm−1 (OH stretch),

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RESULTS AND DISCUSSION Preparation and Characterization of the Hydrogel. The reaction conditions for preparation of the hydrogel are shown in Scheme 1. Briefly, glycogen (GLY) was dissolved in hot water containing ammonium persulfate and a reaction accelerator (TEMED) was added under a nitrogen atmosphere. Acrylic acid was then introduced into the system and stirred for 2 h, followed by the addition of ∼0.3 equiv of acrylamide (the detailed procedure is given in the Experimental Section). The stirring was continued for 10 min, during which the in situ polymerization was complete. The as-synthesized gel was washed several times with distilled water and dried in a desiccator under a vacuum. The hydrogel appeared to be mechanically very much stable yet easily stretchable (Figure 1). In order to understand the molecular interactions in the gel, NMR spectra of the gel (henceforth will be represented by GLY−PAA−PAM) and glycogen−poly(acrylic acid) (GLY− PAA) mixture were recorded. The chemical shift values of −CH and −CH2 protons of poly(acrylic acid) {−(CH-

Figure 2. FT-IR spectra of glycogen, glycogen−poly(acrylic acid) (GLY−PAA), and glycogen−poly(acrylic acid)−poly(acrylamide) hydrogel (GLY−PAA−PAM). 4882

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The Journal of Physical Chemistry B 2925 cm−1 (CH stretch), 1647 cm−1 (OH bending), 1466 and 1426 cm−1 (CH bend), 1000−1250 cm−1 (CO and COH stretch), and 906 and 846 cm−1 (CH bend). Addition of PAA to GLY results in a viscous solution, and the FT-IR spectrum of the GLY−PAA mixture showed additional distinct bands at 1671, 1542, and 1442 cm−1 which correspond to CO stretching vibration, symmetric and asymmetric stretching vibration of the COOH group in PAA. A comparison between the spectra of pure PAA and GLY− PAA indicates that the carboxyl stretching frequency in the mixture has been shifted compared to that in pure PAA (from 1714 to 1671 cm−1), presumably due to the hydrogen bonding of carboxyl groups with hydroxyl groups in the glycogen. The bands at 1542 and 1442 cm−1 correspond to CO symmetric and asymmetric stretching vibration of COOH. In the GLY− PAA−PAM system, the carbonyl and carboxyl stretching vibration are observed at 1633, 1610, and 1729 cm−1 which is attributed to the presence of cross-linked hydrogen bonds between PAA, PAM, and glycogen. The sharp bands observed at 3415 and 3228 cm−1 are from the NH stretching of the amide group. The results indicate that a hydrogen bonded network is generated between PAA and GLY, and upon addition of PAM, a cross-linking network of hydrogen bonds is established in the system. This provides a robust cross-linking in the three-dimensional fashion, where solvent (water) can be trapped effectively, during gel formation. The morphology of the in situ gel has been examined by SEM. Figure 3 shows the morphology of GLY−PAA−PAM at two different concentrations of acrylamide (PAM).

concentrations (0.028 mol) of PAM (Figure 3B). This is attributed to PAM induced cross-linking, which leads to the self-assembly of the globular assemblies to form dense fibers, at an increased concentration of PAM. The porous nature of the surface of GLY−PAA−PAM is clear from the SEM image in Figure 3B. Further changes on the morphology of the gel have been observed upon treating with dye molecules and heavy metal ions. The SEM images of the gel after treating with cadmium ions and methylene blue dye exhibit different morphologies (Figure S3A−D, Supporting Information). In the case of metal ion treated gel (Figure S3A and C, Supporting Information), the system shows a rodlike morphology, and in dye treated gel, the morphology was similar to a disordered bundle of needles (Figure S3B and D, Supporting Information). The morphology change can be attributed to the binding between −COO− groups in the polymer and the metal ions/ dyes, leading to collapse of pores on the surfaces.22,43−45 In order to understand the mechanism of the self-assembly, we have carried out dynamic light scattering (DLS) studies. Figure 4 and Figure S4 (Supporting Information) represent the intensity versus particle size distribution histogram from the DLS study. The experiments were carried out in five batches at 25 °C where the amount of acrylamide was increased from 2 to 4 g from sample 1 to sample 5. As the amount of acrylamide was increased, the distribution of the hydrodynamic diameter of the gel became narrow. This observation corroborates the results from the SEM studies and the hypothesis that increased the concentration of acrylamide (PAM) result in cross-linking the GLY−PAA assemblies. The enhanced cross-linking leads to a narrow size distribution of the self-assembled system. Figure 5 shows the results from rheological study of the material. The strain sweep curve shows that both the storage modulus (G′) and loss modulus G″ are close to 106 (Pa). This suggests the exceptional stability of the hydrogel. As strain increases, both G′ and G″ start decreasing and the values cross over after exceeding a certain strain value which indicates the loss of gel phase (Figure 5A). The dynamic modulus versus time curve (Figure S5, Supporting Information) indicates that there is no considerable variation in moduli values with time at room temperature, which supports the earlier observation of the high stability of the hydrogel. It is clear from the frequency sweep curve (Figure 5B) that the values of storage modulus (G′) and loss modulus (G″) do not strongly depend on frequency changes. G′ is much higher than G″, which is indicative of the high elastic nature of the gel. Swelling properties of the hydrogel were examined as a function of time at two different pH’s, and the results are summarized in Figure 6. The dried hydrogel was first weighed

Figure 3. SEM images of hydrogel at low concentration (A) and high concentration (B) of acrylamide.

The SEM analysis indicated that, at relatively low concentrations (0.014 mol) of PAM, the system exhibits a spherical morphology (Figure 3A). On the other hand, the morphology of the system changes to fiber type at high

Figure 4. Histogram representation of dynamic light scattering data (intensity versus size) for two different concentrations of acrylamide [0.028 mol (A) and 0.056 mol (B)]. 4883

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Figure 5. (A) Strain sweep curve of GLY−PAA−PAM hydrogel. The ratios between acrylic acid and acrylamide are 8:2, 8:3, and 8:4. (B) Plot of elastic modulus (G′) and viscous modulus (G″) of the hydrogel as a function of frequency.

results indicate that self-healing property prevails only under neutral pH conditions.

Figure 6. Variation of the swelling ratio of GLY−PAA−PAM hydrogel with time at two different pH values.

and immersed in doubly distilled water for 8 h. At different time intervals, the swelled gel was taken out and weighed after draining out excess water. Figure 6 shows the swelling behavior of hydrogel under two different pH’s. The graph between swelling degree (details regarding calculation of the swelling degree are given in the Supporting Information) and time indicates that the swelling ability of the gel is remarkably high at pH 8.5, compared to that at acidic pH. The excellent swelling ratio results presumably because of the repulsion between COO− groups in poly(acrylic acid), at basic pH, which causes repulsion at the molecular level, translating into a macroscopic stretching of cross-linking networks. This leads to an expansion in the total space available in the cavities of the gel to accommodate more solvent (water). A maximum swelling ratio of 3250 was obtained for the gel at pH 8.5. A control experiment was performed where glycogen was added after the polymerization of both acrylic acid and acrylamide. The resulting mixture shows no gelation properties, indicating that in situ polymerization has to be carried out in the presence of glycogen, which presumably acts as the backbone template where poly(acrylic acid) and poly(acrylamide) make cross-linking networks to form the gel. Self-Healing Properties. The experimental results taken together suggest that the in situ polymerization cross-linking technique has resulted in good mechanical and swelling properties to the hydrogel. In order to verify the self-healing properties of the gel, the hydrogel was sliced into three parts, and two parts were colored using highly concentrated methyl orange solution. Then, the three different parts of the hydrogel were kept in touch with each other at neutral pH. After 12 h, the three different hydrogels were found to be joined with each other and the joined parts were found to be inseparable even if the gel was stretched (Figure 7A). We have investigated the healing nature of the hydrogel at different pH values, and the

Figure 7. (A) Demonstration of the self-healing by the hydrogel and its pH effect. The healed hydrogel at neutral conditions could withstand stretching and hold its own weight. (B) Schematic representation of hydrogen bonding interactions in the gel leading to self-healing.

This is attributed to the presence of numerous hydrogen bonding interactions between the acid/amide functional groups present in the gel surfaces. The interlinking directional hydrogen bonding involving CO and OH groups of acid and NH2 and CO groups of amides is responsible for this self-healing process (Figure 7B). Under acidic conditions, the repulsion between the protonated amine groups in the surface prevents the hydrogen bonding and self-healing. Similarly, the electrostatic repulsion between the carboxylate groups reduces the chance of self-healing under basic conditions. Optical polarizing microscopic studies and rheology analysis were carried out to prove self-healing. A crack is made in the 4884

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Figure 8. Photograph of GLY−PAA−PAM hydrogel and dyes (MB, MO) before (A and B) and after (C and D) adsorption. UV−vis spectra of dye solution (E) methylene blue and (F) methyl orange during adsorption experiment.

film (mechanically), and it was observed at neutral pH using an optical microscope. The size of the crack decreases as a function of time. After 70 min, the crack was not visible (Figure S7, Supporting Information). This is due to the diffusion of the hydrogen bonding network on the surface and clearly demonstrates self-healing. Next, we have carried out two rheology measurements; one is elastic modulus vs strain, and the other is elastic modulus vs time at a particular strain. In the strain sweep curve, continuous strain is given to gel until it breaks, and the broken gel is allowed to heal. The plot of modulus vs time indicates the regain of the modulus, corroborating the hypothesis of self-healing (Figure S8, Supporting Information). Adsorption of Toxic Heavy Metal Ions. Adsorption Kinetics. The purified hydrogel was treated with NaOH (0.01 M) to convert COOH groups of poly(acrylic acid) (PAA) to the corresponding carboxylate ions. The adsorption kinetics of heavy metal ions and organic dyes were studied by the following method. Hydrogel (10 mg) was stirred with 100 mL of water containing heavy metal ions (100 ppm) or organic dyes (200 ppm) for 1.5 h. At definite time intervals, 5 mL of the sample was taken out and the concentration of the metal ions was monitored by the ICP-OES technique. The adsorption capacity, qe, was found out by the following equation qe =

(C0 − Ce)V m

for removing the toxic metal ions reaches a plateau by 60 min. It is worth mentioning here that a relatively small amount of hydrogel (10 mg/100 mL) has been utilized to eliminate heavy metal ions from water. Kinetic studies were carried out to understand the detailed chemistry behind the metal ion adsorption, which followed the pseudo-second-order kinetic equation given below t 1 t = + qt qe kaqe 2

(2)

where qt is the amount of metal ion adsorbed per unit mass at time t, qe is the amount of metal ion adsorbed at equilibrium, and ka is the rate constant of the pseudo-second-order reaction. The plot of t/qt vs t is shown in Figure S9B (Supporting Information), and the correlation coefficient (R2) indicates that the fit is reasonable. The adsorption efficiency at equilibrium (qe) was also determined from the slope of the curve, which exhibits good agreement with the qe values determined using eq 2. All of the parameters related to the adsorption are tabulated in Table S1 (Supporting Information). Figure S10 in the Supporting Information represents pH dependent variation of adsorption of heavy metal (Cd2+) ions from water, which shows an increase in the adsorption of metal ion with an increase in the pH. After a particular pH (6.5), adsorption reaches a maximum. The increase in the adsorption at basic pH might be due to the formation of an increased number of carboxylate groups, which enhances the metal ion interaction through electrostatic force. Adsorption of Dyes. Next, we examined the adsorption behavior of GLY−PAA−PAM hydrogel toward textile organic dyes in water. We have selected methyl orange (MO) and methylene blue (MB) dyes for the present study. The concentration of dyes was 200 ppm in doubly distilled water. For the case of methylene blue, the pH was fixed at 6.5, and for methyl orange, the pH was fixed at 2.5. The dried hydrogel (10 mg) was added in the dye solution, and the mixture was stirred vigorously for 8 h. A specific amount of the sample (5 mL) was taken out at regular intervals of time and subjected to UV−vis spectra analysis. The absorbance was monitored at each interval of time. The results indicate that more than 90% of MB and MO were removed from water by 8 h. The photographic images of dye solution, before and after adsorption, are given in

(1)

where Ce is the equilibrium concentration, qe corresponds to the adsorption amount at equilibrium, C0 is the initial concentration of metal ions or organic dyes, and V and m are the volume of the solution and the mass of the hydrogel, respectively. We examined the adsorption properties of GLY−PAA−PAM hydrogel for removing Cd2, Hg2+, and Pd2+ ions. All the experiments were carried out in doubly distilled water, and the initial concentration of the heavy metal ions was kept as 100 ppm. Adsorption is rapid in the case of all the heavy metal ions, and the adsorption process reached equilibrium by 60 min. Figure S9A (Supporting Information) shows the effect of contact time on the adsorption efficiency of GLY−PAA−PAM hydrogel. As is evident from the figure, the efficiency of the gel 4885

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Hg2+, and Pd2+) and organic dyes (methyl orange and methylene blue) from water with excellent adsorption capacity. Regeneration and reusability of the material were also investigated, and the results indicate that the eco- and biofriendly polymer based hydrogel is a promising candidate for water purification. The study also suggests that multipolymer self-assembly through in situ polymerization can be an effective strategy for generating mechanically stable gel systems with desired properties for various applications.

Figure 8A−D. The significant decrease in the absorbance can be seen from the UV−vis spectra (Figure 8E and F). It can be concluded that the electrostatic attraction between the hydrogel and the dyes was the major factor for adsorption, since pH has an appreciable influence on the removal of MO and MB from water. The removal of MO was carried out at pH 2.5, and the removal of MB was carried out at pH 6.5. When the pH is 2.5, more amine groups will be protonated so the porous charge as well as surface charge becomes positive. This positive charge can make an electrostatic interaction with anionic MO in the adsorption process. The pH was increased to 6.5 for the case of MB, so that the carboxyl group can play a good role in adsorbing MB by electrostatic attraction due to the negative surface charge. Regeneration and Reusability. Regeneration and reusability of the hydrogel were studied using methyl orange as the adsorbent (100 ppm in 100 mL of water). Upon reaching equilibrium condition, the gel was separated out and dried and methyl orange was extracted using ethanol. The approximate time period for the desorption process was about 20 min. The concentration of the ethanol extracted methyl orange was determined by UV−vis spectroscopy, and the results suggest that 97% of the dye was recovered. The gel after desorption was further used for adsorption studies in the presence of 100 ppm methyl orange in distilled water. Figure 9 contains the plot

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ASSOCIATED CONTENT

S Supporting Information *

Experimental procedure for the preparation of heavy metal ions and organic solutions, adsorption experiments, desorption studies, SEM images of gel after metal ions and dye adsorption, kinetic studies and parameters of adsorption, dynamic light scattering, and rheological measurements. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was done under the financial support of Department of Science and Technology (DST), Government of India {SR/ NM/NS-12/2011 (G)}. We are grateful to SAIF, IIT Madras, for the ICP-OES facility. We thank Prof Abhijit Deshpande, Department of Chemical Engineering, IIT Madras, for the SEM and rheological studies.

Figure 9. Percentage sorption and retrieval of methyl orange.

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between the percentage of the dye removed and the percentage of the dye retrieved versus the number of cycles. As the data suggests, the performance of the hydrogel for five cycles was excellent (90−98%). Figure S11A−D (Supporting Information) contains the photograph of the hydrogel showing the regeneration process. Upon addition of the methyl orange adsorbed gel to ethanol, the whole hydrogel becomes colorless within 20 min. The gel has shrunk due to deswelling, and upon keeping the deswelled gel in water for 1 h, the gel became completely regenerated (Figure S11E−G, Supporting Information). These results suggest that the novel GLY−PAA−PAM hydrogel is a promising candidate for water purification.

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CONCLUSION In summary, a novel, polymer based three-component hydrogel GLY−PAA−PAM was prepared through in situ polymerization of acrylamide and acrylic acid in the presence of glycogen. The experimental data suggested that poly(acrylamide) acts as a cross-linker in a matrix of glycogen chains connected by poly(acrylic acid). In addition to self-healing properties at neutral pH, the hydrogel shows an exceptionally high mechanical strength and swelling ability at room temperature. The hydrogel was used for removal of heavy metal ions (Cd2+, 4886

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The Journal of Physical Chemistry B

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DOI: 10.1021/jp511781e J. Phys. Chem. B 2015, 119, 4881−4887