Zwitterionic copolymer-based and hydrogen bonding-strengthened self-healing hydrogel

Abstract

Self-healing systems that can spontaneously repair their damage would significantly improve the safety and prolong the lifetime of man-made materials. As external energy or healing agents are required in most of the self-healing approaches, non-covalently cross-linked polymeric hydrogels with intrinsic healing nature have attracted much attention recently. We present here a P(AM-co-DMAPS) zwitterionic copolymer based self-healing hydrogel, where AM and DMAPS are acrylamide and 3-((2-(methacryloyloxy)ethyl)dimethylammonio)propane-1-sulfonate, respectively. The electrostatic interactions within DMAPS and hydrogen bonding between AM moieties contribute to the physical cross-linking of the polymer chains and self-healing ability of the hydrogel. The reported strategy represents a general and facile route to construct zwitterionic copolymer based self-healable functional hydrogels for potential applications in the field of enhanced oil recovery (EOR).

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Introduction

Because of their capability to spontaneously repair internal and external mechanical damage, increasing attention has been paid to self-healing polymeric materials, in recent years.^1–3 For their reduced cost, improved safety and extended lifetime, self-healing materials are intensively studied for potential applications such as in drug delivery systems, shape memory materials, aviation and oil industries. As most of the previously reported self-healing systems rely on the input of external energy or healing agents, polymeric materials with automatic and intrinsic healing nature are highly preferred.^2,4–6 Both the dynamic covalent chemical bonds (phenylboronate ester, disulfide, imine, acylhydrazone, etc.) and non-covalent interactions (hydrogen bonding, host–guest, metal–ligand, π–π stacking, electrostatic, hydrophobic interactions, etc.) have been exploited to construct self-healing polymeric hydrogels.^1–14

On the other hand, as the easily recoverable oil running out and alternative energy resources have not yet meet the societies' demand, the development of enhanced oil recovery (EOR) techniques are crucial to deal with the energy issues. Polyacrylamide (PAM) hydrogels with low-cost and microorganisms resistance are usually used in EOR as water shut-off and profile control agents.^15,16 After secondary oil recovery by water injection (Scheme 1a), there is still an amount of oil in the reservoir (marked by yellow dashed zone in Scheme 1b), because of the higher viscosity than that of water. While an injection of polymer hydrogel would increase the viscosity of the injection fluid, which could efficiently prevent from bypassing of oil (Scheme 1c). Furthermore, PAM hydrogels with self-healing ability could repair the damage induced by injecting and transporting and maintain their functionalities in a longer period of time.¹⁷

Scheme 1 Schematic illustration of (a) secondary oil recovery by water injection; (b) after the secondary oil recovery, and (c) enhanced oil recovery by polymer hydrogel.

Previously, Jiang et al.¹⁸ developed a zwitterionic polymer based hydrogel explained by the “zwitterionic fusion” self-healing mechanism.

Emmenegger et al.¹⁹ prepared zwitterionic copolymer based inorganic–organic hybrid self-healing hydrogel, in which 2-hydroxyethyl methacrylate (HEMA) was used as neutral co-monomer and LAPONITE® XLG nanoparticles as the crosslinker.

Herein, we prepared a novel self-healable hydrogels via copolymerization of acrylamide (AM) and betaine-type zwitterionic monomer, 3-((2-(methacryloyloxy)ethyl)dimethylammonio)propane-1-sulfonate (DMAPS). The effects of solid concentration, molar ratio of co-monomers, electrostatic interaction between DMAPS moieties and hydrogen bonding between AM moieties upon the rheological and self-healing properties of hydrogel are investigated and discussed. This design strategy could be taken as a general and facile route to fabricate self-healing hydrogel for potential application in developing enhanced oil recovery techniques.

Experimental section

Materials

2-(Dimethylamino)ethyl methacrylate (DMAEMA) and 1,3-propane sultone (PS) were purchased from Aladdin Reagent Co. Ltd. N,N,N′,N′-Tetramethylethylenediamine (TEMED) was purchased from Sigma Aldrich. Acrylamide (AM), N,N-dimethylacrylamine (DMA), ammonium persulfate (APS) and other reagents were all purchased from Sinopharm. All the materials were used as received without further purification and all experiments were performed using deionized (DI) water.

Zwitterionic monomer synthesis

3-((2-(Methacryloyloxy)ethyl)dimethylammonio) propane-1-sulfonate (DMAPS) was prepared according to the literatures.^20,21 DMAPS was synthesized by the ring-opening reaction of PS with DMAEMA in the presence of acetonitrile. Typically, PS (3.66 g, 0.03 mol) dissolved in acetonitrile (5 g) was added to a solution of DMAEMA (4.71 g, 0.03 mol) in acetonitrile (10 g). The mixture was stirred at 25 °C for 24 h, and then left to stand at 4 °C for another 48 h. The resultant white precipitate was filtered, washed with acetonitrile and acetone, and dried under vacuum for 12 h. DMAPS was obtained as white solid (8.21 g; ∼98% yield).

Preparation of hydrogel

An appropriate amount of monomers, AM (or DMA), DMAPS, initiator, APS (0.5% w/w relative to the total monomers), and accelerator, TEMED (10% w/w, relative to APS) were dissolved in water at a fixed total molar concentration with different molar ratios of the co-monomers. The mixture was cast into tubular moulds (inner diameter 10 mm, length 6.4 mm), and the polymerization was carried out for 24 h at 45 °C. The hydrogel generated in the process of polymerization in situ. After polymerization, the hydrogel was dialyzed against DI water for 1 week for purification and fresh water was replaced approximately every 4 h.

Chemical composition of hydrogels

Hydrogels were freeze-dried and characterized by attenuated total reflectance FTIR spectroscopy (FTIR-ATR) on a BRUKER TENSOR 27 to confirm the successful polymerization and molar ratio of functional groups. The chemical compositions were characterized by ¹H NMR spectra in D₂O obtained from an AC400 Bruker spectrometer operating at 400 MHz.

Measurements of mechanical and self-healing properties

Rheological measurements were conducted on a TA AR-G2 rheometer using a cone-plate of 40 mm diameter. The frequency-sweep spectra were recorded in a constant-strain (1%) mode over the frequency range of 0.01–100 rad s⁻¹ at 25 °C. The strain-sweep spectra were recorded in a constant frequency of 6.28 rad s⁻¹ over the strain range of 0.01–1400 at 25 °C.

To investigate the recovery properties of the samples in response to applied shear forces, the samples were placed between the para-plate and the platform with special care. We used the following programmed procedure (applied shear force, expressed in terms of strain; duration in parentheses): 1% (500 s)/1400% (100 s)/1% (500 s)/1400% (100 s)/1% (500 s)/1400% (100 s)/1% (500 s) 1400% (100 s)/1% (500 s)/1400% (100 s)/1% (500 s)/1400% (100 s).

Temperature-dependent mechanical and self-healing properties measurements were carried out at 25 °C, 35 °C and 45 °C, respectively.

Results and discussion

DMAPS was synthesized and copolymerized with AM as shown in Fig. 1. After polymerization, tube inversion tests were conducted to examine whether hydrogel was formed. As shown in Fig. 2a, when fixing the molar ratio of AM/DMAPS = 1, hydrogel could formed when the total monomer molar concentration is higher than 3.3 M.

Fig. 1 Synthesis of the zwitterionic APS monomer and P(AM-co-DMAPS) based hydrogel stabilized by hydrogen bonds and electrostatic interactions.

Fig. 2 (a) Photography of tube inversions for P(AM_0.5-co-DMAPS_0.5) with different total molar concentration. (b) G′, G′′ and tan δ versus frequencies of P(AM_0.5-co-DMAPS_0.5) hydrogel. Details could be found in Experimental section.

To further confirm the forming of hydrogel, rheological measurements were also conducted using the sample with total monomer concentration of 3.3 M and molar ratio of AM/DMAPS = 1. As shown in Fig. 2b, the storage modulus, G′ are higher than loss modulus, G′′, and the loss tangent (tan δ) values are lower than 1,^22,23 in a board range of frequencies, suggesting the hydrogel formation. And this hydrogel was denoted as P(AM_0.5-co-DMAPS_0.5) based hydrogel.

To find out the linear viscoelastic regime and the gel–sol transform point of the as-prepared hydrogel, the strain-sweep tests were conducted in a constant frequency of 6.28 rad s⁻¹ over the strain ranging from 0.01 to 1400 at 25 °C. As shown in Fig. 3a, the G′ and G′′ remain constant from 0% to 75% strain indicating no apparent damage formed in the hydrogel. When the strain increased over 80%, the G′ and G′′ values decreased sharply, indicating the dislocation and cut-off of polymer chains¹⁸ or physical cross-linking points.²⁴ Further, the G′′ are higher than G′ values when strain increased to 300–400%, representing a gel–sol transition.

Fig. 3 G′ and G′′ versus (a) frequencies and (b) times curves. (c) G′ and recovery times of P(AM-co-DMAPS) and P(DMA-co-DMAPS) hydrogel versus tested cycles. Determine the recovery time as the time cost for G′ to recover to 90% of maximum G′. Details could be found in Experimental section.

Then, the self-healing property of the hydrogel was examined by cyclic tests with toggling strain between 1% and 1400%. As shown in Fig. 3b, when decreasing from 1400% strain back to 1% strain, the G′ and G′′ values of as-prepared hydrogel recovered to their initial states, indicating the self-healing capability of this hydrogel.^18,19

It was found that previously reported zwitterionic copolymer based self-healing hydrogel are usually prepared from amide bonds containing monomers, of which both the electrostatic interaction and hydrogen bonding exist at the meantime.^18,19 We, on the other hand, want to clarify the respective functionalities of such two types of supramolecular interactions in self-healing hydrogel. In the current design, electrostatic interactions exist between zwitterionic DMAPS moieties and hydrogen bonging between AM moieties. The roles of two different interactions could be investigated through different properties of hydrogels with different chemical components.

At first, we prepared another hydrogel without hydrogen bonds as a counterpart, by copolymerization of DMA and DMAPS, (molar ratio, DMA/DMAPS = 1; total monomer molar concentration = 3.3 M), denoted as P(DMA_0.5-co-DMAPS_0.5) based hydrogel, and conducted the rheology tests in the same conditions. From the strain-sweep curve (Fig. 3a), both of the G′ and G′′ values of P(DMA_0.5-co-DMAPS_0.5) based hydrogel are lower than that of P(AM_0.5-co-DMAPS_0.5) based hydrogel, which imply the presence of hydrogen bonds could enhance both of the elastic component and viscous component. Then the cyclic tests with toggling strain were also conducted under the same condition to investigate the influence of hydrogen bonds in self-healing process. As shown in Fig. 3b, when reverted from 1400% strain back to 1% strain, the values of G′ and G′′ in both of the hydrogen-bonded (P(AM_0.5-co-DMAPS_0.5)) and hydrogen-bonding-free (P(DMA_0.5-co-DMAPS_0.5)) samples are able to recover to their initial states, which suggests the self-healing process might not rely on the hydrogen bonds.

From the time–sweep curves of G′ and G′′ values and test cycle dependent G′ and recovery times as shown in Fig. 3b and c, it could be found that, (1) the G′ and G′′ values of P(DMA_0.5-co-DMAPS_0.5) based hydrogel are almost constant; (2) the G′ and G′′ values of P(AM_0.5-co-DMAPS_0.5) based hydrogel are decreased and the decreasing rates slowed gradually along with the cycling times; (3) the recovery of P(DMA_0.5-co-DMAPS_0.5) based hydrogel is relatively fast (recovery time < 9 s) (4) the recovery times of P(AM_0.5-co-DMAPS_0.5) based hydrogel are decreased and the decreasing rates are slowed gradually along with cycling times. Above all, it was deduced that hydrogen bonding between AM moieties in our hydrogel could increase the G′ and G′′ values. However, their contributions to the self-healing ability could be ignored, and the hydrogen bonding might be disrupted during high-frequency shearing which process would be irreversible. And they may even hinder the self-healing process leaded by DMAPS moieties, which is suggested by the accelerated recovery rate with the disruption of hydrogen bonding over cyclic test.

Both of the electrostatic interactions and hydrogen bonding interactions are temperature-dependent. Temperature-dependent mechanical and self-healing properties measurements of P(AM_0.5-co-DMAPS_0.5) based hydrogel were carried out. As shown in Fig. 4a, the G′ and G′′ values decreased as the temperature increasing, revealing the dissociation of the physical crosslinks when heated.^25,26 However, the healing efficiency increased when temperature increased, as shown in Fig. 4b, which is resulted from the accelerated interdiffusion by elevated temperature.^27,28

Fig. 4 G′ and G′′ (a) and healing efficiency (b) versus temperature of P(AM_0.5-co-DMAPS_0.5) hydrogel.

To further understand the influence of electrostatic interaction and hydrogen-bonding on properties of hydrogel, a series of copolymers with different molar ratio of co-monomers (molar ratio of AM/DMAPS = 3 : 7, 5 : 5, 7 : 3; total molar concentration = 3.3 M) were prepared. The successfully preparation of hydrogels were characterized by tube inversion tests, FTIR-ATR and ¹H NMR measurements, as shown in Fig. 5. When decreasing the DMAPS molar ratio, the intensities of sulfonyl bonds (1040 cm⁻¹ and 1121 cm⁻¹), methane groups (1454 cm⁻¹ and 2973 cm⁻¹) and ester bonds (1728 cm⁻¹) decreased, while the intensities of amide bands (3440 cm⁻¹ and 1667 cm⁻¹) increased. ¹H NMR spectra shows the same results (Fig. 5b).

Fig. 5 (a) FTIR-ATR and (b) ¹H-NMR spectra of P(AM-co-DMAPS) hydrogels.

The influences of molar ratios of co-monomers on the properties of hydrogels were investigated systematically. The frequency-sweep tests were conducted in a constant-strain (1%) mode over the frequency range of 0.1–100 rad s⁻¹ at 25 °C to identify whether the hydrogels were frequency-dependent. As shown in Fig. 6a, the G′ values of all of four types of hydrogel (molar ratios, AM/DMAPS = 3/7, 5/5, 7/3, 10/0) increased along with frequency, and faster than the increase of G′′ values. Moreover, the G′ values of hydrogel with higher AM molar content increased faster than that of hydrogel with lower AM molar content. It presumed that the viscoelastic component might be mainly derived from hydrogen bonding, which is consistent with the results of Fig. 6b.

Fig. 6 (a) G′ and G′′ versus frequencies, (b) tan δ versus frequencies, (c) G′ and G′′ versus strains, and (d) G′ and G′′ of the linear viscoelastic region and corresponding tan δ versus strains of all tested P(AM-co-DMAPS) hydrogels. Details could be found in Experimental section.

The strain-sweep tests were also conducted in a constant frequency of 6.28 rad s⁻¹ over the strain range from 0.01 to 1400 at 25 °C, to find out the linear viscoelastic regimes and the gel–sol transform points. As shown in Fig. 6c, there are no evident differences of linear viscoelastic regimes and gel–sol transform points of hydrogels with different co-monomer molar ratios. Further quantitative analysis shown in Fig. 6d indicated that the hydrogel containing 50% (molar ratio) of AM monomers showed the relatively better rheological performance (or elastic component). Taken together, it was hypothesised that, the storage modules (G′) of P(AM-co-DMAPS) copolymer based hydrogel are not simply depend on the molar ratio of AM/DMAPS, and G′ became higher when AM/DMAPS ∼ 1, which means in a certain range (molar ratio, AM/DMAPS ∼ 1), hydrogen bonding could enhance the elastic component by the means of strengthening the elastic network constructed from zwitterionic electrostatic interactions within the hydrogel. The mechanism behind this phenomenon is not quite clearly, and further studies are underway in our group.

Finally, we investigated the self-healing properties of these hydrogels with different monomer molar ratios. The healing efficiency and recovery time were shown in Fig. 7. Comparing with its counterparts, hydrogel with molar ratio of AM/DMAPS = 1, possess relatively high recovery efficiency and the recovery times are all at the acceptable levels, in all three tested cycles, which was consistent with previously conclusions.

Fig. 7 (a) The healing efficiencies and (b) recovery times of hydrogels in three tested cycles.

Temperature-dependent studies on mechanical and self-healing properties of hydrogel with different monomer molar ratios were also carried out, as shown in Fig. S1 and S2,† which is consistent with pervious conclusions. Besides, it is very interesting that the healing efficiency of all the hydrogels increased and the increasing magnitude rose gradually along with the DMAPS molar content, which could also deduced the zwitterionic electrostatic interactions play an important role in the self-healing process.

Conclusions

The commonly used PAM hydrogels in oil industry with self-healing property were prepared via copolymerization of AM and DMAPS monomers. The relationships between structure and property of the hydrogel were investigated. And we believed that P(AM0-co-DMAPS) copolymer based self-healing hydrogels with high efficiency could be a more smart, economic and environmental friendly materials for potential application in oil industry especially for EOR.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant no. 51273189), the National Science and Technology Major Project of the Ministry of Science and Technology of China (2011ZX05010-003), PetroChina Innovation Foundation (2012D-5006-0202) and China Postdoctoral Science Foundation funded project (no. 2013M531513).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra15003a

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