Structure and properties of tough polyampholyte hydrogels: effects of a methyl group in the cationic monomer

Abstract

Two types of polyampholyte (PA) hydrogels are successfully synthesized by a facile one-step random copolymerization method using an ammonium peroxydisulfate (APS)/aliphatic amine [N,N,N′,N′-tetramethylenediamine (TEMED)] redox system. Two cationic monomers, differing only slightly by the presence of a methyl group, [2-(methacryloyloxy)ethyl]trimethylammonium chloride (MATAC) and acryloyloxethyltrimethylammonium chloride (DMAEA-Q), are used to fabricate two hydrogels with the same anionic monomer. The obtained hydrogels show large differences in their mechanical properties. The G-M hydrogels containing MATAC are much stiffer and stronger than the G-D hydrogels containing DMAEA-Q. Particularly, the G-D gels are super stretchable, with a fracture strain of 12 600%, a work of extension at fracture of 27.8 MJ m⁻³, and a stress of 0.5 MPa. The mechanical properties of these hydrogels are comparable to, or even better than, those of the hydrogels prepared under stricter conditions via UV polymerization. The very slight steric bulk of the methyl group exhibits significant effects not only on the strength and distribution of the ionic bonds, but also on the microstructures of the G-M and G-D hydrogels. Although G-M contains more hydrophobic methyl groups, its water content and surficial hydrophilicity are higher than those of G-D. Moreover, the polymer network of G-M stacks more homogenously and stereoscopically with stronger interactions. The results obtained in this study will increase the understanding of structures and properties of tough hydrogels.

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1. Introduction

Hydrogels, three-dimensional networks of soft matter containing large amounts of water, are similar to biological tissues and can be used for different biomedical applications.^1–4 Previously reported hydrogels were found to be mechanically weak materials;^5,6 therefore, only applications which do not require great mechanical strength were developed, such as drug delivery vehicles,⁷ cell cultural scaffolds⁸ and water-absorbent materials.⁹ Recently, high-strength and high-toughness polymer hydrogels have aroused extensive attention due to their great potential in load-bearing soft tissue applications, such as in tendons and ligaments, cartilage, and muscle.¹⁰

To obtain hydrogels with ideal mechanical properties, various hydrogels have been prepared, including slide-ring (SR) hydrogels,^11,12 tetra-PEG hydrogels,¹³ nanocomposite (NC) hydrogels,¹⁴ hydrophobic modified hydrogels,¹⁵ macromolecular microsphere composite (MMC) hydrogels,¹⁶ dipole–dipole or hydrogen bonding-reinforced hydrogels,¹⁷ double-network hydrogels (DN hydrogels),¹⁸ and ionically cross-linked hydrogels.^19–22 SR hydrogels and tetra-PEG hydrogels exhibit high stretching ratios without fracture due to their homogeneous structures. Due to the presence of clay nanoplatelets, which act as multifunctional crosslinkers in the formation of polymer/clay networks, NC hydrogels manifest excellent mechanical properties and deformation mechanisms.^23,24 DN hydrogels can be considered as interpenetrating polymer network (IPN) hydrogels consisting of brittle and ductile networks. In the course of necking deformation, the first (brittle) network fragments into small clusters, and the clusters play a cross-linker role in the second (ductile) network, resulting in effective dissipation of energy and prevention of catastrophic crack propagation upon loading.¹⁰

On the basis of the DN strategy, ionically cross-linked hydrogels replaced undesirable irreversible covalent bonds with sacrificial and reversible non-covalent bonds to allow the sacrificial bonds to be regenerated. Gong's group has reported a new class of physical polyampholyte (PA) hydrogels obtained from oppositely charged cationic monomers and anionic monomers.^19,21,25 The complex ionic structures of PA hydrogels have a wide strength distribution, which enables dynamic crosslinking with an extremely wide lifetime scale. The strong, long-lived bonds composed of numerous ionic bonds provide permanent crosslinking, imparting elasticity, whereas the weak, short-lived bonds composed of a few ionic bonds can break and reform at deformation, providing reversible crosslinking to dissipate energy. By this mechanism, Ihsan et al.^25,26 synthesized a polyampholyte P(NaSS-co-DMAEA-Q) system from p-styrenesulfonate (NaSS) and acryloyloxethyltrimethylammonium chloride (DMAEA-Q) using UV light under argon atmosphere in a glovebox for 8 h. This hydrogel, containing ∼50 wt% water, is highly stretchable and tough, exhibiting a fracture stress of σ_b ∼ 0.4 MPa, a fracture strain of ε_b ∼ 30 and a work of extension at fracture of w_ext ∼ 4 MJ m⁻³; it can be ranked among the toughest materials. Because this method is universal for polyelectrolyte complexes, hydrogels with various combinations of biopolymers and/or synthetic polyelectrolytes may be developed. Therefore, it is highly necessary to understand the structure–property relationships from the original monomer to the final hydrogel for a rational design.

Beyond the advantages of PA hydrogels, it cannot be ignored that the UV polymerization conditions of synthesizing PA gel are quite stringent. For example, the reaction takes place in a glovebox with argon atmosphere (oxygen below 0.1 ppm); therefore, the costs of the facility and the protective gas are quite high, which limit industrial applications. Moreover, the photo-polymerization is heavily dependent on the thickness and transparency of the solution because of the limited transmissivity of UV, which will limit the formation of thick and complex products. To solve these problems, free radical polymerization by a redox system is often used to prepare hydrogels. For example, the most widely employed redox system is ammonium peroxydisulfate (APS) with the water-soluble aliphatic amine [N,N,N′,N′-tetramethylenediamine (TEMED)]. Although APS can be decomposed into free radicals at a high temperature (usually above 70 °C), APS/TEMED redox systems also can be developed below room temperature, even below 0 °C, with bubbling nitrogen.^27–29 Herein, absolutely anaerobic conditions are not necessary for the APS/TEMED redox polymerization system. The existing literature led us to consider that it should not be difficult to use the APS/TEMED redox system to synthesize PA hydrogels.

To simplify the synthesis conditions and understand the effects of the monomer structure on the properties of PA hydrogels, in this study, we synthesized two different PA hydrogels using the APS/TEMED redox system by one-step methods. In one hydrogel, the gel was randomly copolymerized from oppositely charged monomers, sodium p-styrenesulfonate (NaSS) and [2-(methacryloyloxy)ethyl]trimethylammonium chloride (MATAC), without any chemical crosslinking; this is denoted as G-M and is shown in Fig. 1. For the other hydrogel, NaSS and acryloyloxethyltrimethylammonium chloride (DMAEA-Q) systems were prepared under the same procedure as compared, denoted as G-D. It should be noted that only a slight difference of a methyl group exists in the MATAC monomer compared with DMAEA-Q. It is surprising that enormous differences in the structures and properties of the two hydrogels are exhibited. A systematic study of the role of the methyl group in the monomer on the structure and properties of PA hydrogels has been conducted.

Fig. 1 Schematics of physical hydrogels composed of polyampholytes. (a) Illustration of polyampholyte networks with ionic bonds of different strengths. (b) The chemical structures of the monomers used in this study. Cation monomers: MATAC, DMAEA-Q; anionic monomer: NaSS.

2. Materials and methods

2.1 Materials

Sodium p-styrenesulfonate (NaSS, 90%) and methacrylatoethyl trimethyl ammonium chloride (MATAC, 75.0 wt%) were purchased from Aladdin; acryloyloxethyltrimethylammonium chloride (DMAEA-Q; 80.0 wt%) was purchased from J&K (Beijing, China); the thermal initiator, ammonium persulfate ((NH₄)₂S₂O₈, APS), and the co-initiator, N,N,N′,N′-tetramethylethylenediamine (TEMED), were obtained from Nanjing Tianhua Reagent Co. Ltd. NaCl was acquired from Sichuan West Long Chemical Co. Ltd. Unless otherwise specified, all reagents were used as received. Deionized water was used in all the experiments.

2.2 Synthesis of G-M and G-D physical polyampholyte hydrogels

The G-M and G-D hydrogels were synthesized using one-step random copolymerization with the APS/TEMED redox system. Briefly, a mixed aqueous solution with the prescribed total ionic monomer concentration of 2 mol L⁻¹, containing equal molar concentrations of anionic monomers and cation monomers, 1 mol% APS (in relative to the total monomer molar concentration, Table 1) as initiator, 2 mol% TEMED as co-initiator, and 0.5 M NaCl (used to control the ion strength) was injected into a reaction cell (10 cm × 10 cm) consisting of a pair of plates with a 1.5 mm silicone spacer. The polymerization was first carried out in an ice bath under nitrogen atmosphere for 6 h, and the cell was then sealed and placed at 4 °C in a refrigerator for 18 h. After polymerization, the as-prepared gel was immersed in a large amount of water (the water was changed every day for at least a week) to reach equilibrium and wash away residual chemicals and low molecular weight polymers. During this process, the osmotic pressure of the interiors of the hydrogels with NaCl was higher than that outside, resulting in exchange of Na⁺ and Cl⁻ in the water. Therefore, the NaCl and mobile counter-ions of the ionic copolymer were removed from the gel, and the oppositely charged ions in the copolymer formed stable ionic complexes either through intra- or inter-chain interactions.

Table 1 The formula, water content and volume shrinkage of the G-M and G-D gels

Sample	MATAC (g)	DMAEA-Q (g)	NaSS (g)	APS (mg)	TEMED (mg)	Water content (%)		Volume shrinkage (%)
						As-prepared	Equilibrium
G-M	5.28	0.00	4.79	91.3	93.0	62.0 ± 5.3	48.1 ± 5.1	98.3 ± 6.2
G-D	0.00	5.00	4.59	91.3	93.0	66.4 ± 7.1	39.6 ± 3.2	84.7 ± 7.6

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2.3 Characterization

2.3.1 Equilibrium, swelling ratio, and water content measurements. The as-prepared hydrogels, formed in glass plates with rectangle shapes, were cut into samples with fixed sizes and immersed in water to reach the equilibrium state. To verify the charge fractions, the elements of the dehydrated equilibrium samples were tested separately with an XSAM 800 X-ray photoelectron spectrometer (XPS, Kratos) and an elemental analyzer (Flash EA1112, ThermoFisher Scientific, BBOT as a standard sample). For the XPS test, Mg K_α radiation was the X-ray source, with a take-off angle of 90°.

The volume shrinkage ratio, Q_v = V/V₀, was defined as the ratio of the sample volume at swelling equilibrium, V, to that of the as-prepared state, V₀. The water content of the equilibrated gels, q, was calculated according to q = (m_w − m_d)/m_w, where m_w and m_d are the masses of the wet gel and dehydrated gel, respectively. The results reported are the mean values for at least three replicates.

2.3.2 Tensile tests. The tensile measurements were performed on a commercial tensile tester (HZ-1004 Dongguan lixian instrument Scientific Co. Ltd) at a stretching rate of 100 mm min⁻¹ in air with a humidifier6 (KD-2331-6 Kingdom, 25 W). The gels were cut into dumbbell shapes with lengths of 35 mm (L), gauge lengths of 12 mm (L₀), and widths of 2 mm (w). For the cyclic tensile tests, both loading and unloading were performed at a constant velocity of 100 mm min⁻¹; the tests were carried out in a humid environment to prevent water from evaporating from the samples. All the mechanical properties were measured in an air-conditioned room to maintain the temperature (∼25 °C) and humidity (∼70%). The results reported are the mean values for at least three replicates.

2.3.3 Rheometry. To qualitatively evaluate the molecular weights of the polymers, viscosity tests of the dissolved PA solutions were performed using a DHR-3 rheometer equipped with concentric cylinders. The frequency sweep test was performed within the range of 1–100 s⁻¹ at 20 °C. Before the test, 2.5 g of the G-M gel and the G-D gel was respectively dissolved in 100 mL 3 M saline solution at room temperature (20 °C) for two days, and homogeneous aqueous solutions were obtained.

The rheological behaviors of the gels in an equilibrated state were analyzed using a DHR-3 rheometer equipped with 25 mm parallel plates. The frequency sweep tests were performed within the range of 0.01–15.91 Hz, in which the shear strain amplitude was fixed at 0.5% with a temperature range of 0.1–90 °C. The temperature sweep tests were performed in the range of 0.1–96 °C, in which the frequency and strain were fixed at 1 Hz and 0.5%, respectively. The disc-shaped samples with thicknesses of ∼1 mm and diameters of 25 mm were adhered to the plates with super glue and surrounded by water to maintain hydration.

2.3.4 Water contact angle (WCA). WCA measurements were conducted with a Drop Shape Analysis System (DSA 100, Kruss, Hamburg, Germany). Before the test, the gel was stuck on a clear glass plate and the surficial water was wiped away with lens-cleaning papers. Then, the air-facing side of the gel was measured using 3.0 µL deionized water in an air-conditioned room to maintain the temperature (∼25 °C) and humidity (∼70%). The results reported are the mean values for five replicates.

2.3.5 Scanning electron microscopy (SEM). The test specimens were cryogenically shaped in liquid nitrogen to avoid shrinkage in the drying process and then prepared by freeze-drying. Cross-sections of the dried samples were covered with a thin layer (15 nm) of gold and observed with a SEM (Nova Nano SEM FEI Company) operating at 15 kV.

3. Results and discussion

3.1 Equilibrium, swelling ratio and water content

After dialysis, both the G-M and the G-D gels reached an equilibrium state, as confirmed by XPS and element analysis. As shown in Fig. 2a, only elements of C, O, S, and N were detected by XPS. The absence of Na and Cl indicates complete removal of the salt and the mobile counter-ions in the gels. In addition, the true charge ratios, f_true, of the G-M and G-D gels were 0.5, which is the charge balance point, as shown in Table 2. In neutral polyampholytes, Coulomb attraction prevails over repulsion, and the ionic bonds are enhanced during water dialysis.^19,30 Therefore, both the hydrogels shrank, and their water content decreased, as shown in Fig. 2b and Table 1. In their as-prepared states, both G-M and G-D possess water content around 65%. After dialysis (equilibrium), the water content of G-D drops to around 40%; however, that of G-M remains at around 50%, which indicates that the water content of G-D decreases more heavily. Correspondingly, the volume shrinkage ratio of G-D is much lower than that of G-M, as shown in Fig. 2c. In addition, the two hydrogels show very distinct transparencies. G-M is transparent, while G-D is semitransparent, as shown in Fig. 2d. This implies that the phase separation structure in G-M is more uniform than that of G-D. Apart from the above difference, the shrinking behavior and water content of both gels prepared with APS/TEMED redox systems, in this study, were quite similar to those of previously reported gels, which were prepared by strict UV photo-polymerization.²⁵ Notably, the degrees of water decrease and shrinkage of G-D are greater than those of G-M; this is unexpected because the number of methyl groups in the DMAEA-Q monomer used to synthesize G-D is less than that in the MATAC monomer used to synthesize G-M. Generally, higher methyl group content in a polymer results in a more hydrophobic gel, with lower hydration. The interaction of ionic bonds to form the hydrogel should be taken into account to answer this puzzle. Based on the molecular chain of the G-M gel, with the presence of steric hindrance from the methyl groups, the molecular chain of G-M is more hydrophobic than that of G-D. Because the hydrophobic moiety can stabilize ionic bonding in water, the degree of shrinkage of G-D is much greater than that of G-M. Without the steric hindrance of the methyl group in G-D, the polymer chains can stack flexibly, resulting in overlapping and aggregation. Considering the stronger ionic bonds and dense chain stacking of G-D, it is reasonable that the degree of water decrease and shrinkage of G-D is stronger than that of G-M. Moreover, the molecular chain of G-D is more flexible; therefore, it can form a more compacted network with better folding and curling, and is thus semitransparent.

Fig. 2 (a) XPS spectra of the G-M and G-D gels. (b) The water contents of the G-M gel and the G-D gel before and after dialysis. (c) The volume shrinkage ratios of the G-M gel and the G-D gel. (d) Photographs of G-M and G-D samples.

Table 2 Weight percentages of elements in the G-M and G-D gels

Sample	C (wt%)	H (wt%)	N (wt%)	S (wt%)	Cl^b (wt%)	ftrue^a
a ftrue: the true charge fraction of NaSS in the gel by element analysis.b Cl (wt%) by XPS.
G-M	52.68	7.90	3.55	8.23	0	0.50
G-D	52.35	7.51	3.85	8.90	0	0.50

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3.2 Mechanical properties of hydrogels

Fig. 3a shows the tensile stress–strain curves of G-M and G-D in the as-prepared state and at equilibrium. The corresponding data are listed in Table 3. Apparently, the tensile properties of G-M are dramatically different from those of G-D. G-M is much stiffer and stronger than G-D. For the as-prepared samples, both G-D and G-M are very soft and highly stretchable. The elongation at break and the tensile strength of as-prepared G-M are 4900% and 0.06 MPa, whereas the as-prepared G-D is too soft for determination of these properties. After dialysis, both G-M and G-D become tough. The ultimate tensile strength, Young's modulus, elongation at break, and work of extension at fracture of equilibrium G-M are 0.81 MPa, 0.053 MPa, 3743%, and 10.3 MJ m⁻³, respectively. As for G-D, it shows an ultimate tensile strength of 0.5 MPa, Young's modulus of 0.039 MPa, work of extension at fracture of 27.8 MJ m⁻³, and super-high elongation at break of 12 600%, which are superior to a PA hydrogel with the same components synthesized by photo-polymerization²⁵ (ultimate tensile strength: 0.4 MPa, elongation at break of ∼3000%). This result indicates that the tough PA hydrogels were successfully prepared via a more facile and economic redox method. The excellent mechanical properties of both hydrogels are dependent on their charge equilibrations, molecular weights, and the strength and distribution of their ionic bonds, which will be discussed later. Moreover, the extra addition of 0.5 M NaCl in the course of synthesis has some positive effects on both hydrogels. One positive effect is matching with the ions in the copolymer to avoid aggregation of the copolymer molecular chains during polymerization; the other is providing osmotic pressure during the dialysis process to aid the removal of Na⁺, Cl⁻, and other residues inside the hydrogels. Thus, oppositely charged ions in the random copolymer form multiple stable ionic complexes either through intra- or inter-chain interactions.

Fig. 3 Mechanical behaviors of the G-M and G-D hydrogels. (a) The stress–strain curves of G-M and G-D at equilibrium and the as-prepared G-M. The inset plot is a comparison of the elongations at break of G-M and G-D. (b) and (c) Recovery of the stress–strain curves of G-M and G-D for different waiting times performed by cyclic tensile tests. (d) Waiting time dependence of the hysteresis ratio (area ratio of the second hysteresis loop to the first).

Table 3 Physical properties of the G-M and G-D gels

Sample	Young's modulus (MPa)	Ultimate tensile strength (MPa)	Elongation at break (%)	Fracture energy (MJ m^−3)
G-M as-prepared	<0.001	0.06 ± 0.01	4900 ± 301	1.5 ± 0.3
G-M	0.053 ± 0.006	0.81 ± 0.25	3743 ± 342	10.3 ± 1.5
G-D	0.039 ± 0.005	0.50 ± 0.13	12 600 ± 1283	27.8 ± 3.6

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Although the polymerization method of the hydrogels in this study is different from previous reports, the interaction to form crosslink networks is still ionic; ionic bonds show excellent self-recovery after severe deformation.¹⁹ To prove this, the self-recovery phenomena of the two hydrogels are shown in Fig. 3b–d. Both the G-M and G-D hydrogels achieve large hysteresis from the first loading–unloading cycle, as shown in Fig. 3b–c. The samples show manifest residual strain after the first unloading, and the residual strain decreases with increasing waiting time, which indicates the break and recovery of ionic bonds. Considering the recovery rate, the recovery involves both a quick process and a slow process, similar to that of previous PA hydrogels and ionomers.^21,26,31 Notably, the G-M hydrogel recovers faster than the G-D hydrogel, in the same time scale (Fig. 3d), which may be caused by uniform phase separation resulting from the steric hindrance of the methyl group in the monomer.

3.3 Structure analysis of G-M and G-D hydrogels by rheology

The rheology test is a comprehensive technique to study the structures of materials from microstructure to macrostructure. As the polymer chains in the PA hydrogels are tightly bonded by ionic bonds, it is difficult to directly measure the molecular weights of the polymers in the two gels. Alternatively, the viscosities of the two dissolved gels obtained from the rheology tests enable qualitative evaluation of the molecular weights of the polymers. As shown in Fig. 4, the two polymer solutions exhibit very similar viscosities. That is, the viscosity of G-M polymer solution is 0.014 Pa s and the viscosity of G-D polymer solution is 0.016 Pa s. Because of the slightly higher viscosity and greater flexibility of the G-D polymer chains with fewer methyl groups, the molecular weight of the G-D polymer would be slightly higher than that of the G-M polymer. This may be one reason why the G-D hydrogel yields a higher elongation at break.

Fig. 4 Viscosities of the dissolved G-M and G-D solutions at different frequencies.

On the other hand, rheological analysis allows the quantitative evaluation of the elastic and viscous responses of the materials. These values can provide a more direct correlation with the microstructures because the materials can be examined in their at-rest states without causing any disruption of their underlying structures.³² Fig. 5a and b show the master curves of G′, G″ and tan δ for the G-M and G-D hydrogels at different frequencies with a temperature of 30 °C. For both the G-M and G-D gels, G′ is larger than G″ over the entire frequency range from 10⁻⁴ to 10⁸ Hz, indicating that both gels, even without any chemical cross-linking, are always in the gel state, with predominantly elastic properties. The G-M gel shows a main peak at frequencies around 10³ and an upcoming peak beyond 10⁻⁴ Hz in its tan δ relaxation curve, which is attributed to the high-frequency relaxation of loosely bound counterions and the low-frequency relaxation of the linear polymer chain associated with fluctuation of the tightly bound counterions along the polymer chain.³³ On the other hand, the main peak and upcoming peak of the G-D gel shift to higher frequencies, around 10⁴ Hz and close to 10⁻⁴ Hz, respectively, indicating that the chains in G-D are much suppler than those in G-M. To quantitatively evaluate the chain mobility, the apparent activation energy (E_a) is obtained from the Arrhenius plot by depicting the shift factor of the master curve, a_T, to the temperature (1/T) of a sample.^34,35 As shown in Fig. 5c, the Arrhenius plots of the G-M and G-D hydrogels show two linear regions with reflection points ca. 40 °C and 30 °C, respectively, giving two activation energies, E_a. For G-M, E_a appears to be 137.8 kJ mol⁻¹ at a high temperature and 215.8 kJ mol⁻¹ at a low temperature; for G-D, E_a is 87.0 kJ mol⁻¹ at a high temperature and 177.5 kJ mol⁻¹ at a low temperature. All of these E_as are higher than the thermal energy k_BT (room temperature) but much lower than the covalent bond dissociation energy E_c–c of ∼347 kJ mol⁻¹ (140k_BT).³⁶ As a result, the ionic association can sustain a load during the initial deformation; however, these bonds rupture preferentially before the rupture of the covalent bonds of the polymer chains. Meanwhile, the E_a of the G-M gel is higher than that of the G-D gel, indicating the stronger ionic association in the G-M gel. This result is in good agreement with the stronger mechanical properties of G-M. Moreover, the chain mobility character was further determined by temperature sweeps of the two samples, as shown in Fig. 5d. The maximum peak of tan δ of G-M, representing the softening temperature, is also higher than that of G-D, that is, 17.2 °C for the G-M gel and 15.8 °C for the G-D gel. This indicates that the small steric hindrance from the methyl groups of the G-M gel not only decreases the flexibility of its molecular chains but also causes stronger ionic interactions in the bonding zone. It is also noteworthy that the broad character of the relaxation peak and the wide activation energies suggest that the gels have microphase-separated structures with wide distributions of ionic bond strength.

Fig. 5 Dynamic mechanical behaviors of the G-M gel and the G-D gel. (a) and (b) Frequency (Hz) dependence of the storage modulus, G′, the loss modulus, G″, and the loss factor, tan δ, of the G-M gel and G-D gel; these results were obtained by performing classical time–temperature superposition shifts at a reference temperature of 30 °C. (c) Arrhenius plot depicting the temperature dependences of the shift factors for the samples. The apparent activation energy values, E_a, were calculated from the slopes of the curves. (d) Temperature dependences of G′, G″, and tan δ of the G-M and G-D gels.

3.4 Hydrophilicity and morphology analysis of the G-M and G-D gels

The water contact angle (WCA) test is a quick, economical, and relatively simple technique to assess hydrophilicity. As shown in Fig. 6, the WCA of the G-M gel (∼50°) is smaller than that of the G-D gel (∼85°), despite the existence of nonpolar side methyl groups in the backbone of the G-M gel. The contact angle of a liquid on a solid depends on both the surface chemistry and topological structure.^37,38 Therefore, this result implies that the impact of the hydrophobic side methyl group in G-M is weaker than other hydrophilic factors. Herein, two hydrophilic factors should be taken into account. One is the water content in the bulk of the gels. As estimated above, the water content of G-M is much higher than that of G-D, which will increase the hydrophilicity of the surface of G-M. The other may be the structure of the network. It is well known that hydrophilicity is also strongly dependent on the roughness of a surface.³⁹ Because of the steric hindrance of the methyl group in the monomer, the homogenous stacking of polymer chains in G-M, showing transparency, leads to better water compatibility. To understand the microstructures of the gels, Fig. 7 shows the SEM photographs of the internal morphologies of the freeze-dried equilibrium G-M and G-D hydrogels. Three-dimensional (3D) porous network structures are clearly observed for both hydrogels, which is similar to the morphology obtained by UV polymerization in a previous report.³⁴ The pores of G-M range from 0.1 to 0.3 µm in size, slightly smaller than the pores of G-D. The network structure of G-M is stereoscopic, while the structure of G-D is collapsed. In more detail, the wall thickness of the dehydrated polymer of G-M is more homogenous and thinner than that of G-D. Also, some aggregated stacking polymers can be found in G-D, indicating that the phase separation in G-D is more significant than that in G-M. This result may be well explained by the steric hindrance of the methyl groups in the monomers of G-M. Due to the methyl groups, the molecular chains of G-M are more hydrophobic than those of G-D. As the hydrophobic moiety can stabilize the ionic bonding in the water, the ionic bond becomes strong, which leads to a homogeneous pore structure. Without the steric hindrance of methyl groups in G-D, the polymer chains can stack flexibly, resulting in overlapping and aggregation. Furthermore, the differences in the microstructures also correspond well with the distinct water contents and hydrophilicities of the two hydrogels. That is, the stereoscopic stacking polymers in G-M form a more homogeneous three-dimensional network structure which can retain more water and has greater hydrophilicity compared with the G-D gel.

Fig. 6 Water contact angles (WCAs) and corresponding photographs of the G-M and G-D gels.

Fig. 7 SEM photographs of the microstructures of the G-M and G-D gels; the red arrows indicate the aggregated stacking polymers.

3.5 Mechanism of the methyl group in the monomer regarding the structure and properties of the PA hydrogels

As aforementioned, two PA hydrogels were successfully synthesized by a facile one-step random copolymerization method using the APS/TEMED redox system. The mechanical properties of these hydrogels are comparable to, or even better than, those of hydrogels prepared under stricter conditions by UV polymerization. The slight steric bulk of the methyl group in the cationic monomer endows the final hydrogels with notable differences in their structures and properties. First, the slight steric bulk of the methyl group decreases the flexibility of the chains so that the two oppositely charged groups are less prone to form bonds. Due to this hindrance, only strong multiple ionic pairs tend to form bonds. Therefore, the chains with additional methyl groups in G-M stack homogenously and stereoscopically. In contrast, the chains without methyl groups in G-D are so flexible that they pack densely and exclude more water during water dialysis. Secondly, the steric bulk of the methyl group decreases the mobility of the chain, resulting in improvement of the stiffness of the gel. Although the water content of G-M is higher than that of G-D, the modulus, fracture stress, and the soft temperature of G-M are evidently higher than those of G-D. The homogeneous and stereoscopic structure of G-M increases the recovery rate. Thirdly, the steric bulk of the methyl groups would affect the reactivity ratio of copolymerization and the molecular weight of the polymer, which results in different distributions of charge and different ionic bonds strengths. These differences are confirmed by the rheology and microstructure results in Fig. 5 and 7. In addition, the rate of polymerization of G-D is much faster than that of G-M, as observed in our experiment.

In addition to the abovementioned properties, the slight steric bulk of the methyl groups shows distinct effects on the acid and alkali resistance, saline stability, and self-healing behaviors of the hydrogels. In brief, the G-M shows better acid/alkali resistance and saline stability, but slow self-healing. Further studies on those properties will be discussed in our upcoming report.

4. Conclusions

In summary, two different PA hydrogels with very different mechanical properties were successfully synthesized by random copolymerization from oppositely charged monomers using the APS/TEMED redox initiator system. The facile and economical redox initiator method can synthesize hydrogels with excellent properties which are comparable to those of PA hydrogels prepared by UV photo-polymerization. Although the cationic monomer of MATAC in G-M has only a slight difference of methyl groups in its side chains compared to the DMAEA-Q monomer in G-D, dramatic variances in the structures and properties of the two gels were achieved. Distinctively, G-M is strong and tough, while the G-D gel is soft and super stretchable. Due to slight steric hindrance from the methyl groups of the G-M gel, better distribution of the molecular chains in the G-M hydrogel aids the formation of strong ionic bonds and a homogenously stereoscopic structure. Moreover, the hydrophilicity and morphology tests also directly certify that the molecular chain of the G-M gel is folded more homogenously. These systematic studies on the relationships between the structures and properties of hydrogels will promote methods for designing strong, tough hydrogels.

Acknowledgements

This study is supported by the National Natural Science Foundation of China (51673126), the National Science Fund for Distinguished Young Scholars of China (51425305), and Chengdu Huimin Projects of Science and Technology (2015-HM01-00389-SF); this study is also financially supported by the State Key Laboratory of Polymer Materials Engineering (Grant No. sklpme2016-3-04).

References

1. A. S. Hoffman, Adv. Drug Delivery Rev., 2012, 64, 18–23.
2. D. Das, R. Das, P. Ghosh, S. Dhara, A. B. Panda and S. Pal, RSC Adv., 2013, 3, 25340–25350.
3. G. M. Raghavendra, T. Jayaramudu, K. Varaprasad, G. S. Mohan Reddy and K. M. Raju, RSC Adv., 2015, 5, 14351–14358.
4. D. Seliktar, Science, 2012, 336, 1124–1128.
5. J. Bastide and L. Leibler, Macromolecules, 1988, 21, 2647–2649.
6. H. Furukawa, K. Horie, R. Nozaki and M. Okada, Phys. Rev. Lett., 2003, 68, 031406.
7. A. Altunbas, S. J. Lee, S. A. Rajasekaran, J. P. Schneider and D. J. Pochan, Biomaterials, 2011, 32, 5906–5914.
8. L. Almany and D. Seliktar, Biomaterials, 2005, 26, 2467–2477.
9. F. Hua and M. Qian, J. Mater. Sci., 2001, 36, 731–738.
10. J. P. Gong, Soft Matter, 2010, 6, 2583–2590.
11. Y. Okumura and K. Ito, Adv. Mater., 2001, 13, 485–487.
12. K. Kato, K. Karube, N. Nakamura and K. Ito, Polym. Chem., 2015, 6, 2241–2248.
13. T. Sakai, T. Matsunaga, Y. Yamamoto, C. Ito, R. Yoshida, S. Suzuki, N. Sasaki, M. Shibayama and U.-I. Chung, Macromolecules, 2008, 41, 5379–5384.
14. J. Yang, L. Zhu, X. Q. Yan, D. D. Wei, G. Qin, B. Z. Liu, S. Z. Liu and Q. Chen, RSC Adv., 2016, 6, 59131–59140.
15. R. Murphy, T. Borase, C. Payne, J. O'Dwyer, S.-A. Cryan and A. Heise, RSC Adv., 2016, 6, 23370–23376.
16. T. Huang, H. Xu, K. Jiao, L. Zhu, H. R. Brown and H. Wang, Adv. Mater., 2007, 19, 1622–1626.
17. Y. Zhang, Y. Li and W. Liu, Adv. Funct. Mater., 2015, 25, 471–480.
18. J. P. Gong, Y. Katsuyama, T. Kurokawa and Y. Osada, Adv. Mater., 2003, 15, 1155–1158.
19. T. L. Sun, T. Kurokawa, S. Kuroda, A. B. Ihsan, T. Akasaki, K. Sato, M. A. Haque, T. Nakajima and J. P. Gong, Nat. Mater., 2013, 12, 932–937.
20. J. Y. Sun, X. Zhao, W. R. Illeperuma, O. Chaudhuri, K. H. Oh, D. J. Mooney, J. J. Vlassak and Z. Suo, Nature, 2012, 489, 133–136.
21. F. Luo, T. L. Sun, T. Nakajima, T. Kurokawa, Y. Zhao, K. Sato, A. B. Ihsan, X. Li, H. Guo and J. P. Gong, Adv. Mater., 2015, 27, 2722–2727.
22. F. Luo, T. L. Sun, T. Nakajima, T. Kurokawa, A. B. Ihsan, X. Li, H. Guo and J. P. Gong, ACS Macro Lett., 2015, 4, 961–964.
23. J. Yang, L. Zhu, X. Yan, D. Wei, G. Qin, B. Liu, S. Liu and Q. Chen, RSC Adv., 2016, 6, 59131–59140.
24. B. Xu, H. Li, Y. Wang, G. Zhang and Q. Zhang, RSC Adv., 2013, 3, 7233–7236.
25. A. B. Ihsan, T. L. Sun, S. Kuroda, M. A. Haque, T. Kurokawa, T. Nakajima and J. P. Gong, J. Mater. Chem. B, 2013, 1, 4555–4562.
26. A. B. Ihsan, T. L. Sun, T. Kurokawa, S. N. Karobi, T. Nakajima, T. Nonoyama, C. K. Roy, F. Luo and J. P. Gong, Macromolecules, 2016, 49, 4245–4252.
27. F. Plieva, X. Huiting, I. Y. Galaev, B. Bergenstahl and B. Mattiasson, J. Mater. Chem., 2006, 16, 4065–4073.
28. T. Celik and N. Orakdogen, Macromol. Chem. Phys., 2015, 216, 2190–2201.
29. K. Kawabata, Y. Waki, T. Matsumura and S. Umemura, Proc.–IEEE Ultrason. Symp., 2004, 1502–1505.
30. P. G. Higgs and J. F. Joanny, J. Chem. Phys., 1991, 94, 1543–1544.
31. R. J. Varley, S. Shen and S. van der Zwaag, Polymer, 2010, 51, 679–686.
32. J. Y. Kim, J. Y. Song, E. J. Lee and S. K. Park, Colloid Polym. Sci., 2003, 281, 614–623.
33. T. Mitsumata, J. P. Gong, K. Ikeda and Y. Osada, J. Phys. Chem. B, 1998, 102, 5246–5251.
34. F. Luo, T. L. Sun, T. Nakajima, D. R. King, T. Kurokawa, Y. Zhao, A. B. Ihsan, X. Li, H. Guo and J. P. Gong, Macromolecules, 2016, 49, 2750–2760.
35. K. J. Henderson and K. R. Shull, Macromolecules, 2012, 45, 1631–1635.
36. A. A. Zavitsas, J. Phys. Chem., 1987, 91, 5573–5577.
37. F. Guo and Z. Guo, RSC Adv., 2016, 6, 36623–36641.
38. L. Zhai, F. C. Cebeci, R. E. Cohen and M. F. Rubner, Nano Lett., 2004, 4, 1349–1353.
39. C. Yang, U. Tartaglino and B. N. J. Persson, Phys. Rev. Lett., 2006, 97, 116103.

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