Rheological Behavior of Tough PVP-in Situ-PAAm Hydrogels

Oct 21, 2016 - *E-mail: [email protected] (H.W.)., *E-mail: [email protected] (R.A.W.). ... Mechanical and Rheological Behavior of Hybrid Cross-Linked...
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Rheological Behavior of Tough PVP-in Situ-PAAm Hydrogels Physically Cross-Linked by Cooperative Hydrogen Bonding Guoshan Song,† Zhiyang Zhao,‡ Xin Peng,† Changcheng He,† R. A. Weiss,*,‡ and Huiliang Wang*,† †

Beijing Key Laboratory of Energy Conversion and Storage Materials, College of Chemistry, Beijing Normal University, Beijing, 100875, China ‡ Department of Polymer Engineering, University of Akron, 250 South Forge Street, Akron, Ohio 44325-0301, United States S Supporting Information *

ABSTRACT: Rheology studies were performed on tough PVP-in situ-PAAm hydrogels physically cross-linked by cooperative hydrogen bonding to understand their viscoelastic response and, hence, the interactions and microstructure. The viscoelasticity of the PVP-in situ-PAAm hydrogels was strongly affected by the monomer ratio (CAAm/CVP). Hydrogels prepared with a high monomer ratio exhibited weak time, temperature and frequency dependence of the viscoelastic properties, similar to those of chemically cross-linked hydrogels. The storage modulus (G′) of the gels was much greater than the loss moduli (G″) and low loss factor (tan δ < ∼ 0.1), which indicated that they were solid-like, and mostly elastic. These supramolecular gels exhibited a strain- and CAAm/CVP-dependent reversible gel (solid) to viscoelastic liquid transition due to the dynamic nature of the cooperative hydrogen bonds. That transition also coincided with the onset of nonlinear viscoelastic behavior. The addition of a low molecular weight compound, urea, that competes for hydrogen bonding sites weakens the gel by decreasing the effective cross-link density or weakening the intermolecular hydrogen bonding.

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INTRODUCTION Hydrogels have many applications in the pharmaceutical, biomedical, and industrial fields,1−6 so they have been of great interest to scientists for many years. However, most synthetic hydrogels are brittle and therefore poor candidates for many applications. Tough hydrogels, such as slide-ring (SR) gels,7 nanocomposite (NC) gels,8,9 double-network (DN) gels,10,11 tetra-arm poly(ethylene glycol) (PEG) gels,12 and hydrogels based on polyfunctional initiating and cross-linking centers (PFICC),13−16 have been developed in recent years. More recently, tough hydrogels have also been prepared by utilizing supramolecular, physical cross-links, such as hydrogen bonding,17 ionic interactions,18−22 hydrophobic interactions,23,24 crystallization,25 and interpolymer complexation.26,27 Recently, we reported the synthesis and properties of tough PVP-in situ-PAAm hydrogels by the in situ polymerization of acrylamide (AAm) in the presence of a pre-existing polymer (polyvinylpyrrolidone, PVP).28 The PVP-in situ-PAAm hydrogels exhibit excellent mechanical properties, e.g., high tensile (1.20 MPa) and compressive strengths (17.0 MPa at 95% strain), extremely high extensibility (4200%), and excellent resilience. Very recently, several types of tough hydrogels based on the physical cross-linking of hydrogen bonding have also been reported.29−31 The PVP-in situ-PAAm hydrogels are physical gels with supramolecular cross-links due to hydrogen bonding. No chemical cross-linking agents were used that would introduce covalent cross-links during the synthesis. In addition to their very high mechanical strength, which was higher than most chemical hydrogels, they behaved much like chemical gels in © XXXX American Chemical Society

other aspects. For instance, they were highly swollen by water, but they did not dissolve, and they exhibited excellent resilience with very low hysteresis and residual strain during cyclic tensile testing. Comparative synthesis experiments showed that simply mixing PVP and PAAm aqueous solutions did not produce hydrogels, and the copolymerization of N-vinylpyrrolidone (VP) and AAm produced mechanically weak materials that were easily dissolved with water. Those results indicate that the presence of pre-existing PVP chains is crucial for the gel formation and there must be very strong supramolecular interactions between PVP and the in situ polymerized PAAm chainsi.e., intermolecular hydrogen bonding interactions. Molecular modeling confirmed that there should be a significant cooperative effect of H-bonding between PAAm molecules and the PVP. In addition, DSC analyses of the glass transition temperatures (Tg) of the dried PVP-in situ-PAAm with different PVP/PAAm compositions showed that one of the two Tg’s exhibited by the dried hydrogels was always higher than those of the component polymers, which is significantly different than for the case of simple blends of PVP and PAAm where a single Tg that deviated positively from the Fox predictions for a miscible blend was observed.32 Similar results of positive deviation of Tg from the Fox predictions for miscible polymer blends have been observed for blends where there were strong intermolecular interactions between the components.33 The appearance of the abnormally high Tg in the PVPReceived: July 6, 2016 Revised: September 29, 2016

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Rheological Measurements. Linear and nonlinear oscillatory shear measurements were performed with a TA Instruments ARES-G2 rheometer equipped with 8 mm parallel plates and a solvent trap filled with silicone oil to minimize water evaporation. The linear viscoelastic (LVE) response region for each sample was determined at T = 25 °C using a strain sweep from γ = 0.01% to 100% at constant frequency, ω = 6.2 rad·s−1. Frequency-dependent LVE data were obtained at T = 25 °C and a strain amplitude of γ = 0.5% over a frequency range of 0.01− 100 rad·s−1, and temperature-dependent LVE data were obtained at a fixed frequency (ω = 6.2 rad·s−1) and strain (γ = 0.5%) covering a range of T = 25 to 80 °C. The time dependence of the viscoelastic properties was determined by measuring the storage modulus (G′) and loss modulus (G″) as a function of time (t) at 25 °C at a fixed frequency (ω = 6.2 rad·s−1) and a fixed strain (γ = 0.5%). Nonlinear viscoelastic properties were measured at 25 °C, ω = 6.2 rad·s−1 and a range of strain amplitudes γ = 0.01−250%. Cyclic strain sweeps in a shear strain (γ) range of 0.01−200% were performed with a PVP-in situ-PAAm hydrogel at 25 °C and at a fixed frequency (ω = 6.2 rad· s−1). The cycles were successively performed without a time interval.

in situ-PAAm gels is attributed to the strong cooperative Hbonding interactions between the PVP and PAAm chains. Our previous study also showed that the mass ratio of PVP and AAm (or PAAm) is an important factor affecting the formation and the mechanical properties of the gels. Tough hydrogels were only obtained when the mass ratio of PAAm to PVP was greater than 10, suggesting that the fraction of PAAm chains that are not H-bonded with PVP is also very important. At a fixed AAm concentration, an increase of PVP concentration produced a decrease of the elastic modulus and tensile strength of the hydrogel. In the extreme case where the mass ratio of AAm to PVP was less than or equal to 1, a hydrogel was not formed. Those results indicated that the cross-linking between PVP and PAAm differed from conventional chemical and physical gels where the concentration of cross-links increases with increasing cross-linker concentration. As with all physical hydrogels, the PVP-in situ-PAAm gels were viscoelastic materials. Shear rheology is an effective technique for characterizing the viscoelasticity of gel materials by measuring the mechanical response of a sample as it is deformed under a periodic stress (or strain), which establishes the relationships between the mechanical behavior and molecular motions in the materials. In this work, PVP-in situPAAm gels with different monomer ratios were prepared and their linear and nonlinear oscillatory shear behaviors were studied in order to achieve a better understanding of the structure and properties of these hydrogels.

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RESULTS AND DISCUSSION The tough PVP-in situ-PAAm hydrogels are physically crosslinked, mainly by hydrogen bonding (H-bonding) and chain entanglement. There are two types of H-bonding interactions contributing to the formation of the gels, strong cooperative Hbonding interactions between the pre-existing PVP chains and the in situ polymerized PAAm chains and noncooperative Hbonding interactions between the PAAm chains (Scheme 1). Scheme 1. Intermolecular Hydrogen Bond Cross-Links in the PVP-in Situ-PAAm Hydrogels

EXPERIMENTAL SECTION

Materials. Poly(N-vinylpyrrolidone) (PVP, Mw = 4.0 × 10 , high purity grade) and acrylamide (AAm, ultrapure grade) were purchased from Amresco Inc. (Solon, OH). Hydrogel Preparation. The detailed hydrogel preparation procedure can be found in our previous paper.28 Aqueous solutions containing PVP and AAm were transferred into molds made by placing a 2 mm silicone spacer between two glass plates. Dissolved oxygen in the solutions was removed by vacuum evacuation and exchanged with high-purity nitrogen three times. The AAm was polymerized at 56 °C for 36 h to obtain PVP-in situ-PAAm hydrogels. The compositions of the hydrogels are listed in Table 1. The conversion of monomer to 4

Table 1. Composition of the PVP-in situ-PAAm Hydrogels CVP (mol L‑1)

CAAm (mol L‑1)

CAAm/CVP

water content (%)

νe (mmol·L‑1)

0.15 0.15 0.15 0.15 0.15 0.31 0.46 0.61 0.77

2.0 3.0 4.0 5.0 6.0 5.0 5.0 5.0 5.0

13 20 26 33 39 16 11 8.2 6.5

86 81 77 73 69 71 70 68 66

25.6 41.7 63.8 96.7 99.2 62.4 52.6 43.6 33.5

To better understand the contribution of the two types of Hbonding to the viscoelasticity of the gels, two series of PVP-in situ-PAAm hydrogels were prepared by varying the AAm concentration (CAAm) or PVP concentration (CPVP) at a fixed C PVP or C AAm, respectively (Table 1). The monomer concentrations, molar ratios of AAm/PVP and the water contents of the as-prepared hydrogels are summarized in Table 1. The notation used herein for the hydrogels is Gelx, where x is the monomer ratio (CAAm/CVP) of AAm to N-vinylpyrrolidone (VP) (Table 1). Time and Temperature Dependence of the LVE Properties of the Hydrogels. The time and temperature

polymer and the molecular weights of the in situ formed PAAm in the presence of a fixed concentration of PVP (CVP = 0.15 mol L−1) are shown in Table S1. The materials were swollen with water, though not to equilibrium, and the water contents of the as-prepared hydrogels are listed in Table 1. The PVP-in situ-PAAm hydrogels were also swollen with aqueous solutions of urea (1 mol L−1), which competes with AAm for Hbonding with PVP, for 48 h and then dried at ambient temperature until their water contents were the same as the as-prepared hydrogels. B

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Figure 1. Storage modulus (G′), loss modulus (G″), and loss factor (tan δ) of the gel33 as a function of time (a) and temperature (b). All measurements were made with ω = 6.2 rad·s−1 and γ = 0.5%.

Figure 2. (a) Storage modulus (G′), (b) loss modulus (G″), and (c) loss factor (tan δ) as a function of frequency for the PVP-in situ-PAAm hydrogels prepared with a fixed CPVP (0.15 mol L−1) but varying CAAm (2.0−6.0 mol L−1). γ = 0.5%; T = 25 °C.

behavior was similar to a covalent cross-link. The solid-like elastic nature of this gel is also consistent with the very small value of tan δ < 0.02,. Figure 1b shows that G′ was temperature independent and G″ increased only slightly between T = 25−80 °C. Rubber elasticity theory35 predicts that G′ should increase linearly with T and ν22/3, where ν2 is the volume fraction of polymer, so over the range of temperatures probed in these experiments that would produce an increase in G′ by only a factor of ∼1.2, assuming that the water swelling ratio did not change significantly. However, increasing temperature is also expected to weaken the interpolymer hydrogen bonds that produce the supramolecular cross-links, and although it was not measured one might expect that if anything the effective crosslink density of the gel would decrease and the swelling ratio

dependences of the gel mechanical properties provide an indication of how robust is the physical network, since the supramolecular bonds are expected to weaken with increasing temperature.34 The time dependence of G′, G″, and tan δ at 25 °C for the Gel33 is shown in Figure 1a and the temperature dependence of G′ and G″ is shown in Figure 1b. The elastic response, G′, of Gel33 remained constant over a continuous oscillatory shear (ω = 6.2 rad·s−1 and γ = 0.5%) duration of >200 s (Figure 1a), and there was only a very small decrease of G″, which characterizes the viscous response of the gel. The near time-independence of the viscoelastic properties indicates that the physical bonds that form the gel are sufficiently strong that the gel behaved nearly elastically, which is what would be expected from a cross-link that was time-independent over the duration of the experimenti.e., its C

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with higher CAAm of 5.0 and 6.0 mol L−1, which is even lower than those of some covalently cross-linked gels,38 indicating almost no viscous dissipation in the mechanical behavior of the PVP-in situ-PAAm gels. Tan δ of an ideal covalent (i.e., all network chains support stress) gel should be zero,23,38,39 so the extremely low tan δ values of the PVP-in situ-PAAm gels indicate very few imperfections of the cross-linked network (e.g., dangling chains, un-cross-linked chains and/or loops that do not support stress) that produce a viscous response to deformation.23 The frequency dependence of the PVP-in situ-PAAm hydrogels for different temperatures was also measured with the Gel33 (Figure S2). G′ decreased slightly and G″ increased with increasing T. These data indicate that the viscous response of the gel increased with increasing T, but the effect of T was small within the range probed. The solid-like mechanical response of the PVP-in situ-PAAm hydrogels was also demonstrated by the frequency dependence of the complex viscosity (η*, Pa·s), Figure 3. A linear

increase (i.e., ν2 decrease) as the temperature increased. Note that any weakening with temperature of the H−bonded network was not sufficient for dissolving the two gel components. The small, but clear increase of G″ with increasing T in Figure 1b is consistent with a weakening of the network structure, which would increase the viscous response of the gel. However, again, that was a small effect over the T range probed. Frequency Dependence of the Viscoelastic Properties of the Hydrogels. The frequency dependence of G′, G″, and tan δ of the PVP-in situ-PAAm hydrogels with a fixed CVP but varying CAAm is shown in Figure 2. For all hydrogels, G′ (Figure 2a) was much greater than G″ (Figure 2b) over the entire frequency range, which indicates the solid-like, elastic nature of the gels. G′ and G″ increased with increasing CAAm/CVP (Figure 2a,b), which was due to increases in the cross-link density of intermolecular H-bonds and decreases in the equilibrium water swelling, with increasing PAAm concentration, see Table 1. The cross-link density value due to H-bonds reported in Table 1 was calculated from the equation of rubber elasticity,35 ⎡ 2⎤ G′ ≈ G = ⎢1 − ⎥RTvev2 2/3 ⎣ ϕ⎦

(1)

where νe is the cross-link density of the gel, v2 is the volume fraction of polymer in the hydrogel, T is temperature and ϕ is the functionality of the cross-links, which was assumed to be 4 for an intermolecular hydrogen bond. The volume fraction polymer, ν2, was calculated from Equation 2. −1 ⎛ (qF − 1)ρ ⎞ ν2 = ⎜⎜1 + ⎟⎟ d ⎝ ⎠

(2)

where qF is the mass of swollen gel divided by the mass of the dry polymer, ρ is the copolymer density (∼1.15 g/mL) and d is the density of water (1.00 g/mL). The cross-link densities of the gels were calculated using the value of G′ at ω = 100 rad·s−1 and T = 298 K. Unfortunately, for any physical hydrogel, modifications of the structure, such as achieved by changing the composition, change the cross-link density and the equilibrium water swelling of the polymer. The two variables are coupled so it is impossible to vary the composition and have the same crosslink density and polymer concentration for the gel at equilibrium. However, as was discussed in a previous paper,28 the modulus of the gels swollen to the same water content of 90 wt % and the equilibrium swollen state were measured, and those data, Figure S1, clearly show that the modulus increased with increasing cross-link density (i.e., since the gels were not in their equilibrium swollen state, the polymer concentration was held constant so that the only variable left in eq 1 that could change the modulus was the effective cross-link density. The value of G′ of the hydrogels increased an order of magnitude as the monomer ratio increased from CAAm/CVP = 13 to 39. The higher value, G′ = 50 kPa is comparable or even higher than the values reported for chemically cross-linked PAAm hydrogels36 and PAAm-clay nanocomposite hydrogels37 with similar water contents. The tan δ of the hydrogels decreased with increasing CAAm/CVP (Figure 2c), which is consistent with the increasing cross-link density as the PAAm concentration increased, which made the gel more solid-like and elastic. With the exception of the Gel13 prepared with a low CAAm of 2.0 mol L−1, tan δ of the gels was generally less than 0.07 and even lower than 0.02 for the Gel33 and Gel39 prepared

Figure 3. Complex viscosity (η*) as a function of frequency (ω) calculated from the LVE data in Figure 2 for the PVP-in situ-PAAm hydrogels prepared with a fixed CVP (0.15 mol L−1), but varying CAAm (2.0−6.0 mol L−1). γ = 0.5%; T = 25 °C.

relationships between log η* and log ω with a slope of −1 correspond to a perfectly elastic solid, such as an ideal chemically cross-linked gel.23,39 For the PVP-in situ-PAAm hydrogels the slope of the lines in Figure 3 varied from 0.94 to 0.99 and increased with increasing cross-link density (i.e., increasing CAAm/CVP), which again is consistent with the conclusion that the network structure in these gels was highly efficient and the gels were essentially elastic over the frequency range studied. Our previous paper28 showed that, at a given CAAm, CVP has a profound effect on the mechanical properties of the hydrogels, the tensile strength and modulus decreased dramatically when the CAAm/CVP was less than 11. The results in Figure 4 shows the frequency dependence of shear moduli (G′ and G″) and loss factor (tan δ) of the PVP-in situ-PAAm hydrogels prepared at a fixed CAAm (5.0 mol L−1) and varying CVP over a three decades of frequency. Increasing CPVP (i.e., decreasing CAAm/ CVP) lowered G′ and increased G″ and tan δ of the PVP− PAAm hydrogels. In addition, the hydrogels prepared with a lower CAAm/CVP ratio showed more significant frequency dependence. G′ of the hydrogels increased with increasing frequency (Figure 4a), which is a consequence of the viscoelastic nature of the gels. That is, the gel is more elastic at the higher frequency, since there is less time during a stress D

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Figure 4. (a) Storage modulus (G′), (b) loss modulus (G″), and (c) loss factor (tan δ) as a function of frequency for the PVP-in situ-PAAm hydrogels prepared with a fixed CAAm (5.0 mol L−1) but varying CPVP (0.15−0.77 mol L−1). γ = 0.5%; T = 25 °C.

cycle for the material to relax. G″ and tan δ decreased with frequency (Figure 4b,c), with the more significant decrease occurring at ω < 0.1 rad·s−1. Those results are also consistent more relaxation of the gel, i.e., viscous dissipation, occurring at the lower frequenciesi.e., longer time scale. tan δ at the lowest frequency (ω = 0.1 rad·s−1) increased with decreasing CAAm/CVP ratio and reached a value of tan δ ∼ 0.4 for Gel6.5. The significance of the large upturn of tan δ at low frequency is that it is most likely associated with a peak in the loss modulus (and tan δ) representing the characteristic relaxation time of the supramolecular interactions. On the basis of results for other physical hydrogels, that relaxation corresponds to a gel (solid) to liquid transition of the system. The actual peak would occur for ω < 0.1 rad·s−1, so the relaxation time of the supramolecular bonds, τ > 63 s. Nonlinear Viscoelastic Behavior of the Hydrogels. Figure 5(a) plots the G′ and G″ as a function of shear strain (γ). The hydrogels showed a significant strain-dependent viscoelastic response, which was expected for a supramolecular gel. LVE behavior, where the measured stress (or the complex

Figure 5. G′ (solid symbols) and G″ (open symbols) as a function of shear strain (γ) for the PVP-in situ-PAAm hydrogels prepared with a fixed CPVP (0.15 mol L−1) but varying CAAm (2.0−6.0 mol L−1). ω = 6.28 rad/s; T = 25 °C.

(viscous response) and the change of the mechanical properties to G″ > G′ signifies a solid (gel) to a liquid (viscous fluid) transition. Following that transition, the continued decrease of G″ and G′ indicates that the viscosity and elasticity of the liquid decreased with increasing strain amplitude. The presence of a pronounced maximum in the G″ has been classified as type III (weak strain overshoot) behavior by Hyun et al.,40 which is observed in soft glassy systems, concentrated emulsions, suspensions, highly entangled polymer solutions, block copolymer solutions and associative polymer solutions.41 Under a large shear deformation, the molecular arrangements in the hydrogels are moved far from equilibrium. Physical bonds, such as hydrogen bonds and physical entanglements, dissipate and relax leading to the decrease of G′ and the peak in G″. Similar behavior was observed in other physically cross-

modulus, G* = (G′)2 + (G″)2 ) is proportional to strain, persisted to γ ∼ 6% (Figure S3), which is why γ = 0.5% was used in the experiments described earlier in this paper. Within the LVE region, G′ > G″, which is consistent with a solid-like material. That is, when the deformation is sufficiently small the molecular structure of the physical hydrogels remains close to equilibrium and the cross-link structure remains intact. At higher shear strains, however, the hydrogels exhibited nonlinear viscoelastic behavior. G′ decreased by about 2 orders of magnitude when γ increased from 6% to 100% strain, and G″ exhibited a maximum at γ = 10−30% and its value crossed over with that of G′ (i.e., G″ > G′) at higher strains. The peak in G″ in Figure 5 denotes a transition where energy is dissipated E

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Macromolecules linked gel systems.23,34,42,43 At low strains, the hydrogen bonding network is in a dynamic equilibrium with H-bonds breaking and reforming.41 When the strain increases, the rate of breaking H-bonds increases faster than the rate of reformation and G″ decreases. The peak in G″ indicates a new dynamic equilibrium was established, where sufficient concentration of H-bonds are destroyed and the system becomes fluid; i.e., the gel becomes a viscoelastic solution. Self-Healing of the Hydrogel Structure. Figure 6 demonstrates the ability of the H-bonded network to heal

Attempts to synthesize hydrogels using a CAAm/CVP ratio of 1:1 only produced a transparent viscous fluid (Figure S5). In addition, the elastic modulus of the PVP-in situ-PAAm hydrogels (less than 90 kPa28) was much less than those of the hydrogels prepared from DMAA and MAAc (>10 MPa31) at a similar water content of about 67−68%. In the present work, we observed that G′ of the PVP-in situ-PAAm gels decreased with increasing CPVP at a fixed CAAm (Figure 4a), though the solid content of the hydrogels increased. A similar decrease in the tensile strength was reported previously.28 In contrast, the elastic modulus and tensile strength of the hydrogels prepared with PVP and AAc increased with increasing CVP/CAAc ratio up to a ratio of 6/4 (Figure S6). The dramatic differences in the gel formation phenomenon and elastic modulus between the PVP−AAm and DMAA− MAAc systems and the PVP−AAc system suggest that there are significant differences in the cooperative H-bonding and/or the microstructures of gels induced by the H-bonding. One possibility is that the formation of cooperative H-bonding in the PVP−AAm systems is less efficient than for the DMAA− MAAc or PVP−AAc systems. Another, more probable, explanation is that in the PVP−AAm system the AAm molecules are cooperatively H-bonded to the PVP chains one-dimensionally (Scheme 1), i.e., zip-up, as commonly proposed for template polymerization45 and for interpolymer complexes (IPCs),26 while in the DMAA−MAAc or PVP−AAc systems the cooperative H-bonds form three-dimensionally between the monomer molecules as demonstrated by the formation of microsized aggregates.31 The formation of zippedup cooperative H-bonding between PVP and AAm may not be as efficient, due to the steric hindrance of the PVP chains, so they may form multiple, but short cooperatively H-bonded segments on each PVP chain, as proposed by Iliopoulos and Audebert46 and Myung et al.26 for similar H-bonded gels (Scheme S1). As a result, not all potential H-bonding sites participate in the supramolecular network, which would explain the lower elastic modulus of the PVP-in situ-PAAm hydrogels than those of the DMAA−MAAc hydrogels. Note that all compositions of the PVP−PAAm hydrogels with high mechanical strength had a large excess of PAAm with respect to the 1:1 stoichiometric ratio of the PVP−PAAm complex. As a result, the majority of PAAm chains are not cooperatively H-bonded. H-bonding interactions can also be formed among the excess PAAm chains, though they should be weaker than those in the PVP−PAAm complexes. Therefore, the weaker H-bonding and chain entanglements between the noncooperatively H-bonded PAAm chains provide the main contribution to the modulus of the hydrogel. As the CAAm/CVP ratio increases, more H-bonding and physical entanglements increase the effective cross-link density of the network, which increases the modulus and decreases tan δ of the hydrogel (Figures 2 and 4). The cooperatively H-bonded PVP−PAAm IPCs function as strong cross-links only when they are connected by PAAm chains that are not cooperatively H-bonded into a threedimensional network. The connection may be realized by chemical coupling of growing PAAm chains and/or physical interactions between PAAm chains. Therefore, the chemical and physical interactions between the PAAm chains have a vital effect on the mechanical properties of the gels. The hydrogels prepared with a high CAAm/CVP ratio exhibit viscoelastic properties similar to those of chemically cross-linked hydrogels, as additional PAAm chains provide more chemical and physical

Figure 6. Successive strain sweep cycles for the Gel33. T = 25 °C; ω = 6.28 rad/s.

following a nonlinear experiment where the material undergoes a gel to liquid transition. Three successive shear strain sweeps to 200% strain at 25 °C were conducted for the Gel33 with no “rest” period in between each cycle. The G′ and G″ data for each cycle almost superpose, which indicates that the gel to liquid transition was reversible and since the viscoelastic properties of a network are sensitive to the structure of the network, the H-bonded network also appears to be reversible. Network Structure of the Hydrogels. The presence of strong cooperative H-bonding interactions between the preexisting PVP chains and the in situ polymerized PAAm chains has been proven with comparative synthesis experiments, thermal analysis and molecular modeling.28 The formation of cooperative H-bonds between the small molecular weight AAm molecules and the flexible PVP chains is much more efficient than that between two polymers (PAAm and PVP), since the complementary conformations of the PVP chains and AAm molecules can be easily achieved to allow the H-bonding formation, while the formation of cooperative H-bonds between two polymers should be largely inhibited by their strong steric hindrance, as proven by the fact that mixing the aqueous solutions of PAAm and PVP does not produce hydrogels. Note that if a monomer that forms stronger and multiple Hbonding with PVP (e.g., acrylic acid, AAc) was used, threedimensional formation of cooperative H-bonding between the PVP chains and the monomer molecules produced opaque hydrogels at an identical molar concentration of VP (PVP) and AAc and the aggregation and precipitation of the H-bonded PVP−PAAc clusters at higher VP ratios (Figure S4). Similar aggregation of H-bonded interpolymer complexes (IPCs) was also observed by mixing polyoxyethylene (POE) and polymethacrylic acid (PMAAc)44 and hydrogels synthesized from N,N-dimethylacrylamide (DMAA) and methacrylic acid (MAAc).31 F

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Figure 7. Frequency (a) and shear strain (b) dependence of the viscoelastic properties of the Gel33 prepared in the presence of urea and the gel swollen with urea, with comparison to those of the original gel. ω = 6.28 rad/s; T = 25 °C.

interactions. In contrast, for low CAAm/CVP ratio where there are fewer extra PAAm chains and a lower effective cross-link density, G′ decreases (Figure 4a). In addition, the weaker physical interactions break easier when stress is applied, which leads to increases of G″ and tan δ (Figure 4b,c). Tough hydrogels were also formed from PVP−AAm solutions in the presence of urea, suggesting that cooperative H-bonding between PVP and AAm persists in the presence of urea. Although those gels were weakened by the presence of urea, they did not dissolve, even when a large excess of an aqueous solution of urea (1.0 mol L−1) was used. That suggests that although urea can weaken or dissociate noncooperative Hbonds, it did not significantly affect the cooperative H-bonds in the PVP−PAAm IPCs. The reason is that the collective bond energy of the cooperative H-bonds was much larger than the bond energy of noncooperative H-bonds.47,48 To further understand the contribution of the cooperative and noncooperative H-bonding to the formation of the gels and their mechanical properties, rheological studies were carried out on PVP-in situ-PAAm hydrogels synthesized in the presence of urea and on hydrogels swollen with urea postpolymerization. In general, urea can compete for H-bonding sites and, hence, weaken the gel by decreasing its effective cross-link density or weaken the hydrogen bond strength. Figure 7a shows the frequency dependence of Gel33 prepared in the presence of urea and the gel swollen with urea. Note that the water content of the gels was the same as the as-prepared hydrogels. Thus, any difference in the cross-link density of the two gels is strictly proportional to G′. The gel prepared in the presence of urea and the gel swollen with urea exhibited similar viscoelastic behavior, but G′ and G″ of the hydrogels with urea were lower than that of the original hydrogel without urea. The data indicate that the addition of urea decreased the cross-link density by about 40%. Note that each of the NH2 groups of the urea molecule can replace the supramolecular H-bond between PVP and PAAm with a urea cross-link, which would maintain the same cross-link density as the complex in the absence of urea. Note that at high frequency (Figure 7a) or low strain amplitude (Figure 7b), one would not expect the strength of the H-bonds to affect the modulusi.e., the behavior of the supramolecular bond should be strictly elastic. The fact that Figure 7 shows that even under those conditions, G′ decreased ∼40% indicates that at least some of the urea dissociated Hbonds and were ineffective at forming an intermolecular urea cross-link.

Similar results were obtained for Gel26, and Gel39, where the decreases in the cross-link density were 31% and 65%, respectively (Figure S7). The higher percentage of decrease in the modulus and hence cross-link density found in the hydrogel prepared with a higher CAAm/CVP ratio indicates that the H-bonding between the noncooperatively H-bonded PAAm chains is easier to solvate with urea. The hydrogels with urea still exhibited solid-like, elastic response, as G′ was much greater than G″ over the entire frequency range. This observation suggests that the addition of urea did not, or at least not significantly, destroy the cooperatively H-bonded network. The strain-dependent viscoelastic responses of the Gel33 gel prepared in the presence of urea and the gel swollen with urea postpolymerization are shown in Figure 7b and Figure S7. Similar to the original hydrogel without urea, the ureacontaining gels showed a solid to liquid transition, characterized by an order of magnitude decrease in G′ and a peak in G″. However, there was a difference in the onset of the nonlinearity of the viscoelastic behavior when the urea was added in the AAm polymerization step or when it was added to the prepared gel. For the case where urea was used in the polymerization step, the transition strain, defined as the peak in G″, occurred at a lower strain, γ = 20%, compared to γ = 40% for the gel without urea and γ = 80% when the urea was added to the gel. The reason for the effect of urea on the transition strain in the three samples shown in Figure 7b is not yet clear.

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CONCLUSIONS

This study provides an understanding of the rheological behavior of hydrogels cross-linked by cooperative H-bonding. Rheological studies of PVP-in situ-PAAm hydrogels synthesized with different monomer ratios of AAm to PVP (CAAm/CVP) demonstrate that hydrogels prepared with high ratios of CAAm/ CVP are generally solid-like and elastic material. The high concentration of cooperative H-bonds between the two polymers provides a network structure similar to that of an ideal chemically cross-linked hydrogels at low to moderate strain, as indicated by their weak dependence on time, temperature and frequency. However, at relatively high strains the viscoelastic behavior becomes nonlinear and the gels transform from a solid-like gel to a viscoelastic fluid materials. This transformation is due to the physical nature of the Hbonds, which become less resistant to stress when the strain becomes sufficiently large. That is, the H-bonds are weakened and eventually destroyed when the stress becomes comparable G

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and greater than the strength of the H-bonds. In fact, the supramolecular bond, which is reversible, is dynamic and is constantly breaking and reforming, the rate of each process depending on the mobility of the chains that are dependent on temperature, strain rate and strain amplitude. At constant temperature, as the strain amplitude of the oscillatory shear deformation increases the stress exerted on the H-bond by the two chains also increases. That increases the rate of breaking of the H-bond faster than the rate of reformation, such that the integrity of the network is damaged by fewer intact H-bonds carrying the load. At sufficiently high stress, the network structure disappears and the system behaves mechanically much like a high molecular weight amorphous polymer above the glass transition. That process is also affected by the CAAm/CVP ratio, since that controls the concentration of H-bonds in the quiescent gel. For the same applied stress, a gel with higher CAAm/CVP has more H-bonds that can share the load, so each bond experiences a lower applied stress. Thus, the H-bonds in gels with higher CAAm/CVP ratio can survive to higher stresses than those with lower ratios. The breaking and reformation of the physical interactions is also responsible for the nonlinear viscoelastic response and the self-healing property of the hydrogels.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b01448. Table of Yp, Mn, Mw, and PDI of the PAAm’s synthesized with different CAAm, polymer yield and molecular weight measurements, and figures showing the elastic modulus of the hydrogels, storage modulus and loss modulus, G* as a function of shear strain, and photographs showing the polymerized solutions, and a scheme showing the cooperative H-bonding between PVP and PAAm (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.W.). *E-mail: [email protected] (R.A.W.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are thankful for the financial support from the National Science Foundation of China (No. 21274013) and the Program for Changjiang Scholars and Innovative Research Team (PCSIRT) in University for support of this research and for funding short-term visits abroad for doctoral candidates from Beijing Normal University. R.A.W. also thanks the Civil, Mechanical, and Manufacturing Innovation Division of the Engineering Directorate of the National Science Foundation (Grant CMMI-1300212) for support of this work. Professor Xia Dong and Miss Xinran Liu from the Institute of Chemistry, Chinese Academy of Sciences, are acknowledged for their help in doing some measurements.

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REFERENCES

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