Tuning the Viscoelastic Behavior of Hybrid Hydrogels Composed of a


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Tuning the Viscoelastic Behavior of Hybrid Hydrogels Composed of a Physical and a Chemical Network by the Addition of an Organic Solvent Jinkun Hao and R. A. Weiss* Department of Polymer Engineering, University of Akron, Akron, Ohio 44325-0301, United States S Supporting Information *

ABSTRACT: The influence of isopropanol (IPA) addition on the viscoelastic behavior of hybrid hydrogels which were prepared from chemically cross-linked copolymers of N,N-dimethylacrylamide (DMA), 2-(N-ethylperfluorooctanesulfonamido)ethyl methacrylate (FOSM) and cinnamoyloxyethyl acrylate was investigated by dynamic oscillatory shear. The hybrid gels were composed of a supramolecular network formed by phase-separated FOSM nanodomains that served as physical cross-links and a chemical network derived from photo-cross-linking the cinnamate groups. The linear viscoelastic (LVE) behavior of the gels was tunable by changing the solvent ratio (IPA/water) and/or temperature. When the swelling solvent was pure water or pure IPA, the materials were hydrogels and organogels, respectively. When the IPA concentration increased from a molar ratio of IPA:FOSM of zero to 80:1, the crosslink density of the gels decreased due to weakening of the physical network as a result of solvation of the hydrophobic interactions by IPA. Above an IPA:FOSM ratio of 80:1, essentially only the covalent network persisted and the gels behaved as elastic solids. The design of these hydrogels/organogels provides three degrees of freedom for tuning the LVE properties: copolymer composition, temperature, and solvent. They also provide responsive behavior of the gels to changes of temperature and/or solvent.



INTRODUCTION Functional tough hydrogels are a growing contemporary topic of research, largely because gels resemble human tissue and as such have considerable potential biomedical applications.1−5 We previously discussed the structure, mechanical properties, and viscoelastic behavior of physical and hybrid (with physical and covalent networks) hydrogels that were based on poly(alkylacrylamides) that were hydrophobically modified by incorporation of 2-(N-ethylperfluorooctanesulfonamido)ethyl acrylate (FOSA) or N-ethylperfluorooctanesulfonamido)ethyl methacrylate (FOSM) as a comonomer (Figure 1a). The FOSA-DMA and FOSM-DMA copolymer hydrogels exhibited high mechanical strength and toughness.6−10 The microstructure of those materials is notable for the microphase separation of ∼6 nm diameter core−shell nanodomains that had a 4−5 nm fluorocarbon core surrounded by a waterdepleted shell of the poly(alkylacrylamide).7 The water-poor shell includes the chain segments attached to the fluorocarbon repeat units, such that the nanodomains act as multifunctional, water-insoluble physical cross-links. Those hydrogels exhibit temperature- and frequency-dependent viscoelastic behavior and a solid−liquid thermal transition at ∼45 °C.8 Hybrid hydrogels were prepared by introducing covalent bonds into the supramolecular hydrogels based on FOSM and dimethylacrylamide (DMA). That was achieved by incorporating a third comonomer, 2-hydroxyethyl acrylate (HEA), in the © XXXX American Chemical Society

Figure 1. Structures of (a) FOSM-DMA and FOSA-DMA random copolymers and (b) Fx-Cy. Association of the fluorocarbon chains forms a physical network, and a covalent network is formed by reaction of the vinyl bonds of the cinnamate group (arrow).

gel synthesis and converting some of the HEA to a cinnamate species (Figure 1b), which could later be photo-cross-linked. The hybrid hydrogels were also strong and tough and had better dimensional stability than the physical hydrogels.9 The Received: June 13, 2016 Revised: August 15, 2016

A

DOI: 10.1021/acs.macromol.6b01256 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Table 1. Characteristics of the Fx-Cy Hydrogels (Data Reproduced from Refs 8 and 9) compositiona (see Figure 1b) polymer ID

p

r

s

t

water absorbed (wt %)

νeb (mol m−3)

Ec (kPa)

σuc (kPa)

εuc (%)

fracture energy (J m−2)

F5-C6 F10-C3 F10-C6

0.870 0.854 0.808

0.055 0.099 0.096

0.060 0.031 0.064

0.015 0.016 0.034

73 (1) 67 (2) 57 (1)

256 536 631

64 (2) 108 (5) 184 (18)

270 (20) 470 (50) 540 (30)

514 (74) 575 (11) 503 (46)

119 (51) 188 (55)

Mole fraction (see definition of letters in Figure 1). bCross-link density calculated from G′ at 100 rad s−1. cTensile properties: E = Young’s modulus, σu = ultimate stress, and εu = elongation to break (standard deviation in parentheses).

a

(FOSM, 96%) from BOC Sciences and 2,2′-azobis(2-methylpropionitrile) (AIBN, 98%) were purified by recrystallization in methanol. 2-Hydroxyethyl acrylate (HEA, 96%) from Sigma-Aldrich Chemical Co. was purified by dissolving in water (25% w/v), followed by extraction with 10:1 (v/v) of a hexane and water mixture, and then the aqueous solution was salted with 250 g L−1 NaCl. The monomer was separated from the aqueous solution by extracting it four times with ether to remove acrylic acid and dried with CaSO4, distilling off the ether and isolating the monomer was by vacuum distillation. Cinnamoyl chloride (98%, predominantly trans), trimethylamine (99%), 2-hydroxy-4′-(2-hydroxyethoxy)-2-methylpropiophenone) (Irgacure 2959), and dioxane were obtained from Sigma-Aldrich Chemical Co. and used as received. Syntheses of Poly(DMA-co-FOSA), Poly(DMA-co-FOSM-coHEA), and Poly(DMA-co-FOSM-co-HEA-co-CA) Copolymers). The syntheses of a DMA and FOSA copolymer with 9.0 mol % FOSA (DFA9), a DMA and FOSM copolymer with 9.0 mol % FOSM (DFm9), and copolymers of DMA, FOSM, hydroxyethyl acrylate (HEA), and 2-cinnamoyloxyethyl acrylate (CA) (Figure 1) that were used in this paper were reported previously.8,9 All of the copolymers were synthesized by free-radical reaction of the monomers in dioxane at 60 °C for 24 h. The poly(DMA-co-FOSM-co-HEA-co-CA) was prepared by dissolving the poly(DMA-co-FOSM-co-HEA) terpolymer in anhydrous dichloromethane containing trimethylamine, and an excess amount of cinnamoyl chloride in dichloromethane was added dropwise to convert some of the HEA groups to 2-cinnamoyloxyethyl acrylate (CA)15,16 (Figure 1b). Triethylamine was used to remove the byproduct HCl produced. The reaction mixture was stirred at 0 °C for 6 h and then stirred at room temperature for 3 days with the reaction vessel covered with aluminum foil to avoid premature photoreaction of the cinnamate groups. The byproduct triethylammonium was filtered, and the polymer was precipitated into 2:1 hexane/diethyl ether. The cinnamate-containing polymer was stored in an aluminum foil covered bottle and dried in a vacuum oven at 50 °C for 24 h. These samples are denoted as Fx-Cy, where x and y denote the mol % of FOSM and cinnamate rounded to an integer value. The samples used in this study and their water swelling and mechanical properties are summarized in Table 1. Synthesis of the Fx-Cy Hybrid Gels. The Fx-Cy hybrid gels were prepared by photo-cross-linking a 15 wt % solution of poly(DMA-coFOSM-co-HEA-CA) in IPA containing 0.5 wt % of the Irgacure photoinitiator. The solution was transferred to a reaction dish, degassed, and radiated with a UV lamp with a 365 nm wavelength for 40 min. The hybrid gels, ∼1 mm thick, were washed with IPA to remove any non-cross-linked polymer, soaked in isopropanol for 1 day to remove any noncovalent cross-linked polymer, and then soaked in deionized water for at least 7 days with frequent water changes to remove the IPA. The replacement of the IPA with water allowed the hydrophobic associations to take place and re-establish the physical network in addition to the covalent network. Similarly, the gels swollen with IPA/water mixtures were obtained by soaking the hybrid gels in the desired IPA/water mixtures (ranging from 100% IPA (organogel) to 100% water (hydrogel)) for at least 7 days with frequent solvent changes to attain equilibrium. The FOSM groups provided the physical cross-links that had a functionality of f = 60−140 (twice the aggregation number in the nanodomains), based on the neutron scattering data in ref 7, and the cinnamate groups form covalent cross-links with f = 4. As a result, most of the effective cross-link density in the hybrid hydrogels comes

hybrid hydrogels also exhibited thermally activated shape memory behavior with a switching temperature, Tc corresponding to the solid−liquid thermal transition ∼45 °C that was due to softening of the hyrdrophobic nanodomains.10 There have been relatively few reports of the effect of organic solvents on the properties of hydrogels with physical crosslinks.11−14 Da and Hogen-Esch12 reported that graft copolymers composed of 2′-dihydroperfluorodecanoyl groups grafted to a poly(dimethylacrylamide) backbone with poly(ethylene glycol) chains formed not only hydrogels but also organogels with solvents such as dioxane, THF, or methanol. Tsitsilianis et al.13 found that the addition of dimethylformamide to a poly(acrylic acid)−polystyrene triblock copolymer hydrogel changed the rheological behavior by altering the structure from a concentrated suspension of percolating microgels to a more homogeneous physical network. Shi et al.14 were able to reversibly control the viscoelastic behavior of a physical hydrogel formed by N,N′,N″-tris(3-pyridyl)trimesic amide using pH. At pH = 7, the gelator molecules form an ordered structure due to hydrogen-bonding interactions. However, when the pH was lowered, the pyridyl groups were partly protonated, and the gel changed to a branched fibrillar network at pH = 5 and underwent a gel−sol transition at pH < 3. Kim and Cohen15 found that amphiphilic ionomer networks were very sensitive to the composition of the mixed solvent (water/ dioxane) used during formation of the network. The concentration of water influenced the degree of ionic aggregation, which controlled the hydrophilic/hydrophobic balance of the gel. When properly tuned, the network could be highly absorbent and relatively solvent selective. The hybrid gels reported in ref 9 and used in this paper are extremely strong and tough and may be swollen with water (hydrogel) or IPA (organogel). This paper describes how mixed solvents of isopropanol (IPA) and water affect their viscoelastic behavior. Since IPA dissolves FOSA- or FOSMalkylacrylamide copolymers, it was expected that the addition of IPA would weaken or eliminate the physical bonds in a hybrid hydrogel, leaving only covalently cross-linked gel at sufficiently high IPA concentrations. Thus, the hypothesis underlying the research reported herein was that varying the concentration of IPA and water in the swelling solvent would tune the mechanical and viscoelastic properties of hydrogels or organogels formed from these materials. That is the addition of IPA to a Fx-Cy hydrogel was expected to produce a reversible transition from a stiff, strong, and tough gel to a softer, weaker gel. Conversely, adding water to the hybrid organogel should produce a transition from a soft, weak gel to a stiffer, stronger, and tougher gel.



EXPERIMENTAL DETAILS

Materials. N,N-Dimethylacrylamide (DMA, 99%) from SigmaAldrich Chemical Co. was purified by distillation under reduced pressure. 2-(N-Ethylperfluorooctane-sulfonamido)ethyl methacrylate B

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Macromolecules from the physical cross-linking,9 which is also responsible for the high fracture energy,9 ∼100−200 J m−2, which is 1−2 orders of magnitude greater than that of conventional, single network, chemically crosslinked hydrogels.17 Above ∼45 °C, tan δ reaches values of 0.5−0.6 at low frequency that indicates very high energy dissipation, which is responsible for the excellent strength and fracture toughness shown in Table 1. Viscoelastic Property Measurements. Linear viscoelastic properties were measured with a TA Instruments ARES-G2 rheometer equipped with 8 mm parallel plates. A fixture with a solvent trap was used to maintain hydration of the samples during the experiments. The linear response region for the dynamic experiments was determined with a strain sweep, and dynamic shear data were collected over a frequency range of 0.1 to 100 rad s−1 using a strain amplitude of 0.5%.

a soft polymer network is proportional to the density of crosslinks, which explains the modulus increase when the second, covalent, network was added to the already present supramolecular network in the physical hydrogel. When the swelling solvent for the hybrid copolymer was switched from water to IPA, G′ decreased by over an order of magnitude, and it became frequency-independent. Those results are consistent with the loss of some or all of the physical cross-links due to solvation by the IPA. As discussed earlier, the physical crosslinks constitute the majority of the cross-links in the polymer, so their loss leads to a large decrease in the modulus. Also, the frequency-independent modulus of the two organogels (i.e., the IPA-swollen gels) is consistent with a covalently cross-linked network, which does not possess a mechanism for dissipation of energy. In contrast, the fracture toughness of the DFAx and FxCy hydrogels (see Table 1) is a consequence of the reversible nature of the supramolecular cross-links, which can break and provide viscous dissipation when stressed but re-form after the stress relaxes rather than permanently break as in a brittle, covalent network. Figure 3 shows the frequency dependence of G′ and tan δ (= G″/G′) of the F10-C3 gels swollen with different compositions



RESULTS AND DISCUSSION The physical network in the hybrid hydrogels is formed by microphase separation of the FOSM groups into hydrophobic nanodomains, but which can be solvated by IPA. As a result, the addition of IPA to water-swollen Fx-Cy gels is expected to diminish the physical network, and at the extreme where IPA completely dissolves the supramolecular cross-links, the hybrid hydrogel should be left solely with a conventional covalent network. Figure 2 compares G′ for gels of Fx-Cy swollen with water (hydrogels) and swollen with IPA (organogels) (in isopropa-

Figure 3. (a) G′(ω) and (b) tan δ = G″/G′ for F10-C3 gels swollen to equilibrium with different solvent ratios (w/w) of isopropanol/water at 25 °C, 0.5% strain. Figure 2. Comparison of G′(ω) for Fx-Cy copolymers swollen to equilibrium with water or IPA and physical DFx hydrogels with comparable hydrophobe concentration: (a) 5 mol % and (b) 9−10 mol %. T = 25 °C and strain = 0.5%.

of IPA/water mixtures ranging from pure water to pure IPA. (The G″ data are shown in Figure S1.) For all solvent mixtures G′ > G″, which indicates that all the gels were viscoelastic solids. For the F10-C3 swollen only with water, G′ was relatively high, ∼300 kPa at ω = 100 rad s−1, and had a weak frequency dependence. As the IPA/water ratio increased, G′ decreased and asymptoted to a value of ∼5 kPa (ω = 100 rad s−1) for the gel swollen with pure IPA (Figure 4). For the gels swollen with IPA/water ratios greater than 50 wt %, G′ was nearly frequency independent, which as discussed above is expected for a strictly single-network, chemical gel. For IPA/water between 0 and 30 wt %, the frequency sensitivity of G′ was greater than for the hydrogel (100% water). The frequency dependence of the gel first increased with the addition of IPA, reached a maximum frequency dependence at about 20 wt % IPA, and then decreased until the IPA concentration was ∼50 wt %. One explanation for that trend is that at low IPA concentrations, the physical bonds are still active but weakened by the solvation by the IPA. As such, the relaxation time of the physical cross-links decreases sufficiently that the lower frequency oscillations have sufficient time to dissociate the physical interactions, such that they do not contribute to the modulus at low frequencies. However, since the low-frequency modulus of the gel with 50 wt % IPA is still considerably higher than that of the IPA organogel, there

nol) and PDMA-FOSA physical hydrogels (DFAx), where x is the concentration of FOSA. Although there are some differences in the properties of physical hydrogels synthesized with FOSA and FOSM, those differences are small compared with the changes shown in Figure 2. The gels used for Figure 2a have FOSA/FOSM concentrations of 5 mol % and in Figure 2b, 9−10 mol %. For the hybrid (Fx-Cy) and physical (DFAx) hydrogels, G′ > G″ for all frequencies studied, and G′ showed a weak frequency dependence, decreasing by ∼40−50% as the frequency decreased by 3 orders of magnitude. The values of tan δ over the same frequency range were 0.1−0.2. That behavior is characteristic of a viscoelastic solid, though the response was nearly that of an elastic solid, which would exhibit a frequencyindependent G′. G′ of the two hybrid hydrogels was 40 and 100% higher (measured at the highest frequency), respectively, for the gels containing about 5% and 10% FOSA/FOSM than the DFAx hydrogels with the same hydrophobe concentration. The theory of rubber elasticity predicts that the elastic modulus of C

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nanodomains that allows for more mobility of the attached chain segments. The transition is not clearly observed in the frequency response of the F10-C3 hydrogel, but it is responsible for the upturn observed for tan δ at low frequency, which portends a long relaxation time for the hard nanodomain structure that was previously estimated to be λ ∼ 129 h.10 The addition of IPA plasticizes the nanodomains and shortens the relaxation time as the nanodomains become softer. For 10 wt % IPA, the peak for the nanodomain transition is not resolved, but one might infer that tan δ exhibits a peak similar to that for the IPA/water = 2/8 solvent ratio at lower frequency (i.e., longer relaxation time) compared to the IPA/water = 2/8 sample. Based on the shape of the frequency dependence of tan δ for the IPA/water 1/9 gel, if tan δ reaches the same maximum value as for the IPA/water = 2/8 gel, the maximum should occur at ω ∼ 0.01 s−1, which corresponds to a relaxation time of λ ∼ 10 min. When the IPA concentration was increased to 20 wt %, the tan δ peak was clearly resolved, and the relaxation time calculated from the frequency of the peak maximum was λ = 0.31 s. Increasing the IPA further would be expected to shift the transition to even higher frequency, i.e., smaller λ, but the peak frequency would be too high to be resolved by the data in Figure 3. It is clear, however, that the transition occurs at a higher frequency than the maximum used in these experiments, ω = 100 rad s−1, which means for IPA concentrations greater than 20 wt %, λ < 63 ms. Figure 5 compares G′ measured at 100 rad s−1, the equilibrium swelling ratio (mass swollen solvent/mass poly-

Figure 4. Dependence of the dynamic modulus G′: (blue ●) ω = 100 rad s−1, (○) ω = 0.1 rad s−1, and (red ●) tan δ at ω = 100 rad s−1 on the concentration of IPA in the IPA/water solvent used to swell a F10C3 gel. T = 25 °C, strain = 0.5%. The dashed lines correspond to the range of G′ (blue) and tan δ (red) over the frequency range of ω = 0.1−100 rad s−1.

appears to be at least some contribution to the modulus by physical bonds. The extremely low values of tan δ data (0.01−0.04) for the F10-C3 swollen by 100% IPA (Figure 3b) indicate that the organogel was essentially an elastic solid, which would be expected in the absence of the physical cross-links. The frequency dependence of the other gels was more complicated. Tan δ is a measure of the ability of the gels to dissipate energymore specifically, the value of tan δ is the ratio of the energy dissipated as heat to the energy stored during a single dynamic cycle. Figure 4 shows that at the highest frequency (red point), which probes the shorter relaxation times of the gel, tan δ initially decreased with increasing IPA concentration. That result is consistent with the hypothesis that the IPA dissociates the supramolecular cross-links. Adding more IPA decreases the physical cross-links but leaves intact the covalent cross-links, which do not dissipate energy. However, the trend of decreasing tan δ with increasing IPA concentration is broken by the datum point for the gel swollen with a mixture of IPA and water with 20 wt % IPA. In that case, tan δ at ω = 100 rad s−1 was unusually high, indicating that that particular solvent was particularly good at creating a high-energy-dissipating gel. What appears, at first, to be an anomaly is explained by considering the viscoelastic nature of the gels swollen with IPA/water mixtures. Whereas tan δ for the gel swollen with IPA/water = 2/8 exhibited a maximum of tan δ = 0.56 at ω ∼ 20 rad s−1, the gel swollen with IPA/water = 1/9 increased monotonically at low frequency, reaching a tan δ = 0.46 at the lowest frequency measured, ω ∼ 0.1 rad s−1. For gels swollen with IPA/water > 2/8, tan δ decreased with increasing IPA/ water ratio, and for each of those gels, tan δ increased with increasing frequency, though the frequency dependence was much weaker than for the two gels with IPA/water ≤ 2/8. The origin of this seemingly peculiar tan δ behavior is a transition involving the FOSM nanodomains. It is tempting to call this transition a glass transition of the hydrophobic nanodomains because a weak second-order-like transition was observed for a DFm9 copolymer (i.e., a copolymer of DMA and FOSM containing 9 mol % FOSM) hydrogel by differential scanning calorimetry at ∼50 °C as well as a decrease in G′ and a peak in G″ between 50 and 60 °C (Figure S2). Although there is no thermodynamic evidence that this transition is truly a glass transition similar to that observed for glass-forming liquids, the term will nevertheless be used herein to refer to this transition, which appears to be due to a relaxation within the

Figure 5. Effect of the medium composition on: (a) G′ at 100 rad s−1, (b) equilibrium swelling ratio, and (c) modified cross-link density (νe* ≡ νe(1 − 2/ϕ)) of F10-C3 gels calculated from eq 1 with G′ at 100 rad s−1 and 25 °C.

mer), and cross-link density on the composition of the IPA/ water swelling medium (cIPA) for the F10-C3 gels at 25 °C. With increasing cIPA, G′ decreased nearly 2 orders of magnitude, as a consequence of the solvation of the physical bonds. The SR showed the opposite trend, increasing about 280% and asymptoting to a value of 7.6 for cIPA ≥ 70 wt %. That result is also consistent with the elimination of the physical cross-link, as lowering the cross-link density increases the ability of the gel D

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The effect of temperature on tan δ of the F10-C3 gels swollen with IPA/water mixed solvents of varying composition is shown in Figure 7. The behavior of the gels at two

to absorb water/IPA by reducing the stress generated by the network strands when they stretch due to swelling. Since the equilibrium swelling ratio is determined when the restoring stress in the network balances the osmotic stress, the network can accommodate more stretching of the chains and, thus, more solvent. The cIPA had the largest effect on G′ and SR between 0 and 30 wt %, since once the IPA solvates the hydrophobic associations within the nanodomains, adding more IPA should have much less effect on the integrity of the supramolecular cross-links. The influence of cIPA on the effective cross-link density, νe, of the hybrid gels was similar to the effect on G′, and the largest effect was also observed with the first 30 wt % IPA added. νe was calculated from eq 118 ⎛ 2⎞ G = ⎜1 − ⎟VeRTV2 2/3 ϕ⎠ ⎝

(1)

where v2 is the volume fraction of cross-linked polymer in the hydrogel, R and T are the gas constant and absolute temperature, respectively, and ϕ is the functionality of the cross-links. Although the functionality of the nanodomain cross-links is large, as discussed earlier, the precise value is not known. It was assumed here that the functionality did not change appreciably with IPA concentration. In that case, a modified cross-link density, νe* ≡ νe(1 − 2/ϕ), was calculated and plotted against the IPA concentration in the swelling fluid in Figure 5c. As the concentration of isopropanol, cIPA, increased from 0 to 50 wt %, νe* decreased by over an order of magnitude from 537 mol m−3 for F10-C3/water to ∼20 mol m−3 for F10-C3/(IPA/ W = 5/5) and then remained relatively constant for further increases in the concentration of isopropanol. That result is consistent with the conclusions that IPA effectively dissolves the physical cross-links and that the physical cross-links represent the majority of the effective cross-links in the F10-C3 hydrogel. The ratio of the concentration of moles of IPA and FOSM as a function of IPA in the IPA/water solvent used to swell each gel in Figure 5 is plotted in Figure 6. For a solvent mixture of 50% IPA, where the effect of IPA concentration on cross-link density appears to asymptote (see Figure 5), the molar ratio of IPA/ FOSM is about 80, which corresponds to ∼10 IPA molecules per CF2 group needed to completely solvate the hydrophobic supramolecular bonds.

Figure 7. Frequency dependence of tan δ for F10-C3 gels swollen with IPA/water mixtures with different compositions at 25 and 55 °C.

temperatures: (1) at 25 °C, which is below the glass transition of the nanodomains, and (2) at 55 °C, which is above the glass transition. For the hydrogel (F10-C3/water) at 25 °C, tan δ ∼ 0.25 at the highest frequency (ω = 100 rad s−1) and decreased monotonically with decreasing frequency. At 55 °C tan δ for the hydrogel at ω = 100 rad s−1 was similar to the value for the hydrogel at 25 °C, but it increased as the frequency decreased and exhibited a value of tan δ = 0.65 at at the lowest frequency used, ω = 0.1 rad s−1. Similar to the discussion above associated with Figure 3, the upturn for tan δ of the hydrogel at lower frequencies is due to the glass transition of the nanodomains, though the peak related to the transition is not resolved, because it occurs at ω < 0.1 rad s−1. Thus, the relaxation time for the glass transition associated with the nanodomains in the hydrogel at 55 °C was λ > 60 s. It should be noted that the transition referred to herein is time and temperature dependent, so the temperature of the transition moves as the experimental time scale (i.e., the frequency) changes. For F10-C3 swollen with 10 wt % IPA/water, tan δ was low (