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Modulation of the Mechanical Properties of Hydrophobically Modified Supramolecular Hydrogels by Surfactant-Driven Structural Rearrangement Chao Wang,† Clinton G. Wiener,† Ziwei Cheng,‡ Bryan D. Vogt,*,† and R. A. Weiss*,† †

Department of Polymer Engineering, University of Akron, 250 S. Forge St, Akron, Ohio 44325, United States Department of Chemical and Biomolecular Engineering, Catalysis Center for Energy Innovation, University of Delaware, 221 Academy Street, Newark, Delaware 19716, United States

‡

S Supporting Information *

ABSTRACT: A combination of rheology and small-angle neutron scattering (SANS) experiments revealed the mechanism by which a surfactant can stiffen or soften physically cross-linked hydrogels. Here, the structure and rheological properties of a supramolecular hydrogel based on a random copolymer of N,N-dimethylacrylamide (DMA) and 2-(Nethylperfluorooctane-sulfonamido)ethyl methacrylate (FOSM) were modified by the addition of sodium dodecyl sulfate (SDS). The effect of SDS concentration on the microstructure and properties of the hydrogel was determined by two types of experiments: (1) adding SDS by time-dependent, radial diffusion and (2) using samples where uniform loadings, i.e., no concentration gradient, of the SDS were achieved. Nanodomains consisting of FOSM aggregates were responsible for the physical cross-links in the hydrogel, but the formation of an equilibrium supramolecular network of the hydrogel was limited by conformational pinning of the water-swollen polymer segments by the relatively immobile FOSM groups within the nanodomains. The addition of low concentrations of SDS increased the equilibrium swelling of the hydrogel by as much as 3 times, but also increased the network cross-link density and the elastic modulus of the hydrogel. At sufficiently high SDS concentration, however, the surfactant effectively solvated the supramolecular bonds such that the nanodomain structure was partially destroyed, and the sample broke up into smaller pieces that eventually dissolved. The changes in the mechanical properties with addition of SDS corresponded to changes in the nanoscale morphology of the hydrogel measured by SANS.

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INTRODUCTION The unique combination of properties of hydrogels, such as biocompatibility,1 high permeability to small molecules,2 and stimuli responsiveness,3−5 makes them attractive materials for applications ranging from smart materials in tissue engineering to drug delivery and ocular management.6−12 Historically, synthetic hydrogels have been limited by their relatively poor mechanical properties, such as low modulus, low strength, and poor toughness.13 However, recent advances in the development of stiffer, stronger, and tougher hydrogels14 have motivated considerable interest in the development of mechanically robust synthetic hydrogels. The key to the development of toughness in a hydrogel is the inclusion of a mechanism for dissipation of strain energy, which is absent in conventional covalently cross-linked hydrogels. For example, the introduction of reversible supramolecular bonds, such as polyion complexes,15 hydrophobic associations,16 or hydrogen bonds,17 has been used to produce extremely tough hydrogels. Modulating the modulus and strength of a supramolecular network is an especially challenging problem with hydrogels. In general, the mechanical properties of cross-linked hydrogels are © XXXX American Chemical Society

determined solely by hydrophilic nature of the polymer and its cross-link density, which are fixed during the synthesis, but for hydrogels cross-linked by supramolecular interactions, temperature and/or stress can weaken the cross-links, which are noncovalent bonds. This weakening of the cross-links can substantially decrease the modulus and mechanical strength of the hydrogel at elevated temperatures or high stresses. For covalently cross-linked hydrogels or physical hydrogels with supramolecular bonds that are temperature resistant, increasing the temperature can increase the retractive stress in the network chains, which expels water and increases the modulus in accordance with the theory of rubber elasticity.18 When high stresses are exerted on a covalent hydrogel, the only mechanism by which the stress can be relieved is by chain rupture. Thus, most covalent hydrogels are mechanically brittle. In contrast, supramolecular hydrogels can dissipate stress (or strain energy) by dissociating the physical bond, which can reversibly re-form Received: August 19, 2016 Revised: November 3, 2016

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DOI: 10.1021/acs.macromol.6b01813 Macromolecules XXXX, XXX, XXX−XXX

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Figure 1. (a) Chemical structure of the DMA/FOSM copolymer and SDS. (b) Schematic of the nanostructure of dry and water swollen (hydrogel) DMA/FOSM copolymer. Hydrophobic association of FOSM leads to nanodomains that act as the physical cross-links with a water depleted layer of DMA surrounding a FOSM nanodomain core.

The addition of SDS changed the hydrogel microstructure and modulated the dynamic storage and loss moduli of the hydrogel by about 2 orders of magnitude. The structure− property relationships described herein for the SDS-swollen DMA/FOSM hydrogels provide general guidance for controlling the mechanical properties of supramolecular hydrogels with the addition of surfactants.

once the stress has relaxed. That energy dissipation mechanism can produce extraordinarily strong and tough hydrogels. Recently, an isothermal approach for softening or stiffening hydrophobically modified hydrogels was demonstrated by addition of a surfactant.19−21 Tuncaboylu et al.19,20 reported that a hydrocarbon-based surfactant, sodium dodecyl sulfate (SDS), dissolved the hydrophobic cross-links in a hydrogel based on the supramolecular assembly of poly(acrylamide-costearyl methacrylate). The loss of cross-links lowered the effective cross-link density and the modulus of the hydrogel. Liu et al.21 found that adding SDS to a poly(acrylamide-cooctylphenol) polyoxyethylene acrylate (OP10-AC) hydrogel increased the modulus of the hydrogel at low SDS concentrations but decreased the modulus at higher SDS concentrations. They attributed that behavior to the formation of mixed micelles of SDS with OP10-AC at low SDS concentration, which increased the effective cross-link density of the hydrogel.21 When too much surfactant was added to the hydrogel, the modulus decreased. They proposed that the softening was due to phase separation of mixed micelles,21 similar to fluorescence studies by Relógio et al.22 that demonstrates that the hydrophobic groups of a similar hydrophobically modified hydrogel segregated into mixed micelles when surfactants were added. The explanation offered by Liu et al.21 for their conclusion was that the trapping of OP10-AC inside different mixed micelles lowered the effective cross-link density. Although the studies by Tuncaboylu et al. and Liu et al. described a facile method for controlling the mechanical properties of a hydrogel using a surfactant, neither group provided clear evidence that supports their proposed structural changes in the hydrogels. The work described herein aims to provide a more complete description of the structural evolution in a hydrophobically modified supramolecular hydrogel by the addition of surfactant, in particular SDS, and the effect of these structural changes on the mechanical properties. The material used in this work was a supramolecular hydrogel based on a statistical copolymer of a 10/1 molar ratio of N,N-dimethylacrylamide (DMA) and 2-(N-ethylperfluorooctanesulfonamido)ethyl methacrylate (FOSM, Figure 1a). The cross-link junctions were composed of nanodomains formed by hydrophobic associations of the FOSM moieties in the copolymer.23,24 The FOSM nanodomains were surrounded by a thin layer of DMA-rich chains that were water-depleted due to the confined mobility of the polymer segments attached to the FOSM groups, and the continuous phase of the hydrogel was a water-swollen polymer, essentially poly(dimethylacrylamide)23,24 (Figure 1b).

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EXPERIMENTAL SECTION

Materials. N,N-Dimethylacrylamide (DMA, ≥99.0%), 1,1′-azobis(isobutyronitrile) (AIBN, ≥99%), 1,4-dioxane (≥99.0%), and sodium dodecyl sulfate (SDS, ≥99.0%) were obtained from Sigma-Aldrich Chemical Co. DMA was dried with calcium hydride (Sigma-Aldrich Chemical Co., reagent grade, 95%) and then purified by distillation under reduced pressure (635 mmHg) at 60 °C. AIBN was recrystallized from methanol. 2-(N-Ethylperfluorooctanesulfonamido)ethyl methacrylate (FOSM, ≥85%) was purchased from BOC Sciences and recrystallized three times from methanol. Deuterium oxide (D2O, 99.9%) was purchased from Cambridge Isotope Laboratories. Deuterated chloroform (CDCl3, 99.8% D) was purchased from Sigma-Aldrich Chemical Co. All chemicals were used as received unless otherwise noted. The copolymer was synthesized by free radical polymerization of a mixture of DMA and FOSM. First, 10.062 g of DMA and 7.498 g of FOSM was dissolved in 160.0 g of 1,4-dioxane in a round-bottom flask and stirred using a poly(tetrafluoroethylene)-coated magnetic stirrer at 700 rpm. The solution was sparged with dry nitrogen for 30 min using a needle inserted through a rubber septum, and then the solution was heated to 60 °C using an oil bath. AIBN (0.0162 g) was dissolved in 1,4-dioxane (10.0 g), and the solution was injected into the roundbottom flask to initiate the polymerization reaction. After 36 h, the reaction was terminated by cooling to room temperature and exposing to air. The product solution was concentrated to 40% of its original volume using a rotary evaporator at 40 °C and 16 kPa. The DMA/ FOSM copolymer was recovered by precipitating the copolymer in 700 mL of diethyl ether at 0 °C, and the copolymer was dried under vacuum at 50 °C for 48 h. Proton Nuclear Magnetic Resonance (1H NMR) Spectroscopy. The copolymer composition was determined by 1H NMR spectroscopy with a Varian Mercury-300 NMR using a CDCl3 solution. The composition of the copolymer was 9 mol % FOSM and 91 mol % DMA, as determined from the integrated areas of the resonances between 3.383 and 4.392 ppm (−CH2−) and between 2.293 and 3.292 ppm (CH3−N−CH3) (Figure S1a). Hereafter, the nomenclature used for this DMA/FOSM copolymer is DFm9. Sample Preparation. Dry DFm9 is thermoplastic, so film samples were prepared by thoroughly drying the copolymer under vacuum at 60 °C and then compression molding the DFm9 under vacuum at 150 °C with a Technical Machine Products 35 ton vacuum molding machine. The molded films were then swelled with Type 1 ultrapure water (Milli-Q, Millipore Corp.) for at least 7 days to prepare the hydrogels. The change in mass of the hydrogel was monitored gravimetrically to ensure equilibrium swelling was obtained. The average film thickness of the hydrogel after swelling was ∼1.0 mm, and B

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Macromolecules test samples were prepared by cutting 8.0 mm diameter circular disks with a stainless steel hole-punch. Hydrogel samples within a confined geometry to mimic the rheological measurements, where the sample was sandwiched between parallel plates, were prepared by compressing a hydrogel disk between two 25 mm × 75 mm × 1.0 mm glass slides using rubber bands to hold the sample in place and compress the hydrogel to ∼0.60 mm Cross-linked polydimethylsiloxane (PDMS) spacers were used to control the thickness of the sample. In this geometry, diffusion of the SDS solution into the hydrogel was onedimensional with the SDS entering the edges of the hydrogel disk and diffusing radially into the hydrogel. Diffusion Kinetics of SDS. The hydrogel-glass slide assemblies were immersed in 10.0 mL of 5.0 mM SDS solutions in Petri dishes for a prescribed amount of time between 0 and 72 h. The Petri dishes were sealed with Parafilm M (Bemis Company, Inc.) to minimize water evaporation. The SDS solutions were then collected and filtered using syringe filters (VWR) with 0.20 μm polytetrafluoroethylene (PTFE) membrane before analysis. The aqueous SDS samples were mixed with acetonitrile (ACN) to produce SDS solutions with 10% of their original concentrations in ACN/water mixture (v/v = 90/10). Here, ACN was used because it is less viscous than water, which makes it easier for the sample to be pumped through the HPLC system and shortens the time for the separation. The precise SDS concentrations of the SDS/ACN/water solutions were determined by passing the solutions through an Agilent Infinity 1260 HPLC equipped with a Poroshell 120 SB-C18 reverse phase column (4.6 mm diameter, 50 mm length, 120 Å pore size, 2.7 Å packing size), a mobile phase of ACN/water mixture (v/v = 90/10), and a UV detector set at a wavelength of 200 nm (the −S− bond absorption of SDS25) and using an injection volume of 1 μL and a flow rate of 0.3 mL/min at 25 °C. Two measurements were made for each sample, and a control baseline was determined before and after each sample measurement using a 90/10 (v/v) ACN/water mixture. The area under the peaks in the chromatograms of the SDS solutions was calculated using the Agilent ChemStation software, and a calibration curve (Figure S2) was established from measurements of SDS solutions with known concentrations. The hydrogels were immersed in the SDS solution, and the time-dependent absorption of the SDS by the hydrogel was determined by measuring the temporal SDS loss from the solution. To determine if the DFm9 copolymer dissolves in the SDS solution at long immersion times, the hydrogel-glass slide assemblies were immersed in 10.0 mL of 5.0 mM aqueous SDS solutions for 145 h. The SDS solution was then filtered using VWR syringe filters with 0.20 μm PTFE membranes. Two or three drops (∼0.15 mL) of the SDS solution were added to 0.7 mL of D2O, and the 1H NMR spectrum was measured to determine if any DFm9 was dissolved. Optical Characterization. The position dependence of the light transmittance of the confined geometry sample (i.e., the hydrogel-glass slide assembly) was measured with a VASE 2000 M variable angle spectroscopic ellipsometer (J.A. Woollam Co., Inc.). The hydrogelglass slide assembly was mounted in the ellipsometer vertically and perpendicular to the light source using a homemade pedestal, and the sample was translated in 1 mm steps so that transmittance measurements could be made across the diameter of the hydrogel (Figure S3). The hydrogel-glass slide assembly was immersed in 10.0 mL of 5 mM aqueous SDS solution for a prescribed amount of time between 0 and 48 h. Prior to the light transmittance measurements, the glass slide assembly was removed from the SDS solution and wiped with a tissue to remove solution from the surface. The transmittance at each step is reported as the average transmitted light intensity over a wavelength range of 700−800 nm, normalized by the average transmittance through a blank (two glass slides). For comparison, a control sample of the fully swollen hydrogel without SDS was tested under the same conditions. Rheological Measurements. A TA Instruments ARES-G2 rheometer with a parallel plate geometry was used to characterize the change in the linear viscoelastic (LVE) properties of the hydrogel upon exposure to the SDS solution. A compressive force of ∼0.6 N was applied to the hydrogel to prevent slipping. Dehydration of the

sample was prevented by using an 8.00 mm diameter aluminum upper plate and a 43.9 mm diameter aluminum lower plate with a water reservoir. Initially, the water reservoir contained only 10.0 mL of MilliQ water, and the LVE response region of the hydrogel was determined with a strain sweep where the strain, γ, was varied between 0.1 and 100%, at a frequency, ω, of 1 rad/s. A strain amplitude, γ, of 1%, which was within the LVE region, was used to obtain dynamic shear data for all the samples using a frequency range of 0.1−100 rad/s. The water was replaced by 10 mL of a 5 mM SDS solution, and the temporal evolution of the LVE properties of the hydrogel was measured. In the fixture used for rheology measurements, the hydrogel was confined between the two parallel plates so that diffusion of SDS was one-dimensional, similar to the conditions previously described for the diffusion kinetics experiment. Dynamic shear data were collected at 3, 10, 30, 48, and 72 h after the addition of the SDS solution using γ = 1% and ω = 0.1−100 rad/s. During the tests, Milli-Q water was added to the reservoir when approximately 1 mL of water/SDS solution evaporated, so as to maintain a total of ∼10 mL solution. Although it is preferable that a constant concentration of SDS in the reservoir solution be maintained throughout the rheological measurements, adding pure water to the reservoir to maintain a constant volume of solution simplified the experiment without introducing a significant error. In addition to these dynamic experiments where the average properties were measured as SDS diffused into the hydrogel, equilibrium samples were prepared by adding SDS and water to a compression-molded DFm9 to match the swelling ratio obtained in the dynamic samples at the same SDS:DFm9 molar ratio. Small-Angle Neutron Scattering (SANS). The temporal changes in the hydrogel structure were determined by in situ SANS measurements using the NGB 30 m SANS at the NIST Center for Neutron Research (NCNR) in Gaithersburg, MD, with a beam wavelength of 0.6 nm, a circular beam aperture diameter of 12.7 mm, and a wavelength spread of 14%. Three sample-to-detector distances were used: 1.33 m (with 7 neutron guides), 4.00 m (with 5 neutron guides), and 13.2 m (with 1 neutron guide), to cover a scattering vector, q, range of 0.205−3.11, 0.0854−0.830, and 0.0363−0.261 nm−1, respectively (q = 4π sin θ/λ, where θ is one-half the scattering angle and λ is the neutron wavelength). The hydrogel sample was confined between two quartz windows sealed with rubber O-rings in a titanium liquid cell (Figure S4) that was filled with an aqueous SDS solution. In that geometry, the SDS solution could only diffuse into the hydrogel from the edges, similar to the diffusion and rheology experiments. The hydrogel films were originally ∼1.0 mm thick, but they were compressed to ∼0.90 mm using a lead ring spacer between the quartz windows. 5.0 mM SDS solutions were prepared using H2O/D2O mixtures with v/v = 50/50 and 73/27. The solutions were injected into the liquid cell, and SANS measurements were conducted after soaking the hydrogel in this SDS solution for 3, 10, 33, and 63 h. Control SANS measurements were also made on the DFm9 hydrogels without SDS. Data reduction and analysis were done using Igor Pro 6.37 following the procedures described by Kline.27 The details of the microstructure of the nanodomains were resolved with SANS by using the contrast matching technique26 with H2O/ D2O solvent mixtures. Table 1 lists the scattering length density (SLD) of all the materials and solvents used in this study. Note that the continuous water-swollen poly(DMA) phase has a SLD similar to

Table 1. Scattering Length Densities of the Materials and Solvents

C

material

SLD (10−6/nm2)

DMA FOSM SDS H2O/D2O 50/50 (v/v) 73/27 (v/v)

1.159 2.830 0.337 2.870 1.319 DOI: 10.1021/acs.macromol.6b01813 Macromolecules XXXX, XXX, XXX−XXX

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Figure 2. (a) Time dependence of the SDS concentration in the reservoir solution during the SDS absorption experiment for the DFm9 hydrogel. (b) Kinetics associated with the diffusion of SDS into a DFm9 hydrogel fully swollen in water. The red symbols indicate the time where breakup of the hydrogel was obvious. The dashed curve is the model fit to the solution of eq 1 for the diffusion from a solution of limited volume to a cylinder. R ≡ the molar ratio of [SDS]/[FOSM].

Figure 3. Dependence of G′(ω) and G″(ω), for γ = 1%, on the SDS concentration (R ≡ the molar ratio of [SDS]/[FOSM]) in the fully hydrated DFm9 hydrogel. The relationship of R and time is shown in Figure 2. the H2O/D2O solvent mixtures due to the very low polymer concentration of the hydrogel, which makes the scattering from the water-swollen PDMA chains negligible, though there was still contrast between the continuous water-swollen PDMA phase and the waterdepleted PDMA shell of the nanodomains.23,24 A H2O/D2O mixture of 50/50 (v/v) matched the SLD of the FOSM core, which allowed resolution of the water depleted PDMA shell of the nanodomains. A H2O/D2O mixture of 73/27 (v/v) matched the SLD of the PDMA, which allowed independent resolution of the FOSM core of the nanodomains. The DFm9 hydrogel was swollen to equilibrium with either a 50/50 or 73/27 (v/v) H2O/D2O mixture prior to the SANS experiments.

not occur in the absence of SDS. The breakup of the hydrogel is a consequence of the beginning of dissolution of the polymer by the SDS/water solution. The diffusion kinetics of SDS into the DFm9 hydrogels (Figure 2b) was calculated from a mass balance on the hydrogel and the reservoir solution using the data in Figure 2a, where Mt is the mass of SDS that diffused into the hydrogel at time t (i.e., the change in mass of SDS in the reservoir solution) and Meq is the equilibrium mass of absorption of SDS by the hydrogel. Because the hydrogel broke up for long soaking time, only the data up to 30 h were used to calculate the diffusion kinetics, and the equilibrium absorption was estimated by extrapolating the mass of SDS calculated from Figure 2a to constant mass. The data in Figure 2b were fit to the solution for the radial diffusion of a solute with an initial concentration of C0 into a cylindrical sample of constant volume, V, and radius, r (eq 1).29

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RESULTS AND DISCUSSION Diffusion Kinetics of SDS in DFm9 Hydrogel. Figure 2 shows the temporal changes in the SDS concentration of the reservoir solution (Figure 2a) and the calculated SDS mass absorption of the hydrogel (Figure 2b). The latter data are plotted as square root time because the diffusion of the SDS into the hydrogel was Fickian, as was determined from a log− log plot of Mt versus time where the slope for short times was one-half.28 Up to about 30 h, the concentration of the SDS solution decreased monotonically with time (Figure 2a) due to diffusion of the SDS into the hydrogel. At ∼30 h, the SDS concentration in the hydrogel reached equilibrium. However, at longer time, 72 h, the SDS concentration in the reservoir increased. That was due to breakup of the sample, which did

Mt =1− Meq

∞

∑ n=1

4α(1 + α) exp( −Dqn 2t /r 2) 4 + 4α + α 2qn 2

(1)

where the parameter α is calculated from eq 229 Meq VC0

=

1 1+α

(2)

and the qn’s are the positive and nonzero roots of eq 3 D

29

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Macromolecules αqnJ0 (qn) + 2J1(qn) = 0

estimated to be about R = 2.13−2.16. Therefore, the values of R reported herein are the average concentration of SDS in the sample. The assumption underlying this experiment and the data interpretation was that the viscoelastic properties at the average concentration of SDS adequately represents those for a homogeneous hydrogel with a uniform SDS concentration. The validity of that assumption was confirmed by comparing the viscoelastic data for G′ and G″ measured from the experiments above (denoted gradient samples) with that of samples where the hydrogels were swollen to equilibrium, but using a lower value of the SDS concentration than possible if the samples were saturated with SDS (denoted homogeneous samples). The viscoelastic behavior of the gradient samples and the homogeneous samples at same values of R were the same within experimental error (Figure 4 and Figure S5). The

(3)

where J0(qn) and J1(qn) are Bessel functions of the first kind and order zero and one, respectively. This solution considers the diffusion from a stirred solution of limited volume to a cylinder and does not allow for transport through the ends of the cylinder, which is consistent with the experimental geometry where the ends are confined by glass. The fit of eq 1 to the data is shown by the dotted curve in Figure 2b, and the diffusion constant, D, of SDS into the hydrogel calculated from the fit was 3.15 × 10−6 cm2/s. That value is comparable to the value of D = 2.50 × 10−6 cm2/s reported by Valente et al.30 for diffusion of SDS into a water swollen chemically cross-linked polyacrylamide hydrogel. That result suggests that the nanodomain microstructure of the DFm9 hydrogel had a negligible effect on the diffusion constant of SDS in a swollen hydrogel. Dynamic Viscoelastic Behavior. The evolution of the LVE properties of the DFm9 hydrogels with SDS concentration in the hydrogel, reported as R = R(t) ≡ the molar ratio of [SDS]/[FOSM], is shown in Figure 3. In this case, R is controlled by diffusion of SDS into the hydrogel ([SDS] is time-dependent), and the LVE properties were very sensitive to the SDS concentration in the hydrogel. Note that the absolute time frame for diffusion of the SDS into hydrogel depends on the radius of the sample, while R, which includes the time dependence of the SDS concentration, may be considered a normalized time variable in that the FOSM concentration accounts for the size of the sample. Thus, the figures in this paper use R as the dependent variable. Since R decreased with immersion time after equilibrium diffusion and during breakup of the sample (Figure 2), the immersion time was indicated in some figures for clarity. For all times or values of R, G′ > G″, which indicates that the solid-like nature of the hydrogel was preserved by the addition of SDS. G′ and G″ were relatively independent of SDS concentration up to a value of R ∼ 2. Although the data in Figure 3 indicate that G′ and G″ may have increased and then decreased as R increased from 0 to 1.97 (i.e., the first 10 h of the swelling experiment), the viscoelastic data for those three samples are probably the same within experimental error. For R > 1.97 (t > 10 h), G′ and G″ increased with increasing soaking time, and for R = 2.13 (t = 48 h), where the SDS concentration in the hydrogel reached equilibrium, G′ and G″ were more than an order of magnitude greater than for the equilibrium swollen DFm9 hydrogel without SDS. Note that the change of R from 2.16 to 2.13, where G′ and G″ increased by a factor of 2−3, took about 18 h (cf. Figure 2). During the next 24 h (from t = 48 to 72 h), the sample broke up into small pieces, and some polymer may have dissolved, which explains the decrease of the calculated SDS concentration between R = 2.13 and R = 1.69. Similarly, the 2 orders of magnitude decrease of G′ and G″ shown for the sample at 72 h is an artifact due to the breakup/ dissolution of the sample. Note that for all the samples in the transient concentration experiments, i.e., between R = 0 and R ∼ 2.13, the concentration of SDS in the sample is not uniform. The edge of the hydrogel in contact with the reservoir solution should be at the equilibrium concentration of SDS in the hydrogel, but the concentration decreases radially within the sample. With increasing time, the concentration gradient changes as SDS continues to diffuse into the sample and radially within the sample until the entire sample is at equilibrium, which was

Figure 4. Average of G′(ω) over the frequency range (0.1−100 rad/s) studied versus the ratio of concentrations of SDS and FOSM for hydrogels for gradient samples (■) and homogeneous samples (●).

ordinate of Figure 4 is the average of G′(ω)ave, which is the average value of G′ over the frequency range of 0.1−100 rad/s. The advantage of using the gradient method is that it is much simpler to perform and allows direct comparisons of the temporal changes of the hydrogel measured by the diffusion, rheology, and SANS experiments. The changes of R from 2.16 to 2.13 are most likely not significant, and one may conclude that those values are within experimental error of the equilibrium absorption of SDS by the hydrogel. However, the changes in the LVE properties once equilibrium absorption was achieved were substantial and were due to changes in the microstructure of the hydrogel. This leads to the hypothesis that near the equilibrium absorption of SDS the nanodomain structure is able to rearrange, and this structural change increases the elasticity of the hydrogel. At the longer immersion times, where R decreased from 2.13 to 1.69, the microstructure changes weakened the resistance of the copolymer to dissolution, and the hydrogel began to break into small pieces. The value of R = 1.69 is not real, since at this point the sample was not intact. If the hydrogel were held in the SDS/water solution for even longer time, the copolymer partially dissolved, as indicated by the peaks for DFm9 copolymers in the NMR spectra for the SDS/water solution after 145 h of immersion of DFm9 hydrogel (Figure S1b), while the rest of the copolymer formed a suspension with the SDS/water solution. The increase in G′ when near equilibrium sorption of SDS was achieved (e.g., the data for R = 2.16 and 2.13 in Figure 3) E

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The light transmittance at the edge of the sample, d = the radius of the sample, which is the datum point furthest from the center position of d = 0 in Figure 6b, increased more by than 3 orders of magnitude when R changed from 0 to 2.13. Although visually no obvious change was detected by eye with regard to the transparency of the center of the hydrogel disk, the measured transmittance at the center also increased about 3 orders of magnitude when R changed from 0 to 2.13, which suggests that some SDS reaches the center of the disk sample very quickly, but a critical local concentration of SDS was necessary before optical transparency was observed. This conclusion is supported by the calculation from the solution for the radial diffusion of a solute into a cylindrical sample of constant volume (Figure S6). Details about this calculation are discussed in the Supporting Information. The opaque−transparent transition indicates that the addition of SDS reduced the cross-link density fluctuations in the hydrogels network, which since G′ also increased (at least up to t = 48 h) and tan δ at ω = 100 rad/s decreased with increasing SDS concentration (Figures 3 and 5), the reduction of the spatial density fluctuations must be due to rearrangement of the network structure by the increased mobility of the chains that promoted a higher cross-linked hydrogel by decreasing defects in the network, as was discussed earlier in this paper. Figure 6c shows the effect of time and R on the swelling ratio S = (mass of hydrogel)/(mass of dry polymer). With increasing soaking time, S increased monotonically by almost a factor of 4 from S = 4.15 for SDS-free (t = 0 h, R = 0) DFm9 hydrogel to S = 18.4 for DFm9 hydrogel with t = 72 h and R = 1.69. As discussed earlier, this increased swelling is a consequence of the conformational rearrangements of the chains due to weakening of the supramolecular bonds in the nanodomain cross-links, which allows the network to increase the number of supramolecular bonds. However, weakening of the supramolecular bonds also can lead to the loss of effective cross-links by solvating the interactions to sufficiently decrease the number of FOSM moieties in the nanodomains to break the network, which appears to be what occurred at long time. Thus, as shown in Figure 6d, the effective cross-link density increased with increasing S, but after a long time at equilibrium swelling, the supramolecular network became sufficiently weak that the sample broke up into small pieces (red data points in Figures 6c and 6d). The effective cross-link density υe was calculated by eq 418

indicates an increase of the stiffness of the hydrogel, which corresponds to an increase in the effective cross-link density, i.e., more supramolecular bonds. The decrease in G′ at longer sorption times (e.g., the data for R = 1.69) in Figure 3 indicates that the concentration of supramolecular bonds decreased, which would be expected if the SDS dissolved the hydrophobic interactions. The surprising mechanical behavior, first of elasticity increasing and then decreasing, during the 42 h when the concentration of SDS in the hydrogel equilibrated is remarkable, but it may be explained by the increased mobility of the polymer chains due to the solvation of the supramolecular bonds. Before equilibrium absorption of the SDS was achieved, solvation of some of the supramolecular bonds weakened the supramolecular structure sufficiently to allow the polymer conformations to rearrange and promote additional hydrophobic bonds that were inhibited by conformational pinning as the network formed; e.g., network defects such as loops or unassociated FOSM groups were removed by conformational rearrangements. That explanation accounts for the increase of the cross-link density of the network (increased G′), and it is also supported by the simultaneous decrease of tan δ (= G″/G′) at the highest frequency measured (ω = 100 rad/s) (Figure 5), where the mechanical response is expected to be the most elastic.

Figure 5. Effect of time and R on tan δ at ω = 0.1 rad/s (▲) and 100 rad/s (■) of the fully swollen DFm9 hydrogel. The red symbols indicate samples where breakup of the hydrogel was obvious. γ = 1%. The data points represent the average values from at least three independent measurements on different samples. The dashed line indicates when equilibrium swelling was achieved. The R values at t = 48 and 72 h are indicated next to the data points. When R = 1.69 (red symbols), the sample began to break up.

υe =

G

(1 − )RT̅ υ 2 f

2/3

2

(4)

where the elastic shear modulus G ∼ G′, f is the functionality of the cross-links, R̅ is the gas constant, T is the absolute temperature, and υ2 is the volume fraction of the polymer phase in the hydrogel, which can be calculated from the swelling ratio, S, by eq 5

Optical and Microstructural Evolution. The original DFm9 hydrogel (without SDS) was white and opaque in appearance (Figure 6a), probably due to inhomogeneities of the cross-links and the resulting large density fluctuations in the network structure, similar to what has been reported for other hydrogels and rubber networks.31−35 These heterogeneities of the structure cannot be macroscopic phase separation, which is not possible for a single copolymer. That was confirmed by optical microscopy of the DFm9 hydrogel without SDS, which was featureless; i.e., there were no heterogeneities large enough to resolve. As SDS diffused into the hydrogel, the edges of the sample first became transparent, and the position of the opaque-transparent front moved inward with time (Figure 6a).

−1 ⎡ (S − 1)ρ ⎤ υ2 = ⎢1 + ⎥ ⎣ ⎦ d

(5)

where ρ is the density of the hydrogel and d is the density of the solvent. Because the concentration of SDS was very small, the density of the water was used for the density in this calculation. Although the nanodomains represent a multifunctional cross-link in these supramolecular hydrogels, the nanodomains are composed of hydrophobic associations, the F

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Figure 6. (a) Temporal changes in the transparency of the conf ined geometry sample due to diffusion of SDS into the hydrogel from the edges. The purple line denotes the edge of the hydrogel sample, and the brown dashed line denotes the boundary between the opaque and the transparent regions. (b) Time dependence of the light transmittance through the DFm9 hydrogel as a function of position during the SDS diffusion experiment. The numbers in the parentheses are the molar ratio (R) of SDS to FOSM. d = 0 mm is the center of the sample, and the right-most datum point for each time corresponds to the edge of the sample. Note that the radius of the sample increased with time, i.e., with increasing SDS penetration into the sample. The lines connecting the points are intended only to guide the eye. (c) Time dependence of the swelling ratio (S = mass of hydrogel/ mass of dry copolymer) of the DFm9 hydrogels. (d) Time dependence of the effective cross-link density calculated using the values of G′ at 1 rad/s (■) and 100 rad/s (▲). For t = 72 h (red symbols), the sample began to fragment. For (c) and (d), the R values for the SDS swelling time are listed beside each data point.

Figure 7. SANS profiles associated with different R to elucidate changes in (a) the poly(DMA) shell using 50:50 (v:v) D2O:H2O and (b) the FOSM core in nanodomains using 27:72 (v:v) D2O:H2O for contrast matching of the components in the DFm9 hydrogels (see text for contrast-matching details). The solid curves are the model fits. The scattering curves were individually vertically offset by a factor of 1.8 to improve clarity. Schematics insets in both panels illustrate (a) the physical meaning of the Lorentzian screening length from the fitting of the peak at q ∼ 1.5 nm−1 and (b) the physical meaning of d0 from the fitting of the peak at q ∼ 0.82 nm−1.

Since G′ = G′(ω), υe was compared at the two extreme frequencies, ω = 1 rad/s and ω = 100 rad/s, and the results for the SDS-free DFm9 hydrogel and the SDS-swollen hydrogels are shown in Figure 6d. The values of υe and the effect of R on

simplest being a 1:1 association of two FOSM groups with two chains emanating from each. Thus, for the calculations of υ2, a value of f = 4 was assumed. G

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Figure 8. (a) Interdomain spacing and (b) calculated diameters of core−shell nanodomains in the DFm9 hydrogels as a function of time and SDS concentration. The R values at t = 48 and 72 h are indicated beside the data points. When R = 1.69 (red symbols), the mechanical integrity of the hydrogel was lost.

from ∼0.50 to ∼0.93 nm−1 with increasing SDS content, which suggests a wider distribution for the diameters of the nanodomain shells, which may also be associated with the gradient in SDS concentration across the sample. The scattering provides information on the average structure of the hydrogels, which can be used to assess the changes induced by the addition of SDS, but care must be taken in the quantitative interpretation due to the gradient. The upturn in scattering intensity at low q arises from large density inhomogeneities in the swollen poly(DMA) continuous phase,24 which as discussed previously is due to fluctuations in the cross-link density of the hydrogels. Note that the inverse value of the slope of the low-q upturn, i.e., the Porod exponent, decreased from ∼2.08 to ∼1.80. A decreasing Porod exponent suggests a more diffused interface,37 which indicates a more homogeneous structure, i.e., fewer inhomogeneities in the hydrogel. This result is consistent with the opaque−transparent transition of the hydrogel with increasing SDS content. When R ≥ 2.16, a shoulder occurred at q ∼ 0.20 nm−1, and the Lorentzian exponent, m, for the shoulder is ∼3.1 from eq 7. Here, m is related to the excluded volume parameter υ by m = 1/υ, so υ ∼ 0.33, which is consistent with collapsed chains.36,38 This suggests that the shoulder resulted from the form factor of the DMA shells, which is proposed to be dehydrated DMA that is in close proximity of FOSM.23,24 In this case, the Lorentzian screening length ξ0 for the shoulder corresponds to an average diameter of the DMA shells (∼3.2 nm), so the emergence of this shoulder at lower q indicates the formation of smaller nanodomains at the higher SDS concentrations. The scattering patterns for hydrogels containing D2O/H2O mixtures that match the DMA (Figure 7b) also show a low-q upturn. Tian et al. attributed this upturn in similar hydrophobically modified hydrogels to the inhomogeneities in the perfluoronated segment concentration.24 However, the upturn could also be a consequence of imperfect contrast matching of the DMA. Similarly, the Porod exponent of this low-q upturn decreased with increasing SDS content, which suggests less inhomogeneities in the hydrogel. A peak is observed at q ∼ 0.86 nm−1, which is related to the average separation of the FOSM cores. This separation is the center-to-center distance of the nanodomains. As the SDS concentration increased, the intensity of the peak decreased. That result was due to SDS forming mixed micelles with the FOSM, which is consistent

υe calculated for the two frequencies was qualitatively the same. As R changed from 0 to 2.13, υe increased more than an order of magnitude, but when R changed to 1.69 and the sample broke up, υe decreased precipitously. As will be discussed below, the large decrease of υe and the breakup of the hydrogel film were due to changes in the nanostructure. Microstructure Evolution. Figure 7 shows the scattering profiles of the hydrogels for different R at two different contrasts using SANS. The models used to fit the scattering patterns for the FOSM cores and DMA shells are given in eqs 6 and 7.36,37 Equation 6 is a linear combination of a power law model (first term) and a broad peak model (second term). The power law term accounts for the low-q upturn of the scattering intensity due to density fluctuations37 (consistent with the initial opaque hydrogel as shown in Figure 6a, where n is the Porod exponent). The broad peak term accounts for the correlation between the FOSM cores, which provide the scattering inhomogeneities,37 where ξ is a correlation length for the polymer chains, and the peak position, q0, is related to the characteristic distance (center-to-center distance) between the FOSM cores, d0, as shown by eq 8. Equation 7 adds a Lorentzian term (third term) that accounts for the intradomain correlations of the DMA shell, where ξ0 is a correlation length for the polymer chains, and the exponent m is related to the mass fractal of the structure responsible for scattering. For both models, A, C, and D are adjustable constants, and B is the incoherent background. I(q) =

I(q) =

q0 =

A C + +B qn 1 + (|q − q0|ξ)m

(6)

A C D + + +B qn 1 + (|q − q0|ξ)m 1 + (ξ0q)m

(7)

2π d0

(8)

For the SANS profiles associated with scattering from the DMA shell (Figure 7a), a faint peak is present at q ∼ 1.5 nm−1 for the scattering at R = 0. This weak peak is attributed to the average diameter of the nanodomain shell of water-depleted, SDS-free poly(DMA), ∼4.2 nm. The full width at halfmaximum (fwhm) of the peak, calculated as 2/ξ, increased H

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Macromolecules with the results reported by Muto et al. for a mixed hydrocarbon and fluorocarbon surfactants in aqueous solution.39 The decrease in the intensity of the scattering peak for the FOSM nanodomain core is due to the lower SLD of SDS in comparison to FOSM, so the SLD contrast between the nanodomains and the water-swollen DMA continuous phase decreases as the nanodomains are swollen with SDS. The fits of eq 6 to the SANS data indicate that the scattering peak position decreased from ∼0.86 to ∼0.81 nm−1, which shows that the average interdomain spacing of nanodomains increased from ∼7.3 to ∼7.8 nm when R changed from 0 to 1.69 (Figure 8a). Tian et al. reported that for fully swollen DMA/FOSA hydrogel with 9 mol % FOSM the volume fraction of FOSM is ∼0.12.24 The volume fraction of FOSM in DFm9 should be similar, which explains why the average diameter of the DMA shells (∼3.2 nm) and the average interdomain spacing (∼7.3 to ∼7.8 nm) are of the same order of magnitude. In order to support the former interpretations on the shoulder in Figure 7a, it is useful to estimate the average sizes of the core−shell nanodomains at different R from the swelling ratio and interdomain spacing of nanodomains determined from SANS. In this estimation, the neighboring core−shell nanodomains are treated as spheres in adjacent cubes, where the spheres and their corresponding cubes are concentric (Figure S7). Here, the average diameter of DMA shell, ∼4.2 nm, is used as an initial value for estimation, and the calculated average core−shell nanodomain diameters for the fully swollen DFm9 hydrogels with different SDS contents are summarized in Figure 8b. When R changed from 0 to 1.69, the average core−shell nanodomain diameter was estimated to decrease from ∼4.2 to ∼2.7 nm. This suggests that the addition of SDS to the DFm9 hydrogels promoted the reduction of the core− shell nanodomains. From the fit of the low-q shoulder, the average diameter of the poly(DMA) shell decreased from ∼4.2 to ∼3.2 nm when R decreased from 2.16 to 1.69 due to breakup of the hydrogel. The decrease in the average diameter of the core−shell nanodomains and poly(DMA) shell is consistent with the former explanation that SDS solvated the supramolecular bonds. The solvation allows conformational rearrangement of the network that increased the cross-link density and stiffened the hydrogel. Hierarchical Structural Evolution and Structure− Property Relationship. Figure 9 schematically summarizes the mechanisms associated with the hierarchical structural evolution and structure−property relationship of the DFm9 hydrogels as the SDS concentration is varied. When R = 0, Figure 9a, network defects, such as loops (green curves), are hypothesized to create inhomogeneities in the cross-links distribution such that large-scale density fluctuations produced opaqueness of the hydrogel and the upturn in scattering at low q from SANS. With addition of SDS, the supramolecular bonds in the network were solvated by SDS, but limited structural changes occurred at low SDS concentration (Figure 9b). When the SDS concentration was R > 1.97, the solvation of the FOSM by SDS increased the mobility of the polymer chains to facilitate rearrangement of the nanodomains to form additional supramolecular bonds, which increased the cross-link density, promoted smaller nanodomain cross-links, and simultaneously increased the swelling ratio and modulus of the hydrogel (Figure 9c). The network structure became more homogeneous and the hydrogel film became more transparent. However, above a critical SDS content and sufficient time, the supramolecular network partially dissolved to sufficiently

Figure 9. Schematic of the macroscopic, microscopic and nanoscopic evolution and the associated change in G′ at 1 rad/s (■) and 100 rad/ s (▲) for the fully swollen DFm9 hydrogels with different molar ratios of SDS to FOSM. The R value at t = 72 h is indicated beside the datum point. Four stages were identified: (a) unaltered hydrogel, (b) solvation of the supramolecular bonds, (c) formation of new supramolecular bonds, and (d) local dissolution of copolymer and weakening of hydrogel.

weaken the material, such that the hydrogel broke up into small pieces (Figure 9d).

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CONCLUSIONS The addition of a hydrocarbon-based surfactant, sodium dodecyl sulfate, to a hydrophobically modified supramolecular hydrogel, DFm9, led to a significant enhancement in the stiffness of the hydrogel despite an increase in the swelling ratio of the hydrogel. This enhancement is attributed to the solvation of the supramolecular bonds in the network by SDS and the subsequent increase in the mobility of the polymer chains. With increasing mobility, the polymer chains rearranged to form new supramolecular bonds and smaller nanodomains by eliminating the network defects. The cross-link density of the hydrogel increased, and the hydrogel stiffened. However, weakening of the supramolecular bonds eventually produced breakup of the hydrogel sample and some dissolution of the copolymer at long soaking times at sufficiently high SDS concentration. The addition of SDS also changed the appearance of the DFm9 hydrogel. The unaltered hydrogel was opaque due to the large density fluctuations in the network structure resulting from the inhomogeneities of the cross-links. An opaque-totransparent transition occurred when SDS reached a critical concentration in the hydrogel. The transparency was due to the elimination of network defects and suppression of the spatial density fluctuations. The stiffening and weakening of the DFm9 hydrogel by surfactant reported herein suggests a facile method for modulating the mechanical properties of supramolecular I

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hydrogels through careful selection of the surfactant concentration.

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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.6b01813. NMR spectra of copolymer and SDS solution; calibration curves for chromatography; schematic for transmittance measurements across the hydrogels; drawing of SANS cell; raw rheological data for “gradient” and “homogeneous” hydrogels; time dependence of the SDS content and transmission at the center of the hydrogel; schematic of two neighboring core−shell nanodomains (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (R.A.W.). *E-mail [email protected] (B.D.V.). ORCID

Chao Wang: 0000-0002-5205-9771 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Civil, Mechanical and Manufacturing Innovation (CMMI) Division in the Directorate for Engineering of the National Science Foundation, Grant CMMI-1300212. This work utilized facilities supported in part by the National Science Foundation under Agreement No. DMR-1508249. We acknowledge the support of the National Institute of Standards and Technology, U.S. Department of Commerce, in providing the neutron research facilities used in this work. We thank Yiming Yang for his help on material synthesis and Chongwen Huang for his help on rheology measurements. We thank Prof. Dionisios G. Vlachos for use of the HPLC and Basu Saha for assistance with these HPLC measurements.

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