High-Toughness Polycation Cross-Linked Triblock Copolymer

Article pubs.acs.org/Macromolecules

High-Toughness Polycation Cross-Linked Triblock Copolymer Hydrogels Yaoyao Chen and Kenneth R. Shull* Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, United States S Supporting Information *

ABSTRACT: Poly(methyl methacrylate)−poly(methacrylic acid)−poly(methyl methacrylate) (PMMA−PMAA−PMMA) triblock copolymers can self-assemble into well-defined elastic hydrogels in water by forming glassy PMMA micelle cores connected with PMAA bridges. The stiffness and toughness of these hydrogels are enhanced substantially by introducing partially quaternized poly(4-vinylpyridine) (QVP) into the system. Interactions between the QVP and PMAA molecules provide an energy dissipation mechanism, with fracture energies in excess of 1000 J/m2 obtained in some cases. The materials are fully self-assembled and are formed by exposing liquid solutions in DMSO to small amounts of water. The simplicity of gel formation, and the ability to adjust the chemical and physical characteristics of these materials over a wide range, make them excellent model systems for fundamental investigations of the mechanical response of ion-containing gels.

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INTRODUCTION Double network (DN) hydrogels, described originally by Gong et al.,1 can have fracture energies in excess of 1000 J/m2 while consisting primarily of water. The double network concept has been extended from chemical−chemical network gels to gels containing chemical−physical and physical−physical networks.2−5 An in-depth understanding of the energy dissipation mechanisms active during deformation of these materials enables the design of new gels with high toughness and other special functionalities including, for example, a self-healing capability.6 Weak interactions, such as electrostatic interactions,7−10 hydrophobic associations,11 metal−ligand coordinate complexation,12,13 micelle cross-links,14 and hydrogen bonding,15,16 require less energy to break or to re-form, and they are attracting more and more attention due to their flexibility in modifying gel performance. Polyelectrolyte-based hydrogels are of particular interest because of their responses to pH, salt, and temperature.7,9,17−19 The complexation of oppositely charged polyelectrolytes is an entropy-driven process, during which polycations and polyanions release counterions to form structures with properties spanning the continuum from liquid-like to solidlike behavior.20−23 Environmental conditions, such as salt content and temperature, can affect the disassociation and complexation behavior of the polyelectrolytes. Previous investigations of polyelectrolyte complexes provide a foundation for extending this concept to the design of stronger hydrogels with better performance. More specifically, it has been demonstrated that amphiphilic polyelectrolyte hydrogels benefit from the breaking−reforming mechanism of electro© XXXX American Chemical Society

static interactions, enabling the formation of stimulusresponsive materials with good mechanical properties.17,24,25 A number of studies have focused on small molecule crosslinked hydrogels using metal ions such as Zn2+, Ca2+, and Fe3+.2−4,12,26−30 The metal ions in these cases act as reversible cross-links, enhancing the strength, toughness, and self-healing ability of these hydrogels. Instead of small molecule species (e.g., metal ions), large molecules can also be incorporated into hydrogels to form a transient network. These polymers can be viewed as multivalent cross-links with some potential advantages because of their smaller diffusion coefficients in comparison to atomic ions.31 This idea has been realized by immersing hydrogels in oppositely charged polymer solutions32 and by in situ polymerization of H-bonding forming monomers within an existing hydrogel network.15 Triblock copolymers with hydrophobic end blocks and hydrophilic midblocks are able to self-assemble into elastic hydrogels in water. The well-defined triblock copolymer poly(methyl methacrylate)−poly(methacrylic acid)−poly(methyl methacrylate) (PMMA−PMAA−PMMA) hydrogel has been studied extensively in our group.3,33−36 Divalent ions can be utilized to strengthen and toughen the triblock copolymer network due to the complexation between metal ions and carboxylate groups.3 In this work, we take a step further by using quaternized poly(4-vinylpyridine) (QVP) polycations as cross-linkers based on electrostatic interaction Received: February 10, 2017 Revised: April 14, 2017

A

DOI: 10.1021/acs.macromol.7b00304 Macromolecules XXXX, XXX, XXX−XXX

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80 °C for 6 h. The experimental details of polymerization and hydrolysis process have been previously described by Guvendiren et al.33 and Henderson et al.3 PMMA−PMAA−PMMA (tMAM). The triblock copolymer PMMA− PtBMA−PMMA [MW = 43K−387K−43K (g/mol, PDI = 1.12)] was also synthesized by anionic polymerization. The molecular weight and polydispersity were determined by GPC referenced to a polystyrene standard. The lengths of different blocks were decided by comparing the NMR signals of methoxy protons in PMMA and tert-butyl protons in PtBMA, and this procedure has been described by Henderson et al.3 This block copolymer was hydrolyzed to form a PMMA−PMAA− PMMA triblock copolymer [MW = 43K−234K−43K (g/mol)] in the same way as hydrolysis of PtBMA. The resulting PMMA−PMAA− PMMA copolymer was then dissolved in dimethyl sulfoxide (DMSO, Sigma-Aldrich) to make a 10 wt % solution. Quaternized Poly(4-vinylpyridine) (QVP10 and QVP20). QVP refers to quaternized poly(4-vinylpyridine), obtained by the reaction between P4VP [Scientific Polymer Products Inc., MW = 200 000 g/ mol] and ethyl bromide (EtBr, Sigma-Aldrich) in DMSO at 40−50 °C for at least 2 days. As the reaction proceeded, the yellowish solution first turned into green and then changed to dark yellow or orange. The chemistry of this reaction is illustrated in Figure 2a, and the expected charge ratio of QVP is 10% or 20%, controlled by amount of EtBr added to the solution. We refer to these samples with 10% and 20% quaternization as QVP10 and QVP20, respectively. In order to assess the efficiency of the quaternization reaction, P4VP was dissolved in DMSO-d6, and EtBr was subsequently added to the solution, at a molar ratio of pyridine groups to EtBr of 10:1 or 5:1. The reaction proceeded for 2 days, and the product was analyzed by H1 NMR, with spectra obtained from a Bruker Avance III 500 MHz system, Ag500. The spectra in Figure 3 correspond to P4VP and a QVP sample with an expected degree of quaternization of 10%, based on the composition of the solution used for the quaternization reaction. The actual degree of quaternization calculated from the integration of the appropriate peaks in the NMR spectrum is 9.1% in this case. The sample with 20% expected charge ratio has an actual charge ratio of 19.3%, analyzed by integrating NMR peaks in the same way. These samples are denoted as QVP10 and QVP20 in the remainder of this article. Mechanical Tests. Rheology. Rheological properties of the hA/ QVP20 and hA/QVP10 systems were tested on an Anton Paar MCR 302 rheometer using a stainless steel cone−plate geometry (50 mm in diameter, 2° angle). PMAA and QVP20 (or QVP10) were dissolved in DMSO−water mixed solvents with water contents of 0−30 wt % to form viscous liquids. The overall polymer concentration in the solutions was 8.6 wt %, consisting of a stoichiometric ratio balance of methacrylic acid units and vinylpyridine units. Samples were first warmed up to ∼50 °C to reduce the viscosity, and then loaded on the rheometer. After being cooled and equilibrated to 25 °C, they were sheared at the frequency of 10 rad/s (in the Supporting Information) to determine the linear viscoelastic regime. The frequency responses were further investigated by performing frequency sweeps from 0.1 to 100 rad/s in the linear viscoelastic regime (strain amplitudes less than 2%). Indentation. Equal volumes of a 7.3 wt % PMAA solution in DMSO and a 10 wt % solution of QVP20 in DMSO (with a matched molar concentration of methacrylic acid and 4-vinylpyridine units) were well mixed in a vial to form a homogeneous, viscous solution with an overall polymer concentration of 8.6 wt %. This solution was then poured on a Teflon-coated Petri dish and placed in a sealed water vapor saturated chamber for 3 days. In this process, water vapor diffused into the polymer solution to induce gelation. The solution gradually changed from a viscous liquid to a stiffer hydrogel, which could be easily taken out from the dish. To control the ionization degree of PMAA, the materials were soaked in an aqueous solution with controlled pH and allowed to equilibrate under these conditions. The Young’s moduli of hA/QVP20 hydrogels formed in this manner were obtained from indentation tests. A hemispherical indenter (radius = 3 mm) was used to indent a flat hydrogel specimen with thickness of ∼2 mm at an indentation speed of 20 μm/s. Moduli were obtained by

between polycations and methacrylic acid groups to toughen the swollen midblocks. Both types of cross-links in this novel structure, micelle cross-links and electrostatic interactions, are physical (Figure 1). We first investigate the transient network

Figure 1. Design of triblock copolymer-based tough hydrogels.

formed between PMAA and QVP by performing rheological tests on solutions in mixed solvents of DMSO and water. We then investigate the mechanical properties of PMMA−PMAA− PMMA/QVP double network hydrogels, which combine this transient network with micellar cross-links of the PMAA component. Our hydrogels have unique energy dissipation mechanisms during deformation as a result of the QVP/PMAA transient network. The transient network can be easily tailored by adjusting the polycation content, the pH, and the salt concentration, giving a response that depends both on the structure of the gel and the environment in which it is being used.

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MATERIALS AND METHODS

Materials. The materials used in these investigations include ethylquaternized poly(4-vinylpyridine) (QVPxx in our notation, where xx is the expected percent quaternization), poly(methacrylic acid) (PMAA) homopolymers, and triblock copolymers with poly(methyl methacrylate) (PMMA) end blocks and a poly(methacrylic acid) midblock. We use the letter “M” to refer to PMMA and the letter “A” to represent PMAA. We also use “h” to refer to a homopolymer and “t” to refer to a triblock copolymer. The individual materials used in this work are QVP10, QVP20, hA, and tMAM. The degrees of polymerization and names of the different materials used in this study are summarized in Table 1.

Table 1. Summary of the Polymers Used To Make the Materials Investigated in This Work name

degrees of polymerization

QVP10

1900

QVP20

1900

hA tMAM hA/QVP10, hA/QVP20

800 430−3000−430

comments 9.1% quaternization ratio with ethyl bromide 19.3% quaternization ratio with ethyl bromide PMAA homopolymer PMMA−PMAA−PMMA triblock stoichiometric balance of methacrylic acid and pyridyl groups

PMAA (HA). PtBMA [PtBMA: poly(tert-butyl methacrylate)], with a molecular weight (MW) 102 kg/mol (PDI = 1.08), was synthesized via anionic polymerization. The molecular weight and polydispersity were determined by GPC referenced to a polystyrene standard. The PtBMA was then hydrolyzed to PMAA (MW = 62 kg/mol) by hydrochloric acid (Sigma-Aldrich) in 1,4-dioxane (Sigma-Aldrich) at B


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Figure 2. Chemical structures of P4VP, QVP, and PMMA−PMAA−PMMA.

Figure 3. 1H NMR spectra of P4VP and QVP10. averaging at least three tests, and the calculation was based on the relationship between contact area, load, and displacement for indentation of adhesive thin films developed by Long et al.37 Tensile Tests. tMAM/QVP10 and tMAM/QVP20 gels were formed by dissolving equal amounts of a 10 wt % of triblock copolymer solution in DMSO and a 10 wt % solution of either QVP10 or QVP20 in DMSO, corresponding to a stoichiometric balance of methacrylic acid and pyridyl groups. These solutions were then mixed and the mixture was placed in a closed chamber saturated with water vapor. The gel formation and subsequent equilibration processes were identical to those used to form hA/QVP gels, but the tMAM polymer assembled into a micellar structure (PMMA micelle cores and swollen PMAA bridges) as water vapor diffused into the polymer solution.33 Tensile tests were carried out on samples with dimensions of approximately 15 mm × 15 mm × 1.5 mm. The initial distance between two clamps was 2.5 mm, and the stretching rate was 30 mm/ min (strain rate: 20%/s). The total strain energy density, Wf, obtained by integration of the tensile stress/strain curve up to the failure strain, ϵb, provides a measure of the toughness of the material: Wf =

∫0

ϵb

σN dϵN

ΓC = H0

∫0

ϵc

σN dϵN

(2)

Here H0 is the initial distance between clamps, and the stress−strain relation is taken from the loading curve for a specimen in the same geometry without a crack. The calculation of fracture energy is illustrated in Figure 4.

(1) Figure 4. Fracture energy calculation schematic. The top curve is the stress−strain curve for a specimen without a precrack. The bottom curve is the force−strain relationship in a precracked specimen, illustrating the crack initiation point at a strain, ϵc, obtained from video taken during the experiment.

Fracture Tests. In order to obtain a more quantitative measurement of the fracture energies of the tMAM/QVP10 and tMAM/QVP20 gels, hydrogels with dimensions of 15 mm × 15 mm × 1.5 mm were used to conduct fracture tests loaded in a pure shear geometry. The gels were produced by the same solvent exchange process used to produce the gels for the tensile tests. The initial distance between clamps was 2.5 mm, and the sample width was 15 mm. A 7.5 mm precrack was made by a razor before being loaded on the stage. One clamp moved at the rate of 30 mm/min (strain rate: 20%/s) to stretch the specimen and crack propagation was recorded by a Nikon camera at the capture rate of 24 fps. Crack initiation was decided from the recorded video at the point where the crack started to move forward. The fracture energy ΓC was calculated from the expression:38

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RESULTS AND DISCUSSION Transient Network Dynamics: hA/QVp20. Mixed Solvents. We find that QVP is fully soluble in DMSO for all degrees of quaternization and is soluble in water when the degree of quaternization is larger than ≈5%. The good solubility of QVP in both water and organic solvents makes

C


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Figure 5. Rheological responses of the hA/QVP20 system in mixed solvents (DMSO and water).

Figure 6. Solvent-frequency superposition of the hA/QVP20 system in mixed solvents and the dependence of solvent shift factor on water content. In (a), G′ values correspond to the filled symbols and G″ values correspond to the open symbols. (b) describes the dependence of both aS and η0 on water content.

the solvent composition in Figure 6b, where aS is defined to be equal to one for pure DMSO as the solvent. We also plot the zero shear viscosity, η0, which is proportional to aS and is obtained from the rheological behavior in the low frequency limit:

it an easily controllable and processable charged system for model studies of polyelectrolyte behavior. The dynamics of transient networks formed by PMAA and QVP20 in DMSO− water mixed solvents were investigated by measuring the linear viscoelastic properties of the equimolar solutions of PMAA and QVP20, at a total polymer concentration of 8.6 wt %. The results are shown in Figures 5 and 6. For the low water content in the solvents used for these experiments (30 wt % or less), these solutions are viscous solutions with a phase angle near 90° for the lowest frequencies, transitioning to a more solid-like behavior with a lower viscoelastic phase angle at higher frequencies. Addition of water increases the viscosity of the material, with an increased value in the |G*|, the magnitude of the complex shear modulus, as illustrated in Figure 5a. This increase in viscosity corresponds to an increase in the characteristic relaxation time of the gels, giving a transition to a more elastic response at a lower frequency as the water content within the solvent increases (Figure 5b). The effect can be quantified by introducing a solvent-dependent shift factor, aS, in order to superpose the frequency dependence of loss modulus, G″, for different solvent compositions. This superposition is illustrated in Figure 6a, where the storage and loss moduli are plotted against the solvent-shifted frequency. The shift factors used to generate these curves are plotted against

η0 = lim

ω→ 0

G″ ω

(3)

The effect of added water on the viscosity is significant, with η0 increasing by a factor of ≈7000 as the water content in the solvent increases from 0 to 30 wt %. The gelation in the hA/QVP20 system is induced by water addition, which increases the strength of the interactions between PMAA and QVP. This process involves the changes of different types of interactions, including electrostatic interactions, hydrogen bonding, and the association of hydrophobic groups in unquaternized P4VP. Water addition enhances the ionization ability of PMAA, increasing the electrostatic interaction between QVP and PMAA.23 At the same time, the hydrophobic groups (unquaternized pyridine) in QVP might associate as water content increases, and this phenomenon would contribute the gelation process as well. The observed sol−gel transition results from the combination of these interactions. D


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Macromolecules To determine the roles of different interactions in the gelation process, the effect of QVP charge ratio is examined in hA/QVP. We first performed experiments on PMAA/P4VP system in mixed solvents, in which electrostatic interaction between PMAA and quaternized P4VP is not involved, and find that the strong hydrophobicity of P4VP will lead to the precipitation phenomenon as water content increases to 10 wt %. Thus, the quaternization plays an important role in keeping QVP in solution so that a uniform, well-mixed solution remains thermodynamically stable across the full composition range from DMSO to pure water. When the charge ratio of QVP increases to 100% (hA/QVP100), in which hydrogen bonding and hydrophobic association are minimized, single phase, flowable systems are obtained in DMSO/water mixtures with water contents up to 40 wt %. A more quantitative illustration of the effect of QVP charge fraction is illustrated in Figure 7, where we plot the solvent

Table 2. pH Dependence of Polymer Volume Fraction and Young’s Modulus for hA/QVp20 Gels Equilibrated in Water at Different pH Conditions pH

ϕp

E (MPa)

5 6 7

0.03 0.17 0.33

0.002 0.014 1.1

Triblock Copolymer/Polyelectrolyte Hybrids: tMAM/ QVP. The toughness of the hA/QVP materials described above is limited by the fact that energy dissipation during fracture is restricted to a very small region in the vicinity of a propagating crack tip. The material can be strengthened by incorporation of a secondary network with a low cross-link density, thereby transferring stresses away from the crack tip, increasing the region within the sample in which energy is dissipated by breaking intermolecular complexes, and resulting in much larger fracture energy.41,42 This general mechanism is responsible for the toughness of the original double network gels formed by the Gong group1 and applies qualitatively to a broad category of toughened gels, including the materials described here. In our case the loose network originates by the addition of water-insoluble poly(methyl methacrylate) blocks (M blocks in our terminology) to the ends of the poly(methacrylic acid) polymer, producing what is in our terminology a tMAM triblock copolymer. The M blocks aggregate into micelle cores when a small amount of water is added, forming an elastic network consisting of the bridging A blocks. The activation energies of PMMA aggregates are in the order of 100 kJ/mol;34,43 thus, this elastic network could be considered as a permanent network bridged by strong interactions. The formation of this elastic network is described by a solvent shift factor, as, similar to the solvent shift factor used to describe the rheological data in Figure 6. The value of as for the tMAM triblock system is quite high, rising to ≈106 for water content in the solvent of only 8 wt %.34 This result indicates that the structure of the triblock gels is formed at an overall polymer concentration very close to the initial concentration of polymer in the original DMSO solution. An important consequence of this behavior is that materials with reproducible properties are readily obtained and that these properties can be modified simply by changing the initial concentration of the polymers in DMSO. Here we describe the effects of pH and salt content on the behavior of these materials. pH Effects. The effects of pH on polycation cross-linked triblock copolymer hydrogels are examined in two sets of hydrogels consisting of equal weight fractions of tMAM triblock copolymer and either QVP10 or QVP20. The properties of these materials after equilibration in salt-free solutions at different pH conditions are listed in Table 3. In Figure 8 we plot the pH dependence of the moduli and fracture energies for the tMAM/QVP10 and tMAM/QVP20 systems. For comparison, we also include the pH dependence of the hA/QVP20 system from the previous section. We use these data to illustrate several points. First, the moduli and fracture energy values are correlated with one another; stiffer materials also have higher fracture energy values, with both properties depending to a large extent on ϕp, the overall polymer volume fraction in the gel. In addition, the modulus increases with increasing pH, but this increase is much less prevalent for gels incorporating QVP10 than gels incorporating QVP20. Our

Figure 7. Comparison of zero shear viscosity of QVP10 and QVP20 with different water contents.

dependence of the viscosity for the hA/QVP20 and hA/QVP10 systems. The addition of water to the solvent mixture increases the viscosity in both cases, but the overall viscosity decreases as the charge ratio increases. Our view at this point is that the quaternization of the P4VP does not play a substantial direct role in the rheological response of the materials but that the dominant interactions are between the unquaternized groups and the methacrylic acid groups. These groups interact with each other more strongly as water is added to the solvent. Similar behavior has been observed in hydrophobically modified poly(methacrylic acid) in water/propylene glycol mixtures.39,40 The degree of quaternization does affect the equilibrium water content in the gels, which as we show below correlates strongly with the modulus and mechanical strength of the gels. pH Effects. The hA/QVP10 and hA/QVP20 materials both have a solid-like character in water due to the strength of the intermolecular complexes that are formed between the QVP and poly(methacrylic acid) molecules. The obtained hydrogels were equilibrated by immersing them in salt-free solutions at controlled values of the pH. The elastic moduli (measured by indentation) and polymer volume fractions of this set of hydrogels are listed in Table 2. At low pH, methacrylic acids are mostly in uncharged state. The strong polyelectrolyte QVP20 in the system possesses excess charges, leading to significant swelling. As the pH increases, the fraction of the carboxylate groups in the charged state increases, with consequent deswelling of the gel, an increase in polymer volume fraction, and a corresponding increase in the modulus of the gel. E


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from the previous cycle, as is not the case when the bonds are not able to reform. This self-healing capability is one of the important characteristics of the tMAM/QVP gels. Salt Effects. When considering the salt concentration in the gels, it is useful to compare to the concentration of methacrylic acid groups (or pyridnyl groups) in the gel as a point of reference. The tMAM polymer has a molecular weight, Mn, of 320 000 g/mol and a density, ρ, of 1.1 g/cm3. A tMAM molecule has 3000 methacrylic acid groups, corresponding to the degree of polymerization of the PMAA block, so the average molar concentration of methacrylic acid groups in the dry polymer is 3000ρ/Mn. When converted to molar units, this gives a concentration, Cp0, of 10.3 M. For a polymer gel with a polymer volume fraction of ϕp this concentration gets reduced to ϕpCp0/2. The factor of 2 here originates from the fact that in our case half of the total polymer fraction corresponds to the tMAM polymer, with the other half corresponding to the QVP polymer. We define a normalized salt fraction, where the concentration of salt in the external solution is divided by this internal concentration of acid groups within the gel:

Table 3. Properties of the Salt-Free tMAM/QVP10 and tMAM/QVP20 Gels specimen

pH

ϕp

E (MPa)

σb (MPa)

Wf (MJ/m3)

Γc (J/m2)

tMAM/QVP10 tMAM/QVP10 tMAM/QVP10 tMAM/QVP10 tMAM/QVP20 tMAM/QVP20 tMAM/QVP20

5 6 7 8 5 6 7

0.09 0.13 0.13 0.14 0.05 0.10 0.22

0.18 0.44 0.43 0.56 0.055 0.081 0.69

0.094 0.12 0.14 0.15 0.044 0.066 0.24

0.31 0.41 0.40 0.51 0.12 0.15 0.64

86 130 260 200 44 88 590

interpretation of this result is that a 10% quaternization balances the different polymer/polymer interactions in a way that results in a material that is relatively insensitive to changes in the pH. Compared with QVP10 cross-linked system, tMAM/QVP20 has higher concentration of unbalanced charges at low pH and thus swells more. The nonlinear mechanical properties of the materials under different pH conditions are illustrated by the uniaxial extension curves shown in Figure 9. The behavior is qualitatively similar to that observed with the corresponding zinc cross-linked hydrogels.3 The low strain modulus is determined by the density of ionic cross-links or interpolymer complexes. As the strain increases, these secondary bonds are broken and reformed, giving a progressive leveling off of the stress−strain curve. The behavior of the tMAM/QVP10 sample during cyclic loadings to progressively higher strains is shown in Figure 9b. The sample exhibits behavior similar to the Mullins effect in filled rubbers, where the behavior after cyclic loading depends on the largest strain to which the sample has previously been loaded,44 an effect that has been observed in fully covalent double network gels as well.45 In our materials, however, a substantial fraction of the strength is recovered as noncovalent bonds in the material are reformed. As a result, the loading curve for any cycle does not overlap with the unloading curve

fsalt =

2csalt ϕpC0p

(4)

Table 4 and Figure 10 show the effect of salt on Young’s modulus and fracture energy, which are both correlated to the swelling behavior of the gels. For NaCl the stiffest hydrogel can be formed when NaCl concentration is 1.1 M, corresponding to E = 4.9 MPa, with the highest fracture energy of 1085 J/m2. At the salt concentration as high as 4 M, the hydrogel still has a relatively high modulus of ∼2 MPa. The properties of the gels are only weakly dependent on the KBr concentration but depend more strongly on the NaCl concentration, with ϕp, E, and Γc all maximized for fsalt ∼ 1. In all cases we find a much smaller dependence of the properties on the salt concentration that is observed with coacervate systems, obtained from the equimolar mixtures of oppositely charged polymers.21

Figure 8. Effect of pH on the modulus (a) and fracture energy (b) of the gels in the absence of added salt. F


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Figure 9. (a) Tensile loading behavior of tMAM/QVP10 and tMAM/QVP20 hydrogels equilibrated at different pH conditions. (b) Response of a tMAM/QVP10 gel equilibrated at pH 5 during four consecutive tensile loading cycles to increasing strains. The curve labeled “part a” is the continuous loading curve for this same sample from part a.

higher than the salt concentration in the surrounding medium. The hydrogel expands the network by absorbing water to balance the osmotic pressure, resulting in significant swelling. As the salt concentration increases, the osmotic driving force for swelling is reduced. At still higher salt concentrations, corresponding to a roughly balanced ion concentration inside and outside the network, a transition from deswelling (collapse) to reentrant swelling is observed. At these high salt concentrations the electrostatic interactions between ion pairs are weakened due to salt screening, leading to reentrant swelling. It is reported that KBr is more effective in terms of breaking electrostatic pairs,21 and this may play a role in the difference that we observed between KBr and NaCl. The stoichiometry of polycations and polyanions plays an important role in the salt responses of polyelectrolyte hydrogels. Most charge balanced amphiphilic hydrogels only exhibit negligible gel collapse at low salt,7,47 while gel collapse is more distinct in charge unbalanced hydrogels. Also, Gao et al.48 demonstrated that unbalanced polyelectrolyte hydrogels show less swelling (softening) at high salt compared with balanced polyelectrolyte hydrogels. In our QVP cross-linked system the quaternization ratio of P4VP is 10% or 20%, so there is

Table 4. Salt Effects on Mechanical Properties of QVP20 Cross-Linked Triblock Copolymer Hydrogels salt

ϕp

pH

Csalt (M)

fsalt

E (MPa)

Γc (J/m2)

NaCl NaCl NaCl NaCl NaCl NaCl KBr KBr KBr KBr KBr

0.18 0.19 0.25 0.30 0.28 0.23 0.30 0.29 0.27 0.26 0.21

7 7 7 7 7 7 7 7 7 7 7

0.055 0.22 0.49 1.1 2.0 3.9 0.05 0.20 0.49 0.99 3.1

0.06 0.22 0.38 0.72 1.4 3.2 0.032 0.13 0.34 0.74 2.9

0.69 3.2 4.1 4.9 3.5 2.1 2.5 2.6 2.6 1.8 1.5

405 782 735 1085 589 499 638 745 584 576 465

The qualitative effects of salt can be understood in terms of the osmotic swelling and deswelling of the gels in different salt concentration regimes.3,46 At low salt concentrations we observe a deswelling of the gels, followed by a swelling of the gels at higher concentrations. At low salt concentrations, the concentration of counterions inside the hydrogels is much

Figure 10. Dependence of the elastic modulus (a) and fracture energy (b) on the concentration of either NaCl or KBr for QVP20 gels at pH 7. The dashed lines correspond to the properties of the corresponding gels in salt-free conditions. G


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Figure 11. Cross-plots of E vs ϕp (part a) and Γc vs E (part b) for the different gels from Tables 3 and 4.

where E is the modulus in MPa and Γc is in the unit of J/m2. This expression is very similar to the expression used by Luo et al. to describe the fracture energy of polyampholyte gels,7 where similar physical mechanisms are responsible for the material behavior:

substantial excess negative charges at pH values large enough so that the acid groups exist in their deprotonated, charged state. Overall Scaling of the Mechanical Response. During crack propagation tests, we observe crack blunting at the beginning, followed by crack initiation. This phenomenon has been widely reported in other hydrogel networks7,49 and contributes to the high toughness of hydrogels in a nontrivial way. The physical nature of the interactions in these system leads to a reversible capability as mechanical loading is applied, enlarging the highly strained zone in crack tip vicinity, dissipating more energy during crack propagation. The fracture of QVP cross-linked hydrogels happens by first breaking weak interaction based network and then rupturing micelle network. Our picture of this system is a double network hydrogel with a stronger network based on micelle cross-links and a weaker network built upon interactions, including electrostatic interactions, hydrophobic association, and hydrogen bonding, while the parameters we use to tune hydrogel properties are related to cross-link densities of these two networks. The loose network corresponds to the micellar network with a modulus determined by the total molecular weight of the polymer and can be described by the following expression:3 E = 0.72

ρRT 1/3 ϕ Mn p

Γc = 1230E 0.53

This expression is shown as the dashed line in Figure 11b, and describes the relationship between Γc and E for all of the polyampholyte gels studied by Luo et al. except for the stiffest gels, for which crack blunting is not observed.7 Our gels have values of Γc that are about a factor of 3.5 times lower than the gels of Luo et al.7 when compared at the same value of the modulus, presumably because the detailed molecular interactions responsible for the toughening are different in these two cases, as are the details of the molecular morphology. For example, the micellar cross-links in ours can be viewed as permanent interactions, giving a system that will not dissolve at any concentration of salt, whereas the polyampholyte and the closely related polyelectrolyte complex systems studied by the Gong group9 do not have permanent cross-links and are able to flow when exposed to high concentrations of salt.17 The desired behavior depends, of course, on the applications that one has in mind for these types of materials. The primary point that we are making by plotting the data as shown in Figure 11 is that for a given system good estimates of the mechanical response (both E and Γc) can be obtained by measurements of the equilibrium value of the polymer volume fraction, ϕp, at the conditions of interest, which will typically involve different values of pH, ionic strength and identity of the ionic species in the surrounding medium, and temperature.

(5)

This expression for the modulus is shown as the solid line in Figure 11a, where we plot the moduli of the gels as a function of their polymer volume fractions. The enhancement of the modulus in comparison to this value is as large as 3 orders of magnitude and is representative of the much higher density of the “weaker” bonds that make up the denser network of interactions between the QVP and PMAA segments. These bonds are broken in a reversible manner during deformation of the sample and are responsible for the enhanced toughness of these materials. Because both the elastic modulus and fracture energy of the gels depend on the density of these secondary bonds, which is in turn determined by the polymer volume fraction, we expect both of these properties to scale with the polymer volume fraction in some well-controlled way, resulting in a correlation between the fracture energy and modulus. This correlation is illustrated in Figure 11b. The solid line in this figure is the following fit to the data: Γc = 350E 0.6

(7)

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CONCLUSIONS We have developed a fully self-assembled hydrogel system with high toughness that consists of a loose network of strong bonds and a tighter network of weaker bonds. The loose network originates from the aggregation of poly(methyl methacrylate) end blocks of a high molecular weight triblock copolymer that has a poly(methacrylic acid) midblock. The weak bonds are formed by interactions of partially quaternized poly(4-vinylpyridine) with the methacrylic acid groups of the triblock copolymer. The interactions (hydrophobic associations, hydrogen bonding) between unquaternized groups and PMAA play essential roles in the formation the hydrogels, while the

(6) H


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Macromolecules

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electrostatic interaction between charged QVP and PMAA provides good processability and stimuli-responsive capabilities. These weak bonds formed by QVP and PMAA are responsible for the energy dissipation process, giving a material response that is less dependent on the salt concentration than that of traditional polyelectrolyte complexes. Measured values of the fracture energy are as large as 1 kJ/m2, scaling with the elastic modulus in a way that is consistent with the behavior of related gels where toughness is derived from the fracture of a high concentration of relatively weak, reversible bonds. The ability to tailor the materials by controlling the degree of quaternization and the chemical identity of the quaternizing agent provides the scope to optimize the material response for a wide range of applications.

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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.7b00304. Figures S1 and S2 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (K.R.S.). ORCID

Yaoyao Chen: 0000-0002-9217-5769 Kenneth R. Shull: 0000-0002-8027-900X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported through NSF DMR Polymers Program (DMR-1410968). This work made use of the MatCI Facility which receives support from the MRSEC Program (NSF DMR-1121262) of the Materials Research Center at Northwestern University. Also, this work made use of the IMSERC at Northwestern University, which has received support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF NNCI-1542205); the State of Illinois and International Institute for Nanotechnology (IIN). The authors want to thank Lingqiao Li’s help on running GPC tests to decide the molecular weight of PtBMA and also want to thank Dr. Qifeng Wang for his help with the indentation tests and for a variety of fruitful discussions.

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High-Toughness Polycation Cross-Linked Triblock Copolymer

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