Fabrication of Tough Hydrogels from Chemically Cross-Linked

Nov 23, 2016 - Increasing the number of polymeric networks significantly changed the mechanical properties of pseudo-IPN hydrogels even though the che...
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Fabrication of Tough Hydrogels from Chemically Cross-Linked Multiple Neutral Networks S. Shams Es-haghi and R. A. Weiss* Department of Polymer Engineering, The University of Akron, 250 S. Forge St., Akron, Ohio 44325-0301, United States ABSTRACT: A simple method was developed to fabricate tough hydrogels from a chemically cross-linked neutral hydrogel. Loosely cross-linked pseudoand true-interpenetrating polymer network (IPN) hydrogels with double-, triple-, and quadruple-network structures were synthesized from acrylamide (AAm), and their mechanical properties were studied. Increasing the number of polymeric networks significantly changed the mechanical properties of pseudo-IPN hydrogels even though the chemical composition and polymerization procedure of each individual network was the same. The SN and DN hydrogels showed behavior similar to extensible soft tissue, but the TN and QN hydrogels exhibited strain localization during tensile deformation. Loading−unloading−reloading tensile experiments indicated that tensile loading causes no damage to the SN and DN hydrogels. For pseudo-IPN hydrogels where strain localization occurred, unloading before strain localization resulted in no damage, but unloading after strain localization showed a large hysteresis due to the energy dissipation from damage to the internal structure of the sample. No damage occurred prior to failure in the SN, DN, and TN hydrogels during uniaxial compression, but QN hydrogel did suffer damage during compression.

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INTRODUCTION The idea of free-radical polymerization of a water-soluble monomer inside a highly cross-linked polyelectrolyte network developed by Gong et al.1 opened a new perspective for research on soft wet materials. This hydrogel architecture that was originally believed to be an interpenetrating or semiinterpenetrating polymer network (IPN or SIPN) of a soft neutral polymer network within a highly cross-linked polyelectrolyte network was termed a double-network (DN) hydrogel. Recently, infrared spectra evidence for grafting of the second polymer to the skeleton of the first network was reported by Shams Es-haghi et al.,2 who also found that the grafting between the two networks is essential for achieving toughness of a DN hydrogel. In fact, the actual microstructure of a tough DN hydrogel is either a pseudo-IPN or pseudo-SIPN, the word pseudo being used because the two interpenetrating polymers that comprise the double network are at least partially bonded to each other. In synthesis of conventional DN hydrogels, as invented by Gong et al.,1 the first gel is a highly cross-linked polyelectrolyte network. Despite its very high cross-link density, a polyelectrolyte network has a very high swelling ratio, and therefore it can absorb a large amount of the reaction mixture for a second polymerization step. A DN hydrogel synthesized by polymerization of a neutral first network using the same composition used for polyelectrolyte gels is brittle. Recently, Nakajima et al.3 showed that by using a linear polyelectrolyte “molecular stent” it was possible to generalize the DN concept and synthesize tough hydrogels from a neutral first network. They used the molecular stent to increase the swelling ratio of the neutral gel. Nakajima et al.3 reported that the molecular stent does not contribute to the mechanical behavior; it only increases the © XXXX American Chemical Society

osmotic pressure and accordingly the swelling ratio of the neutral gel. The generalization of the DN concept by Nakajima et al.3 raises the question of whether the participation of an ionic component is necessary for making a tough chemically crosslinked hydrogel. That is, can one design a tough chemically cross-linked hydrogel without using an ionic component? Recently, Argun et al.4 synthesized DN and triple-network (TN) hydrogels from acrylamide (AAm) and N,N-dimethylacrylamide (DMAAm). They synthesized the first network, a single-network (SN) gel, from AAm or DMAAm, and then prepared the DN by swelling the first network in an aqueous solution of AAm or DMAAm without a cross-linking monomer and then thermally polymerizing the solution. The latter process was repeated to polymerize the third network. According to the chemistry that was used to synthesize their DN hydrogel (i.e., there remained some residual unsaturation in the first network), some grafting of the second network to the first must have occurred, and therefore by the notation used in the present paper, those were pseudo-SIPNs. Since, based on the results reported in ref 2, there was probably no residual unsaturation in the polymerization of the second network in the TN hydrogels of ref 4, the third polymerization should have produced linear chains. Thus, those materials would have long chains grafted to a cross-linked network (i.e., the second polymer grafted to the first network), plus ungrafted linear chains from the third polymerization. In that design, the second and third polymers also formed supramolecular (physical) networks by chain entanglements and for the AAm polymer Received: October 18, 2016 Revised: November 16, 2016

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DOI: 10.1021/acs.macromol.6b02264 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules hydrogels, also hydrogen bonding between AAm groups. Argun et al.4 measured uniaxial compression properties and reported that their TN hydrogel sustained extremely high compressive stress. This paper aims to extend the work of Argun et al.4 to multiple-network hydrogels with nonionic species where in contrast to the gels reported by the Argun et al.4 all of the networks were covalently cross-linked; i.e., the gels were pseudo-IPNs. In the pseudo-SIPN design of Argun et al.,4 the grafted linear second polymer chains and the ungrafted linear third polymer chains had physical “cross-links” that would be expected to contribute toughness because the supramolecular bonds can be reversibly broken when stressed. That provides a convenient mechanism for energy dissipation without rupturing covalent bonds.5 The question then is whether a hydrogel design of multiple covalently cross-linked networks can produce a tough material. Note that in all the DN and TN designs used by other researchers who follow the synthetic protocols of Gong et al.,1 at least the second network is covalently attached to the first network (i.e., a pseudo-IPN or pseudo-SIPN) due to the presence of residual double bonds after the polymerization of the first network. As discussed in an earlier paper,2 for such a system the polymerization of the second polymer, whether it is intended to be a linear or crosslinked polymer, at least partially grafts the second polymer to the first by polymerization occurring from the residual double bonds on the first network or from chain-transfer reactions during the second polymerization step. In the present work, very low concentrations of cross-linking monomer were used in the second and subsequent polymerization steps so as to achieve loosely cross-linked polymer networks. AAm was used for all of the network polymerization steps. The objective of the study was to determine if two, three, and four network pseudo-IPNs could produce tough hydrogels. Mechanical behavior in tension and compression is reported. A second objective of this research was to compare the mechanical properties of a multi-network pseudo-IPN to a multi-network IPN, i.e., a multiple-network hydrogel where the residual unsaturation of a network was removed before the polymerization of a subsequent network.

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Scheme 1. Synthesis of a Poly(acrylamide) Single Network (SN)

hydrogel, and the TN hydrogel was used to make a quadruple-network (QN) hydrogel. SN, DN, and TN hydrogels were also synthesized from HEA using the same reaction compositions and conditions for FTIR characterization. This was done to determine whether the residual unsaturation remains in the hydrogels after polymerization. HEA hydrogels were used because for hydrogels made of AAm, the infrared bending vibrations for the N−H group in AAm occur in the same spectral region as does the band for vinyl unsaturation, which complicates the identification of residual double bonds in those gels. This is not a problem for HEA gels, since HEA has no N−H bond. Since both gel chemistries involve conventional free-radical vinyl polymerization, it is most likely that the results and conclusions for the HEA system are also applicable to other hydrogels, such as those made of AAm. Synthesis of Loosely Cross-Linked True IPNs. For the synthesis of loosely cross-linked true IPNs, the residual unsaturation following any polymerization step was removed prior to the next polymerization step. A procedure for accomplishing that was described in an earlier paper.2 After the first network, SN, was prepared, it was immersed into a deoxygenated 0.1 M aqueous solution of VA-044. When the (VA044)-swollen gel equilibrated with the solution, it was heated for 1 h at 60 °C. The sample was then cooled slowly to room temperature and washed at least seven times with fresh DI water over a period of 7 days. The deactivated SN, denoted as (VA-044)-r-SN, was immersed into a reaction mixture of AAm until an equilibrium swelling was achieved. The AAm-swollen (VA-044)-r-SN gel was then placed between two parallel glass slides and exposed for 20 min to 365 nm ultraviolet (UV) light (15 mW/cm2) (Scheme 1). The resulting gel is called a deactivated DN, DDN. The same process was conducted on DDN to produce DTN and DQN. The recipes used to synthesize the different networks in this study are denoted as AAm(w,x,y,z), where w is the concentration (M) of AAm in the reaction solution, x is the OXGA initiator concentration (mol % with respect to AAm), y is the MBAA cross-linking monomer concentration (mol % with respect to AAm), and z is the UV dose (J/ cm2) used for the polymerization. Therefore, AAm(4,0.1,0.01,18) corresponds to the polymerization of a 4 M AAm aqueous solution using 0.1 mol % OXGA, 0.01 mol % MBAA, and a UV dose of 18 J/ cm2. The hydrogels synthesized and used in this study are summarized in Table 1. The resulting hydrogel samples were immersed in DI water, which was replaced a number of times with fresh DI water, to remove any unreacted monomer. Polymer Characterization. Fourier transform infrared (FTIR) spectroscopy was used to determine if residual unsaturation remained in the HEA networks after UV curing. A Nicolet 380 FTIR spectrometer was used with 32 scans and a resolution of 4 cm−1. Samples were washed at least five times with deionized water to remove any unreacted monomer and then dried before the FTIR measurements. Tensile Testing. Uniaxial tensile tests were performed with an Instron 5567 universal testing machine equipped with a 100 N load cell using a constant crosshead speed of 50 mm/min at room temperature. The samples were cut into a dumbbell shape using an ASTM D-638 Type V cutting die. Samples with the thickness more than 2 mm were cut using an ASTM D-638 Type IV cutting die. Sandpaper was used to prevent slippage of the samples in the grips.

EXPERIMENTAL SECTION

Materials. Acrylamide (AAm) was purchased from Sigma-Aldrich Chemical Co. and used as received. 2-Hydroxyethyl acrylate (HEA) was obtained from Sigma-Aldrich Chemical Co. and was vacuum distilled prior to use. The cross-linking agent N,N′-methylenebis(acrylamide) (MBAA) was purchased from Sigma-Aldrich Chemical Co. and was recrystallized from ethanol. A photoinitiator, 2-oxoglutaric acid (OXGA), was obtained from Fluka Chemical Co. and used as received. The initiator 2,2′-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride (VA-044) was purchased from Wako Chemical Co. and used as received. Synthesis of Loosely Cross-Linked Pseudo-IPNs. A reaction mixture was prepared by adding OXGA (0.1 mol % with respect to AAm) and MBAA (0.01 mol % with respect to AAm) to a 4 M solution of AAm in deionized (DI) water. The reaction solution was then injected into a glass mold made of two parallel glass slides, which was then exposed for 20 min to 365 nm ultraviolet (UV) light (15 mW/cm2) (Scheme 1). The resulting SN hydrogel was immersed into the same reaction mixture until an equilibrium swelling was achieved. The AAm-swollen SN gel was then placed between two parallel glass slides and exposed for 20 min to 365 nm UV light (15 mW/cm2) (Scheme 1). A TN hydrogel was synthesized using the same procedure with the DN B

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the cross-link density, one can achieve optimum mechanical properties for a particular chemistry, but for a covalently crosslinked gel, the stiffness and extensibility are coupled; i.e., as stiffness increases, extensibility decreases, so it is not possible to independently optimize both values for a conventional gel. Since high cross-link density produces brittle behavior, i.e., low extensibility, all the networks synthesized in this research were loosely cross-linked and can undergo large deformations. The initial polymeric network is denoted as SN, and subsequent networks synthesized within a previous network are designated as DN (double network), TN (triple network), and QN (quadruple network). The SN, DN, and TN hydrogels were used as precursors for the DN, TN, and QN hydrogels, respectively. Since the networks interpenetrate each other (though each network is chemically bonded to the previous network, as will be discussed later in this paper), adding each subsequent network in a unified architecture increases the effective or average cross-link density of the resulting gel. As such, it is expected that the greater the number of networks, i.e., polymerization steps, the greater is the load the sample should sustain. Since the cross-links are distributed among multiple networks and not on a single network, catastrophic failure at low sample extension is prevented as the cross-linking density of the assembled network increases. Free-radical synthesis of cross-linked acrylic polymers, such as shown in Scheme 1, generally produces inhomogeneous networks as a consequence of undesirable side reactions that may leave some residual vinyl bonds in the final network.8,9 Evidence of residual carbon−carbon double bonds can be found using FTIR spectroscopy. However, AAm is a primary amide, and absorption of the N−H bending (amide II band) at ∼1610 cm−1 overlaps with the vinyl IR absorption at ∼1650 cm−1,10 which complicates the resolution of a low concentration of CC in gels made from AAm. In order to demonstrate that the residual unsaturation occurred in the AAm polymerizations used in this work, multiple polymerization of HEA were also carried out to synthesize SN, DN, and TN with the same compositions as used for the AAm multiplenetwork gels. The advantage of using the HEA monomer is that there are no IR absorptions with HEA than overlap with the vinyl unsaturation, so resolution of any residual unsaturation is unambiguous. Figure 2 shows the FTIR spectra of HEA-SN and HEA-DN hydrogels. The CC absorption band at ∼1650 cm−1 in both spectra is due to residual CC unsaturation. The same result was observed for the HEA-TN hydrogel. The assumption here

Table 1. Recipe Used To Synthesize Hydrogels recipe

hydrogel

AAm(4,0.1,0.01,18) SN/AAm(4,0.1,0.01,18) DN/AAm(4,0.1,0.01,18) TN/AAm(4,0.1,0.01,18) (VA-044)-r-SN/AAm(4,0.1,0.01,18) (VA-044)-r-DDN/AAm(4,0.1,0.01,18) (VA-044)-r-DTN/AAm(4,0.1,0.01,18) HEA(4,0.1,0.01,18) HEA-SN/HEA(4,0.1,0.01,18) HEA-DN/HEA(4,0.1,0.01,18)

SN DN TN QN DDN DTN DQN HEA-SN HEA-DN HEA-TN

The tensile data are reported as engineering stress versus stretch ratio, where the engineering stress, σe, was obtained by dividing the force by the original cross-sectional area of the specimen and the stretch ratio, λ ≡ L(t)/L0, where L(t) is the instantaneous sample length and L0 is the original sample length. Note that λ = εe + 1, where εe is the engineering tensile strain. Compression Testing. Uniaxial compression tests were performed with an Instron 5567 universal testing machine equipped with a 10 kN load cell, using a constant crosshead speed of 5 mm/min at room temperature. Cylindrical samples having length-to-diameter ratios, L/D, less than 2.0 were used to avoid buckling and provide accurate measurements.6 The surface of the samples was dried and then lubricated with silicone oil to reduce friction.7 The compression data are reported as engineering compressive stress versus compressive strain, where the engineering stress, σe, was obtained by dividing the force by the original cross-sectional area of the specimen and the compressive strain, ε ≡ (Δh/h0) × 100%, where Δh is the instantaneous change in sample thickness and h0 is the original sample thickness.

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RESULTS AND DISCUSSION Chemically cross-linked hydrogels usually have poor mechanical properties, which is due to their inability to dissipate energy of deformation when they are exposed to an external force. Depending upon the concentration of cross-linking agent used to make chemically cross-linked gels, the tensile mechanical behavior of these materials lies between two extremes, Figure 1.

Figure 1. Tensile mechanical behavior of a chemically cross-linked hydrogel lies between two extreme cases depending on the extent of cross-link density.

When the cross-link density is very high the material is extremely brittle, red line in Figure 1. Often, the sample actually fails in the grips of a tensile machine. When the cross-link density is very low, the sample can be extended to a high stretch ratio but it cannot achieve very high stress, blue curve in Figure 1. Therefore, the cross-link density is the key factor that controls the mechanical behavior of the sample. By changing

Figure 2. FTIR spectra of SN and DN hydrogels made of HEA. The intensities are normalized at carbonyl wavenumber. C

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Macromolecules is that since the reaction kinetics and statistics of monomer addition are similar for any free-radical polymerization reaction, the SN, DN, and TN hydrogels prepared from AAm should have produced similar residual unsaturation in the SN and each subsequent multiple network. Therefore, the multiple AAm network hydrogels reported herein were actually pseudo-IPNs, where the networks interpenetrate each other, but are grafted to each other as shown in Figure 3.

Figure 4. Engineering tensile stress versus stretch ratio for SN, DN, TN, and QN AAm hydrogels. Arrows show the strain localization in TN and QN hydrogels.

underestimated since they were calculated from the data prior to the sample slipping. The mechanical properties of the hydrogels are summarized in Table 2. Apart from the high tensile strength achieved by the Table 2. Mechanical Properties of Loosely Cross-Linked Pseudo-IPN Hydrogels

Figure 3. Schematic representation of loosely cross-linked pseudointerpenetrating polymer networks: (a) SN, (b) DN, (c) TN, and (d) QN. The black chains show the grafted links between the different networks.

An important distinction between this study and previous reports of multiple-network hydrogels is that the other research groups polymerized linear networks after the initial cross-linked SN (i.e., they studied pseudo-SIPN hydrogels) while this study focused on pseudo-IPN structures, where cross-linked networks were synthesized in each polymerization step (see Figure 3). Conventional DN hydrogels, such as reported in the original work of Gong1 and repeated by many other research groups, are composed of a brittle first network (i.e., high cross-link density) and a softer second network (lower cross-link density) that are covalently attached to each other, as discussed earlier in this paper. Covalent bonding between the two networks is required to achieve a tough hydrogel.2 In this study, however, none of the individual networks were brittle, though it will be shown later in this paper that some grafting between loosely cross-linked gels is also necessary for achieving toughness of these structures. Figure 4 compares the mechanical behavior of the SN, DN, TN, and QN hydrogels prepared from loosely cross-linked AAm networks. The SN shows the typical behavior expected for a loosely cross-linked network. It is highly extensible but does not support a high load. The second polymerization step results in a small reduction in the extension at break, but the tensile strength increased by a factor of 4.40 and the work of deformation of the sample (i.e., the area under the stress−strain curve) increased by a factor of 2.25. After the initial reduction of the ultimate elongation from the SN to the DN, increasing the number of polymerization steps did not further decrease the sample’s extensibity, but the strength and strain energy increased markedly. Note that the QN sample did not fail. For λ > 8 the samples slipped out of the clamp fixture, so the properties reported for the QN hydrogel are actually

hydrogel

λ at break

σe at break (MPa)

work of deformation (MJ/m3)

work of deformation ratio with respect to SN

SN DN TN QNa

10.2 8.18 7.74 7.88

0.220 0.970 1.13 1.83

0.590 1.33 2.69 5.13

1.00 2.25 4.56 8.69

a

QN did not break.

QN hydrogel, an intriguing feature of the mechanical behavior of these materials was that the TN and QN hydrogels exhibited strain localization during finite deformation, as is denoted by the arrows in Figure 4. This occurred at a single stretch ratio during in the stress−strain experiment for the TN hydrogel and at two separate stretch ratios for the QN hydrogel. That phenomenon in the TN and QN hydrogels is a consequence of local inhomogeneities of the cross-link junctions which produces clusters (microgel),11 which occurs from increasing the number of polymerization steps or from translation of the cross-link junctions in the loosely cross-linked networks during deformation. The cross-link junctions, even in a loosely crosslinked network, represent an impediment to mobility of other parts of the network. As a consequence, although the relatively small segments of a network chain may have mobility similar to a polymer chain in solution, the translation of a junction and the segments of the four network chains attached to the junction may be suppressed, especially when they approach another junction with its four attached network chains. The suppression of the junction and the adjoining chains produces heterogeneities in the network where junction points form clusters due to increasing local density of the cross-links. This, in turn, may result in strain localization that can produce a localized fracture of the cluster structure, but since the average cross-link density in the neighborhood of the cluster is lower, the fracture of the cluster does not propagate. As a result, although brittle localized fracture of the clusters may occur, but D

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higher extension was needed for the strain localization to occur. The observation of two strain localizations for the virgin QN hydrogel suggests some heterogeneity in the structure of that gel. The first strain localization occurred at a very low stretch ratio, and its disappearance after applying an initial compressive load is most likely due to brittle regions of high local cross-link density. The second strain localization at a higher stretch ratio, which can persist after applying a compressive load, occurs when the local cross-link density increases under tension, most likely due to rearrangements of the cross-link distribution when the chains were stretched. Further insight into the strain localization was obtained from loading−unloading−reloading tensile tests, shown for the SN and DN hydrogels in Figure 7. The samples were first loaded to an arbitrarily high stretch ratio, then unloaded, and immediately reloaded to the failure point. If mechanical loading causes damage, the mechanical properties of the sample will exhibit hysteresis, and the reloading deformation path will deviate from the initial loading path. For the SN and DN hydrogels, where no strain localization behavior was observed, no hysteresis was observed (Figure 7). For both samples, the loading− unloading−reloading data completely coincided indicating that tensile preloading produced no damage to the samples. In contrast to the mechanical behavior of the SN and DN hydrogels, the loading−unloading−reloading experiment for the TN and QN hydrogels, where strain localization occurred, showed considerable mechanical hysteresis (Figures 8 and 9). Strain localization, which damaged the internal structure of the hydrogel, was responsible for the hysteresis. For Figure 8a, the TN sample was first loaded and unloaded before reaching the strain localization point, and it was then reloaded to failure. In that case, no hysteresis was observed. However, when the TN hydrogel was loaded to beyond strain localization and then unloaded and reloaded to failure, a large mechanical hysteresis occurred and the strain localization occurred at a higher stretch ratio (Figure 8b). Moreover, the reloading path continued along the unloading path until it approached the stretch ratio used in the unloading experiment. At that point, the stress deviated from the unloading path with a lower stress, but eventually at higher stretch ratios the stress coincided with the stress−strain path of the original sample that was not preloaded beyond strain localization; cf. the blue data points in Figures 8a and 8b for λ > ∼6. Similar tensile mechanical behavior was observed for the QN hydrogel (Figure 9). When the sample was loaded to beyond the first strain localization (black data points), but before the second strain localization, and then unloaded (red data) and reloaded (blue data) (Figure 9a), a small hysteresis occurred that corresponded to the loss of the first strain localization at relatively low strain, but the second strain localization persisted in the reloading experiment. When the QN hydrogel was loaded to beyond the second strain localization and then unloaded and reloaded (Figure 9b), a greater hysteresis was observed and the second strain localization moved to higher stretch ratio. The hysteresis observed for the TN and QN hydrogels in Figures 8 and 9 is the direct consequence of damage to their microstructure, and the area enclosed by the hysteresis loop is the amount of energy dissipated per unit volume to damage the material. The increase in the stretch ratio at which the strain localization was observed for the TN and QN hydrogel when the sample was prestrained to beyond the stretch ratio at strain localization is also a consequence of the damage to the sample by the preload. In that case, some of the

the macroscopic gel is still ductile, which explains the rapid stress hardening behavior of the TN and QN hydrogels after strain localization occurs. A previous study12 showed that necking, which is the manifestation of strain localization, in the early DN hydrogel formulations1 is due to the brittle structure of the first network, which was highly cross-linked. When those original DN hydrogels were predamaged by a compressive load, no necking was observed because the strain localization was eliminated by fracture of the first network by the compression. As a consequence, the application of a compressive load on the sample prior to tensile deformation may be considered a convenient mechanical test for checking the clustering hypothesis. If the clusters are very brittle, they will break under a compressive load, and therefore the effect will disappear in a subsequent tensile test. Figure 5 compares the

Figure 5. Engineering tensile stress versus stretch ratio for virgin TN and a TN hydrogel after being compressed by a 10 kN load. The arrow shows the strain localization in the samples.

tensile mechanical behavior of an as-made TN hydrogel and the same hydrogel after being precompressed with a 10 kN load. Strain localization is seen at the same stretch ratio for both samples. Thus, after the third polymerization step in the synthesis of a multiple network, the clusters will not be predamaged under a compressive load. The near coincidence of the red and blue data in Figure 5 indicates that for the TN hydrogel the damage due to compression is negligible. Figure 6 compares the tensile mechanical behavior of a virgin QN hydrogel with the behavior of the same gel, but after first being compressed with a 10 kN load. The first strain localization disappeared after compression, and the second strain localization shifted to a little higher strain, indicating that

Figure 6. Engineering tensile stress versus stretch ratio for virgin QN hydrogel and a QN hydrogel after being compressed by a 10 kN load. Arrows show the strain localization in the samples. E

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Figure 7. Loading−unloading−reloading tensile experiments for (a) SN and (b) DN hydrogels synthesized from AAm. Black triangles show the loading path, red triangles show unloading path, and blue triangles show the reloading after unloading. The coincidence of data points in these deformations indicates that tensile loading results in no damage to the sample.

Figure 8. Loading−unloading−reloading tensile experiments on TN hydrogel (a) before strain localization and (b) after strain localization. Black triangles show the loading path, red triangles show unloading path, and blue triangles show the reloading after unloading. Before strain localization no damage occurs in the sample; however, after strain localization a large hysteresis is observed which is attributed to the damage to internal structure of the gel.

Figure 9. Loading−unloading−reloading tensile experiments on QN hydrogel (a) after the first strain localization and (b) after the second strain localization. Black triangles show the loading path, red triangles show unloading path, and blue triangles show the reloading after unloading. A hysteresis is observed in both cases, indicating that strain localization results in damage to the internal structure of the material.

cross-link structure was probably destroyed, and therefore, a higher stretch ratio was required to form a cross-link cluster that caused strain localization. Figure 10 shows the mechanical behavior for the SN, DN, TN, and QN hydrogels for uniaxial compression. Two consecutive uniaxial compression tests were conducted on each sample, and none of the samples broke during the tests. The two sets of data for the SN, DN, and TN hydrogels in Figure 10 overlapped, which indicates that compressive loading did not damage the samples. However, the stresses for second compression test data for the QN hydrogel were lower than for the first compression test. These results are consistent with the data in Figures 5 and 6, where precompression with a 10 kN load had little effect on the tensile behavior of the TN hydrogel, but it eliminated the first tensile strain localization and

significantly reduced the tensile stress prior to the second tensile strain localization for the QN hydrogel. A previous study2 reported that in a conventional DN hydrogel synthesized from the polymerization of a loosely cross-linked network inside a highly cross-linked polyelectrolyte network, grafting between the first and second network is necessary for achieving a tough hydrogel. True-IPN DN hydrogels, where there are no covalent bonds between the two networks, are brittle.2 The pertinent question then is: how do the connections between successive networks in a multiplenetwork hydrogel produce toughness? The building blocks of all the hydrogels discussed in the current study were loosely cross-linked and highly extensible, weak polymeric networks. In contrast to conventional DN hydrogels, none of the individual network components of the F

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link density in a network to make a hydrogel that undergoes large deformations and achieve very high stress. A DN structure of two networksone with very high crosslink density and the other with very low cross-link densitycan produce a tough hydrogel.1 Generally, however, this requires hydrogel design where the first network is a highly cross-linked polyelectrolyte network and the first and second network must be covalently grafted to each other.2 In this paper, a simple method was described for fabricating tough chemically cross-linked hydrogels using neutral polymer networks. In this method sequential free-radical polymerization was used to make multiple loosely cross-linked network structures from one chemical composition. As with the conventional DN design, the multiple, loosely cross-linked networks must be chemically linked. That occurs naturally in the sequential free-radical syntheses of the different networks due to residual double bonds in each sequential network. As a result, the molecular architecture of the multiple-network hydrogels reported herein is that of a pseudo-interpenetrating polymer networks (pseudo-IPNs). Since the multiple networks interpenetrate each other and each network is chemically bonded to the previous network, increasing the number of polymeric networks increases the effective or average cross-link density of the resulting structure. However, unlike true IPN hydrogels that are brittle, the pseudo-IPN structure with loosely cross-linked networks produces high extensibility and high stress, the latter due to the high average cross-link density. The number of polymerization steps did not significantly affect the sample’s extensibity, but the cross-link density increased with increasing polymerization steps, which allowed the gel to support higher stresses. Thus, this approach to the design of a multiple-network pseudo-IPN hydrogel allows one to independently optimize stress and strain. The SN and DN hydrogels showed stress−strain behavior similar to an extensible soft tissue, but as the cross-link density increased with subsequent polymerization steps, the hydrogels exhibited strain localization during tensile deformation due to the clustering of cross-links.11 Increasing the number of loosely cross-linked networks, even when the cross-link density in each network is very low, leads to heterogeneities in the distribution of cross-link density, which creates clusters that produce brittle regions of high local crosslink density where local failure of the sample may occur. Despite the fact that only a single chemistry, i.e., AAm polymers, was used in this study, it is expected that the general conclusions with regard to the effect of multiple networks will also apply to multiple-network gels prepared with different monomers. That is based on the similarity of the free radical reactions expected. However, the chemistry of the monomer will affect properties, such as the equilibrium water absorption, the molecular weight of linear chains in the case of pseudo-SIPN multiple-network gels, and whether or not supramolecular interactions such hydrogen bonding occur. As a result, the actual mechanical properties of multiple-network hydrogels based on different monomers, but produced in a manner similar to the methods used herein, are expected to be different.

Figure 10. Compressive stress versus compressive strain for SN, DN, TN, and QN hydrogels. Blue lines: first compression test on the samples; red lines: second compression after the first run on the same sample.

hydrogels discussed herein were brittle. Figure 11 compares the mechanical behavior of loosely cross-linked pseudo-IPNs and

Figure 11. Engineering tensile stress versus stretch ratio for DN, TN, QN, DDN, DTN, and DQN hydrogels. Note that the QN hydrogel did not break during the tensile experiment.

loosely cross-linked true-IPNs, the latter prepared by deactivation of the residual double bonds in the precursor structure as described in the Experimental Section. The true-IPNs had lower ultimate strain, tensile strength, and strain energy to break than their pseudo-IPN counterparts, which is consistent with the earlier conclusion2 that grafting between networks plays a critical role in achieving toughness. Note that for the true-IPN hydrogels the ultimate strain and stress for TN were greater than for DN, but the ultimate strain and stress decreased dramatically from TN to QN, which was much brittler than the other gels. Increasing the number of successive loosely cross-linked networks for a pseudo-IPN hydrogel increased toughness, while increasing the number of loosely cross-linked interpenetrating networks for a true-IPN hydrogel actually embrittled the material to the extent that it behaved similar to a highly cross-linked single network.

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CONCLUSIONS The mechanical behavior of a conventional, chemically crosslinked hydrogel is controlled by its cross-link density. Depending on the cross-link density the tensile mechanical behavior of a chemically cross-link hydrogel can change from a soft extensible gel to an extremely brittle material. Because of the coupling between extensibility and stiffness in a chemically cross-linked network, it is not possible to optimize the cross-

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (R.A.W.). Present Address

S.S.: Birck Nanotechnology Center, Purdue University, West Lafayette, IN 47907. G

DOI: 10.1021/acs.macromol.6b02264 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by a grant, CMI-1300212, from the Civil, Mechanical and Manufacturing Innovation (CMMI) Division within the Engineering Directorate of the National Science Foundation.

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DOI: 10.1021/acs.macromol.6b02264 Macromolecules XXXX, XXX, XXX−XXX