Deconstructing the Double-Network Hydrogels - American Chemical

Jun 30, 2014 - synthesized from SAPS and acrylamide (AAm) indicate that linear and ... grafting of chains in the polymerization of the second network ...
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Deconstructing the Double-Network Hydrogels: The Importance of Grafted Chains for Achieving Toughness S. Shams Es-haghi, A. I. Leonov,* and R. A. Weiss* Department of Polymer Engineering, The University of Akron, 250 South Forge Street, Akron, Ohio 44325-0301, United States ABSTRACT: This paper aims to shed light on the microstructure of tough, “double-network” (DN) hydrogels synthesized by free-radical polymerization of a monomer within a highly cross-linked polyelectrolyte hydrogel and to discuss the most efficient topological microstructure for toughness enhancement. Fourier transform infrared (FTIR) characterization of a hydrogel synthesized from the potassium salt of 3-sulfopropyl acrylate (SAPS) and 2-hydroxyethyl acrylate (HEA) demonstrated that polymer chains synthesized during the second polymerization step of a conventional DN hydrogel are grafted to the skeleton of the polyelectrolyte network. Uniaxial tensile tests performed on hydrogels synthesized from SAPS and acrylamide (AAm) indicate that linear and nonlinear polymerization of a second monomer within a network without grafting to the first network, i.e., forming a semi-interpenetrating or interpenetrating network, does not produce a tough hydrogel. Toughness enhancement of a covalent hydrogel was optimized by grafting high molecular weight polymer chains with a free end to a first, highly cross-linked polyelectrolyte network with residual unsaturation. The concentration of the grafted chains is a crucial factor in determining the mechanical behavior of the hydrogel.

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Unfortunately, the work of Nakajima et al.6 and subsequent work by the Gong research group did not provide any experimental evidence for the occurrence of grafting or lack of grafting during the second polymerization in the preparation of a DN hydrogel. Hitherto, the possibility of grafting the second polymer or network to the first network or the definitive synthesis of a true IPN or SIPN hydrogel have not been confirmed. Nakajima et al.6 also described experiments during which they purportedly removed the residual vinyl unsaturation from the first network by swelling it with an initiator solution (0.1 M 2-oxoglutaric acid aqueous solution) and exposing the mixture to UV light (365 nm) for 10 h.6 They concluded that “almost all” of the residual unreacted double bonds in the crosslinked PAMPS gels, which would arise from incomplete reaction of the N,N′-methylene bis(acrylamide) (MBAA) cross-linking agent, were eliminated by their procedure, which produced “inert-PAMPS gels” that they called i-PAMPS gels. Again, no experimental evidence was reported to substantiate their presumption of eliminating the residual unsaturation from the first network. All the conclusions in that paper and subsequent papers on DN hydrogels were based on the assumption that the procedure described in ref 6 was sufficient to remove residual unsaturation and that they were able to synthesize true IPN or SIPN DN hydrogels. Infrared spectra evidence for the presence of residual vinyl unsaturation in the first, polyelectrolyte network in the synthesis of a DN hydrogel was recently reported,8 and that

INTRODUCTION The similarity of hydrogels to biological tissues makes them attractive candidates for applications such as soft robotics, molecular filters, drug delivery, and tissue engineering.1 The main deficiency of hydrogels is their usually poor mechanical properties, which is due to their large water content.2 The problem of poor mechanical properties was resolved by Gong et al.3 who developed a new hydrogel architecture that was originally believed to be an interpenetrating polymer network (IPN) of a soft neutral polymer network within a more highly cross-linked polyelectrolyte network, which they termed a double-network (DN) hydrogel. Those hydrogels were prepared by a two-step sequential free-radical polymerization. In the first step, a highly cross-linked polyelectrolyte network was synthesized and in the second step the polyelectrolyte network was swollen with a water-soluble monomer that was then polymerized within the first network. The second polymerization step was conducted with or without adding a crosslinking agent−that is, the product was thought to be either an IPN or a semi-IPN (SIPN). According to the papers by Gong and co-workers,4,5 DN hydrogels prepared without using a cross-linking agent in the second polymerization step form tougher hydrogels than when two cross-linked networks are used. In 2009, the IPN model for the microstructure of DN hydrogels was questioned by Nakajima et al.6 who considered that the presence of residual vinyl (CC) unsaturation in the first network (poly(2acrylamido-2-methylpropanesulfonic acid) gels, PAMPS) may result in grafting of the second polymer to the skeleton of the first network. Those authors called such materials “connected double networks” (c-DN). © XXXX American Chemical Society

Received: April 17, 2014 Revised: June 13, 2014

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Scheme 1. Synthesis of SAPS network

result provides credence to the concern by Nakajima et al.6 that grafting of chains in the polymerization of the second network of a DN hydrogel is a distinct possibility. Because of the experimental and modeling work that has been done on DN hydrogels and the technological importance of these materials, it is imperative that the microstructure of these materials be resolved with experimental evidence. For example, in the phenomenological models proposed by Tanaka2 and Brown7 to explain the extraordinary toughness and high fracture energy of DN hydrogels, these gels were considered as SIPNs and IPNs as originally reported by Gong et al.3 The objectives of the work described in this paper were to develop a better understanding of the microstructure of tough hydrogels synthesized by free-radical polymerization of a monomer within a highly cross-linked polyelectrolyte hydrogel and to determine the molecular architecture necessary for toughness enhancement. The results herein show that at least some chains produced by the second polymerization during synthesis of a conventional DN hydrogel do indeed graft to the skeleton of the first network and that a true SIPN has poorer mechanical properties than a DN with high molecular weight linear chains grafted to the first network. These results raise questions concerning the validity of mechanical models that have thus far been used to describe the significant strength and toughness improvement exhibited by DN hydrogels, since those models assumed IPN or SIPN microstructure. This paper also demonstrates the importance of the microstructure in determining the mechanical properties of DN hydrogels. Fourier transform infrared spectroscopy (FTIR) characterization of a hydrogel synthesized from the potassium salt of 3sulfopropyl acrylate (SAPS) and 2-hydroxyethyl acrylate (HEA) showed that the residual vinyl unsaturation observed in the first network (SAPS) disappeared after a second polymerization step, which indicated that the double bonds participated in the second polymerization to produce polymer chains grafted to the skeleton of the SAPS network. The FTIR results also indicate that the procedure proposed by Nakajima et al.6 to “deactivate” the residual carbon−carbon double bonds in the first network is questionable and cannot guarantee that the first network will be completely deactivated. SIPN and IPN hydrogels synthesized from the polymerization of acrylamide

(AAm) within a SAPS network with no residual unsaturation in the SAPS network produced very brittle hydrogels. Synthesis of linear PAAm within a partially deactivated SAPS gel also produced a brittle hydrogel. However, synthesis of a loosely cross-linked PAAm network inside the same partially deactivated SAPS produced a tough hydrogel, though its toughness was lower than that of a pseudo-IPN made of the same nondeactivated SAPS gel. This paper also shows that long linear polymer chains with one end grafted to the first network are necessary for achieving high toughness in DN hydrogels. In fact, a molecular architecture where the linear chains or second network are grafted to the first network represents the most efficient microstructure for mechanical dissipation and toughness enhancement.

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EXPERIMENTAL SECTION

Materials. The potassium salt of 3-sulfopropyl acrylate (SAPS) and acrylamide (AAm) were 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 Pseudo-SIPN and Pseudo-IPN. Pseudo-SIPN, i.e., where linear polymer chains are attached to a cross-linked polyelectrolyte network, was synthesized by a two-step sequential free-radical polymerization. The first network was prepared by adding OXGA and MBAA to a 1 M solution of SAPS in deionized (DI) water. Dry nitrogen gas was bubbled through the reaction mixture for 5−10 min to remove oxygen and the solution was then injected into a glass mold made of two parallel glass slides that was then exposed for 10 min to 365 nm ultraviolet (UV) light (15 mW/cm2), Scheme 1. The resulting SAPS gel was then immersed into a 2 M solution of AAm, with no MBAA, in DI water containing the photoinitiator that had already been deoxygenated with N2 gas. When equilibrium swelling was achieved, the AAm-swollen SAPS gel was placed between two parallel glass slides and exposed for 9 h to 365 nm UV light (3 mW/cm2), Scheme 2. Pseudo-IPN was synthesized by the same procedure except for using very low concentration of MBAA (0.01 mol % with respect to the AAm) in the second polymerization step, Scheme 3. B

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and then dried. Infrared spectra were obtained prior to and after the reaction of the gel with VA-044. Hereafter, SAPS gels deactivated by this procedure are referred to as (VA-044)-r-SAPS (VA-044 reacted SAPS). Synthesis of SIPN and IPN. Two OXGA-r-SAPS gel samples were immersed separately in 2 M solutions of AAm in DI water, one containing only the OXGA photoinitiator (0.1 mol % with respect to the monomer) and another containing OXGA (0.1 mol % with respect to the monomer) and cross-linking agent (0.01 mol % with respect to the monomer). These are the procedures used by Nakajima et al.6 to presumably prepare a SIPN, Scheme 2, and an IPN hydrogel, Scheme 3, respectively. The AAm-swollen OXGA-r-SAPS gels were then placed between two parallel glass slides and exposed to 365 nm UV light to initiate the second polymerization. The resulting hydrogels were immersed in DI water, which was replaced a number of times with fresh water to remove any unreacted monomer. As will be discussed later in this paper, the OXGA-r-SAPS gel was not completely deactivated (i.e., residual double bonds remained), though it is most likely partially deactivated. To designate the gels for which the double bonds were not completely deactivated as SIPNs or IPNs (depending on whether the second polymerization was linear or nonlinear), as did Nakajima et al.,6 is confusing and misleading. In this study these are referred to as N-SIPN and N-IPN to denote that the procedure of Nakajima et al.6 was followed in the synthesis. True SIPN and IPN were synthesized by the same procedure using (VA-044)-r-SAPS hydrogel. Two UV intensities, 15 and 3 mW/cm2, were used for the photopolymerization reactions, and the radiation exposure times for each intensity were 10 min (dose = 9 J/cm2) and 9 h (dose = 97 J/ cm2), respectively. The sample notation contains the information about the recipes used for the synthesis of the different networks. Each individual network is described by the name of the monomer used, followed by four numbers in parentheses that indicate (1) the molar concentration of monomer in DI water, (2) the mol % of photoinitiator with respect to monomer, (3) the mol % of crosslinking agent with respect to monomer and (4) UV dose (J/cm2) used for the reaction (i.e., intensity × time of exposure). Thus, SAPS(1,2,2,9) corresponds to the polymerization of a 1 M SAPS solution using 2 mol % OXGA, 2 mol % MBAA and a UV dose of 9 J/ cm2. The molar ratio of SAPS to AAm in DN hydrogels prepared in this study was about 1/20. Polymer Characterization. FTIR spectroscopy was used to characterize residual unsaturation. A Nicolet 380 FTIR spectrometer was used with 32 scans and a resolution of 4 cm−1. Samples were washed at least seven times with DI 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. Rectangular samples with length =40 mm, width =10 mm, thickness = 2−3 mm and a gauge length of 30 mm were used. Sandpaper was used to prevent slippage of the samples in the grips. The tensile data are reported as engineering stress versus stretch ratio,

Scheme 2. Synthesis of Linear Poly(acrylamide) Chains

The resulting DN hydrogel samples were immersed in DI water, which was replaced a number of times with fresh water to remove any unreacted monomer. A conventional DN hydrogel synthesis was also conducted with cross-linked SAPS as the first network and HEA, without MBAA, as the monomer for second polymerization step. This was done to determine whether the synthesis of the second network in the presence of residual saturation in the first network produced grafting of the second network to the first network. The reason for using SAPS and HEA in that experiment was that for DNs of AMPS and AAm the infrared bending vibrations for the N−H group in AMPS and in AAm absorb in the same infrared region as does the band for vinyl unsaturation, which complicates the identification of residual double bonds in the AMPS/AAm DN. This is not a problem for SAPS/HEA DNs, since neither SAPS nor HEA have N−H groups, and, therefore, resolution of the vinyl unsaturation in the SAPS/HEA hydrogels was easier to assess. Since both gel chemistries involve conventional freeradical vinyl polymerization, it is most likely that the results and conclusions for the SAPS/HEA system are also applicable to other DN hydrogels, such as AMPS/AAm. Deactivation of Vinyl Unsaturation with OXGA (Procedure Proposed by Nakajima et al.6). After making the first network from SAPS, it was immersed into a 0.1 M solution of OXGA in DI water, which had already been deoxygenated with N2 gas, until equilibrium swelling was achieved. The OXGA-swollen SAPS gel was then placed between two parallel glass slides and exposed to 365 nm UV light (3 mW/cm2) for 10 h. After radiation, the sample was washed at least 7 times with fresh DI water over a period of 7 days and then dried before an infrared spectrum was obtained. Nakajima et al.6 used PAMPS for the first network in their work, but as is discussed later in this paper, this procedure does not guarantee that the residual unsaturation in a SAPS network is completely deactivated. It is also doubtful that the nature of the first network, PAMPS versus SAPS, should make a difference in the ability of OXGA to deactivate the double bonds. Hereafter, SAPS gels prepared after applying this procedure are referred to as OXGA-r-SAPS (OXGA reacted SAPS). Deactivation of Vinyl Unsaturation with a Thermal Initiator. An alternative procedure to the use of a photoinitiator to deactivate the double bonds in the first network,6 involved the use of a thermal initiator, VA-044. After the first network, SAPS, was prepared, it was immersed into a deoxygenated 0.1 M aqueous solution of VA-044. When the VA-044-swollen SAPS gel equilibrated with the solution, it was heated at 60 °C for 1h. The sample was removed from the heating source and allowed to cool slowly to room temperature, after which it was washed at least 7 times with fresh DI water over a period of 7 days

Scheme 3. Synthesis of Poly(acrylamide) Gel

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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)/Lo, where L(t) was the instantaneous sample length and Lo was the original sample length. Note that λ = εe + 1, where εe is the engineering tensile strain.

conditions in the Experimental Section), SAPS(1,2,2,9)/ HEA(2,0.1,0,97), red curve in Figure 1. This result clearly indicates that the double bonds reacted during the polymerization of the HEA and, therefore, some poly(2-hydroxyethyl acrylate) (PHEA) chains must be grafted to the SAPS network. It is expected that this result is general for the synthesis of any DN hydrogel prepared from free-radical polymerization of vinyl monomers using a similar procedure as used in this study. Given the fact that, in preparing a DN hydrogel, the second network is synthesized in the presence of residual carbon− carbon double bonds, there are different possibilities for the topology of the PAAm chains to grow within the first network, Figure 2. If no cross-linking agent is used in the synthesis of the

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RESULTS AND DISCUSSION The 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 can leave some residual vinyl bonds from the cross-linking reagent in the final network.9,10 FTIR spectroscopy can identify the chemical bonds in a polymer, in this case the focus was on the carbon−carbon double bond. AMPS and AAm are secondary and primary amides, respectively, and absorption (∼1610 cm−1) of the N− H bending (amide II band) overlaps with the vinyl IR absorption,11 which complicates the resolution of a low concentration of CC in the first network made of AMPS and its DN hydrogel made with AAm. In a previous study,8 we showed that vinyl unsaturation remains in a free-radical polymerization of a SAPS network that does not have an N− H bond, but when a DN was made from that SAPS network with AAm, it was not possible to unambiguously determine whether the CC bonds disappeared upon the polymerization of AAm, because of overlap of the CC and amide II absorptions. Thus, it was not possible to prove that grafting of polyacrylamide (PAAm) chains to the first network occurred. In order to determine whether grafting of polymer chains to the skeleton of the first network occurs during the second polymerization step in a DN, the present study used SAPS and HEA to synthesize a DN hydrogel. Since both of these monomers are esters and neither contains an N−H bond, resolution of the absorbance band for the vinyl unsaturation in the SAPS/HEA DN was unambiguous. Figure 1 shows the FTIR spectra of SAPS(1,2,2,9) and SAPS(1,2,2,9)/HEA(2,0.1.0,97) hydrogels. The CC absorp-

Figure 2. Schematic representation of different topological possibilities for PAAm chains to grow within the first network (blue network).

second polymer, the chain may initiate at a residual double bond of the first network. Unless that chain terminates at another double bond on the skeleton of the first network, which is statistically unlikely, a grafted linear chain is formed, see the red chain in Figure 2. Even if a chain did initiate and terminate at double bonds in the first network (the black chains in Figure 2), that would simply become another network chain in the first network, but it is highly improbable that all the PAAm chains initiated and terminated in that way. In addition to grafted chains, the second monomer can also initiate in the aqueous phase to produce linear chains. If those chains terminate by reaction with a vinyl bond on the first network, a grafted chain identical to the red chain in Figure 2 will form. However, statistically it is more likely that a linear chain initiated in solution will not terminate with a double bond on the skeleton of the first network. In that case, a nonbonded linear chain is formed, i.e., the green chain in Figure 2. If all the second polymer chains were nonbonded (i.e., green chain in Figure 2), an SIPN is produced. However, since disappearance of the double bonds was confirmed for the SAPS/HEA DN, these materials most likely contained both green and red chains and are designated pseudo-SIPNs. Separate polymer synthesis of purely linear chains using the identical formulations that were used for the second network in these DN hydrogels produced very high molecular weight polymers (M ∼ 106 g/mol).8 That result suggests that the nongrafted linear polymer chains in the pseudo-SIPN were probably highly entangled with the network chains and might be able to support stress on at least the time-

Figure 1. FTIR spectra: (blue) SAPS(1,2,2,9); (red) HEA-swollen SAPS(1,2,2,9) radiated by 365 nm UV light of intensity 3 mW/cm2 for 9 h.

tion band at ∼1650 cm−1 in the SAPS(1,2,2,9) spectrum (blue curve in Figure 1) is due to residual CC unsaturation, which is consistent with our previous results8 and the hypothesis of Nakajima et al.6 that some double bonds remained after the polymerization of the first network. The CC absorption band, however, disappears for the DN hydrogel (see reaction D

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scales used for the mechanical property experiments. In fact, removal of the linear chains by soaking the DN hydrogel in water for a sufficiently long time that allows them to diffuse out of the hydrogel has a detrimental effect on the mechanical properties of the hydrogel. If a cross-linking agent is used in the synthesis of the second network, such as for the formulation of a DN hydrogel as originally described by Gong et al. in their 2003 paper3, at least part of the second network will be intimately grafted to the skeleton of the first network. In effect, that produces a single polymer network with two distributions of cross-links. It may also produce a nonbonded, interpenetrating second network, and characterization of the structure of such a product or the relative effects of the bonded and unbonded second networks on the mechanical properties are not easy to resolve. An important conclusion of the these results is that the AMPS/ AAm DN’s that were developed by Gong et al.3 and replicated by many other research groups are not simple IPNs. One might consider them pseudo-IPNs with a complicated network made up of the two different polymers and cross-link distributions and containing a second interpenetrating network. It is important at this point to note that the DN hydrogels developed by the Gong group are definitely robust hydrogels. Nothing reported here should be interpreted as diminishing the importance of that group’s discovery. However, a key caveat resulting from this paper is that modeling of the mechanical properties of DN hydrogels must move away from the assumption of an IPN and consider structural features reported herein. Although we believe we now understand the general features of the DN microstructure, we do not know the specifics of structural parameters such as the relative fractions of grafted and ungrafted network chains (or linear chains), the two sets of cross-link densities (or molecular weight of network chains) for the first network that includes a grafted second network, the cross-link density of the ungrafted nework and the molecular weights of any linear chains that are grafted or ungrafted. A second question addressed by this paper is what topology of the second polymer is the most efficient for achieving a tough hydrogel, i.e., IPN, SIPN, pseudo-SIPN or pseudo-IPN, the latter two being the conventional DNs. Unfortunately, the term DN has become too common in the literature to discard it for a more accurate description of the actual morphology, so hereafter in this paper, DN will be used to describe hydrogels that are synthesized in a manner similar to that used in ref 3, i.e., a single network with large compositional and cross-link density heterogeneity. The answer to the question above involves a comparison of the mechanical properties of each type of hydrogel with a fixed overall composition, which is not a trivial task since the first two (IPN and SIPN) require the synthesis of a first network without residual unsaturation or a method to deactivate the residual double bonds so they cannot participate in the second polymerization. Figure 3 shows the FTIR spectra of SAPS(1,2,2,9), OXGAswollen SAPS(1,2,2,9) and OXGA-r-SAPS(1,2,2,9). The reaction procedure was similar to the “deactivation” method for eliminating the residual double bonds in PAMPS used by Nakajima et al.6 Unfortunately, since their paper does not report the UV lamp intensity used, we could not exactly replicate their procedure. The CC absorption band at 1650 cm−1, signifying the presence of vinyl unsaturation is clearly seen in the spectrum of SAPS(1,2,2,9), the blue curve in Figure 3. Although a distinct peak is not resolved at 1650 cm−1 for the

Figure 3. FTIR spectra: (blue) SAPS(1,2,2,9); (red) OXGA-swollen SAPS(1,2,2,9) radiated by 365 nm UV light of intensity 3 mW/cm2 for 10 h; (green) OXGA-swollen SAPS(1,2,2,9) before radiation.

OXGA-r-SAPS(1,2,2,9), red curve, there is a broad and strong shoulder that overlaps the 1650 cm−1 region on the low frequency side of the 1720 cm−1 carbonyl peak of SAPS. The green curve shows that the OXGA exhibits a very broad carbonyl absorption due to its carboxylic acid and ketone groups that may be responsible for the shoulder in the 1650 cm−1 region of the red curve. However, it is not possible from these data to conclude that the additional OXGA reaction used here and by Nakajima et al.6 was sufficient to eliminate the residual double bonds of the first network. Figure 4 shows deconvolutions of the FTIR spectra for SAPS(1,2,2,9), SAPS(1,2,2,9) swollen with OXGA and OXGAr-SAPS(1,2,2,9) for the spectral region of 1850−1500 cm−1. MagicPlot Pro (MagicPlot Systems, LLC) was used to deconvolute the FTIR spectra using Lorentzian shape line curves12 with peak frequencies assigned based on the spectra of neat SAPS and OXGA. The effectiveness of the deconvolutions was judged by how well the actual spectrum was reproduced, i.e., visual comparison of the blue and dashed red curves. Figure 4a shows the deconvolution of the carbonyl and vinyl absorptions for SAPS(1,2,2,9). In order to improve the deconvolution, the weak broad absorptions at frequencies of 1840 and 1535 cm−1 were specified in the calculations for all the parts of Figure 4. Figure 4b shows the carbonyl and vinyl absorptions for the SAPS and the carbonyl for the OXGA in the deconvoluted spectrum of the OXGA-swollen SAPS(1,2,2,9). No attempt was made to deconvolute the OXGA carbonyl peak into an acid and ketone peak. Figure 4c shows the FTIR spectrum of reaction product OXGA-r-SAPS(1,2,2,9). The deconvolution of Figure 4c assumed complete reaction of the residual double bonds, but note that the composite fit (dashed red line) is relatively poor compared with the fits in Figure 4, parts a and b. The deconvolution of the reacted OXGA-swollen SAPS(1,2,2,9) was greatly improved by adding another peak at 1650 cm−1 for unreacted double bonds, Figure 4d, and that result indicates that the reaction of excess OXGA with SAPS(1,2,2,9) did not completely eliminate the excess double bonds. That conclusion does not agree with the claim by Nakajima et al.,6 though they provided no experimental evidence to support their assertion that the reaction of OXGA with PAMPS “deactivated” the double bonds. Figure 5 shows the FTIR spectra of SAPS(1,2,2,9) and the reaction product (VA-044)-r-SAPS(1,2,2,9). That reaction was E

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Figure 4. Deconvoluted FTIR spectra: (a) SAPS(1,2,2,9); (b) OXGA-swollen SAPS(1,2,2,9) before radiation; (c) OXGA-swollen SAPS(1,2,2,9) radiated by 365 nm UV light of intensity 3 mW/cm2 for 10 h, assuming complete reaction of the residual double bonds; (d) OXGA-r-SAPS(1,2,2,9) including the absorption at 1650 cm−1 for unreacted double bonds. Blue curves are the actual spectra and the red dashed curves are the calculated spectra. The numbered peaks correspond to (1) the carbonyl stretching in SAPS, (2) vinyl unsaturation, (3, 4) the weak, broad peaks at 1535 and 1840 cm−1 in SAPS, and (5) the carbonyl stretching in OXGA.

Figure 5. FTIR spectra: (blue) SAPS(1,2,2,9); (red) (VA-044)swollen SAPS(1,2,2,9) heated at 60 °C for 1 h.

Figure 6. Deconvoluted FTIR spectrum for (VA-044)-r-SAPS(1,2,2,9). The absorbance shown by red curve about 1650 cm−1 is due to the overlap of the CO and CN peaks.

more efficient at eliminating the double bonds in SAPS(1,2,2,9), as evident by the disappearance of the 1650 cm−1 absorption (red curve). The absorption intensity for (VA-044)r-SAPS(1,2,2,9) between 1630 and 1680 cm−1 is due to the overlap of the CO and CN peaks, as shown by the deconvolution in Figure 6. These FTIR results indicate that the deactivation process used by Nakajima et al.6 is not an efficient way to eliminate the residual unsaturation, and their N-SIPN and N-IPN hydrogels were not true SIPNs or IPNs. In contrast, the complete disappearance of the 1650 cm−1 absorption in Figure 5 clearly indicates that hydrogels made by polymerizing AAm within (VA-044)-r-SAPS (1,2,2,9) were true SIPNs and IPNs. The difference in the efficiency of OXGA and VA-044 at deactivating the double bonds may be a consequence of better

homogeneity of the reaction using the thermal initiator compared with the photoinitiator. While it is relatively easy to achieve isothermal or near isothermal conditions in a thin hydrogel film, it is more difficult to achieve a homogeneous reaction using radiation, since the intensity of radiation decreases with the inverse square of the distance. That is, the hydrogel at the bottom of the film (i.e., furthest from the radiation source) receives a lower dose rate, by a factor of the inverse thickness squared, than the material nearest to the source. As a result, the reaction of the double bonds nearer the top of the sample is expected to occur with greater frequency than those nearer to the bottom, and it is possible that accounts F

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with the materials that were previously reported as SIPN’s.4,5,8,13 The FTIR spectrum of OXGA-r-SAPS(1,2,2,9), the red curve in Figure 3, indicates that the hydrogel made of OXGA-rSAPS(1,2,2,9), and AAm(2,0.1,0.01,97) was not a true-IPN, since double bonds remained in the OXGA-r-SAPS(1,2,2,9). Instead, that hydrogel was a pseudo-IPN, where at least some chains in the AAm(2,0.1,0.01,97) network were bonded to the OXGA-r-SAPS(1,2,2,9). The N-IPN, OXGA-r-SAPS(1,2,2,9)/ AAm(2,0.1,0.01,97), hydrogel was tough, but its toughness (defined here as the strain energy density, i.e., the area under the stress−strain curve) was much smaller than that of a pseudo-IPN made from SAPS(1,2,2,9) and AAm(2,0.1,0.01,97); cf., the red and green data in Figure 8. Data for the corresponding pseudo-SIPN, SAPS(1,2,2,9)/AAm(2.0,1,0,97), is also shown in Figure 8 (blue data).

for the failure to completely deactivate the double bonds in the sample. In contrast, the thermal reaction is more or less equal throughout the sample, which increases the probability of reacting all the double bonds. Depth profiling studies are needed to assess the validity of that explanation, but in any case, it is clear from the FTIR data that the thermal reaction was more efficient at ensuring that the double bonds in the first network were deactivated prior to the synthesis of the second part of the DN hydrogel. The SIPN and IPN hydrogels made from (VA-044)-rSAPS(1,2,2,9) were brittle. Their structure was so weak that they exhibited a diffusion-induced fracture when swollen with DI wateri.e., parts of the sample broke during swelling process. These may be the first true IPN and SIPN DN hydrogels that have been prepared, and their brittleness is in stark contrast to the excellent mechanical properties of pseudoSIPNs4,5,8,13 made with very high molecular weight PAAm. These results also stress the critical importance of the role of grafted polymer chains for developing toughness of DN hydrogels. For comparison, even when a relatively low molecular weight PAAm was used for the linear chains in the formulation SAPS(1,2,2,9)/AAm(2,0.1,0,9),14 the pseudo-SIPN was considerably stronger than the (VA-044)-r-SAPS(1,2,2,9)/AAm(2,0.1,0,97) true-SIPN, even though the pseudo-SIPN broke before it achieved large deformations. Once again, this observation conflicts with the results for N-IPN hydrogels reported by Nakajima et al.,6 and it leads us to conclude that their material was not a true IPN hydrogel; i.e., their “deactivated” PAMPS retained at least some double bonds and some chains of the second, linear polymer were grafted to the first network. That conclusion is supported by the mechanical behavior of hydrogels made from OXGA-rSAPS(1,2,2,9). A hydrogel made from OXGA-r-SAPS(1,2,2,9) and AAm(2,0.1,0,97) was brittle, as shown in Figure 7. The samples broke in the grips of the tensile machine, so it was not possible to measure their tensile properties. That result is consistent with the report by Nakajima et al.6 that a “true”-SIPN of PAMPS and PAAm had poor mechanical properties compared

Figure 8. Engineering tensile stress versus stretch ratio: blue circles, pseudo-SIPN made of SAPS(1,2,2,9) and AAm(2,0.1,0,97); red circles, N-IPN made of OXGA-r-SAPS(1,2,2,9) and AAm(2,0.1,0.01,97); green circles, pseudo-IPN made of SAPS(1,2,2,9) and AAm(2,0.1,0.01,97).

Although the deactivation procedure described by Nakajima et al.6 did not completely remove the double bonds in the OXGA-r-SAPS(1,2,2,9), it is likely that some, perhaps most, of the double bonds were eliminated. As a result, the graft concentration that resulted from the second polymerization step was less than it would have been if the hydrogel were prepared by the original DN synthesis process, i.e., with no deactivation of the residual double bonds. For the OXGA-rSAPS(1,2,2,9)/AAm(2,0.1,0,97) N-SIPN hydrogel the concentration of grafted chains was insufficient for achieving a tough hydrogel, but when a N-IPN was made, by adding a crosslinking agent during the second polymerization, i.e., OXGA-rSAPS(1,2,2,9)/AAm(2,0.1,0.01,97), a tough hydrogel was produced. While the concentration of grafted chains was probably no different than for the N-SIPN, the additional crosslinks in the second network increased the overall cross-link density of the hydrogel. However, a higher cross-link density of the second network is not by itself sufficient to improve the toughness of the gel. It is imperative that the second network be connected to the first networkin this case, by a covalent bond. The structure and toughness of DN hydrogels made using a first network without CC double bonds and with two different concentrations of double bonds are summarized in Table 1. The two different concentrations of double bonds

Figure 7. OXGA-r-SAPS(1,2,2,9)/AAm(2,0.1,0,97) N-SIPN prepared after reaction of residual CC unsaturation of the first network, SAPS(1,2,2,9) with OXGA for 10 h is brittle. G

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Table 1. Characteristics of DN Hydrogels made from unreacted and initiator-reacted SAPS networks hydrogel

topology

mechanical behavior

DN Hydrogels Made From Initiator-Reacted SAPS Network (VA-044)-r-SAPS(1,2,2,9)/AAm(2,0.1,0,97) SIPN (VA-044)-r-SAPS(1,2,2,9)/AAm(2,0.1,0.01,97) IPN OXGA-r-SAPS(1,2,2,9)/AAm(2,0.1,0,97) N-SIPN (low grafting concentration) OXGA-r-SAPS(1,2,2,9)/AAm(2,0.1,0.01,97) N-IPN (low grafting concentration) DN Hydrogels Made From Unreacted SAPS Network SAPS(1,2,2,9)/AAm(2,0.1,0,97) pseudo-SIPN (higher grafting concentration) SAPS(1,2,2,9)/AAm(2,0.1,0.01,97) pseudo-IPN (higher grafting concentration)

tougher tougher

The synthesis of true SIPN or IPN hydrogels requires eliminating the residual double bonds from the first network prior to polymerization of the second polymer or network. Although this can be done, in principle, by deactivation of the double bonds by reaction with excess free-radical initiator, in practice the free-radical reaction of excess initiator with the first network may not be sufficient for accomplishing the complete elimination of the residual double bonds. For example, a photochemical reaction with 2-oxoglutaric acid that was previously used to deactivate the residual double bonds.6 That reaction does not efficiently remove all the unsaturation from the first network, though thermally activated free-radical reaction with 2,2′-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride (VA-044) does. Although, the available data on the elimination of the residual double bonds in the first network of a DN hydrogel is limited, one potential problem with photochemical deactivation may be the thickness dependence of the radiation intensity that may produce a nonuniform reaction. That explanation is, at the moment, speculative, but it suggests that more studies be carried out on the use of photoand thermally activated reactions to determine not only whether one or both is efficient at deactivating the residual unsaturation, but also whether there are significant differences in the heterogeneity of the products when one or the other process is used to prepare a DN hydrogel. A necessary requirement for preparing strong and tough DN hydrogel is that long chains or a second network are grafted to the skeleton of the first network. True SIPN and IPN hydrogels are mechanically weak, while DN hydrogels with grafted linear chains (pseudo-SIPNs) or networks (pseudo IPNs) are very tough if there is sufficient graft density. If the residual double bond concentration in the first network is too low, polymerization of linear polymer inside that network produces a brittle hydrogel. However, for the same residual double bond concentration in the first network, polymerization of a loosely cross-linked network is mechanically tough, though not as tough as a pseudo-SIPN made with a first network with higher residual double bond concentration. For the pseudo SIPNs, the covalent connection of a linear chain to the skeleton of the first network holds the sample under small deformations and assumes the load after failure of the first network. If there is no or insufficient grafting of the linear chains (or second network) to the first network, the sample will fail even at very small stretch ratios due to the catastrophic failure of the first brittle network. In effect, the sufficient grafting between two networks is necessary to hold the sample at small deformations when the first network breaks and also the molecular weight of polymer chains of second network must be high enough to assume the load at large deformations. Disentanglement of the linear chains provides a dissipation mechanism which is effective at large deformations

were achieved by either using the as-polymerized SAPS network or partially deactivating it by the photochemical reaction with OXGA. The weak mechanical behavior of the true SIPN and IPN hydrogels made from (VA-044)-r-SAPS and the excellent mechanical properties of pseudo-SIPNs4,5,8,13 prepared from SAPS with a sufficiently high concentration of C C bonds clearly demonstrate the importance of grafting either long linear chains or a second network to the first (SAPS) network to achieve toughness of a DN hydrogel. One of the reviewers of this paper commented that the poor toughness of the (VA-044)-r-SAPS(1,2,2,9)/AAm(2, 0.1, 0.01,97) sample was due to a poorly developed cross-linked second network due to the use of too high concentration of the initiator, instead of the absence of grafting the second network to the first. Following the reviewer’s suggestion to lower the initiator concentration for the second polymerization to 0.01%, a, (VA-044)-r-SAPS(1,2,2,9)/AAm(2,0.01,0.01,97) DN hydrogel was made. It, too, however, was brittle and the sample broke in the grips of the tensile testing machine. For a pseudo-SIPN, the chains from the second polymerization are covalently attached to the first, brittle, network from one end and the chains are highly entangled, which provides a dissipation mechanism by chain sliding and disentanglement at large deformations. Because of the high molecular weight and high entanglement density of the grafted linear chains, they can hold the sample in small deformations and assume the load bearing role after failure of the first network. As discussed earlier, pseudo-SIPNs made of low molecular weight PAAm chains can support load only up to the yield point.8 At that point, the first network breaks, and since the low molecular weight polymer chains of the second polymer provide no mechanical support, catastrophic failure of the DN hydrogel occurs before large deformations are achieved. The ability of grafted polymer chains to dissipate energy is reduced by using a cross-linking agent in second polymerization step, because those cross-links restrict the motion of polymer chains at high stretch ratios. Therefore, hydrogels prepared without using a cross-linking agent in the second polymerization step form tougher hydrogels than when two cross-linked networks are used, as has been previously observed for pseudo-SIPNs.4,5

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brittle brittle brittle tough

CONCLUSIONS

Free-radical polymerization of cross-linked AMPS or SAPS hydrogels using a divinyl cross-linker typically contains residual unsaturation due to inefficient reaction of the cross-linking agent. When such networks are used to synthesize a doublenetwork (DN) hydrogel, the polymerization of the second monomer results in grafting of some chains (or network) to the skeleton of the first network. The microstructures of most, if not all, of the previous DN hydrogels reported in the literature have not been IPNs or SIPNs. H

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provided that the molecular weight of the chain is very high. For pseudo-IPNs, the efficiency of the dissipation process is reduced by cross-linking in the second polymerization step, though these DNs are also very tough. This study used covalent bonding for connecting the grafted chains (network) to the first network. Although it was not considered in this work, it is likely that supramolecular bonding, e.g., ionic bonds or hydrophobic associations, of long chains or a second network to the first network will also be effective at producing tough hydrogels. In effect, that has been demonstrated by the reports of tough hybrid hydrogels produced with a combination of covalent and physical bonds.15

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AUTHOR INFORMATION

Corresponding Authors

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by a grant from the Civil, Mechanical and Manufacturing Innovation Division of the Engineering Directorate of the National Science Foundation, Grant CMMI1300212.

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REFERENCES

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dx.doi.org/10.1021/ma500815d | Macromolecules XXXX, XXX, XXX−XXX