Do Physically Trapped Polymer Chains Contribute to the Mechanical

Oct 5, 2017 - The reduction of mechanical properties of a DN hydrogel aged for 12 ..... Systems and Techniques Based on Double-Network Principle Bull...
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Do Physically Trapped Polymer Chains Contribute to the Mechanical Response of a Host Double-Network Hydrogel under Finite Tensile Deformation? 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: This paper describes the effect of physically trapped polymer chains (PTPCs) on the mechanical behavior of pseudo-semi-interpenetrating (pseudo-SIPN) and pseudo-interpenetrating (pseudo-IPN) double-network (DN) hydrogels synthesized from 3-sulfopropyl acrylate potassium salt (SAPS) and acrylamide (AAm). DN hydrogel containing a low concentration of very high molecular weight poly(ethylene oxide) (PEO) had markedly higher tensile strength and elongation at break than the neat DN hydrogel. DN hydrogels with trapped ex situ prepared polyacrylamide (PAAm) chains also exhibited toughness enhancement. This effect was attributed to the role of PTPCs as a molecular release agent that reduced internal friction among polymer chains during large deformations and therefore increased energy dissipation and elongation at break. The reduction of mechanical properties of a DN hydrogel aged for 12 months was consistent with that conclusion in that the PTPC diffused out of the pseudo-SIPN during aging. Increasing temperature also accelerated the diffusion and reduced the mechanical properties improvements due to the PTPCs. The effect of PTPCs on the toughness enhancement of pseudo-SIPNs was more efficient than their effect on pseudo-IPNs.

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methods reported by Gong et al.3 with a cross-linked second network or a linear second polymer, respectively. For pseudo-SIPN hydrogels, in addition to grafting of the second polymer to the skeleton of the first network, chains formed in the second polymerization step that neither initiate nor terminate at a residual unsaturation site on the first network will be linear, and high molecular weight chains will be highly entangled with the first network. The existence of physically trapped linear polymer chains (PTPCs) raises the question of whether those chains contribute to the mechanical response of the host hydrogel. Nakajima et al.4 asserted that “linear PAAm chains in the gel likely do not sustain the load and transfer the force as linear chains are dragged out of the first network during deformation”. Tsukeshiba et al.,7 however, concluded that molecular entanglements between the PTPCs and the first cross-linked network play a crucial role in the toughening mechanism of DN gels. Nakajima et al.8 later showed that a linear polyelectrolyte physically trapped within a neutral crosslinked hydrogel produced tough hydrogels. Those authors stated that the polyelectrolyte “molecular stent” increased the swelling ratio of the neutral gel, but it did not contribute to the mechanical behavior. Clearly, there are inconsistencies in the literature with regard to the role PTPCs play in toughening

INTRODUCTION

The similarity of hydrogels to biological tissues makes them attractive for applications such as drug delivery, tissue engineering, and soft robotics.1 Conventional hydrogels, however, are elastic solids and are relatively brittle because they do not possess a mechanism for dissipating energy upon deformation.2 One solution to the poor toughness of hydrogels is the double-network (DN) hydrogel design that is synthesized by a two-step sequential free-radical polymerization in which a second hydrophilic monomer is polymerized within a previously formed highly cross-linked polyelectrolyte network.3 The DN hydrogel was considered either an IPN or a semi-IPN (SIPN) depending on whether the second polymerization product was a cross-linked network or linear chains. The IPN microstructure for DN hydrogels was first questioned by Nakajima et al.,4 who suggested that the second network could be grafted to the skeleton of the first network by free-radical reactions of the growing second polymer to the residual carbon−carbon double bonds in the first network. That scenario possibility was later confirmed from infrared spectra evidence5 that showed that conventional DN hydrogels were not IPNs or SIPNs. It was also shown that the grafting between first and second networks is actually necessary for achieving a tough hydrogel.6 As a result, the IPN and SIPN nomenclature that has been generally used in the DN literature is incorrect, and herein we refer to DN hydrogels as pseudo-IPNs or pseudoSIPNs to denote DN hydrogels synthesized by the conventional © XXXX American Chemical Society

Received: June 28, 2017 Revised: September 12, 2017

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DOI: 10.1021/acs.macromol.7b01382 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules hydrogels, and the confusion is especially remarkable considering that refs 4, 7, and 8 are from the same laboratory. The work described in the current paper aims to gain a better understanding of the effect of PTPCs on the mechanical properties of DN hydrogels, specifically for pseudo-SIPN hydrogels where no cross-linking agent was used in the second polymerization step. Two types of PTPCs were considered: (1) in situ polymerized PTPCs, where linear chains result naturally from the second polymerization step for a pseudo-SIPN, and (2) embedded PTPCs, where intentionally added high molecular weight linear chains are trapped by molecular entanglements with the first network prior to the second polymerization step, similar to the molecular stent approach described by Nakajima et al.8

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Scheme 2. Synthesis of Linear PAAm Chains

Scheme 3. Synthesis of PAAm Hydrogel

EXPERIMENTAL SECTION

Materials. Acrylamide (AAm), 3-sulfopropyl acrylate potassium salt (SAPS), and poly(ethylene oxide), PEO (Mv ∼ 4 000 000 g· mol−1) were purchased from Sigma-Aldrich Chemical Co. and used as received. 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. Synthesis of Polymeric Networks. Pseudo-SIPN hydrogels were synthesized by a two-step sequential free-radical polymerization. The first network was prepared by adding MBAA and OXGA and 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 injected into a glass mold made of two parallel glass slides, which was then exposed to 365 nm ultraviolet (UV) light to prepare a covalently cross-linked first network (see Scheme 1).

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. Each individual network is shown 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 cross-linking agent with respect to monomer, and (4) the nominal UV dose used for the reaction (i.e., intensity × time of exposure). The first polymer network (see SAPS formulation in Table 1) for all the DN hydrogels discussed herein were exactly the same. The DN hydrogels differ from one another with regard to whether a cross-linking monomer was used in the second polymerization and/or whether or not embedded PTPCs were used. Note that in situ polymerized linear polymer results from a second polymerization that does not include a cross-linking agent during the synthesis of a DN hydrogel. In that case, some of the chains graft to the first network and nongrafted chains are the in situ produced linear chains. In that case, the actual concentration of in situ produced linear chains is not known. The sample notation for hydrogels, DN-n-m-y/z, contains the information about the recipes used for the synthesis of the second networks and presence or absence of in situ polymerized and embedded PTPCs. Here, n is the concentration (in pphm) of crosslinking agent, m is UV dose (in J/cm2), y is the in situ polymerized linear polymer, and z is the externally polymerized embedded polymer (0.01 wt %) used in the first polymerization step, respectively. DN0.01-9 corresponds to a DN hydrogel containing no PTPCs where 0.01 mol % MBAA and a UV dose of 9 J/cm2 were used in the second polymerization step; no in situ polymerized linear chains were produced, and no externally polymerized linear polymer was added to the DN hydrogel. DN-0.01-97/PEO is a DN hydrogel where 0.01 mol % MBAA and a UV dose of 97 J/cm2 were used in the second polymerization step; no in situ linear chains were produced, and 0.01 wt % of externally polymerized linear PEO was added to the DN hydrogel. DN-0-97-PAAm/PAAm corresponds to a hydrogel containing in situ polymerized linear chains of PAAm, and 0.01 wt % of externally polymerized PAAm chains was added to the DN hydrogel. No cross-linking agent and a UV dose of 97 J/cm2 were used in the second polymerization step. The DN hydrogels synthesized in this study are summarized in Table 1. Ex Situ Synthesis of Linear PAAm Chains. A 2 M aqueous solution of AAm containing 0.1 mol % photoinitiator with respect to the monomer was used to synthesize a linear PAAm using a UV dose of 97 J/cm2 (see Scheme 2). Polymer Characterization. The molecular weights of PEO, ex situ prepared PAAm, and PAAm extracted from a DN hydrogel were determined using a Tosoh HLC-8320 gel permeation chromatograph (GPC) equipped with a refractive index detector. The column temperature was set at 35 °C, and a QC buffer composed of 0.05 M KH2PO4, 0.05 M Na2HPO4·12H2O, 0.1 M Na2SO4, and 0.008 M

Scheme 1. Synthesis of SAPS Network

A very low concentration of PTPCs was incorporated into some SAPS networks by adding 0.01 wt % of a very high molecular weight linear polymer (PEO or ex situ prepared PAAm) to the SAPS reaction mixture before making the first network. The resulting SAPS gel containing PTPCs was then immersed into a 2 M solution of AAm in DI water deoxygenated with N2 gas and containing the photoinitiator, until equilibrium swelling was achieved (about 1 h). The AAm-swollen gels were then placed between two glass slides and exposed to 365 nm UV light to polymerize the AAm and prepare the pseudo-SIPN (see Scheme 2). Pseudo-IPN hydrogels were also made using the same procedure, but including a low concentration of MBAA (0.01 mol % with respect to the AAm monomer) in the second polymerization step (Scheme 3). The resulting DN hydrogel samples were kept in DI water, which was replaced a number of times with fresh DI water to remove any unreacted monomer. Two UV intensities, 15 and 3 mW/cm2, were B

DOI: 10.1021/acs.macromol.7b01382 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Table 1. DN Hydrogels Synthesized in This Study formulation of SAPS gels (pphm)a DN hydrogel notation DN-0-97-PAAm DN-0-97-PAAm/PEO DN-0-97-PAAm/PAAm DN-0.01-97 DN-0.01-97/PEO DN-0.01-9 DN-0.01-9/PEO a

OXGA 4 4 4 4 4 4 4

MBAA 2 2 2 2 2 2 2

2

UV dose (J/cm ) 9 9 9 9 9 9 9

formulation of AAm gels (pphm)a OXGA

MBAA

0.1 0.1 0.1 0.1 0.1 0.1 0.1

0 0 0 0.01 0.01 0.01 0.01

b

UV dose (J/cm2) 97 97 97 97 97 9 9

in situb polymerized PTPCs

embeddedc PTPCs

d

PAAm PAAmd PAAmd

PEOe PAAmf PEOe PEOe c

In parts per hundred parts monomer. Physically trapped polymer chains polymerized during second polymerization step. Intentionally added high molecular weight chains that are trapped by molecular entanglements within the first network. dNongrafted linear PAAm chains that are formed during second polymerization. eLinear PEO chains (Mv = 4000000 g/mol) added before synthesis of the first network fLinear PAAm chains (Mv = 1648000 g/mol)5 added before synthesis of the first network NaN3 was used as the eluent with a flow rate of 0.35 mL/min at a pressure of 1.02 MPa. Tensile Testing. Uniaxial tensile tests were performed using an Instron 5567 universal testing machine equipped with a 100 N load cell, using an extension speed of 50 mm/min at room temperature. Rectangular bar specimens (40 mm long, 10 mm wide, and 2−3 mm thick) and a gauge length of 30 mm were used. Sandpaper between the grips and the sample was used to prevent slippage. Tensile data are reported as engineering stress versus stretch ratio, where the engineering stress, σe, is the force divided by the original crosssectional area of the sample, and the stretch ratio is λ ≡ L(t)/Lo, where L(t) is the instantaneous specimen length and Lo is the original specimen length. Note that λ = εe + 1, where εe is the engineering tensile strain.

microstructure of pseudo-SIPN hydrogels (Figure 1), it is difficult to determine the effect of in situ polymerized PTPCs,

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RESULTS AND DISCUSSION During the free-radical synthesis of a conventional DN hydrogel, the second network or linear polymer is grafted to the residual unsaturation in the first network.5,6 When no crosslinking agent is used in the second polymerization step, a pseudo-SIPN is formed, where some linear chains are grafted to the first, highly cross-linked polymer network and some nongrafted chains entangle with the first network. In this case, an in situ polymerized PTPC is formed. The molecular weight of the second polymer can be controlled by the formulation used in the second polymerization step. For a conventional free-radical polymerization, the molecular weight should be proportional to the monomer concentration and inversely proportional to the square root of the initiator concentration, provided that the rate constants for initiation, propagation, and termination do not change.9 In the case of very high molecular weight polymer chains, the second network is a transient network made up of highly entangled linear chains, some of which are covalently attached to the skeleton of the first network and some of which are in situ polymerized PTPCs. The crucial role of grafted chains for achieving toughness in a DN hydrogel was recently demonstrated by Shams Es-haghi et al.6 Absent grafted chains, a DN hydrogel is brittle. However, if a sufficient concentration of polymer chains is grafted to the first network, even relatively low molecular weight grafted chains can sustain a tensile load up to the onset of plastic deformation, though the low molecular weight chains will restrict the magnitude of the total strain of the DN hydrogel. As discussed earlier in this paper, the interest here is whether PTPCs can contribute to the mechanical behavior of a pseudoSIPN hydrogel under finite deformation. Because of the complexity of the second polymerization step and the

Figure 1. Schematic representation of microstructure of a pseudoSIPN hydrogel. In a pseudo-SIPN hydrogel, some of second polymer chains (blue chains) are covalently attached to the skeleton of the first network (black network) and some of the chains are physically trapped inside the gel (red chain). The latter chains are referred to in the text as in situ polymerized PTPCs. This representation of pseudo-SIPNs is not meant to imply that every chain in the second polymerization is grafted or that every grafted chain initiates at an unsaturation site on the first polymerized network. The simplified picture was drawn simply to indicate the possibility of grafting the second polymer to the first network.

i.e., trapped, ungrafted linear chains, on the mechanical behavior of the hydrogel. One problem is that the grafting density and the molecular weight of the grafted chains are difficult to analyze. A second problem is that entangled chains can diffuse out of the gel, though the diffusion process is very slow for high molecular weight polymers.7 Tsukeshiba et al.7 studied the diffusion of PTPCs out of DN hydrogels made of 2acrylamido-2-methylpropanesulfonic acid (AMPS) and AAm. They observed that at room temperature chains with M > 106 g/mol did not diffuse out of a pseudo-SIPN DN hydrogel that was aged for at least one month. An alternative approach for achieving PTPCs in a DN hydrogel is to intentionally add low concentration of high C

DOI: 10.1021/acs.macromol.7b01382 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules molecular weight, linear chains prior to the second polymerization step. In that way the concentration and molecular weight of the linear chains are known. If the molecular weight of these embedded PTPCs is sufficiently high, e.g., >106 g/mol, their diffusion out of the first network may be suppressed long enough to resolve their effect on the load-bearing properties of the DN hydrogel. The mechanical behavior of a pseudo-SIPN hydrogel, DN-097-PAAm, made from SAPS(1,4,2,9) as the first network and AAm(2,0.1,0,97) as the second network is compared in Figure 2 to the same formulation with 0.01 wt % of trapped PEO

Figure 3. Engineering tensile stress versus stretch ratio for DN-0-97PAAm and DN-0-97-PAAm/PAAm hydrogels that shows the effect of embedding linear PAAm chains on the properties of the DN hydrogel made from SAPS and AAm. Black circles: DN-0-97-PAAm DN hydrogel; gray circles: the same DN hydrogel with trapped ex situ prepared PAAm chains.

occurs in the second regime of deformation, where the embedded chains disentangle,10 thus providing another energy dissipative mechanism in addition to that from disentanglements of the grafted linear chains. One would expect that disentanglement of the embedded chains would have a greater toughening effect because for sufficient strain they can fully disentangle, whereas complete disentanglement of the grafted chains is restricted by the covalent connection with the skeleton of the first network. The caveat here, however, is that the disentanglement process will produce plastic flow of the hydrogel, which then is expected to be more significant for gel with the embedded chains. Moreover, the very high molecular weight of PTPCs used in this study makes them to be highly entangled to the body of material specifically at higher stretch ratios and therefore results in high fracture stress observed in experiments. A comparison between mechanical behavior of DN-0-97-PAAm/PEO and DN-0-97-PAAm/PAAm hydrogels (Figure 4) indicates that the efficiency of the PEO PTPCs at improving the mechanical behavior of the host hydrogel was better than the PAAm. That difference may be due to the differences in molecular weight (∼4 M vs (1.6 M g mol−1)5), but there may also be some influence from the inherent higher

Figure 2. Engineering tensile stress versus stretch ratio for DN-0-97PAAm and DN-0-97-PAAm/PEO hydrogels that shows the effect of embedding linear PEO chains on the properties of the DN hydrogel made from SAPS and AAm. Black circles: DN-0-97-PAAm DN hydrogel; gray circles: the same DN hydrogel with trapped PEO chains.

(DN-0-97-PAAm/PEO). The tensile data for both of hydrogels coincided up to the failure of the hydrogel without PEO chains. However, the hydrogel with trapped linear PEO chains exhibited much higher elongation at break and failure stress compared with the conventional pseudo-SIPN. The work of deformation calculated from the area under the stress− deformation curve is much higher with the embedded PTPCs. Figure 3 compares the mechanical behavior of two pseudoSIPN hydrogels, formed using the same recipes to synthesize the first SAPS(1,4,2,9) and second AAm(2,0.1,0,97) networks, one with 0.01 wt % of trapped ex situ prepared PAAm chains and the other without the PAAm chains. The stress−strain behavior of the two systems is qualitatively similar to the results shown in Figure 2. The effects on the mechanical properties of adding a small concentration of linear PEO or PAAm to a pseudo-SIPN DN hydrogel were similar (cf. Figures 2 and 3). The tensile data for the host hydrogel and hydrogel containing the embedded PTPCs coincided up to the failure of the host hydrogel, and the addition of the embedded PTPC improved the amount of work of fracture. As was previously discussed5,10 for pseudo-SIPN hydrogels that exhibit necking, the onset of necking is the demarcation of two regimes of deformation with different damage intensities. The first regime of deformation is related to the damage of the first network, and the second one is associated with the damage during neck propagation. The coincidence of tensile data up to the failure of the host hydrogel in Figures 2 and 3 indicates that the embedded PTPCs contribute little or nothing to the mechanical behavior within the first regime of deformation. A significant difference in the mechanical behavior, however,

Figure 4. Engineering tensile stress versus stretch ratio for DN hydrogels made of the same first and second networks except for embedded PTPCs. Black circles: PEO as embedded PTPCs; gray circles: ex situ prepared PAAm as embedded PTPCs. D

DOI: 10.1021/acs.macromol.7b01382 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules flexibility of PEO chains (C∞ = 4.1)11 compared with PAAm (C∞ = 14.8).11 It should be noted that an important reason to use very high molecular weight polymers as PTPCs is to prevent diffusion of these chains out of the first network during the swelling process used to prepare the second network. The effect of the molecular weight of the PTPCs can be better appreciated by considering Figure 5, which shows the

Figure 6. Engineering tensile stress versus stretch ratio for a pseudoSIPN hydrogel synthesized from SAPS(1,1,2,9) as the first network and AAm(2,0.1,0,97) as the second network: (circle) gel as prepared and (triangle) gel aged for 12 months.

comparison between the mechanical behavior of a fresh DN-097-PAAm pseudo-SIPN hydrogel and the same hydrogel aged for 24 h in DI water at 60 °C is shown in Figure 7. A Figure 5. Engineering tensile stress versus stretch ratio for DN-0.01-9 and DN-0.01-9/PEO pseudo-IPN DN hydrogels that shows the effect of embedding linear PEO chains on the properties of the DN hydrogels made from AAm(2,0.1,0.01,9) as the second network. Gray circles: DN-0.01-9 DN hydrogel; black circles: the same DN hydrogel with trapped PEO chains.

effect of embedded PEO chains on the mechanical behavior of a DN-0.01-9, pseudo-IPN DN hydrogel that fails at a very low stretch ratio. The addition of 0.01 wt % of the high molecular weight PEO makes the brittle hydrogel very strong, which allows it to undergo large deformation and break at a very high stretching ratio. The hydrogel without PEO chains exhibited necking, but the neck did not propagate because of the inability of the second network, AAm(2,0.1,0.01,9), to support the load after failure of the first network. As with the pseudo-SIPN DN hydrogels discussed above, the effect of the PTPCs occurs in the second regime of deformation, after neck initiation. The soft network provided by the highly entangled PEO chains improves the load carrying capability of the second covalent network and allows the gel to undergo finite deformation. This experiment also demonstrates the importance of the molecular weight of the PTPCs as a decisive factor for neck propagation. Linear chains should eventually diffuse out of the hydrogels, though the diffusion time is strongly dependent on the molecular weight, so that the removal of the embedded PTPCs described in the preceding discussions by molecular diffusion should be very slow. Nonetheless, one would expect that regardless of the molecular weight used, linear PTPCs will diffuse out of the gel, which will produce a time-dependent reduction of the mechanical properties. That was not an issue for the results described above where the mechanical properties of the gels were measured relatively soon after making the samples. Figure 6, however, shows the effect of aging a pseudoSIPN DN hydrogel, with in situ polymerized PTPCs, immersed in DI water at room temperature for 12 months. The reduction of the toughness is clear, and this observation consistent with the conclusions discussed earlier in this paper regarding the effects of PTPCs. The slow diffusion process can, of course, be accelerated by increasing the temperature at which the gel is stored. A

Figure 7. Engineering tensile stress versus stretch ratio for DN-0-97PAAm pseudo-SIPN hydrogel. Gray circles show data points for the same hydrogel kept in DI water at 60 °C for 24 h.

remarkable reduction in the toughness of sample was observed, which is due to the diffusion of in situ polymerized PAAm chains. The results of GPC measurements on PEO, ex situ prepared PAAm chains, and extracted PAAm chains after aging at 60 °C are tabulated in Table 2. The molecular weight Table 2. Molecular Weight of Polymers Synthesized and Extracted from the DN Hydrogels polymer

Mw (g mol−1)

Mw/Mn

PEO ex situ prepared PAAm extracted PAAm

6.7 × 106 4.3 × 106 1.4 × 106

5.1 14.0 19.3

averages of the PAAm chains extracted from the hydrogel were high. The high polydispersity of molecular weights can be attributed to the solution free-radical polymerization,9 and the difference between the as-polymerized and extracted samples is due to the fact that the latter sample does not represent the entire sample that was added to the gel. Figure 8 shows the effect of embedded PEO chains on mechanical properties of a pseudo-SIPN and a pseudo-IPN E

DOI: 10.1021/acs.macromol.7b01382 Macromolecules XXXX, XXX, XXX−XXX

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entangled linear polymer chains, some of which are covalently attached to the skeleton of the first network and some that are physically trapped inside the hydrogel. In a pseudo-IPN, the second network is cross-linked, and at least part of the second network is covalently grafted to the skeleton of the first network. Therefore, a pseudo-IPN does not have physically trapped polymer chains (PTPCs). However, some ex situ prepared PTPCs can be embedded in the structure of both pseudo-SIPNs and pseudo-IPNs. This can be done by trapping high molecular weight polymer chains inside the first polyelectrolyte network before the second polymerization step. The PTPCs in a pseudo-SIPN hydrogel that are the results of second polymerization step are called in situ PTPCs. Those PTPCs that are embedded inside the first network before the second polymerization step are called embedded PTPCs. The literature is not clear about the contribution of PTPCs to the mechanical behavior of DN hydrogels under large tensile deformations. In effect, one can see the inconsistency between the viewpoints regarding the contributions of PTCPs to the mechanical properties of DN hydrogels in refs 4, 7, and 8. Our findings indicate that the contribution of PTPCs to the mechanical response of a hydrogel under finite deformation becomes important in the second regime of deformation of DN hydrogels where the second network assumes the load-bearing role, and disentanglement of polymer chains can be an efficient dissipation mechanism. During this dissipation process, the more flexible polymer chains physically trapped inside the gel can contribute more efficiently to the disentanglement process and enhance toughness. Obviously, in a pseudo-SIPN, PTPCs can dissipate energy of deformation much better than polymer chains tethered to the skeleton of the first network. In a pseudo-SIPN, that has in situ PTPCs, the aging of the hydrogel can reduce the mechanical properties because the in situ PTPCs can diffuse out of the gel during the storage time in water. The DN hydrogels made with low concentration of embedded PTPCs exhibit a significant increase in the toughness. In a pseudo-IPN, the presence of embedded PTPCs can enhance the mechanical behavior of the hydrogel, but the efficiency of the PTPCs is lower than what is observed in pseudo-SIPNs. That is due to the chemical cross-links of the second network in a pseudo-IPN hydrogel that restrict the motion of PTPCs during the disentanglement process.

Figure 8. Engineering tensile stress versus stretch ratio for pseudoSIPN and pseudo-IPN hydrogels with and without trapped PEO chains. Black circles: DN-0.01-97 pseudo-IPN hydrogel; gray circles: the same pseudo-IPN hydrogel with trapped PEO chains; blue circles: DN-0-97-PAAm pseudo-SIPN hydrogel; red circles: the same pseudoSIPN hydrogel with trapped PEO chains.

hydrogel. The effect of the embedded PTPC on the toughness of the hydrogel decreased when the second polymer was slightly cross-linked. That is, the PTPC had a greater effect on a pseudo SIPN than on a pseudo-IPN of similar composition. Because of very high molecular weight of the PTPCs, in a loosely cross-linked network they can be highly entangled with the network (Figure 9), and it prevents them from sliding under finite deformation and therefore their efficiency to reduce internal friction is reduced.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (R.A.W.). Figure 9. Schematic representation of a very high molecular weight polymer chain trapped inside a pseudo-IPN. Black lines: first network; blue lines: polymer chains of the second network; red chain: high molecular weight polymer chain physically trapped inside the gel.

ORCID

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

S. Shams Es-haghi: 0000-0001-6659-2828 R. A. Weiss: 0000-0002-5700-6871 Present Address

CONCLUSIONS Double-network (DN) hydrogels prepared by the procedure described by Gong et al.,3 i.e., where two cross-linked networks are sequentially prepared or a linear polymerization is carried out on monomer within a water-swollen polymer network (usually a polyelectrolyte network), are classified as either a pseudo-IPN or pseudo-SIPN. In a pseudo-SIPN, the second network is a transient network made of a mixture of highly

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for the support of this work by grants from the Engineering Directorate of the National Science Foundation, CMMI-1300212 and CBET-1606685. F

DOI: 10.1021/acs.macromol.7b01382 Macromolecules XXXX, XXX, XXX−XXX

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REFERENCES

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DOI: 10.1021/acs.macromol.7b01382 Macromolecules XXXX, XXX, XXX−XXX