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Double Networks Based on Amphiphilic Cross-Linked Star Block Copolymer First Conetworks and Randomly Cross-Linked Hydrophilic Second Networks Maria Rikkou-Kalourkoti,† Elina N. Kitiri,† Costas S. Patrickios,*,† Epameinondas Leontidis,† Marios Constantinou,‡ Georgios Constantinides,‡ Xiaohan Zhang,§ and Christine M. Papadakis§ †

Department of Chemistry, University of Cyprus, P.O. Box 20537, 1678 Nicosia, Cyprus Research Unit for Nanostructured Materials Systems, Department of Mechanical Engineering and Materials Science and Engineering, Cyprus University of Technology, P.O. Box 50329, 3603 Limassol, Cyprus § Fachgebiet Physik weicher Materie, Physik-Department, Technische Universität München, James-Franck-Str. 1, 85748 Garching, Germany ‡

S Supporting Information *

ABSTRACT: This study presents the preparation and characterization of double networks (DN) based on a first amphiphilic polymethacrylate conetwork (APCN) and a second polyacrylamide network. The APCN first network comprised interconnected “in−out” star copolymers of 2-(dimethylamino)ethyl methacrylate (DMAEMA, hydrophilic ionizable monomer) and 2-ethylhexyl methacrylate (EHMA, hydrophobic comonomer) or lauryl methacrylate (LauMA, second hydrophobic comonomer), synthesized using group transfer polymerization, following one-pot, sequential, monomer, and hydrophobic cross-linker (ethylene glycol dimethacrylate, EGDMA) additions. The second network was prepared by the aqueous photopolymerization of acrylamide (AAm) at two different concentrations, 2 and 5 M, and N,N′-methylenebis(acrylamide) cross-linker in the presence of the fully ionized (via HCl addition) APCN. After synthesis, all DNs and single (first and second) (co)networks, equilibrium-swollen in water, were characterized in terms of their mechanical properties in compression. The DNs exhibited improved mechanical properties (stress and strain at break, and elastic modulus) compared to the corresponding single networks. Better reinforcement was achieved in the DNs whose APCN first networks bore a lower hydrophobic content and whose hydrophobic monomer was EHMA rather than LauMA. The best DN exhibited stress at break above 8 MPa and strain at break nearly 80%, close to the values of the best DNs in the literature. Nanoindentation studies were also performed on selected DNs which proved again the enhanced mechanical properties of the present DNs, manifested as high resistance to penetration and low creep displacement. Small-angle X-ray scattering (SAXS) indicated a broad correlation peak for all APCN first networks, suggestive of microphase separation with short-range order, arising from the presence of the hydrophobic segments. The single correlation peak was preserved in the SAXS profiles of the DNs, which was, however, shifted to lower q-values, consistent with further network swelling. Despite the SAXS evidence for only weak phase separation on the nanoscale in the DNs, half of the water-swollen DNs (the ones with a 5 M AAm concentration in the second network) exhibited strong birefringence which probably arose from the stretching of the charged DMAEMA segments rather than the presence of anisotropic nanophases.

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The first DN was reported by Gong et al. in 2003 and comprised a first chemically cross-linked network of poly(2acrylamido-2-methylpropanesulfonic acid) (PAMPS) and a second chemically cross-linked network of polyacrylamide (PAAm).8 The cross-linking densities in the two constituting networks were optimized to maximize the mechanical strength. In fact, this DN system still displays the best mechanical properties among all the (“truly-double”) DNs reported to date, with a record of compression stress and strain at break of

INTRODUCTION Polymeric hydrogels are cross-linked hydrophilic polymers, capable of absorbing large amounts of water. This property leads to their major commercial uses in wastewater management, agriculture, drug delivery, and tissue engineering.1,2 However, the poor mechanical strength of hydrogels is an obstacle for extending these uses and needs to be overcome. To date, three different types of mechanically enhanced hydrogels have been developed,3−5 namely, slide-ring networks,6 nanocomposite networks,7 and double networks (DN).8 Among the three types of mechanically enhanced hydrogels, DNs8−14 stand out as they exhibit the greatest improvement in mechanical strength, and they are the easiest and least expensive to prepare. © XXXX American Chemical Society

Received: November 17, 2015 Revised: February 8, 2016

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DOI: 10.1021/acs.macromol.5b02490 Macromolecules XXXX, XXX, XXX−XXX

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Figure 1. Chemical structures and names of the monomers, the cross-linkers, the initiators, and the catalyst used for the syntheses of the first and the second networks.

17.2 MPa and 92%, respectively,8 and tensile stress and strain at break of 0.9 MPa and 800%, respectively.15,16 Both networks in this system were randomly cross-linked, as they were prepared by conventional free radical copolymerization of monomer and cross-linker. This is also the case with most DNs reported to date. Two notable exceptions are the DNs prepared at two instances by the research teams of Frank17 and Gong,18 where the first (but not the second) networks were well-defined and comprised end-linked poly(ethylene glycol) diacrylate macrocross-linkers17 and end-linked four-arm poly(ethylene glycol) star (tetraPEG) polymers,18 respectively. The controlled structure was expected to minimize heterogeneities in the first network and further enhance the mechanical properties of the final DNs. This was indeed the case, with a 2-fold increase in the tensile stress and strain at break in the case of a PAMPS− PAAm DN reinforced with a homogeneous tetraPEG gel first network (essentially a “triple” network)18 as compared to a standard, optimized PAMPS−PAAm DN.15,16 Thus, perfecting the network structure, even partially, provides a powerful means for further improving the mechanical strength of hydrogels. Motivated by the above conclusion, in this work we decided to prepare DNs using as first networks a particular type of welldefined19 hydrogel we developed20,21 during the past 15 years and evaluate the mechanical properties of the resulting DNs. The hydrogels to be used as first networks are amphiphilic polymer conetworks (APCNs),22−26 comprising segments of hydrophilic and hydrophobic polymer chains. The presence of hydrophobic segments has several important implications, namely capability of swelling in organic solvents (in addition to water), lower swelling in water compared to the fully hydrophilic polymer network counterparts, and self-organization in water with the formation of hydrophobic, micelle-like, water-free domains.27 This self-organization may further enhance the mechanical properties of the targeted DNs.28−30 This enhancement would be in addition to that imparted through the homogeneity of these materials which are prepared

via quasi-living/controlled polymerization chemistries,31 including group transfer polymerization (GTP),32 atom transfer radical polymerization (ATRP),33 and reversible addition− fragmentation chain transfer (RAFT)34 polymerization. The APCNs employed as first networks in this study had two other features aiming at even better mechanical properties. One was that they comprised interconnected amphiphilic star block copolymers21 rather than end-linked amphiphilic ABA triblock copolymers;20 the much higher molecular weight of the constituting star copolymers compared to that of the linear counterparts (the arms of the stars) would provide further polymer network reinforcement. The other feature was the use of monomers giving rubbery rather than glassy polymers, in order to avert network brittleness. Thus, instead of using monomers bearing esters of aromatic or very short aliphatic alcohols, monomers with esterified relatively long aliphatic alcohols were employed as the hydrophobic monomers. Large enhancement in the mechanical properties of the water-swollen DNs compared to those of their APCN first networks was determined using bulk compression testing, with the DNs based on the least hydrophobic APCNs exhibiting the largest enhancement. This enhancement was despite the weaker phase separation on the nanoscale of the DNs compared to the APCN first networks, as revealed using small-angle X-ray scattering (SAXS). Finally, nanoindentation measurements were performed on a selected set of DN samples to determine their resistance to penetration and creep response, which were found comparable to those of classical PAMPS−PAAm DNs.

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EXPERIMENTAL SECTION

Materials and Methods. All chemical reagents were purchased from Aldrich, Germany. Figure 1 shows the chemical structures and names of the monomers used for the preparation of the first and the second networks. All first networks were prepared via sequential GTP,32 whereas all second networks were prepared through conventional cross-linking photopolymerization in the presence of the first networks. For the first network, the monomers 2B


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terized in terms of their absolute molecular weights using dual static light scattering (SLS) and RI detection in a GPC configuration (GPCSLS system). Again, experimental details and results are given in the Supporting Information. Measurement of the Degrees of Swelling. The degrees of swelling (DS) of the APCN first networks and the DNs were measured in water as a function of pH. Details of the experimental procedure and the relevant results are provided and discussed in the Supporting Information. Small-Angle X-ray Scattering (SAXS). SAXS experiments were performed using a GANESHA SAXS instrument (SAXSLAB, Copenhagen) with a GENIX 3D microfocus X-ray source and point collimation. The wavelength was λ = 1.54 Å. A 2D Pilatus 300 K detector was placed at sample-to-detector distances (SDD) of 401 and 1051 or 1081 mm, which resulted in a q-range from 0.06 to 2.00 nm−1, where q = 4π sin(θ/2)/λ and θ is the scattering angle. The background was determined by measuring the empty sample holder with mica windows. The dried and milled samples were swollen with deionized water for 24 h, and the excess water was removed using filter paper. Then, the swollen gel pieces were mounted between two thin sheets of mica (10 mm diameter) in a so-called “sandwich” sample holder. Measurements were carried out at room temperature. The exposure times were 2400 and 3600 s for SDDs of 401 and 1051/1081 mm, respectively. The SAXS curves of the first networks were fitted using a model including a Porod contribution to describe the upturn at low qvalues, a form factor for spherical core−shell particles with a polydisperse core to describe the hydrophobic domains and their surroundings, a hard-sphere structure factor to describe the correlation between these domains, and an Ornstein−Zernike structure factor describing the polymer−polymer correlations in the swollen gel matrix. In the case of the DNs, a form factor of polydisperse homogeneous spheres was used to describe the hydrophobic domains. All operations were carried out using the NIST small-angle neutron scattering (SANS) package 7.04 within the Igor Pro 6.1 environment.36 The details will be published in a forthcoming paper.37 Polarized Light Microscopy. Samples from both single and double networks, in powder form (produced by crushing dried network material), in both the dry and water-swollen states, were examined at room temperature (∼20 °C) using a Leica DM LSP polarized light microscope under both parallel and crossed polarizers, at an overall magnification of 100×, and an object field of the order of 0.5 mm. After examining a certain network sample in the dried state, a drop of water was added to the sample to fully cover it with water, and the sample was re-examined using the microscope a few minutes later (to secure equilibration of the network powder in water). Mechanical Properties. The mechanical properties of the networks were characterized using two different methods. First, classical uniaxial compression testing was employed for all DNs and single networks to determine the Young’s modulus, and the stress and strain at break. Second, selected DNs were characterized using nanoindentation to determine the resistance to penetration and creep response. Compression Testing. Water-swollen (to equilibrium) single conetworks and DNs were characterized in terms of their mechanical behavior in compression, at a constant displacement rate of 10 mm min−1, using an EZ20 Lloyd testing machine (Sussex, UK). Rectangular blocks were cut from each swollen network using a blade. The stress and strain at break were taken as the point of failure in the stress−strain curve, whereas the compressive Young’s modulus, E, was determined as the slope of the stress−strain curve at 10% strain. Nanoindentation Testing. Nanoindentation was performed using a NanoTest Platform (Micromaterials Ltd., UK). A three-sided pyramidal diamond indenter (Berkovich type) with an angle between its central axis and one of its faces of 65.3° and a nominal tip curvature of about 10 nm was used. The samples were subjected to a maximum load of 0.5 mN, the loading time was 5 s, the hold time at maximum load was 30 s, and the unloading time was 2 s. During the test, the load, P, and the depth of penetration, h, were continuously monitored. For each sample, a total of six indentation experiments were performed. The hardness, H, and reduced elastic modulus, Er, of the

(dimethylamino)ethyl methacrylate (DMAEMA, 99%), 2-ethylhexyl methacrylate (EHMA, 98%), and lauryl methacylate (LauMA, 96%), the cross-linker ethylene glycol dimethacrylate (EGDMA, 98%), the initiator 1-methoxy-1-trimethylsiloxy-2-methylpropene (MTS, 95%), and the catalyst tetrabutylammonium bibenzoate (TBABB) were used. For the second network, acrylamide (AAm, ≥98%) was used as the monomer, N,N′-methylenebis(acrylamide) (MBAAm, 99%) was employed as the cross-linker, and 2-oxoglutaric acid (OA) (≥99%) served as the photoinitiator. DMAEMA, EHMA, LauMA, and EGDMA were highly purified in order to enable their oxyanionic polymerization by GTP, following three steps: first, passage through basic alumina (98%) columns to remove the polymerization inhibitors and any other acidic impurities, and subsequent stirring overnight over calcium hydride (CaH2, 90−95%) to remove the last traces of moisture and protonic impurities; to prevent undesirable thermal polymerization, the 2,2-diphenyl-1-picrylhydrazyl hydrate (DPPH, 95%) free radical inhibitor had been added before stirring was initiated. The third and final purification step involved vacuum distillation just prior to the polymerization. AAm and MBAAm were recrystallized from chloroform and ethanol, respectively. TBABB was prepared according to the literature32g and was stored under vacuum until use. Tetrahydrofuran (THF, 99.8%, both HPLC and reagent grade) was purchased from Scharlau, Spain. Reagent grade THF was dried by being refluxed over a sodium/potassium eutectic alloy for 3 days and was freshly distilled prior to use as polymerization solvent. Polymerizations. Preparation of the First Networks. All amphiphilic cross-linked star copolymer first conetworks of this study were prepared using sequential GTP at room temperature, following our previously published procedures.21,35 Details for the polymer syntheses are provided in the Supporting Information. Two families of (first) conetworks were synthesized: one using EHMA as the hydrophobic monomer and the other using LauMA. The EHMAcontaining family comprised three conetworks of different compositions, while the LauMA-containing family was composed of five conetworks with different compositions. Composition variation was achieved by changing the comonomer feed ratio. In addition to the amphiphilic cross-linked star copolymer first conetworks, three crosslinked star homopolymer first networks of DMAEMA, LauMA, and EHMA were also prepared. Preparation of the Second Networks. For the preparation of the double networks, complete ionization of the tertiary amine groups of the DMAEMA units in the first networks was necessary to confer a rather large aqueous degree of swelling which would help accommodate the high polymer volume fraction required for the second network. Thus, two pieces from each APCN (after washing in THF, followed by vacuum drying) were placed in glass vials containing deionized water, where they were left for 1 week. Subsequently, a precalculated amount of fuming HCl was added to achieve full ionization of the DMAEMA units, leading to a pH of ∼2. Then, all conetwork pieces were again dried in a vacuum oven for 3 days at room temperature and were subsequently allowed to equilibrium-swell in an aqueous solution of AAm (2 or 5 M) also containing MBAAm cross-linker at a concentration of 0.1 mol % relative to AAm and OA photoinitiator also at 0.1 mol % relative to AAm. The second network (AAm-MBAAm) was formed by irradiating the swollen networks for 4 h at 365 nm using a commercial UV lamp assembly from Vivo Ltd. model NW107RG-T3296 with four UV lamps of total power of 36 W. Determination of the Sol Fraction in the First APCNs and in the Final DNs. The percentage of the extractables from the (co)networks was determined; the molecular weight and composition characteristics of the extractables were measured using gel permeation chromatography with single refractive index detection (GPC-RI system) and 1H NMR spectroscopy, respectively. Details of the experimental procedure and the relevant results are provided and discussed in the Supporting Information. Characterization of the APCN Precursors. Similarly, all soluble precursors to the first (co)networks, i.e., the linear and the star (co)polymers, were also characterized in terms of their molecular weights and composition using GPC-RI and 1H NMR spectroscopy, respectively. Furthermore, the star (co)polymers were also characC


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Scheme 1. Schematic Representation of the Synthetic Procedure Followed for the Preparation of the APCN First Network

segments in the APCN first networks to reduce brittleness and facilitate morphology equilibration. To this end, the monomers EHMA and LauMA, bearing relatively long aliphatic side chains, with or without branching, respectively, were chosen. The glass transition temperatures, Tg, of polyEHMA and polyLauMA are −12 °C39 and −45 °C,40 respectively, which must be compared with those of polyMMA21a−c,e and poly(benzyl methacrylate)21d of 100 and 54 °C, respectively, employed in some of our previous work. Eleven polymer (co)networks were synthesized in total: three homopolymer (DMAEMA, EHMA, and LauMA) networks, three EHMA−DMAEMA conetworks, and five LauMA−DMAEMA conetworks. In the two conetwork families, the hydrophobic content was systematically varied (12.5−75 mol %) in the secondary arms whose total nominal degree of polymerization was fixed at 40. Scheme 1 illustrates the preparation of the APCN first networks in a five-step, onepot, sequential procedure via GTP. In the first step, DMAEMA was polymerized to yield linear homopolymers, which were converted to “arm-first” star homopolymers by the addition of EGDMA cross-linker in the second step. In the third step, DMAEMA (indicated here in dark blue rather than in light blue to distinguish it from that used in the first step) was again polymerized to convert the “arm-first” star homopolymers to “in−out” star homopolymers. The hydrophobic monomer was added and polymerized in the fourth step, yielding amphiphilic “in−out” star block copolymers, which were covalently endlinked by the addition and polymerization of cross-linker a second time in the fifth and final step. The hydrophobic monomer was either LauMA or EHMA. The resulting APCN first networks consisted of dangling DMAEMA homopolymer chains (being elastically inactive and hydrophilic, these dangling chains would enhance aqueous swelling in the first network, thus facilitating the introduction of more PAAm in the second network, necessary for mechanical property enhancement), prepared in the first polymerization step, and an equal number of amphiphilic diblock copolymer elastic chains covalently bridging the star copolymers.

materials were extracted from the nanoindentation load−displacement curves using:

H=

Pmax A

(1)

Er =

π 1/2S 2A1/2

(2)

where Pmax is the maximum applied load, A is the projected contact area at maximum load, Er is the reduced elastic modulus (1/Er = (1 − vs2)/Es + (1 − vi2)/Ei), vs is the Poisson’s ratio of the sample, vi is the Poisson’s ratio for the diamond indenter (0.07), Es is the sample elastic modulus, Ei is the indenter elastic modulus (1141 GPa), and S is the contact stiffness found from the slope of the unloading portion (dP/ dh) of the load−displacement curve at Pmax. The hold time at maximum load gives access to the creep deformation (change in displacement during time) of the materials, quantifying the timedependent material characteristics.

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RESULTS AND DISCUSSION Polymer Design and Synthesis. The general aim in this work is to create networks with enhanced mechanical properties, whereas the specific aim is to improve the mechanical strength of APCNs. These aims were pursued by simultaneously exploiting several concepts of mechanical property enhancement.38 The first concept was that of DNs,3−5,8−18 with the APCN first network complemented with an AAm-MBAAm second network. The second one was that of network homogeneity secured by the use of a quasiliving/controlled polymerization method31−34 for the preparation of the first network. The third idea was the amphiphilicity of the first network whose phase separation on the nanoscale and resulting hydrophobic nanodomains were expected to provide another mechanism for energy dissipation.28−30 The fourth principle concerned the building blocks of the first network, which were made unusually large to confer greater strength; thus, the first network consisted of interlinked star block copolymers21 rather than end-linked linear block copolymers.20 And, finally, the fifth concept involved the employment of rubbery rather than glassy hydrophobic D


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Macromolecules The APCN first networks were transferred from THF to water and were acidified by the addition of an aqueous HCl solution to fully protonate (pH ∼ 2) the DMAEMA units and make the networks swell. They were subsequently used as the first networks for the preparation of the DNs via the photopolymerization of an AAm−MBAAm mixture. The AAm concentration in the mixture was kept at two levels, 2 and 5 M. Thus, from each first network, two DNs were prepared, each corresponding to these two AAm concentrations. Size and Composition of the Conetwork Precursors. All soluble precursors (linear and star polymers) to the first networks were characterized in terms of their molecular weights and composition, and the results are provided and discussed in the Supporting Information. Figure S1 plots the GPC traces of all soluble precursors to a typical APCN first network, whereas Table S1 summarizes the molecular weights and composition characteristics of the linear and star precursors to all the APCN first networks. Characterization of the absolute molecular weights of the star polymer precursors to the conetworks indicated a range of values between 30 700 and 1 200 000 g mol−1, corresponding to 7 and 281 arms per cross-linking node. Percentage, Size, and Composition of the Extractables from the Networks. These results are presented and discussed in the Supporting Information (Table S2). Briefly, the percentage of the extractables from the first networks was lower than 14% (lower than 10% in most cases), and that from the DNs was even lower, lower than 1%. Swelling Behavior of the Conetworks in Water. The pH and network composition dependence of the equilibrium DSs in water of the first and the DNs was studied and is provided and discussed in detail in the Supporting Information (Figures S2−S5). In short, for the first (co)networks, the aqueous DSs increased as the pH was lowered and as the content in hydrophobic monomer repeating units was also lowered. These dependencies were weakened in the corresponding DNs, also bearing nonionic hydrophilic AAm monomer repeating units as the main component of the second network. Nanophase Separation. Small-Angle X-ray Scattering. Figure 2 displays the SAXS profiles of six out of the eight APCN first networks and most (all but two) of their corresponding DNs. Similar to the SANS profiles of previously studied APCNs,20b,d−f,i−k,21a−c the present APCN first networks also displayed a correlation peak at intermediate q-values, within the range of 0.2−0.3 nm−1, and an upturn toward low qvalues. The latter feature indicates the presence of large-scale inhomogeneities, whereas the former feature is the signature of internal organization within the present system, yielding hydrophobic domains placed at rather well-defined distances of 21−31 nm (= 2π/qmax) and in good agreement with the molecular dimensions of the arms of the stars possessing a (nominal) DP of 40, corresponding to a contour length of 10.1 nm (40 × 0.252 nm; 0.252 nm is the contribution of one monomer repeating unit to the chain contour length41), which should be and indeed is about one-half of the distance between the hydrophobic domains from SAXS; this agreement also implies rather stretched chains, justified by the large number of arms in the star polymers (∼10−200). As expected, the correlation peaks were more pronounced in the cases of the more hydrophobic APCNs. In addition to the (main) correlation peak, the SAXS profiles of the APCN first networks also exhibited a shallower peak (bump) in the q-range from 0.4

Figure 2. SAXS profiles, together with the fitted curves (lines), of the APCN first networks in water and the corresponding DNs in water at room temperature. (a) EHMA-containing APCN first networks, (b) LauMA-containing APCN first networks, (c) DNs based on EHMAcontaining APCN first networks and AAm 2 M second networks, (d) DNs based on LauMA-containing APCN first networks and AAm 2 M second networks, (e) DNs based on EHMA-containing APCN first networks and AAm 5 M second networks, and (f) DNs based on LauMA-containing APCN first networks and AAm 5 M second networks. D, E, L, A, and s in the curve labels are further abbreviations for DMAEMA, EHMA, LauMA, AAm, and star, respectively.

to 1 nm−1. The bumps are located at q-values approximately 3 times those of the primary peaks and may arise from the form factor of the cross-linkers and the densely packed hydrophobic blocks around them. The SAXS profiles of the DNs did not exhibit the upturns at low q-values, suggesting that the gel homogeneity was improved with the introduction of the AAm-based second network at the length scales studied here. The presence of the AAm second network was also the reason why the correlation peaks in the parent APCN first networks were significantly weakened in the corresponding DNs and remained visible in the SAXS profiles only for the DNs based on the most hydrophobic APCN first networks. The position of these peaks was shifted to lower q-values as compared to the first networks, a result of the introduction of the AAm second network and the ionization of the first networks. It is noteworthy that a SANS study on the standard PAMPS−PAAm DNs determined that no peak was present in the SANS profile, unless the DN was highly compressed (50% strain).42 Birefringence. Given the phase separation at the nanoscale observed in the APCN first networks in water from their SAXS profiles, we decided to examine whether any of those nanophases presented birefringence. To this end, dried samples of the (co)networks were finely ground, placed on microscope glass slides and examined in the dried state or immersed in water using polarized light microscopy. Figure 3 shows the images from polarized light microscopy for the six out of the eight APCN first networks, plus the three homopolymer first E


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Figure 3. Polarized light microscopy images for six of the first (co)networks and their corresponding DNs in the water-swollen and dried states (100× magnification; image length: 312 μm). D is a further abbreviation for DMAEMA.

Figure 4. Compressive stress−strain profiles for the six APCN and the two hydrophobic first (co)networks.

networks, as well as the DNs corresponding to these six APCNs. The images in the figure exhibiting birefringence were enclosed in a yellow frame. As shown in the figure, the three homopolymer first networks did not exhibit any color variation in the dried or in the water-swollen states because they could not microphase separate at all. Similar to the homopolymer first networks, the six APCN first networks in the dried state presented no birefringence. However, the six water-swollen APCN first networks clearly exhibited birefringence. The

intensity of birefringence increased with the conetwork composition in hydrophobic monomer, LauMA or EHMA, with the LauMA-bearing APCNs displaying a stronger effect than the EHMA-bearing ones at the same molar composition. This latter observation can be attributed to the higher hydrophobic volume fraction in the case of the LauMA-based APCNs as compared to the EHMA-based ones of the same molar composition, arising from the higher molecular weight of LauMA compared to EHMA. F


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Macromolecules Figure 3 also shows the images from polarized light microscopy for the DNs corresponding to the six APCN first networks, in both the dried and the water-swollen states. Unlike the first networks that did not present birefringence in the dried state, all DNs exhibited intense (in most cases) color variation in the dried state. Furthermore, the water-swollen DNs whose second network was produced from a 5 M AAm solution also exhibited birefringence. This may be due either to the formation of anisotropic morphologies, i.e., lamellae or cylinders, or to the (internal) stretching of the material arising from the ionization of the DMAEMA units. As the former possibility can be excluded, based on the finding of spherical hydrophobic domains by SAXS in Figure 2, the birefringence of the water-swollen DNs should be due to ionization-induced strain. Mechanical Properties of the Networks. The mechanical properties of all the networks swollen in water were investigated using bulk compression measurements to determine the stress and strain at break and the Young’s modulus. Nanoindentation studies were also performed on selected DNs to determine the penetration resistance and the creep displacement. Stress and Strain at Break and Young’s Modulus of the APCN First Networks. Figure 4 presents the compressive stress−strain measurements for the six APCN first networks, plus the two hydrophobic first networks, with Figure 4a plotting the results for the EHMA-based (co)networks and Figure 4b displaying those for the LauMA-based (co)networks. Each curve ends at the point where the material failed (was fractured). Figure 5 summarizes the results for the APCN first networks calculated from the stress−strain curves in Figure 4 and includes the stress and strain at break and the low-strain

Young’s modulus. The Young’s moduli, calculated at 10% strain, increased monotonically, and nearly linearly, with the content in hydrophobic monomer, LauMA or EHMA, as the presence of more hydrophobic units implied a lower aqueous DS (Figure S4) and hence a higher polymer volume fraction and, consequently, a higher concentration of elastic chains, responsible for network resistance. The EHMA-based APCNs were stiffer (higher modulus) than the LauMA-based ones owing to the higher Tg of polyEHMA as compared to that of polyLauMA, with the respective values being −12 °C39 and −45 °C.40 The fracture strain for the EHMA-based APCNs presented a trend opposite to that of the modulus, with the stiffer materials breaking at lower deformation, as expected. On the other hand, the fracture stress for the same APCNs exhibited a maximum, suggesting that moderately stiff materials can bear a higher load. The fracture strain for the APCNs containing the more rubbery LauMA was higher than that for the EHMA-based ones with the same composition, owing to the more compliant characteristics of polyLauMA. In fact, the fracture strain for the LauMA-containing networks displayed a minimum with respect to composition, with the LauMA homopolymer network presenting the highest fracture strain. The LauMA homopolymer network also exhibited the highest, by far, fracture stress among the first networks bearing it, with the curve presenting a minimum at intermediate LauMA contents. Stress and Strain at Break and Young’s Modulus of the DNs. The mechanical properties of the DNs synthesized in the present study were also characterized in compression. Figure 6

Figure 6. Compressive stress vs strain profiles of the two DNs based on the DMAEMA20-star-(DMAEMA30-b-EHMA10)-network as well as the three corresponding single networks DMAEMA20-star-(DMAEMA30-b-EHMA10)-network, PAAm-2-0.1, and PAAm-5-0.1.

presents the stress−strain profiles for the two DNs based on the DMAEMA20-star-(DMAEMA30-b-EHMA10)-network as first network, DMAEMA20-star-(DMAEMA30-b-EHMA10)/PAAm2-0.1 and DMAEMA20-star-(DMAEMA30-b-EHMA10)/PAAm5-0.1, together with those of the three corresponding single networks: DMAEMA20-star-(DMAEMA30-b-EHMA10)-network, PAAm-2-0.1, and PAAm-5-0.1. As with Figure 4, the stress− strain curves in Figure 6 are shown until the point of failure. From these curves, it can be seen that the strain at break of the two DNs were at ∼80%, higher than that of the parent APCN single network of ∼60%, even slightly higher than those of the PAAm single networks of ∼75%. However, much more significant was the enhancement in the stress at break. In particular, the stress at break of the two DNs rose to 6.6 and 8.4 MPa, from the value of the parent APCN first network of 0.4 MPa and those of the PAAm single networks of 0.11 MPa (2 M

Figure 5. Effect of hydrophobic monomer type and content in the APCN first networks on their Young’s modulus and their stress and strain at break. G


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original DN publication,8 a 2 M AAm concentration in the polymerization mixture for the second network was optimal, with the higher AAm concentration of 5 M giving poorer results in most cases; the only exception was the case of the very hydrophobic APCN with 50 mol % LauMA units, whose low DS did not allow the introduction of sufficient PAAm when the AAm monomer concentration was only 2 M. The values of the stress at break of the DNs were highest for lowest hydrophobe contents, decreasing with the percentage of the EHMA or the LauMA units (with the exception of the 50 mol % LauMA with 5 M AAm), whereas the stress at break for the APCN first networks increased with the EHMA content and decreased with the LauMA content. There was also enhancement in the strain at break of the DNs relative to the APCN first networks in several, but not all, cases, and this enhancement was not as high as the enhancement in the stress at break. Finally, there was enhancement in the Young’s modulus at 10% strain of the DNs relative of their APCN precursors. The DN with the structure DMAEMA20-star-(DMAEMA30b-EHMA10)-network/PAAm-2-0.1 exhibited the best mechanical properties, with stress and strain at break of 8.37 MPa and 76%, respectively. These values are comparable to those of the best DN reported in the literature, with respective values of 17.2 MPa and 92%.8 This result is very promising given that the present first APCNs were based on polymethacrylates which are rather fragile. Changing the chemistry of the first networks from methacrylates to polyacrylates or polyacrylamides, which are mechanically more robust, may readily further enhance the mechanical properties of the final DNs. Nonetheless, it is noteworthy that the DN principle successfully operated and significantly enhanced the mechanical properties of the polymethacrylate APCNs. One further possibility for enhancing the mechanical properties of the present APCNs might be through the concept of triple networks, recently applied in totally hydrophobic43 and totally hydrophilic44 networks. Resistance to Penetration and Creep Displacement. The mechanical properties of selected DNs were also characterized using nanoindentation. In this experiment, a diamond probe is pushed against the surface of the material being studied, and the applied load vs the corresponding depth of penetration is continuously recorded. An inverse analysis of the loading− penetration depth (P−h) curve gives access to hardness, H (a measure of the resistance to penetration), while an analysis of the unloading slope gives access to the elastic modulus of the material. If one adds a dwell time at maximum load, access is

AAm) and 0.046 MPa (5 M AAm). Thus, the DN concept is also in action in the case of APCNs as first networks, manifesting itself by the strengthening of these important materials. From all recorded stress−strain curves for the DNs, the stress and strain at break, and the Young’s modulus at 10% strain were calculated, and the results are plotted in Figure 7.

Figure 7. Effect of hydrophobic monomer content in the APCN first network on the stress and strain at break, and the Young’s modulus of the final DNs, as well as of the APCN first networks of this study.

The figure also contains the results for the corresponding APCN first networks for comparison. By examining the results in Figure 7, it can be observed that in most cases the mechanical properties of the DNs were superior to those of the parent APCN first networks. The stress at break of the DNs was significantly enhanced relative to that of the respective APCNs, especially for lower hydrophobic contents in the parent APCNs. This last observation can again be attributed to the aqueous DSs of the parent APCNs which were higher for lower hydrophobic compositions, allowing a greater amount of the reinforcing PAAm to be introduced in the DN. As in the

Figure 8. Load−displacement responses for the two APCN-based DNs characterized by nanoindentation: (a) DMAEMA20-star-(DMAEMA10-bEHMA30)/PAAm-5-0.1; (b) DMAEMA20-star-(DMAEMA30-b-EHMA10)/PAAm-2-0.1. The arrows indicate the direction of deformation. H


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Nanoindentation hardness, elastic modulus, and creep deformation were found to be similar for the DMAEMA20star-(DMAEMA10-b-EHMA30)/PAAm-5-0.1 and the DMAEMA20-star-(DMAEMA30-b-EHMA10)/PAAm-2-0.1 samples. The results are displayed in Table 1. Table 1 also shows the nanoindentation results for two PAMPS-1-4/PAAm-2-0.1 DN samples, of which one was synthesized and kindly provided by Prof. J. P. Gong of the University of Hokkaido in Japan, and the second was synthesized in our Laboratories at the University of Cyprus following the literature procedure.8 As shown in Table 1, the sample made in Japan presented the highest resistance to penetration as evidenced by the highest hardness and lowest depth of penetration. Additionally, comparing the two PAMPS/ PAAm DNs, the sample synthesized in Cyprus presented twice as large creep displacement compared to the sample synthesized in Gong’s lab, suggesting a lower time-dependent resistance to penetration. Differences are probably due to the different configurations in the photopolymerizations, being in the form of a thin film in Japan and in bulk solution in Cyprus. Although DMAEMA20-star-(DMAEMA30-b-EHMA10)/PAAm2-0.1 (DN with the best mechanical properties synthesized in this study) presented a larger, by 56%, creep displacement compared to the DN sample synthesized in Japan, its creep displacement was by 24% lower than that of the DN sample synthesized (in Cyprus) following a procedure more similar to its own, indicating that the enhancement of mechanical properties in our system was comparable to that in the literature.8 From the nanoindentation experiments, we also calculated the fraction of plastic work, i.e., the energy dissipated through plastic deformation compared to the total energy dissipated. The results from these calculations are also included in Table 1, which show that the plastic work fraction determined for the DNs based on APCNs was about twice as high that determined for the classical PAMPS/PAAm DNs. Furthermore, the DN based on the more hydrophobic APCN, DMAEMA20-star(DMAEMA10-b-EHMA30)/PAAm-5-0.1, exhibited a higher (by 20%) plastic work fraction compared to that based on the more hydrophilic one, DMAEMA20-star-(DMAEMA30-b-EHMA10)/ PAAm-2-0.1. Thus, the inclusion of a rubbery, hydrophobic component in the DNs helps the material dissipate more energy through greater plastic deformation (ductility), and this is more so for the DN possessing a higher hydrophobic content. Importantly, this is despite the fact that the stress at break under uniaxial compression, also summarized in Table 1, follows the opposite trend.

granted to time-dependent deformation characteristics (creep). The load−displacement curves (six repetitions at different locations) for the two DNs investigated, DMAEMA20-star(DMAEMA10-b-EHMA30)-network/PAAm-5-0.1 and DMAEMA20-star-(DMAEMA30-b-EHMA10)-network/PAAm-2-0.1, are shown in Figure 8. The very good repeatability of the P−h curves testifies toward the homogeneous nature of the polymeric materials. It is evident that the maximum penetration depth of the indenter for the DMAEMA20-star-(DMAEMA30-b-EHMA10)/PAAm-20.1 sample was lower than the value corresponding to DMAEMA20-star-(DMAEMA10-b-EHMA30)/PAAm-5-0.1, suggesting a higher resistance to penetration for the former sample. These results agree with those from the compression experiments which indicated that DMAEMA20-star-(DMAEMA30-b-EHMA10)/PAAm-2-0.1 was the DN with the best mechanical properties (stress at break). The creep measurements for DMAEMA20-star-(DMAEMA10-b-EHMA30)/PAAm-5-0.1 and DMAEMA20-star-(DMAEMA30-b-EHMA10)/PAAm-2-0.1 are shown in Figure 9. The

Figure 9. Creep deformation for the two APCN-based DNs, DMAEMA20-star-(DMAEMA10-b-EHMA30)/PAAm-5-0.1 and DMAEMA20-star-(DMAEMA30-b-EHMA10)/PAAm-2-0.1, characterized by nanoindentation. D and E in the curve labels are further abbreviations for DMAEMA and EHMA, respectively.

total creep deformation for DMAEMA20-star-(DMAEMA30-bEHMA 10 )/PAAm-2-0.1 was smaller than that for the DMAEMA20-star-(DMAEMA10-b-EHMA30)/PAAm-5-0.1 sample. This could be potentially attributed to the low percentages of EHMA and/or the higher volume fraction of the second to the first network.

Table 1. Hardness, Elastic Modulus, Creep Displacement, and Fraction of Plastic Work for the DNs As Measured Using Nanoindentation; Elastic Modulus and Stress at Break as Measured Using Uniaxial Bulk Compression Tests Are Also Shown for Comparison indentation tests network structure DMAEMA20-star-(DMAEMA10-b-EHMA30)/ PAAm-5-0.1 DMAEMA20-star-(DMAEMA30-b-EHMA10)/ PAAm-2-0.1 PAMPS-1-4/PAAm-2-0.1 (Japan sample) PAMPS-1-4/PAAm-2-0.1 (Cyprus sample)

hardness (MPa)

elastic modulus (MPa)

1.3 ± 0.4

a

uniaxial compression tests

creep displacement (nm)

plastic work fraction

elastic modulus (MPa)

maximum stress (MPa)

7.2 ± 0.8

1650 ± 110

0.59 ± 0.004

5.9

1.3

1.9 ± 0.1

5.4 ± 0.2

1370 ± 65

0.472 ± 0.009

0.8

8.4

3.7 ± 0.1 1.3 ± 0.1

4.5 ± 0.1 1.5 ± 0.1

878 ± 77 1800 ± 140

0.281 ± 0.013 0.298 ± 0.016

N/A N/A

17.2 N/A

a

Elastic moduli are caclculated from the experimentally measured Er, assuming a Poisson’s ratio of 0.5, a reasonable assumption for incompressible materials like hydrogels. I


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Macromolecules To further show the beneficial role of the presence of hydrophobic segments on the mechanical properties of hydrogels, we performed additional nanoindentation experiments on the four samples listed in Table 1. These experiments were “depth recovery tests” that involved a loading−unloading cycle, applying a maximum load of 0.5 mN and followed by a holding load of 0.1 mN, while monitoring depth recovery. The depth recovery curves are displayed in Figure S7. In the figure, full recovery would correspond to a zero y-value. The curves indicate that the extent of recovery after deformation improves with the content in hydrophobic component. In particular, the poorest recovery was exhibited by the very hydrophilic classical PAMPS/PAAm DNs, whereas the two APCN-based DNs displayed better recoveries, with the more hydrophobic one presenting the best recovery behavior. Thus, although between the DMAEMA20-star-(DMAEMA10-b-EHMA30)/PAAm-5-0.1 and the DMAEMA20-star-(DMAEMA30-b-EHMA10)/PAAm2-0.1 DNs, the latter exhibited a higher (and the highest among its homologues in this study) stress at break, it was the former that displayed better behavior in terms of recovery to deformation, arising from its higher content in hydrophobic (and rubbery) units. In addition to the stress at break, Table 1 also lists the elastic moduli measured through uniaxial compression and compares these values to the ones derived from the nanoindentation experiments. The latter values were consistently higher than the former ones. This expected result relates to the nonlinear elastic response exhibited by the DN hydrogels which show significant stiffening with increasing strain. While the elastic moduli in the compression tests are calculated at 10% strain, the highly triaxial stress state generated during an indentation test is expected to cause strains well in excess of 10%. It has been reported that an average strain of ∼30% is generated for Berkovich indenters.45 Hardness, as defined in eq 1, relates to the mean contact pressure generated between the indenter and the indented material. In general, hardness relates to the resistance of the material to plastic deformations. For ductile metals, Tabor46 has shown that hardness is linearly related to the yield stress of materials, as obtained in a conventional uniaxial compression test:

H = Cσy

For linear elastic materials, an analytical solution exists which relates hardness to its elastic properties:48

H=

(5)

where E and ν are the elastic modulus and Poisson’s ratio of the indented material, and β is the angle generated by the outer inclined edge of the indenter and the indented surface (β = 19.7° in our case). Figure 10 shows eqs 3−5 on a H/σy vs E/σy graph. For materials exhibiting a linear elastic response, the ratio E/σy

Figure 10. Behavior of the three DN samples characterized using nanoindentation, represented in a graph with axes normalized hardness, H/σy, vs E/σy. The dependence of H/σy on E/σy according to three models (elastic, plastic, and elastic−plastic) is also shown in the figure. D and E in the curve labels are further abbreviations for DMAEMA and EHMA, respectively.

corresponds to the inverse of their yield strain which controls the mechanical response of the system. While our synthesized hydrogels are highly nonlinear, this ratio continues to serve as a good measure of the overall mechanical response. Materials with large E/σy values tend to behave plastically while materials with low E/σy values tend to behave elastically. In between, there is a transition domain where materials deform with a combination of elastic and plastic strains. Our experimental data suggest that DMAEMA20-star-(DMAEMA30-b-EHMA10)/ PAAm-2-0-1 and PAMPS-1−4/PAAm-2-0.1 (Japan sample) possess very low E/σy values (E/σy < 1) which dictate a primarily elastic response. This is also in line with the very good correspondence between our experimental data and eq 5 which represents a purely elastic response. DMAEMA20-star-(DMAEMA10-b-EHMA30)/PAAm-5-0-1 has an E/σy ≈ 4, coming closer to eq 4, suggesting a combined elastic-plastic deformation mode.

(3)

where C is a constant whose value is about 3, depending, to a certain extent, on the geometry of the indenter. This equation, which has been verified computationally and experimentally, is based on the theory of indentation on rigid plastic solids. When indenting on very soft materials, like hydrogels, with elastic moduli appreciably lower than those of metals, the resulting indentation contact pressure (hardness) is much lower than what would be predicted by eq 3. In such circumstances, the elastic and plastic strains in the indented region become comparable in magnitude and should be taken into consideration. Utilizing an observation on the mode of deformation which simulates an expansion of internally pressurized spherical cavity, Marsh found a relation which links hardness to the elastic and plastic components of the indented material:47 ⎛ E⎞ H ≈ ⎜⎜0.07 + 0.6 ln ⎟⎟σy σy ⎠ ⎝

E tan β 2 1 − ν2

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CONCLUSIONS In conclusion, the successful synthesis of amphiphilic polymer conetworks (APCN) based on interconnected “in−out” star copolymers of DMAEMA−EHMA or DMAEMA−LauMA, as well as their double networks (DN) with acrylamide, was accomplished. Small-angle X-ray scattering (SAXS) indicated phase separation on the nanoscale within the APCN first networks in water, which faded away in most corresponding

(4) J


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DN materials. The DNs presented enhanced mechanical properties compared to the APCN first networks and the PAAm single networks. The stress at break was particularly enhanced for the DNs based on APCN first networks with a lower hydrophobic content, and more so for the ones based on EHMA rather than LauMA units. The mechanical properties of the best DN in this study compare favorably with those of the best DN in the literature,8 with the present material displaying a compressive stress at break equal to 8.37 MPa and a compressive strain at break equal to 76%.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b02490. Experimental details on polymer synthesis, characterization of the (soluble) polymer network precursors and the extractables from the networks in terms of their molecular weights (gel permeation chromatography) and composition (1H NMR spectroscopy), aqueous swelling degrees of all single and double networks as a function of pH, transmission electron microscopy (TEM) images on selected networks, and indentation depth recovery experiments on selected DNs (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (C.S.P.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the European Regional Development Fund and the Republic of Cyprus for cofunding, through the Cyprus Research Promotion Foundation, projects NEA YPODOMH/ NEKYP/0311/27, NEA YPODOMH/NEKYP/0308/02 (500 MHz NMR spectrometer), and DIDAKTOR/0311/80 (postdoctoral research fellowship to M.R.-K.). We are also grateful to the A. G. Leventis Foundation for a generous donation that enabled the purchase of the 300 MHz NMR spectrometer of the University of Cyprus. Moreover, we acknowledge with thanks our colleague Professor J. P. Gong of the University of Hokkaido in Japan for kindly providing the PAMPS−PAAm DN sample. Furthermore, we are indebted to our colleagues Mr. M. Nicolaou and Dr. E. Psimolophitis of CNE Ltd., Nicosia, Cyprus, for their contribution to the mechanical property characterization. Finally, we thank our colleague Dr. K. Kyriakos of the Technische Universität München for his help with the SAXS measurements and Ms. M. Nearchou and Dr. K. Kyriacou of the Cyprus Institute of Neurology and Genetics for kindly performing for us the transmission electron microscopy (TEM) measurements.

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REFERENCES

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