Article pubs.acs.org/Macromolecules
Facile Fabrication of Tough Hydrogels Physically Cross-Linked by Strong Cooperative Hydrogen Bonding Guoshan Song,† Lei Zhang,‡ Changcheng He,† De-Cai Fang,*,‡ Philip G. Whitten,§ and Huiliang Wang*,† †
Beijing Key Laboratory of Energy Conversion and Storage Materials, College of Chemistry, Beijing Normal University, Beijing 100875, China ‡ Key Laboratory of Theoretical and Computational Photochemistry, Ministry of Education, College of Chemistry, Beijing Normal University, Beijing 100875, China § School of Mechanical, Materials & Mechatronic Engineering, Faculty of Engineering, University of Wollongong, Wollongong NSW 2522, Australia S Supporting Information *
ABSTRACT: Novel hydrogels with excellent mechanical properties have prompted applications in biomedical and other fields. The reported tough hydrogels are usually fabricated by complicated chemical and/or physical methods. To develop more facile fabrication methods is very important for the practical applications of tough hydrogels. We report a very simple yet novel method for fabricating tough hydrogels that are totally physically cross-linked by cooperative hydrogen bonding between a pre-existing polymer and an in situ polymerized polymer. In this work, tough hydrogels are prepared by heating aqueous acrylamide (AAm) solution in the presence of poly(Nvinylpyrrolidone) (PVP) but without any chemical initiators or covalent bonding cross-linking agents. Mechanical tests of the asprepared and swollen PVP-in situ-PAAm hydrogels show that they exhibit very high tensile strengths, high tensile extensibility, high compressive strengths, and low moduli. Comparative synthesis experiments, DSC characterization, and molecular modeling indicate that the formation of strong cooperative hydrogen bonding between the pre-existing PVP and the in situ formed PAAm chains contributes to the gel formation and the toughening of the hydrogels. The unique microstructure of the gels with evenly distributed flexible cross-linking sites and long polymer chains attached to them endow the hydrogels with an excellent mechanism of distributing the applied load. (α-CD) molecules with PEG chains threaded through.7 NC gels employ nanoscopic clay slabs as polyfunctional crosslinking centers (PFC), on which chemical initiators are adsorbed to initiate the polymerization of monomer molecules and the formed polymer chains are physically adsorbed.8 For the PFICC-based gels,11 part of the polymer chains is covalently joined to the cross-linking centers. DN gels are synthesized via a two-step sequential free-radical polymerization process by combining a rigid and brittle first network with a soft and ductile second network.9 The toughing mechanism of DN gels is different to the above-mentioned tough hydrogels, in which the first brittle network serves as sacrificial bonds, which breaks into small clusters to efficiently disperse the stress.12 There is a current trend to fabricate tough synthetic hydrogels based on physical cross-linking. The physically reversible interactions capable of constructing and maintaining the gel network include hydrogen bonding,13 ionic inter-
1. INTRODUCTION Hydrogels have many applications in pharmaceutical, biomedical, and industrial fields;1 they are used as superabsorbents,2 contact lenses, drug delivery systems,3 sensors and actuators,4 scaffolds for tissue engineering,5 etc. However, conventional synthetic hydrogels are generally mechanically very weak due to the structural inhomogeneity in the gel network. To broaden the applications of hydrogels, especially in load bearing applications, many efforts have been devoted to develop novel mechanically strong hydrogels.6 Typical tough hydrogels include slide-ring (SR) gel,7 nanocomposite (NC) gel,8 double-network (DN) gel,9 tetra-arm poly(ethylene glycol) (PEG) gel,10 and the hydrogels synthesized by using polyfunctional initiating and cross-linking centers (PFICC).11 The main strategy used for fabricating these tough hydrogels is to synthesize or introduce special and uniformly distributed cross-linking points or centers, which are effective in distributing the applied load, into the gels. Tetra-arm gels have a homogeneous network structure, which is created by using two kinds of tetrahedron-like macromonomers with monodispersed chain extenders.10b SR gels are made by using sliding cross-linkers, which are covalently joined α-cyclodextrin © XXXX American Chemical Society
Received: May 21, 2013 Revised: September 2, 2013
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dx.doi.org/10.1021/ma401053c | Macromolecules XXXX, XXX, XXX−XXX
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action,14 hydrophobic interactions,15 crystallization,16 inclusion complexation,17 and interpolymer complexation.18 However, in most cases, chemical cross-linking is combined with physical cross-linking to improve the mechanical properties of the gels. For example, Suo and co-workers19 reported the synthesis of highly stretchable and tough hydrogels from polymers forming ionically and covalently cross-linked networks. Only a few tough hydrogels are totally physically cross-linked. One example is the ionically cross-linked triblock copolymer hydrogels;14b another example is poly(vinyl alcohol) (PVA) hydrogels made by the repeated freezing−thawing method, in which part of PVA chains form crystallites held by very strong hydrogen bonds (H-bonds).16,20 The synthesis methods for the chemically and/or physically cross-linked tough gels are usually multistepped and timeconsuming, and for some methods monodispersed polymers10 or triblock copolymers with ionic domains14b are required. To develop more facile fabrication methods is very important for the practical applications of tough hydrogels. It is well-known that there is a cooperative effect in H-bonds of small molecules and polymers which exist in chains, rings, or other shapes where the molecule can participate concertedly as a donor and as an acceptor.21 The cooperative effect significantly enhances the H-bond strength. A nonlinear or higher-order cooperativity is even found in the hydrogen bonding between two polymers.22 Hydrogen-bonding cooperativity plays an important role in stabilizing secondary and tertiary structures of proteins, DNA duplexes, and ligand− receptor complexes.23 For example, the strong cooperative hydrogen bonding between the base pairs of the double helix of DNA is the main reason for the formation of chiral assemblies that are stable in water, where solvent molecules can compete effectively for hydrogen bonds.21d The H-bond cooperativity between two polymers leads to the formation of interpolymer complexes with properties entirely different to the properties of their component polymers.24 There are some reports on the fabrication of hydrogels utilizing the interpolymer complexation as one of the cross-linking mechanisms, usually combined with chemical cross-linking.18b,25 However, these hydrogels usually do not typically exhibit high mechanical strengths. One exception is the interpenetrating polymer network (IPN) systems reported by Frank and co-workers, in which interpolymer complexes are formed between a cross-linked poly(acrylic acid) (PAA) network and a chemically cross-linked poly(ethylene glycol) (PEG) network.18c−e There is no report on the fabrication of tough hydrogels physically cross-linked by only interpolymer complexation (cooperative hydrogen bonding) yet. Herein, we report a very simple yet novel method for fabricating tough hydrogels cross-linked by cooperative hydrogen bonding. We found with some surprise that when a polymerpoly(N-vinylpyrrolidone) (PVP)and a monomeracrylamide (AAm)were dissolved into water to form a solution, a hydrogel could be formed when the deaerated solution was kept at a moderately elevated temperature without using any specific chemical initiators or covalent bonding crosslinking agents. Tensile and compressive tests show that the mechanical properties of the hydrogels are among the highest ever reported. Comparative synthesis experiments, molecular modeling, and DSC characterization were performed to understand the formation and toughening mechanisms of the hydrogels.
2. EXPERIMENTAL SECTION Materials. Poly(N-vinylpyrrolidone) (PVP, Mw = 4.0 × 104, high pure grade) and acrylamide (AAm, ultra pure grade) were purchased from Amresco Inc. (OH). Polyacrylamide (PAAm, Mw = 8.3 × 105, 10% in water) was purchased from TCI (Shanghai, China). NMethylpyrrolidone (AR grade) was from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). N-Vinyl-2-pyrrolidone (AR grade) was from Acros Organics (Morris Plains, NJ); it was redistilled before use. Hydrogel Synthesis. PVP was dissolved in deionized water, and then AAm was added to the PVP solution. The concentrations of PVP and AAm are specified in the corresponding main text. The homogeneous transparent PVP−AAm solution was transferred into a mold made by placing a silicone spacer with a height of 2 or 4 mm between two glass plates. Dissolved oxygen in the solution was then removed by three cycles of vacuum evacuation and exchange with high-purity nitrogen. Finally, the mold was kept at 56 °C for 36 h to transform the solution into a hydrogel. The hydrogels synthesized by this approach are referred to here as PVP-in situ-PAAm gels. For comparison, aqueous solutions containing N-methylpyrrolidone and AAm, N-vinyl-2-pyrrolidone and AAm, and that with only AAm were also prepared, and they were treated in the same way described above. Polymer Yield and Molecular Weight Measurements. To measure the conversion of monomer to polymer and the molecular weights of the in situ formed PAAm in the presence of pre-existing PVP, the PAAm need to be isolated first. The PVP-in situ-PAAm hydrogels were dissolved in water in the presence of urea (5 mol L−1) at 75 °C in 6 days. And then the dissolved solutions were dialyzed against pure water by using dialysis bag with the molecular weight cutoff of 5.0 × 105. The dialysis was performed for a week with refreshing water twice a day. Since the molecular weight of PVP used in our work is 4.0 × 104, the dialysis process is believed to be able to separate the pre-existing PVP from the in situ formed PAAm. The dialyzed solutions were collected for determining polymer yield (Yp) and for GPC analysis. The molecular weights and polydispersity indices (PDIs) of the PAAm’s were measured with a Waters 515 gel permeation chromatograph (GPC) equipped with a Wyatt DAWN HELEOS-II (laser 658.0 nm) laser light scattering detector and a Wyatt Optilab rEX refractive index detector using water as the eluent at a flow rate of 0.5 mL min−1. Mechanical Tests. An Instron 3366 electronic universal testing machine (Instron Corporation, Norwood, MA) was used for mechanical testing. The as-prepared hydrogels were cut into dumbbell-shaped specimens, in accordance to DIN-53504 S3 (inner width: 2 mm; gauge length: 10 mm, thickness: 2 mm) for tensile testing. Tensile tests were performed at a cross-head speed of 800% min−1. The tensile stress σt was calculated as follows: σt = load/tw (t and w were the initial thickness and width of the dumbbell-shaped hydrogel sample, respectively). The tensile strain εt is defined as the change in the grip separation relative to the initial length, εt = [Δl/l0] × 100% (l is the grip displacement during testing and l0 is the length of the sample before tests). Tensile fracture stress or tensile strength (σb) and the tensile fracture strain or elongation at break (εb) are the tensile stress and strain at which the specimen breaks, respectively. Stress− strain data between εt = 10%−30% were used to calculate initial tensile elastic modulus (E). Tensile mechanical properties of the PVP-in situ-PAAm hydrogels swollen to 90% water content and equilibrium swollen samples were also measured. To prepare gel samples with 90% water content, the asprepared gel samples were swollen to the required water content, and then the swollen samples were kept in sealed plastic bags for 7 days to ensure the uniform swelling throughout the samples. To prepare equilibrium swollen samples, dumbbell-shaped specimens (standardized as DIN-53504 S3) cut from the as-prepared hydrogels were immersed in deionized water for 8 days, and their dimensions (length, width, and height) were recorded. Uniaxial compression tests were performed on cylindrically shaped specimens with a thickness of 4 mm and a diameter of 20 mm at a B
dx.doi.org/10.1021/ma401053c | Macromolecules XXXX, XXX, XXX−XXX
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Figure 1. Tensile mechanical properties of the hydrogels synthesized with a fixed PVP concentration (0.017 g mL−1) and different monomer concentrations (CAAm). (a−c) Typical stress−strain (σt−εt) curves of the as-prepared (a), swollen to 90% water content (b), and equilibrium swollen hydrogels (c); (d−f) the tensile strength (σb), elongation at break (εb), and elastic modulus (E) of the hydrogels as a function of CAAm, respectively. crosshead speed of 4 mm min−1. The compressive stress (σc) was calculated by σc = load/πr2, where r is the original radius of the specimen. The strain (εc) under compression was defined as the change in the thickness (h) relative to the original thickness (h0) of the freestanding specimen, εc = (h0 − h)/h0. Prior to mechanical testing, the gel specimens were coated with a thin layer of silicon oil to prevent the evaporation of water. For each sample, at least four specimens were tested. Computational Methods. The geometric parameters were optimized using B3LYP/6-311+G(d) methods and confirmed with vibrational analysis to ensure they are of correct curvatures in the corresponding potential energy surfaces. Single-point energy corrections have been performed with MP2/cc-PVQZ in order to consider the basis set effect and dispersion interaction. The optimized structures were also employed to perform the basis set superposition errors (BSSE) correction using counterpoise method26 at the MP2/cc-PVQZ
level, in order to calculate accurately the hydrogen-bonding interaction energies between donating polymers. DSC Characterizations. Glass transition temperatures of vacuumdried samples were determined with differential scanning calorimetry (DSC). DSC analyses were performed with DSC 1 (Mettler-Toledo, Switzerland) over the temperature range of 100−220 °C at a heating rate of 10 °C min−1 in a nitrogen atmosphere. Two consecutive scans were performed on each sample, and the middle point value of the transition in the second scan was taken as the glass transition temperature.
3. RESULTS AND DISCUSSION Mechanical Properties of the Hydrogels. Hydrogels were made by heating the aqueous acrylamide (AAm) solution in the presence of poly(N-vinylpyrrolidone) (PVP) but without C
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with CPVP until 0.017 g mL−1 and then decreased as CPVP was further increased; an abrupt decrease was found at 0.067 g mL−1. E remained almost constant as the concentration was increased from 8.3 × 10−3 to 0.05 g mL−1; then it also abruptly decreased when the concentration was increased beyond 0.05 g mL−1. By calculation, the mass ratio of PAAm to PVP is about 10 at the CAAm of 5 mol L−1 and the CPVP of 0.05 g mL−1. When the mass ratio of AAm to PVP was close to 1 or less, no tough hydrogels could be formed. In addition, when CPVP was less than 8.3 × 10−3 g mL−1, no tough hydrogels could be formed either. These results suggest a proper PVP concentration range is required for forming tough hydrogels. The compressive mechanical properties of the as-prepared PVP-in situ-PAAm hydrogels were also measured. Figure 3a shows the compressive stress−strain (σc−εc) curves of the PVP-in situ-PAAm hydrogels synthesized with different CAAm, and Figure 3b gives the σc’s of the gels at a εc of 0.95. The PVPin situ-PAAm hydrogels did not fracture during the tests, and they could recover their original shapes immediately after release of the load. The σc’s increased rapidly with increasing CAAm from 2 to 5 mol L−1 and then slowly, and the highest one at 6 mol L−1 is about 17.0 MPa (Figure 3b). The PVP-in situ-PAAm hydrogels also exhibited excellent shape recoverability. After being elongated to the fracture strain, the hydrogel specimens had almost the same length as the original specimens, and the gels also exhibited low hysteresis ratio and residual strains during the cyclic tensile tests (Figure S2). The PVP-in situ-PAAm hydrogels contain a high volume fraction of water, and they exhibit rubber-like elastic deformation over strains up to the breaking strain, implying that the cross-linked points are stable. In contrast, a concentrated polymer solution containing polymer chains of high molar mass would exhibit viscoelastic deformation under similar conditions as the entanglements translate as a function of time. Hence, for the hydrogels reported here, the interaction energy at the cross-linked points is much higher than the thermal energy. Both tensile and compressive mechanical testing results show that the PVP-in situ-PAAm hydrogels have excellent mechanical properties. By comparing our testing results with reported values,6c it is observed that the mechanical properties of the PVP-in situ-PAAm hydrogels are much better than those of conventional synthetic hydrogels and are comparable to those of the toughest reported hydrogels. For example, the σb’s of the as-prepared PVP-in situ-PAAm hydrogels are in the range 0.25− 1.20 MPa, which are among the highest values for tough hydrogels, and are 2−3 orders of magnitude higher than those of the conventional synthetic hydrogels (∼10 kPa). The asprepared PVP-in situ-PAAm hydrogels show extremely high extensibility, with εb generally more than 3000%, similar to the most extensible NC gels and the graphene oxide nanocomposite hydrogel based on PFICC.11c The PVP-in situPAAm hydrogels show relatively low E (
