Tough and self‐recoverable hydrogels crosslinked by triblock copolymer micelles and Fe3+ coordination

Tough hydrogels have great potentials in soft robotics, artificial muscles, tissue replacement, and so on. Here we introduce novel tough hydrogels crosslinked by triblock copolymer (F127DA) micelles and metal coordination. The gels showed outstanding tensile strength (∼1–11 MPa), toughness (∼4–32 MJ m⁻³), and excellent self‐recovery properties (∼56.8–87.2% toughness recovery in 9 min at room temperature). The mechanical and self‐recovery properties could be manipulated by varying contents of micelles and/or COO⁻ groups. Dynamic mechanical analysis of the hydrogels revealed apparent activation energy and relaxations for both physical interactions. In situ small‐angle X‐ray scattering measurements on hydrogels upon stretching revealed micelle deformations. XPS measurements on hydrogels before and after stretching revealed significant changes in the binding energy of Fe³⁺ ions in the gels, suggesting the rupture of coordination bonds. The experimental results strongly suggest a synergistic effect from the micelle‐crosslinking and Fe³⁺–COO⁻ coordination on the strength, toughness, and self‐recovery of the hydrogels. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 865–876     

Bioactive composites fabricated by freezing‐thawing method for bone regeneration applications

Hydrogels are widely used for controlled delivery of therapeutic agents. However, hydrogels lack bioactivity to encourage bone formation and mechanical integrity. Moreover, chemically crosslinked hydrogels exhibit cytotoxic effect. To overcome these limitations poly‐vinyl alcohol (PVA) and poly‐acrylic acid (PAA) blends were combined with ceramic materials based on β tricalcium phosphate, wollastonite, and magnesium silicate with different pore size distributions. The final 3D matrix was physically crosslinked using various freeze thawing (F/T) cycles. FTIR and SEM analysis showed that ceramics were dispersed within the polymer matrix and formed hydrogen bonds. Swelling studies in buffer solution pH 7.4 showed an increase in polymer swelling when ceramic was added. Furthermore, rheological testing demonstrated that incorporation of ceramics caused an increase in mechanical properties which varies with different pore size distributions of ceramics grains added. DSC thermograms showed increased T_g values for samples containing ceramics. Antimicrobial activity containing ciprofloxacin was tested against a pathogen associated with osteomyelitis and presented positive results with ciprofloxacin. The combination of increased strength and ability to encapsulate a clinically relevant antimicrobial agent indicates that the composite tested in this study has potential for the treatment of osteomyelitis. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 761–773     

Injectable supramolecular hybrid hydrogels formed by MWNT‐grafted‐poly(ethylene glycol) and α‐cyclodextrin

Poly(ethylene glycol)‐grafted‐multiwalled carbon nanotube (MWNT‐g‐PEG) was synthesized by a coupling reaction and formed inclusion complexes (ICs) after selective threading of the PEG segment of the MWNT‐g‐PEG through the cavities of α‐cyclodextrins (α‐CDs) units. The polypseudorotaxane structures of the as‐obtained hydrogels were confirmed by ¹H NMR, X‐ray diffraction and DSC analyses. The complexation of the PEG segments with α‐CDs and the hydrophobic interaction between the MWNT resulted in the formation of supramolecular hybrid hydrogels with a strong network. Thermal analysis showed that the thermal stability of the hydrogel was substantially improved by up to 100 °C higher than that of native hydrogel. The resultant hybrid hydrogels were found to be thixotropic and reversible, and could be applied as a promising injectable drug delivery system. The mechanical strength of the hybrid hydrogels was greatly improved in comparison with that of the corresponding native hydrogels. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 3145–3151, 2010     

The mechanical and electrical properties of carbon nanotube‐grafted polyimide nanocomposites

Polyimide (PI)‐based nanocomposites containing aminophenyl functionalized multiwalled carbon nanotubes (AP‐MWCNTs) obtained through a diazonium salt reaction was successfully prepared by in situ polymerization. PI composites with different loadings of AP‐MWCNTs were fabricated by the thermal conversion of poly(amic acid) (PAA)/AP‐MWCNTs. The mechanical and electrical properties of the AP‐MWCNTs/PI composites were improved compared with those of pure PI due to the homogeneous dispersion of AP‐MWCNTs and the strong interfacial covalent bonds between AP‐MWNTs and the PI matrix. The conductivity of AP‐MWNTs/PI composites (5:95 w/w) was 9.32 × 10^?1 S/cm which was about 10¹⁵ times higher than that of Pure PI. The tensile strength and tensile modules of the AP‐MWCNTs/PI composites with 0.5 wt % of AP‐MWCNTs were increased by about 77% (316.9 ± 10.5 MPa) and 25% (8.30 ± 1.10 GPa) compared to those of pure PI, respectively. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 960–966     

One‐pot fabrication of robust interpenetrating hydrogels via orthogonal click reactions

Interpenetrating polymer network (IPN) hydrogels have been fabricated through a facile one‐pot approach from tetra/bifunctional telechelic macromonomers with epoxy, amine, azide, and alkyne groups by orthogonal double click reactions: epoxy‐amine reaction and copper‐catalyzed azide‐alkyne cycloaddition. Both the crosslinked networks are simultaneously constructed in water from the biocompatible poly (ethylene glycol)‐based macromonomers. The crosslinking density of each network was finely tuned by the macromonomer structure, permitting control of network molecular weights between crosslinks of the final gels. Compared to corresponding single network gels, the IPN gels containing both tightly and loosely crosslinked networks exhibited superior mechanical properties with shear moduli above 15 kPa and fracture stresses over 40 MPa. The synthetic versatility of this one‐pot approach will further establish design principles for the next generation of robust hydrogel materials. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 1459–1467     

Semi-Interpenetrating Hydrogels of Polyelectrolytes,Polymer-Metal Complexes and Polymer-Protected Palladium Nanoparticles

Summary: Semi-interpenetrating hydrogels (SIHs) of polyelectrolytes consisting of poly(acrylamide) hydrogel (PAAH) as matrix and sodium poly(styrenesulfonate) (NaPSS), poly(N,N′-dimethyl-N,N′-diallylammonium chloride) (PDMDAAC), stoichiometric interpolyelectrolyte complex of NaPSS-PDMDAAC, and linear polyethyleneimine-metal complexes (PMC) as well as polymer-protected palladium nanoparticles were prepared by in situ polymerization, e.g. crosslinked acrylamide chains were formed in aqueous solutions of NaPSS, PDMDAAC, NaPSS-PDMDAAC (1:1 mol/mol), PMC and palladium nanoparticles protected by poly(N-vinylpyrrolidone) (PVP), poly-(vinyl alcohol) (PVA), poly(acrylic acid) (PAA), and linear poly(ethyleneimine) (LPEI) respectively in the presence of N,N′-methylenebisacrylamide. For each system the equilibrium swelling degree α and the parameters n and k describing the mechanism of water transport into SIHs volume was determined. Catalytic properties of SIHs were studied in allyl alcohol hydrogenation and cyclohexane oxidation reactions.     

Double-networks based on interconnected amphiphilic “in–out” star first polymer conetworks prepared by RAFT polymerization

Double-network hydrogels were prepared using well-defined first networks comprising interconnected amphiphilic “in-out” star copolymers synthesized via sequential reversible addition-fragmentation chain transfer (RAFT) polymerization, and second networks based on a photopolymerized mixture of acrylamide and N,N′-methlyenebisacrylamide. All first and double-network hydrogels were characterized in terms of their aqueous degrees of swelling and mechanical properties in compression. The most hydrophobic first and double-network hydrogels exhibited the best mechanical properties, which may be attributed to their low aqueous swelling degrees and good mesoscale organization in water as revealed using small-angle neutron scattering (SANS) which showed that the size of the formed hydrophobic domains could be controlled by the polymer conetwork structure. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 2161–2174     

High water content clay–nanocomposite hydrogels incorporating guanidinium‐pendant methacrylamide: Tuning of mechanical and swelling properties by supramolecular approach

Novel clay–polymer composite hydrogels with high water content (up to 98 wt %) are developed, in which mechanical properties are reinforced by the formation of multiple ion‐pairs between the polymer chains and clay nanosheets (CNS). When a small amount of guanidinium‐pendant methacrylamide (0.1–0.2 wt %) is copolymerized with a neutral monomer (0.5–2.0 wt %) in an aqueous dispersion of CNS (1.0–3.0 wt %), a self‐standing hydrogel with satisfactory mechanical toughness and elasticity results, despite its high water content (95–98 wt %). The mechanical properties and swelling behaviors of the hydrogels can be tuned by the amount of the guanidinium‐pendant acrylamide. A systematic study indicates that the ion pairs, formed between the guanidinium groups in the polymer chains and the oxyanions on the surfaces of the CNS, serve as crosslinking points in the three‐dimensional network developed in these hydrogels. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 839–847     

Thermosensitive behavior of amphiphilic triblock copolymers based on poly(acrylic acid) and poly(propylene oxide)

Self‐association in aqueous solution of amphiphilic poly(acrylic acid)‐b‐poly(propylene oxide)‐b‐poly(acrylic acid) (PAA‐b‐PPO‐b‐PAA) copolymers having various outer PAA block lengths are presented. These copolymers show two thermosensitive behaviors. The first one, due to hydrogen bonds between PAA and PPO resulting in large aggregates, was observed by visible spectroscopy. The second one, due to the association of PPO middle block into aggregates, was evidenced by dynamic light scattering and pyrene fluorescence. These critical temperatures both depend on the ionization and the length of PAA blocks. The characterization of the aggregates above the critical aggregation concentration by fluorescence quenching experiments showed a very low aggregation number corresponding to dimers or trimers association depending on the conditions. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1507–1514     

Physically cross‐linked networks of POSS‐capped poly(acrylate amide)s: Synthesis,morphologies, and shape memory behavior

In this work, we synthesized a novel organic–inorganic semitelechelic polymer from polyhedral oligomeric silsesquioxane (POSS) and poly(acrylate amide) (PAA) via reversible addition‐fragmentation chain transfer (RAFT) polymerization. The organic–inorganic semitelechelic polymers have been characterized by means of nuclear magnetic resonance spectroscopy, thermal gravimetric analysis, and dynamic mechanical thermal analysis. It was found that capping POSS groups to the single ends of PAA chains caused a series of significant changes in the morphologies and thermomechanical properties of the polymer. The organic–inorganic semitelechelics were microphase‐separated; the POSS microdomains were formed via the POSS–POSS interactions. In a selective solvent (e.g., methanol), the organic–inorganic semitelechelics can be self‐assembled into the micelle‐like nanoobjects. Compared to plain PAA, the POSS‐capped PAAs significantly displayed improved surface hydrophobicity as evidenced by the measurements of static contact angles and surface atomic force microscopy. More importantly, the organic–inorganic semitelechelics displayed typical shape memory properties, which was in marked contrast to plain PAA. The shape memory behavior is attributable to the formation of the physically cross‐linked networks from the combination of the POSS–POSS interactions with the intermolecular hydrogen‐bonding interactions in the organic–inorganic semitelechelics. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 587–600     

Recovery stress and work output in hyperbranched poly(ethyleneimine)‐modified shape‐memory epoxy polymers

In this study a series of hyperbranched modified shape‐memory polymers were subjected to constrained shape recoveries in order to determine their potential use as thermomechanical actuators. Materials were synthesized from a diglycidyl ether of bisphenol A as base epoxy and a polyetheramine and a commercial hyperbranched poly(ethyleneimine) as crosslinker agents. Hyperbranched polymers within the structure of the shape‐memory epoxy polymers led to a more heterogeneous network that can substantially modify mechanical properties. Thermomechanical and mechanical properties were analyzed and discussed in terms of the content of hyperbranched polymer. Shape‐memory effect was analyzed under fully and partially constrained conditions. When shape recovery was carried out with fixed strain a recovery stress was obtained whereas when it was carried out with a constraining stress the material performs mechanical work. Tensile tests at Tg^E′ showed excellent values of stress and strain at break (up to 15 MPa and almost 60%, respectively). Constrained recovery performances revealed rapid recovery stress generation and unusually high recovery stresses (up to 7 MPa) and extremely high work densities (up to 750 kJ/m³). The network structure of shape‐memory polymers was found to be a key factor for actuator‐like applications. Results confirm that hyperbranched modified‐epoxy shape memory polymers are good candidates for actuator‐like shape‐memory applications. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1002–1013     

A novel sodium carboxymethylcellulose/poly(N‐isopropylacrylamide)/Clay semi‐IPN nanocomposite hydrogel with improved response rate and mechanical properties

A novel semi‐IPN nanocomposite hydrogel (CMC/PNIPA/Clay hydrogel) based on linear sodium carboxymethylcellulose (CMC) and poly(N‐isopropylacrylamide) (PNIPA) crosslinked by inorganic clay was prepared. The structure and morphology of these hydrogels were investigated and their swelling and deswelling kinetics were studied in detail. TEM images showed that the clay was substantially exfoliated to form nano‐dimension platelets dispersed homogeneously in the hydrogels and acted as a multifunctional crosslinker. The CMC/PNIPA/Clay hydrogels swell faster than the corresponding PNIPA/Clay hydrogels at pH 7.4, whereas they swell slower than the PNIPA/Clay hydrogels at pH 1.2. The CMC/PNIPA/Clay nanocomposite hydrogels showed much higher deswelling rates, which was ascribed to more passway formed in these hydrogels for water to diffuse in and out. The deswelling process of the hydrogels could be approximately described by the first‐order kinetic equation and the deswelling rate decreased with increasing clay content. The mechanical properties of the CMC/PNIPA/Clay nanocomposite hydrogels were analyzed based on the theory of rubber elasticity. It was found that with increasing clay content, the effective crosslink chain density, v_e, increased whereas the molecular weight of the chains between crosslinks M_c decreased. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1546–1555, 2008     

Polyelectrolyte complexes of linear copolymers and hydrogels based on 2‐[(methacryloyloxy)ethyl]trimethylammonium chloride and N‐isopropylacrylamide

The formation of polyelectrolyte complexes of linear copolymers and hydrogels based on copolymers of 2‐[(methacryloyloxy)ethyl]trimethylammonium chloride with N‐isopropylacrylamide (MADQUAT–NIPAAM) and poly(acrylic acid) (PAA) has been studied. The composition of the copolymer has been found to affect the composition of the polyelectrolyte complexes significantly, and the molecular weight of PAA influences their aggregation stability. Hydrogels of MADQUAT–NIPAAM immersed in solutions of PAA undergo contraction because of the formation of gel–polymer complexes. The rate of contraction and the final swelling degree of the gel–polymer complexes depend on the concentration of PAA in solution and its molecular weight. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1506–1513, 2004     

Antimicrobial loading into and release from poly(ethylene glycol)/poly(acrylic acid) semi‐interpenetrating hydrogels

Electrostatic interactions within a semi‐interpenetrating network (semi‐IPN) gel can control the postsynthesis loading, long‐term retention, and subsequent release of small‐molecule cationic antibiotics. Here, electrostatic charge is introduced into an otherwise neutral gel [poly(ethylene glycol) (PEG)] by physically entrapping high‐molecular‐weight poly(acrylic acid) (PAA). The network structure is characterized by small‐angle neutron scattering. PEG/PAA semi‐IPN gels absorb over 40 times more antibiotic than PAA‐free PEG gels. Subsequent soaking in physiological buffer (pH 7.4; 0.15 M NaCl) releases the loaded antibiotics for periods as long as 30 days. The loaded gels elute antibiotics with diffusivities of 4.46 × 10^?8 cm²/s (amikacin) and 2.08 × 10^?8 cm²/s (colistin), which are two orders of magnitude less than those in pure PEG gels where diffusion is controlled purely by gel tortuosity. The release and hindered diffusion can be understood based on the partial shielding of the charged groups within the loaded gel, and they have a significant effect on the antimicrobial properties of these gels. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 64–72     

Preparation and characterization of poly(ethyl acrylate)/bentonite nanocomposites by in situ emulsion polymerization

Transparent poly(ethyl acrylate) (PEA)/bentonite nanocomposites containing intercalated–exfoliated combinatory structures of clay were synthesized by in situ emulsion polymerizations in aqueous dispersions containing bentonite. The samples for characterization were prepared through direct‐forming films of the resulting emulsions without coagulation and separation. An examination with X‐ray diffraction and transmission electron microscopy showed that intercalated and exfoliated structures of clay coexisted in the PEA/bentonite nanocomposites. The measurements of mechanical properties showed that PEA properties were greatly improved, with the tensile strength and modulus increasing from 0.65 and 0.24 to 11.16 and 88.41 MPa, respectively. Dynamic mechanical analysis revealed a very marked improvement of the storage modulus above the glass‐transition temperature. In addition, because of the uniform dispersion of silicate layers in the PEA matrix, the barrier properties of the materials were dramatically improved. The permeability coefficient of water vapor decreased from 30.8 × 10^?6 to 8.3 × 10^?6 g cm/cm² s cmHg. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1706–1711, 2002     

Strain stiffening and negative normal stress in alginate hydrogels

Alginate hydrogels are polysaccharide biopolymer networks widely useful in biomedical and food applications. Here, we report nonlinear mechanical responses of ionically crosslinked alginate hydrogels captured using large amplitude oscillatory shear experiments. Gelation was performed in situ in a rheometer and the rheological investigations on these samples captured the strain‐stiffening behavior for these gels as a function of oscillatory strain. In addition, negative normal stress was observed, which has not been reported earlier for any polysaccharide networks. The magnitude of negative normal stress increases with the applied strain amplitude and can exceed that of the shear stress at large‐strain. Fitting a constitutive relationship to the stress‐strain curves reveals that the mode of deformation involves stretching of the alginate chains and bending of both the chains and the junction zones. The contribution of bending increases near saturation of G blocks as Ca²⁺ concentration was increased. The results presented here provide an improved understanding of the deformation behavior of alginate hydrogels and such understanding can be extended to other crosslinked polysaccharide networks. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1767–1775     

A facile method to fabricate tough hydrogel with ultra-wide adjustable stiffness,stress, and fast recoverability

In this article, we report a synergistic strategy to develop dual physically cross-linked tough hydrogels via one-pot bulk copolymerization of N-vinyl-2-pyrrolidone, acrylic acid, and stearyl methylacrylate (SMA) without any adscititious surfactant. Due to synergic effects of hydrogen bonding and hydrophobic association, the resulted dual physically cross-linked hydrogels (DP Gel) with ultra-wide range adjustable Young's modulus (0.08–45.6 MPa), tensile stress (0.7–6.9 MPa), and toughness (3.3–23.1 MJ m⁻³). Stretching to 300%, DP Gel exhibited fast recoverability that remained ~95% of initial dissipated energy after resting in 60 °C for 3 min. Finally, scanning electron microscopy revealed that the microstructure of hydrogel changed from phase separation structure to micro phase separation as SMA added, which accounted for excellent performance of DP Gel. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 1469–1474     

A Double Network Hydrogel with High Mechanical Strength and Shape Memory Properties

Double network (DN) hydrogels as one kind of tough gels have attracted extensive attention for their potential applications in biomedical and load-bearing fields. Herein, we import more functions like shape memory into the conventional tough DN hydrogel system. We synthesize the PEG-PDAC/P(AAm-co-AAc) DN hydrogels, of which the first network is a well-defined PEG (polyethylene glycol) network loaded with PDAC (poly(acryloyloxyethyltrimethyl ammonium chloride)) strands, while the second network is formed by copolymerizing AAm (acrylamide) with AAc (acrylic acid) and cross-linker MBAA (N, N'-methylenebisacrylamide). The PEG-PDAC/P(AAm-co-AAc) DN gels exhibits high mechanical strength. The fracture stress and toughness of the DN gels reach up to 0.9 MPa and 3.8 MJ/m³, respectively. Compared with the conventional double network hydrogels with neutral polymers as the soft and ductile second network, the PEG-PDAC/P(AAm-coAAc) DN hydrogels use P(AAm-co-AAc), a weak polyelectrolyte, as the second network. The AAc units serve as the coordination points with Fe³⁺ ions and physically crosslink the second network, which realizes the shape memory property activated by the reducing ability of ascorbic acid. Our results indicate that the high mechanical strength and shape memory properties, probably the two most important characters related to the potential application of the hydrogels, can be introduced simultaneously into the DN hydrogels if the functional monomer has been integrated into the network of DN hydrogels smartly.     

Poly(ethylene glycol)–poly(L‐lactide) star block copolymer hydrogels crosslinked by metal–ligand coordination

The aqueous solution behavior and thermoreversible gelation properties of pyridine‐end‐functionalized poly(ethylene glycol)–poly(L ‐lactide) (PEG–(PLLA)₈–py) star block copolymers in the presence of coordinating transition metal ions were studied. In aqueous solutions, the macromonomers self‐assembled into micelles and micellar aggregates at low concentrations and formed physically crosslinked, thermoreversible hydrogels above a critical gel concentration (CGC) of 8% w/v. In the presence of transition metal ions like Cu(II), Co(II), or Mn(II), the aggregate dimensions increased. Above the CGC, the gel–sol transition shifted to higher temperatures due to the formation of additional crosslinks from intermolecular coordination complexes between metal ions and pyridine ligands. Furthermore, as an example, PEG–(PLLA)₈–py hydrogels stabilized by Mn(II)–pyridine coordination complexes were more resistant against degradation/dissolution when placed in phosphate buffered saline at 37 °C when compared with hydrogels prepared in water. Importantly, the stabilizing effect of metal–ligand coordination was noticeable at very low Cu(II) concentrations, which have been reported to be noncytotoxic for fibroblasts in vitro. These novel PEG–(PLLA)₈–py metallo‐hydrogels, which are the first systems to combine metal–ligand coordination with the advantageous properties of PEG–PLLA copolymer hydrogels, are appealing materials that may find use in biomedical as well as environmental applications like the removal of heavy metal ions from waste streams. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Nanoreinforced hemicellulose-based hydrogels prepared by freeze–thaw treatment

A novel hybrid hydrogel was prepared from hemicelluloses, polyvinyl alcohol, and chitin nanowhiskers by the freeze–thaw technique. The hydrogel was characterized by Fourier-transform infrared (FT-IR) spectrometry, scanning electron microscopy, X-ray diffraction analysis, cross-polarization (CP)/magic-angle spinning (MAS) ¹³C nuclear magnetic resonance (NMR), and swelling property and compressive strength measurements. Atomic force microscopy images of chitin implied that the size of whiskers reached nanometer level with average length of about 200 nm and width of 40 nm. The FT-IR and NMR results indicated that physical cross-linking rather than chemical reaction occurred during the gelation process. The mechanical properties of the hydrogels were significantly improved with increasing proportion of chitin nanowhiskers, with the highest compressive stress of 9.6 MPa being found for Gel-0.5. The results showed that the repeated freeze–thaw cycles induced physical cross-linking of packed chains by hydrogen bonds among the polymers, and the concentration of chitin nanowhiskers affected the hydrogel morphology and properties. It is suggested that hydrogels with good mechanical properties can be successfully prepared by this physical method, offering promise for tissue engineering applications.