Nanoclay Reinforced Self‐Cross‐Linking Poly(N‐Hydroxyethyl Acrylamide) Hydrogels with Integrated High Performances

Despite recent significant progress in fabricating tough hydrogels, it is still a challenge to realize high strength, large stretchability, high toughness, rapid recoverability, and good self‐healing simultaneously in a single hydrogel. Herein, Laponite reinforced self‐cross‐linking poly(N‐hydroxyethyl acrylamide) (PHEAA) hydrogels (i.e., PHEAA/Laponite nanocomposite [NC] gels) with dual physically cross‐linked network structures, where PHEAA chains can be self‐cross‐linked by themselves and also cross‐linked by Laponite nanoplatelets, demonstrate integrated high performances. At optimal conditions, PHEAA/Laponite NC gels exhibit high tensile strength of 1.31 MPa, ultrahigh tensile strain of 52.23 mm mm^?1, high toughness of 2238 J m^?2, rapid self‐recoverability (toughness recovery of 79% and stiffness recovery of 74% at room temperature for 2 min recovery without any external stimuli), and good self‐healing properties (strain healing efficiency of 42%). The work provides a promising and simple strategy for the fabrication of dual physically cross‐linked NC gels with integrated high performances, and helps to expand the fundamentals and applications of NC gels.     

Poly (vinyl alcohol)/graphene oxide nanocomposite films and hydrogels prepared by gamma ray

ABSTRACT

Poly (vinyl alcohol)/graphene oxide (PVA/GO) gamma irradiated nanocomposite films and hydrogels were prepared. In composite films, GO was initially irradiated by gamma ray in order to improve interactions between GO and PVA. The film containing 1?wt-% GO was very strong where tensile modulus and tensile yield strength were 45 and 115% higher than those of pure PVA. In the second set of experiments PVA/GO hydrogels were made by irradiating PVA/GO suspensions by gamma ray at various doses. It was an interesting finding that GO increased the gel portion of hydrogels through contribution of H-bonds between PVA and GO. The hydrogels prepared at 20?kGy had remarkable water swelling ratio that reached as high as 20 at water temperature of 80°C. The hydrogel metal ion adsorption capability was tested on Cu²⁺ ions. It was shown that the GO contributed significantly to the adsorption capacity of PVA hydrogels.     

Physicochemical evaluation of nanocomposite hydrogels with covalently incorporated poly(vinyl alcohol) functionalized graphene oxide

To avoid the negative effect of graphene oxide (GO) nanosheets aggregation in aqueous solutions on physicochemical properties of GO incorporated nanocomposite hydrogels, poly(vinyl alcohol)-functionalized GO (GO-es-PVA) are synthesized and are used for preparation of nanocomposite hydrogels. By graft copolymerization of GO-es-PVA with poly(AA-co-AAm) chains, the nanocomposite hydrogel samples with covalently incorporated GO-es-PVA are achieved. FTIR spectroscopy, XRD analysis, and SEM and EDAX techniques confirm successful synthesis process. It is clear that GO-es-PVA content has significant effect on physicochemical properties of nanocomposite hydrogels, such as improvement of the water uptake properties, porosity, and gel strength. The hydrogel sample with 1:80 mass ratio of GO-es-PVA/AAm has the best physicochemical properties due to the optimum amount of GO-es-PVA, which gives the hydrogel proper viscoelasticity as well as fine porosity and water uptake rate. Interpenetration of PVA chains into the polymeric networks makes the movement of the polymer chains easier, which leads to softer polymeric networks. This phenomenon is called plasticizing effect. The plasticizing nature of PVA and its high hydrophilicity are the main reasons for the fine physicochemical properties. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 48025.     

High clay content nanocomposite hydrogels with surprising mechanical strength and interesting deswelling kinetics

A series of high hectorite content nanocomposite Poly (N-isopropylacrylamide), (PNIPAAm), hydrogels have been successfully synthesized by choosing a special kind of hectorite (Laponite XLS) modified by tetrasodium pyrophosphate. It was found that these hydrogels show surprising mechanical properties (i.e. tensile strength: 1 MPa, elongation at break: 1400%) and complicated deswelling behavior, which are due to the high clay content of the hydrogels and ionic dispersant contained in Laponite XLS, respectively.     

Hydrophobically modified nanocomposite hydrogels with self-healing ability

Several strategies have been developed in the past two decades to increase the mechanical performance of the hydrogels, and to generate self-healing function within the polymer network. Here, we combine two of these strategies to create hydrophobically modified nanocomposite (NC) hydrogels with high mechanical strength and self-healing efficiency. The hydrogels were prepared by in situ copolymerization of N,N-dimethylacrylamide and n-octadecyl acrylate (C18A) in the presence of 2 w/v % Laponite clay nanoparticles in an aqueous solution of worm-like sodium dodecyl sulfate micelles. Incorporation of hydrophobic C18A segments into the gel network significantly increases both the storage and loss moduli of NC hydrogels indicating increasing elasticity and energy dissipation. An improvement in the mechanical performance and self-recoverability of NC hydrogels was also observed after hydrophobic modification. The compressive fracture stress and Young's modulus increase with increasing amount of C18A, and they become 9 ± 1 MPa and 30 ± 2 kPa, respectively, at 4 mol % C18A. Incorporation of hydrophobic segments also provides a larger energy dissipation under large strain as compared to the traditional NC hydrogels providing a self-healing efficiency of 90 ± 10% in mechanically strong NC hydrogels. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48853.     

Influence of graphene oxide on thermal,electrical, and morphological properties of new achiral polyimide

In this study, a new diamine N‐[2‐(1H‐indol‐3‐yl)ethyl]‐3,5‐diaminobenzamide (IEDAB) was synthesized using tryptamine as starting material and characterized by FT‐IR, ¹H‐NMR, ¹³C‐NMR, and mass spectroscopy. Then, it was polymerized with 3,3',4,4'‐benzophenone tetracarboxylic dianhydride (BTDA) via thermal imidization to produce polyimide (PI). A series PI/GO nanocomposite films were prepared by incorporating different ratios (1, 3, and 5 wt%) of synthesized GO by solution casting method. The synthesized PI was confirmed by Ubbelohde viscometer and FT‐IR spectroscopy. SEM and Raman spectroscopy showed that GO was well dispersed in the PI matrix. XRD patterns indicated the PI and PI/GO nanocomposite films were highly amorphous in nature. The synthesized PI and their nanocomposites show high thermal stability as their T_10% weight loss are in the range of 498 to 563°C with 30.6 to 40% of char yield and the glass transition temperatures (T_g s) are in the range of 188 to 262°C. The limited oxygen index (LOI) values increased from 31.4% to 56.0% with increases of 5% GO content in the PI/GO nanocomposite. They have high dielectric constant in the range of 2.6 to 5.1 at 1 MHz and also good mechanical properties with tensile strength of 81 to 116 MPa, elongation at break 5 to 9%. POLYM. ENG. SCI., 2017. © 2017 Society of Plastics Engineers     

An inorganic cross-linked quadruple shape memory hydrogel with high mechanical performance

Shape memory hydrogels (SMHs) have been the subject of great interest in recent years. However, there were few reports on the simultaneous multiple shape memory and high mechanical performance. Therefore, a novel nanocomposite (NC) hydrogel poly (acrylic acid)/Chitosan/ Laponite (PAA/CS/Laponite) was developed by using Laponite as physical cross-linker. In order to achieve three temporary shapes, the PAA/CS/Laponite was soaked in iron chloride hexahydrate (FeCl₃), sodium hydroxide (NaCl) and sodium hydroxide (NaOH) respectively to (a) form metal coordination; (b) gain the chains entanglement of chitosan; (c) get the microcrystalline structure of chitosan. The maximum shape fixity ratio of PAA/CS/Laponite can reach 100% in 1 minute and it can be restored its original shape within 5 minutes. Moreover, PAA/CS/Laponite showed excellent mechanical performance. The maximum tensile and compressive strengths were 0.73 MPa and 13.1 MPa. By comparison with PAA/CS obtained from our previous work, the tensile strength, elongation at break and compressive strength increased by 2.21 times, 1.46 times, and 3.26 times respectively. Scanning electron microscopy (SEM) showed that the obtained sample has uniform honeycomb network structures which can effectively explain why the gel has strong mechanical performance. These characteristics make PAA/CS/Laponite have huge application potential in reality.     

The effects of graphene oxide on the properties and drug delivery of konjac glucomannan hydrogel

Konjac glucomannan (KGM) hydrogel has good potential application in food and medical science, although to achieve this, the physical and mechanical properties need further improvement. In this study, graphene oxide (GO) was used to improve the functionality of KGM hydrogel. KGM/GO hydrogels were prepared by freezing the alkaline KGM/GO sols. Rotational rheometer was used to study the rheological properties of different alkaline KGM/GO sols. Fourier transform infrared, Raman, differential scanning calorimetry, thermogravimetric analyses, and scanning electron microscopy were used to evaluate the structure and properties of the hydrogels. In addition, different pH solutions and an in vitro assay were used to study the swelling property and the release behavior of KGM/GO hydrogels, respectively. The result revealed strong hydrogen‐bond interaction between KGM and GO. The incorporation of GO highly improved the gel properties of KGM/GO sol, higher thermal stability, and more compact structure of KGM/GO hydrogels. KGM/GO hydrogels showed better swelling properties in deionized‐distilled water and pH 7.2 PBS. The release of 5‐aminosalicylic acid (5‐ASA) from KGM/GO (KG4) hydrogel was different in various pH media, but the initial burst release effect was very severe. Therefore, incorporation of GO have a good potential in enhancing the properties of KGM hydrogel, but KGM/GO hydrogel is not an ideal carrier for 5‐ASA release. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45327.     

Synthesis and dual response of ionic nanocomposite hydrogels with ultrahigh tensibility and transparence

Ionic nanocomposite hydrogels cross-linked by hectorite Laponite XLS with high tensile strength and ultrahigh tensibility were successfully synthesized for the first time via in situ copolymerization of N-isopropylacrylamide (NIPAm) and sodium methacrylate (SMA). The pH and temperature response, transparency, and mechanical properties of the ionic hydrogels were investigated. The results showed that the addition of only 2 mol% of SMA endowed the nanocomposite hydrogels with pH response, while the temperature response remained in the whole pH range. All the as-prepared hydrogels, even with 10 mol% of SMA, demonstrated transparency higher than 75%. The tensile strength evidently decreased from 60 kPa to 45 kPa when the SMA content was higher than 6 mol%. The elongation at break increased with increasing SMA content and 2800% was achieved for the sample containing 10 mol% of SMA. The effective network chain density was estimated from the tensile stress at elongation of 200% and the equilibrium storage modulus. The low chain density was the intrinsic origin of the ultrahigh tensibility for these ionic NC gels. This work provides a new way to prepare dual responsive hydrogels with ultrahigh tensibility and high transparency.     

Dual pH‐ and temperature‐responsive hydrogels with extraordinary swelling/deswelling behavior and enhanced mechanical performances

pH‐ and temperature‐responsive semi‐interpenetrating nanocomposite hydrogels (NC hydrogels) were prepared with surface‐functionalized graphene oxide (GO) as the crosslinker, N‐isopropylacrylamide (NIPAM) as the monomer, and chitosan (CS) as an additive. The effects of 3‐(trimethoxysilyl)propylmethacrylate‐modified GO sheets and CS content on various physical properties were investigated. Results show that PNIPAM/CS/GO hydrogels undergo a large volumetric change in response to temperature. Swelling ratios of PNIPAM/CS/GO hydrogels are much larger than those of the conventional organically crosslinked PNIPAM hydrogels. The deswelling test indicates that the deswelling rate was greatly enhanced by incorporating CS into the hydrogel network and using the surface‐functionalized GO as the crosslinker. The pH‐sensitivity of PNIPAM/CS/GO hydrogels is evident below their volume phase transition temperature. Moreover, the PNIPAM/CS/GO hydrogels have a much better mechanical property compared with traditional hydrogels even in a high water content of 90%. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41530.     

Mechanically strengthened double network composite hydrogels with high water content: a preliminary study

A novel poly[(1,2-ethylenediamino) (2-hydroxy-1,3-propanedily) chloride]/ Laponite/polyacrylic acid (PEDAECH/Laponite/PAA) hydrogel was synthesized by two-step solution polymerization combining nanocomposite (NC) strategy with double network (DN). The structural characteristics of resulting hydrogels were investigated by Fourier Transform infrared spectrum (FTIR) and Transmission Electron Microscopy (TEM). A core shell structure was observed in PEDAECH/Laponite composite. The swelling and mechanical strength of the resulting hydrogels were measured when PEDAECH/Laponite composite dose varied. The novel hydrogel achieved a high compressive stress of 148.0 KPa even in higher water content of 98.7% when the PEDAECH/Laponite composite dose is 0.05 ml, the dose of AA was 3.6 ml, N, N??-methylenebisacrylamide (MBAM) dose was 0.04 wt% (based on the weight of AA) and reaction temperature was 0 °C, Based on the cyclic compression studies, there is a small decline in the maximum stress of the hydrogels at the fixed strain of 45% even under three cyclic compressions.     

Unusual swelling behavior of polymer-clay nanocomposite hydrogels

The swelling behavior and the elastic properties of nanocomposite hydrogels have been investigated. The hydrogels were prepared by free-radical polymerization of the monomers acrylamide (AAm), N,N-dimethylacrylamide (DMA), and N-isopropylacrylamide (NIPA) in aqueous clay suspensions at 21 °C. Laponite with a radius of gyration in distilled water of 20 nm was used as clay particles in the hydrogel preparation. The reactions with AAm monomer were carried out in the presence of the chemical crosslinker N,N′-methylenebis(acrylamide) (BAAm). It was found that the volume of nanocomposite hydrogels immersed in water rapidly increases and attains a maximum value after about one day. Surprisingly, further increase in the swelling time results in the deswelling of the gels until they reach a limiting swelling ratio after about 5 days. This unusual swelling behavior is observable only when the clay concentration in the hydrogel is above the overlap threshold c^∗. Swelling measurements combined with the elasticity tests show that the effective crosslink density first decreases, but then increases with increasing time of swelling of the hydrogels. The results were explained in terms of the rearrangements of the highly entangled polymer chains and clay particles during the gel volume change.     

Improved mechanical and swelling behavior of the composite hydrogels prepared by ionic monomer and acid-activated Laponite

A composite hydrogel (CH) with much improved mechanical and swelling properties was prepared using an ionic monomer and acid-activated Laponite XLS which was used as a cross-linking agent. Addition of acid-activated clay solved the gelation problem when ionic monomers were added to clay mineral dispersions. Reaction of Laponite XLS with sulfuric acid yielded amorphous silica. A dispersion of the acid-activated Laponite and the monomers was used to synthesize composite hydrogels by in-situ polymerization. The FT-IR spectra and rheological results of the composite hydrogels demonstrated the formation of a network. The equilibrium swelling ratios of composite hydrogels (> 6000 g/g) were more than 18 times larger than traditional organic cross-linked hydrogels. The moduli G′ and G″ in the observed frequency range were about 4 and 10 times larger than those of organic cross-linked hydrogel (OR gel). The improvement in both the equilibrium swelling ratio and mechanical strength was attributed to the homogeneous cross-linked network structure.     

Network chain density and relaxation of in situ synthesized polyacrylamide/hectorite clay nanocomposite hydrogels with ultrahigh tensibility

Nanocomposite hydrogels (NC gel) D-AM and S-AM were synthesized through in situ polymerization of acrylamide (AM) with hectorite clays of Laponite RD and RDS, respectively. The tensile performance of the NC gel was observed at different crosshead speeds and all of the NC gels exhibited an extremely high tensibility, e.g., the elongation at break even higher than 4000%, except for two samples with the lowest Laponite content of 1 w/v%. Strong tensile hysteresis was observed in the elongation-reversion curve, indicating a slow relaxation in the NC gels. Dynamic moduli G′ and G″ within linear viscoelasticity illustrated that the network structure was formed in these gels with the junction of Laponite platelets. The Laponite RD showed stronger gelation capability than the tetrasodium pyrophosphate modified Laponite RDS. The relaxation modulus G(t) for the NC gels was found to be similar to the slow rubber relaxation with the critical exponent n of about 0.16, much lower than 0.66-0.71 for the critical gel at the sol-gel transition. In comparison, the chemically cross-linked hydrogel showed almost no relaxation during the same time interval. The effective network chain density of the NC gel was determined from equilibrium shear modulus, which was evidently lower than that of the chemically cross-linked hydrogels. The present results reveal that the high deformability of these NC gels comes from their low effective network chain density with moderate relaxation.     

Calcium ion‐induced self‐healing pattern of chemically crosslinked poly(acrylic acid) hydrogels

Poly(acrylic acid) hydrogels crosslinked with N,N′‐methylenebisacrylamide were synthesized by free radical polymerization. Polymerization conditions had a significant influence over the gel content and swelling behaviour of the hydrogels. The incorporation of calcium ions led to the origin of a self‐healing feature. The self‐healing behaviour and mechanical performance of the hydrogels were systematically investigated. The hydrogels showed good tensile strength of 1 MPa and excellent stretchable behaviour where hydrogels regained instantaneously. Hydrogel pieces joined together to become an integrated matrix as soon as two cut pieces were brought in contact. The hydrogels possessed a marked healing efficiency of 97% within 6 h at room temperature without any external intervention. The results are explained in terms of the dynamic mobility of calcium ions within the dual‐crosslinked networks of the poly(acrylic acid) hydrogels. © 2017 Society of Chemical Industry     

A transparent Laponite polymer nanocomposite hydrogel synthesis via in-situ copolymerization of two ionic monomers

The poor stability of clay dispersion in the presence of ionic species presents a challenge for preparing ionic clay polymer nanocomposite (CPN) transparent hydrogel with desired strength. The transparent and tough ionic hydrogels are highly demanded as potential material options for contact lens or ophthalmic implants. Here we reported an ionic CPN hydrogel with combined high transparency and mechanical properties synthesized via in-situ copolymerization of 2-acrylamido-2-methylpropanesulfonic acid (AMPS) and acrylic acid (AA) in Laponite dispersion. The ionic transparent CPN can have strength of 189 kPa and a strain of 1780% which is likely a result of uniformly dispersed Laponite platelets in the structure. A synergistic interaction between acylamino and sulfonic acid functional groups was found to play a key role in the stable dispersion of Laponite. This work provided a new way to prepare transparent ionic CPN hydrogels.     

Effect of the intercalation agent content of montmorillonite on the swelling behavior and drug release behavior of nanocomposite hydrogels

A series of novel dual functional nanocomposite hydrogels were prepared from N‐isopropylacrylamide (NIPAAm), acrylic acid (AA) that is neutralized 50 mol % by sodium hydroxide (SA50), and montmorillonite (MMT). MMT was intercalated with three different contents of intercalation agent of (3‐acrylamidopropyl) trimethyl ammonium chloride (TMAACl). Investigation of the effect of intercalated MMT with three contents of intercalation agent (TMAACl) in the present nanocomposite hydrogels on the swelling and drug release behaviors is the main purpose in this study. The microstructure was identified by X‐ray diffraction (XRD). Results showed that the swelling ratio for the present nanocomposite hydrogels decreased with an increase in the content of the intercalation agent. The gel strength of the present gels did not change obviously with an increase in the content of intercalation agent. XRD results indicated that exfoliation of MMT was achieved in the dry and swollen gel state. Finally, the drug release behaviors for these gels were accessed also. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 74–82, 2004     

Radiation synthesis of graphene oxide/composite hydrogels and their ability for potential dye adsorption from wastewater

A high yield of graphene oxide (GO) was chemically synthesized from graphite powder utilizing adjusted Hummer's method. The contents of acidic functional groups in GO were determined using potentiometric titration. Composite hydrogels dependent on graphene oxide/poly(2-acrylamido-2-methylpropanesulfonic acid)/polyvinyl alcohol (GO/PAMPS/PVA) were synthesized utilizing a ⁶⁰Co gamma irradiation source at different doses. The synthesized graphene oxide and composite hydrogels were portrayed via X-ray diffraction, thermogravimetric analysis, and Fourier transform infrared analysis. The morphology of composite hydrogels was characterized by scanning electron microscope. The gel % and swelling % for the prepared hydrogel demonstrated that the swelling % of hydrogel increased with raising AMPS content. Whereas the increment of GO and increasing the irradiation dose lead to a reduction in the swelling %. The influences of pH, GO percentage, initial dye concentration, the adsorbent dosage, contact time, and temperature on the adsorption of basic blue 3 dye were evaluated and the adsorption capacity was 194.6 mg/g at optimum conditions; pH = 6, GO/PAMPS/PVA composite hydrogels with 5 wt% of GO, initial dye concentration = 200 mg/L, adsorbent dose = 0.1 g, solution volume = 50 mL after 360 min at room temperature (25°C). The adsorption of dye onto the GO/PAMPS/PVA composite hydrogels follows Pseudo-second-order adsorption kinetics, fits the Freundlich adsorption isotherm model.     

Equilibrium swelling behavior and elastic properties of polymer–clay nanocomposite hydrogels

Nanocomposite hydrogels were prepared by free‐radical polymerization of the monomers acrylamide (AAm), N,N‐dimethylacrylamide (DMA), and N‐isopropylacrylamide (NIPA) in aqueous clay dispersions at 21°C. Laponite XLS was used as clay nanoparticles in the hydrogel preparation. The hydrogels based on DMA or NIPA monomers exhibit much larger moduli of elasticity compared with the hydrogels based on AAm monomer. Calculations using the theory of rubber elasticity reveal that, in DMA‐clay or NIPA‐clay nanocomposites, both the effective crosslink density of the hydrogels and the functionality of the clay particles rapidly increase with increasing amount of Laponite up to 10% (w/v). The results suggest that DMA‐clay and NIPA‐clay attractive interactions are stronger than AAm‐clay interactions due to the formation of multiple layers on the nanoparticles through hydrophobic associations. It was also shown that, although the nanocomposite hydrogels do not dissolve in good solvents such as water, they dissolve in dilute aqueous solutions of acetone or poly(ethylene oxide) of molecular weight 10,000 g/mol, demonstrating the physical nature of the crosslink points. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Ultrastretchable,Super Tough,and Rapidly Recoverable Nanocomposite Double‐Network Hydrogels by Dual Physically Hydrogen Bond and Vinyl‐Functionalized Silica Nanoparticles Macro‐Crosslinking

It remains a challenge to develop tough hydrogels with recoverable or healable properties after damage. Herein, a new nanocomposite double‐network hydrogel (NC‐DN) consisting of first agar network and a homogeneous vinyl‐functionalized silica nanoparticles (VSNPs) macro‐crosslinked polyacrylamide (PAM) second network is reported. VSNPs are prepared via sol‐gel process using vinyltriethoxysilane as a silicon source. Then, Agar/PAM‐SiO₂ NC‐DN hydrogels are fabricated by dual physically hydrogen bonds and VSNPs macro‐crosslinking. Under deformation, the reversible hydrogen bonds in agar network and PAM nanocomposite network successively break to dissipate energy and then recombine to recover the network, while VSNPs in the second network could effectively transfer stress to the network chains grafted on their surfaces and maintain the gel network. As a result, the optimal NC‐DN hydrogels exhibit ultrastretchable (fracture strain 7822%), super tough (fracture toughness 18.22 MJ m^‐3, tensile strength 431 kPa), rapidly recoverable (≈92% toughness recovery after 5 min resting at room temperature), and self‐healable (can be stretched to 1331% after healing) properties. The newly designed Agar/PAM‐SiO₂ NC‐DN hydrogels with tunable network structure and mechanical properties by multi‐bond crosslinking provide a new avenue to better understand the fundamental structure‐property relationship of DN hydrogels and broaden the current hydrogel research and applications.