Nanocomposite smart hydrogels with improved responsiveness and mechanical properties: A mini review

Responsive hydrogels have the ability to change their volume, transparency, or other properties in response to external chemical and/or physical stimuli. The responsiveness properties including responsive rate and degree, as well as mechanical properties such as Young's modulus, toughness, breaking strength, and breaking strain are crucial parameters of the smart hydrogels that determine the scope of hydrogel applications such as soft actuators, artificial muscles, and tissue engineering scaffolds. In this paper, the development of the nanocomposite smart hydrogels, which can achieve both improved responsiveness and mechanical properties, is reviewed. First, the fabrication approaches for building the nanocomposite networks by doping organic or inorganic nanomaterials via crosslinking or blending strategies are introduced. Then, the mechanisms used to improve both responsiveness and mechanical properties of nanocomposite responsive hydrogels are discussed. Finally, the perspectives as well as current challenges of such nanocomposite responsive hydrogels are addressed. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 1306–1313     

Self‐healing in Nanocomposite Hydrogels

Polymer hydrogels with characteristics distinct from those of solid materials are one of the most promising candidates for smart materials. Here, we report that a nanocomposite hydrogel (NC gel) consisting of a unique polymer/clay network structure, can exhibit complete self‐healing through autonomic reconstruction of crosslinks across a damaged interface. Mechanical damage in NC gels can be repaired without the use of a healing agent, and even sections of NC gels separated by cutting, from whichever the same or different kinds of NC gel, perfectly (re‐)combine by just contacting the cut surfaces together at mildly elevated temperatures. In NC gels, the autonomic fusion of cut surfaces as well as the self‐healing could be achieved not only immediately after being cut but also after a long waiting time.

     

Synthesis and mechanical strength of a novel double network nanocomposite hydrogel with core‐shell structure

A novel poly[(dimethylimino)(2‐hydroxy‐1,3‐propanedily)chloride]/Laponite/polyacrylic acid (PDMIHPC/Clay/PAA) hydrogel was synthesized by two‐step solution polymerization combining the strategies of both nanocomposite (NC) gels and double network (DN) gels. The chemical composition and core‐shell structure of the hydrogels were confirmed by Fourier transform infrared (FTIR) and transmission electron microscopy (TEM). The mechanical strength was examined by varying the reaction temperature, PDMIHPC/Clay composite dose, N,N′‐methylenebisacrylamide (MBAM) dose and water content. When the reaction temperature was 35°C, PDMIHPC/Clay composite/AA is 1:10 and MBAM dose was 0.050 wt% (based on the weight of AA), the novel hydrogel achieved a best compressive strength of 100.05 KPa with a water content of 98.8%. Copyright © 2011 John Wiley & Sons, Ltd.     

Effect of solvent–matrix interactions on structures and mechanical properties of micelle‐crosslinked gels

Previous studies on hydrogels crosslinked by acrylated PEO₉₉–PPO₆₅–PEO₉₉ triblock copolymer (F127DA) micelles demonstrate outstanding strength and toughness, which is attributed to the efficient energy dissipation through the hydrophobic association in the micelles. The current study further focuses on how the solvent property affects the structures and the mechanical properties of F127DA micelle crosslinked polyacrylamide gels. Binary solvents comprised of dimethyl sulfoxide (DMSO) and water are used to adjust the polymer/solvent interactions, which consequently tune the conformations of the polymer chains in the network. The presence of DMSO significantly decreases the strength but increased the stretchability of the gels, whereas the overall tensile toughness remained unchanged. In situ small‐angle X‐ray scattering measurements reveal the deformation of micelles along with the stretching direction. A structure evolution mechanism upon solvent change is proposed, according to the experimental observations, to explain influence of solvent quality on the mechanical properties of the micelle‐crosslinked gels. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 473–483     

Elaboration of open‐cell microcellular nanocomposites

Stable water‐in‐oil high internal phase emulsions, containing styrene and divinylbenzene monomers and exfoliated montmorillonite, were prepared and polymerized to obtain nanocomposite microcellular materials. The porous structure was investigated by scanning electron microscopy, mercury intrusion porosimetry, and nitrogen adsorption/desorption analyses. The exfoliation of clay was investigated by X‐ray diffraction and transmission electron microscopy analyses. The presence of inorganic filler did not modify the microcellular structure of the composite, while the use of modified clay significantly enhanced its mechanical properties. No influence on the thermal degradation was noted, except for materials with high clay content that tended to deteriorate at lower temperature than the other materials. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4193–4203, 2007     

Injectable poloxamer/graphene oxide hydrogels with well‐controlled mechanical and rheological properties

Although significant progress has been made in the design and application of injectable hydrogels for biomedical applications, concurrent control of rheological and mechanical properties of injectable hydrogels has remained as an open challenge to the researchers. In this work, we introduce and put into practice a photo‐curable poloxamer (also known as Pluronic)/graphene oxide (Plu/GO) injectable hydrogel with well‐controlled rheological and mechanical properties. Acrylate group was anchored to hydrogel structure to endow photo‐crosslinking ability through decelerating degradation rate of poloxamer hydrogels after injection. It was found that the modified Plu remains stable in biological media for a long‐term period without significant weight loss. Rheological properties of hydrogels were also carried out as essential prerequisite for an ideal injectability via frequency sweep, flow curve, recovery, and yield stress before and after modification, signifying shear‐thinning behavior of Plu/GO hydrogels with high recoverability. The viscosity of shear‐thinning‐like hydrogels dropped at higher shear stress, which facilitated injection process. Moreover, mechanical behavior of Plu was optimized by manipulating the content of Plu, degree of modification with reactive precursor, curing, and particularly incorporation of GO without deteriorating effects on rheological behavior of Plu.     

Effects of Na‐sulfopropyl groups on the mechanical properties and morphology of polystyrene‐co‐methacrylate ionomers

The dynamic mechanical properties and morphology of poly(styrene‐co?3‐sulfopropyl sodium‐methacrylate) SSPMANa ionomers were investigated. It was found the increasing rate of ionic moduli of the SSPMANa ionomer was very low, and the cluster T_g of the ionomers remained more or less constant with increasing ion content. A well‐developed SAXS peak was seen for low ion content SSPMANa ionomers and the peak position changed slightly with ion content. Thus, it was suggested that the presence of the alkyl ester side chains made the ion pairs form multiplets more easily at their prevalent distances, and the small‐agglomerated multiplets were dispersed in the polymer matrix relatively evenly. The interpretation of ionic moduli using a number of theories implied that the multiplets and clusters acted as effective crosslinks and filler particles, respectively, and the size and shape of the clusters were irregular. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1043–1053     

Crosslinking behavior and enhanced mechanical properties of acrylonitrile‐butadiene rubber composites by incorporating aluminum ammonium sulfate particles

The crosslinked structure formed by the metal coordination bonding provides excellent and new properties for rubber materials. Herein, the crosslinking of acrylonitrile‐butadiene rubber (NBR) is induced by introducing aluminum ammonium sulfate (NH₄Al(SO₄)₂·12H₂O) particles. The crosslinking behavior, morphology, mechanical properties, and the Akron abrasion resistance of NBR/NH₄Al(SO₄)₂·12H₂O composites were fully explored. The results show that the three‐dimensional crosslinking structure is held together by metal–ligand coordination bonds between the nitrile group and AI(III). The coordination crosslink density exhibits a considerable increase with the addition of NH₄Al(SO₄)₂·12H₂O. Thus, the mechanical properties and abrasion resistance of the obtained composites are better than that of NBR/sulfur system. Interestingly, the elongation at break for NBR/NH₄Al(SO₄)₂·12H₂O composites is over 2000% due to the nature of coordination bonds. The abrasion volume loss decreases to 0.4 cm³ for NBR/NH₄Al(SO₄)₂·12H₂O composites with 20 phr NH₄Al(SO₄)₂·12H₂O particles as compared to 0.75 cm³ for NBR/sulfur system. The obtained NBR composites with facile preparation and excellent mechanical properties make the composites based on metal coordination bonding attractive for practical use. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 879–886     

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     

Design of polyphosphazene hydrogels with improved structural properties by use of star-shaped multithiol crosslinkers

The feasibility of using a star-shaped crosslinker to produce a hydrogel with controlled mechanical properties and degradation rates was investigated. The aqueous blends of functional polymers and crosslinkers formed a solution at low temperature and a hydrogel with desired mechanical properties at body temperature. The introduction of star-shaped crosslinkers affected the microscopic and macroscopic properties of the hydrogel. The fabricated hydrogels could be suitable for many potential biomedical applications because of their injectability, tunable mechanical properties, controlled degradation rate and gel formation under physiological conditions.     

The addition of clay on the mechanical properties of surface‐treated CF‐filled PA66 composites

Polyamide 66 (PA66) composites filled with clay and carbon fiber (CF) were prepared by twin‐screw extruder in order to study the influence of nanoparticle reinforcing effect on the mechanical behavior of the PA66 composites (CF/PA66). The mechanical property tests of the composites with and without clay were performed, and the fracture surface morphology was analyzed. The results show that the fracture surface area of the clay‐filled CF/PA66 composite was far smoother than that of the CF/PA66 composite, and there formed a tense interface on the CF surface after the addition of clay. The tensile and flexural strength of CF/PA66 composites with clay was improved. The impact strength decreased because of the high interfacial adhesion. In conclusion, the addition of clay favored the improvement of the higher interface strength and so had good effect on improving the tensile and flexural properties of the composites. Copyright © 2017 John Wiley & Sons, Ltd.     

Towards high‐performance biopackaging: barrier and mechanical properties of dual‐action polycaprolactone/zinc oxide nanocomposites

The properties of nanocomposites of biodegradable polycaprolactone containing zinc oxide (ZnO) nanoparticles with diverse morphologies, that is, ZnO nanospheres, nanorods, and nanodisks are investigated. It is demonstrated for the first time that the dual action of the ZnO nanoparticles reduces the gas permeability of the nanocomposites via two mechanisms: first by the creation of a tortuous path and second by gas adsorption. Depending on the morphology of the particles, the oxygen permeability can be reduced by more than 60%. Tensile tests show that the nanocomposites remain very ductile. The nominal strain for all nanocomposites is higher than 500% before fracture occurs. The Young's modulus and tensile strength of the nanocomposites increase at higher ZnO concentrations. This behavior is more pronounced in the case of ZnO nanorods. As a result, the incorporation of ZnO nanoparticles into (bio)polymers provides an opportunity to manufacture polymer‐based nanocomposite materials, resulting in the production of high‐performance (bio)packaging. Copyright © 2011 John Wiley & Sons, Ltd.     

Grafting of polymer matrix to exfoliated montmorillonite nanoplatelets in nanocomposite film cast from soap‐free emulsion polymerized latex and its fortified mechanical properties

Poly(methyl acrylate‐co‐methyl methacrylate) [P(MA‐co‐MMA)] nanocomposite film containing 1 wt % of montmorillonite (MMT) exhibited unusual higher ductility, higher strain recovery ratio after creep, and higher modulus and strength compared to neat P(MA‐co‐MMA) as they were cast from their individual latices fabricated by soap‐free emulsion polymerization. The fortified mechanical properties were attributed to the MgO components of exfoliated MMT nanoplatelets being grafted by P(MA‐co‐MMA) chains as verified by FTIR and XPS spectroscopies, which to the best of our knowledge is the first time in the literature providing the direct evidence for the polymer chains grafting onto the exfoliated MMT. TEM investigation of the stretched nanocomposite film revealed that the microcracks in the nanocomposite film appeared mainly in the bulk region of polymer matrix, implying that the interfacial strength between P(MA‐co‐MMA) and its grafted MMT nanoplatelets was higher than the cohesion strength of P(MA‐co‐MMA). © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 5891–5897, 2009     

Molecular Characteristics of Poly(N‐isopropylacrylamide) Separated from Nanocomposite Gels by Removal of Clay from the Polymer/Clay Network

The extraordinary mechanical and swelling/deswelling properties of nanocomposite (NC) gels are attributed to their unique organic (polymer)/inorganic (clay) network structure. In this study, poly(N‐isopropylacrylamide) (PNIPA) was successfully separated from an NC gel network by decomposing the clay (hectorite) using hydrofluoric acid (HF). A very low HF concentration (0.2 wt.‐%) was adequate for the decomposition of the clay without causing any damage to PNIPA. The separated PNIPA had a high (=5.5 × 10⁶ g · mol⁻¹). Also, was almost constant regardless of the clay concentration (C_clay = 1–25 × 10⁻² mol · l⁻¹), even though the properties of the NC gel varied widely over this C_clay range. Comparisons of NC gels, PNIPA, and SiO₂‐NC gels indicated that the clay platelets specifically play an important role in NC gels.

     

Synthesis and studies of thermal,mechanical and electrical properties of MWCNT–cyclodextrin as a nanoparticle in polyamide matrix based on 2,2‐Bis[4‐(4‐aminophenoxy)phenyl] propane

In the present research, polyamide (PA) ( 6 ) was synthesized by the polycondensation reaction of 2,2‐Bis[4‐(4‐aminophenoxy)phenyl] propane as a diamine ( 4 ) with adipic acid ( 5 ) in the optimized condition. The resulting PA was characterized using Fourier transform infrared spectroscopy, Proton Nuclear Magnetic Resonance (¹H NMR) spectroscopy, inherent viscosity (η_inh), X‐ray diffraction, and solubility tests. Also, the thermal property of the new PA ( 6 ) was investigated by using Thermogravimetric analysis. To apply multiwall carbon nanotube (MWCNT) as an effective reinforcement in polymer composites, it is essential to have appropriate proper dispersion, interfacial adhesion between the MWCNT and polymer matrix, and increasing solubility. With this end particularly, functionalized MWCNTs were combined with a soluble molecule, and a series of modified MWCNT with cyclodextrin (Cy) known as PA/MWCNT‐Cy composite film (2, 5, and 8 wt%) were prepared by a solution intercalation technique. Field emission scanning electron microscopy images showed that MWCNT‐Cy was well dispersed in the PA matrix. Thermogravimetric analysis indicated an increase in thermal stability of nanocomposites as compared with the pristine PA. Anisotropic structure of the synthesized films and dispersed MWCNT‐Cy in the films approved by use of X‐ray diffraction and field emission scanning electron microscopy. The resultant PA/MWCNT‐Cy composite films were electrically conductive, which is favorable for many practical uses. Measurements of mechanical properties of these composite films showed high strength in 8% MWCNT‐Cy content. Also, results showed increases in Young's modulus and tensile strength. Copyright © 2016 John Wiley & Sons, Ltd.     

Preparation and morphological,electrical, and mechanical properties of polyimide‐grafted MWCNT/polyimide composite

Precursor of polyimide, polyamic acid has been prepared sucessfully. Acid‐modified carbon nanotube (MWCNT) was grafted with soluble polyimide then was added to the polyamic acid and heated to 300 °C to form polyimide/carbon nanotube composite via imidation. Morphology, mechanical properties and electrical resistivity of the MWCNT/polyimide composites have been studied. Transmission electron microscope microphotographs show that the diameter of soluble polyimide‐grafted MWCNT was increased from 30–60 nm to 200 nm, that is a thickness of 70–85 nm of the soluble polyimide was grafted on the MWCNT surface. PI‐g‐MWCNT was well dispersed in the polymer matrix. Percolation threshold of MWCNT/polyimide composites has been investigated. PI‐g‐MWCNT/PI composites exhibit lower electrical resistivity than that of the acid‐modified MWCNT/PI composites. The surface resistivity of 5.0 phr MWCNT/polyimide composites was 2.82 × 10⁸ Ω/cm² (PI‐g‐MWCNT) and 2.53 × 10⁹ Ω/cm² (acid‐modified MWCNT). The volume resistivity of 5.0 phr MWCNT/polyimide composites was 8.77 × 10⁶ Ω cm (PI‐g‐MWCNT) and 1.33 × 10¹³ Ω cm (acid‐modified MWCNT).Tensile strength and Young's modulus increased significantly with the increase of MWCNT content. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 3349–3358, 2007     

Influence of varying hard segments on the properties of chemically crosslinked moisture‐cured polyurethane‐urea

Polyurethane prepolymers are widely used in the reactive hot melt adhesives and moisture‐cured coatings. The chemically crosslinked moisture‐cured formulation based on PEG‐1000 and isophorone diisocyanate was prepared with NCO/OH ratio of 1.6:1.0. Trimethylol propane was used as a crosslinking agent. The excess isocyanate of the prepolymer was chain extended in the ratio of 2:1 (NCO/OH) with different aliphatic diols, and 4:1 with different aromatic diamines. The polymer network maturation during moisture cure was followed by dynamic mechanical thermal analyzer (DMTA) instrument. The thermal and dynamic mechanical properties of the crosslinked polymers were evaluated using thermogravimetric analysis, differential scanning calorimetric analysis and DMTA. Surface properties were evaluated through angle‐resolved X‐ray photoelectron spectroscopy. The present article discusses the physical properties of moisture‐cured polyurethane‐urea (MCPU) containing chemical crosslinks in the hard segment. The complete moisture‐cured polymers showed amorous results toward room temperature modulus, tensile strength, hardness, thermal stability, and transparency. The surface properties showed the enrichment of soft segments. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 102–118, 2006     

Mechanical and thermo‐mechanical studies of double networks based on thermoplastic elastomers

A new approach to prepare and characterize double network elastomeric systems was investigated. A styrene‐ethylene‐co‐butylene‐styrene (SEBS) triblock copolymer system containing physical crosslinks was used to achieve a double network by additional crosslinking using ultra‐violet (UV) light. An ethylene–propylene–diene monomer (EPDM) terpolymer system containing chemical crosslinks was used to achieve a conventional double network using UV crosslinking. Properties from conventional monotonic tensile tests, dynamic mechanical analysis, and thermomechanical properties were investigated. These double network elastomers show a transition between competitive and collaborative behavior in their mechanical properties and lower coefficients of thermal expansion arising from a competition of the networks. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 778–789, 2010     

Synthesis and properties of a novel double network nanocomposite hydrogel

A novel polyacrylamide/polyacrylic acid (PAAm/PAA) double network (DN) nanocomposite (NC) hydrogel had been synthesized by two‐step solution polymerization. The PAAm network was crosslinked by inorganic clay while the PAA network was crosslinked by a chemical crosslinker. The chemical structure of the network was confirmed by Fourier transform infrared (FTIR), X‐ray diffraction (XRD), and transmission electron microscopy (TEM). The swelling and mechanical strength properties of PAAm/PAA hydrogels were examined. The results showed that a DN hydrogel achieved both a high swelling capacity of 1219 g/g in deionized water and 124 g/g in 0.9 wt% NaCl solution and high compressive stress of 21.5 kPa in a high water content of 99.58%. Copyright © 2008 John Wiley & Sons, Ltd.     

Physical properties of glycosaminoglycan hydrogels

The chemical composition of glycosaminoglycan (GAG) hydrogels was found to have a profound effect on the physical properties of gels. Hyaluronan (HA) and chondroitin sulfate (CS) were each modified with adipic dihydrazide (ADH) with carbodiimide chemistry. The resulting polymer was crosslinked with various concentrations of poly(ethylene glycol) dialdehyde (PEG‐diald) to produce a series of hydrogels. The physical properties of these GAG hydrogels varied in a concentration‐dependent fashion. Maximal crosslinking was observed at a theoretical crosslinking of 50% for the HA‐ADH‐PEG‐diald hydrogels and 75% for the CS‐ADH‐PEG‐diald hydrogels. Adding PEG‐diald beyond the optimum for crosslinking prolonged the in vitro enzymatic degradation time of the hydrogels. The swelling of the crosslinked GAG hydrogels was correlated with the amount of PEG‐diald used rather than with the crosslinking density. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 4344–4356, 2004