Super Bulk and Interfacial Toughness of Amylopectin Reinforced PAAm/PVA Double‐Network Hydrogels via Multiple Hydrogen Bonds

A simple, multiple‐hydrogen‐bond approach to fabricating physically crosslinked, Amylopectin reinforced polyacrylamide/poly(vinyl alcohol) (Amy/PAAm/PVA) double‐network (DN) hydrogels with super toughness in bulk and at solid interfaces is reported. The Amy/PAAm/PVA DN hydrogels exhibit high tensile strength (854.1 kPa), high extensibility (≈eight times), high bulk toughness (4094.8 kJ m^?3), good self‐recovery property (≈92% of self‐recovery at room temperature), and strong adhesion to nonporous glass surfaces (≈158 kPa). Such tough and adhesive DN hydrogels have great potential for various applications in engineering artificial soft tissues, flexible electronics, and wearable devices.     

A novel designed high strength and thermoresponsive double network hydrogels cross-linked by starch-based microspheres

A new kind of nanocomposite double network (DN) hydrogels consisting of starch-based microspheres cross-linked oligo (ethylene glycol) methyl ether methacrylate) (POEGMA) as soft network and diethylene glycol dimethacrylate (DEGMA) cross-linked poly(2-(2-methoxyethoxy) ethyl methacrylate (PMEO₂MA) as brittle network (named POEGMA/PMEO₂MA DN hydrogels) were synthesized by a two-step free radical polymerization. The chemical structure of DN hydrogels was characterized by ¹H NMR, the temperature sensitive properties were measured by the lower critical solution temperature (LCST) tested by UV-Vis spectrophotometer as a function of temperature, the mechanical properties were measured by tensile test. The LCST showed only one transition at 20.2 °C measured by the transmittance variation as a function of the ambient temperature from 5 to 70 °C. The fracture toughness and the hysteresis behaviors were also tested and showed that they were affected by the content of starch-based microspheres cross-linker in the soft POEGMA network, the content of small-molecular cross-linkers and monomer concentration in the brittle PMEO₂MA network. They are related to perfect network and physical adherence and entanglements between microspheres and the networks brought by AAS microspheres, the increment of “sacrifice bond” brought by DEGMA and polymer chains entanglement brought by MEO₂MA. These studies will provide theoretical support for the future research of DN hydrogel and macromolecular microspheres cross-linked hydrogel.     

Double‐network hydrogel with high mechanical strength prepared from two biocompatible polymers

Novel double‐network (DN) hydrogels with high mechanical strength have been fabricated with two biocompatible polymers, poly(vinyl alcohol) (PVA) and poly(ethylene glycol) (PEG), through a simple freezing and thawing method. Some properties of the obtained hydrogels, such as the mechanical strength, rheological and thermodynamic behavior, drug release, and morphology, have been characterized. The results reveal that in sharp contrast to most common hydrogels made with simple natural or synthetic polymers, PVA/PEG hydrogels can sustain a compressive pressure as high as several megapascals, highlighting their potential application as biomedical materials. In addition, a model for describing the structural formation of PVA/PEG DN hydrogels is proposed: the condensed PVA‐rich phase forms microcrystals first, which bridge with one another to form a rigid and inhomogeneous net backbone to support the shape of the hydrogel, and then the dilute PEG‐rich phase partially crystallizes among the cavities or voids of the backbone; meanwhile, there are entanglements of molecular chains between the two polymers. Moreover, a mechanism is also proposed to explain the high mechanical strength of PVA/PEG DN hydrogels. It is suggested that the free motion of PEG clusters in the cavities of PVA networks can prevent the crack from growing to a macroscopic level because the linear PEG chains in the cavities effectively absorb the crack energy and relax the local stress either by viscous dissipation or by large deformation of the PEG chains. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Thermodynamic investigation of thermoresponsive xanthan‐poly (N‐isopropylacrylamide) hydrogels

Polysaccharide‐based hydrogels, such as xanthan maleate/poly(N‐isopropylacrylamide) (PNIPAAm) interpenetrated polymer networks, are thermostimulable materials of interest for the controlled release of biologically active components due to conformation changes at the low critical‐solution temperature (LCST) PNIPAAm phase transition. The phase transition of these interpenetrated polymer network hydrogels, where PNIPAAm is in a ‘confined’ environment, was examined by high resolution magic angle spinning nuclear magnetic resonance and differential scanning calorimetry. High resolution magic angle spinning nuclear magnetic resonance spectroscopy allows the accurate determination of LCST and an evaluation of the corresponding thermodynamic data. More particularly, the evolution of these data as a function of the composition of the hydrogel, and of the external parameters such as pH and ionic strength, was considered. LCST shows a minimal value with increasing xanthan content. Moreover, it was possible to calculate, as a function of temperature, the fraction of NIPAAm which remains uncollapsed. The data obtained for pure PNIPAAm hydrogels are in good agreement with recently published results. The phase transition of PNIPAAm in a diphasic hydrogel is broader when PNIPAAm is ‘confined’ within an interpenetrated polymer network than in a pure PNIPAAm crosslinked network. The widening of the transition with increasing xanthan content indicates a reduction of the PNIPAAm interchain aggregation in a network structure. Copyright © 2011 Society of Chemical Industry     

Dual-responsive release behavior of pH-sensitive PVA/PAAc hydrogels containing temperature-sensitive PVA/PNIPAAm microcapsules

Both temperature and pH responsive drug delivery system was prepared by combining temperature-sensitive poly(vinyl alcohol) (PVA)/poly(N-isopropylacrylamide) (PNIPAAm) microcapsules and pH-sensitive PVA/poly(acrylic acid) (PAAc) hydrogels. The release of drug from the composite hydrogels increased as the pH increased due to the repulsion among the carboxylate anions in the PVA/PAAc hydrogels. The release of drug from the composite hydrogels also increased as the temperature decreased due to the higher hydrophilicity generated below the lower critical solution temperature of PNIPAAm. The compression moduli of composite hydrogels increased with increasing the content of PVA/PNIPAAm microcapsules. The biocompatibility of composite hydrogels was confirmed by the cytotoxicity test.     

Synthesis and Characterization of Temperature-Sensitive Poly(N-isopropylacryamide) Hydrogel with Comonomer and Semi-IPN material

A series of temperature and pH sensitive hydrogels were synthesized using N-isopropylacrylamide (NIPAAm) as main monomer, sodium alginate (SA) as semi-IPN material, ethyl acrylate (EA) and acrylic acid (AA) as comonomer, and N-maleyl chitosan (N-MACH) as cross-linker. The temperature and pH sensitive behavior, swelling/deswelling kinetics of the hydrogels were investigated. And the mechanism of the phase transition was summed up. Sodium alginate/Poly(N-isopropylacryamide) semi-interpenetrating polymer network (SA/PNIPAAm semi-IPN) hydrogels exhibited a lower critical solution temperature (LCST) at about 32 °C with no significant deviation from the conventional PNIPAAm hydrogels. Poly(N-isopropylacryamide-co-ethyl acrylate) (P(NIPAAm-co-EA)) hydrogels exhibited LCST at 29–31°C, increasing the amount of EA in the hydrogel gradually decreased the LCST. Poly(N-isopropylacryamide-co-acrylic acid) [P(NIPAAm-co-AA)] hydrogels exhibited LCST at 34–39°C, with decreasing NIPAAm/AA from 96/4 to 92/8 and 90/10, the LCST increased from 34°C to 37°C and 39°C. In the swelling/deswelling kinetics, all the dried hydrogels exhibited fast swelling/deswelling behavior, which might be attributed to macroporous structures of the hydrogels.     

Synthesis and characterization of temperature‐responsive poly(vinyl alcohol)‐based copolymers

A poly(vinyl alcohol) (PVA)/sodium acrylate (AANa) copolymer was synthesized to improve the water solubility of PVA at the ambient temperature. Furthermore, a series of temperature‐responsive acetalyzed poly(vinyl alcohol) (APVA)‐co‐AANa samples of various chain lengths, degrees of acetalysis (DAs), and comonomer contents were prepared via an acid‐catalysis process. Fourier transform infrared and ¹H‐NMR techniques were used to analyze the compositions of the copolymers. The measurement of the turbidity change for APVA‐co‐AANa aqueous solutions at different temperatures revealed that the lower critical solution temperature (LCST) of the copolymers could be tailored through the control of the molecular weight of the starting PVA‐co‐AANa, DA, and comonomer ratios. Lower LCSTs were observed for APVA‐co‐AANa with a longer chain length, a higher DA, and fewer acrylic acid segments. In addition, the LCSTs of the APVA‐co‐AANa aqueous solutions appeared to be salt‐sensitive. The LCSTs decreased as the concentration of NaCl increased. Moreover, atomic force microscopy images of APVA‐co‐AANa around the LCST also proved the temperature sensitivity. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Synthesis of poly(acrylic acid)–Fe3+/gelatin/poly(vinyl alcohol) triple‐network supramolecular hydrogels with high toughness,high strength and self‐healing properties

Hydrogels with good mechanical and self‐healing properties are of great importance for various applications. Poly(acrylic acid)–Fe³⁺/gelatin/poly(vinyl alcohol) (PAA‐Fe³⁺/Gelatin/PVA) triple‐network supramolecular hydrogels were synthesized by a simple one‐pot method of copolymerization, cooling and freezing/thawing. The PAA‐Fe³⁺/Gelatin/PVA triple‐network hydrogels exhibit superior toughness, strength and recovery capacity compared to single‐ and double‐network hydrogels. The mechanical properties of the synthesized hydrogels could be tailored by adjusting the compositions. The PAA‐Fe³⁺/Gelatin/PVA triple‐network hydrogel with 0.20 mmol Fe³⁺, 3% gelatin and 15% PVA could achieve good mechanical properties, the tensile strength and elongation at break being 239.6 kPa and 12.8 mm mm^?1, respectively, and the compression strength reaching 16.7 MPa under a deformation of about 91.5%. The synthesized PAA‐Fe³⁺/Gelatin/PVA triple‐network hydrogels have good self‐healing properties owing to metal coordination between Fe³⁺ and carboxylic groups, hydrogen bonding between the gelatin chains and hydrogen bonding between the PVA chains. Healed PAA‐Fe³⁺(0.20)/Gelatin3%/PVA15% triple‐network hydrogels sustain a tensile strength of up to 231.4 kPa, which is around 96.6% of the tensile strength of the original samples. Therefore, the synthesized triple‐network supramolecular hydrogels would provide a new strategy for gel research and expand the potential for their application. © 2019 Society of Chemical Industry     

Study on a new polymer/graphene oxide/clay double network hydrogel with improved response rate and mechanical properties

pH‐ and temperature‐responsive double network hydrogels (DN hydrogels) were prepared by using poly (N‐isopropylacrylamide) (PNIPAM) as a tightly crosslinked network (1st network), polyacrylic acid (PAA) as a loosely crosslinked network (2nd network), with clay and graphene oxide as effective crosslinkers and reinforcing fillers. The structure and morphology of the hydrogels were characterized by SEM, FTIR, DSC, and TGA. The synergetic effects of clay, GO and DN structure on various physical properties were investigated. With the increasing of crosslinking densities, the swelling ratios of DN hydrogels gradually decreased by increasing the contents of graphene oxide and PAA. While the DN hydrogels had much better mechanical properties than that of the conventional chemically cross‐linked PNIPAM hydrogels. POLYM. ENG. SCI., 55:1361–1366, 2015. © 2015 Society of Plastics Engineers     

POEGMA hydrogel cross-linked by starch-based microspheres: synthesis and characterization

Hydrogels with high mechanical strength and controllable stimuli responses are highly desirable in the biomedical field. Herein, starch-based microspheres were used as macrosized cross-linkers to synthesize a series of extremely tough and thermosensitive poly[2-(2-methoxyethoxy) ethyl methacrylate-co-oligo (ethylene glycol) methyl ether methacrylate] (POEGMA) hydrogels. Scanning electron microscopy and confocal laser scanning microscopy showed that the starch-based microspheres were uniformly distributed in the hydrogel network. Compression test results indicated that the POEGMA hydrogel exhibits strength of 3.0 MPa, which is ten times greater than that of conventional hydrogels cross-linked using small molecules. This improvement in mechanical strength is attributable to the even distribution of the cross-linking points in the hydrogel, because of which the length of the flexible polymer chains between the microspheres was similar. As a result, the polymer network can readily dissipate stress. Moreover, the mechanical strength of the POEGMA hydrogel can be regulated efficiently by varying the amount of microspheres used. In addition, the POEGMA hydrogel exhibited a lower critical solution temperature (LCST) of 37°C when the 2-(2-methoxyethoxy)ethyl methacrylate (MEO₂MA)/oligo(ethylene glycol) methyl ether methacrylate (OEGMA₃₀₀) mass ratio was 70/30. Further, the LCST of the POEGMA hydrogel can also be adjusted by adding salt or ethanol. The LCST decreased in the presence of sodium chloride but increased in the presence of ethanol.     

Facile synthesis of extremely biocompatible double‐network hydrogels based on chitosan and poly(vinyl alcohol) with enhanced mechanical properties

An easy and ecofriendly method for designing double‐network (DN) hydrogels based on chitosan and poly(vinyl alcohol) (PVA) with high mechanical performance is described. When covalent bonds in the networks are used as crosslinking agents in the achievement of a higher mechanical strength, the irreversible deformation of these hydrogels after a large force is applied is still one of the most important obstacles. To overcome this problem, we used physical crosslinking for both networks. The mechanical strength, surface morphology, and cytotoxicity of the films were studied by tensile testing, scanning electron microscopy analysis, and an MTT assay. The synthesized chitosan–PVA DN hydrogels showed a large improvement in the tensile strength to 11.52 MPa with an elongation of 265.6%. The surface morphologies of the films demonstrated the effective interactions between the two networks and a suitable porosity. Also, because of the use of a natural polymer and honey as a plasticizer, the cell culture indicated that the synthesized DN hydrogels had good biocompatibility (with 327.49 ± 11.22% viability) and could be used as capable biomaterials. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45752.     

Mechanical and tribological behaviors of PVA/PAAm double network hydrogels under varied strains as cartilage replacement

The strain change of double network (DN) hydrogels during compressive mechanical and frictional tests is crucial for their performances. The positive effect of the sacrificed network for prohibiting crack in DN hydrogel is likely to be initiated by large strain. In this study, the mechanical and tribological properties of polyvinyl alcohol/polyacrylamide (PVA/PAAm) DN hydrogels are investigated from the viewpoint of strain. The compressive tangent modulus of PVA/PAAm DN hydrogel with 15 wt% AAm shows a sudden increase in the strain of 60% due to the sacrificed PAAm network. The optimized friction behavior is obtained from PVA/PAAm hydrogel with 5 wt% of AAm content, which is not consistent with the optimal compressive modulus at 15 wt% of AAm content. The variation of frictional coefficient of PVA/PAAm DN hydrogels with load is quite different for migrating and stationary contact configurations. The biphasic lubrication mechanism transited to solid–solid contact dominant mechanism is also induced by the high strain at heavy load.     

Preparation and characterization of dual responsive sodium alginate‐g‐poly(vinyl alcohol) hydrogel

Poly(vinyl alcohol) (PVA) was chosen as a controllable gelator to prepare sodium alginate (SA)‐based physically cross‐linked dual‐responsive hydrogel by three steps. First, polyvinyl acetate (PVAc) was grafted onto SA via radical copolymerization. Then, the copolymer was subsequently converted into SA‐g‐poly(vinyl alcohol) (SAPVA) by alcoholysis reaction. PVA content of SAPVA was tailored by controlling the graft percentage of PVAc, i.e. through varying the amount of vinyl acetate during copolymerization. Finally, SAPVA hydrogels were formed by freezing‐thawing cycles. The structure of the graft copolymers was verified with FTIR spectroscopy. X‐ray diffraction analysis results revealed that the crystallinity of SAPVA hydrogels depended on the PVA content of SAPVA. The swelling test showed that SAPVA hydrogels were pH‐responsive, and the swelling was reversible. SAPVA hydrogels also behaved electric‐responsive. In addition, the pH‐sensitivity of SAPVA hydrogels was able to be controlled with the composition of the hydrogels. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Simultaneous lower and upper critical solution temperature‐type co‐nonsolvency behaviour exhibited in water–dioxane mixtures by linear copolymers and hydrogels containing N‐isopropylacrylamide and N,N‐dimethylacrylamide

The co‐nonsolvency behaviour in water–dioxane mixtures of linear copolymers and hydrogels consisting of N‐isopropylacrylamide (NIPAM) and N,N‐dimethylacrylamide (DMAM) was studied as a function of solvent composition and temperature. The composition of the copolymers, P(NIPAM‐co‐DMAMx), in DMAM units, x, varies from x = 0 up to x = 100%. It is shown that the copolymers combine the lower critical solution temperature (LCST)‐type co‐nonsolvency behaviour of poly‐NIPAM with the upper critical solution temperature (UCST)‐type co‐nonsolvency behaviour of poly‐DMAM. Depending on x, both the LCST‐ and UCST‐type co‐nonsolvency behaviour may be simultaneously observed in water‐rich and dioxane‐rich solvent mixtures, respectively. Due to this complex phase separation behaviour, the variation of the reduced viscosity of the linear copolymers, as well as the swelling–deswelling behaviour of the respective hydrogels, are shown to be temperature‐ and solvent‐sensitive. Copyright © 2006 Society of Chemical Industry     

Hydrogels reinforced with nanoclays with improved response rate

Two series of semiinterpenetrating networks (SIPN) based on linear hydrophilic poly(vinyl alcohol) (PVA) and thermo‐responsive poly(N‐isopropylacrylamide) (PNIPA), physically crosslinked with inorganic clay, are presented. The hydrogels with different crosslinking densities were prepared by varying the content of clay from 1 to 6 wt % and contained linear interpenetrant, PVA in the range of 0.5–1.5 wt %. The effect of clay content on swelling/deswelling behavior and phase transition in PNIPA gels, as well as the feasibility of reinforcing the gels with high molecular weight PVA, were analyzed. The thermal response of hydrogels, followed by DSC, confirmed that the insertion of hydrophilic PVA did not have a significant effect on the onset of the volume phase transition temperature, while the response was faster. The equilibrium degree of swelling of SIPNs and PNIPA hydrogels was in the range of 9–79 and decreased with increasing content of clay. The internal morphology and surface wettability of the hydrogels were investigated by scanning electron microscope analysis and contact angle measurements, respectively. The network structural parameters of the PNIPA and SIPN nanocomposites hydrogels, such as the average molecular weight between crosslinks, M_c, and effective crosslinking density, N_e, were determined by dynamic mechanical analysis. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44535.     

Gelation of a new hydrogel system of atactic‐poly(vinyl alcohol)/NaCl/H2O

a‐PVA/NaCl/H₂O hydrogels have been prepared by gelation of aqueous atactic‐poly(vinyl alcohol) (a‐PVA) solutions in the presence of NaCl. The gelling temperature, melting temperature and the preservation of water of the hydrogels have been measured. The effect of the addition of NaCl to the hydrogels on gelling temperature and melting temperature is marked when the NaCl concentration is over a certain value. High NaCl concentration favours high‐melting‐point hydrogels. When the NaCl concentration is high enough (such as 11%), high‐melting‐point and white turbid opaque a‐PVA/NaCl/H₂O hydrogels can be prepared, regardless of the PVA concentration. Similarly, the low gelling temperature of a‐PVA/NaCl/H₂O solutions comes from low NaCl concentration, while high gelling temperature ranges from 50 to 70 °C when the NaCl concentration is 11%. In appearance, the types of syneresis of a‐PVA/NaCl/H₂O are χ‐type or a mixture of χ‐ and n‐types; water release of the hydrogels is slowed down by the addition of NaCl. © 2002 Society of Chemical Industry     

Thermal phase transition of poly(N‐propionylethyleneimine) hydrogel

This article presents the preparation of the hydrogel of poly(N‐propionylethyleneimine) and its interpenetrating polymer network (IPN) hydrogel containing polyacrylamide by means of γ‐ray radiation and a study of the phase transition temperature of these hydrogels. As a result, the hydrogel of the crosslinked poly(N‐propionylethyleneimine) exhibited swelling below and shrinking above the phase transition temperature (about 61°C), as well as the lower critical solution temperature (LCST) of the liner polymer–water system. The experiment also showed that the LCST of the IPN hydrogel could be adjusted by the incorporation of the second component polyacrylamide. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 2457–2461, 1999     

温度及pH敏感生物水凝胶的研究

运用互穿网络技术,合成了具有温敏性的聚(N 异丙基丙烯酰胺)(PNIPAm)和生物大分子明胶(gelatin)的互穿网络聚合物(PNIPAm/Gelatinsemi IPN和PNIPAm/GelatinIPN)水凝胶,该水凝胶的最低临界溶液温度(LCST)与PNIPAm水凝胶的LCST基本相同,均为33℃左右,但在LCST以下的平衡溶胀率减小、相变区域略微变宽。在此基础上,通过N 异丙基丙烯酰胺(NIPAm)与丙烯酸(AAc)交联共聚,改变了水凝胶的LCST,在pH=4 0的缓冲溶液中,各水凝胶的溶胀行为基本一致,与AAc含量无关,LCST都为28℃左右;在pH>4 0的缓冲溶液中,LCST随AAc组分含量的增加而增加,但温敏性减小。同时,AAc的加入,使水凝胶具有pH敏感性,敏感点为pH=4 5左右。还考察了该水凝胶降解的特点:戊二醛(GA)交联后的明胶网络,保留了明胶的生物降解性,但互穿网络水凝胶在实验条件下几乎未被胃蛋白酶和胰蛋白酶降解,在pH=9 6的碱性条件下,水凝胶可发生化学降解。     

Characterization of thermo-sensitive hydrogels based on poly(<Emphasis Type="Italic">N</Emphasis>-isopropylacrylamide)/hyaluronic acid

In this study, an hydrogel was synthesized from the monomer N-isopropylacrylamide (NIPA), generating the poly(N-isopropylacrylamide) (PNIPA) and other formulations were synthesized in the presence of 1, 2, and 3% hyaluronic acid (HA) for obtain an interpenetrating polymer network. For all the obtained hydrogels, the thermo-sensitive response was studied since the lower critical solution temperature (LCST) and was analyzed by differential scanning calorimetry (DSC), nuclear magnetic resonance (NMR), and oscillatory rheology at constant frequency as a function of temperature. The LCST transition temperature (Tt onset) was found between 34.4 and 35.5 °C. By scanning electron microscopy (SEM) PNIPA-HA formulations showed a porous morphology. The applicability of the hydrogels as injectable and non-toxic materials was verified, respectively, by rheology results and by cytotoxicity studies through an in vitro test of cell hemolysis on blood agar.     

Dually responsive multinetworked hydrogels with improved performances by covalently bonding poly(N‐isopropylacrylamide)‐based microgels and poly(vinyl alcohol) skeleton

To prepare intelligent hydrogels with both good mechanical performance and fast response, a series of multinetworked composite hydrogels with pH and thermo‐responsivity were designed and fabricated combining the advantages of both interpenetrating polymer network hydrogels and microgels. To begin with, precipitation polymerization of N‐isopropylacrylamide, methacrylic acid and polyethyleneglycol methacrylate was carried out in poly(vinyl alcohol) (PVA) aqueous solution, yielding thermo‐ and pH‐responsive copolymeric microgels. Then glutaraldehyde was added to the obtained microgel dispersion to covalently bond the microgel particles and PVA through acetal reaction, leading to the formation of multinetworked hydrogels with dual responses. The morphology of microgels and the internal microstructure of composite hydrogels were characterized by transmission electron microscope and scanning electron microscope, respectively. The influence of the PVA content and glutaraldehyde dosage on the mechanical behavior of the prepared hydrogels was investigated by means of uniaxial compressive tests. Experimental results reveal that the as‐prepared composite hydrogels exhibit enhanced mechanical performance compared to conventional hydrogels. Investigation onthe swelling and deswelling behaviors shows that they possess good temperature‐ and pH‐sensitivity andswell/deswell more rapidly than conventional hydrogels. POLYM. COMPOS., 2012. © 2013 Society of Plastics Engineers