Composite double network hydrogels with thermoresponsive colloidal nanoemulsions

We report the formulation and mechanical characterization of double network (DN) composite hydrogels. The first network consists of covalently crosslinked poly(ethylene glycol diacrylate) (PEGDA), which forms a strong, brittle network that provides elasticity to the gel. The second network, sodium alginate, is ionically crosslinked with Ca²⁺ to allow increased dissipation of mechanical energy. The novelty of this system over existing DN hydrogels is the additional incorporation of a third mesoscale network, composed of thermoresponsive poly(dimethyl siloxane) (PDMS) nanoemulsions, which undergo colloidal gelation through the bridging of the PEGDA hydrophobic end groups into the PDMS droplets. The colloidally gelled microstructures are photopolymerized into a solid hydrogel by crosslinking the precursors with ultraviolet (UV) light. Tensile mechanical experiments performed on the crosslinked DN nanoemulsion hydrogels show that their rupture stress (0.17–0.34 MPa), fracture energy (144–421 J/m²), and Young's modulus (1–2.1 MPa) are comparable to similar systems in the literature. These mechanical measurements suggest that the gels may be suitable for manufacturing processes in which large shear rates and deformations are encountered.     

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.     

High strength and toughness of double physically cross-linked hydrogels composed of polyvinyl alcohol and calcium alginate

The weak mechanical properties of hydrogels, especially physically cross-linked hydrogels are usually a major factor to hinder their application. To solve this problem, in this work, we prepared a high strength and toughness of double physically cross-linked (PDN) hydrogels composed of crystalline domain cross-linked polyvinyl alcohol (PVA) and Ca²⁺-cross-linked alginate (Alg). With a further annealing treatment, the noncovalent cross-linked network via the formed crystalline promote the as-prepared PDN PVA/Alg hydrogel to exhibit well mechanical properties with the tensile strength of ~1.94 MPa, elongation at break of ~607% and Young's modulus of ~0.45 MPa (above 70 wt% of water content). By analyzing the mechanism of improving the hydrogel mechanical properties, it is found that annealing can effectively improve the crystallinity of PVA in the hydrogel, and then greatly improve the mechanical properties of the hydrogel. This provides a general method for improving the mechanical properties of PVA PDN hydrogels. In addition, the PDN PVA/Alg hydrogel was also proved to have good ionic conductivity of 1.70 S m⁻¹. These desirable properties make the prepared physically cross-linked hydrogels promising materials for medical and biosensing fields.     

Synthesis of multifunctional high strength,highly swellable,stretchable and self‐healable pH‐responsive ionic double network hydrogels

The multifunctional double network (DN) soft hydrogels reported here are highly swellable and stretchable pH‐responsive smart hydrogel materials with sufficient strength and self‐healing properties. Such multifunctional hydrogels are achieved using double crosslinking structures with multiple physical and chemical crosslinks. They consist of a copolymer network of acrylamide (AM) and sodium acrylate (Na‐AA) and other reversible network of poly(vinyl alcohol)–borax complex. They were characterized by Fourier transform IR analysis and studied for their hydrogen bonding and ionic interaction. The degree of equilibrium swelling was observed to be as high as 5959% (at pH 7.0) for a hydrogel with AM/Na‐AA = 25/75 wt% in the network (GS‐6 sample). The highest degree of swelling was observed to be 6494% at pH 8.5. The maximum tensile strength was measured to be 1670, 580 and 130 kPa for a DN hydrogel (GS‐2 sample: AM/Na‐AA =75/25 wt% with 20, 40 and 60 wt% water content, respectively). The self‐healing efficiency was estimated to be 69% for such a hydrogel. These multifunctional DN hydrogels with amalgamation of many functional properties are unique in hydrogel materials and such materials may find applications in sensors, actuators, smart windows and biomedical applications. © 2018 Society of Chemical Industry     

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.     

新型淀粉基微球对Cu~(2+)、Pb~(2+)吸附性能的研究

研究了新型淀粉基微球对Cu2+、Pb2+的吸附性能,结果表明,淀粉基微球对Cu2+的吸附能力强于Pb2+,对Cu2+、Pb2+的吸附率可达到85%以上,是一种有效处理重金属离子废水的处理剂。     

Super tough double network hydrogels and their application as biomaterials

The double network (DN) technique, developed by authors’ group, provides an innovative and universal pass way to fabricate hydrogels with super high toughness comparable to rubbers. The excellent mechanical performances of DN hydrogels originate from the specific combination of two networks with contrasting structures. The first brittle network serves as sacrificial bonds, which breaks into small clusters to efficiently disperse the stress around the crack tip into the surrounding damage zone, while the second ductile polymer chains act as hidden length, which extends extensively to sustain large deformation. Based on the principle of DN hydrogel, the author’s group recently has developed several novel systems and techniques, which has greatly expanded the practical accessibility of DN technique for practical use. The DN principle and the DN gel have already attracted much attention in the soft matter community. Inspired by the DN principle, many research groups have also designed and developed some innovative hydrogels with large enhancement in their mechanical strength and toughness. Some tough hydrogels fabricated by the DN technique also exhibit good biocompatibility and low friction resistance with promising prospective in industrial and medicine fields, especially for load-bearing artificial soft tissues such as artificial cartilage. In this feature article, we address the major concept and toughening mechanism of DN gel, then we describe some recent novel hydrogel systems based on the DN concept, and finally the applicability of DN gel as soft biomaterials is discussed.     

Chitosan-based double network hydrogels with self-healing and dual-responsive shape memory abilities

Chitosan-based single network hydrogels with imine bonds have excellent self-healing capability, while poor mechanical properties limit their applications. Here, chitosan-polyacrylamide-based double network hydrogels were prepared via in situ free-radical polymerization of acrylamide in the presence of N-carboxyethyl chitosan (CEC) and dibenzaldehyde-terminated telechelic poly(ethylene glycol), which had excellent mechanical properties, self-healing, and dual-responsive shape memory abilities. The maximum tensile strength and elongation at break could reach 460 kPa and 4600%, respectively. Meanwhile, owing to the reversibility of imine bonds, elongation and strength at break of hydrogels could heal by 84.2 and 93.2% under alkali stimulation at 35 °C, respectively. Furthermore, the hydrogels also had good shape memory abilities for pH-stimuli responsiveness of the imine bonds and metal ions stimuli responsiveness of CEC. The prepared chitosan-based functional hydrogels have great potential application prospects in tissue scaffolds, actuators, and wearable devices. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 48247.     

An examination of the thermorheological and drug release properties of zinc tetraphenylporphyrin-containing thermoresponsive hydrogels, designed as light activated antimicrobial implants

This study describes the thermorheological, mechanical and drug release properties of novel, light-activated antimicrobial implants. Hydrogels, based on N-isopropylacrylamide (NIPAA) and hydroxyethylmethacrylate (HEMA) and either devoid of or containing zinc tetraphenylporphyrin, were prepared by free radical polymerisation and characterised using oscillatory rheometry and texture profile analysis. Drug release was studied at both 20 and 37 °C. Hydrogels containing NIPAA exhibited a sol-gel temperature (Tm), which increased as the proportion of HEMA increased and was . The viscoelastic properties (storage modulus G^′, loss modulus G^″, loss tangent and dynamic viscosity η^′) were affected by hydrogel composition and temperature, with the copolymers exhibiting lower values of G^′,G^″ and η^′ values than either homopolymer. Similar relationships between the composition of the hydrogels and the textural/mechanical properties were observed. At 37 °C rheological structuring (increased G^″,G^″,η^′ and reduced loss tangent) occurred for all NIPAA-containing polymers and increased as the NIPAA content increased. At 20 °C drug release was diffusion controlled, the rate of which was similar for all NIPAA-containing polymers and was lower than drug release from p(HEMA), despite the greater elasticity of this homopolymer. At 37 °C drug release from the NIPAA-containing hydrogels was initially non-diffusion controlled, following which drug release levelled. Drug release decreased as the NIPAA content increased and correlated to hydrogel elasticity. It is suggested that the ability to engineer the release of Zn-TPP from these hydrogels, in conjunction with their acceptable mechanical properties, may be clinically advantageous for the treatment of infection.     

A novel approach to prepare reticulated porous TiN ceramics with high strength and high porosity by in-situ synthesis

Titanium nitride (TiN) with high porosity (90%) was successfully in-situ prepared by a novel approach with the combination of carbothermic reduction nitriding method and replication template method. The microstructure of porous TiN prepared with different temperature and phenolic resin (PF) content were revealed by XRD, Raman spectrum, SEM, TEM, respectively. The results show that when the mass ratio of PF and TiO₂ is 1:2 and the sintering temperature is 1850 ℃, porous TiN with high purity and ideal strength could be synthesized. In addition, the synthesis path and thermodynamic mechanism of porous TiN were analyzed by TG-DSC and Gibbs free energy calculation. The mechanical properties and corrosion resistance were preliminarily explored.     

Chemical modification and synthesizing conditions of nanocomposite hydrogels with high mechanical strength crosslinked by hydrophilic reactive microgels

Series of polyacrylamide hydrogels with high mechanical strength were synthesized using hydrophilic reactive microgels (HRM) with C?C double bonds as crosslinkers. The hydrophilic microgels were prepared by inverse emulsion photopolymerization and then were chemical modified by N‐methylolacrylamide (NMA) to obtain HRM. Chemical‐modifying conditions affecting the HRM double bound content were investigated. The maximum double‐bond content was 1.82% at the optimum conditions of NMA 8 g, hydrochloric acid 0.8 mL, reaction temperature 60°C, and time 4 h. The mechanical properties of the hydrogels were significantly enhanced by using HRM as crosslinkers instead of the conventional crosslinkers. These HRM hydrogels were studied by varying such parameters as HRM content, monomer concentration, HRM double‐bond content, and the initiator dosage. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011.     

Removal of methylene blue from aqueous solution by sorption on lignocellulose-g-poly(acrylic acid)/montmorillonite three-dimensional cross-linked polymeric network hydrogels

Batch lignocellulose-g-poly(acrylic acid)/montmorillonite (LNC-g-PAA/MMT) hydrogel nanocomposites were applied as adsorbents. The nanocomposites were characterized by FTIR, XRD, SEM, and TEM. The results showed that montmorillonite (MMT) could react with the monomers and change the structure of polymeric network of the traditional superabsorbent materials, an exfoliated structure was formed in the nanocomposites. The effect of process parameters such as MMT content (wt%), contact time (t), initial concentration of dye solution (C ₀), adsorption temperature (T), and pH value (pH) of the dye solution for the removal of methylene blue (MB) from aqueous solution were also studied. The results showed that the adsorption capacity for MB increased with increasing contact time, initial dye concentration, and pH value, but decreased with increasing MMT content and temperature. The adsorption kinetics were better described by the pseudo-second-order equation, and their adsorption isotherms were better fitted for the langmuir equation. By introducing 20 wt% MMT into LNC-g-PAA polymeric network, the obtaining hydrogel composite showed the high adsorption capacity 1994.38 mg/g and economic advantage for MB. The desorption studies revealed that the composite provided the potential for regeneration and reuse after MB dye adsorption, which implied that the composite could be regarded as a potential adsorbent for cationic dye MB removal in a wastewater treatment process.     

A novel fabrication procedure for producing high strength hydroxyapatite ceramic scaffolds with high porosity

Hydroxyapatite (HA) scaffolds were fabricated using the space holder method with a pressureless sintering process in a systematically developed manner at different fabrication stages to increase the strength of the scaffold at high porosity. Polyvinyl alcohol (PVA) and Polymethyl methacrylate (PMMA) were used as binders and space holder agents, respectively. The physical properties of the HA scaffolds were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), linear shrinkage test, and porosity measurements. The mechanical properties of the HA scaffolds were analyzed using compressive strength measurements. The results revealed that the HA scaffold met the expected quality requirements with a compressive strength of 2.2 MPa at a porosity of 65.6% with pore sizes distributed in the range of 126–385 μm. The shrinkage of the scaffold diameter occurred by 20.27%, this diameter shrinkage predominantly to the shrinkage of the HA scaffold caused by sintering. Besides, suspect that a higher PMMA concentration causes pore size shrinkage upon sintering. The formation of pore interconnections was evidenced by SEM observations and the ‘translucent light method’ developed in this study. The results of the scaffold phase test using XRD showed that the final scaffold consisted only of the HA phase, as the PVA and PMMA phases burned out during the sintering process.     

Highly stretchable,ionic conductive and self-recoverable zwitterionic polyelectrolyte-based hydrogels by introducing multiple supramolecular sacrificial bonds in double network

Zwitterionic hydrogels have been explored for applications in electrochemical devices very recently due to their high water retention ability and interesting electrochemical properties. The use of zwitterionic hydrogels in devices requires them tough and recoverable or healable from fatigue damage. Herein, a double network zwitterionic hydrogel contains a reversible noncovalent interaction crosslinked polyvinyl alcohol (PVA) first network, together with a covalent/noncovalent hybrid crosslinked acrylamide and sulfobetaine methacrylate copolymer (P(AM-co-SBMA)) second network, was fabricated by a simple two-steps methods of copolymerization and freezing/thawing. The reversible hydrogen bonds, crystalline domain, and electrostatic interactions in the double networks work as sacrificial bonds to dissipate energy and toughen the materials when hydrogel deforms. The broken bonds can reform upon unloading endowing the recovery of hydrogels' properties with the assistance of the elastic covalent network. The optimal hydrogels are highly stretchable (fracture strain 970%), tough (fracture toughness 693 kJ m⁻³), rapidly recoverable (65% toughness recovery and 75% stiffness recovery after resting 5 min at room temperature) and with widely tunable mechanical properties by multibond crosslinking. Meanwhile, the zwitterionic counterions of SBMA moieties endow the tough and recoverable hydrogels extremely high intrinsic ionic conductivities (7.49 S m⁻¹) at room temperature. This work not only provides a simple strategy for fabricating tough and recoverable zwitterionic hydrogels but also demonstrates multifunctional properties of the zwitterionic hydrogels, which possess a great potential to fulfill flexible devices applications. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47783.     

A novel cross-linked polyimide film: synthesis and dielectric properties

A novel cross-linked polyimide (CPI) has been prepared by imidization of cross-linked poly(amic acid) (CPAA). In this work, the Ac conductivity and dielectric properties of this polyimide are presented comparitively with those of conventional polyimide (PI), in the 0.2-100 kHz frequency range and 300-463 K temperature interval. Although the frequency and temperature dependencies of dielectric constant of both conventional and cross-linked polyimides show the same behaviour, the dielectric constant of CPI takes lower values. The Ac conduction studies suggest that electron hopping is responsible for conduction of the PI and CPI films. The activation energy calculated in 296-353 K temperature interval and the ß-relaxation was also observed for CPI.     

Thermoresponsive nanocomposite hydrogels with high mechanical strength and toughness based on a dual crosslinking strategy

The practical application of thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) hydrogels are severely limited by their poor mechanical properties. Herein, we reported a series of dual crosslinked (DC) PNIPAM hydrogels with superior mechanical properties prepared by simple copolymerization of N-isopropylacrylamide and sodium acrylate (SA) in the laponite RDS suspension, following by a soaking process in multivalent metal cations (e.g., Ca²⁺, Al³⁺, Fe³⁺) aqueous solutions to form ionic coordination interactions with  COO groups of copolymer side chains. The effect of laponite RDS, AANa (sodium acrylate), and metal cation (e.g., Fe³⁺) concentrations on the mechanical properties and deswelling properties of the DC hydrogels are evaluated. The DC hydrogel prepared with 10 w/v% laponite RDS, 0.25 mol/L AANa and 0.45 mol/L Fe³⁺ possesses the best mechanical properties (ca. 1.1 MPa of tensile strength, 9.1 MPa of compression strength at 80% of compression strain, 1.4 MPa of elastic modulus and 1.3 MJ/m³ of toughness). Moreover, we also discovered that the DC hydrogels crosslinked by Fe³⁺ showed better mechanical properties due to the larger charge and ion radius of Fe³⁺.     

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.     

A novel,efficient route for the crosslinking and creep improvement of high modulus and high strength polyethylene fibres

A new route is presented for the chemical crosslinking of solution‐spun, ultra‐drawn Ultra‐High‐Molecular‐Weight Polyethylene (UHMW‐PE) fibres. UHMW‐PE fibres with a range of draw ratio's, Young's moduli and tensile strengths were impregnated with a radical initiator using supercritical carbon dioxide as a carrier. After impregnation, the drawn fibres were crosslinked with ultra‐violet light and fibres with a high gel content (> 90%) were obtained. It was found that the chemical crosslinking strongly reduces the plateau creep rate of the fibres and that the threshold stress for irreversible creep is enhanced. Simultaneously, the high Young's modulus and the high tensile strength of the drawn fibres are preserved which illustrates that the long term properties of the fibres (i. e. creep) are improved without a large sacrifice short term mechanical properties such as Young's modulus.     

聚环氧乙烷对PAMPS/PAM双网络水凝胶结构和性能的影响

采用紫外光引发聚合制备了含聚环氧乙烷（PEO）的聚(2-丙烯酰胺-2-甲基丙磺酸)(PAMPS）/聚丙烯酰胺（PAM）双网络（DN）水凝胶。使用扫描电子显微镜（SEM）观察了PAMPS单网络水凝胶的结构;测定了PEO改性前后双网络水凝胶的压缩及拉伸性能。PEO改性DN凝胶的第一网络网孔上由于PEO片晶结构引起不同程度的褶皱,这种褶皱起支撑作用;PEO的分子量达到5万时,褶皱的支撑作用最佳,DN凝胶的力学性能最佳;DN凝胶的力学性能随PEO加入量先提高后下降,在PEO加入量为0.1%时,PEO片晶结构加固了DN凝胶的物理交联点,力学性能达到最大,压缩应力达到31.6 MPa;加入更多的PEO阻碍了第一网络的凝胶化,造成网络结构的不连续,从而使DN凝胶的力学性能下降。