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Polyampholyte (PA) hydrogels are randomly copolymerized from anionic and cationic monomers, showing good mechanical properties owing to the existence of numerous ionic bonds in the networks. However, relatively tough PA gels can be synthesized successfully only at high monomer concentrations (CM), where relatively strong chain entanglements exist to stabilize the primary supramolecular networks. This study aims to toughen weak PA gels with relatively weak primary topological entanglements (at relatively low CM) via a secondary equilibrium approach. According to this approach, an as-prepared PA gel is first dialyzed in a FeCl3 solution to reach a swelling equilibrium and then dialyzed in sufficient deionized water to remove excess free ions to achieve a new equilibrium, resulting in the modified PA gels. It is proved that the modified PA gels are eventually constructed by both ionic and metal coordination bonds, which could synergistically enhance the chain interactions and enable the network toughening. Systematic studies indicate that both CM and FeCl3 concentration (CFeCl3) influence the enhancement effectiveness of the modified PA gels, although all the gels could be dramatically enhanced. The mechanical properties of the modified PA gel could be optimized at CM = 2.0 M and CFeCl3 = 0.3 M, where the Young’s modulus, tensile fracture strength, and work of tension are improved by 1800%, 600%, and 820%, respectively, comparing to these of the original PA gel. By selecting a different PA gel system and diverse metal ions (i.e., Al3+, Mg2+, Ca2+), we further prove that the proposed approach is generally appliable. A theoretical model is used to understand the toughening mechanism. This work well extends the simple yet general approach for the toughening of weak PA gels with relatively weak chain entanglements.

The poor adhesion performance of typical gels still remains a challenge to find a simple method to achieve strong and reversible adhesion with the existence of water. Here, a poly(acryloyloxyethyl trimethyl ammonium chloride-co-2-vinyl-4-6-diamino-1,3,5-triazine) (P(DAC-co-VDT)) gel with high and adjustable interfacial adhesion is fabricated by combining cation-triazine π interaction and multiple hydrogen bonding and through a one-pot route. Characterization of the gels reveals that the two types of interactions are introduced into the gel network and that the gel-gel and gel-glass interfacial adhesion can be readily adjusted in a wide range from 15.98 to 123.60 kPa. This approach enables the creation of high-strength composites using P(DAC-co-VDT) gel as matrix, anionic monomer sodium p-styrene sulfonate as ion concentration adjustor, and discrete quartz sands as filler with easy and repeated moldability and self-healing capability.

The application of big data in the field of ecological environment has achieved remarkable results, but there are still problems such as insufficient innovation and lack of shared service ability. This research takes the big data platform as the technical support and makes full use of advanced technologies such as internet of things, cloud computing, and big data to realize the integration of ecological environment big data resources and real-time data sharing. Through network technology, the ecological environment metadata can be automatically obtained from the internet and stored. When sharing ecological environment metadata, realize the data response and declassification of geospatial data for different user access levels, and finally safely feedback the ecological environment spatial big data integrated with the target ecological environment metadata and declassified geospatial data to the access initiator, so as to realize the centralized sharing of decentralized environmental information resources and give full play to the supporting role of big data, so that big data can be effectively applied in environmental monitoring. Find out the changes of pollution sources and pollutants through massive data, so as to provide necessary data support for the whole environmental monitoring.

The use of glycidyl methacrylate (GMA) and acrylic acid (AAc) as grafting monomers to improve the mechanical properties, particularly tensile strength, of recycled polyethylene (R-PE) is re...

A thermal and photo dual-responsive drug delivery system is newly designed for controlled anticancer drug delivery. The concept of this design is to encapsulate upconversion nanoparticles in a phot...

Hydrogels that possess adequate toughness in bulk and at soft interface are highly desirable. We report a fully physically crosslinked dual Amylopectin (Amy, the first network)/poly(N-hydroxyethyl acrylamide) (PHEAA, the second network) hydrogel, where both networks were hydrogen bonding (H-bonding) crosslinked, with super toughness in bulk and tunable adhesion at hydrogel-soft tissue interface. Through adjustment of hydrogen bond density, strong and tough hydrogels were prepared that exhibited different levels of adhesion strength and three adhesive failure mechanisms. We replaced Amy with oxidized starch (OSA) to synthesize dual and hybrid chemically/physically crosslinked OSA/PHEAA double network (DN) hydrogels that were capable of forming Schiff's bases at hydrogel-hydrogel or hydrogel-tissue interfaces, aside from interfacial H-bonding, incurring sustained, high interfacial toughness (1850 J m−2 hydrogel-hydrogel and 610 J m−2 hydrogel-tissue) in wet environments. Such tough and adhesive DN hydrogels have great potential in various applications such as engineering of artificial soft tissues or implantable devices that are intended to adhere to internal organs or tissues.

Rewritable information record materials usually demand not only reversibly stimuli-responsive ability, but also strong mechanical properties. To achieve one photochromic hydrogel with super-strong mechanical strength, hydrophobic molecule spiropyran (SP) has been introduced into a copolymer based on ion-hybrid crosslink. The hydrogels exhibit both photoinduced reversible color changes and excellent mechanical properties, i.e., the tensile stress of 3.22 MPa, work of tension of 12.8 MJ m-3 , and modulus of elasticity of 8.6 MPa. Moreover, the SP-based Ca2+ crosslinked hydrogels can be enhanced further when exposed to UV-light via ionic interaction coordination between Ca2+ , merocyanine (MC) with polar copolymer chain. In particular, hydrogels have excellent reversible conversion behavior, which can be used to realize repeatable writing of optical information. Thus, the novel design is demonstrated to support future applications in writing repeatable optical information, optical displays, information storage, artificial intelligence systems, and flexible wearable devices.

A versatile ionic crosslink lithography (ICL) approach is reported to achieve geometry transitions of strong polyvinyl alcohol/sodium alginate (PVA/SA) hydrogels in a controllable and programmable manner. Specifically, localized PVA/SA and PVA/SA/Fe3+ hydrogel domains of significantly different swellabilities (i.e., in-plane gradient) are created by patterning and selective ionic crosslinking of one single type of PVA/SA hydrogel. A simple two-step sequential pre- and free-swelling, or each alone, directs the patterned, inhomogeneous hydrogel to transform in various programmable and quasi-quantitative ways through local bulging and/or global buckling. All types of shape changing are reversible and repeatable due to the reversible nature of ionic coordination in the hydrogel networks. The flexibility and versatility of 3D printing is also demonstrated in creating through-thickness gradient in PVA and PVA/SA hydrogel assemblies with similar morphing capability. The ICL approach developed in this work may help shed some light on developing strong and shape morphing hydrogels as soft sensors and actuators and for potentially biomimetic transformations. The ICL approach may also be transferable to fabrication of many other types of hydrogel materials for similar applications.

As promising structural materials, various tough hydrogels have been developed recently by incorporating various kinds of bonds. An important challenge is to use dual physical cross-linking to develop both toughness and self-recovery in a single material. Here we report smart, strain-responsive hydrogels composed of a fully physically linked agarose/poly(acrylic acid)-ferric ion (agar/PAAc-Fe3+) double network (DN) with high toughness and pH-sensitivity. These hydrogels were fabricated in a one-pot reaction to generate dual physical cross-linking through, first, a hydrogen-bonded cross-linked agarose network, and, second, a physically linked PAAc-Fe3+ network via Fe3+ coordination interactions. The DN hydrogels possessed high toughness, with breaking strain of 1130%, fast self-recovery properties in ambient conditions (100% recovery in 30 min) and self-healing properties (the healed hydrogels can be manually stretched up to 700% of their original length after self-healing for 60 h from the cut-off state). In addition, the hydrogels exhibited pH-sensitivity due to the dissociation of ionic coordinate bonds between –COO− ions of the PAAc chains and Fe3+ ions. Double-layer hydrogel strips with two different concentrations of PAAc formed a “C”-shaped material when initially immersed in pH 7 solution and then soaked in a pH 3 solution. These characteristics make the hydrogels attractive candidates for tissue engineering, soft actuators and flexible electronics.

We report a dual ionic cross-linking approach for the preparation of double-network hydrogels with robustness, high strength, and toughness, sodium alginate/poly(acrylamide-co-acrylic acid)/Fe3+ (SA/P(AAm-co-AAc)/Fe3+), in a facile “one-step” dual ionic cross-linking method. We take advantage of the abundant carboxyl groups on alginate molecules and the copolymer chains and their high coordination capacity with multivalent metal ions to obtain hydrogels with high strength and toughness. The optimal SA/P(AAm-co-AAc)/Fe3+ (SA 2 wt % and AAc 5 mol %) hydrogels showed a remarkable mechanical performance with 3.24 MPa tensile strength and 1228% strain, both of which remained stable with 76% water content and were highly swelling resistant in an aqueous environment. The hydrogels possessed high fatigue resistance, self-recovery, pH-triggered healing capability, shape memory, and reversible gel–sol transition facilitated by pH regulation. Moreover, they show three-dimensional (3D) printing processability by prop...

In this study, metallocene linear low density polyethylene (abbreviated as mPE) was grafted with glycidyl methacrylate (GMA) and acrylic acid (AAc) forming long-chain branched polymers, which act as an interfacial modifier of recycled high-density polyethylene (abbreviated as rPE)/low-density polyethylene (LDPE) blends. AAc was shown to increase GMA grafting efficiency onto mPE producing mPE-g-GMA/AAc. The mechanical properties, crystalline structure, rheological properties, and phase structure of rPE/LDPE/mPE-g-(GMA-co-AAc) (abbreviated as rPE/LDPE/mPE-g-G/A) blends were investigated using uniaxial tensile, differential scanning calorimeter, rheometer, and X-ray diffraction experiments. The tensile strength of rPE/LDPE/mPE-g-G/A was 34.51 MPa and the multiphase polymer displayed co-crystallization characteristics indicating good compatibility between the two components. The changes in the interface layer thickness and dispersed phase size further indicate the existence of partial compatibility among the multiphase polymer.

To take advantage of the polymer’s special properties, we synthesized PVDAT coated polystyrene (PVDAT@PS) microspheres through semi-continuous precipitation polymerization, and demonstrated the possibility of controlling the size of microspheres by using different sized PS cores. Mechanism of the microsphere synthesis was investigated. Preliminary application studies indicated that the synthesized microspheres adsorbed double-stranded DNAs (dsDNAs) comprising A-T base pairs in pH7.4 phosphate buffered saline, and the adsorption capacity was in a range of 3.28mg to 5.58mg per gram PVDAT@ PS microspheres depending on A-T base pair percentage and chain length of the dsDNAs.

In this study, dual-responsive polymeric nanocapsules, in which the state (swelling or collapse) can be repeatedly controlled by external stimuli (i.e., temperature and light), have been designed and prepared through distillation–precipitation polymerization. Temperature sensitive monomers of N-isopropylacrylamide are cross-linked by photoresponsive bis(methacryloylamino) onto a silica nanospherical template to form a core–shell (SiO2–PNIPAM/Azo) structure. The silica core is then removed by hydrofluoric acid to produce PNIPAM/Azo nanocapsules (P/ANCs) of diameter ∼238 nm at ∼25 °C. The size of the nanocapsule is temperature responsive and, as such, its diameter could be reduced to ∼182 nm on increasing the temperature to 40 °C. In addition, the permeability of nanocapsules can be adjusted by UV irradiation. The cis–trans transformation of modified azobenzene allowed us to perform both the encapsulation and controlled release of molecules. Rhodamine B (RhB) was successfully encapsulated using the photomec...

The low heat dissipation efficiency and serious security issues related to polymer-based thermal management materials (TMMs) have seriously limited their practical applications. However, simultaneously improving their thermal conductivity (TC) and flame retardancy is still a challenge due to performance limitations and complicated processing. Here, non-covalent ionic liquid flame retardant-functionalized boron nitride nanosheets (ILFR-fBNNSs) were used as a multifunctional nano-additive for fabricating epoxy (EP)-based nanocomposites with both superior TC and flame retardancy. Owing to its structural uniqueness and multifunctionality, the ILFR-fBNNS triggers resin crosslinking at a given temperature, while conferring significant improvements in dispersion and interfacial adhesion, thereby forming a thermally conductive network with reduced interfacial phonon scattering and a high-efficiency nano-barrier network acting synergistically with ILFR-induced char residues during thermal degradation. Therefore, the ILFR-fBNNS not only functions as a curing agent for fabricating the nanocomposites, but also acts as a thermal conductor and flame retardant for the nanocomposites. The optimal comprehensive performances of EP/ILFR-fBNNS nanocomposite are achieved with a 12.1 vol% ILFR-fBNNS content with TC enhancement of 478%, and reductions of up to 68.9% and 42.3% by microscale combustion calorimetry (42.4% and 37.7% by cone calorimeter) in peak heat release rate (PHRR) and total heat release (THR) respectively compared to neat EP.

Strong and tough poly(vinyl alcohol) (PVA)/alginate hydrogen-bonded-ionic dual-physical double-network (DN) hydrogels have been successfully prepared by a facile route of a freeze–thaw (25–25–25 °C) cycle followed by concentrated (1.0 mol L−1 of) aqueous-Ca2+ immersion of PVA/Na alginate (SA) mixed aqueous solutions. It was found that, at mole ratios of the PVA- to SA repeat units of 20/1 to 80/1, the DN gels likely evolved a semi-interpenetrating polymer network (IPN) morphology of rigid alginate networks dispersed in while interlocking with ductile PVA network to accomplish DN synergy that gave their high strength and toughness, where the high alginate rigidity originated probably from its dense cross-link induced syneresis and dispersion along crosslink-defective voids to result in little internal stress concentration. Tentatively mechanistically, as the 20/1–80/1 DN gels were stretched steadily, their mechanical response was gradually differentiated into distinct synergistic states: the sparsely hydrogen-bonded PVA served as a ductile matrix to bear small fractions of the established stresses at its large elongations; whereas the densely ionically (i.e. Ca2+) cross-linked alginate functioned as a rigid skeleton to sustain the remaining larger stresses upon its smaller local strains. Promisingly, this ductile-rigid matrix-skeleton synergistic mechanism of semi-IPN morphology may be universally extended to all A/B DN hydrogels of large A–B rigidity (or cross-link density) contrast, whether the cross-link nature of network(s) A or B is covalent, ionic, hydrogen bonded or van der Waals interacted. The strong and tough DN gels also displayed satisfactory self-recovery of viscoelastic behaviour, in that their Young's modulus and dissipated energy in the uniaxial tensile mode and dynamic storage and loss moduli in the oscillatory shear mode all recovered significantly from non-linear viscoelastic regimes despite different degrees of failure to revert to (quasi)linear viscoelasticity.

Broader applications of polymer hydrogels are largely constrained by their poor mechanical performance. Here, we report our strategy of achieving ultrahigh mechanical strength, toughness, and self-healing properties simultaneously in a hybrid dual crosslinked polyacrylic acid (PAAc) hydrogel. The hybrid PAAc hydrogel was fabricated in a one-pot reaction with two types of crosslinking points, i.e. the primary chemical cross-linkers that create covalent cross-linking among PAA chains and the secondary physical cross-linkers Fe 3+ that introduce ionic coordinates between Fe 3+ and −COO − groups. The resulting hybrid PAA hydrogel was subsequently subject to a simple saline solution soaking to become highly mechanically robust as a result of chain entanglement within the network. Favorable mechanical properties possessed by such hybrid hydrogels included high tensile strength (ca. 1.794 MPa), large elongation at break (ca. 13.209 times), and excellent work of extension (ca. 11.475 MJ m −3 ). In addition, the hydrogels exhibited self-healing properties because of their inter-chain interactions at ambient conditions (tensile strength can recover approximately 30% of the initial after self-healing for 48 h from a cut-off state). This work demonstrates a simple and promising strategy of preparation of novel ductile and doughty hydrogels through dual crosslinking and saline solution soaking.

A poly(styrene-ran-methyl acrylate) (S-MA) (75/25 mol/mol), synthesized by surfactant-free emulsion copolymerization, was used as a compatibilizer for polystyrene-b-polybutadiene-b-polystyrene (SBS)-toughened polylactide (PLA) blends. Upon compatibilization, the blends exhibited a refined dispersed-phase morphology, a decreased crystallinity with an increase in their amorphous interphase, improved thermal stability possibly from the thicker, stronger interfaces insusceptible to thermal energy, a convergence of the maximum decomposition-rate temperatures, enhanced magnitude of complex viscosity, dynamic storage and loss moduli, a reduced ramification degree in the high-frequency terminal region of the Han plot, and an increased semicircle radius in the Cole&ndash, Cole plot due to the prolonged chain segmental relaxation times from increases in the thickness and chain entanglement degree of the interphase. When increasing the S-MA content from 0 to 3.0 wt %, the tensile properties of the blends improved considerably until 1.0 wt %, above which they then increased insignificantly, whereas the impact strength was maximized at an optimum S-MA content of ~1.0 wt %, hypothetically due to balanced effects of the medium-size SBS particles on the stabilization of preexisting crazes and the initiation of new crazes in the PLA matrix. These observations confirm that S-MA, a random copolymer first synthesized in our laboratory, acted as an effective compatibilizer for the PLA/SBS blends.

A poly(styrene-ran-methyl acrylate) (S-MA) (75/25 mol/mol), synthesized by surfactant-free emulsion copolymerization, was used as a compatibilizer for polystyrene-b-polybutadiene-b-polystyrene (SBS)-toughened polylactide (PLA) blends. Upon compatibilization, the blends exhibited a refined dispersed-phase morphology, a decreased crystallinity with an increase in their amorphous interphase, improved thermal stability possibly from the thicker, stronger interfaces insusceptible to thermal energy, a convergence of the maximum decomposition-rate temperatures, enhanced magnitude of complex viscosity, dynamic storage and loss moduli, a reduced ramification degree in the high-frequency terminal region of the Han plot, and an increased semicircle radius in the Cole–Cole plot due to the prolonged chain segmental relaxation times from increases in the thickness and chain entanglement degree of the interphase. When increasing the S-MA content from 0 to 3.0 wt %, the tensile properties of the blends improved considerably until 1.0 wt %, above which they then increased insignificantly, whereas the impact strength was maximized at an optimum S-MA content of ~1.0 wt %, hypothetically due to balanced effects of the medium-size SBS particles on the stabilization of preexisting crazes and the initiation of new crazes in the PLA matrix. These observations confirm that S-MA, a random copolymer first synthesized in our laboratory, acted as an effective compatibilizer for the PLA/SBS blends.

The research on multiple hydrogen bonds (H-bonds) hydrogels have gradually aroused wide interest. In this paper, a multiple H-bonds-reinforced poly(acrylamide-co-2-vinyl-4,6-diamino-2-vinyl-1,3,5-triazine)/tannic acid (P(Am-co-VDT)/TA) hydrogels are prepared. The results suggest that the prepared hydrogel has two types of H-bonds crosslinking regions: A "soft" region of H-bonds between the diaminotriazine (DAT) moieties on the polymer chains and the TA pyrogallol/catechol groups, and a "hard" region of H-bonds forming by DAT moieties with itself. The hard crosslinking region exhibits significantly higher activation energy than the soft region. Such soft and hard dual physically crosslinked networks dramatically enhance the mechanical properties of P(Am-co-VDT)/TA hydrogels in a synergistic manner (tensile strength is 2.34 MPa, elongation at break is 410%). Due to the multiple hydrogen bonds, the hydrogel has good pH sensitivity and rapid response to shape memory within a few minutes. In addition, the hydrogels have the capacity of physical adsorption of the anti-inflammatory drug diclofenac sodium and other molecules with a specific spatially arranged chemical composition. These hydrogels with high mechanical strength, excellent shape memory behavior, and capacity of adsorption of anti-inflammatory drug could be attractive candidates for applications in the fields of biomedicine, tissue engineering, and medical materials.

Rheological behaviour of suspensions of straw fibre in low density polyethylene (LDPE) melts was investigated at a certain temperature over a wide range of frequencies using a plate rheometer operated in dynamic mode. The viscoelastic of composite melt changed gradually with the straw fibre content increasing, for example at the low frequency region the slope of the G′ decreased from 1.67 of the plain LDPE to 0.889 of 50 wt-% straw fibre plastic composite. The addition of compatibiliser doubled the tensile strength, and the relaxation time reduced from 405 to 214 s. It was observed that the presence of compatibiliser may be attributed to improve interaction between the polymer and filler, which reduces the resistance of the agglomeration of straw fibres, lessening the relaxation time. Consequently, the straw fibre plastic composite is easier to produce.

A rapid prototyping technology, formed by three-dimensional (3-D) printing and then crosslinked by spraying Ca2+ solution, is developed to fabricate a sodium alginate (SA) hydrogel scaffold. The porosity, swelling ratio, and compression modulus of the scaffold are investigated. A friction mechanism is developed by studying the reproducible friction behavior. Our results show that the scaffold can have 3-D structure with a porosity of 52%. The degree of swelling of the SA hydrogel scaffold is 8.5, which is nearly the same as bulk SA hydrogel. SA hydrogel exhibits better compressive resilience than bulk hydrogel despite its lower compressive modulus compared to bulk hydrogel. The SA hydrogel scaffold exhibits a higher frictional force at low sliding velocity (10−6 to 10−3 m/s) compared to bulk SA hydrogel, and they are equal at high sliding velocity (10−2 to 1 m/s). For a small pressure (0.3 kPa), the SA hydrogel scaffold shows good friction reproducibility. In contrast, bulk SA hydrogel shows poor reproducibility with respect to friction behavior. The differences in friction behaviors between the SA hydrogel scaffold and bulk SA hydrogel are related to the structure of the scaffold, which can keep a stable hydrated lubrication layer.

With the deepening of research on high-strength hydrogels, the multi-functional study of hydrogels has become a hot spot. In this paper, a dual cross-linked physical high-strength hydrogels is prepared by a relatively simple method. 2-Vinyl- 4,6-Diamino-2-vinyl-1,3,5-triazine (VDT) induces the formation of the first cross-linking points through the interaction of hydrogen bonds with poly(acrylamide-co-acrylic acid) (PAm-co-Ac) chains, then the secondary physical cross-linkers Fe3+ that introduce ionic coordinates between Fe3+ and -COO- groups. Due to the synergistic effect of hydrogen bonding and ionic coordination, hydrogels possess high tensile strength (approx. 4.34 MPa), large elongation (approx. 17.64 times), and good healing properties under alkali solution after cutting into two pieces. Meanwhile, VDT contains diaminotriazine functional groups that easily form hydrogen bonds so that the polymer of hydrogels could absorb 5-fluorouridine. In addition, the contribution of ionic polymer segments enables pH to be sensitive to hydrogels and facilitates the adsorption of a large number of ionic monomers to form ionic conductive networks, the prepared hydrogel capacitor device has very high sensitivity to pressure and deformation, and can detect the movement behavior of the human body. The dual-physical cross-linked hydrogels had a selective adsorption to biological small molecules and could be assembled into a flexible wearable device with high sensitivity.

Structural and frictional behaviours of sodium alginate (SA)/polyacrylamide (PAAm) ionic-covalent hybrid, sequential double-network (DN) hydrogels against glass have been investigated in water, NaCl and CaCl2 aqueous solutions using a rotational rheometer. Dilution of adsorptive elastohydrodynamic friction for the PAAm covalent network with repulsive hydrodynamic lubrication for the minor SA ionic network was found to control the frictional stresses of the SA/PAAm gels within between those of the SA and PAAm single-network gels. A tentative qualitative model was proposed to describe the impact of ionic environmental solution on the frictional behaviour of the hybrid gel by selectively affecting the SA-network structure and friction. It was revealed that strong Debye shielding in the NaCl solution significantly reduced the thickness of the electric double layer for hydrodynamic lubrication of the SA network, which made the SA/PAAm gel's friction the highest among the three solutions. Dramatically increased ionic cross-linking of the SA network in the CaCl2 solution, although effectively mediated by the PAAm-network flexible skeleton, still functioned partially to conserve a portion of the SA fractional boundary-friction at the interface, making the friction of the hybrid gel intermediate among the three solutions. In contrast, extreme hydration of the SA network in water sharply increased the volume fraction of its unshielded hydrodynamic lubrication at the interface, which greatly reduced the SA/PAAm's friction to the lowest among the three solutions. We have thus incorporated for the first time both super-lubrication (frictional coefficients of below 10(-2) over low sliding-velocities of 3 × 10(-5) to 2 × 10(-3) m s(-1)) and previously reported high fracture energy (over 9000 J m(-2)) into a single ionic-covalent hybrid DN hydrogel, which is the SA/PAAm (∼1/8.5 w/w) gel in water. Effects of inversion of DN-formation sequence further indicated that frictional behaviours (i.e. frictional stress-sliding velocity profiles) of the hybrid sequential DN hydrogels (SA/PAAm and PAAm/SA), respectively, were primarily determined by those of the second networks (PAAm and SA), presumably due to the formation of first-second network "core-shell" structures at the blob scale. Frictional stress of the SA/PAAm gel was increased monotonically with external normal pressure at all of the sliding velocities investigated in the three solutions, which was in agreement with the predictions from the repulsion-adsorption model proposed by Gong et al.