Biological connective tissues, such as cartilage and corneal stroma, are essentially hydrogels consisting of fibrous collagen and proteoglycans. Little is known of the surface properties of the hydrogel, although we observe fascinating tribological behavior in biological soft tissues, such as extremely low friction between animal cartilages. We consider that the role of the solvated polymer network existing in the extracellular matrix as a gel state is critically important in the specific frictional behavior of cartilages. In order to elucidate the general tribological features of a solvated polymer matrix, the friction of various kinds of hydrogels has been investigated, and very rich and complex frictional behaviors are observed. The friction force and its dependence on the load differ with the chemical structure of the gels, surface properties of the opposing substrates, and the measurement conditions, which are totally different from those of solids. Most importantly, the coefficient of friction of gels, μ, varies over a wide range and exhibits very low values (μ
				≈ 10−3–10−4), which cannot be obtained from the friction between two solid materials. A repulsion–adsorption model has been proposed to explain the gel friction, which says that the friction is due to lubrication of a hydrated layer of polymer chains when the polymer chain of the gel is non-adhesive (repulsive) to the substrate, and the friction is due to elastic deformation of the adsorbed polymer chain when it is adhesive to the substrate.

Friction and lubrication of hydrogels—its richness and complexity

We have recently developed a novel type of gel called the slide-ring gel or topological gel that is different from physical and chemical gels by using the supramolecular architecture with topological characteristics. In this gel, polymer chains with bulky end groups exhibit neither covalently cross-links as in chemical gels nor attractive interactions as in physical gels but are topologically interlocked by figure-of-eight cross-links. Hence, these cross-links can pass along the polymer chains freely to equalize the tension of the threading polymer chains similarly to pulleys; this is called the pulley effect. The slide-ring gel is a new cross-linking concept for the polymer network as well as a real example of a slip-link model or sliding gel, which was previously considered only theoretically. In this study, we review the synthesis, structure, and mechanical properties of the slide-ring gels with freely movable cross-linking junctions based primarily on our recent studies. The pulley effect of the slide-ring gels has been recently confirmed by mechanical measurements, small-angle neutron scattering (SANS), small-angle X-ray scattering (SAXS), quasi-elastic light scattering (QELS), etc. This concept can be applied to not only gels but also to a wide variety of polymeric materials without solvents.

/pdf/novel-cross-linking-concept-of-polymer-network-synthesis-129fipqbyg.pdf

Novel Cross-Linking Concept of Polymer Network: Synthesis, Structure, and Properties of Slide-Ring Gels with Freely Movable Junctions

The fracture energy G of double network (DN) gels, consisting of poly(2-acrylamido-2-methylpropanesulfonic acid) (PAMPS) as the first network and poly(acrylamide) (PAAm) as the second network, was measured by the tearing test as a function of the crack velocity V The following results were obtained: (i) The fracture energy G ranges from 10(2) to approximately 10(3) J/m2, which is 100-1000 times larger than that of normal PAAm gels (10(0) J/m2) or PAMPS gels (10(-1) J/m2) with similar polymer concentrations to the DN gels (ii) G shows weak dependence on the crack velocity V (iii) G at a given value of V increases with decreasing of cross-linking density of the 2nd network The measured values of G were compared with three theories that describe different mechanisms enhancing the fracture energy of soft polymeric systems A mechanism relating to a heterogeneous structure of the DN gel is convincing for the remarkable large values of G

Determination of fracture energy of high strength double network hydrogels.

Synthesis of hydrogels with extremely low surface friction.

The dynamic aspect of double network (DN) gels showing an extremely high mechanical strength has been investigated by dynamic light scattering (DLS) measurements The DN gels are formed from highly cross-linked poly(2-acrylamido-2-methylpropanesulfonic acid) (PAMPS) as the first network and loosely cross-linked (or non-cross-linked) polyacrylamide (PAAm) as the second network The results of DLS show that the presence of slow mode besides the gel mode (fast mode) enhances the strength of DN gels at the low cross-linking density of the second network The dynamics of the slow mode cannot be explained in terms of reptational motion of the second component in the first network, but it is similar to the translational motion of PAAm polymers in a semidilute solution A strong velocity dependence of the mechanical strength is observed at a shear rate close to the inverse of the relaxation time of the slow mode These results suggest that large “voids” of the first network may exist, and PAAm polymers that exist

Structural Characteristics of Double Network Gels with Extremely High Mechanical Strength

The sliding friction of various kinds of hydrogels has been investigated, and it has been found that the frictional behaviors of these hydrogels do not conform to Amonton's law F = μW, which well describes the friction of a solid. Instead, the friction force F of the gel shows slight dependence on the load W in the investigated load range, while it strongly depends on the sliding velocity. Additionally, the frictional coefficient μ of the gel reaches a value as low as ∼10-3, which is smaller than those observed in solid materials.

Friction of gels

Frictions of several kinds of hydrogels, poly(vinyl alcohol) gel, gellan gel, poly(2-acrylamido-2-methylpropanesulfonic acid) gel, and its sodium salt gel, sliding on glass plates as well as on Teflon plates have been investigated both in air and in water. The frictional force and its dependencies on the load are quite different depending on the chemical structures of the gels, surface properties of the opposing substrates, and the measurement condition. The gel friction is explained in terms of interface interaction, either attractive or repulsive, between the polymer chain and the solid surface. Surface adhesion between glass particles and gels measured by atomic force microscopy showed a good correlation with the friction, which supports the polymer repulsion−adsorption model proposed by authors.

Friction of Gels. 3. Friction on Solid Surfaces

Surface friction of polymer gels: 1. Effect of interfacial interaction

The sliding friction of various kinds of hydrogels has been studied and it was found that the frictional behaviors of the hydrogels do not conform to Amonton's law F equals (mu) W, which well describes the friction of solids. The frictional force and its dependencies of on the load are quite different depending on the chemical structures of the gels, surface properties of the opposing substrates, and the measurement condition. The gel friction is explained in terms of interfacial interaction, either attractive or repulsive, between the polymer chain and the solid surface. According to this model, the frictional is ascribed to the viscous flow of solvent at the interface in the repulsive case. In the attractive case, the force to detach the adsorbing chain from the substrate appears as friction. Surface adhesion between glass particles and gels measured by AFM showed a good correlation with the friction, which support the repulsion- adsorption model proposed by authors.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Friction of polymer gels and the potential application as artificial cartilage

The sliding friction of various kinds of hydrogels has been studied and it was found that the frictional behaviors of the hydrogels do not conform to Amonton's law F=μW, which well describes the friction of solids. The frictional force and its dependencies on the load are quite different depending on the chemical structures of the gels, surface properties of the opposing substrates, and the measurement condition. The gel friction is explained in terms of interfacial interaction, either attractive or repulsive, between the polymer chain and the solid surface. According to this model, the frictional force is ascribed to the viscous flow of solvent at the interface in the repulsive case. In the attractive case, the force to detach the adsorbing chain from the substrate appears as friction. Surface adhesion between glass particles and gels measured by AFM showed a good correlation with the friction, which support the repulsion-adsorption model proposed by authors.

Yoshiyuki Iwasaki

Papers

Friction of gels

Friction of Gels. 3. Friction on Solid Surfaces

Surface friction of polymer gels: 1. Effect of interfacial interaction

Friction of polymer gels and the potential application as artificial cartilage

Surface friction of hydrogels