Abstract
Since human body joints have a gel-like structure with low friction that persists for several decades, hydrogels have attracted much interest for developing low-friction materials. However, such advantages can hardly be realized in industrial usage because water in the gel evaporates easily and the gel deswells. The substitution of water with an ionic liquid (IL) is one of the effective ways to overcome this problem. In this study, we substituted water in a double network (DN) hydrogel with 3-ethyl-1-methyl-imidazolium ethylsulfate (EMI-EtSulf), a hydrophilic IL, via a simple solvent exchange method to obtain a DN ion gel. A compressive test and thermogravimetric analysis showed that the DN ion gel has a high compression fracture stress and improved thermal properties, with the difference in 10% loss of temperature being ΔT10 = 234 °C. A friction test conducted using a reciprocating tribometer showed that the friction of a glass ball/DN ion gel was relatively higher than that of a glass ball/DN hydrogel. Because the minimum coefficient of friction (COF) value increased after substitution, the increase in polymer adhesion caused by the electrostatic shielding of the surface moieties of glass and poly 2-acrylamidomethylpropanesulfonic acid (PAMPS) was considered the main contributor to the high friction. As the COF value decreased with increasing temperature, the DN ion gel can achieve low friction via the restriction of polymer adhesion at high temperatures, which is difficult in the DN hydrogel owing to drying.
Highlights
Human body joints have a coefficient of friction (COF) as low as 10−3 at pressures exceeding atm, and they can maintain their lubrication for several decades
Compression tests revealed that the double network (DN) ion gel was relatively softer than the DN hydrogel, indicating backbone
Compression tests revealed that the DN ion gel was relatively softer than the DN hydrogel, the effect of electrostatic shielding of sulfate moieties on the poly 2-acrylamidomethylpropanesulfonic acid (PAMPS) network
Summary
Human body joints have a coefficient of friction (COF) as low as 10−3 at pressures exceeding atm, and they can maintain their lubrication for several decades. They are considered a useful model for designing low-friction materials [1]. Gels are soft materials comprising cross-linked polymers and a large amount of swelling agent. They have a high solvent content and a flexible structure; in addition, various functionalities, such as shape memory [5] and self-oscillation [6], can be realized through appropriate design of the chemical structure of the polymer backbone or solvent. The present study aims to clarify gel properties and the potential application of gels as artificial biomaterials
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