Effects of degree of cross-links on adhesion curves of cross-linked polymers observed by a point-contact method.

The effects of degree of cross-links on the adhesion properties of polydimethylsiloxane (PDMS) were investigated with the use of a point-contact method in air at room temperature. The degree of cross-links, rho, was varied in a wide range, inducing changes in the rheology of the cross-linked PDMS from viscous solution to elastic gel, and the adhesion properties were examined by changing the separation velocity under a constant normal load and waiting period prior to separation. As a result, the adhesion force, F(A), strongly depended on rho and showed a maximum at a characteristic rho around the loss tangent (tan delta) = 1. In addition, the shapes of the adhesion curves were found to depend strongly on rho, and they were categorized into four types. The different states of cross-linked polymers, i.e., elastic-gel type, weak-gel type, and solution type, showed the different shapes. In the case of the weak gel, two types of characteristic adhesion curves were obtained, which resulted from the different separation mechanisms with and without cavitations near the surface due to the interface and cohesive failures. In the intermediate range of rho, viscous or elastic fingering was detected, which resulted in the characteristic shape of the adhesion curve. The factors determining the type of the adhesion curves are discussed in terms of the microscopic surface, bulk properties, and the different shapes of the adhesive front at the contact surface.

Swelling behavior of poly(sodium acrylate) gels crosslinked by aluminum ions.

The cylindrical poly(sodium acrylate) gel (SA gel) was synthesized in the glass capillary using aluminum ions as the crosslinker. The swelling ratio of the gel was measured after the repeated exchange of solvent (distilled deionized water, about pH 5.8). The gel exhibited two relaxation processes; at first the gel swells rapidly as exchange of water (the swelling process), then shrinks very slowly (the shrinking process). In order to reveal the microscopic structural change (especially, the formation of hydrogen bonding) by water exchange, attenuated total refraction (ATR) Fourier transform infrared (FT-IR) spectroscopy was applied to the gels with different swelling ratio. The IR absorption peaks of the gel were assigned based on those of poly(sodium acrylate) aqueous solutions at different pH. On the swelling process, the carboxyl groups were gradually protonated, and the intermolecular hydrogen bonding started to form in the gel with maximum swelling ratio. On the shrinking process, the formation of hydrogen bonding gradually increased with long-time repeated water exchange which resulted in the shrinkage of the gel. Effects of the repeated water exchange on the swelling behavior were discussed in terms of the exchange of counter ions and the formation of hydrogen bonding.

[X-ray crystallographic analysis of picrates].

Picric acid forms stable picrates with various organic molecules through pi- bonding or ionic bonding, and such picrates have been very useful for identification and qualitative analysis. As it seemed desirable to determine the crystal structures and the bonding mode of picrates of basic organic compounds, we have investigated the crystal structures of aromatic hydrocarbons, aromatic amino compounds, heterocyclic compounds and so on. A series of our studies on the crystal structure of basic organic compounds have shown that the complexes of picric acid and aromatic hydrocarbons are formed through pi-bonding, and those of aromatic heterocyclic compounds are formed through ionic and hydrogen bonding; in addition, some of them also have pi-bonding.