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.

Temperature-dependent absorption/desorption behavior of lower critical solution temperature (LCST) polymers on various substrates

We have been studying adsorption and retention (resistance to desorption) behavior of temperature sensitive LCST polymers on different substrates as a function of temperature. According to our studies with Poly 64 (a copolymer of 60% (mol) NIPAAm and 40% (mol) NnBAAm, LCST = 8.5 degrees C in water), the copolymer retention depends on the rinse temperature. When the rinse temperature is above the LCST, the polymer adheres well to most surfaces. On the contrary, at rinse temperatures below the LCST, most of the adsorbed polymer is easily rinsed off. These studies are relevant to our work on the thermally reversible adsorption of LCST polymers conjugated to peptides and proteins, such as affinity ligands, for uses in immunoassays and affinity separations. The interaction between the LCST polymer and most hydrophobic polymer surfaces is mainly due to hydrophobic interactions, and the critical surface tension (gamma c) and the solubility parameter (delta) of the solid polymer substrate are the most important factors which influence the LCST polymer adsorption and retention. The critical surface tension appears to correlate best with the LCST polymer adsorption levels on different substrates, while the solubility parameter correlates best with the retention of the adsorbed polymer. According to our preliminary study, n-butyl groups probably interact more strongly with the substrates than isopropyl groups because of the greater hydrophobic surface area of the former groups.