Adhesion of polydimethylsiloxane during molecular cross-linking

The transformation of polydimethylsiloxane (PDMS) material from a non-cross-linked liquid to a fully cross-linked solid state is investigated from the point of view of contact mechanics according to the classical Johnson-Kendall-Roberts scheme (JKR). The PDMS material sample was produced by mixing two liquids, namely PDMS base and cross-linking agent in a predefined ratio, and a further continuous measurement of the adhesion was performed during a chemical cross-linking reaction. It is demonstrated how adhesion between a glass ball and a PDMS sample is dependent on the PDMS cross-link density. The sample was in the liquid state for the first 16 hours. In the liquid state, the adhesive interaction can be quantitatively described as the result of the formation of a capillary bridge with an insignificant influence of the viscosity of the liquid up to the gel point. Then the sample turned into a soft gel-like structure, which corresponds to the formation of a percolated molecular network. It is shown that during the transformation of a liquid into a gel, its viscosity increases by several orders of magnitude (similar to 3 orders of magnitude in about 8 hours). The adhesive force as a function of time also shows a sharp increase when approaching the gel point of PDMS, where PDMS manifests itself as a tacky material, similar to pressure sensitive adhesives. During this transition period, the contact mechanics is characterized by the formation of "strings" and irreversible deformation, which are not observed either in the initial liquid state or in the completely bound state. Furthermore, the gel PDMS material continues to solidify, that is associated with changing of dynamical parameters of contact interaction as well as decreasing of adhesive "wear" of the PDMS surface.