Direct Measurement of Interfacial Interactions Between Semispherical Lenses and Flat Sheets of Poly( Dimethylsiloxane) and Their Chemical Derivatives
Author(s): Chaudhury, Manoj K., George M. Whitesides
Journal: Langmuir (1991) 7: 1013-1025.
Abstract:
The deformations resulting on contacting small (1-2 mm) semispherical lenses of elastomeric poly-(dimethylsiloxane) (PDMS) with the flat sheets of this material were measured in air and in mixtures of water and methanol. The measurements in air were carried out in two ways: as a function of external loads, and under zero load but with variations in the sizes of the lenses. The measurements in liquids were carried out under zero load and varied the composition of the liquid mixtures. These experimental data were analyzed by using a theory of Johnson, Kendall and Roberts to obtain the works of adhesion between PDMS surfaces in the air and liquid media. The strength of interaction between PDMS surfaces decreased in mixtures of water and methanol as the concentration of methanol increased. A small interaction persisted even in pure methanol. The interfacial free energies (γsl) of the PDMS-liquid interfaces obtained from these measurements, together with the contact angles of these liquids on PDMS, were analyzed by using Young's equation. This analysis provided an estimate of the surface free energy of the polymer (γsv) that was consistent both with the value obtained from measurements made in air and with the value estimated from the analysis of the contact angles of nonpolar liquids on PDMS using the Good-Girifalco-Fowkes equation. This research also developed ways to modify the surface of PDMS chemically and thus to control its properties. The chemically derivatized poly(dimethylsiloxanes), in the form of lenses and flat sheets, were subjected to load-deformation measurements similar to those used for unmodified PDMS. These functionalized PDMS surfaces exhibited hysteresis in contact deformations, whereas no hysteresis was detected for unmodified PDMS. The origin of this hysteresis is not clear at present. The observation of hysteresis at solid-solid interfaces is relevant to understanding adhesion to these surfaces.