Contact creep behavior of polydimethylsiloxane and influence of load, tip size, and crosslink density

Time-dependent indentation creep behaviors of polydimethylsiloxane (PDMS) samples of different crosslink densities were studied through contact creep tests loaded with silica tips. Step loads from 0.1 to 10 mN were applied and held for 600 s. The data of penetration depth versus time were recorded during the holding period. A Hertz-type viscoelastic model was used to compute the creep compliance of the samples and the Johnson-Kendall-Roberts (JKR) theory was used to obtain the initial equivalent modulus, infinite equivalent modulus, and work of adhesion between the tested each pair of the PDMS and fused silica tip surfaces. The comparison between initial and infinite equivalent moduli obtained from the Hertz viscoelastic theory and the JKR theory shows that the adhesion between the tip and the sample surface plays an importance role in affecting the analysis results when the indentation strain is small. The influences of crosslink density, applied load, and tip size on the localized PDMS properties are discussed.