Measurement of adhesion between soft elastomers with different mixing ratios

The JKR method is widely used to measure the work of adhesion between soft materials. In this report, the JKR theory is summarized and three dimensionless parameters are proposed as prerequisites to determine sample dimensions in designing experiments. Also, the work of adhesion between two commonly used soft elastomers PDMS (Sylgard 184) and Ecoflex 0300 are obtained with the measured pull-in and pull-off forces from a dynamical mechanical analyzer. The Young’s moduli of pristine PDMS are also calculated with a two point formula and the results are compared with that from tensile tests. Our results for the work of adhesion of pristine PDMS 10:1 agree well with those reported in the literature. The pull-off work of adhesion of pristine PDMS increases significantly as the mixing ratio increases from 10:1 to 20:1. With further increasing mixing ratios, the pull-off work of adhesion does not change much. However, for PDMS samples extracted with chloroform, the pull-off work of adhesion increases monotonically as the mixing ratio varies from 10:1 to 50:1. A similar trend is also observed for the case of contacts between pristine PDMS lenses and Ecoflex substrates. For the pull-in work of adhesion, the results are almost independent of the mixing ratios. An adhesion mechanism is proposed to explain these complex adhesion behaviors. It is concluded that the entanglement with each other and penetration into networks of tethered chains during the contact could enhance the pull-off work of adhesion. With both ends uncross-linked, free chains do not enhance the pull-off work of adhesion as significant as tethered chains. Therefore, for pristine PDMS with higher mixing ratios, the existence of more free chains reduces the chance of the entanglement and penetration of tethered chains, which compensates for the enhancement by more tethered chains.