Contributions of network topological structures to the mechanical properties of PDMS elastomers

Silicone elastomers (poly dimethyl siloxane, PDMS) are widely used in the fabrication of MEMS devices and in mechanobiological studies. Mechanical properties of the elastomers may be varied by changing the crosslinking density and physical entanglements in the networks. Other factors like dangling chains and uncrosslinked sol fractions also influence the network mechanics. We investigated the role of crosslinks, entanglements and sol fraction in the mechanical properties of PDMS prepared with different ratios of base to crosslinker. Results from compression tests show greater elastic modulus for the highly crosslinked networks. We removed uncrosslinked sol, altered the entanglement density with xylene extraction, and used data from mechanical tests to assess predictions from the Frenkel-Flory-Rehner model. These studies show that the polymer-solvent interaction parameter, χ, varied linearly with the equilibrium volume fraction. Dynamic mechanical analyses of extracted PDMS samples showed that crosslinking density, solvent presence, and sol fraction affect their overall viscoelasticity properties. We also compared experimental data with scaling laws and demonstrate that trapped entanglements in the networks contribute to the mechanical properties of PDMS. Finally, we show from these studies that swelling behaviors of PDMS in xylene follow affine deformation.