Tuning the Bulk and Surface Properties of PDMS Networks through Cross-Linker and Surfactant Concentration

The elastic modulus and hydrophilicity of cross-linked poly(dimethylsiloxane) (PDMS) are tunable via cross-linker concentration and the addition of a simple surfactant, C₁₂E₄, before curing. However, the surfactant concentration, [C₁₂E₄], reduces the elastic modulus (73% lower for 6.3% w/w) because it reduces the extent of curing. This is likely because the hygroscopic surfactant results in water poisoning of the catalyst. Three distinct time-dependent hydrophilicity profiles were identified using water contact angle analysis with [C₁₂E₄] determining which profile was observed. This indicates the concentration-dependent phase behavior of C₁₂E₄ within PDMS films. Changes in phase behavior were identified using small-angle neutron scattering (SANS) and a compatibility study. No surface excess or surface segregation of surfactant was observed at the PDMS-air interface. However, a surface excess revealed by neutron reflectivity against a D₂O interface indicates that the increase in hydrophilicity results from the migration of C₁₂E₄ to the film interface when exposed to water.