Influence of precursor molecular weight on swelling and elastic modulus of cross-linked PDMS films

The influence of precursor molecular weight on the structure and properties of cross-linked poly(dimethylsiloxane) (PDMS) thin films was investigated at a constant cross-linker ratio. Linear telechelic PDMS precursors with molar masses ranging from 0.8 to 10 kg mol^-1 were cured into supported films and characterized by ellipsometry, solid-state ²⁹Si NMR, and nanoindentation. Swelling experiments in n-hexane were analyzed within the Flory-Rehner framework, using a concentration-dependent interaction parameter χ(ϕ). This approach enabled the estimation of the molecular weight between cross-links (M_c) and its dependence on precursor chain length. Solid-state ²⁹Si NMR confirmed the consumption of trimethoxysilane end-groups and the formation of Si-O-Si junctions, while also revealing restricted mobility near cross-linking sites. Nanoindentation provided independent measurements of Young's modulus, which were compared with predictions from the Flory-Rehner model. The results demonstrate that increasing precursor molecular weight decreases cross-link density, enhances swelling, and reduces modulus, providing a consistent picture of the network architecture in thin PDMS films. Beyond polymer physics, this strategy offers a simple means to modulate elasticity without altering surface chemistry, with potential implications for applications such as biointerfaces, microfluidics, and soft lithography.