Chain-length dependence of PDMS’ solubility parameter and its correlation with surface tension, density, and viscosity

This work aims to clarify the understanding of polydimethylsiloxane fluids' (PDMS) solubility by reporting experimental measurements of the chain length dependence of the solubility parameter (δ), and the correlation of δ with density (ρ), surface tension (γ), and viscosity (η). Indeed, the few measurements of δ that are found in the literature for PDMS are often conflicting despite this information being critical for designing modern siloxane-based materials and coatings. γ, δ, and ρ were shown to display a chain length dependence which varies for molecular weights but reaches an asymptote at high molecular weights. This experimental chain length dependence of δ, and ρ, was appropriately fitted using a recent model originally presented for γ, and linear relationships between γ, δ, and ρ were highlighted, which could be exploited to estimate δ of PDMS from knowledge of γ or ρ. Additionally, an alternative method to determine δ from variations of ρ (not restricted to PDMS) was exposed by modifying Fedors’ relationship. Finally, the crossover from short chain behavior to long chain behavior was found to coincide with the entanglement molecular weight, but we point out that the cause of the crossover is not the entanglements themselves.