Molecular Structure of Omniphobic, Surface-Grafted Polydimethylsiloxane Chains

The unique surface properties of grafted polydimethylsiloxane (PDMS) chains, particularly their omniphobicity and low friction, are influenced by molecular structure and tethering density. Despite molecularly smoothness and homogeneity, these surfaces exhibit significant variability in wettability and contact angle hysteresis (CAH). This work uncovers the molecular structure of grafted PDMS chains. Grafted PDMS chains synthesized using a difunctional chlorosilane initiator, which exhibits CAH <2° on silicon wafers, adopt a brush-to-mushroom conformation with a molecular weight ≈7,800 g mol^-1, a grafting density of 0.22 ± 0.4 chains nm^-2, and a thickness of ≈3 nm. Each PDMS chain terminates with a silanol group, and ≈96% of substrate silanols remain unreacted. The presence of these terminal silanols is confirmed with time-of-flight secondary ion mass spectroscopy, as is their removal when exchanged for trimethylsilyl groups, both on the substrate and terminating the PDMS chains. Quartz crystal microbalance with dissipation measurements show that this "capping" procedure exchanges ≈1.5 silanols nm^-2; capping occurs at the substrate and PDMS chain end. The findings suggest that grafted, capped PDMS chains of this molecular weight are able to achieve excellent omniphobic properties even when the majority of surface silanols remain unreacted, which may aid in the design of future omniphobic materials.