Molecular Structure of Omniphobic, Surface-Grafted Polydimethylsiloxane Chains

Area of Science:

• Surface Science
• Polymer Chemistry
• Materials Science

Background:

• Polydimethylsiloxane (PDMS) chains exhibit unique surface properties like omniphobicity and low friction.
• Surface properties are influenced by molecular structure and tethering density.
• Variability in wettability and contact angle hysteresis (CAH) exists despite surface homogeneity.

Purpose of the Study:

• To uncover the molecular structure of grafted PDMS chains.
• To understand the role of terminal silanol groups in PDMS surface properties.
• To investigate the impact of 'capping' procedures on PDMS chain conformation and surface characteristics.

Main Methods:

• Synthesis of grafted PDMS chains using a difunctional chlorosilane initiator.
• Characterization of chain conformation, molecular weight, grafting density, and thickness.
• Time-of-flight secondary ion mass spectroscopy (ToF-SIMS) to detect terminal silanol groups.
• Quartz crystal microbalance with dissipation (QCM-D) measurements to quantify silanol exchange during capping.

Main Results:

• Grafted PDMS chains adopted a brush-to-mushroom conformation (MW ≈7,800 g mol⁻¹, density ≈0.22 chains nm⁻², thickness ≈3 nm).
• Chains terminated with silanol groups, with ≈96% of substrate silanols remaining unreacted.
• A 'capping' procedure using trimethylsilyl groups removed terminal silanols and exchanged ≈1.5 silanols nm⁻² at the substrate and chain ends.
• Low CAH (<2°) was observed on silicon wafers with these grafted PDMS chains.

Conclusions:

• Grafted, capped PDMS chains achieve excellent omniphobic properties.
• The presence of terminal silanol groups is crucial for omniphobicity, even with unreacted substrate silanols.
• Findings can guide the design of advanced omniphobic materials.