Engineering poly(dimethylsiloxane) grafted chains for enhanced droplet mobility and abrasion resistance

Abstract

This thesis presents a comprehensive study of surface-grafted polydimethylsiloxane (PDMS) brushes, focusing on their structure-property relationships and potential applications as omniphobic coatings. By tuning molecular parameters such as chain length, grafting density, and terminal functionality, the wetting behavior, mechanical durability, and contact angle hysteresis (CAH) of PDMS surfaces were systematically analyzed. These coatings aim to replace the traditional fluorinated materials, offering a more environmentally friendly alternative for omniphobic applications. The study begins with a background on oleophobic coatings, highlighting PDMS brushes as an ideal candidate due to their unique properties. Current techniques for characterizing polymer brushes at the nanoscale are reviewed, addressing challenges in determining their structural and interfacial properties. Using a vapor deposition method, omniphobicity was demonstrated on smooth gold surfaces grafted with PDMS chains. Cleaving these chains for molecular weight analysis revealed a value of ~7800 g/mol, corresponding to a transition between the mushroom and brush regimes, with ~96% of the surface remaining unreacted. Terminal hydrophilic silanol groups were identified in the uppermost layer, which were partially capped with trimethylsilyl groups to enhance hydrophobicity. High dynamic droplet friction, a common issue with PDMS brushes, was mitigated by capping silanol groups. This reduced chain relaxation times from seconds to milliseconds, significantly improving droplet mobility. Additionally, altering the surrounding vapor to pentane enhanced droplet velocity on a 10° tilted surface by over 50 times compared to air. Addressing low abrasion resistance, thicker PDMS brush layers were achieved through vapor deposition, which led to the formation of nanodroplets via autophobic dewetting. Stepwise growth of these nanodroplets was controlled, resulting in enhanced abrasion tolerance and improved liquid repellency, withstanding up to 1,000 abrasion cycles, whereas a PDMS-grafted surface without nanodroplets lost its omniphobicity after 500 cycles. This thesis demonstrates that surface-grafted PDMS brushes are a promising material for next-generation omniphobic coatings, offering potential solutions for a range of applications, including self-cleaning surfaces, anti-fouling coatings, and liquid-repellent materials. The findings provide a pathway for the development of durable, fluorine-free alternatives to existing surface technologies, with a focus on sustainability and long-term performance.