High grafting density polydimethylsiloxane polymer brushes: Synthesis and Characterization

Abstract

Surface modification of a substrate using an ultra-thin functional coating is a common approach for attaching or growing polymer chains. These polymer chains are called polymer brushes as one end of the chain is tethered to the substrate. In particular, polydimethylsiloxane (PDMS) brushes have garnered significant attention for their application in liquid-repellent surfaces due to their low surface energy, high segmental motion, and liquid-like behavior at room temperature. Factors like the grafting density, chain length, and uniform brush distribution play a key role in determining the conformation of PDMS brushes and their liquid repelling behavior. To fabricate PDMS brushes, either (1) self-assembled monolayers (SAMs) are used as a functional coating to attach pre-synthesized PDMS chains by the “grafting-to” method, or (2) free hydroxyl groups on the substrate surface are used for polymerization of siloxane compounds by the “grafting-from” method. Though the grafting-to method with SAMs offers an advantage over prior characterization, it offers less control over grafting density, with limited long-term stability of SAMs in the presence of various reagents. The polycondensation-based grafting-from method offers high grafting density but synthesizes uneven PDMS brushes with lower chain length. To overcome these challenges, we recently developed an approach for the fabrication of PDMS brushes from ultra-thin polymer coatings prepared by spin coating a random copolymer consisting of a polymerizable initiator (inimer), a crosslinker, and a nonreactive comonomer. This inimer- based coating approach, together with ring-opening polymerization, allows for control over the brush length and, more importantly, grafting density by varying the inimer content in the random copolymer. The thermal stability of the inimer and the mechanical stability of the inimer coating are analyzed by TGA and ultrasonication experiments. Surface characterization techniques like ellipsometry, XPS, and AFM are employed to characterize the PDMS chain length, the effect of inimer content on grafting density of PDMS brushes, and surface roughness. These fundamental studies would help in fabricating liquid-like PDMS brushes for liquid-repellent surfaces with better control over brush conformation.