Low-surface-free-energy materials based on polybenzoxazines.

Hydrophobicity and oleophobicity are highly desirable properties for materials that are used in a wide range of applications. In this context, poly(tetrafluoroethylene) (teflon) is considered to be the “benchmark” low-surfacefree-energy material as it combines water repellence with other desirable properties. Unfortunately, the use of teflon comes with certain disadvantages, such as those resulting from its low oil repellence, high cost, and poor processability. Considerable research efforts have therefore been directed towards the development of nonwettable, low-surface-freeenergy polymeric materials that exhibit good film-forming characteristics. 15] Intraand intermolecular interactions are important for the surface properties of polymers. Jiang et al. have reported that at temperatures above the lower critical solution temperature (LCST), the compact, collapsed conformation of poly(N-isopropylacrylamide) (PNIPAAm) chains induced by intramolecular hydrogen bonding between the C=O andN H groups in the main PNIPAAm chains leads to a low surface free energy and high contact angle for water. When the temperature is below the LCST, however, the predominantly intermolecular hydrogen bonding between the main PNIPAAm chains and water molecules leads to a higher surface free energy and lower water contact angle. In addition, Chung et al. have reported that the presence of amide groups in a fluorinated-main-chain liquid-crystalline polymer system tends to induce strong intermolecular hydrogen bonding, which results in higher surface free energies and higher degrees of hydrophilicity. According to these previous studies, it appears that intramolecular hydrogen bonding tends to lower the surface free energy more than intermolecular hydrogen bonding between main polymer chains. Polybenzoxazines have many unique properties, such as low water absorption, high moduli, and high glass-transition temperatures. It is generally believed that hydrogen bonding between polybenzoxazine polymer chains plays an important role in promoting these attractive properties. We report here that these readily available polybenzoxazines display extremely low surface free energies, even lower than that of pure teflon. We used two bifunctional benzoxazine monomers, namely 2,2-bis(3-phenyl-3,4-dihydro-2H-1,3-benzoxazinyl)propane (BA-a) and 2,2-bis(3-methyl-3,4-dihydro-2H-1,3-benzoxazinyl)propane (BA-m). By spin-coating, we produced films that were subsequently cured in an oven to obtain ultrathin films with a thickness of about 150 nm. Table 1 lists the surface