Multifunctional polyimides by direct silyl ether reaction of pendant hydroxy groups: Toward low dielectric constant, high optical transparency and fluorescence

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Unsatisfied overall properties and high-cost production are major obstacles for the commercialization of polyimide (PI) films. This work aims to develop a convenient way with low cost to prepare multifunctional PI films with excellent combined properties of low dielectric constant (k), high transparence and fluorescence by the direct structural modification on easily available PIs containing active groups. Thus, a series of fluorene-based PIs with tert-butyldimethylsiloxy (TBS) side groups (PI-TBSs) were successfully synthesized by a direct silyl ether reaction of hydroxyl-containing PIs (PI-OHs) with tert-butyldimethylsilyl chloride (TBSCl). The resulted PI-TBSs with high yield are readily soluble in many common solvents, even in low-boiling-point solvents at room temperature. Due to the presence of the cardo-type fluorene structure, PI-TBS films still possess the excellent thermal properties with a glass transition temperature (Tg) of 311.2–338.9 °C and a 5% weight loss temperature (T5%) of 469.5–506.4 °C in N2 and the good mechanical properties with a tensile strength (TS) of 57.7–79.1 MPa. In addition, PI-TBS films exhibit the light color with the high optical transparence (over 80% at 450 nm), the low dielectric constant (as low as 2.44 at 1 MHz for PI-TBS-2F) and the bright fluorescence because of the reduced charge transfer (CT) interaction in PI chains chemically bonded by bulky non-polar TBS groups. As the result, this work provides a convenient way to prepare multifunctional PI films with the excellent overall properties, which show high potential applications of these PI films in microelectronic, electric and photonics industries.