ABSTRACT
The rapid development of advanced high-frequency mobile communication techniques has advanced urgent requirements for polymer materials with high-temperature resistance and good dielectric properties, including low dielectric constants (low-Dk) and low dielectric dissipation factors (low-Df). The relatively poor dielectric properties of common polymer candidates, such as standard polyimides (PIs) greatly limited their application in high-frequency areas. In the current work, benzoxazole units were successfully incorporated into the molecular structures of the fluoro-containing PIs to afford the poly(imide-benzoxazole) (PIBO) nano-fibrous membranes (NFMs) via electrospinning fabrication. First, the PI NFMs were prepared by the electrospinning procedure from organo-soluble PI resins derived from 2,2'-bis(3,4-dicarboxy-phenyl)hexafluoropropane dianhydride (6FDA) and aromatic diamines containing ortho-hydroxy-substituted benzamide units, including 2,2-bis[3-(4-aminobenzamide)-4-hydroxylphenyl]hexafluoropropane (p6FAHP) and 2,2-bis[3-(3-aminobenzamide)-4-hydroxyphenyl]hexafluoropropane (m6FAHP). Then, the PI NFMs were thermally dehydrated at 350 °C in nitrogen to afford the PIBO NFMs. The average fiber diameters (dav) for the PIBO NFMs were 1225 nm for PIBO-1 derived from PI-1 (6FDA-p6FAHP) precursor and 816 nm for PIBO-2 derived from PI-2 (6FDA-m6FAHP). The derived PIBO NFMs showed good thermal stability with the glass transition temperatures (Tgs) over 310 °C and the 5% weight loss temperatures (T5%) higher than 500 °C in nitrogen. The PIBO NFMs showed low dielectric features with the Dk value of 1.64 for PIBO-1 and 1.82 for PIBO-2 at the frequency of 1 MHz, respectively. The Df values were in the range of 0.010~0.018 for the PIBO NFMs.
ABSTRACT
Protection of intrinsically brittle quartz chromatographic columns (CCs) from breakage or property deterioration in gas chromatography (GC) analysis has become an important research topic regarding high-temperature GC techniques. Polyimide (PI) has proved to be the most suitable protective coating for quartz CCs. In the current research, a series of novel high-temperature-resistant PI coatings for quartz CCs operated over 320 °C have been successfully prepared. For this purpose, the aromatic diamine with a rigid skeleton structure 2-(4-aminophenyl)-5-aminobenzimidazole (APBI) was copolymerized with two aromatic dianhydrides-3,3',4,4'-benzophenotetracarboxylic acid dianhydride (BTDA) and 4,4'-oxydiphthalic anhydride (ODPA)-and an aromatic diamine with flexible ether linkages-4,4'-oxydianiline (ODA)-by a two-step polymerization procedure via soluble poly(amic acid) (PAA) precursors, followed by thermal imidization at elevated temperatures. The developed PI coatings exhibited good comprehensive properties, including glass transition temperatures (Tg) as high as 346.9 °C, measured by dynamic mechanical analysis (DMA), and coefficients of linear thermal expansion (CTEs) as low as 24.6 × 10-6/K in the range of 50-300 °C. In addition, the PI coatings exhibited good adhesion to the fused quartz capillary columns. No cracking, delamination, warpage, or other failures occurred during the 100-cycle thermal shock test in the range of 25-320 °C.
