Hongyan Yao

Design and synthesis of hyperbranched polyimide containing multi-triphenylamine moieties for memory devices

A novel triamine monomer, N,N′,N′′-tris(4-methoxyphenyl)-N,N′,N′′-tris(4-phenylamino)-1,3,5-benzenetriamine, was designed and synthesized. A hyperbranched polyimide (HBPI) was prepared by reacting the triamine monomer with 4,4-(hexafluoroisopropylidene)diphthalic anhydride for application in memory devices. The resulting HBPI exhibited excellent organo-solubility and high thermal stability. A memory device with a sandwich structure of indium tin oxide (ITO)/HBPI/Al was fabricated by using HBPI as an active layer. The device exhibited static random access memory (SRAM) behavior with a relatively low switching voltage of −1.90 V. Moreover, the device showed good stability in both the OFF and ON states, which could be retained as long as 1 × 104 s under a constant voltage stress of −1.00 V with an ON/OFF current ratio reaching up to 1 × 106. Molecular simulation results suggested that efficient charge transfer between the triamine moieties and hexafluoropropylidene phthalimides moieties in HBPI exist, which is responsible for the improved electrical memory performance. Such a HBPI was expected to be potentially useful in polymer memory applications.

Highly sulfonated co-polyimides containing hydrophobic cross-linked networks as proton exchange membranes

A novel diamine monomer bearing double hydrophobic cross-linkable tetrafluorostyrol side-groups has been successfully synthesized. Based on this monomer along with 4,4′-diaminodiphenyl ether-2,2′-disulfonic acid (ODADS) and 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTDA), a series of cross-linked highly sulfonated co-polyimide (CSPIy-6FATFVPx) membranes with IEC values ranging from 2.07 to 2.41 meq g−1 were prepared via a high temperature poly-condensation, followed by a thermal cross-linking reaction. The SPI were synthesized by high temperature polymerization. The CSPI membranes were obtained from the SPI membrane by thermal crosslinking reation. The polymerization and crosslinking reation were not performed in one pot. The CSPIy-6FATFVPx membranes showed significantly excellent performance, especially the high proton conductivity (0.153–0.210 S cm−1 at 80 °C), low water uptake (54.1%–87.1% at 80 °C) and swelling ratio (15.5%–23.0% at 80 °C). Furthermore, the CSPIy-6FATFVPx membranes also exhibited outstanding thermal stability (5% weight loss when the temperature exceed 320 °C), excellent hydrolytic stability and mechanical properties. The results indicate that CSPIy-6FATFVPx are promising candidates as proton exchange membranes in fuel cell technology.

Highly sulfonated co-polyimides containing cross-linkable hydrophobic tetrafluorostyrol side-groups for proton exchange membranes

A series of novel highly sulfonated co-polyimides (SPI-20s) bearing cross-linkable hydrophobic tetrafluorostyrol side-groups have been successfully synthesized. The cross-linking reaction of the tetrafluorostyrol groups in the SPI-20s was performed at 260 °C without any additive. The glass transition temperatures of the cross-linked membranes were determined by differential scanning calorimetry and dynamic mechanical analysis and found to be higher than the SPI-20s proving the formation of cross-linked networks. There was no elimination of sulfonic acid groups during cross-linking reaction. The cross-linked membranes (Cured-SPI-20s) showed significantly enhanced performances, particularly high proton conductivity (0.103–0.179 S cm−1 at 80 °C), low water uptake (31.2–53% at 80 °C) and swelling ratio (6.4–14.1% at 80 °C). Furthermore, the Cured-SPI-20s exhibited a greatly reduced methanol permeability (3.99–4.93 × 10−7 cm2 s−1), which was lower than that of Nafion 117 (2.94 × 10−6 cm2 s−1) at room temperature. The Cured-SPI-20 membranes also exhibited improved glass transition temperatures (254–277 °C), thermal stability (5% weight loss temperature exceed at 300 °C) and excellent oxidative stability, chemical resistance and mechanical properties. The results indicate that the Cured-SPI-20s were promising candidates as proton exchange membranes in fuel cell technology.

Microporous polyimide networks constructed through a two-step polymerization approach, and their carbon dioxide adsorption performance

Based on a dianhydride monomer 2,5-bis(3,4-dicarboxyphenoxy)-4′-phenylethynyl biphenyl (PEPHQDA) containing a crosslinkable phenylethynyl pendant group, microporous polyimide networks (HBPI-CRs) were prepared through a two-step pathway combining polymerization and crosslinking reactions. Specifically microporous features such as surface morphology, microporous structure and uniform nanometer-sized pore channels were introduced to the polyimide networks through this two-step pathway. The HBPI-CR networks exhibited a BET surface area (385–497 m2 g−1) as well as a comparable CO2 uptake (1.65–2.04 mmol g−1 at 273 K and 1 bar) and enthalpy of adsorption (28.6–30.0 kJ mol−1) to that of other microporous polyimides derived from rigid tri-dimensional monomers.

Phenylethynyl- and naphthylethynyl-terminated hyperbranched polyimides with low melt viscosity

In the past decades, 4-phenylethynyl phthalic anhydride has been the most commonly used endcapper for polyimide (PI). A series of different groups of terminated hyperbranched PIs (HBPIs) were synthesized from 1,3,5-tris(2-trifluoromethyl-4-aminophenoxy)benzene, bis[4-(3,4-dicarboxyphenoxy)phenyl]ether dianhydride, and 4-phenylethynylphthalic anhydride or 4-(1-naphthylethynyl)phthalic anhydride. The glass transition temperatures (T gs) of the polymers varied from 166°C to 193°C. The 5% weight loss temperatures of polymers were in the range of 548 to 571°C and those of the endcapped HBPIs increased to the range of 600 to 601°C after curing. The PIs exhibited rather low melt viscosity at processing temperature and the complex viscosity increased continuously with the temperature increase. Because there is a large temperature difference between the Tg and cross-linking exothermic temperature, HBPIs provided a wide processing window.