High Thermally Stable and Melt Processable Polyimide Resins Based on Phenylethynyl-Terminated Oligoimides Containing Siloxane Structure

A series of 4-phenylethnylphthalic anhydride (PEPA)-terminated oligoimides were prepared by co-oligomerizing isomeric dianhydrides, i.e., 2,3,3′,4′-biphenyltetracarboxylic dianhydride (a-BPDA), 2,3,3′,4′-benzophenonetetracarboxylic dianhydride (a-BTDA) or 2,3,3′,4′-diphenylethertetracarboxylic dianhydride (a-ODPA), with diamines mixture of bis(4-aminophenoxy)dimethyl silane (APDS) and 2,2′-bis(trifluoromethyl) benzidine (TFDB). The effects of siloxane content and dianhydride structure on the rheological properties of these oligoimides and thermal stability of the corresponding cured polyimide resins were investigated. The results indicated that the introduction of the siloxane structure improved the melt processability of the oligoimides, while the thermal stability of the cured polyimide resins reduced. The oligoimide derived from a-ODPA revealed better melt processability and melt stability due to the existence of a flexible dianhydride structure. The oligoimide PIS-O10 derived from a-ODPA gave the lowest minimum melt viscosity of 0.09 Pa·s at 333 °C and showed the excellent melt stability at 260 °C for 2 h with the melt viscosity in the range of 0.69–1.63 Pa·s. It is also noted that the thermal stability of these resins can be further enhanced by postcuring at 400–450 °C, which is attributed to the almost complete chemical crosslinking of the phenyethynyl combined with oxidative crosslinking of siloxane. The PIS-T10 and PIS-O10 resins that were based on a-BTDA and a-ODPA, respectively, even showed a glass transition temperature over 550 °C after postcuring at 450 °C for 1 h.

Phenylethynyl-terminated oligoimides with ultra-low melt viscosity derived from 1,4-bis(3,4-dicarboxy phenoxy)benzene dianhydride

Most of the existing polyimide oligomers for resin transfer molding (RTM) processing exhibited high melt viscosity, which can only maintain below 1 Pa·s at 280°C for 2 h, leading to very high process temperatures. So novel RTM-type oligomers with lower and stable melt viscosities are more desirable. Three series of thermoset oligoimides derived from 1,4-bis(3,4-dicarboxy phenoxy)benzene dianhydride and three different aromatic diamines were prepared herein. The diamines included 4,4′-oxydianiline, 2,2′-bis(trifluoromethyl)benzidine (TFDB), and 2-phenyl-4,4′-diaminodiphenyl ether ( p-ODA). 4-Phenylethynylphthalic anhydride was used as an endcapping reagent. Effects of the chemical structures and molecular weights of the oligoimides on their aggregated structures, melt processability, and the thermal and mechanical properties of the cured films were then systematically investigated. X-Ray diffraction results indicated that ODA series oligoimides and TFDB series oligoimides showed crystallinity in various degrees. However, the asymmetric p-ODA enables the p-ODA series oligoimides to exhibit amorphous forms. It was found that the p-ODA-based oligoimide with a molecular weight of 750 g mol−1 showed very low melt viscosity of 1 Pa·s even at 210°C, and the melt viscosity could maintain below 1 Pa·s after isothermal aging for 2 h at any temperature in the range of 200–280°C by rheological measurements. The cured film also showed a high glass transition temperature of 355°C by dynamic mechanical analysis, very good thermal stability by thermogravimetric analysis, and good mechanical properties. It might be more suitable for RTM processes in the future.

Synthesis and characterization of a carborane-containing monofunctional imide monomer as a modifier for imide oligomer

A novel carborane-containing imide monomer (CB-PEPA) was synthesized and used as a modifier for phenylethynyl-terminated imide oligomer (PI-mPDA) to study the effect on the performance of the oligomers and thermosetting polyimide materials. The curing behavior of the blends of CB-PEPA and PI-mPDA was investigated by differential scanning calorimetry (DSC). It showed that CB-PEPA had a melting point at 183°C. Nonisothermal DSC showed that imide monomer had higher activation energy than that of bifunctional imide oligomer. Rheometer study showed that the monomer CB-PEPA could be used as a reactive diluent for PI-mPDA to reduce the minimum melt viscosity and broaden the process window of imide system. Thermogravimetric analysis showed that the char yield of resin increased with the addition of CB-PEPA. The resin derived from a 25 wt% CB-PEPA loading in PI-mPDA exhibited a high char yield of 81.0% at 800°C in nitrogen. Due to the steric hindrance and rigid group of carborane, the Tg of composites also increased with the addition of CB-PEPA. Composites derived from a 10 wt% CB-PEPA loading in PI-mPDA exhibited the relatively higher flexural strength and interlaminar shear strength.