Processable Polyimides with High Glass Transition Temperature and High Storage Modulus Retention at 400°C

Low-melting Phthalonitrile Oligomers: Preparation, Polymerization and Polymer Properties

A series of low-melting phthalonitrile oligomers were prepared in which variable-length multiple aromatic ether linkages interconnect the terminal phthalonitrile units. These materials were designed to address the need for a processable resin system with good high-temperature properties. The melt-processable oligomers are obtained using a modified-Ullman ether reaction between a bisphenol and a dihalobenzene to form a hydroxyl-terminated oligomeric intermediate that is endcapped by reaction with 4-nitrophthalonitrile. Viscosity measurements show that the phthalonitrile oligomers are polymerized at a moderate temperature (200°C) using the typical aromatic diamine curing additives, bis[4-(4-aminophenoxy)phenyl]sulfone and 1,3-bis(3-aminophenoxy)benzene. The oligomeric phthalonitrile/diamine mixtures exhibit a low complex melt viscosity (0.01-0.1 Pa s) at 200°C. Differential scanning calorimetric analysis is used to follow the polymerization as the oligomeric phthalonitrile/diamine mixtures are heated to elevated temperatures. Thermal and dynamic mechanical properties of thermally-cured oligomeric phthalonitrile polymers are systematically evaluated and compared with those of two other high temperature thermosetting phthalonitrile polymers, 4,4~-bis(3,4-dicyanophenoxy)biphenyl and 1,3- bis(3,4-dicyanophenoxy)benzene. After thermal treatment at 425°C for 8 h, the oligomeric phthalonitrile polymers exhibit char yields of 70% when heated to 1000°C in flowing nitrogen and decomposition temperatures in excess of 500°C when heated in either flowing nitrogen or air. Rheometric measurements indicate that the fully cured oligomeric phthalonitrile polymers do not soften or exhibit a glass transition temperature upon heating to 450°C. Overall, studies on the phthalonitrile oligomers and the corresponding polymers reveal an attractive combination of processability, thermal and thermo-oxidative stability and good dynamic mechanical properties for these materials

Preparation and Characterization of Mono-End-Capped PMR Polyimide Matrix Resins for High-Temperature Applications

Mono-end-capped PMR-type polyimide matrix resins were prepared by incorporation of carbonyl groups into the polymer backbone of second-generation PMR polyimide by two synthetic pathways. The matrix resin features, thermal processing characteristics of B stage resins as well as thermal and thermooxidative stability of the thermally cured polyimide materials, were systemically investigated. Experimental results indicated that the matrix resins were lowviscosity and stable homogeneous solutions with 50% solid content in ethanol, possessing good characteristics for preparation of carbon or graphite fiber prepregs or B-stage molding compounds. The thermally cured polyimide materials exhibited glass transition temperatures (Tg) of >470 0C and onset temperatures of the storage modulus of >440 'C, 100 'C higher than those of commercial materials. Continuous carbon fiber-reinforced composites were fabricated, which have very high Tg and onset temperature of storage modulus at 446 'C. Preliminary results demonstrated that the composites laminate exhibited high mechanical strength and mechanical property retention at 371 'C, superior to the properties of commercial composite materials (CF/PMR-1I-50).

Thermoset polyimide matrix resins with improved toughness and high Tg for high temperature carbon fiber composites

Thermoset polyimide matrix resins with high melt processability for high-temperature carbon fiber composites were prepared from the diethyl ester of 3,3′,4,4′-benzophenonetetracarboxylic acid (BTDE) and the aromatic diamine mixtures consisting of 4,4′-bis(4-amino-2-trifluoromethylphenoxy)biphenyl (6FBAB) and p-phenylenediamine ( p-PDA) with the monoethyl ester of 5-norbornene-2,3-dicarboxylic acid (NE) as the molecular-weight-controlling and reactive endcapping agent. The effects of diamine mixture compositions on the melt processability of the B-staged oligoimides and the thermal and mechanical properties of the thermally cured polyimide resins were systematically investigated. Experimental results indicated that the polyimide matrix resins with p-PDA concentration of ≤ 40% in 6FBAB + p-PDA showed lower melt viscosities, corresponding to a higher melt processability, than the conventional thermoset polyimide (PMR-15). After thermal curing, the thermoset polyimides exhibited a very good combination of thermal and mechanical properties with the glass transition temperature (Tg) of as high as 353 °C determined by differential scanning calorimetry and impact strength of 15.2 kJ m−2. Carbon fiber-reinforced composite derived from the representative polyimide resin showed good mechanical properties at temperatures of as high as 288 °C.