Thermosetting Polyimides with Improved Impact Toughness and Excellent Thermal and Thermo-Oxidative Stability

Preparation and Properties of Modified Phenylethynyl Terminated Polyimide with Neodymium Oxide

Modified phenylethynyl terminated polyimides (PIs) were successfully prepared by using neodymium oxide (Nd₂O₃) via high-speed stirring and ultrasonic dispersion methods. In addition, the structure and properties of the Nd₂O₃-modified imide oligomers as well as the thermo-oxidative stability of the modified polyimides (PI/Nd₂O₃ hybrid) and its modification mechanism were investigated in detail. The thermogravimetric analysis (TGA) results indicated that the 5% decomposition temperature (Td5%) of the PI/Nd₂O₃ hybrids improved from 557 °C to 575 °C, which was also verified by the TGA-IR tests. Meanwhile, the weight loss rate of the PI/Nd₂O₃ hybrids significantly decreased by 28% to 31% compared to that of pure PI under isothermal aging at 350 °C for 450 h when the added content of Nd₂O₃ was between 0.4 wt% and 1 wt%, showing outstanding thermo-oxidative stability. Moreover, the mechanism of the enhanced thermo-oxidative stability for the modified PIs was analyzed by scanning electron microscopy (SEM) and X-ray diffraction (XRD).

Interlaminar Fracture Behavior of Carbon Fiber/Polyimide Composites Toughened by Interleaving Thermoplastic Polyimide Fiber Veils

Carbon fiber reinforced thermosetting polyimide (CF/TSPI) composites were interleaved with thermally stable thermoplastic polyimide (TPPI) fiber veils in order to improve the interlaminar fracture toughness without sacrificing the heat resistance. Both of the mode I and mode II interlaminar fracture toughness (G_IC and G_IIC) for the untoughened laminate and TPPI fiber veils interleaved laminates were characterized by the double cantilever beam (DCB) test and end notch flexure (ENF) test, respectively. It is found that the TPPI fiber veils interleaved laminates exhibit extremely increased fracture toughness than the untoughened one. Moreover, the areal density of TPPI greatly affected the fracture toughness of laminates. A maximum improvement up to 179% and 132% on G_IC and G_IIC is obtained for 15 gsm fiber veils interleaved laminate, which contributes to the existence of bicontinuous TPPI/TSPI structure in the interlayer according to the fractography analysis. The interlaminar fracture behavior at elevated temperatures for 15 gsm fiber veils interleaved laminate were also investigated. The results indicated that the introduction of thermally stable TPPI fiber veils could enhance the fracture toughness of CF/TSPI composites by exceeding 200% as compared to the untoughened one even as tested at 250 °C.

Phenylethynyl-terminated imide oligomers derived from thioetherdiphthalic anhydride isomers with decreased melt viscosities

A series of oligomers based on the mixture of thioetherdiphthalic anhydride isomers and 4,4′-oxydianiline with 4-phenylethynylphthalic anhydride as reactive endcapping reagent were prepared. The calculated molecular weights were in the range of 1150–5070 g mol−1 with different degrees of polymerization ( n = 1, 3, 5, 7, and 9). The effect of molecular weight of the aromatic oligomers on their processability and solubility as well as the thermal and mechanical properties of the thermal-cured polyimides (PIs) was systematically investigated. The typical oligomer (Oligo-1) could be melted at temperatures of 289–334°C to yield stable molten fluid with melt viscosity below 1.0 Pa s. The melt viscosity of the oligomers increased with the increasing molecular weight. After thermally curing at 370°C, the thermoset PIs exhibited good thermal properties. The glass transition temperatures of oligomers measured by differential scanning calorimetry were in the range of 286–326°C, and the temperature of 5% weight loss was higher than 524°C. The cured films also showed good mechanical properties with tensile strength above 72 MPa and modulus more than 2.5 GPa.

Mechanical, thermal and dielectric properties of graphene oxide/polyimide resin composite

Graphene oxide (GO) sheets have captured the attention of the scientific community because of its excellent performance and applicability. Hence, studying its reinforcing effects on polyimide (PI) resin is an important research topic. In this study, samples of GO-reinforced PI resin were prepared by hot pressing. The effects of GO as nanofiller on the structure and morphology as well as on the mechanical, thermal, and dielectric properties of the GO/PI resin composites were investigated carefully to provide a practical strategy for the use of the polymer-based composites. The GO nanosheets were dispersed uniformly into the PI matrix by ultrasonication, as illustrated by scanning electron microscopic images (SEM). Compared with pure PI, the GO/PI resin composite loaded with 1 wt% GO showed improved tensile strength by 38.9%, flexural strength by 24.8%, and impact strength by 40.7%. Dynamic mechanical analysis test showed that the addition of GO (1 wt%) increased the glass transition temperature by nearly 9.1°C. In addition, the thermal stability and the dielectric constant were also enhanced by adding only a small amount of GO. This approach provides a strategy for developing simple and cost-effective GO-polymer resin composite materials.

Processing and Properties of Carbon Fiber-reinforced PMR Type Polyimide Composites

Carbon fiber-reinforced PMR-type polyimide composites were prepared from T-300 carbon fibers and polyimide matrix resin derived from 4,4′-methylenediamine (MDA), p-phenylenediamine ( p-PDA), diethyl ester of 3,3′,4,4′-oxydiphthalic acid (ODPE) and monoethyl ester of cis-5-norbornene-endo- 2,3-dicarboxylic acid (NE). The rheological properties of the PMR polyimide matrix resin were investigated. Based on this information, an optimized molding cycle was designed for fabricating T-300/PI composites. The composites exhibited high thermal stability and good mechanical properties. The glass transition temperature ( Tg) determined by dynamic mechanical analysis and the decomposition temperature at 5% of weight loss ( T5) were higher than 360 and 540°C, respectively. The flexural strength and shear strength of the composites were 1560 and 95 MPa, respectively. The composite also exhibited good thermal-oxidative stability and hygrothermal resistance as evaluated under the conditions of isothermal aginginairat320°C and hygrothermal aging at 120°C under 2 atm. The unaged thermal and mechanical properties of the composites were hardly changed after isothermal aging for 500 h and hygrothermal aging for 100 h. The scanning electron microscopic analyses for both the isothermally and hygrothermally aged composites revealed that the matrix was bonded to the fibers very well, implying that no interface degradation occurred during the aging experiments

Short Carbon Fiber-Reinforced PMR Polyimide Composites with Improved Thermo-oxidative and Hygrothermal Stabilities

A series of short carbon fiber-reinforced polyimide composites (SCF/PI) with different SCF loadings was prepared by thermal curing of B-staged molding powders, which were obtained by impregnating short carbon fibers with PMR monomer solution in ethyl alcohol, followed by thermally baking up to 220°C. The B-staged molding powders possess good melt-flowing characteristics and processing properties. Experimental results demonstrated that the fully-cured SCF/PI composites exhibited high thermal stability with the glass transition temperatures determined by dynamic mechanical analysis (DMA) of 412-432°C and the decomposition temperatures at 5% of weight loss in the range 497-512°C. The SCF/PI composites also had good mechanical properties, which were affected by the SCF loading. The tensile strength and the flexural strength of SCF/PI composites increased as the SCF loadings increased and then decreased gradually. SCF-20/PI has the highest tensile strength of 101 MPa and SCF-10/PI gave the highest flexural strength of 154 MPa. The SCF/PI composites exhibited a great improvement in impact toughness in comparison with PMR-15. Despite the impact strength of SCF/PI being slightly decreased as the SCF loading was increased, it was still within the range of 15-23 J cm~2.Furthermore, SCF/PI composites exhibited excellent thermal-oxidative and hygrothermal stabilities. The SCF/PI composites showed a weight loss of less than 6% after isothermal aging for 500 h at 320°Cinair. There was no significant difference in thermogravimetric analysis (TGA) and DMA curves for SCF-20/PI after hygrothermal treatment at 120°C under 2 atm for 50 h

Highly soluble phenylethynyl-endcapped imide oligomers derived from thioetherdiphthalic anhydride isomers

Novel imide oligomers (calculated molecular weights: 1250–5000 g mol−1) based on the mixture of thioetherdiphthalic anhydride isomers ( m-TDPA) with 2-phenyl-4,4′-diaminodiphenyl ether ( p-ODA) in the presence of 4-phenylethynylphthalic anhydride (PEPA) as reactive endcapping agent were prepared. Then pyromellitic dianhydride (PMDA) was introduced to the polymerization of m-TDPA/ p-ODA (mole m-TDPA:molePMDA = 1:1). The effect of molecular weights and polymer chemical structures of the aromatic oligomers on their processability and solubility as well as the thermal and mechanical properties of the thermal-cured polyimides (PIs) was systematically investigated. All the oligomers showed good solubility (more than 30 wt%) in N-methyl-2-pyrrolidone and very low melt viscosities. The minimum melt viscosities of Oligo-5 was 61.5 Pa s at 321°C. The melt viscosity of the oligomers increased with the increase of molecular weight. After thermally curing at 370°C for 1 h, the thermoset PIs exhibited good thermal properties. The glass transition temperatures of oligomers measured by differential scanning calorimetry were in the range of 264–337°C, and the temperature of 5% weight loss was higher than 505°C. The cured films also demonstrated good mechanical properties with tensile strength and modulus greater than 52 MPa and 2.9 GPa, respectively.

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.

Fluorinated PMR polyimides with improved melt processability and impact toughness

Fluorinated in situ polymerization of monomeric reactants (PMR) polyimide resins with improved melt processability and impact toughness were prepared from the diethyl ester of 3,3',4,4 '-benzophenonetetracarboxylic acid and the aromatic diamine 4,4 '-bis(4-amino-2-trifluoromethylphenoxy)biphenyl (6FBAB) with the monoethyl ester of 5-norbornene-2, 3-dicarboxylic acid as the molecular-weight-controlling and reactive endcapping agent. Effects of the calculated molecular weights (calc’d Mn values) on the resin melt processability and the thermal and mechanical properties of the thermally cured polyimides were systematically investigated. It was found that the fluorinated PMR resins with calc’d Mn of ≤ 2500 g mol-1 exhibited higher melt processability than the conventional PMR-15. After thermally cured, the thermoset polyimides displayed high thermal stability with the decomposition temperature at 5% weight loss ( T5) of > 62; 450°C, the glass transition temperature ( Tg) of 273—326° C, and good mechanical properties. Carbon-fiber-reinforced composites fabricated from the representative polyimide showed outstanding mechanical properties both at ambient and elevated temperatures as well as high impact toughness at room temperature.