Synthesis and Characterization of Modified Phenylethynyl Imides

IM7/LARC™ MPEI-1 Polyimide Composites

LARC™ MPEI-1 (Langley Research Center™ modified phenylethynyl imide-1) phenylethynyl containing aromatic polyimide, is based on the reaction of biphenyl dianhydride (BPDA), 3,4′-oxydianiline (3,4′-ODA), 1,3-bis(3-aminophenoxy)benzene (APB), 2,4,6-triaminopyrimidine (TAP) and 4-phenylethynyl phthalic anhydride (PEPA), presumably resulting in a mixture of linear, branched and star shaped phenylethynyl containing imides which was evaluated as a matrix for high-performance composites. The poly(amid acid) solution of MPEI-1 in N-methypyrrolidinone was synthesized at 35% and 42% solids. Unidirectional prepreg was fabricated from these solutions and Hercules IM7 carbon fibre utilizing NASA-Langley’s multipurpose prepreg machine. The temperature-dependent volatile depletion rates, thermal crystallization behaviour and resin rheology were characterized. Based on this information, a composite moulding cycle was developed which yielded well consolidated, voidfree laminates. Composite mechanical properties such as short beam shear strength, longitudinal and transverse flexural strength and flexural modulus, longitudinal tensile strength and notched and unnotched compression strengths were measured at room temperature (RT) and elevated temperatures. These mechanical properties are compared with those of IM7/LARC™ PETI-5 composites.

Amine Reactivity Changes in Imide Formation from Heterocyclic Bases

The inclusion of aza-heterocyclic di- or tri-amines in aromatic polyimide formulations can hinder the formation of high molecular weight products. Model studies using the reactions of pyridine, pyrimidine and triazine polyamines with phthalic anhydride and substituted phthalic anhydrides have shown that the combined deactivating effects of multiple aza-substitution and successive imidization of the amino groups can be sufficient to seriously impede or even prevent the formation of fully imidized derivatives. Model co-polyimides incorporating 2, 4, 6-triamino-pyrimidine were found to have lowered molecular weights and to contain persistent free amino groups. Other polyimides containing these amines could be expected to have similar non-ideal structures (defects), which could extend into the structures of products formed from these resins, thereby potentially lessening the extent of any beneficial effects arising from use of the heterocyclic amines.

Modification of PETI-5K Imide Oligomers: Effect on Viscosity

A phenylethynyl (PE) end-capped oligoimide of number average molecular weight Mn 5000, known as PETI-5K was modified by: (1) varying molecular weight; (2) addition of phenylethynyl (PE) end-capped reactive diluents to PETI oligomides; (3) incorporation of 1,4-cyclohexanyl diamine with PETI-5K diamines; (4) incorporation of 2,4,5-triaminopyrimidine with PETI-5K; and (5) replacement of the PETI-5 diamines, 3,4′-ODA and APB with 2,2′-bis [4-(4-amino-phenoxy) phenyl] hexafluoropropane (4-BDAF) to determine the effects on glass transition temperature and minimum melt viscosity. All approaches caused a reduction in complex minimum melt viscosity, but the minimum melt temperatures were only sufficiently low enough for the material to have melt stability for a hot-melt coating application in methods (2), (4), and (5). The reduction in the complex minimum melt viscosity of oligomer prepared by method (4) may be due to unreacted monomers and a lower oligomer molecular weight, Mn, than calculated.

Shape memory polymers with high and low temperature resistant properties

High temperature shape memory polymers that can withstand the harsh temperatures for durable applications are synthesized, and the aromatic polyimide chains with flexible linkages within the backbone act as reversible phase. High molecular weight (Mn) is demanded to form physical crosslinks as fixed phase of thermoplastic shape memory polyimide, and the relationship between Mn and glass transition temperature (Tg) is explored. Thermoset shape memory polyimide shows higher Tg and storage modulus, better shape fixity than thermoplastic counterpart due to the low-density covalent crosslinking, and the influence of crosslinking on physical properties are studied. The mechanism of high temperature shape memory effects based on chain flexibility, molecular weight and crosslink density is proposed. Exposure to thermal cycling from +150 °C to -150 °C for 200 h produces negligible effect on the properties of the shape memory polyimide, and the possible mechanism of high and low temperature resistant property is discussed.

Preparation of new phenylethynyl terminated imide oligomers from pyromellitic dianhydride

Phenylethenyl end-capped imide oligomers based on pyromellitic dianhydride (PMDA), 2, 5-bis(4-aminophenoxy)-biphenyl ( p-TPEQ) and 4-phenylethynylphthalic anhydride (PEPA) were prepared. The thermal properties and melt viscosity of oligomers were investigated by differential scanning calorimetry, thermogravimetric analysis and rheological studies, respectively. The oligomers bearing pendant phenyl groups exhibited much lower melt viscosities than PETI-5, and provided broader processing window. Thermal curing processing was monitored by Fourier transform infrared spectrometry, crosslink reaction of phenylethynyl end group completed at 370 °C in 20 min. Glass transition temperatures of cured oligomers determined by DMA were about 273 to 330 °C, which was much higher than PETI-5. In addition, these cured imide oligomers exhibited excellent tensile properties with strength about 100 MPa and modulus more than 2 GPa. Finally, the films cured by oligo-6 were aged at 177 °C for different times and they still exhibited excellent thermo-oxidative stabilities after aging. It is concluded that using inexpensive PMDA not only lowers the cost, but also increases the glass transition temperature after curing. The excellent properties of these phenylethynyl end-capped imide oligomers demonstrate a promising potential for future aerospace applications.

Synthesis of phthalic end-capped copolyimides and their adhesive properties

A series of molecular-weight-controlled aromatic copolyimides based on anhydrides 4,4′-oxydiphthalic anhydride (ODPA) or/and 2,3,3′,4′-biphenyltetracarboxylic dianhydride (α-BPDA) were prepared by polycondensation reaction with mixture of diamines 1,4-bis(4-amino-2-trifluoromethylphenoxy)benzene (6FAPB) and 4,4′-diaminodiphenylsulphone (DDS) in the presence of phthalic anhydride (PA) as end-capping agent. The effect of the chemical structure of the copolyimides on solubility, rheological behavior, thermal stabilities and adhesive properties was investigated. Experimental results suggested that the solubility and thermal properties of copolyimides could be improved by incorporation of asymmetric α-BPDA. The copolyimide TPI-B (α-BPDA/6FAPB-DDS/PA) exhibited better solubility with the solid content as high as 45 wt.% in N, N-dimethylacetamide and high thermal stability with the Tg of 306 °C. TPI-A (ODPA/6FAPB-DDS/PA) and TPI-C (α-BPDA-ODPA/6FAPB-DDS/PA) could be produced into glass-cloth-supported adhesive films. TPI-A adhesive film gave better adhesive properties at room temperature with lap shear strength of 16.41 MPa and T-peel strength of 2.1 kN m−1, which attributed to its flexible structure and good melt processability. TPI-C adhesive film exhibited higher adhesive properties at elevated temperature with the lap shear strength exceeding 9 MPa at 280 °C. The copolyimides with combination of rigid and flexible structures could afford good melt flow behavior and high thermal stability, which yielded good adhesive properties at high temperature.

Phenylethynyl-endcapped oligomides with low melt viscosities and high Tgs: effects of the molecular weights

Phenylethynyl-endcapped imideoligomers (oligomides) with low melt viscosities and high glass transition temperatures ( Tg) were prepared by the high temperature polycondensation of 2,3,3′,4′-biphenyltetracarboxcylic anhydride (a-BPDA), 4,4′-methylenebis(2,6-dimethylaniline) (MBDA) and 4-phenylethynyl-phthalic anhydride (4-PEPA). Experimental results indicated that the oligomides were mixtures of biphenylethynyl-endcapped imide oligomers with different degrees of polymerization (DP = 0, 1, 2, 3, 4), corresponding to the molecular weights ( Mn) of 737 to 2787 determined by MALDI-TOF. The melt viscosities of the oligomide were reduced by lowering the calculated molecular weight (Calc’d Mn). The thermally cured polyimide neat resins showed glass transition temperatures ( Tg) of > 400 °C and 5% weight loss temperature ( T5) of > 499 °C. It was also found that the mechanical properties of the thermally cured polyimide neat resins were dependent on the Calc’d Mn values of the oligomide, and improved gradually with increasing Calc’d Mn.