TOUGH, HIGH PERFORMANCE, ADDITION-TYPE THERMOPLASTIC POLYMERS

Abstract: A tough, high performance polyimide is provided by reacting a triple bond conjugated with an aromatic ring in a bisethynyl compound with the active double bond in a compound containing a double bond activated toward the formation of a Diels-Adler type adduct, especially a bismaleimide, a biscitraconimide, or a benzoquinone, or mixtures thereof. Addition curing of this product produces a highly linear polymeric structure and heat treating the highly linear polymeric structure poduces a thermally-stable aromatic addition-type thermoplastic polyimide, which finds utility in the preparation of molding compounds, adhesive compositions, and polymer matrix composites.

Description: TOUGH, HIGH PERFORMANCE, ADDITION-TYPE THERMOPLASTIC POLYMERS Origin of the Invention The invention described herein was made by an employee of the United States Government and may be manufactured and used by or for the Government for governmental purposes without the payment of any royalties thereon or therefor. Background of the Invention 1. Field of the Invention The present invention relates generally to tough, high performance, thermoplastic polymers. It relates particularly to addition-type thermoplastic polymers, which are polymers having an addition curing which leads to a linear structure. 2. Description of Related Art The development of toughened high performance thermosetting polymers such as toughened epoxies. SU bismaleimides, PMR polyimides and acetylene-terminated resins, dominates the present day approach to the preparation of new polymer composites. These polymers can be toughened in several ways, including: (1) reducing crosslink density, (2) incorporation of flexibilizing linkages, (3) rubber toughening, (4) synthesis of addition-type thermoplastics, (5) polymer blends, and (6) semi-interpenetrating polymer networks. One characteristic common to most of the above methods is that toughness is gained at the considerable cost of lowering the Tgs of these polymers which, in turn, adversely effects their high temperature mechanical performance. Of the above toughening methods, the synthesis of addition-type thermoplastics (ATTs) is considered to be a very attractive approach, but remains the least explored area for the following reasons. An ATT is defined as a polymer that has an addition curing which leads to a linear structure. Such a polymer is non-classical in that it has similarities to two major classical categories: thermosets (addition curing with a crosslinked structure) and thermoplastics (condensation reaction cured having a linear structure) . Because of their addition curing and linear structure, ATT polymers can have toughness (like thermoplastics) and can be easily processed (like thermosets) . It is the primary object of the present invention to provide a synthetic reaction which forms stable aromatic rings in the backbone of an ATT polymer, thereby combining high temperature performance and thermo- oxidative stability with toughness and easy processibility, and minimizing or eliminating the necessity for property tradeoffs often observed in conventional polymer synthesis. It is another object of the present invention to provide novel monomeric materials for use in attaining the primary object above. Summary of the Invention According to the present invention, the foregoing and additional objects are attained by reacting a triple bond conjugated with an aromatic ring in a bisethynyl compound with the active double bond in a compound containing a double bond activated toward the formation of a Diels- Alder type adduct. Especially good results have been obtained if the latter compound is one of or a mixture of a bismaleimide, a biscitraconimide, and a benzoquinone. A highly linear polymer structure is produced if the reaction product is addition cured; and a thermally-stable aromatic addition-type thermoplastic polyimide is produced by heat treating this highly linear polymeric structure. The bisethynyl compound and the compound containing a double bond activated toward the formation of a Diels-Alder type adduct are reacted in stoichiometric quantities, as well as in off- stoichio etric quantities, especially in a mole ratio range between about 7:1 and 1:7. The tough, high performance polyimides according to the present invention find special utility in the preparation of molding compounds, adhesive compositions, and polymer matrix composites. Novel monomeric materials used in the preparation of the polyimides according to the present invention have the following general structural formula: SUBSTITUTE SHE Brief Description of the Drawings For a more complete understanding of the present invention, including its objects and attending benefits, reference should be made to the Detailed Description of the Preferred Embodiments, which is set forth below. This Description should be read together with the accompanying drawings, wherein: FIG. 1 is a reaction equation showing possible mechanisms for ' the synthesis of addition-type thermoplastic polymers; FIG. 2 is a reaction equation showing the synthesis of a polyimide product according to the present invention (LaRC-RP80) from a commercially available starting material (Thermid 600) and a novel monomeric material according to the present invention (CA/MDA/6F) ; FIO. 3 is a reaction equation showing possible mechanisms for the reaction of a bisethynyl compound with a bismaleimide; FIG. 4 shows FTIR spectra of (a) the polyimide product of FIG. 2, in accord with the present invention, (b) the commercially available starting material of FIG. 2, and (c) the novel monomeric material of FIG. 2, in accord with the present invention; FIG. 5 shows the results of thermomechanical analyses of (a) the polyimide product of FIG. 2 in the dry state; (b) the polyimide product of FIG. 2 in the wet state; and (c) the polyimide product of FIG. 2 which has been aged for six hours at 371"C in air; FIG. 6 is a scanning electron micrograph of the polyimide product of FIG. 2; FIG. 7 shows the results of thermogravimetric analyses of (a) the polyimide product of FIG. 2; (b) the commercially available starting material of FIG. 2; and (c) the novel monomeric material of FIG. 2; and FIG. 8 shows the results of isothermal (371°C) thermogravimetric analyses of (a) the polyimide product of FIG. 2; (b) the commercially available starting material of FIG. 2; and (c) the novel monomeric material of FIG. 2. Description of the Preferred Embodiments The concept of the ATT synthesis is schematically depicted in FIG. 1. The synthesis may proceed through the cycloaddition of an acetylene-terminated prepolymer with a compound containing a double bond activated toward the formation of a Diels-Alder type adduct, such as a bismaleimide, a biscitraconimide, or a benzoquinone. The reaction sites are the triple bond conjugated with an aromatic ring in a bisethynyl compound and the active double bond in a compound containing a double bond activated toward the formation of a Diels-Alder type adduct. The cycloaddition may proceed via at least two reaction pathways as shown in (a) and (b) of FIG. 1. Both involve a concerted process. Pathway (a) forms a highly strained intermediate (3) containing an allene functionality from the Diels-Alder reaction of the 4π electrons in the conjugated triple bond with the 2π electrons in the maleimide double bond. To release the ring strain, compound (3) would most likely quickly rearrange itself to give the more stable compound (4) through a [1,3] sigmatropic hydride shift. Alternatively, compound (4) can be directly formed from the interaction of the 2π electrons in the triple bond UB with the 2τr electrons in the maleimide double bond and a concomitant [1,5] sigmatropic hydride shift (Pathway b) . To enhance thermo-oxidative stability, compound (4) is heat treated to achieve aromatization leading to compound (5) . If a linear thermoplastic material is to be prepared, the synthesis must utilize stoichiometric quantities of the reactants. Otherwise, the presence of an excess reactant can result in the formation of semi- interpenetrating polymer networks. Since a large number of acetylene-terminated bismaleimide, biscitraconimide, and benzoquinone compounds are currently available, and because these compounds can be combined in various ways, the present invention is very versatile and should lead to the development of a wide variety of new products suitable for composite and adhesive applications. The above concept was demonstrated by synthesizing and characterizing a new polyimide designated as LaRC-RP80. EXAMPLES 1. Materials Thermid LR-600 was purchased from National Starch. The 4,4'-methylenedianiline (MDA) from Eastman was used as r e c e i v e d . T h e 2 , 2 - b i s ( 3 , 4 - dicarboxyphenyl)hexafluoropropane dianhydride (6F) from American Hoechst was recrystallized from acetic anhydride/toluene (20/80 volume ratio) , m.p.245 β C-246°C. Citraconic anhydride (CA) from Aldrich was freshly distilled. 2. Synthesis of Biscitraconimide The new biscitraconimide, CA/MDA/6F, the chemical structure of which is shown in FIG. 2, was prepared in two steps. Step one concerns the preparation of the diamine MDA/6F. Step two deals with the reaction of CA and MDA/6F according to the following procedure. To a refluxing and stirred solution of the diamine MDA/6F (0.05 mole) in 200 ml of a solvent mixture consisting of methylene chloride and acetone in a 1:1 volume ratio, a solution of CA (0.1 mole) in 100 ml of the same solvent mixture was added over a 15 minute period. After refluxing for ten minutes, the reaction solution changed color from dark brown to yellow, and the solid material, identified to-be the amic acid precursor, was precipitated. After one-half hour, sodium acetate (5 g) and acetic anhydride (100 ml) were added to chemically imidize the amic acid into the corresponding i ide. Immediately following the addition of acetic anhydride and sodium acetate, the reaction solution changed color from yellow back to dark brown, and the solid material dissolved to give a clear brown solution. The progress of the reaction was followed by FTIR. After one hour the reaction product was worked up by washing three times with 200 ml of saturated sodium carbonate aqueous solution, drying the organic materials with anhydrous magnesium sulfate, and then evaporating the organic solvents. This afforded the crude biscitraconimide in 99% yield. After recrystallization from acetone/water, a light gray solid (overall yield 78%) was obtained, m.p. 190°C-192 β C. Its FTIR spectrum had the following characteristic absorption bands: 3100 (C = C-H maleimide) , 1775 (C = O imide in-phase) and 1720 (C = O imide out-of-phase) , 1635 (C = C maleimide) , 1375, 1260, 1140, and 1100 cm 1 (C-F) . 3. Resin Preparation and Characterization FIG. 2 shows the synthesis of LaRC-RP80. The commercial Thermid LR-600 and the previously prepared CA/MDA/6F were dissolved in stoichiometric quantities in acetone to give a 50% w/w dark brown solution. The solution was concentrated at 100 * C in a nitrogen atmosphere for one and one-half hours, followed by staging at 250 β C in air for one hour. This afforded LaRC-RP80 molding powder, the DSC of which showed one endotherm at 210 β C and one exother at 240 β C. Interestingly, this exothermic peak was not found in the DSC scans of the Thermid 600 molding powder prepared under the same condition as LaRC-RP80 and CA/MDA/6F prepolymer. This suggests that this exotherm is due to the chemical structure resulting from the reaction of Thermid 600 with the biscitraconimide, rather than the homopolymerization of the constitutent materials. The LaRC-RP80 molding powder (15.50 grams) was placed in a cold matched metal die. This was then inserted into a press preheated to 288 β C. A thermocouple was attached to the die to determine the temperature profile. When the die temperature reached 225 β C, 2000 psi pressure was applied. The temperature was raised to 288 β C at a rate of 2 β C/minute. The neat resin was cured at 288"C in air under 2000 psi pressure for one hour and removed from the press when the die temperature cooled to 177°C. The resin was postcured at 288 ° C in air for four hours. This afforded a neat resin having dimensions of 3.2 cm by 3.2 cm by 1.0 cm and a density of 1.35 g/cc. The optical microscopic examination of the cross-section of the neat resin showed no detectable voids or defects. This molding was then used as a compact tension specimen and characterized. 4. Adhesive Bonding and Testing The 50% w/w solution of Thermid LR-600 and biscitraconimide (1:1 molar ratio) in acetone was brush coated onto a 112 E-glass (A1100 finish) cloth which was stretched over a metal frame. The scrim cloth was dried between coatings at 60"C in air for one-half hour. After the fourth coating, the cloth was staged at 100 β C, 150°C and 177 β C in air for one hour at each temperature. Single lap shear bond specimens were prepared using 25.4 mm wide, 1.27 mm thick 6A1-4V titanium adherends. The bond area of the adherend was surface treated with Pasa Jell 107, which is marketed by SEMCO, Glendale, California, primed with the resin solution and heated in the same manner as the scrim cloth prepared above. Sandwiching the S-staged scrim cloth between the primed adherends having a 12.7 mm overlap, the lap shear specimens were bonded as follows: (1) raise temperature from room temperature to 250 β C at 4°C/min, (1) apply 200 psi at 250 β C and raise temperature to 288 1 °C at 4°C/min, (3) hold one hour at 288°C under 200 psi pressure and (4) cool to room temperature under pressure. The bonded specimens were postcured at 288 * C in air for four hours. The lap shear tests were performed on an Instron universal testing machine according to ASTM D-1002. 5. Reaction Mechanisms FIG. 3 shows that the reaction of an acetylene terminated compound with a maleimide can occur in three ways: (a) the individual homopolymerization of each of the two reactants leads to a mixture of crosslinked networks; (b) the cycloaddition reaction of the acetylene with the maleimide forms an ATT via one of the two routes shown in FIG. 1; and (c) the addition of the maleimide double bond across the acetylene triple bond gives a highly crosslinked material. Only pathway (b) forms a tough linear thermoplastic material. The other two routes produce brittle crosslinked polymers. This is an important distinction. 6. Evidence for ATT Of the above three reaction mechanisms set forth in FIG. 3, pathway (b) is consistent with the following five findings. First, the FTIR spectrum of cured LaRC-RP80 neat resin showed five new absorption bands which are consistent with the formation of a cycloaddition adduct. These new bands are marked with an arrow shown in FIG. 4. For comparison purposes, the FTIR spectra of Thermid 600 and CA/MDA/6F polymers cured under the identical condition as LaRC-RP80 are also shown in FIG. 4. The new bands and their assignments are 3115 cm "1 due to stretching vibration of C = C-H in cyclohexene, 1645 cm "1 due to stretching vibration of C = C in cyclohexene, 1510 cm "1 due to aromatic ring adjacent to cyclohexene, 1140 cm "1 due to C-N-C succinimide. Next LaRC-RP80 is significantly tougher than the constituent polymers (G 1c 324 J/m 2 compared to 32 J/m 2 for CA/MDA/6F and 85 J/m* for Thermid 600) . Such high toughness characteristics are in line with the behavior of a linear thermoplastic, but not with the behavior of a highly crosslinked polymer. Third, the DSC scan of LaRC-RP80 molding powder shows an exothermic peak around 240°C, which is not seen in the DSC scans of the constituent materials. This suggests that pathways (b) and (c) , but not (a) , are occurring. Fourth, only one Tg was observed in the TMA thermogram (see FIG. 5) and confirmed in the TBA spectrum of LaRC- RP80. From this, it follows that LaRC-RP80 is a one- phase system. Such a morphology is consistent with both pathways (b) and (σ) , but not (a) . Finally, AS-4/LaRC- RP80 composite can be reprocessed to correct flaws. On the basis of the foregoing discussion, the evidence supporting the formation of an ATT through pathway (b) is strong. 7. Processing State-of-the-art BMIs are known for their ease of processing. However, their processing cycles often require long curing and postcuring time involving several steps. LaRC-RP80 can be processed easily and quickly for the following three reasons. The reacting components are readily soluble in a low boiling solvent, such as acetone, making solvent removal easy. It has an addition curing mechanism, which eliminates voids caused by evolution of volatile by-products during the critical final stage of curing. Lastly, the curing takes place rapidly at a moderately high temperature. The cure cycle for LaRC-RP80 is: cure one hour at 288"C and postcure four hours at 288 β C. U 8. Resin Properties Table 1 below gives the neat resin properties of LaRC- RP80. • TABLE 1 NEAT RESIN PROPERTIES OF LaRC-RP80 Physical Properties Tg 1 , Dry/Wet 2 , *C 268/254 Density, g/cc 1.33 2 3 Moisture /Solven Absorption, 2. 2.60/2.40 Toughness G lc , J/m 2 (in-lbs/in 2 ) 4 324 (1.85) Thermo-oxidative Stability by TGA in Air Onset temperature, β C 484 Temperature at 52. wt loss, *C 514 t loss after 50 hours at 371*C, % 18 Determined by TMA. 2 Specimens immersed in water at room temperature for two weeks. 3 Specimens immersed in boiling CH„C1«, for 60 hours. 4 Calculated from K. using Thermid 600 tensile modulus at room temperature 6.0 x 10 5 psi, average of two runs with variability 3Z. As shown in FIG. 5, LaRC-RP80 had dry and wet Tgs at 268°C and 254°C, respectively. Isothermal aging at 371°C in air for four hours increased the Tg to 312°C. While having a high Tg, LaRC-RP80 also showed exceptional toughness characteristics. The value of G 1c was found to be 324 J/m 2 . Since high toughness and high Tg are desirable properties, and because the former is often achieved at the expense of the latter, it is interesting to compare both of the properties simultaneously. The values of G lc for state-of-the-art BMIs having Tgs in the range of 230°C to 290 β C vary from 34 to 260 J/m 2 . Therefore, LaRC-RP80 ranks highest in toughness among these BMIs having a comparable Tg. FIG. 6 shows the scanning electron micrograph of the fracture surface of LaRC-RP80. The fractography of LaRC- RP80 reveals a dendritic pattern. The initial propagation region shows extended arrays and lines which run in the direction of crack propagation and extend over a considerable distance with a high degree of regularity. Clearly, this is a ductile fracture. As shown in FIG. 7, LaRC-RP80 has a 5% weight loss temperature of 514 β C. This represents the highest thermo-oxidative stability ever observed for BMIs developed to date. In addition, the data of FIG. 7 and FIG. 8 indicate that the thermo-oxidative stability of LaRC-RP80 is equivalent to that of Thermid 600, and is substantially better than that of the biscitraconimide CA/MDA/6F. LaRC-RP80 also exhibited outstanding moisture resistance. Typical BMIs have equilibrium moisture absorptions which range from four to six percent. A value of 2.6 percent was obtained for LaRC-RP80. The good moisture resistant characteristics of this material are reflected in the high wet Tg mentioned previously and the excellent hot/wet lap shear strength presented below in Table 2. 9. Adhesive Properties Table 2 summarizes the adhesive properties of LaRC- RP80, along with those of Thermid 600 for comparison purposes. TABLE 2 ADHESIVE PROPERTIES OF LaRC-RP80 AND THERMID 600 TITANIUM-TO-TITANIUM Lap Shear Strength, MPa (psi) RT 232 β C Thermid. 10.5 15.5 600 (1515) (2243) Average of four tests with variability 6Z; average bondline thickness 0.005". 2 Fracture surfaces are smooth without visible voids, Failure adhesive. Failure cohesive/adhesive. Specimen immersed in water at room temperature for two weeks. Failure cohesive. 7 Λfter aging for 500 hours at 288 β G in air. The room temperature lap shear strength of LaRC-RP80 was 2078 psi, using titanium as an adherend. Moisture absorption increases the lap shear strengths at both room temperature and elevated temperature. Moreover, elevated temperature tests also resulted in higher lap shear strengths for both dry and wet conditions. This was unexpected. Invariably, the specimens tested at room temperature in dry conditions showed adhesive failure, whereas the moisture saturated samples tested at 232 β C showed cohesive failure. With 2963 psi lap shear strength at 232°C in wet condition, LaRC-RP80 retains 143 percent of its room temperature properties. State-of- the-art BMIs have considerably poorer adhesive properties by comparison. 10. Synthesis of Biscitraσonimides and Bismalei ides Table 3 shows the chemical structures and designations of five biscitraconimides and five bismaleimides which were prepared for subsequent polymer synthesis described hereinafter. The following is a general synthetic procedure used for the preparation of the above ten compounds. The synthesis involves two steps. Step one concerns the preparation of the diamine from 4,4'- (hexafluoroisopropylidene)bis(o-phthalic anhydride) , hereinafter referred to as 6F dianhydride, and the corresponding aromatic diamine. For example, 4,4'- [2,2,2-trifluoro-l-(trifluromethyl)ethylidene]bis(N-[α_- (p_-aminophenyl)-p_-tolyl]phthalimide] , hereinafter referred to as MDA/6F, was prepared by refluxing 4,4'- methylenedianiline (MDA) (0.48 mole) and 6F dianhydride (0.24 mole) in N-methy1-pyrrolidone (350 ml) for four hours. The cooled reaction mixture was poured onto an ice-water mixture (500 ml) , and the solid was filtered, washed with distilled water (5 x 100 ml) , and dried in vacuum at 100 "C to yield diamine MDA/6F in 99% yield. Using the same procedure given above, the following four diamines were also prepared: 4,4'-[2,2,2-trifluoro-1- (trifluoromethyl) ethylidene] bis [N_- [p_- (p_- aminophenoxy)phenyl]phthalimide] (0DA/6F) , 4,4'-[2,2,2- trifluoro-1-(trifluoromethyl) ethylidene]bis[N.-)p_- sulfanilylphenyl)phthalimide] (DDS/6F) , 4,4 « -[2,2,2- trifluoro-1-(trifluoromethyl) ethylidene]bis[N.-(p_- aminophenyl) hthalimide] (PD/6F) and 4,4*-[2,2,2- trifluoro-1-(trifluoromethyl)ethylidene]bis[N-(12- aminododecyl)phthalimide] (DDA/6F) . Step two is exemplified by the preparation of biscitraconimide CA/MDA/6F as described in the following Example 10(A) : TABLE 3 THE CHEMICAL STRUCTURES AND DESIGNATIONS OF THE BISMALEIMIDES AND BISCITRACONIMIDES PREPARED 10(A) . To a refluxing and stirred solution of the diamine MDA/6F (0.05 mole) in 200 ml of a solvent mixture consiting of methylene chloride and acetone in 1:1 volume ratio, a solution of CA (0.1 mole) in 100 ml of the same solvent mixture was added over a 15 minute time period. After refluxing for ten minutes, the reaction solution changed color from dark brown to yellow and the solid material, identified to be the amic acid precursor, was precipitated. After one-half hour sodium acetate (5 g) and acetic anhydride (100 ml) were added to chemically imidize the amic acid into the corresponding imide. Immediately following the addition of acetic anhydride and sodium acetate, the reaction solution changed color from yellow back to dark brown and the solid material dissolved to give a clear brown solution. The progress of the reaction was followed by FTIR. After one hour the reaction product was worked up by washing three times with 200 ml saturated sodium carbonate aqueous solution, drying the organic materials with anhydrous magnesium sulfate and evaporating the organic solvents. This afforded the crude biscitraconimide CA/MDA/6F in 99% yield. After recrystallization from acetone/water, a pale yellow solid (overall yield 78%) was obtained, m.p. 190 β C-192 β C; IR (CHC1 3 ) 3100, 1775, 1720, 1635, 1375, 1260, 1140 and 1100 cm "1 . Analysis: Calcd. for C gg H^N^O g : C, 66.53; H, 3.43; N, 5.65, F, 11.49. Found: C, 64.48; H, 3.62; N, 5.51; F, 12.31. 10(B). As in Example 10(A), the reaction of CA (0.1 mole) and ODA/6F (0.05 mole) afforded the crude CA/0DA/6F in 99% yield, m.p. 138 β C-143 β C. After recrystallization, a dark brown solid was obtained, m.p. 180°C-182 β C; IR (CHC1 3 ) 3050, 1775, 1725, 1640, 1225, 1375, 1260, 1140 and 1100 cm '1 ; » H NMR: δ 2.09, 6.80, 7.20, 7.35, 7.91. Analysis: Calcd. for C 53 H 30 N 4 F 6 O 10 : C, 63.86; H, 3.01; N, 5.62; F, 11.45. Found: C, 62.35; H, 3.29; N, 5.31; F, 13.26. 10(C). As in Example 10(A), the reaction of CA (0.1 mole) and DDS/6F (0.05 mole) afforded the crude CA/DDS/6F in 98% yield, m.p. 174 β C-180'C. After recrystallization, a gray solid was obtained, m.p. 210*C-211'C; IR (CHC1 3 ) 3030, 1770, 1720, 1350 and 1140 cm "1 . Analysis: Calcd. for C 53 H 30 N 4 F 6 O 12 S 2 : C, 58.24; H, 2.75; N, 5.13; F, 10.44; S, 5.86. Found: C, 57.39; H, 3.30; N, 4.73; F, 10.67; S, 5.98. 10(D). As in Example 10(A), the reaction of CA (0.1 mole) and PD/6F (0.05 mole) afforded the crude CA/PD/6F in 99% yield, m.p. 208*C-212 β C. After recrystallization, a dark purple solid was obtained, m.p. 230°C-232 β C; IR (CHC1 3 ) 3030, 1760, 1715, 1640, 1375, 1260, 1140 and 1100 cm "1 . Analysis: Calcd. for C 41 H 22 N 4 F 6 0 8 : C, 60.59; H, 2.71; N, 6.90 F, 14.04. Found: C, 60.32; H, 2.87; N, 6.75; F, 14.27. 10(E). As in Example 10(A), the reaction of CA (0.1 mole) and DDA/6F (0.05 mole) afforded the crude CA/DDA/6F in 99% yield, m.p. 121 β C-125"C. After recrystallization, a pale yellow solid was obtained, m.p. 132"C-134 β C; IR (CHC1 3 ) 3300, 1760, 1720, 1375, 1260, 1140 and 1100 cm "1 . Analysis: Calcd. for C 43 H 5a N 4 F 6 0 4 : C, 62.82; H, 7.35; N, 6.10; F, 11.89. Found: C, 63.86; H, 7.18; N, 6.93; F, 12.01. 10(F). As in Example 10(A), the reaction of maleic anhydride (MA) (0.1 mole) and MDA/6F (0.05 mole) afforded the crude MA/MDA/6F, m.p. 138 β C-143 β C, in gold color. 10(G). As in Example 10(A), the reaction of MA (0.1 mole) and 0DA/6F (0.05 mole) afforded the crude MA/ODA/6F, m.p. 130 β C-134°C in dark brown color. 10(H). As in Example 10(A), the reaction of MA (0.1 mole) and DDS/6F (0.05 mole) afforded the crude MA/DDS/6F, m.p. 158 β C-163 β C in off-white color. 10(1). As in Example 10(A), the reaction of MA (0.1 mole) and PD/6F (0.05 mole) afforded the crude MA/PD/6F, m.p. 189 β C-193'C in purple color. 10(J). As in Example 10(A), the reaction of MA (0.1 mole) and DDA/6F (0.05 mole) afforded the crude CA/DDA/6F in 89% yield, m.p. 105 β C-108 β C in light yellow color. Example 11 A solution of the commercial Thermid LR-600 having 50% w/w solid in N-methylpyrrolidone (NMP) (18.89 g, 0.008 mole) in acetone (5 ml) was added in one portion to a solution of CA/MDA/6F prepared (8.0 g, 0.008 mole) in acetone (5 ml) . The mixture was stirred at room temperature for one-half hour to give a 50% w/w dark brown solution. The solution was concentrated at 100 β C in N 2 atmosphere for one and one-half hours followed by staging at 250*C in air for one hour. This afforded molding powder whose DSC showed one endotherm at 210"C and one exotherm at 240*C. The molding powder (15.50 grams) was placed in a cold matched metal die. This was then inserted into a press preheated to 288"C. A thermocouple was attached to the die to determine the temperature profile. When the die temperature reached 225 β C, 2000 psi pressure was applied. The temperature was raised to 288 β C at a rate of 2 β C/minute. The neat resin was cured at 288"C in air under 2000 psi pressure for one hour and removed from the press when the die temperature cooled to 177'C. The resin was postσured at 288*C in air for four hours. This afforded a neat resin having dimension of 3.2 cm by 3.2 cm by 1.0 cm and a density of 1.35 g/cc. The optical microscopic examination of the cross-section of the neat resin showed no detectable voids or defects. The polymer is designated LaRC-RP80. Example 12 As in Example 11, a polymer having an off stoichiometric composition was also prepared from the same reaσtants as in Example 11, in order to evaluate the effect of stoichiometry on the properties of the polymer. According to the procedure of Example 11, a polymer was prepared from Thermid LR-600 (0.008 mole) and CA/MDA/6F (0.0053 mole). This polymer is designated LaRC-RP80-A. Example 13 As in Example 11, a polymer having an off stoichiometric composition was prepared from Thermid LR- 600 (0.0053 mole) and CA/MDA/6F (0.008 mole). This polymer is designated LaRC-RP80-B. Example 14 As in Example 11, the in-situ polymerization of Thermid LR-600 (0.008 mole) and CA/0DA/6F (0.008 mole) yielded a void-free neat resin having dimensions of 3.2 cm x 3.2 cm x 1.5 cm and a density of 1.37 g/cc. This polymer is designated LaRC-RP83. Also, a polymer consisting of Thermid LR-600 (0.008 mole) and CA/MDA/6F (0.0053 mole) was prepared and is designated as LaRC-RP83-A. Example 15 As in Example 11, the reaction of Thermid LR-600 (0.001 mole) and CA/DDS/6F (0.001 mole) formed a void-free neat resin disc having a diameter of 2.54 cm and thickness of 1 cm, a density of 1.36 g/cc and Tgs of 269 β C dry and 265"C wet. This polymer is designated LaRC-RP-56. Example 16 As in Example 11, the reaction of Thermid LR-600 (0.001 mole) and CA/PD/6F (0.001 mole) gave a void-free neat resin disc having 2.54 cm in diameter and 1 cm in thickness, a density of 1.37 g/cc and Tgs of 310"C dry and 288"C wet. This polymer is designated LaRC-RP57. Example 17 As in Example 11, the reaction of Thermid LR-600 (0.001 mole) and CA/DDA/6F (0.001 mole) afforded a void-free neat resin disc, with a density of 1.29 g/cc and Tgs of 139 β C dry and 133*C wet. This polymer is designated LaRC-RP58. Example 18 As in Example 11, the reaction of Thermid LR-600 (0.008 mole) and MA/MDA/6F (0.008 mole) produced a void-free neat resin having dimensions of 3.2 cm x 3.2 cm x 1.3 cm, a density of 1.37 g/cc and Tgs of 265°C dry and 253"C wet. This polymer is designated LaRC-RP98. Example 19 As in Example 11, the reaction of Thermid LR-600 (0.008 mole) and MA/ODA/6F (0.008 mole) afforded a void-free neat resin having dimensions of 3.2 cm x 3.2 cm x 1.4 cm, a density of 1.33 g/cc and Tgs of 253"C dry and 251°C wet. This polymer is designated LaRC-RP99. Example 20 As in Example 11, the reaction of Thermid LR-600 (0.008 mole) and MA/DDS/6F (0.008 mole) yielded a void-free neat resin having a density of 1.36 g/cc and Tgs of 267"C dry and 250"C wet. This polymer is designated LaRC-RPlOO. Example 21 As in Example 11, the reaction of Thermid LR-600 (0.008 mole) and MA/PD/6F (0.008 mole) produced a void-free neat resin with a density of 1.37 g/cc and Tgs of 278°c dry and 271°C wet. This polymer is designated LaRC-RPlOl. Example 22 As in Example 11, the reaction of Thermid LR-600 (0.008 mole) and MA/DDA/6F (0.008 mole) gave a void-free neat resin having a density of 1.28 g/cc and Tgs of 121"C dry and 119 "C wet. This polymer is designated LaRC-RP102. Example 23 As in Example 11, the reaction of Thermid LR-600 (0.001 mole) and p-benzoquinone (0.001 mole) gave a neat resin with a density of 1.23 g/cc. This polymer is designated LaRC-RP103. Example 24 As in Example 11, the reaction of the commercial Thermid FA-700 (0.001 mole) and MA/MDA/.6F (0.001 mole) yielded a void-free neat resin disc having a density of 1.41 g/cc. This polymer is designated LaRC-RP104. Example 25 As in Example 11, the reaction of an ethynyl terminated arylene ether oligomer (ETAE) having " an inherent viscosity of 0.35 dL/g (0.001 mole) and CA/MDA/6F (0.001 mole) formed a void-free neat resin disc with a density of 1.31 g/cc. This polymer is designated as LaRC-RP105. Tables 4 and 5 show the processing and properties of the neat resins. TABLE 4 Resin Cure Cycle All of the polyimides One hour cure at 288 β C prepared according to the present invention Four hours postcure at 288 β C Commercial Nine hours cure at 80 β C-204°C Bismaleimides 4-24 hours postcure at 220 β C-260°C TABLE 5 Thermo-oxidative Stability by TGA Glass Transition Wt % Isothermal Temperature Fracture Moisture Temperature, wt. loss, X Tg, β C Toughness Uptake β C after 50 2 Dry Wet G. , J/m at 25 β C at 5% wt hours at for two loss in 371 β C in Resin weeks air air 2.6 514 18 2.1 490 19 1.2 514 18 4.5 •347 Example 26 The 50% w/w solution of Thermid LR-600 and CA/MDA/6F (1:1 molar ratio) in acetone was brush coated onto a 112 E-glass (A1100 finish) cloth which was stretched over a metal frame. The scrim cloth was dried between coatings at 60 β C in air for one-half hour. After the fourth coating, the cloth was staged at 100"C, 150 β C and 177*C in air for one hour at each temperature. Single lap shear bond specimens were prepared using 25.4 mm wide, 1.27 mm thick 6A1-4V titanium adherends. The bond area of the adherend was surface treated with Pasa Jell 107 (trademark of a product marketed by SEMCO in Glendale, California) , primed with the resin solution and heated in the same manner as the scrim cloth prepared above. Sandwiching the β- staged scrim cloth between the primed adherends having a 12.7 mm overlap, the lap shear specimens were bonded as follows: (1) raise temperature from room temperature to 250*C at 4*C/min, (2) apply 200 psi at 250*C and raise temperature to 288*C at 4 β C/min, (3) hold one hour at 288"C under 200 psi pressure and (4) cool to room temperature under pressure. The bonded specimens were postcured at 288*C in air for four hours. The lap shear tests were performed on an Instron universal testing machine according to * ASTM D-1002. Table 6 shows the adhesive properties. SUBSTIT Resin LaRC-RP80 14.3 (2078) 20.5(2975) 19.2(2786) 20.5 (2963) 12.6 (1820) LaRC-RP80-A 13.8(2004) 16.8(2440 -- 9.5(1375) 8.9(1283) LaRC-RP83 16.9(2446) 21.1 (3063) 20.5(2975) 20.4 (2950) 10.5 (1528) LaRC-RP83-A 14.0(2034) 16.9 (2450) 17.6 (2552) 17.0(2465) 17.2(2493) Thermid 600 TM (National Starch) 10.5 (1515) — 15.5 (2243) — Kerimid 601 TM (Rhone-Poulenc) 8.3(1210) — 4.3(620) — — Example 27 A prepreg was prepared by drum winding AS-4 unsized graphite yarn followed by brush application of the 50% w/w solution of Thermid LR-600 and CA/MDA/6F in 1:1 molar ratio in acetone. The quantity of the resih solution was calculated to yield finished composite containing 60 volume percent fiber. The tapes were dried on the rotating drum at room temperature for three hours, removed from the drum and cut into 1.9 cm x 7.6 cm plies. The prepreg showed excellent tack and drape characteristics. Twelve plies were stacked unidirectionally and then staged at 80"C for one hour in an air-circulating oven. The staged lay-up was placed in a cold matched metal die. This was then inserted into a preheated 288 β C press. A thermocouple was attached to the matched die to determine the temperature. When the die temperature reached 135'C, 200 psi pressure was applied. The temperature was raised to 288*C at a rate of 4"C/minute. The composite was cured at 288"C in air under 200 psi pressure for one hour and removed from the press when the die temperature reached 100*C. The composite was then postcured at 288 β C in air for four hours. The ultrasonic c-scan of the composite showed no detectable voids.' Also, the composite can be reprocessed to correct flaws. This procedure was used for making ten composite systems using various resins. Table 7 gives the composite properties. TABLE 7 Thermo-Oxidative Stability Glass Transition Wt Z Moisture Isothermal Temperature Temperature Uptake at 25*C wt loss, 1 at 52 wt Tg, β C for two weeks after 500 loss in air Dry Wet hours at by TGA 232 β C in Composite Air

Claims: What is claimed is: Claims 1. A process for the preparation of a tough, high performance polyimide, which process comprises reacting a triple bond conjugated with an aromatic ring in a bisethynyl compound with the active double bond in a compound containing a double bond activated toward the formation of a Diels-Alder type adduct. 2. The process according to claim 1, wherein the compound containing a double bond activated toward the formation of a Diels-Alder type adduct is a member selected from the group consisting of bismaleimides, biscitraconimides, and benzoquinones. 3. The process of claim 2, which comprises the additional procedural step of addition curing the reaction product to produce a highly linear polymeric structure. 4. The process of claim 3, which comprises heat treating the highly linear polymeric structure to form a thermally-stable aromatic addition-type thermoplastic polyimide. 5. The process of claim 2, wherein the bisethynyl compound and the member selected from the group consisting of bismaleimides, biscitraconimides, and benzoquinones are reacted in stoichiometric quantities. 6. The process of claim 2, wherein the bisethynyl compound and the member selected from the group consisting of bismaleimides, biscitraconimides, and benzoquinones are reacted in off-stoichiometric quantities. 7. The process of claim 6, wherein the bisethynyl compound and the member selected from the group consisting of bismaleimides, biscitraconimides, and benzoquinones are present in a mole ratio between about 7:1 to 1:7. 8. A tough, high performance polyimide prepared by reacting a triple bond conjugated with an aromatic ring in a bisethynyl compound with the active double bond in a compound containing a double bond activated toward the formation of a Diels-Alder type adduct. 9. The polyimide according to claim 8, wherein the compound containing a double bond activated toward the formation of a Diels-Alder type adduct is a member selected from the group consisting of bismaleimides, biscitraconimides, and benzoquinones. 10. A tough, high performance, highly linear addition- type thermoplastic polyimide prepared by reacting a triple bond conjugated with an aromatic ring in a bisethynyl compound with the active double bond in a member selected from the group consisting of bismaleimides, biscitraconimides, and benzoquinones, followed by addition curing the reaction product. 11. A tough, thermally-stable, high performance, highly linear aromatic addition-type thermoplastic polyimide prepared by reacting a triple bond conjugated with an aromatic ring in a bisethynyl compound with the active double bond in a member selected from the group consisting of bismaleimides, biscitraconimides, and benzoquinones, followed by successive addition curing and heat treating the reaction product. 12. The polyimide of claim 9, wherein the bisethynyl compound and the member selected from the group consisting of bismaleimides, biscitraconimides, and benzoquinones are reacted in stoichiometric quantities. 13. The polyimide of claim 9, wherein the bisethynyl compound and the member selected from the group consisting of bismaleimides, biscitraconimides, and benzoquinones are reacted in off-stoichiometric quantities. 14. The polyimide of claim 13,-wherein the bisethynyl compound and the member selected from the group consisting of bismaleimides, biscitraconimides, and benzoquinones are present in a mole ratio between about 7:1 and 1:7. 15. A molding compound comprising the polyimide of claim 8. 16. An adhesive composition comprising the polyimide of claim 8. 17. A polymer matrix composite comprising the polyimide of claim 8. 18. A bismaleimide having the general formula wherein R, and R ϊ are a monovalent hydrogen, alkyl, or aryl; R 3 is a divalent aryl radical or a divalent alkyl radical; and n has a value from 1 to 20. 19. A process for the preparation of a bismaleimide, which comprises reacting an anhydride with a diamine in a solvent having a boiling point of less than about 70°C to form an amic acid, chemically imidizing the amic acid in the same solvent to form the bismaleimide, wherein the anhydride is a member selected from the group consisting of maleic anhydride and citraconic anhydride, and the diamine is a compound having the general formula wherein R 3 is a divalent aryl radical or a divalent alkyl radical and n has a value from 1 to 20, and wherein the solvent is a member selected from the group consisting of acetone and ethylene chloride.

Inventors: PATER RUTH SHIEH, PATER, RUTH, SHIEH
Applicants: US GOVERNMENT, UNITED STATES GOVERNMENT, AS REPRESENTED BY THE N
Application year: 1989
Main classification: C08F26
IPC codes: C07D403, C08F2, C08F22, C08F38, C08G61, C08G73