Abstract:
The compound N-(3-ethynylphenyl)maleimide (NEPMI) was used to prepare thermally stable, glassy polyimides which did not exhibit glass transition temperatures below 500° C. NEPMI was blended with the maleimide of methylene dianiline (BMI) and heated to form the polyimide. NEPMI was also mixed with Thermid 600®, a commercially available bisethynyl oligomeric material, and heated to form a thermally stable, glassy polyimide. Lastly, NEPMI was blended with both BMI and Thermid 600® to form thermally stable, glassy polyimides.

Description:
ORIGIN OF THE INVENTION 
     The invention described herein was made in the performance of work under a NASA contract and is subject to the provision of Section 305 of the National Aeronautics and Space Act of 1958, as amended, Public Law 85-568 (72 Stat. 435; 42 USC 2457). 
    
    
     CROSS-REFERENCE 
     This application is a continuation-in-part of copending application Ser. No. 528,666, filed May 18, 1990, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to polyimides which are suitable for high temperature applications as a result of their high or nondetectable glass transition temperatures and their high thermooxidative stabilities. It relates particularly to three types of polyimides which are prepared from monomers containing carbon-carbon double and/or triple bonds, in particular N-(3-ethynylphenyl)maleimide (NEPMI). 
     2. Description of Related Art 
     Polymers prepared from the reaction of organic compounds containing pendant ethynyl groups or organic oligomers containing pendant ethynyl groups are well known for their thermooxidative stability and exceptionally high glass transition temperatures. Likewise, organic compounds or oligomers containing activated carbon-carbon double bonds and imide moieties (i.e. maleimides) can be used to prepare thermally stable polymers. 
     Hergenrother et al. (U.S. Pat. No. 4,889,912 and U.S. Pat. No. 4,937,356) prepared acetylene terminated aspartimides by reacting an aromatic diamine with NEPMI. These acetylene terminated aspartimides were blended with acetylene terminated polyarylene ethers or sulfone oligomers to yield polymers with improved mechanical properties such as fracture toughness. 
     Hergenrother et al. (U.S. Pat. No. 4,861,882) also prepared ethynyl terminated imidothioethers by reacting a dimercaptan with NEPMI. These ethynyl terminated imidothioethers were then blended with other ethynyl terminated oligomers to produce resin blends which are useful as adhesives, coatings, and films. The glass transition temperatures of these blends ranged from 193°-245° C. 
     SUMMARY OF THE INVENTION 
     The compound N-(3-ethynylphenyl)maleimide (NEPMI) has the following structural formula: ##STR1## This monomer is highly reactive due to its two types of unsaturation (double and triple bonded carbons). The ethynyl group allows for copolymerization with other ethynyl-containing monomers and oligomers and the maleimide group allows for copolymerization with maleimide monomers and oligomers. 
     The highly reactive nature of NEPMI is demonstrated when it is heated past its melting point of 130° C. Depending on the heating rate, the liquid form of NEPMI exhibits an intense exotherm in the region of 155°-248° C. This exotherm is so strong that active cooling is necessary in order to dissipate the heat. The resulting polymers do not exhibit glass transitions below 500° C. These characteristics are also demonstrated by compounds having the following general structural formula: ##STR2## where Ar is any aromatic moiety. 
     An object of the present invention is to provide high temperature polyimides by reacting NEPMI with bisethynyl-containing monomers and/or oligomers and heating them to a temperature where they react. 
     Another object is to provide high temperature polyimides by reacting NEPMI with bismaleimide monomers and/or oligomers and heating them to a temperature where they react. 
     Another object is to provide high temperature polyimides by reacting NEPMI with mixtures of bisethynyl and bismaleimide monomers and/or oligomers and heating them to a temperature where they react. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     EXAMPLE 1 
     High temperature polymers were prepared by heating 5-10 mg samples of NEPMI in an open pan at heating rates of 1° C./min., 10° C./min., 20° C./min., 50° C./min. , and 100° C./min. using a DuPont 1090 Thermal Analyzer. Table 1 lists the melt onset, exotherm temperature and intensity, and temperature for 5% weight loss in air for each polymer prepared. For comparison, samples of the maleimide of methylene dianiline (BMI), and samples of Thermide 600®, a bisethynyl oligomeric compound available from National Starch, were heated at heating rates of 10° C./ min. and 50° C./min. using the same thermal analyzer. These results are also recorded in Table 1. 
     
                       TABLE 1______________________________________Pure Reactants                     Ex-           Temp.            Melt     otherm Ex-    5% Wt.  Heating   Onset    Temp.  otherm loss inSample Rate      (°C.)                     (°C.)                            Intensity                                   air______________________________________NEPMI   1° C./min            130.5    155    low    405° C.NEPMI  10° C./min            131.2    200    low    412° C.NEPMI  20° C./min            131.7    215    moderate                                   420° C.NEPMI  50° C./min            135.7    244    high   410° C.NEPMI  100° C./            135.2    248    high   412° C.  minBMI    10° C./min            151.7    246    very low                                   420° C.BMI    50° C./min            154.7    246    very low                                   410° C.Thermid  10° C./min            Not      250    low    508° C.600 ®        observedThermid  50° C./min            Not      297    moderate                                   503° C.600 ®        observed______________________________________ 
    
     EXAMPLE 2 
     NEPMI was blended in various ratios with the maleimide of methylene dianiline (BMI) and thermally polymerized as described in Example 1. BMI has the following structural formula: ##STR3## Table 2 shows the heating rate, melt temperature, exotherm temperature, temperature of 5% weight loss in air, and glass transition temperature (Tg) for the resulting polymer blends and is compared to BMI and Thermid 600® control polymers. It is evident from the data that the presence of NEPMI offers a significant improvement in the Tg of these polymers. 
     
                       TABLE 2______________________________________Polymers from NEPMI-BMI Blends                            Temp.Ratio of         Melt    Exotherm                            5%NEPMI/ Heating   Temp.   Temp.   wt. loss                                   TgBMI    Rate      (°C.)                    (°C.)                            (°C.)                                   (°C.)______________________________________90/10  10° C./min            122.8   210     418    &gt;50090/10  50° C./min            131.3   240     432    &gt;50080/20  10° C./min            107.9   208     415    &gt;500            12080/20  50° C./min            110     250     410    &gt;500            13570/30  10° C./min            109.4   214     430    &gt;500            12070/30  50° C./min            111.9   250     420    &gt;500            13360/40  10° C./min            109.4   211     410    &gt;500            12660/40  50° C./min            113     251     420    &gt;50050/50  10° C./min            109.9   212     442    &gt;50050/50  50° C./min            113     248     418    &gt;50080/20  10° C./min            110     224     428    &gt;500            12980/20  50° C./min            113     252     418    &gt;500            137BMI    10° C./min            151.7   246     420     290Thermid  10° C./min            None    250     508     305600 ®______________________________________ 
    
     Although BMI was used in this example, other bismaleimides can also be used which have the following structure: ##STR4## where Ar is any aromatic moiety and n=1-50. 
     EXAMPLE 3 
     NEPMI was also blended in various ratios with the ethynyl-containing oligomeric compound Thermid 600® and thermally polymerized as described in Example 1. Thermid 600® has the following structural formula: ##STR5## where n=1-10. 
     Table 3 contains data on the resulting NEPMI-Thermid 600® blends as compared to BMI and Thermid 600® control polymers. 
     
                       TABLE 3______________________________________Polymers from NEPMI-Thermid 600 ®Ratio of                          Temp.NEPMI/           Melt     Exotherm                             of 5%Thermid  Heating   Temp.    Temp.   wt. loss                                    Tg600 ®  Rate      (°C.)                     (°C.)                             (°C.)                                    (°C.)______________________________________90/10  10° C./min            127.6    210     415    &gt;50090/10  50° C./min            131.7    246     438    &gt;50080/20  10° C./min            127.1    218     432    &gt;50080/20  50° C./min            133.1    254     396    &gt;50070/30  10° C./min            128      218     435    &gt;50070/30  50° C./min            132      252     440    &gt;50060/40  10° C./min            127      228     378    &gt;50060/40  50° C./min            129      276     432    &gt;50050/50  10° C./min            128      230     410    &gt;50050/50  50° C./min            130      278     462    &gt;50020/80  10° C./min            128      246     455    &gt;50020/80  50° C./min            130      288     500    &gt;500BMI    10° C./min            151.7    246     420     290Thermid  10° C./min            Not      250     508     305600  ®       Observed______________________________________ 
    
     Although Termid 600® was used in this example, other bisethynyl terminated oligomers known to those skilled in the art may also be employed. These compounds have the following general structure: 
     
         HC.tbd.C--(Ar).sub.n --C.tbd.CH 
    
     where Ar is any aromatic moiety and n=1-50. 
     EXAMPLE 4 
     Polymers were made by heating blends of NEPMI, Thermid 600®, and BMI in various ratios. The data is presented in Table 4 and compared to BMI and Thermid 600® control polymers. 
     
                       TABLE 4______________________________________Ternary BlendsRatio of                          Temp.NEPMI/                            of 5%BMI/             Melt     Exotherm                             weightThermid  Heating   Temp.    Temp.   loss  Tg600 ®  Rate      (°C.)                     (°C.)                             (°C.)                                   (°C.)______________________________________1/1/1  10° C./min            105      230     468   &gt;5001/1/1  50° C./min            114      260     458   &gt;5001/0.5/3  10° C./min            111      247     450   &gt;5001/0.5/3  50° C./min            125      290     482   &gt;5001/3/2  10° C./min            107      236     462   &gt;5001/3/2  50° C./min            116      256     450   &gt;500BMI    10° C./min              151.7  246     420    290Thermid  10° C./min            None     250     508    305600 ®______________________________________ 
    
     The utility of these polymers is dependent on their ability to be formed from monomers and oligomers which melt and polymerize without the evolution of volatiles. The presence of NEPMI in the polymer results in an increase in crosslink density, and thus, they do not exhibit a glass transition (Tg) below 500° C. These polymers are useful as high temperature composite matrix resins for supersonic aircraft structures and mainframe and engine applications where high Tg is a requirement.