Abstract:
Molecular dispersions of rigid-rod polybenzobisazole polymer/copolymer in thermoplastic matrices in a weight ratio of about 1:99 to 50:50 are prepared by dissolving the rigid-rod polymer in an alcoholic or aprotic solvent, dissolving the thermoplastic polymer in an alcoholic or aprotic solvent, combining the two solutions and recovering the resulting molecular composite from the combined solution. These molecular dispersions consist essentially of rigid-rod polybenzobisazole polymer/copolymer having sulfonic acid groups pendant to the polymer/copolymer backbone and thermoplastic polymers selected from the group consisting of polyvinylpyridines, poly(arylene ether ketone)s with a pyridyl functionality, aromatic poly(pyridine ether)s and poly(pyridine ether sulfone) copolymers, and polyimides with a pyridyl functionality.

Description:
RIGHTS OF THE GOVERNMENT 
     The invention described herein may be manufactured and used by or for the Government of the United States for all governmental purposes without the payment of any royalty. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to polymeric composites of rigid-rod aromatic heterocyclic polymers and flexible thermoplastic polymers and a method for the preparation of composite articles with rigid-rod aromatic heterocyclic polymers molecularly dispersed in a continuous thermoplastic polymeric matrix. 
     Polybenzobisazole polymers, and particularly the so-called &#34;rigid-rod&#34; benzobisazole polymers, are noted for high tensile strength, high tensile modulus and high thermal stability. However, many benzazole polymers are difficult to fabricate into useful articles. Rigid and semi-rigid polybenzobisazoles do not have glass transition temperatures at any temperature at which they are stable. Therefore, the polymers are ordinarily spun from solution to form fibers, which serve as reinforcement within a thermosetting matrix, such as epoxy resins, to form composites. However, the fibers and the cured composites are not moldable or thermoformable. 
     Many moldable and thermoformable polymers are known. Exemplary polymers include thermoplastic polyamides, polyimides, polyquinolines, polyquinoxalines, poly(aromatic ether ketones) and poly(aromatic ether sulfones). However, those polymers do not have the high tensile strength and modulus which are characteristic of polybenzobisazole polymers. 
     Attempts have been made to synthesize articles which combine the processability of the thermoplastic polymer with the superior mechanical properties of the polybenzobisazole polymers. To this end, molecular composites of rigid rod polybenzobisazole and flexible polymers have been studied. A molecular composite is defined as a molecular dispersion of a rigid-rod polymer in a flexible polymer matrix such that the rigid-rod polymer acts as the reinforcing elements. To ensure molecular dispersion of the rigid-rod polymer in the blend, processing of the composite can only be carried out from solution at a concentration lower than the critical concentration, C cr , to prevent segregation of the rigid-rod reinforcement. At a concentration above C cr , the solution becomes biphasic where the rigid-rod segregates into liquid crystalline domains which are dispersed in an entangled flexible matrix. Such molecular composites are described in numerous references, such as Helminiak et al, U.S. Pat. No. 4,207,407, issued Jun. 10, 1980; Helminiak et al, U.S. Pat. No. 4,377,546, issued Mar. 22, 1983; Hwang et al, U.S. Pat. No. 4,631,318, issued Dec. 23, 1986; Nishihara et al, U.S. Pat. No. 4,749,753, issued Jun. 7,1988; Uy, U.S. Pat. No. 4,972,028, issued Nov. 20, 1990; Arnold et al, U.S. Pat. No. 4,977,223, issued Dec. 11, 1990; Tan et al, U.S. Pat. No. 5,086,120, issued Feb. 4, 1992; Arnold et al, U.S. Pat. No. 5,095,075, issued Mar. 10, 1992 and Hwang et al, U.S. Pat. No. 5,217,809, issued Jun. 8, 1993. 
     However, polybenzobisazoles, and particularly rigid-rod polybenzobisazoles, are incompatible with many thermoplastic polymers. When dopes containing polybenzobisazole and a thermoplastic polymer are coagulated, the thermoplastic agglomerates and/or phase separates. The resulting shaped articles either have poorer properties in all directions than the corresponding thermoplastic alone, or have superior properties in one direction and inferior properties in all other directions. Such compositions may be useful for fibers, but not for molded articles. 
     What are needed are materials and processes which can be used to make molded articles containing reinforcing amounts of polybenzobisazole polymer which have superior properties in at least two dimensions and/or are not substantially phase separated. 
     Accordingly, it is an object of the present invention to provide molecular dispersions of rigid-rod polybenzobisazole polymer in a thermoplastic matrix. 
     It is another object of the present invention to provide molecular dispersions of rigid-rod polybenzobisazole polymer in a thermoplastic matrix above the critical concentration to a degree at which the polymer blend can be cast from solution. 
     It is yet another object of the present invention to provide a method for preparing molecular dispersions of rigid-rod polybenzobisazole polymer in a thermoplastic matrix above the critical concentration to a degree at which the polymer blend can be cast from solution. 
     Other objects, aspects and advantages of the present invention will be apparent to those skilled in the art from a reading of the following detailed disclosure of the invention. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention there are provided molecular dispersions of rigid-rod polybenzobisazole polymer/copolymer in thermoplastic matrices in a weight ratio of about 1:99 to 50:50. These molecular dispersions consist essentially of rigid-rod polybenzobisazole polymer/copolymer having sulfonic acid groups pendant to the polymer/copolymer backbone and thermoplastic polymers as defined hereinafter. The rigid-rod polybenzobisazole polymers/copolymers having sulfonic acid groups pendant to the polymer/copolymer backbone are soluble in alcoholic and aprotic solvents when solubilized with an organic tertiary amine. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The rigid-rod polybenzobisazole polymers and copolymers suitable for use in the present invention are disclosed in the following U.S. Pat. Nos.: 5,312,876, issued May 17, 1994 to Dang et al; 5,344,896, issued Sept. 6, 1994 to Dang et al; and 5,508,376, issued Apr. 16, 1996 to Dang et al, which are incorporated by reference herein. 
     Dang et al, U.S. Pat. No. 5,508,376, disclose polymers and copolymers having repeating units of the formulae I or II, below: ##STR1## wherein x has a value of 0.0 to 1.00 and Q is a benzobisazole of the formula: ##STR2## wherein X is --O-- or --S--; and ##STR3## wherein y has a value of 0.05 to 1.00; Q is as described above; R is selected from the group consisting of: ##STR4## and R&#39; is selected from the group consisting of alkyl having 1 to 5 carbon atoms, alkaryl having 7 to 12 carbon atoms, aralkyl having 7 to 12 carbon atoms and substituted aromatic having 1 to 3 substituent groups selected from the group consisting of nitro, ethynyl, phenylethynyl, and the like. Examples of suitable alkyl groups include methyl, ethyl, butyl and the like; suitable alkaryl and aralkyl groups include benzyl, tolyl, and the like; suitable substituted aromatic groups include nitroaniline, ethynylaniline, phenylethynylaniline and the like. Referring to formula I, above, the polymer where x is 1.0, prior to solubilization with an amine, is hereinafter referred to as 2-sulfo-PBI. 
     Dang et al, U.S. Pat. Nos. 5,312,876 and 5,344,896 disclose polymers and copolymers having repeating units of the formula: ##STR5## wherein y has a value of 0.05 to 1.00, M is an alkali metal and Q is a benzobisazole as described above; and R&#39;&#34; is selected from the group consisting of: ##STR6## 
     Another rigid-rod polymer that can be employed in the current invention has repeating units of the formula: ##STR7## wherein x, Q and R&#39; are as described previously. 
     The polymers set forth above are soluble in alcoholic and aprotic solvents when solubilized with an organic tertiary amine. Such solubilization is accomplished by adding an organic tertiary amine to a suspension of the polymer. Suitable alcohols include methyl, ethyl, propyl and butyl alcohols. Suitable aprotic solvents include N,N-dimethylacetamide (DMAC), dimethylsulfoxide (DMSO), N,N-dimethylformamide (DMF) and N-methyl-2-pyrrolidone (NMP). Suitable organic tertiary amines have the formula NR&#39; 3 , wherein R&#39; is as previously defined. The quantity of organic tertiary amine is generally a stoichiometric amount, although a greater quantity may be employed. The rate of solubilization of the polymer may be increased by heating the mixture to reflux. 
     The thermoplastic polymers suitable for use in the present invention include polyvinylpyridines, poly(arylene ether ketone)s with a pyridyl functionality, aromatic poly(pyridine ether)s and poly(pyridine ether sulfone) copolymers, and polyimides with a pyridyl functionality. The polyvinylpyridine polymers are available commercially from Scientific Polymer Products, Ontario,New York. Exemplary vinylpyridine polymers include: ##STR8## 
     The poly(arylene ether ketone)s with pyridyl functionality are disclosed by Hergenrother, P. M., Jensen, B. J. and Havens, S. J., Polymer, 1988, Volume 29, pp 358-368, which is incorporated herein by reference. Briefly, these polymers are prepared by the reaction of activated aromatic dihalides and potassium bisphenates in DMAC (or DMSO) at 155° C. The polymers shown below are prepared by the reaction of 2,6-di(4-fluorobenzoyl)pyridine (2,6-FBP) with 9,9-bis(4-hydroxyphenyl)fluorene (9,9-HPF) or 2,2-bis(4-hydroxyphenyl)propane (BPA). 
     These polymers include, for example, the following: ##STR9## 
     The aromatic poly(pyridine ether)s and poly(pyridine ether sulfone) copolymers are disclosed by Kricheldorf, H. R., Schwarz, G. and Erxleben, J, J. Makromol. Chem., (1989), 189, 2255-2267, Kricheldorf, H. R. and Jahnke, P., J. Makromol. Chem., (1990), 191, 2027-2035, and Kricheldorf, H. R. and Jahnke, P., Macromolecules, (1992), 25, 1382-1386, which are incorporated herein by reference. The aromatic poly(pyridine ether)s have repeating units of the formula ##STR10## 
     The polyimides with a pyridyl functionality are disclosed by Kurita, K. and Williams, R. L., Journal of Poly. Sci., Chem. Ed., (1974), 12, 1809-1822. These polymers have repeating units: ##STR11## 
     The polymeric composites of this invention are prepared as follows: First, solutions of the rigid-rod polymer and the matrix polymer are prepared. Next, the two solutions are combined and, finally, the resulting polymeric composite is recovered from solution. 
     The solutions of the rigid-rod polymer and the matrix polymer can be prepared by the skilled artisan following procedures known in the art, together with the general guidelines given heretofore as well as the examples which follow. The concentrations of these solutions should be consistent with ease of handling and desired results. The rigid-rod polymer and matrix polymer solutions are then combined in a desired ratio. Upon mixing these solutions, by a process of ionic interchange, the basic thermoplastic replaces the tertiary amine, so that the rigid-rod polymer remains in solution. 
     The resulting molecular composite can be recovered by (a) removing the solvent or (b) coagulating the composite in a nonsolvent. Under either approach, it is preferred practice to heat the composite solution to an elevated temperature determined by the solvent(s) used, under a nitrogen purge to facilitate evaporation of the solvent and the tertiary amine. For example, when using methanol as the solvent and triethyl amine as the tertiary amine, the composite solution may be heated to about 60°-80° C. for several hours to remove the volatile components. It may be necessary to add solvent back to the solution to prevent the concentrated solution from gelling, keeping the polymer blend in homogeneous solution until the solution is free of the tertiary amine. 
     To recover the composite by removing the solvent, the solution is transferred to a casting dish and the solvent is completely removed, using elevated temperature and/or vacuum, as necessary, leaving a film of molecular composite. 
     To recover the composite by coagulation, the solution is poured or otherwise transferred to an excess of nonsolvent for the polymer(s), such as distilled water, to precipitate the composite as a fine aggregate. The resulting aggregate is then filtered, washed and dried. The filtered and washed aggregate may be extracted with the nonsolvent to ensure complete removal of the solvent, then dried. 
    
    
     The following examples illustrate the solubilization of the rigid-rod and the formation of various thermoplastic rigid-rod molecular composite films as well as bulk composite aggregates with different rigid-rod/thermoplastic matrix w/w ratios. 
     EXAMPLE I 
     Methanol-soluble 2-Sulfo-PBI Composition 
     2-Sulfo-PBI (0.30 g) (η=4.0 dl/g, methanesulfonic acid, 30° C.) was suspended in 300 ml anhydrous methyl alcohol and a large molar excess (4.0 g) of triethylamine was added to the red suspension. Dissolution of the polymer started to occur and the solublization was completed under methanol reflux conditions in a dry nitrogen atmosphere overnight. A clear, geenish, yellow solution of the triethylammonium salt of 2-sulfo-PBI in methanol was obtained. Various volumes of this solution were used for the formation of thermoplastic rigid-rod film blends of different compositions. The wt of the polymer in solution was approximately 0.10%. 
     EXAMPLE II 
     Methanol-soluble 2-Sulfo-PBI Composition 
     2-Sulfo-PBI (0.50 g) (η=7.0 dl/g, methanesulfonic acid, 30° C.) was suspended in 300 ml anhydrous methyl alcohol and with the addition of triethylamine (4.0 g), instant solubilization of the polymer in MeOH was observed. The greenish yellow solution was refluxed overnight under a dry nitrogen atmosphere to complete the dissolution. This solution (0.16 wt % polymer) was used in the formation of precipitated blend compositions which were thermally consolidated by compression molding for dynamic mechanical analysis. 
     EXAMPLE III 
     Molecular Composite Film from 2-sulfo-PBI and poly(4-vinylpyridine) with 75/25 w/w rod/coil composition 
     To 40 ml methanolic solution of the triethylammonium salt of 2-sulfo-PBI (0.04 g of the polymer) from example I was added 10 ml methanolic solution containing 0.0137 g poly(4-vinylpyridine). The mixture was vigorously stirred in a three-necked flask fifted with an inlet and an outlet for passage of dry nitrogen. The homogeneous, greenish yellow solution was heated on a hot oil bath in the temperature range 60°-80° C. for several hours, driving off the volatile methanol and triethylamine; the blend solution in methanol was concentrated to a viscous solution. The solution was pipetted into a glass casting dish and the volatiles were evaporated overnight in an evacuated hood. 
     A homogeneous, clear, orange film was obtained and isolated. This film composition corresponded to 1:1 molecular stoichiometry based on ionic interaction between the sulfonic acid pendant of the aromatic heterocyclic rigid-rod and the pyridyl functionality of the thermoplastic matrix. 
     EXAMPLE IV 
     Molecular Composite Film from 2-sulfo-PBI and poly(4-vinylpyridine) with 50/50 w/w rod/coil composition 
     To 25 ml methanolic solution of the triethylammonium salt of 2-sulfo-PBI (0.025 g of the polymer) from example I was added 10 ml methanolic solution containing 0.025 g poly(4-vinylpyridine). After evaporation of the volatiles under a nitrogen purge for several hours at the oil bath temperature in the range 60°-80° C., the homogeneous, viscous methanolic solution was transferred to a glass casting dish and the solvent was evaporated in a vacuum hood overnight, leaving behind a transparent yellowish orange film composite with a 50/50 w/w rod/coil composition. The IR spectrum of the film delineated the structural features of both poly(4-vinylpyridine) and 2-sulfo-PBI. 
     EXAMPLE V 
     Molecular Composite Film from 2-sulfo-PBI and poly(4-vinylpyridine) with 10/90 w/w rod/coil composition 
     To 17 ml methanolic solution of the triethylammonium salt of 2-sulfo-PBI (0.017 g of the polymer) from example I was added 10 ml methanolic solution containing 0.153 g of poly(4-vinylpyridine). After evaporation of the volatiles under a nitrogen purge for several hours at the oil bath temperature in the range 60-80° C. , the homogeneous, viscous methanolic solution was transferred to a glass casting dish and the solvent was evaporated in an evacuated hood overnight, leaving behind an optically clear yellow molecular composite film with a 10/90 w/w rod/coil composition. IR spectrum was used to delineate the structural features of the thermoplastic-rich rod/coil molecular composite. 
     EXAMPLE VI 
     Molecular Composite Film from 2-sulfo-PBI and poly(4-vinylpyridine) with 20/80 w/w rod/coil composition 
     To 30 ml methanolic solution of the triethylammonium salt of 2-sulfo-PBI containing 0.03g of the polymer from example I was added 0.12 g of poly(4-vinylpyridine) dissolved in 10 ml methanol. After evaporation of the volatiles under a nitrogen purge for several hours at the oil bath temperature in the range 60°-80° C., the homogeneous, viscous methanolic solution was transferred to a glass casting dish and the solvent was evaporated in a vacuum hood overnight, leaving behind an optically clear yellow molecular composite film with a 20/80 w/w rod/coil composition. 
     EXAMPLE VII 
     Molecular Composite Film from 2-sulfo-PBI and poly(2-vinylpyridine) with 50/50 w/w rod/coil composition 
     To 20 ml methanolic solution of the triethylammonium salt of 2-sulfo-PBI containing 0.02g of the sulfonic acid polymer from example I was added 0.02 g of poly(2-vinylpyridine) in anhydrous methanol. The stirred homogeneous solution of the polymers was heated on an oil bath in the temperature range 60°-80° C. under a dry nitrogen flow to obtain a homogeneous viscous solution which was pipetted out into a glass casting dish. It was left to dry in the fume hood overnight to obtain an optically clear yellow film with a 50/50 w/w rod/coil composition. 
     EXAMPLE VIII 
     Aggregated Molecular Composite from 2-sulfo-PBI and poly(4-vinylpyridine) with 5/95 w/w rod/coil composition 
     To 10 ml methanolic solution of the triethylammonium salt of 2-sulfo-PBI containing 0.0166 g of the sulfonic acid polymer from example II was added 0.3166 g of poly(4-vinylpyridine) in 20 ml methanol The yellow solution was heated under a nitrogen purge on an oil bath at 80° C. to facilitate evaporation of both methanol and triethylamine. Methanol was added intermittently to prevent the concentrated solution from gelling, keeping the polymer blend in a homogeneous solution. After several hours, the condensate from the solution was free of the triethylamine base (pH indicator). The solution was concentrated further and the viscous solution was pipetted out into a large excess (1 liter) of de-ionized, distilled water to precipitate the polymer blend as a fine suspension. After filtration using a 10-20μ fritted funnel, the composite was Soxhelet extracted with boiling water for 48-72 hours and finally dried in vacuum at 100° C. for 48 hours to provide a dry, yellow fibrous composite specimen. The weight of the recovered, dried sample was 0.260g. 
     EXAMPLE IX 
     Aggregated Molecular Composite from 2-sulfo-PBI and poly(4-vinylpyridine) with 10/90 w/w rod/coil composition 
     To 25 ml methanolic solution of the triethylammonium salt of 2-sulfo-PBI containing 0.0416 g of the sulfonic acid polymer from example II was added 0.375 g of poly(4-vinylpyridine) in 25 ml methanol. The solution was heated under a nitrogen purge on an oil bath at 80° C. to let the volatiles consisting of methanol and triethylamine escape. Methanol was added intermittently to prevent the concentrated solution from gelling, keeping the polymer blend in a homogeneous solution. After several hours, the condensate from the solution was free of the triethylamine base (pH indicator). The solution was concentrated further and the viscous solution was pipetted out into a large excess (1 liter) of de-ionized, distilled water to precipitate the polymer blend as a fine aggregate. After filtration using a 10-20μ fritted funnel, the composite was Soxhelet extracted with boiling water for 48-72 hours and finally dried in vacuum at 100° C. for 48 hours to provide a dry, yellow fibrous composite specimen. The weight of the recovered, dried sample was 0.30 g. 
     EXAMPLE X 
     Aggregated Molecular Composite from 2-sulfo-PBI and poly(4-vinylpyridine) with 20/80 w/w rod/coil composition 
     To 45 ml methanolic solution of the triethylammonium salt of 2-sulfo-PBI containing 0.075 g of the sulfonic acid polymer from example II was added 0.300 g of poly(4-vinylpyridine) in 25 ml methanol. The solution was heated under a nitrogen purge on an oil bath at 85° C. to let the volatiles consisting of methanol and triethylamine escape. Anhydrous methanol was added intermittently to prevent the concentrated solution from gelling, keeping the polymer blend in a homogeneous solution. After several hours, the condensate from the solution tested free of the basic triethylamine (pH indicator). The solution was concentrated further and the viscous solution was coagulated with a large excess (1 liter) of de-ionized, distilled water to precipitate the polymer blend as a fine aggregate. After filtration using a 10-20μ fritted funnel, the composite was Soxhelet extracted with boiling water for 48-72 hours and finally dried in vacuum at 100° C. for 48 hours to provide a dry, yellowish orange fibrous composite specimen. 
     EXAMPLE XI 
     Compression molded poly(4-vinylpyridine)/2-Sulfo-PBI composites 
     The aggregated composite samples with low rod contents (5/95 and 10/90 w/w 2-sulfo-PBI/poly(4-vinylpyridine)) were thermally consolidated by hot pressing in the temperature range 210°-220° C. at 5000 psi pressure. The matrix polymer poly(4-vinylpyridine) was thermally consolidated at 195° C. (reported T g  =158° C.) at 3000 psi. The thermomechanical properties of the thermoplastic and the composites were investigated by dynamic mechanical analysis. The glass transition temperatures of the matrix polymer versus the 2-sulfo PBI/poly(4-vinylpyridine) composites are shown in Table I, below: 
     
                       TABLE I______________________________________Rod/Thermoplastic ratio (w/w)               T.sub.g (° C.)______________________________________0/100               1535/95                17610/90               210______________________________________ *Obtained from Dynamic Mechanical Analysis (tan δ measurements) 
    
     EXAMPLE XII 
     Preparation of poly(1,7-dihydropyrimido(1,2-d:4,5-d&#39;) diimadazo-2,6-diyl(2-(2-sulfo)-p-phenylene)) 
     In a 150 mL resin flask equipped with a mechanical stirrer, nitrogen inlet/outlet, and pressure regulator, was placed 3.6930 g (15.0 mmol) of 2-sulfoterephthalic acid, 3.9983 g (15.0 mmol) of 1,2,4,5-tetraaminopyridine trihydrochloride monohydrate, and 8.67 g polyphosphoric acid (PPA) (77% P 2  O 5 ). The resulting mixture was dehydrochlorinated under reduced pressure (176 mm) while heating slowly to 65° C. The reaction temperature was maintained at 65° C. for 16 hr, raised to 80° C. for 4 hr, then cooled to 60° C. and 10.97 g of P 2  O 5  was added to the mixture to bring the polymer concentration to 18%. The mixture was headed under a nitrogen atmosphere at 60° C. for 2 hr, 100° C. for 16 hr and 170° C. for 16 hr. As the temperature increased, stir opalescence began to occur at about 165° C. The mixture was finally heated to 190° C. for 16 hr. The polymer was precipitated in water, broken up with a blender, collected by suction filtration, washed with water and dried under reduced pressure (0.05 mm) at 100° C. The polymer exhibited an intrinsic viscosity of 17 dl/g in methanesulfonic acid at 30° C. 
     Various modifications may be made to the invention as described without departing from the spirit of the invention or the scope of the appended claims.