Source: {"pile_set_name": "USPTO Backgrounds"}

The present invention relates to polymeric blends comprising crosslinking oligomer(s) and polymer(s) wherein the backbones of the oligomer(s) and polymer(s) are from different chemical families.
Recently, chemists have sought to synthesize oligomers for high performance advanced composites suitable for aerospace applications. These composites should exhibit solvent resistance; be tough, impact resistant, and strong; be easy to process; and be thermoplastic. Oligomers and composites that have thermo-oxidative stability and, accordingly, can be used at elevated temperatures are particularly desirable.
While epoxy-based composites are suitable for many applications, their brittle nature and susceptibility to thermal and hydrolytic degradation make them inadequate for many aerospace applications, especially those applications which require thermally stable, tough composites or service in harsh conditions. Accordingly, research has recently focused on polyimide composites to achieve an acceptable balance between thermal or hydrolytic stability, solvent resistance, and toughness. Still the maximum temperatures for use of the polyimide composites, such as PMR-15, are about 600-625xc2x0 F., since they have glass transition temperatures of about 690xc2x0 F. PMR-15 also suffers from brittleness.
There has been a progression of polyimide sulfone compounds synthesized to provide unique properties or combinations of properties. For example, Kwiatkowski and Brode synthesized maleic-capped linear polyarylimides as disclosed in U.S. Pat. No. 3,839,287. Holub and Evans synthesized maleic- or nadic-capped, imido-substituted polyester compositions as disclosed in U.S. Pat. No. 3,729,446. We synthesized thermally stable polysulfone oligomers as disclosed in U.S. Pat. No. 4,476,184 or U.S. Pat. No. 4,536,559, and have continued to make advances with polyetherimidesulfones, polybenzoxazolesulfones, polybutadienesulfones, and xe2x80x9cstarxe2x80x9d or xe2x80x9cstar-burstxe2x80x9d multidimensional oligomers. We have shown surprisingly high glass transition temperatures yet reasonable processing and desirable physical properties in many of these oligomers and their composites.
Polybenzoxazoles, such as those disclosed in our U.S. Pat. No. 4,965,336 (to Lubowitz and Sheppard) and U.S. Pat. No. 4,868,270 (to Lubowitz, Sheppard, and Stephenson), may be used at temperatures up to about 750-775xc2x0 F., since these composites have glass transition temperatures of about 840xc2x0 F. Some aerospace applications need composites which have even higher use temperatures while maintaining toughness, solvent resistance, ease of processing, formability, strength, and impact resistance.
Multidimensional oligomers, such as disclosed in U.S. Pat. No. 5,210,213, are easier to process than some advanced composite oligomers since they can be handled at lower temperatures. Upon curing, however, the oligomers chemically crosslink through their end caps in addition polymerization so that the thermal resistance of the resulting composite is markedly increased with only a minor loss of stiffness, matrix stress transfer (impact resistance), toughness, elasticity, and other mechanical properties. Glass transition temperatures above 950xc2x0 F. are achievable.
Commercial polyesters, when combined with well-known diluents, such as styrene, do not exhibit satisfactory thermal and oxidative resistance to be useful for aircraft or aerospace applications. Polyarylesters (i.e., arylates) are often unsatisfactory, also, since the resins often are semi-crystalline which may makes them insoluble in laminating solvents, intractable in fusion, and subject to shrinking or warping during composite fabrication. Those polyarylesters that are soluble in conventional laminating solvents remain so in composite form, thereby limiting their usefulness in structural composites. The high concentration of ester groups contributes to resin strength and tenacity, but also makes the resin susceptible to the damaging effects of water absorption. High moisture absorption by commercial polyesters can lead to distortion of the composite when it is loaded at elevated temperature.
High performance, aerospace, polyester advanced composites, however, can be prepared using crosslinkable, end capped polyester imide ether sulfone oligomers that have an acceptable combination of solvent resistance, toughness, impact resistance, strength, ease of processing, formability, and thermal resistance. By including Schiff base (xe2x80x94CHxe2x95x90Nxe2x80x94), imidazole, thiazole, or oxazole linkages in the oligomer chain, the linear, advanced composites formed with polyester oligomers of our copending application U.S. Ser. No. 07/137,493, now U.S. Pat. No. 5,705,598, can have semiconductive or conductive properties when appropriately doped.
Conductive and semiconductive plastics have been extensively studied (see, e.g., U.S. Pat. Nos. 4,375,427; 4,338,222; 3,966,987; 4,344,869; and 4,344,870), but these polymers do not possess the blend of properties which are essential for aerospace applications. That is, the conductive polymers do not possess the blend of (1) toughness, (2) stiffness, (3) elasticity, (4) ease of processing, (5) impact resistance (and other matrix stress transfer capabilities), (6) retention of properties over a broad range of temperatures, and (7) high temperature resistance that is desirable on aerospace advanced composites. The prior art composites are often too brittle.
Thermally stable multidimensional oligomers having semiconductive or conductive properties when doped with suitable dopants are also known and are described in our copending applications (including U.S. Ser. No. 07/212,404, now U.S. Pat. No. 5,817,738, to Lubowitz, Sheppard, and Torre). The linear arms of the oligomers contain conductive linkages, such as Schiff base (xe2x80x94Nxe2x95x90CHxe2x80x94) linkages, between aromatic groups. Sulfone and ether linkages are interspersed in the arms. Each arm is terminated with a mono- or difunctional end cap (i.e. an end cap having one or two crosslinking functionalities) to allow controlled crosslinking upon heat-induced or chemically-induced curing. Other xe2x80x9csemiconductivexe2x80x9d oligomers are described in our other copending applications.
Polyamide oligomers and blends are described in our U.S. Pat. Nos. 4,935,523; 4,847,333; and 4,876,328, and polyetherimide oligomers and blends are described in our U.S. Pat. No. 4,851,495.
Polyamideimides are generally injection-moldable, amorphous, engineering thermoplastics which absorb water (swell) when subjected to humid environments or immersed in water. Polyamideimides are generally described in the following patents: U.S. Pat. Nos. 3,658,938; 4,628,079; 4,599,383; 4,574,144; or 3,988,344. The thermal integrity and solvent-resistance can be greatly enhanced by capping amideimide backbones with monomers that present one or two crosslinking functionalities at each end of the oligomer, as described in U.S. Pat. No. 5,104,967.
Blends of these oligomers are described in many of our earlier applications and comprising a mixture of an oligomer and a compatible polymer, generally of the same family, of substantially the same backbone. The polymer is formed by an analogous condensation generally substituting a noncrosslinking end-cap monomer (such as phenol, benzoic acid chloride, or aniline) for the crosslinking end cap used in the oligomers.
Interpenetrating or semi-interpenetrating networks are also known, such as those described by Egli et al. in xe2x80x9cSemi-Interpenetrating Networks of LARC-TPIxe2x80x9d available from NASA-Langley Research Center.
Mixed polymer blends, such as an amideimide/phenoxyphenylsulfone blend, are described in U.S. Pat. No. 3,658,939.
Blends present promise for tailoring the mechanical properties of composites while retaining ease of processing. The present invention comprises advanced composite blends that are mixed chemical blends of a linear or; multi-dimensional crosslinking oligomer(s) of one chemical family, such as a heterocycle, and corresponding linear or multidimensional polymer(s), unable to crosslink, from a different chemical family, such as ethersulfone. Generally the polymer has an average formula weight that is initially higher than that of the oligomer, but the formula weight of the oligomeric portion of the blend will increase appreciably during curing through addition (i.e. homo-) polymerization between the crosslinking functionalities. The ratio of oligomer(s) to polymer(s) can be varied to achieve the desired combination of physical properties. Usually the ratio is such that the addition polymer formed during curing constitutes no more than about 50 mol % of the composite.
Briefly, therefore, the present invention is directed to an advanced composite blend comprising a mixture of at least one crosslinkable oligomer and at