HIGH COMPRESSIVE STRENGTH LIQUID CRYSTALLINE POLYMERS AND FIBERS AND FILMS THEREOF

Abstract: The present invention provides fibers and films of articulated liquid crystalline polymers which have improved compressive properties, as well as articulated liquid crystalline polymers having less than about 5 mole % articulated monomer units.

Description: HIGH COMPRESSIVE STRENGTH LIQUID CRYSTALLINE POLYMERS AND FIBERS AND FILMS THEREOF BACKGROUND OF THE INVENTION This invention relates to fibers and films of rigid rod heterocyclic liquid crystalline polymers having improved compressive strength. Ordered polymers are polymers having an "ordered," orientation in space i.e., linear, circular, star shaped, or the like, imposed thereon by the nature of the monomer units making up the polymer. Most ordered polymers possess a linear "order" due to the linear nature of the monomeric repeating units comprising the polymeric chain. Linear ordered polymers are also known as "rod-like" polymers. As a result of their rigid-rod-like molecular structures, these materials form liquid crystalline solutions, and they are also known as liquid crystalline polymers. For example, U.S. Patent No. 4,423,202 to Choe, discloses a process for the production of para-ordered, aromatic heterocyclic polymers having an average molecular weight in the range of from about 10,000 to 30,000. U.S. Patent No. 4,377,546 to Helminiak, discloses a process for the preparation of composite films prepared from para-ordered, rod-like, aromatic, heterocyclic polymers embedded in an amorphous heterocyclic system. U.S. Patent Nos. 4,323,493 and 4,321,357 to Keske et al., disclose melt prepared, ordered, linear, crystalline injection moldable polymers containing aliphatic, cycloaliphatic and araliphatic moieties. U.S. Patent No. 4,229,566 to Evers et al., describes para-ordered aromatic heterocyclic polymers characterized by the presence of diphenoxybenzene "swivel" sections in the polymer chain. U.S. Patent No. 4,207,407 to Helminiak et al., discloses composite films prepared from a para-ordered, rod-like aromatic heterocyclic polymer admixed with a flexible, coil-like amorphous heterocyclic polymer. U.S. Patent No. 4,108,835 to Arnold et al., describes para-ordered aromatic heterocyclic polymers containing pendant phenyl groups along the polymer chain backbone. Ordered polymer solutions in polyphosphoric acids (including PBZT compositions) useful as a dope in the production of polymeric fibers and films are described in U.S. Patent NOS. 4,533,692, 4,533,693 and 4,533,724 (to Wolfe et al.). U.S. Patent Nos. 4,939,235, 4,973,442, and 4,963,428 disclose films comprising multiaxially oriented liquid crystalline polymers and apparatus for making such films. The disclosures of each of the above described patents are incorporated herein by reference. Polybenzazole ("PBZ") polymers are one class of liquid crystalline polymers currently of great interest in the art. Such PBZ polymers include polybenzoxazole ("PBO") , polybenzothiazole ("PBZT"), and polybenzimidazole ("PBI") . Polybenzazole polymers and their synthesis are described at length in numerous references, such as Wolfe, Liquid Crystalline Polymer Compositions, Process and Products. U.S. Patent 4,533,693 (August 6, 1985) and W.W. Adams et al., The Material Science and Engineering of Rigid-Rod Polymers (Materials Research Society 1989) , which are incorporated herein by reference. The major problems confronting PBZT film and fiber based composites are low compressive strength compared with tensile properties and poor interlaminar adhesion. This is attributed to buckling of the fibrillar network, as evidenced by kinked regions which are localized deformation. "Microbuckling" appears to be the cause of low compressive strength in fibers and is also observed in films. Properties of a typical PBZT fiber are listed in Table 1 by way of example. Table 1 - Properties of PBZT Fiber Tensile Strength > 500 Ksi Tensile Modulus > 55 Msi Compressive Strength 30-50 Ksi Density = 1.6 g/cc Thermal Stability > 650°C (Nitrogen) Flexible at -196°C Electrically Insulating Accordingly, ways of improving the compressive strength of liquid crystalline polymers are being sought. BRIEF DESCRIPTION OF DRAWINGS Fig. 1 is a schematic depicting the synthesis of APBTZ. Figs. 2 and 3 are schematic representations of the articulated linkages formed in APBTZ. Fig. 4 is a diagrammatic representation of the morphology of PBZT versus APBTZ. SUBSTITUTE SHEET Fig. 5 is a schematic representation showing the synthesis of one perferred articulated monomer in accordance with the present invention. Fig. 6 is a graph showing mole percent articulation versus intrinsic viscosity. Fig. 7 is a diagrammatic representation of one fiber spinning apparatus for use in the present invention. Fig. 8 is a diagrammatic representation of fiber drying for use in the present invention. Fig. 9 is a schematic representation of a tensile test device for use in practicing the present invention. Fig. 10 is a graph showing mole percent articulation versus compressive strength. Fig. 11 is an illustration of one polymerization apparatus for use in the present invetion. SUMMARY OF THE INVENTION The present invention provides fibers and films having improved compressive strength and methods of making such fibers and films. In one embodiment of the present invention, the fiber or film comprises a rigid-rod, heterocyclic liquid crystalline polymer comprising up to about 25 mole % of at least one articulated monomer unit. In one preferred embodiment of the present invention, the compressive strength of the fiber or film is improved by a factor of about 3 to 4 over the compressive strength of a comparable fiber or film consisting of the rigid-rod, heterocyclic liquid crystalline polymer. Fibers and films wherein the articulated monomer unit is present at up to about 10 mole % are preferred in accordance with the present invention. Up to about 15 mole % articulated monomer unit is particularly preferred, with a mole % of from about 0.5 to about 2.5 mole % being especially preferred. Articulated monomer units for use in the present invention are selected on the basis of compatibility with the properties of the liquid crystalline polymer into which they will be incorporated. The monomer unit must be compatible, for example, with the morphology and thermal properties of the host liquid crystalline polymer, as well as with the chemical properties in so far as how the material is polymerized. The articulated monomer unit must provide the correct spacing and length to allow the articulated polymer formed to fit within the polymer crystal structure. Preferred articulated monomer units for use in the present invention include a 3,3'-biphenyl, 3,3'-triphenyl, 2,2'-bipyridyl, and bis(oxyphynylene)benzene units. Polybenzazole (PBZ) liquid crystalline polymers are one class of preferred rigid-rod, heterocyclic liquid crystalline polymers for use in the present invention. Preferred PBZ polymers are selected from the group consisting of polybenzoxazole (PBO) , polybenzothizole (PBZT) and polybezimidazole (PBI) polymers and random, sequential or block copolymers thereof. The present invention also provides liquid crystalline polymers having less than about 5 mole % articulation, 0.5 to 2.5 mole % articulation being preferred. SUBSTITUTE SHEET DETAILED DESCRIPTION OF THE INVENTION The improvements which are obtained by the rigid rod heterocyclic liquid crystalline polymer structures of the present invention are predicated upon the unexpected discovery that the compressive properties of such structures are surprisingly improved by incorporation of articulated linkages within the polymer backbone. The present invention provides structures, such as fibers and films, comprising ordered polymers having improved mechanical properties by the incorporation in small amounts of articulated monomer units between long, ordered polymer chain segments. Articulated monomers units for use in the present invention impart a three-dimensional order to the desired liquid crystalline polymer that resists compressive failure and interlaminar shear due to microbuckling. In one embodiment of the present invention, the preferred articulated monomer unit comprises a "flexible swivel group." Such groups are disclosed in U.S. Patent No. 4,229,566, supra; Evers and Moore, J. Polymer Sci. , 24 (1986) 1863-1877; and U.S. Patent No. 4,359,567. The disclosures of each of the above references are incorporated herein by reference. In accordance with the present invention, small quantities of articulated monomer units, e.g., flexible swivel groups, are incorporated between long segments of the rigid rod, ordered liquid crystalline polymer chain. This produces microfibrillar structures that contain covalent linkages between the bundles of S microfibrils and have a three-dimensional network of interconnected molecules. The resulting polymer morphology produces articulated liquid crystalline polymer structures, e.g., fibers and films, that are more resistant to microbuckling when subjected to compressive loads than liquid crystalline polymer structures containing no such articulated units. Fibers and films prepared from articulated liquid crystalline polymer dopes will exhibit enhanced compressive strength and higher interlaminar shear strength as a direct result of three-dimensional molecular interconnectivity. Articulated monomers units for use in the present invention are selected on the basis of compatibility with the properties of the liquid crystalline polymer into which they will be incorporated. The monomer unit must must be compatible, for example, with the morphology and thermal properties of the host liquid crystalline polymer, as well as with the chemical properties in so far as how the material is polymerized. The articulated monomer unit must provide the correct spacing and length to allow the articulated polymer formed to fit within the polymer crystal structure. Polybenzazole (PBZ) liquid crystalline polymers are one class of preferred ordered polymers for use in the present invention. Preferred PBZ polymers are selected from the group consisting of polybenzoxazole (PBO) , polybenzothizole (PBZT) and polybenzimidazole (PBI) polymers and random, sequential or block copolymers thereof. PBZ polymers typically contain a plurality of mer units that are AB-PBZ mer units, as represented in Formula 1(a) , and/or AA/BB-PBZ mer units, as represented in Formula 1(b) Kα) AB-PBZ Kb) AA/BB- PBZ wherein: Each Ar and Ar' represents an aromatic group. The aromatic group may be heterocyclic, such as a pyridinylene group, but it is preferably carbocyclic. The aromatic group may be a fused or unfused polycyclic system. The aromatic group preferably contains no more than about three six-membered rings, more preferably contains no more than about two six-membered rings and most preferably consists essentially of a single six-membered ring. Examples of suitable aromatic groups include phenylene moieties, biphenylene moieties and bisphenylene ether moieties. Each Ar and Ar' is most preferably a 1,2,4,5-phenylene moiety, except wherein a predetermined percent of Ar groups is replaced with articulated monomer units in accordance with the present invention for use in producing structures having enhanced compressive strength. Each Z is independently an oxygen atom, a sulfur atom or a nitrogen atom bonded to an alkyl group or a hydrogen atom. Each Z is preferably oxygen or sulfur (the polymer is preferably PBO, PBZT or a copolymer thereof) ; Each DM is independently a bond or a divalent organic moiety that does not interfere with the synthesis, fabrication or use of the polymer. The divalent organic moiety may contain an aliphatic group (preferably C^ to C 12 ) ' but tne divalent organic moiety is preferably an aromatic group (Ar or Ar') as previously described. The nitrogen atom and the Z moiety in each azole ring are bonded to adjacent carbon atoms in the aromatic group, such that a five-membered azole ring fused with the aromatic group is formed. The azole rings in AA/BB-PBZ mer units may be in cis- or trans-position with respect to each other, as illustrated in 11 Ency. Polv. Sci. & Eog., 601, at 602, (J. Wiley & Sons 1988) which is incorporated herein by reference. The PBZ polymer may be rigid rod, semirigid rod or flexible coil. It is preferably rigid rod in the case of an AA/BB-PBZ polymer or semirigid in the case of an AB-PBZ polymer. It more preferably consists essentially of AA/BB-PBZ mer units. Exemplary highly preferred unmodified mer units, i.e., before articulation in accordance with the present invention, are illustrated in Formulas 2 (a)-(f). The unmodified polybenzazole polymer most preferably consists essentially either of the mer units illustrated Formula 2(a) (cis-PBO) or of the mer units illustrated in Formula 2(c) (trans-PBZT) . Each unmodified polymer preferably contains on average at least about 25 mer units, more preferably at least about 50 mer units and most preferably at least about 100 mer units. The intrinsic viscosity of unmodified cis-PBO or trans-PBZT in methanesulfonic acid at 25°C is preferably at least about 10 dL/g, more preferably at least about 20 dL/g and most preferably at least about 30 dL/g. The structures of the present invention will be illustrated with respect to PBZ liquid crystalline polymers. However, the invention is not so limited. It will be appreciated by those skilled in the art that the preferred liquid crystalline polymer will be selected depending upon its ultimate use. The present invention will be specifically illustrated by the incorporation of articulated monomer units comprising 3,3'-biphenyl linkages into the backbone of PBZT. However, the invention is not limited to this articulated monomer. Preferred articulated monomers for use with PBZT polymers are disclosed, for example, in U.S. Patent Nos. 4,229,566; and Evers and Moore; and U.S. Pat. No. 4,359,567; supra. Preferred monomer units for use in the present invention include 3,3'-biphenyl, 3,3'-triphenyl, 2,2'-bipyridyl, and bis(oxyphenylene) benzene units. Articulated monomer units comprising 3,3'-biphenyl units are particularly preferred for preparing modified, i.e., articulated. polymers in accordance with the present invention. The articulated monomer unit chosen must have the correct dimensions to align with two adjacent layers of polymers in the film. The mole percent of articulated monomer unit incorporated into rigid-rod, heterocyclic liquid crystalline polymers in accordance with the present invention will vary depending upon the polymer and monomer selected, as well as the desired end use for such articulated polymer. Up to about 25 mole % articulation is expected to be useful in the practice of the present invention. A preferred mole % articulation is between 0.1 to 10 mole %, 0.5 to 2.5 mole % articulation being particularly preferred. Fig. 1 illustrates one embodiment of the present invention wherein the articulated monomer unit 3,3'-bis(carboxy)biphenyl is incorporated into the polymer backbone of PBZT. The 3,3'-bis(carboxy)biphenyl monomer unit was chosen for this embodiment of the present invention, because it exhibited bond lengths and bond angles that enabled the development of the desired APBZT morphology shown in Figs. 3 and 4. In contrast to PBZT crystallines, where individual modules were aligned in parallel, APBZT crystallines contain molecules linking the aligned PBZT rigid-rod molecules. The all-aromatic structure of this monomer also provided thermal and oxidative stability that was comparable to the aromatic PBZT molecule itself. A morphological study indicated that articulated biphenyl units fit well into the PBZT crystal lattice, causing only minor decreases in lateral crystalline size and a 1% decrease in crystalline density. Polymerizing equimolar amounts of DABDT and terephthalic acid monomers produced PBZT, which is shown as the first repeating unit, M, in Fig. 1. By removing a molar amount of terephthalic acid monomer and replacing it with an equimolar amount of the articulated monomer, a polymer with the second repeating unit, N, was created. The articulated monomer has the correct dimensions and, being chemically similar to the terephalic acid monomer, polymerized readily into the polymer as is shown in Fig. 2. With the articulation present within the polymer, the rigid rod is able to rotate and swivel at this linkage. With one portion of the PBZT molecule in the plane, another portion of the molecule can rotate and protrude out of the plane as shown in Fig. 3. The APBZT has significantly increased intermolecular and interlayer interaction through this three-dimensional reinforcement. Fig. 4 illustrates the interaction between articulated molecules versus non-articulated molecules of PBZT. One method for preparing 3,3'-bis(carboxyl)biphenyl monomers for use in the present invention is disclosed in Evers and Moore, supra. It has been found, however, that 3,3'-bis(chlorocarbonyl)biphenyl monomer is preferred and the present invention also provides an improved method of synthesis for this compound which is shown in Fig. 5. In this synthesis, the articulated monomer 3,3'-bis(chlorocarbonyl)biphenyl was synthesized in high purity, via a safer and higher yield route than previous published procedures. In a typical preparation of articulated polymers for use in the present invention, lOOg batches of articulated PBZT ("APBZT") polymer were polymerized using terephthalic acid, the more convenient articulated 3,3'-bis(chlorocarbonyl) biphenyl monomer, and terephthaloyl chloride. DABDT monomer, was typically added to 77 percent PPA and completely dehydrochlorinated by heating the reaction in stages to 80°C. Vacuum-degassing facilitated removal of HC1 from the mixture. Then, a stoichiometric amount of terephthalic acid ("TPA") comonomer was added to the flask along with sufficient P2°5 to produce PPA of 83 percent P 2 0 5 content and the flask was heated rapidly to the polymerization temperature of 170°C. After 24 hours at that temperature, the reaction flask was heated to 195°C for an additional 24 hours to complete the polymerization reaction. At this point, a viscous, yellow-green dope of APBZT formed. PBZT polymer was also prepared to serve as the control for comparison with APBZT polymers during fiber and film testing. PBZT and APBZT polymers of high molecular weight were produced as evidenced by high intrinsic viscosity values measured for PBZT and APBZT polymers. APBZT polymers were prepared containing 2.5, 5, 7.5, 10, and 15 mole percent articulated units within the PBZT backbone. Figure 6 illustrates the variation of intrinsic viscosity (IV) with degree of articulation for APBZT polymers prepared. Samples of 0, 5, and 10 mole percent APBZT polymer dopes in polyphosphoric acid solution were extruded into fibers and vacuum-cast into films. PBZT and APBZT polymer dopes were spun into fibers from 15 wt % solids solutions in PPA, using the apparatus shown in Fig. 7 and dried using the apparatus of Fig. 8. The resultant fibers were characterized with respect to tensile and compressive strength. Fig. 9 illustrates the structure of a fiber compression test specimen. Incorporation of articulated monomers within the PBZT polymer backbone increased the compressive strength of the fibers by a factor of about 3 to 4 over the compressive strength of PBZT fibers as indicated by fiber recoil compression testing. This surprising increase in compressive strength was observed even though the fibers contained extrusion defects and voids from the rapid drying process. It will be possible to realize even higher compressive strengths and higher mechanical performance through the absence of defects within the fibers. This can be accomplished for example, by extruding APBZT fibers under processing conditions that are more appropriate for highly viscous materials, such as higher temperatures, lower pressures and longer air gaps to facilitate fiber cooling before coagulation. APBZT fiber compressive strength rapidly increased, then decreased with increasing articulated linkage content, the best results being obtained at 2.5 mole % articulated monomer content as shown in Fig. 10. Tensile strength decreased linearly with increasing degree of articulation. However, it is expected that higher tensile strength values will be obtained at articulation levels lower than 2.5 mole %, between 1 and 2 mole %, as well as improved compressive strength. Uniaxial films of APBZT were cast from solution using conventional methodology. Films studies showed that the cast APBZT films contained the articulated PBZT morphology illustrated in Fig. 4. Multiaxially oriented films comprising articulated rigid-rod, heterocyclic liquid crystalline polymers and having improved compressive strength, e.g., by a factor of 3 to 4, over comparable films of the non-articulated polymer, may be prepared following the teachings of U.S. Patent Nos. 4,939,235; 4,973,442; and 4,962,428, suora. The invention will be further understood with reference to the following examples, which are purely exemplary in nature, and are not to be utilized to limit the scope of the invention. Materials used in the following examples were obtained from readily available commercial sources or made in accordance with the indicated procedures or publications. EXAMPLE I - Preparation of PBZT and APBZT Polymers A. The Apparatus The custom-built apparatus shown in Fig. 11 or one similar thereto was used for all polymerization experiments. A curved argon inlet tube 1 directed the constant argon purge toward the bottom of the flask. Argon was chosen in place of nitrogen because it was heavier than air and tended to drift toward the bottom of the flask, keeping the contents blanketed with inert atmosphere at all times. A vacuum-regulator 2 allowed degassing of the entire flask under high vacuum while continuing to purge with argon makeup gas. The high torque stirrer motor 3 was required to stir viscous PBZT dopes continuously, even at very low rotational speeds. An outlet tube 4 on the reactor vessel directed argon and volatiles from the SUBSTITUTE SHEET flask and into a gas bubbler 5 to indicate flow rate of purge gas, or alternatively by opening and closing the appropriate valves, into a cold-trap attached to the vacuum pump 6. A fourth opening to the flask 7 was used for the addition of P 2 °5 and PBZT monomers. Monomers and P 2 0 5 were added to the flask through a dry glass tube fitted with a wide funnel to prevent spillage. The tube was extended into the flask to a level close to the surface of the reactant mixture to ensure placement of monomer directly onto the surface of the reactant mixture. A thin wire was used to dislodge small amounts of monomer if they became trapped within the addition tube. The entire reaction flask was oven-dried at 100°C for several hours before use, along with any glassware that was to be used in the experiment. In order to ensure the dryness of monomers used in the polymerization, 2,5-diamino-l,4-benzendithiol dihydrochloride (DABDT) and micronized terephthalic acid (TPA) monomers were vacuum dried at 60°C. This temperature accelerated drying yet did not cause monomer decomposition. Argon was used to break the vacuum when drying was complete. This produced dry monomers that contained adsorbed argon rather than air which might have caused decomposition during polymerization. PPA solvent was used at two P2°5 concentrations during the PBZT polymerizations. The first function of the PPA was to dehydrochlorinate the DABDT. Preparing a 77 percent P 2 0 5 content PPA solution produced a solution strong enough for dehydrochlorination but still of sufficiently low viscosity for rapid devolatization of evolving HC1. The second function of PPA was to solubilize the PBZT polymer. A P 2 0s content PPA of 83 percent was required to maintain solubility of the PBZT polymer, but at the 83 percent level was too viscous for dehydrochlorination. The requisite P 0 5 contents will be achieved by adding P 2 °5 powder directly before the polymerization but after the dehydrochlorination was completed. B. Recrystallization of TPC In order to prepare about 15 grams for monomer-grade TPC from commercial sources, the recrystallization began with 25 grams of TPC. Due to the severe lachrymatory properties of TPC, all the work below was performed under a fume hood. In a 250-ml Erlenmeyer flask, 25 grams of TPC was dissolved by several increments of boiling methylene chloride totalling 150 ml. When the TPC was completely dissolved, an additional 25 ml of methylene chloride was added. After adding 0.5 gram decolorizing carbon to the methylene chloride/TPC solution, the hot solution was filtered with No. 40 Whatman fluted filter paper. The filter paper was preheated with 20 ml of hot methylene chloride before the solution was filtered and collected in a 250-ml beaker situated on a hot plate. After filtration, the clear solution was reduced in volume to 45 ml by boiling off the solvent. Slow cooling the beaker induced crystallization at a slow enough rate to form crystals with no entrapped solvent. A slow cool to room temperature for 16 hours produced diffused needle-like white crystals. The mother liqueur was decanted off and saved while a fresh 70 ml methylene chloride was added to recrystallize the product. Boiling the solution down to 35 ml was sufficient to induce crystallization upon cooling. Again, a diffused mass of white needle-like crystals formed after 16 hours. The mother liqueur was decanted into the first aliquot and saved. The crystals were filtered, air dried quickly, and vacuum dried for 2 hours at 60°C argon replaced the vacuum in the drying pistol. The yield of crystals (MP 149°C) was about 70 percent for a recovered weight of 17.86 grams. The monomer-grade TPC crystals were then immediately weighed for addition to the polymerization. C. Recrystallization of Articulated Biphenyl Diacid Chloride Because 3,3'-bis(chlorocarbonyl)biphenyl is not stable in vacuum storage, it was necessary to synthesize sufficient quantities for this reaction. The 3,3'- bis(chlorocarbonyl)biphenyl is converted into the desired diacid chloride monomer by reaction with thionyl chloride. Trace amounts of dimethylformamide (DMF) effectively catalyzed the reaction to over 75 percent yield. The crude product was recrystallized twice from methylene chloride/hexane forming large needles (M.P. 148°C) . Vacuum drying the pulverized crystals at 60°C for 200 hours prepared the articulated biphenyl diacid chloride for immediate reaction addition. D. Polymerization Preparation of 5 mole % APBZT is described below. Other mole % APBZT was prepared following the described procedure. Into the assembled, vacuum-dried, argon purged reaction vessel, 41.235 grams of 86.1 percent H 3 P0 4 and 26.260 grams of P 2 0 5 were reacted to form 67.495 grams of 77.0 percent P ' 2 ( - ) 5 polyphosphoric acid. This solution was stirred for 22 hours at room temperature, degassed, heated to 80°C, and again vacuum degassed. The clear solution, strong enough to dehydrochlorinate the DABDT, had reacted to completion. After the flask cooled to room temperature, 17.2611 grams (0.0703966 mole) of DABDT monomer were added to the flask. The solution, stirred at room temperature for 16 hours, rose to 55°C for 22 hours and plateaued at 80°C for 85 hours. After vacuum degassing, the solution turned from a cloudy yellowish solution to the desired clear yellow. The reaction flask cooled to 55°C and 0.9825 grams (0.0035199 mole) 3,3'-bis(chlorocarbonyl)biphenyl, the articulated monomer, was added to the flask. This mixture was mixed for 1 hour at 55°C, and heated to 80°C where it stirred for 24 hours to complete the dehydrochlorination of the articulated monomer. The viscosity was detectably higher after the articulated monomer addition, due to the amine-thiol trimers forming in solution. Again, this mode of addition was used to maximize the chain distance between articulated linkages on each PBZT polymer chain. The solution was vacuum degassed, releasing volatiles, but when the foaming ended, a clear yellow viscous solution had not developed. After vacuum degassing at 80°C, 13.5778 grams (0.066877 mole) of terephthaloyl chloride was added with a glass tube ensuring the addition onto the PPA surface instead of sticking to the walls, destroying the needed stochiometry for the success of the reaction. For 88 hours the reaction stirred liberating gaseous hydrochloric acid which the argon purge removed. Vacuum degassing after 77 hours of stirring the solution while the temperature remained at 80°C did not produce much foaming. Next, 36.093 grams of 2 0s were added to the reaction flask. Almost immediately, the flask contents began to foam. The stirring rate was increased greatly to diminish the degassing that the addition of the final p 2°5 ------ c use( i. The foam coated the walls of the flask but drill press motion of the stirrer shaft broke the sticky coating form the walls and affixed the solution to the stirrer. The solution then fell to the bottom of the flask and mixed thoroughly with the rest of the reaction components. No color change was noticeable. From this observation, there was no degradation of the monomers in the reaction. After stirring for 77 hours to ensure homogeneity, the flask was heated from 80°C to 170°C and remained for 48 hours. The polymer formed was removed by using the Haake Buchler Mixer Rheometer, the dope was later upgraded outside the flask. A stainless steel grade 304 stirrer. was used in the polymerizations. E. Intrinsic Viscosity Measurements A sample of APBZT dope was pressed and coagulated in a neutral water bath for 48 hours. After the acid was extracted out of the APBZT dope, the disc of pressed dope was fibrillated and vacuum filtered. When the pH of the wash water was over 5.0, the fibers were vacuum air dried for another half an hour. At 165°C, the fibers were vacuum dried for 5 hours to remove any residual moisture. Into 72.38 ml of methanesulfonic acid (MSA) , 0.1448 grams of PBZT fibers were solubilized after 72 hours of stirring. The intrinsic viscosity was tested to be 21.5 dl/g. After Haake Buchler mixing, two batches of APBZT prepared by this procedure measured intrinsic viscosities of 25.2 and 33.8. The metal stirrer, using the Haake Buchler Mixer, and using the acid chloride form of terephthalic acid all contributed positively to the increased viscosity of the polymer. EXAMPLE 2: Fiber Extrusion The specialized fiber extrusion apparatus illustrated in Figure 7 and located at the Air Force Materials Laboratory, WRDC, Dayton, Ohio, was used to extrude PBZT and APBZT fibers. PBZT and APBZT dopes prepared in accordance with Example 1 were pressure-filtered and degassed prior to fiber extrusion. The fiber extrusion equipment included a screw driven ram extruder with a funnel shaped lOμ extrusion die. This was coupled to a water bath and take-up system that maintained tension in the fiber at all times, while it was drawn on a pre-set draw ratio and wound upon a 10 inch diameter drum. The large diameter of the drum prevented kinking and damage to the fiber due to compressive failure. The extruder was filled with APBZT do and fibers were extruded at two different draw ratios, namely, 10 to 1 and 5 to 1. It was predicted at this time that the lower draw ratio would produce a higher compressive strength fiber, since more three-dimensional order would be preserved. However, in some cases a higher draw ratio may be desirable where it is sought to maximize compressive as well as tensile strength. Several hundred feet of each articulated polymer fiber were extruded (0, 2.5, 5, 7.5, 10 and 15 mole % articulation) . This was wound upon a collecting drum which was immersed in distilled water to extract the residual amount of PPA. The drum was allowed to soak in distilled water to extract residual phosphoric acid from the fiber. During this period, the rinse water was changed several times to ensure removal of all traces of acid which would otherwise weaken the fibers. After the fibers had soaked in distilled water overnight, they were stage-dried and heat-treated in the apparatus illustrated in Fig. 8. Drying and heat-treating steps were conducted in separate operations and at different temperatures, by passing the tensioned fiber through the heated tube oven at a speed of approximately 15 feet per minute. Preliminary drying of wet fiber was conducted at a temperature of 200°C, a temperature that had been successfully applied to the drying of previous PBZT fibers. Heat-treatment was performed at 535°C, since that temperature had also produced PBZT fibers with the highest mechanical properties Fiber tension was maintained throughout each operation to prevent damage due to abrasion or kinking. EXAMPLE 3: Fiber Recoil Compression and Tensile Strength Heat-treated PBZT and APBZT f bers prepared in accordance with Example 2 were tested for compressive strength using the single fiber recoil compression test procedure developed by the Air Force Materials Laboratory (See, e.g., Takahashi et al, J. Appl. Poly. Sci. 28, 579-586 (1983); DeTeresa et al., J. Mat. Sci. 19, 57-72 (1984); Allen, J. Mat. Sci. 22, 853 (1987); DeTeresa et al, J. Mat. Sci. 20, 1645 (1985) ; and DeTeresa, J. Mat. Sci. 23, 1886 (1988)). This test was conducted as follows: A fiber specimen was mounted within a tensile test fixture as illustrated by Fig. 9. The fiber was then examined under the microscope for the absence of kink bands or other defects and measured with respected to average fiber diameter along its length. The fiber was tensioned to a predetermined load which was somewhere below the ultimate tensile failure load for the fiber. An electric arc was used to sever the fiber instantaneously, without causing spikes in the fiber tensile load. The severed fiber specimen was re-examined under the microscope for the presence of kink bands at either side of the specimen. The presence of a dark, swollen kink band, usually situated at the junction between fiber and epoxy potting droplet, was indication of compressive failure for that end of the fiber. Fail/No Fail notations were recorded for gradually changing tensile loads, with a load being reached where neither end of the fiber developed any kink bands. This load, divided by the average fiber diameter, represented the fiber recoil compression strength of the fiber specimen. Fig. 10 illustrates the results of the fiber recoil compression strength tests involving PBZT and APBZT fibers. The tests indicated significant improvement in PBZT fiber compressive strength by incorporation of articulated linkages, at low loadings, within the polymer backbone. APBZT fiber tensile strength measurements were conducted using the same test apparatus. The test results showed that tensile strength decreased by SUBSTITUTE SHEET approximately 14% in 2.5 mole % APBZT fibers in comparison to PBZT fiber. However, improved tensile strength values are expected to be obtained at articulation levels lower than 2.5 % (between 1-2%). It is understood that the examples and embodiments described herein are for illustrated purposes only, and that various modifications and changes in light thereof that will be suggested to persons skilled in the art are to be included in the spirit and purview of this application and the scope of the approved claims. SUBSTITUTE SHEET

Claims: What is Claimed is: 1. A fiber having improved compressive strength, wherein the fiber comprises a rigid-rod, heterocyclic liquid crystalline polymer comprising up to about 25 mole % of at least one articulated monomer unit. 2. A fiber in accordance with claim 1, wherein the compressive strength of the fiber is improved by a factor of about 3 to 4 over the compressive strength of a comparable fiber consisting essentially of the rigid-rod, heterocyclic liquid crystalline polymer. 3. The fiber of claim 1, wherein the articulated monomer unit is present at up to about 10 mole %. 4. The fiber of claim 1, wherein the articulated monomer unit is present at up to about 15 mole %. 5. The fiber of claim 1, wherein the articulated monomer unit is present at from about 0.5 to about 2.5 mole %. 6. The fiber of claim 1, wherein the articulated monomer unit is a 3,3'-biphenyl, 3,3'-triphenyl, 2,2'-bipyridyl, or bis(oxyphenylene)benzene unit. 7. The fiber of claim 1, wherein the rigid-rod, heterocyclic liquid crystalline polymer comprises polybenzazole. SUBSTITUTE SHEE 8. The fiber of claim 7, wherein the polybenzazole is polybenzoxazole, polybenzothiazole or polybenzimidazole. 9. The fiber of claim 7, wherein the polybenzazole is polybenzothiazole. 10. A fiber in accordance with claim 6, having a compressive strength about three to four times greater than a comparable fiber consisting essentially of polybenzazole. 11. A process for making a fiber having improved compressive strength, the process comprising: (a) providing a dope (i) of a rigid-rod, heterocyclic liquid crystalline polymer, or (ii) from a polymerization mixture of a rigid-rod, heterocyclic liquid crystalline polymer, the polymer comprising up to about 25 mole % of at least one articulated monomer unit, and a solvent; and (b) extruding the dope through a die to form a fiber. 12. The process of claim 11, wherein the compressive strength of the fiber is improved by a factor of 3 to 4 over the compressive strength of a comparable fiber prepared without the articulated monomer unit. 13. The process of claim 11, wherein the liquid crystalline polymer is polybenzazole. 14. The process of claim 13, wherein the liquid crystalline polymer is polybenzoxazole, SUBSTITUTESHEET polybenzothiazole, or polybenzimidazole. 15. The process of claim 11, wherein the liquid crystalline polymer is polybenzothiazole. 16. A fiber produced by the process of claim 11. 17. The fiber of claim 16, wherein the compressive strength of the fiber is improved by a factor of about 3 to 4 over the compressive strength of a comparable fiber prepared without the articulated monomer unit. 18. A fiber produced by the process of claim 15. 19. The fiber of claim 18, wherein the fiber has a compressive strength greater than about 50 Ksi. 20. The fiber of claim 18, wherein the compressive strength of the fiber is improved by a factor of about 3 to 4 over the compressive strength of a comparable fiber consisting essentially of polybenzazole. 21. A film having improved compressive strength, wherein the film comprises an articulated rigid-rod, heterocyclic liquid crystalline polymer comprising from about 0.1 to about 25 mole % of at least one articulated monomer unit. 22. A film in accordance with claim 21, wherein the compressive strength of the film is improved by a factor of about 3 to 4 over the compressive strength of a comparable film consisting essentially of the rigid-rod, heterocyclic liquid crystalline polymer. SUBSTITUTE SHEET 23. The film of claim 21, wherein the articulated monomer unit is present at up to about 25 mole %. 24. The film of claim 21, wherein the articulated monomer unit is present at up to about 10 mole %. 25. The film of claim 21, wherein the articulated monomer unit is present at from about 0.5 to about 2.5 mole %. 26. The film of claim 21, wherein the articulated monomer unit is a 3,3'-biphenyl, 3,3'-triphenyl, or 2,2'-bipyridyl or bis(oxyphenylene)benzene unit. 27. The film of claim 21, wherein the rigid-rod, heterocyclic liquid crystalline polymer comprises polybenzazole. 28. The film of claim 27, wherein the polybenzole polymer is polybenzoxazole, polybenzothiazole or polybenzimidazole. 29. The film of claim 27, wherein the polybenzazole is polybenzothiazole. 30. A film in accordance with claim 27, having a compressive strength about three to four times greater than a comparable film consisting essentially of polybenzazol . 31. A process for making a film having improved compressive strength, the process comprising: (a) providing a dope (i) of a rigid-rod, heterocyclic liquid crystalline polymer, or (ii) from a polymerization mixture of a rigid-rod, heterocyclic liquid crystalline polymer, the polymer comprising up to about 25 mole % of an articulated monomer unit, and a solvent; and (b) extruding the dope through a die to form a film. 32. The process of claim 31, wherein the compressive strength of the film is improved by a factor of 3 to 4 over the compressive strength of a film prepared without the articulated monomer unit. 33. The process of claim 31, wherein the liquid crystalline polymer is polybenzazole. 34. The process of claim 33, wherein the liquid crystalline polymer is polybenzoxazole, polybenzothiazole, or polybenzimidazole. 35. The process of claim 31, wherein the liquid crystalline polymer is polybenzothiazole. 36. A film produced by the process of claim 31. 37. The film of claim 36, wherein the compressive strength of the film is improved by a factor of 3 to 4 over the compressive strength of a comparable film prepared without the articulated monomer unit. 38. A film produced by the process of claim 35. Su 39. The film of claim 38, wherein the film has a compressive strength greater than about 50 Ksi. 40. The film of claim 38, wherein the film has a compressive strength of about three to four times greater than a comparable film consisting essentially of polybenzazole. 41. A rigid rod heterocyclic liquid crystalline polymer comprising less than about 5 mole % of at least one articulated monomer unit. 42. The polymer of claim 41, wherein the articulated monomer unit is present at up to about 2.5 mole %. 43. The polymer of claim 41, wherein the articulated monomer unit is present at from about 0.5 to about 2.5 mole %. 44. The polymer of claim 41, wherein the articulated monomer unit is a 3,3'-biphnyl, 3,3'-triphenyl, 2,2'-bipyridyl , or bis(oxyphylene) benzene unit. 45. The polymer of claim 41, wherein the liquid crystalline polymer comprises polybenzazole. 46. The polymer of claim 45, wherien the polybenzazole polymer is polybenzoxazole, polybenzothiazole or polybenzimidazole. 47. The polymer of claim 45, wherein the polybenzazole is polybenzothiazole.

Inventors: EVERS ROBERT C, KOVAR ROBERT F, LUSIGNEA RICHARD W, EVERS, ROBERT, C., KOVAR, ROBERT, F., LUSIGNEA, RICHARD, W.
Applicants: FOSTER MILLER INC, FOSTER-MILLER, INC.
Application year: 1992
Main classification: B28B3
IPC codes: B29C55, B29C67, B29D7, C08G73, C08G75, C08J5, D01F6