Abstract:
The invention relates to a class of copolyesters which display optical anisotropy in the molten state and to the shaped articles, fibers and films obtained from the optically anisotropic melts. the copolyesters comprise recurring structural units (a) independently each occurrence selected from the group consisting of Formulas I and II; and recurring structural units (b) independently each occurrence selected from the group consisting of Formula III; ##STR1## wherein R independently each occurrence is a chemically inert substituent.

Description:
FIELD OF THE INVENTION 
     The invention relates to a class of polyesters and copolyesters which display optical anisotropy in the molten state and to the shaped articles, fibers and films obtained from the optically anisotropic melts. 
     BACKGROUND OF THE INVENTION 
     Liquid crystalline polymers (LCPs) are macromolecules possessing significant orientation in either the molten state or in concentrated solution. The state of their solution (lyotropic) or melt (thermotropic) is between the boundaries of solid crystals and isotropic liquids. In the solid state these highly ordered polymers display exceptional strength properties in the direction of orientation. By designing molecules containing only relatively inert chemical bonds, preparation of thermally and oxidatively stable high-performance materials is possible. 
     A review of thermotropic LCPs can be found in Kwolek et al., &#34;Liquid Crystalline Polymers&#34;, &#34;Encyclopedia of Polymer Science and Engineering&#34;2nd Ed, Vol. 9, pp 23-55 (1987). Among those listed are polyesters. Many liquid crystalline polyesters display several of the desirable attributes of these compounds. Unfortunately, most have too high of a melt temperature for economical melt fabrication. 
     There is a growing need in the thermoplastic engineering industries to provide for new and improved polyesters and copolyesters which possess a high degree of processability while concurrently exhibiting superior mechanical properties. 
     SUMMARY OF THE INVENTION 
     The invention concerns copolymers capable of forming an optically anisotropic melt and comprising recurring structural units (a) independently each occurrence selected from the group consisting of Formulas I and II: and recurring structural units (b) independently each occurrence selected from the group consisting of Formula III: ##STR2## wherein R independently each occurrence is a chemically inert substituent. 
     DETAILED DESCRIPTION 
     Preferably, R is independently selected from the group consisting of hydrogen, halo, lower alkyl, methoxy and phenyl. Most preferably, R is each occurrence hydrogen. 
     The copolymers of the invention preferably comprise from 5 to 70 mole percent of the recurring structural units of Formula I or II or mixtures thereof and from 95 to 30 mole percent of the recurring structural units of Formula III. More preferably, from 15 to 55 mole percent of the units of Formula I or II or mixtures thereof and from 85 to 45 mole percent of the units of Formula III and most preferably, from 20 to 40 mole percent and from 80 to 60, respectively. 
     The copolyesters may be formed by a variety of ester-forming techniques from difunctional organic compounds possessing functional groups which upon polycondensation form the requisite recurring units. For example, the functional groups of the organic aromatic compounds may independently contain carboxylic acid groups or acid halide groups and functional groups reactive therewith such as hydroxyl or acyloxy groups. In a preferred embodiment, the organic reactants comprise lower acyloxy and carboxylic acid functionality. For example, lower acyl esters of 4-(4&#39;-hydroxyphenoxy)benzoic acid, 3-(4&#39;-hydroxyphenoxy)benzoic acid and 4-hydroxybenzoio acid wherein the hydroxy group is esterified are more preferred as reactants. The lower acyl groups preferably have from 2 to 4 carbon atoms. Most preferably, the acetate esters are useful. 
     The organic compounds may be allowed to react under anhydrous conditions in an inert atmosphere via a melt acidolysis procedure, in a suitable solvent via a solution procedure, or in a heat exchange medium via a slurry polymerization as described in Calundann, U.S. Pat. No 4,067,852. Additional suitable reaction conditions are described in Schaefgen, U.S. Pat. No. 4,118,372. The teachings of the foregoing U.S. patents are incorporated herein by reference. A preferable technique is the melt acidolysis technique. 
     A catalyst may or may not be used in the polymerization process. If used, representative catalysts for use in the process include dialkyl tin oxide (e.g., dibutyl tin oxide), diaryl tin oxide, titanium dioxide, alkoxy titanium silicates, titanium alkoxides, Lewis acids, hydrogen halides (e.g., HCl), alkali and alkaline earth metal salts of carboxylic acids (e.g., sodium acetate) The quantity of catalyst utilized typically is from 0.001 to 1 weight percent based upon total reactant weight, and most commonly from 0.01 to 0.2 weight percent. In a preferred embodiment, no catalyst is used in the preparation thereof. 
     The melts of the copolyesters have low viscosity and unusual interactions with light and electrical fields. Melts of the copolyesters of the present invention can be processed by thermoplastic shaping such as injection molding or extrusion to produce articles. Articles comprise mouldings, boards, sheets, fibers and films which have desirable mechanical properties such as high tensile strength and high impact resistance. These articles also exhibit high resistance to solvents and chemicals. The articles are useful, for example, in the aircraft and aerospace industries, for military applications, in telecommunications systems and for high performance electrical components. 
     Conventional additives and processing aids can be added to the copolyester melts of the invention. Examples of additives are oxidation stabilizers; heat stabilizersultraviolet light (UV) stabilizers; lubricants; mold release agents: dyes and pigments: fibrous or powdered fillers and reinforcing agents; nucleating agents; and plasticizers. 
     Examples of oxidation stabilizers and heat stabilizers are halides of metals of group I of the Periodic Table, used alone and used as a mixture with copper (I) halides or sterically hindered phenols in concentrations from 0.001 to 1 weight percent based on the weight of the copolyester composition. 
     Examples of UV stabilizers are substituted resorcinols, salicylates, benzotriazoles, benzophenones and mixtures of these, which are added, for example, in amounts from 0.001 to 2 weight percent based on the weight of the copolyester composition. 
     Dyes and pigments are used, for example, in amounts from 0.001 to 5 weight percent based on the weight of the copolyester composition. Examples are nigrosine, titanium dioxide, cadmium sulfide, phthalocyanine dyes, ultramarine blue and carbon black. 
     Examples of fillers and reinforcing agents are carbon fibers, glass fibers, amorphous silica, calcium silicate, aluminum silicate, magnesium carbonate, kaolin, chalk, powdered quartz, mica and feldspar, which may be present in a concentration from 0.5 to 70 weight percent, based on the total weight of the filled material. 
     Examples of nucleating agents are talc, calcium fluoride, sodium phenylphosphanate, alumina and finely divided polytetrafluoroethylene. Suitably, the nucleating agent may be present in an amount from 0.001 to 1 percent by weight. 
     For example, from 0.0001 to 20 weight percent, based on the weight of the composition can be plasticizers, such as phthalates, hydrocarbon oils and sulfonamides. 
     Also included in the composition of the invention, in addition to or in partial replacement of the reactants of Formula I, II and III are small amounts of other aromatic polymerizable units whose presence do not interfere with the excellent mechanical properties of these copolyesters. Examples of such aromatic units comprising these additional repeating units are isophthalic acid, 4-4&#39;-dihydroxybiphenyl, resorcinol, 4,4&#39;-isopropylidenediphenol and 3-hydroxybenzoic acid. 
     Preparation of 3-(4&#39;-Acetoxyphenoxy)benzoic Acid 
     Ethyl 3-hydroxybenzoate (34.0 grams (g), 0.205 mole) (Aldrich Chemical Company, Milwaukee, Wis.), 4-bromoanisole (26.0 milliliters (ml)), (38.8 g, 0.208 mole) (Aldrich Chemical Company), anhydrous potassium carbonate (17.8 g, 0.129 mole) and copper powder (10.0 g, 0.157 mole), (Fisher Scientific, Pittsburgh, Pa.) were heated under nitrogen to 190° C. with slow stirring. Water generated during the reaction was removed by means of a short-path distillation apparatus attached to the reaction flask. After 20 hours, the reaction mixture was allowed to cool and 20 percent aqueous sodium hydroxide (NaOH) (50 ml) was added. Ester hydrolysis was complete after 1 hour at reflux. The cooled reaction mixture was acidified with concentrated hydrogen chloride (HCl), partitioned between methylene chloride (CH 2  Cl 2  (300 ml)) and water (50 ml), and filtered to remove copper powder. The phases were separated and the organic layer was washed with saturated sodium chloride (NaCl) (2×100 ml), dried with magnesium sulfate (MgSO 4 ), filtered, and solvent evaporated to afford a light brown solid. The crude product was recrystallized from 60 percent aqueous acetic acid to afford 3-(4&#39;-methoxyphenoxy)benzoic acid as off-white crystals (40.77 g, 0.167 mol) with a melting point (m.p.) of 120° C. to 125° C. 
     An amount of 3-(4&#39;-methoxyphenoxy)benzoic acid (40.7 g, 0.167 moles) was dissolved in 200 ml of acetic acid to which 150 ml of a 48 percent solution of hydrogen bromide (HBr) was added (Aldrich Chemical Company). The reaction mixture was heated to reflux under nitrogen for 14.5 hours. The cooled reaction mixture was then diluted with 400 ml of ethyl acetate (EtOAc), washed with saturated NaCl (3×150 ml), dried with MgSO 4 , filtered, and solvent evaporated to afford a dark brown solid. The crude product was dissolved in methanol, treated with activated carbon, filtered through a celite pad, and evaporated. Recrystallization of the product (1:1 acetic acid/water) gave 3-(4&#39;-hydroxyphenoxy)benzoic acid as tan crystals with a m.p. of 163° C. to 166° C. 
     An amount of 3-(4&#39;-hydroxyphenoxy)benzoic acid (9.772 g, 42.45 millimoles (mmoles)) was dissolved in 6M NaOH (18 ml, 108 mmoles) and cooled in an ice bath. Acetic anhydride (8.0 ml, 11.1 g, 109 mmoles) was added cautiously to the basic solution at such a rate as to maintain 0° to 5° C. and then stirred vigorously for an additional 15 minutes (min) at 0° C. The reaction mixture was then allowed to warm to room temperature over 1 hour. The aqueous solution was acidified with 6M HCl (18 ml, 108 mmoles) and extracted with 75 ml of ethyl acetate. The organic solution was washed three times with a 50 ml saturated solution of NaCl, dried with MgSO 4 , filtered, and solvent evaporated. The crude product was taken up in methanol, treated with activated carbon, filtered through a celite pad, and evaporated. Recrystallization of the product gave 9.83 g of 3-(4&#39;-acetoxyphenoxy)benzoic acid as an off-white powder with a m.p. 141° C. to 143° C. 
     Preparation of 4-(4&#39;-Acetoxyphenoxy)benzoic Acid 
     A mixture of (136 g, 1.10 moles) of 4-hydroxy-anisole and (65 g, 1.00 mole) of potassium hydroxide was heated at 120° C. to 130° C. until a hazy solution was formed. A vacuum was applied and the temperature was slowly increased to 170° C. over a 1 hour period. The reaction mass was cooled to room temperature and 200 ml of dimethylsulfoxide was added. The mixture was heated to 120° C. to 130° C. under nitrogen to dissolve the solid. Over a 45 minute period a solution of 4-chlorobenzo-nitrile (137 g, 1.0 mole) in 250 ml of dimethylsulfoxide was slowly added. The temperature was held at 120° C. to 130° C. for an additional two hours. The reaction mass was cooled to room temperature and was diluted with 200 ml of 1.1N aqueous potassium hydroxide solution. An off-white solid separated and was collected and washed for 1 hour with 2 L of water and dried in a vacuum oven at 75° C. for 5 hours. There remained 4-(4&#39;-methoxyphenoxy)benzonitrile with a m.p. of 106° C. to 109° C. 
     A solution of 4-(4&#39;-methoxyphenoxy)benzonitrile (101.2 g, 0.45 mole) in 750 ml of hot acetic acid was slowly diluted with 500 ml of 48% aqueous hydrobromic acid. This solution was refluxed for 25 hours, then cooled. A light brown solid separated. The solid was collected, stirred in 800 ml of fresh water, collected again and dried in a vacuum oven at 90° C. for 4 hours. There remained 4-(4&#39;-hydroxyphenoxy)benzoic acid with a m.p. of 191° C. to 193° C. The acid was dissolved in 500 ml of aqueous base, the solution was filtered and the hydroxy acid reprecipitated by acidifying the solution with aqueous hydrochloric acid. The solid was collected and dried in a vacuum oven at 100° C. for 24 hours. There remained a tan solid with a m.p. of 193° C. to 194° C. 
     The purified hydroxy acid (61.3 g, 0.266 mole) was dissolved in 500 ml of water containing 0.572 mole of sodium hydroxide. The solution was cooled to 9° C. in an ice bath and acetic anhydride (31.1 g, 0.305 mole) was added. In one minute a solid began to precipitate. The slurry was stirred at 0° C. to 9° C. for one hour then it was acidified with concentrated HCl. The light tan solid was isolated by filtration and washed with water, then dried in a vacuum oven for 16 hours at 60° C. There remained 4-(4&#39;-acetoxyphenoxy)benzoic acid with a m.p. of 150° C. to 151° C. The crude acetoxy acid was recrystallized from 700 ml of toluene yielding 60.6 g of purified off-white product with a m.p. of 151.5° C. to 152.5° C. 
     Preparation of 4-Acetoxybenzoic Acid 
     An amount of 4-hydroxybenzoic acid (92.1 g, 0.67 mole) was dissolved in a solution of sodium hydroxide (53.4 g, 1.33 moles) and 1.33 liters (L) of water in a 4 L beaker. The solution was stirred and cooled to a temperature of 0° C., then acetic anhydride (102.1 g, 1.00 mole) was added. The temperature was maintained at -2° C. for 1 hour. A solution of concentrated HCl (144.7 g, 1.42 mole) in 267 ml of water was added. The slurry was stirred briefly and filtered. The product was washed twice by stirring it with 2 L portions of fresh water then filtered and dried in a vacuum oven at 80° C. for 16 hours. After recrystallization from methyl isobutyl ketone, the product consisted of white crystals with a m.p. of 192° C. to 192.5° C. 
    
    
     EXAMPLE I 
     Preparation of a Copolyester from 4-Acetoxybenzoic Acid and 4 -(4&#39;-Acetoxyphenoxy)benzoic Acid 
     The polymerization was run in a 1 L, single neck, round bottom flask fitted with a two neck adapter upon which were mounted a paddle stirrer and a 13 cm distillation column, distillation head, condenser and receiver. An amount of 4-acetoxybenzoic acid (200.7 g, 1.11 moles) and 4-(4&#39;-acetoxyphenoxy)benzoic acid (163.4 g, 0.60 mole) were added to the flask. The apparatus was evacuated and refilled with nitrogen. The flask was immersed in a molten salt bath preheated to 250° C. When the solid reactants had melted, stirring was started and the temperature was slowly increased to 297° C. over a 50 minute period. In the next 33 minutes the pressure was reduced to 2 mm Hg and the temperature was increased to 340° C. The viscous, opaque, off-white reaction mass was stirred for an additional 20 minutes at 340° C. and 2 mm Hg then cooled to room temperature. The opaque, off-white copolyester plug was sawed into 0.5 cm slices then ground in a Wiley mill. The polymer had recurring units ##STR3## 
     Physical and General Characteristics of a Copolyester of 4-(4&#39;-Acetoxyphenoxy)benzoic Acid/4-Acetoxybenzoic Acid 
     The inherent viscosity of the copolyester prepared as described above was measured from a dilute solution (0.1 g per deciliter (dL)) in a mixture of 65 volume percent methylene chloride and 35 volume percent trifluoroacetic acid at 25.0° C. The viscosity was 2.28 dL g  -1 . 
     Melt temperature analysis was carried out using differential scanning calorimetry (DSC) on a 15 mg compressed pellet at a heating and cooling rate of 20° C. min  -1  on a Mettler DSC-30 low temperature cell with a Mettler TC10A thermal analysis processor (Mettler Instrument Corp., Hightstown, N.J.). The copolyester showed peak melting points of 270° C. and 288° C. and glass transition temperatures of 135° C. and 126° C. on the first and second heating scans, respectively, of the DSC. 
     The copolyester was injection molded into standard 1/8 inch tensile test bars using a Boy* 30-M Injection Molding Machine, (Boy Machines, Inc., Exton, Pa.). The barrel temperature was held at 290° C., the mold temperature at 82° C. and the pressure at 15 bars. The table below lists the tensile and impact properties of these bars measured using procedures described in ASTM Test No&#39;s D638 and D256, respectively. 
     
         ______________________________________PROPERTY               VALUE______________________________________Tensile Strength       13,100 psiElongation             3.5%Tensile modulus        6.1 × 10.sup.5 psiNotched Izod impact strength                  5.0 ft lb/in______________________________________ 
    
     EXAMPLE II 
     Preparation of a Copolyester from 4-Acetoxybenzoic Acid and 3-(4&#39;-Acetoxyphenoxy)benzoic Acid 
     An amount of 3-(4&#39;-acetoxyphenoxy)benzoic acid (0.90 g, 0.0033 mole) and 4-acetoxybenzoic acid (0.59 g, 0.0033 mole) were added to a 15 mm I.D. polymerization tube which was equipped with a capillary tube and combined distillation head and condenser. The reaction vessel was alternately evacuated and purged with nitrogen and left under a mild flow of nitrogen. The reaction vessel was heated to 240° C. and after the monomers had melted, the capillary tube, with a slow nitrogen bleed, was inserted into the melt to facilitate mixing. The reaction temperature was maintained at 240° C. for 2 hours and gradually increased to 340° C. over the next 2 hours. While maintaining 340° C. a vacuum was applied of less than 3 mm Hg for one-half hour. The vacuum was then released under nitrogen and the reaction vessel was cooled to room temperature. The resulting copolyester was opaque and tan in color. The DSC analysis of the copolyester showed a peak melting temperature of 250 ° C. and a glass transition temperature of 101° C. on the first heating scan. The copolyester gave a crystallization peak at 210° C. on cooling and a peak melting temperature of 240° C. on the second heating scan. The heating/cooling rate for each scan was 20° C. per minute. A film of the copolyester was optically anisotropic when observed through a polarizing microscope above its DSC-determined melting temperature. The copolyester had recurring units of ##STR4## 
     Measurement of the Anisotropic Properties of the Copolyesters 
     Optical anisotropy of the copolyester melts can be determined by examination of the materials with the use of an optical microscope. The equipment used for determining the optical anisotropy of the copolyesters of the present invention included a TH 600 hot stage, (Linkham Scientific Instruments LTD, Surrey, England) and a Nikon Optiphot Microscope equipped with crossed polarizers and a 35 mm camera (Nikon Instrument Group, Nikon, Inc., Garden City, N.Y.). Using the method of example II, other copolyesters were prepared. Tables I and II show the anisotropic properties and melting temperatures of examples of these copolyesters of the invention. At the concentration ratios of reactants in which the polymeric melt assumed anisotropic properties, the copolyester was said to exhibit liquid crystalline properties. 
     
                       TABLE I______________________________________Properties of the Example I Copolymers ##STR5##X       Y                           Liquid(mole   (mole                       Crystallinepercent)   percent)  Tg(°C.)                       Tm(°C.)                               Properties______________________________________ 0      100       150       407     no10      90        153       388     no20      80        140       375     no30      70        138       374     no35      65        131       366     yes40      60        137       355     yes50      50        137       320     yes60      40        130       305     yes65      35        122       290     yes70      30        127       323     yes75      25        138       367     yes80      20        none      380     yes______________________________________ 
    
     
                       TABLE II______________________________________Properties of the Example II Copolymers ##STR6##X       Y                           Liquid(mole   (mole                       Crystallinepercent)   percent)  Tg(°C.)                       Tm(°C.)                               Properties______________________________________ 0      100       136       288     no10      90        136       250     no20      80        139       244     no30      70        132       242     no40      60        105       248     yes50      50        101       250     yes60      40        105       242     yes70      30        107       338     yes80      20        107       366     yes90      10        121       389     yes______________________________________