as described hereinabove , polyimides of the following structures are known : ## str7 ## where r is : ## str8 ## also , it has been known to react the respective monomers 2 , 2 - bis ( 3 - or 4 - aminophenyl ) hexafluoropropane and 4 , 4 &# 39 ;- hexafluoroisopropylidene [ bis ( phthalic anhydride )] in the presence of a solvent such as those enumerated hereinbefore , with the reactants being mixed at about room temperature . in practice , the initial product formed following the prior art is the polyamic acid precursor of the polyimide . the polyamic acid can be converted to the polyimide by solvent evaporation followed by heating at about 250 ° c . in carrying out the polyimide preparation process embodiment of this invention , the respective monomers are reacted as heretofore with the substitution of a cresol as the solvent , and with the use of heat to aid dissolution and to form the polyimide . although not entirely understood , the use of a cresol solvent allows the polycondensation reaction to proceed beyond the point reached in practice with the employment of the solvents of dupont and bilow , bringing the polyimide molecular weight up to a level where a free - standing film which can be manufactured into useful forms is obtainable . further , with the use of cresol solvent , the reaction in solution proceeds past the precursor polyamic acid stage to form an isolatable polyimide . in an effort to understand the unique effect cresol exerts on the 6fda / 6f diamine polyimide reaction , it is believed that the following factors may be involved : ( 1 ) cresol is mildly acidic , which would extenuate the basicity of 6f diamine , ( 2 ) cresol has a high boiling point which facilitates the formation of the cyclic imide structure from the precursor amic acid , and ( 3 ) cresol is non - reactive with the polyimide at the elevated temperatures necessary to drive the reaction to completion . in the cresol solvent , first the polyamic acid is formed at a lower temperature ( about ambient temperature ). upon heating to a higher temperature ( up to 210 °- 220 ° c . ), the polyamic acid cyclodehydrates to the polyimide . the polyimide is completely soluble in the cresol solvent . the 6f diamine / 6fda polyimide of this invention is characterized by molecular weight of about 10 , 000 to 130 , 000 , preferably about 25 , 000 to 50 , 000 (&# 34 ; n &# 34 ; of about 15 to 200 , preferably about 40 to 80 ) and inherent viscosity of about 0 . 1 to 0 . 6 , preferably about 0 . 4 . inherent viscosity is measured at 30 ° c . as a 0 . 5 % solution in a suitable solvent such as n , n - dimethyl acetamide or tetrahydrofuran . as used herein , the term &# 34 ; cresol &# 34 ; is inclusive of ortho - cresol , meta - cresol , para - cresol , or mixtures thereof . in the practice of the present invention , meta - cresol is preferred since it is the easiest of the cresol isomers to remove and it does not leave a residue which is potentially deleterious . for example , when para - cresol is oxidized , a colored material is produced which would be deleterious if the desired polyimide product needed to be colorless . however , commercial grade materials , which do contain impurities , are usable in the practice of this invention . commercial grade &# 34 ; cresol &# 34 ; is often a mixture of the three isomers plus impurities . it is preferable to dry and vacuum distill the commercial grade cresols to remove water and colored impurities . moreover , the cresol solvent used in practicing the present invention may be mixed with a co - solvent which does not adversely affect the preparation of high molecular weight polyimides and which has a boiling point that is compatible with the temperature needed to produce the reaction to form the polyimide product . typical co - solvents include benzene , toluene , and xylene . in addition , the latter co - solvents could facilitate formation of the polymer product by allowing water to be removed from the reaction mixture by azeotropic distillation and thus preventing break - up of the polymer by water . the co - solvent may comprise up to 95 % by volume of the reaction solvent , with the cresol component achieving the desired results in accordance with the present invention by a catalytic effect . the polyimide can be isolated by admixing the cresol solution thereof with sufficient non - solvent , such as absolute methanol , or ethanol , or by driving off the cresol solvent by vacuum distillation . thereafter , following further purification if desired , a film can be formed by a standard solvent film casting process , for example by dissolving the polymer in dimethylacetamide or a mixture of dimethylacetamide with acetone , followed by casting onto a stationary or moving warmed substrate and heating to remove remaining solvent . a recoverable , transparent film is obtained which can be used in the place of the quartz or glass covers of individual solar cells . one could bond the film to a solar cell using the polyimide in the form of a lacquer with solvents such as tetrahydrofuran , acetone , dimethylacetamide , and similar solvents or mixtures . the following non - limiting example illustrates the preparation process of this invention . a 100 ml , round bottom , three - necked flask was fitted with an oil bath , magnetic stirrer , reflux condenser , nitrogen gas inlet , and nitrogen gas outlet to a mercury bubbler . the flask was charged with 6fda ( 4 . 44 grams , 0 . 010 mole ) and freshly distilled and dried meta - cresol ( 35 ml ). the 6fda formed a light yellow solution and it was necessary to warm the meta - cresol to effect solution . the solution was cooled to room temperature and 3 - 6f diamine ( 3 . 34 grams , 0 . 010 mole ) was added portion by portion . the solution was stirred at ambient temperature for 30 minutes and then the oil bath placed under the flask . the reaction mixture was heated under nitrogen with the oil bath temperature at 210 °- 220 ° c . there was some frothing during the early part of heating , probably due to the evolution of water from the imidization step . the reaction mixture was heated for 1 hour , cooled to ambient temperature and the polymer isolated by pouring into 300 ml of absolute methanol . the polymer was filtered and washed with fresh methanol and dried at 90 ° c . in vacuum . the polymer was then dissolved in 200 ml of tetrahydrofuran , treated with several grams of activated charcoal , filtered and reduced in volume to 75 ml . then 200 ml of absolute methanol was added and the precipitated polymer filtered . the polymer was washed with methanol and dried at 100 ° c . to yield 4 . 1 gms , molecular weight ( mw ) of 34 , 400 . after standing , additional polymer came out of solution , mw of 13 , 800 . a good quality colorless free - standing film was cast from the 34 , 400 mw polymer by dissolving in dimethylacetamide , drying at 100 ° c . in a forced air oven and then up to 300 ° c . in nitrogen . the film when heated in air at 300 ° c . showed very little color change . it was also soluble in dimethylacetamide and acetone after the 300 ° c . heat treatment , which makes this material unique among polyimides . such a film of thickness of about 0 . 1 to 2 . 0 mils ( 2 . 54 × 10 - 4 to 5 . 08 × 10 - 3 cm ) can be used to protect the active surfaces of solar cells . variations of the invention will be apparent to the skilled artisan . for example , it is contemplated that the high molecular weight polyimide of the present invention could be directly cast from a solution thereof onto a substrate such as a solar cell without first forming the free - standing film , and the polyimide film of the present invention could be used in other applications , such as in the printed circuit industry , including use as an alpha particle barrier in semiconductor device manufacture . in these applications , at times a free - standing film would be used and at other times a polymer solution would be applied , such as by dipping , electrocoating , spraying , electrostatic spraying and the like . the polyimide herein lends itself to various manufacturing techniques . also , the unique solubility properties of the polyimide will suggest other uses , such as in preserving art objects , i . e . stained glass windows .