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1. A fiber comprises a phosphonated polybenzimidazole (PBI) polymer with a phosphoric acid pick-up (APU) in the range of 1-25%, an LOI (Limiting Oxygen Index) of about ≥47%; and having a denier no greater than 6 denier per filament (dpf), having an initial thermal decomposition temperature of about 590° C., wherein the PBI polymer is selected from the group consisting of poly-2,2′-(biphenylene-2″2′″)-5,5′-bibenzimidazole: poly-22′-(biphenylene-4″4″′)-5,5′-bibenzimidazole: poly-2,2′-(1″,1,3″trimethylindanylene)-3″5″-p-phenylene-5,5-bibenzimidazole; 2,2′-(m-phenylene)-5,5′-bibenzimidazole/2,2-(1″,1″,3″-trimethylindanvlene)-5″,3″-(p-phenylene)-5,5′-bibenzimidazole copolymer: 2,2′-(m-phenylene)-5,5-bibenzimidazole-2,2′-biphenylene-2″,2′-5,5′-bibenzimidazole copolymer; poly-2,2′-(furylene-2″,5″)-5,5′bibenzimidazole; poly-2,2′-(naphthalene-1″,6″) 5,5′-bibenzimidazole; poly-2,2′-(naphthalene-2″,6″)-5,5′-bibenzimidazole: poly-2,2-amylene-5,5′-bibenzimidazole poly-2,2′-octamethylene-5′-bibenzimidazole: poly-2,2′-(m-phenylene)-diimidazobenzene: poly-2,2′-cyclohexenyl-55′-bibenzimidazole poly-2,2-(m-phenylene)-5,5-di(benzimidazole)ether; pol-2,2′-(m-phenylene)-5,5-di(benzimidazole)sulfide: poly-2,2′-(m-phenylene)-5,5′-di(benzimidazole)sulfone: poly-2,2′-(m-phenylene)-5,5-di(benzimidazole)methane poly-2,2″-(m-phenylene)-5.5″-di(benzimidazole)propane-2,2; and poly-ethylene-1,2-2,2″-(m-phenylene)-5,5″-dibenzimidazole)ethylene-1,2 where the double bonds of the ethylene groups are intact in the PBI polymer.
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