A polymer alloy has been developed comprising a polysulfone and a vinyl lactam polymer. The resulting alloy has excellent thermal characteristics and even in the presence of substantial quantities in vinyl lactam polymers, has solvent resistance to both organic and aqueous solvent materials. The materials, when dissolved in solvents, can be spun from a variety of solvents into a variety of useful fiber materials. The resulting fine fiber, microfiber and nanofiber materials have excellent thermal and chemical resistance for a variety of fiber applications. The polymer alloys of the invention can be spun into nanofiber mats that can act as a filtration media and can also be combined into conventional substrate materials for fabrication into filter structures.