Described herein is a continuous process for modifying the properties of polyester and polyester based fibers, such as a poly(butylene terephthalate) (PBT) fiber, comprising subjecting the PBT fiber to alkaline hydrolysis, and optionally further comprising functionalizing the PBT fiber by solution grafting such as fluorination. The alkaline hydrolysis and optionally subsequent functionalization such as fluorination process can be continuous, following the melt blowing/spinning or spun-bonding process. Also described is a nonwoven PBT fiber mat obtained by the surface modification process. Further described is a filtration device comprising the nonwoven PBT fiber mat.