Electrospinning of a series of soft segment-free biocidal polyurethanes was undertaken to facilitate examination of the influence of fiber size and morphology on resulting biocidal activity. Although the fibrous nanomaterials greatly outperformed films of the same composition, tunability of resulting activities was also accomplished by controlling crystallization of the nanofibers through electrospinning parameter modulation. While inaccessibility of quaternary ammonium biocides driven by extensive fiber crystallization limited overall antimicrobial activity, biocide structure and electrospinning parameters were utilized to overcome material limitations in order to generate highly antimicrobial materials. Overall, processing conditions were shown to be decisive elements in dictating activities as specific polymer compositions were characterized as having antimicrobial activities spanning over 4 orders of magnitude depending on electrospinning conditions and up to 7 orders of magnitude when compared to films of the same composition. Published by Elsevier Ltd.