Quantifying Surface-Accessible Quaternary Charge for Surface Modified Coatings via Streaming Potential Measurements

Prior research established that P[AB]-copolyoxetane polyurethanes with soft blocks having A = trifluoroethoxy (CF3CH2-O-CH2-, 3FOx) and B = dodecylammonium-butoxy (C12) are highly effective as polymer surface modifiers (PSMs). These PSMs displayed high contact antimicrobial efficiency against spray challenge that was attributed to surface concentration of quaternary charge. Herein, using a novel cell design and polymer coating process, streaming potential (SP) measurements arc reported or estimating accessible surface charge density. Fused-silica capillaries were embedded in flat polypropylene sheets, and the inner capillary walls were coated with neat HMDI-BD(30)-P[(3FOx)(C12)-87;13-5100] (PU-1) and 1 wt% PU-1 in HMDI-BD(50)-PTMO-1000 (base polyurethane 2). Effects of annealing (60 degrees C) and electrolyte now cycles on near-surface quaternary charge concentration were determined. Neat PU-1 had a constant SP that was cycle-independent and actually increased on annealing. As-cast 1 wt% PU-1 showed initial SPs about half those for neat PU-1, with substantial attenuation over 16 measurement cycles. St's for annealed 1 wt% PU-1 displayed lower initial values that attenuated rapidly over multiple cycles. Zeta potentials and surface charge densities were calculated from SPs and discussed relative to contact antimicrobial properties. Tapping mode atomic force microscopy (TM-AFM) imaging was employed for investigation of 1 wt% PU-1 surface morphology. Ailicroscale phase separation occurs on annealing 1 wt% PU-1 for 24 h at 60 degrees C. Surprisingly, phase separation was also observed after short immersion of 1 wt% PU-1 coatings in water. The morphological changes are correlated with instability of near-surface charge found by SP measurements. A model is proposed for near-surface spinodal decomposition of metastable as-cast 1 wt% PU-1. The formation of a fluorous modifier rich phase apparently sequesters near-surface quaternary charge and accounts for temporal instability of antimicrobial properties. The results are important in providing a facile method for screening polycation-based, contact antimicrobial coatings for accessible charge density and in assessing durability.