2, The University of Chicago, Chicago, Illinois, United States
Understanding the role of interfaces is critical to catalysis, surface physics, corrosion, nanoscience, tribology, geochemistry and electrochemistry, and energy production. Resonant X-ray Scattering (RXS) offers a unique element, site and valence specific probe to study spatial modulations of molecular orbital degrees of freedom on the nanoscopic length scale. This unique sensitivity is achieved by merging small angle x-ray scattering and x-ray absorption spectroscopy into a single experiment, where the scattering provides information about spatial modulations and the spectroscopy provides sensitivity to the molecular anisotropy. We applied RXS to biomimetic poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) zwitterionic polymer (ZWP) brushes at solid-liquid interfaces. ZWPs, on which each monomer segment bears both a positive and a negative charge, are an understudied class of polyelectrolyte macromolecules, most of which are simple polyelectrolytes (SPE) that bear a single sign of charge on each monomer. As each monomer has no net charge, the chains are not in extended configurations at low salt concentrations. In contrast to SPE, they expand when salt is added since the local attractions between positive and negative ions are screened, thereby producing a rich science base in understanding the configurations, ionic distribution, and in interfacial interactions of ZWP brushes in a variety of relevant and important ionic environments. The technical opportunity stems from observations that ZWP surface layers are particularly resistant to the nonspecific accumulation of proteins and microorganisms, making them excellent candidates for a wide range of antifouling applications, ranging from biocompatible medical devices to marine coatings. PMPC is a unique member in polyzwitterionic families and it has an ultrahigh affinity to water, leading to no detectable shrinks in aqueous solutions even with low ionic strengths, in contrast to those sulfobetaine and carboxybetaine polymer brushes. In this work, we systemically synthesized highly dense PMPC brushes via surface initiated atom transfer radical polymerization and thoroughly characterized the configurational behavior and lateral charge correlation of PMPC ZWPs at interfaces under a variety of ionic (mono- and multivalent) conditions with different concentrations. PMPC polymers only weakly interact with biomembranes via van der Waals forces as seen in force and neutron spin echo measurements, indicative of the nonspecific binding nature of PMPC polymers. The ion-induced changes of PMPC chain configurations are not apparent, in contrast to common polyelectrolytes and other ZWPs, such as poly(carboxybetaine) and poly(sulfobetaine). This, in turn, establishes structure-property relationships between surface chemistry and the ability of a thin film to resist biofoulant adhesion for the design and optimization of antifouling materials.