Immobilization and characterization of poly(acrylic acid) graft layers

This study reports the covalent immobilization of poly(acrylic acid) (PAAC) onto thin films deposited using radio-frequency glow discharge from a vapor of n-heptylamine (HApp). The successful immobilization of PAAC onto HApp was demonstrated by XPS analyses and AFM colloid probe force measurements. The force profiles obtained between silica particles and the PAAC coating were repulsive in nature, roughly exponentially decaying and of long range. The interaction measurements for the PAAC surfaces were not purely electrostatic in nature but also a result of compression of the covalently attached PAAC layer by the silica surface (i.e., electrosteric). The concentration of EDC/NHS added during the coupling influenced both the range and magnitude of the interaction force profiles between the silica colloid probe and the grafted PAAC layer; i.e., the concentration of EDC/NHS added during coupling of PAAC influenced the structure of the layer formed. This result was in good qualitative agreement with XPS analyses. The repulsive forces measured between the silica sphere and the PAAC layers exhibited a significant electrolyte concentration, pH, and molecular weight dependencies. For example, the magnitude and range of the interactions were constant over a pH range of 4.3-9.4. However, at a pH value of 3.1-3.3, the magnitude and range of the interaction were drastically reduced. The results obtained were compared to expectations based on theoretical studies and simulations, and good qualitative agreement was observed.