Controlled Nitroxide-Mediated Styrene Surface Graft Polymerization with Atmospheric Plasma Surface Activation

Polymer layer growth by free radical graft polymerization (FRGP) and controlled nitroxide-mediated graft polymerization (NMGP) of polystyrene was achieved by atmospheric pressure hydrogen plasma surface activation of silicon. Kinetic polystyrene layer growth by atmospheric pressure plasma-induced FRGP (APPI-FRGP) exhibited a maximum surface-grafted layer thickness (125 angstrom after 20 h) at an initial monomer concentration of [M](0) = 2.62 M at 85 degrees C. Increasing both the reaction temperature (T = 100 degrees C) and initial monomer concentration ([M](0) = 4.36 M) led to an increased initial film growth rate but a reduced polymer layer thickness, due to uncontrolled thermal initiation and polymer grafting from solution. Controlled atmospheric pressure plasma-induced NMGP (APPI-NMGP), using 2,2,6,6-tetramethyl-l-piperidinyloxy (TEMPO), exhibited a linear increase in grafted polystyrene layer growth with time due to controlled surface graft polymerization as well as reduced uncontrolled solution polymerization and polymer grafting, resulting in a polymer layer thickness of 285 angstrom after 60 h at [TEMPO] = 10 mM, [M](0) = 4.36 M, and T = 120 degrees C. Atomic force microscopy (AFM) surface analysis demonstrated that polystyrene-grafted surfaces created by APPI-NMGP exhibited a high surface density of spatially homogeneous polymer features with a low root-mean-square (RMS) surface roughness (R-rms = 0.36 nm), similar to that of the native silicon surface (R-rms = 0.21 nm). In contrast, polymer films created by APPI-FRGP at [M](0) = 2.62 M demonstrated an increase in polymer film surface roughness observed at reaction temperatures of 85 degrees C (R-rms = 0.55 nm) and 100 degrees C (R-rms = 1.70 nm). The present study concluded that the current approach to APPI controlled radical polymerization may be used to achieve a grafted polymer layer with a lower surface roughness and a higher fractional coverage of surface-grafted polymers compared to both conventional FRGP and APPI-FRGP.