Pulsed AC plasma polymerization for the chemical modification of glassy carbon surfaces in model composite interface studies

Pulsed AC plasma polymerization was used for surface modification of glassy carbon (GC) substrates, which are chosen as a model for the surface chemistry of carbon fibres used in composites. The pulsed AC plasma system, which is custom built, is powered by a 50 Hz 2-phase AC power supply from the grid and phase 1 is shifted 180° from phase 2. The resulting voltage between the electrodes (the plasma voltage) is the sum of phase 1 and 2. Depending on pressure and gas composition, a voltage of around 250 V is needed to ignite the plasma, resulting in a (100 Hz) pulsing plasma current that has an effective power input of less then 1 W/L. This opens up the possibilities of generating plasma polymer layers by plasma-induced radical polymerisation. We utilise maleic anhydride (ppMAH) and 1,2-methylenedioxybenzene (ppMDOB) to generate both homo-plasma polymer and co-plasma polymer layers surfaces, in the power range from 0.3 to 1.3 W/L, to generate stable layers with varying degrees of functional integrity. X-ray Photoelectron Spectroscopy (XPS) and Attenuated Total Reflectance Fourier Transform Infrared spectrometry (ATR-FTIR) were used to monitor surface chemistries, and toluidine blue (TB) staining to determine the number of reactive acid groups after hydrolysis. Atomic Force Microscopy was used to monitor both topography and layer thicknesses using a polymeric masking method. The results show that the properties of the plasma polymer layers can be subtly varied by the plasma conditions. Unexpected results were obtained using the TB staining, and indicate that the hydrolysis of MAH groups is much more difficult to achieve than previously reported. Finally, the interfacial fracture properties were determined using a Double Cantilever Beam (DCB) method. It was found that differences in fracture energy are related to differences in surface properties, and that in all cases, the plasma polymer coatings increased the fracture energy, compared to the uncoated GC