Pulsed plasma discharge polymer coatings

The authors studied a pulsed radiofrequency glow discharge polymer film deposition method (pRFGD) on polyethylene terephthalate (PET), silicon (Si), and potassium chloride (KCl) surfaces, with the aim of better controlling film uniformity and homogeneity. A pulse generator was used to control a conventional 13.56 MHz RFGD circuit to provide plasma on and off times throughout a wide range of duty cycles. Starting monomers included fluorocarbon monomers (C₈F₁₆O and C₃F₆O) and more conventional unsaturated monomers [acrylonitrile (C₃H₃N) and vinyl trimethyl silane (C₅H₁₂Si)]. With each of these monomers progressive, large scale changes in the molecular structure of the plasma deposited films were noted with systematic variations in the RF duty cycle. Film characterizations were performed using electron spectroscopy for chemical analysis (ESCA) and fourier transform infrared (FTIR) analysis. In the case of fluorocarbon (FC) films, systematically decreasing plasma on time at a constant off time resulted in enhanced CF₂ and CF₃ content compared with that seen with the less highly fluorinated groups. There was virtually no oxygen atom incorporation in the FC films obtained from the oxygen containing monomers. Overall, a dramatic decrease in cross-linking of the FC polymer films was observed with decreasing RF duty cycles. A highly ordered Teflon-like structure was obtained for the lowest duty cycles. In the silane experiments, a systematic variation in the ratio of Si-H/Si-CH₃ groups was observed, with this ratio increasing as the RF duty cycle decreased. Experiments with C₃H₃N revealed an increasing surface density of -CN groups with decreasing RF duty cycle. In general, film deposition rates per unit absorbed RF energy increased markedly with plasma off times, suggesting that significant chemistry and film growth occurred during plasma relaxation. FC coated PET samples were subjected to albumin binding studies. Albumin retention after dilute sodium dodecyl sulfate treatment was significantly increased for FC- PET compared with that seen with PET controls, suggesting that improved surface passivation is possible. The results demonstrate the utility of the variable duty cycle pRFGD method in controlling film chemistry during surface modifications. The authors think this technique offers many advantages, compared with competing approaches, for improving polymer film formation on medical device surfaces.