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This work arises from the possibility of changing the surface properties of materials with the use of plasma. It proved to be a very good method for treating surfaces, it is in fact able to modify surface properties of materials without altering their bulk properties. In particular, with the Plasma Enhanced Chemical Vapour Deposition (PECVD) is feasible sustaining the polymerization of a specific monomer depositing thin films containing interesting chemical groups. For these reasons, this technique was chosen for modifying polypropylene substrates through the polymerization of acrylic acid, an organic acid carrying the carboxy group (COOH). This chemical group is of particular interest because it has biocompatibility properties itself and can be used for a further grafting reaction allowing further surface modifications. During this works two types of plasma reactor were optimized for obtaining the best working condition, namely, creating a stable coating of poly-acrylic acid resistant to Phosphate Buffer Saline (PBS) and water washing. The physical and chemical properties of the deposited thin films were studied by means of Fourier Transform Infrared Attenuated Total Reflectance (FTIR-ATR), Water Contact Angle (WCA), XRay Photoelectron Spectroscopy (XPS), Atomic Force Microscope (AFM). The density of COOH groups into the polypropylene surface were evaluated by ion-exchange reaction with Thionin Acetate (THA). A simple model for explaining the particular physical composition and stratification of the films is presented along with a computer simulation. The reactor geometry effect onto the film properties was investigated. Furthermore, some applications of the process were then discussed. Part of the present work consisted in the use of the AFM for internal scopes and extramural collaborations. Among these, the measurement of the thickness of different PEG chains bounded to the plasma treated surface in support of Monte Carlo simulations of their polymer dynamics.

Deposition of linear polymers, such as polyethylene glycol (PEG), on a plasma-treated surface has been studied experimentally and theoretically by means of Monte Carlo (MC) simulations. Acrylic acid is deposited on a silicon wafer in the presence of argon at a pressure of 10 Pa by applying 30W external power. Active carboxyl sites are obtained having a surface number density of ∼2 sites nm-2. A homogeneous PEG solution is brought into contact with the treated surface (over 24 h) and a thin film of attached PEG chains is formed. Two different PEGs having molecular weights of 3000 and 5000 g mol -1, respectively, are considered. The corresponding thin film widths, W, are measured, yielding W(3000) =4.3 ±3.1 nm and W(5000) = 8.8 ± 1.8 nm. For the MC simulations, the linear polymers are modeled as an ensemble of self-avoiding walks of length N (number of monomers) on a simple cubic lattice, executing worm-like or reptation dynamics, which can become attached at an active carboxyl site on the surface. The numerical results for the film widths are in good agreement with the experimental findings. We find that less than 20% of active sites are effectively occupied by attached chains, corresponding to less than 5% of the total available surface sites. Scaling arguments predict universal power-law dependences of the film density, ρ(N), as a function of polymer length, i.e.ρp (N) ∼ c/Nv, with c ≃ 5 g cm-3 and v ≃ 0.6. The model also predicts a dependence of the prefactor c on the density of carboxyl active sites. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.