Attachement of polymer chains on plasma treated surfaces: experiments and modeling

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.