Modification of PTFE using low-pressure and atmospheric-pressure plasma methods

Abstract

As an inherently chemically inert and physically stable polymer, PTFE has the potential to be used in medical applications as replacement ligaments or vascular stents. In the work presented in this thesis, atmospheric and low-pressure plasma processes were used to modify PTFE surfaces without altering the bulk properties
of the substrate.

The coupling of two low-pressure gas plasma treatments together into a two-step process was investigated as a method of producing a stable hydrophilic PTFE surface. A roughening oxygen plasma treatment was used to create a high water contact angle (WCA) Cassie-Baxter surface, before an ammonia plasma treatment transformed it into a hydrophilic Wenzel state. Although these surfaces initially exhibited a WCA of <10°, solvent washing caused significant hydrophobic recovery which was attributed to the washing off of low molecular weight oxidised species (LMWOS).

Economically, an atmospheric-pressure plasma process is industrially favourable to low-pressure methods. The simple equipment required for a dielectric barrier discharge (DBD) process means that PTFE modification could be carried out in situ to prevent contamination or hydrophobic recovery being an issue in surgeries. The work presented here produced surfaces with a stable surface potential, the polarity of which was determined by the feed gas. Doping in water and/or ammonia molecules into inert feed gases was found to change the polarity of
the surface potential.

The use of the theory of electrowetting to decrease the WCA of DBD plasma-treated surfaces was successful, although only a small decrease in WCA was observed on the charged surfaces. However, the surface potential of the
substrates was used to initialise the grafting and subsequent polymerization of a number of monomers, as well as deposition of a sulfobetaine zwitterionic layer. The lowest WCA was produced by the dipping of DBD-charged PTFE substrates into an aqueous sulfobetaine solution which produced a WCA of <10° recovering to 39°
after solvent washing.

The methods described in this thesis present a number of ways in which stable hydrophilic PTFE surfaces can be produced: an effective low-pressure treatment altered the wetting state of the surface using roughening effect, and DBD plasma-treated surfaces used the surface potential imparted by the plasma to initialise further grafting processes to achieve stable hydrophilicity.