REPOSITORY > RESULTS

Abstract

In the present thesis, the problem of the functionalisation of fluoropolymers was addressed.
As chemical methods for the functionalisation of fluoropolymers apply environmentally
harmful chemicals, the research was directed to low-pressure non-equilibrium
radiofrequency (RF) inductively coupled plasmas of environmentally friendly gasses. The
research was focused on polytetrafluoroethylene (PTFE), the most representative
fluoropolymer. The interactions between low-pressure cold hydrogen plasma and the PTFE
surface were studied using X-ray photoelectron spectroscopy (XPS), Secondary Ion Mass
Spectrometry (SIMS), and goniometry (water contact angle - WCA). After optimising the
PTFE surface functionalisation with hydrogen plasma, we studied the interaction of pretreated
samples with low-pressure cold oxygen plasmas. A cobalt catalytic probe was used
for O-atom density measurements. The subsequent treatment of PTFE with low-pressure
hydrogen and oxygen plasmas enabled us to obtain a superhydrophilic PTFE surface with
a water contact angle of about 5°. By selective exposure of PTFE to vacuum ultraviolet
(VUV) radiation in hydrogen plasma, we discovered that VUV is the main cause of C−F
bond breakage in the PTFE surface layer, resulting in the formation of dangling bonds.
We found H-atoms of great importance for terminating the dangling bonds and binding
the released fluorine into HF molecules. Hydrogen plasma treatment resulted in nearly
instant formation (after 1 s) of a few nm thick polyolefin-like layer containing almost no
fluorine. The surface of samples treated with hydrogen plasma exhibited a WCA of about
95°. The PTFE with polyolefin-like surface layer was then treated in the afterglow of
oxygen plasma. We found that superhydrophilicity (WCA of 5°) of the pre-treated PTFE
surface occurred instantly within 0.2 to 0.3 s with an O-atom fluence of about 1×1023 m−2.
The oxygen content in the treated surface as determined by XPS was about 20 at.% and
remained constant over a broad range of O-atom fluences. Oxygen was bound in
hydrophilic and hydroneutral functional groups such as hydroxyl, carboxyl or carbonyl.
The WCA dependence on the O-atom fluence exhibited a minimum, followed by a dramatic
hydrophobic recovery. Ultimately, after prolonged treatment with oxygen plasma, the F/C
ratio of the untreated polymer was achieved. The complete hydrophobic recovery indicates
that there are simultaneous competitive processes of functionalisation and etching. From
the constant oxygen content over a broad range of atom fluences around the WCA
minimum, it was concluded that the WCA did not depend only on the chemical
composition of the surface, but also on the surface roughness controlled by etching. The
method for hydrophilisation of PTFE samples was proved in a small experimental plasma
system. To bring our research closer to industrialisation, an industrial size reactor
sustaining plasma with four coils coupled in parallel to the same RF generator was
developed. Such a reactor can be used for the treatment of polymers of almost arbitrary
shape and size. The specialty of this system is the ability to equalise the impedances of
leads for each coil separately using sliders. We mounted RF coils into the reactor, and
constructed innovative dielectric cups submerged into the reactor, which enabled
positioning of the coils in a space sustained at atmospheric pressure, thus preventing arcing
and the interference of electromagnetic fields with metallic parts of the reactor. The
innovative solution enabled high atom density in the metallic chamber due to minimised
recombination of neutral atoms. We managed to achieve an almost perfect equalisation of
the power distribution among coils and thus steady transitions of plasma modes in all coils
at elevated RF power. Transitions between plasma modes refer to the number of coils
sustaining plasma in a specific mode. The even distribution of power among coils enabled
a transition from E- to H-mode for all four coils at reasonable power. The proper operation
of the newly constructed plasma system was proved by systematic measurements of Oatom
densities within the reactor vessel. We observed tangible gradients close to the plasma
exhausts of the dielectric cups, but 21 cm below the dielectric cups we found homogeneous
distribution of neutral oxygen atom density of about 1.2×10[SUP]21 m−3.