Page 185 - Biodegradable Polyesters

7.3 Improving the Bioactivity of Electrospun Polyesters 163

temperature, and reactor geometry. In consequence, it is difficult to achieve a
good understanding of the mechanisms involved between the plasma species
and the surface.
3) It is difficult to control the concentration of a particular functionality formed
on a polymer surface.
4) Usually, polymer substrates modified by plasma have to be further processed
after treatment, from a question of hours to a few days because polymers com-
monly recover pristine properties in a short period of time.

The use of direct current (DC) in a plasma requires that electricity has to be
conducted through the electrodes, where in one of them, the sample is set for
treatment. Frequently, polymer films are insulators which charge up the electrode
and terminate the discharge. To solve the problem, an AC power source is used
because positive charges accumulated during one half-cycle can be neutralized by
electron bombardment during the next cycle. To provide a continuous discharge,
a frequency of 50–100 kHz is sufficient to obtain many of the qualitative features
observed in a DC glow discharge. An RF plasma can be initiated and sustained
by external electrodes outside the reactor vessel or the coil wound around the
reactor vessel. A typical plasma system, shown in Figure 7.2, consists of a vacuum
pumping system, a reactor vessel, a matching network, a power source, and a gas-
handling or flow-control system. In general, the interaction of the reactive species
generated in a gas plasma can react with a polymer surface in accordance with the
following processes [62]:
1) Surface reactions: Reactive plasma gas-phase species can react directly
with the polymer surface. Alternatively, surface species can produce new
functional groups after exposure to oxygen or other gases. In addition, those
reactive species can give place to cross-linking reactions at the surface.
Examples of these reactions include plasma treatment by argon, ammonia,
carbon monoxide, carbon dioxide, fluorine, hydrogen, nitrogen, nitrogen
dioxide, oxygen, and water.
2) Plasma polymerization: A thin film on the surface of a polymer is formed via
polymerization of an organic monomer, such as CH ,C H ,C F ,C F ,or
4
6
3 6
2 4
2
RF R
hν
R R e M # H
M # O OH HO Si O H H H
e e C O N
R R O
High vacuum

Figure 7.2 Schematic diagram of a parallel-plate cold-plasma reactor representing the
#
*
plasma-generated species. RF: radio frequency generator; R : radical species; M :metastable
excited species; h : UV and VUV emitted radiation.