Chemistry Reference
In-Depth Information
7.3.4 Plasma Treatment
When a gas is subjected to extreme heat, or an electric or electromagnetic
field applied from a laser or microwave generator, this causes the molecules
to ionize thus turning the gas into plasma composed of charged ions,
electrons, radicals, and neutral species.
108
When the generated plasma has a
uniform thermal equilibrium with its components at the same temperature,
such plasma is described as thermal plasma. On the other hand, the plasma
is termed a non-thermal having a strongly deflected kinetic equilibrium
when excited electrons temperature is higher than the ions and the neu-
trals.
109
Because of their high temperature, diverse density and complex
compositions, plasma interacts with a polymer when it is physically bom-
barded by excited electrons resulting in polymer surface modification
bearing chemical or ionic groups that could participate in further reactions
such as cross-linking, grafting, etching, roughening and functionalization.
Like ion implantation, plasma treatment has the advantage of modifying the
material's surface layer only, without tempering with the polymer's intrinsic
mechanical properties.
110
The high discharging e
ciency of radio frequency
(RF) plasma makes it an excellent choice in this regard.
108
Despite its excellent biodegradability, compatibility and diverse adjustable
mechanical properties,
7
the highly crystalline PHB is strongly hydrophobic,
resulting in limited applications due to its slow degradation rate, bior-
esorbability and poor cell adhesion.
26,28
Surface treatment by plasma is
considered as an effective approach to increase the hydrophilicity and
wettability of such polymers.
111
Recently, Mirmohammadi et al.
108
compared
the biocompatibility of the PHB surface upon treatment with O
2
and CO
2
plasma at 50 W discharge for 3 min, and found that O
2
plasma treated PHB
showed much improvement. The results indicated that neat PHB with an
irregular and coralloid surface was modified to a more regular surface
morphology with improved roughness (nano-protrusion and nano-in-
dentation) upon plasma treatment. This modification resulted in enhanced
cell-polymer electrostatic interactions, and improved growth of attached
L929 fibroblasts. Based on FTIR-ATR analysis, it was observed that the sur-
faces of plasma treated PHB were functionalized via endowment of oxygen
functional groups such as -OH, COO- and -CO- that improved the wett-
ability of the treated surfaces.
108
Hasirci et al.
112
studied the influence of
oxygen RF-plasma treatment on the surface and bulk properties of PHBV.
Their findings showed that the plasma-treated films absorbed more water
than the untreated films, and the degree of absorption depends on the ap-
plied plasma discharge power. They further observed a decrease in water
contact angles and an increase in oxygen-carbon atomic ratio upon treat-
ment, indicating improved hydrophilicity due to an increase in the oxygen-
containing functional groups on the surface of the polymer.
112
Radio frequency glow discharge (RFGD) plasma generated at 100 kHz was
used to modify a PHO surface that was subsequently grafted with acrylamide
in aqueous solution.
110
The amount of grafted amide and wettability were
d
n
2
r
4
n
g
|
1
.
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