Biomedical Engineering Reference
In-Depth Information
instead, the peroxy/hydroxyl grafting method is preferred since it does not
require exclusion of oxygen between the irradiation step and the grafting step;
in fact, exposure to oxygen (e.g., air) is required. When the radiation source
used to introduce the peroxy radicals is plasma (typically Ar or O
2
plasma) the
process is often referred to as 'plasma-induced graft polymerisation'. The
surface concentration of peroxy radicals can be measured by the diphenylpi-
crylhydrazyl (DPPH) method
35
and have been reported for various substrates to
be up to 2.9
10
11
mol cm
2
for PTFE films,
36
up to 3.5
10
10
mol cm
2
for
polyethylene (PE) films
35
and up to 2.5
10
8
mol cm
2
for poly(vinylidene
fluoride) membranes.
37
The concentration of these peroxy radicals will be
dependent on the polymer substrate (as discussed above in Section 11.2.1) and
the source of radiation, the radiation dose and dose rate. With respect to high
energy radiation details were provided in Section 11.2.1 while for plasma-in-
duced grafting additional aspects of the type of plasma reactor and position of
the sample relative to the plasma zone will also affect the incorporation of
radicals and the reader is referred to articles on plasma modification by Des-
met et al.
2
and Morent et al.
38
for further details.
It is important to realise that the different irradiation sources have dif-
ferent penetration depths into the polymer substrate with gamma rays and
electron beam penetrating the entire material as described above while a
plasma source will affect a depth of up to 60 nm when using Ar or O
2
. This
has impact not only on the position of the graft co-polymer (it will form
where activated species, e.g., radicals, are present in contact with the
monomer) but also on the bulk properties of the polymer. While the use of a
plasma source will leave the bulk properties practically intact, ionising ra-
diation will lead to molecular changes as discussed above in Section 11.2.1.
The produced hydroperoxides or diperoxides species decompose to oxy-
gen-centred radicals in the second step after immersion of the activated
polymer substrate in a degassed monomer solution either at elevated tem-
perature or upon exposure to UV radiation resulting in grafting. As for the
simultaneous radiation method, the grafting process is essentially a radical
polymerisation reaction from the surface. However, since it is a two-step
process and the monomer (solution) is not exposed to radiation, linear (ra-
ther than branched or crosslinked) grafted chains of a broad molecular mass
distribution and with well-defined polymer chemistry will result from this
approach as illustrated in Scheme 11.1B. In the second grafting step the
solvent used to prepare the monomer solution and the monomer concen-
tration will impact on the grafting outcome as outlined in Box 1. In addition,
the reaction temperature and reaction time potentially affect the grafting
outcome when grafting at elevated temperature as indicated in Box 2. As an
alternative to performing the second grafting step in solution, a monomer
vapour can be exposed to the activated surface. This can be done simply by
leaving the sample in the plasma reactor and at reduced pressure, bleeding
in the monomer in the gaseous state. In this type of grafting process the
reaction time or monomer pressure will affect the grafting yield.
44
While the
monomer AA has most extensively been grafted on a large variety of
d
n
3
r
4
n
g
|
2
.
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