Biomedical Engineering Reference
In-Depth Information
with respect to introduction of grafted chains is to be achieved it is crucial to
understand both the exact chemistry introduced, the physical properties that
result from introduction of the grafted chains and the position of the graft
co-polymer in the substrate.
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11.3.1 Chemical Characterisation of Surfaces
The most commonly used techniques for evaluation of chemical changes to
the surface of a polymeric substrate are Fourier transform infrared spec-
troscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). While FTIR
spectra are recorded at ambient conditions XPS requires vacuum conditions.
These two techniques are somewhat complementary with XPS giving infor-
mation regarding the elemental composition (as well as the element's 'oxi-
dation state') while FTIR will give information regarding the functional
groups present. It should be noted, that when the chemistry of the substrate
and the grafted chains are chemically distinct, e.g., PAA chains on PTFE,
then it is fairly straight forward to characterize the grafted material using
either technique.
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However, when the chemistry is similar, e.g., PAA chains
on PCL, then it is more challenging and this is illustrated in Box 4.
A limitation of XPS is that it cannot be used directly to assess for
example, carboxylate groups on a polyester substrate due to the similar
binding energies of COOH and COOR (289.2-289.2 eV and 288.6-289.2,
respectively
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) and FTIR cannot easily be used to e.g., quantify different
nitrogen-containing groups due to overlap in band positions.
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Some of
these shortcomings can be overcome using derivatisation methods, most
commonly in conjunction with XPS although it has also successfully been
used with Raman spectroscopy.
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Examples of commonly used probes and
the functional groups with which they react are given in Table 11.5. Some of
these probes are used in a gas phase reaction in which a sealed container
with the substrate as well as the probe molecule (sometimes at elevated
temperature) is reacted. Other probes react with the functional group in a
solution phase reaction, in some cases assisted by other reagents (e.g., some
probes require carbodiimide chemistry for coupling). Some studies have
found that probes in the gas phase will react only with the functional groups
exposed to the surface
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while solution based probes may penetrate further
in to the substrate if the solution is capable of swelling the grafted chains
and/or the polymer substrate. Some of these probes display high selectivity
towards a specific functional group; however, others are reactive towards
many different groups, e.g., trifluoroacetic anhydride (TFAA) is reactive
towards amines, carboxyl, epoxides and hydroxyl groups. An example of a
highly selective probe is trifluoromethyl benzaldehyde (TFBA) used to detect
amine groups. This probe was found to yield the same information with
regards to percent amine groups relative to all nitrogen species as that
obtained from narrow scan fitting of the N 1s spectrum.
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A technique which is gaining increasingly more use is time-of-flight sec-
ondary ion mass spectrometry (ToF-SIMS).
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.
This technique provides
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