ADHESION
SCIENCE
\ 5 9
fluoro-carbons has
been shown
to
produce
a
drastic reduction
in
adhe-
sive
bond strength.
Once
the
bond
has
been made,
the
task
of
examining
the
interfacial
chemistry
is
extremely
difficult.
Various methods
of
approaching
the
interface
have been developed, involving
the
removal
of the
polymer
in
a
suitable solvent,
or the
dissolution
of the
iron substrate
in a
metha-
nolic
iodine solution followed
by
sputter depth profiling through
the
oxide towards
the
interface,
but
both
suffer
from their
own
particular
problems. Careful ultramicrotomy followed
by
STEM used
in
conjunc-
tion with windowless EDX, EELS,
or
electron
diffraction
may be
more
useful
where
an
interphase
has
developed,
but the
analysis
of the
inter-
face
on an
atomic scale
by
these methods
is
still some
way
off.
The
most
productive approach
to
probing
the
interface chemistry directly
is the
use
of
thin layers
of
model compounds deposited from very dilute solu-
tions
or
even
the use of
dilute solutions
of
multi-component commercial
products.
As an
example
of the
former method, Figure 5.44 shows
a set
of
C 1s
spectra
recorded
from thin
films ( <2 nm) of
poly(methy meth-
acrylate)
applied
to
various oxidized metal substrates.
Subtle
differences
in the C 1s
spectra
are
observed which
are
ascribed
to the
nature
of the
interactions
between
the
polymer
and the
oxide
substrates.
The
three substrates
are
silicon, aluminium,
and
nickel
whose oxides
are
acidic, weakly basic,
and
strongly basic respectively.
The
interactions
that
occur
are
shown
in
Figure 5.45
and are
hydrogen
bonding
(silicon)
bidentate interaction
(aluminium)
and
acyl nucleophil-
ic
attack
(nickel),
which
are
established
on the
basis
of the C 1s
spectra
of
Figure 5.44.
In
a
similar vein Figure 5.46 shows
the N 1s
spectrum
of
diethanol-
arnine
(DEA which
is a
convenient analogue
for a
cured epoxy
resin)
adsorbed
on
oxidised aluminium treated with
the
adhesion promoter
glycidoxypropyl
trimethoxy silane
(GPS).
The two
components repre-
sent nitrogen with
a
partial charge,
(<5
+
), at the
lower binding energy
and
quaternary nitrogen
(—C—NH
2
—C—)
represented
by the
higher
binding
energy component. This indicates
the two
different
modes
of
interaction experienced
by the DEA
molecule.
The
partial
charge results
from
intermolecular (hydrogen bonding) between adjacent
DEA
mole-
cules
whilst
the
quaternary component represents
a
formal interaction
between
the DEA
molecule
and the GPS
treated
aluminium substrate.
The
change
in
relative intensities
of the
higher binding-energy compo-
nent
results
from
a
change
in
conformation
of the
adsorbed molecules: