Biomedical Engineering Reference
In-Depth Information
representations of the curves used to account for the
amorphous halo, as well as the areas associated with
each peak. Diffraction peaks are visible at 2
h
approximately equal to 19
,21
,23
,29
,33
, and
39
for PEEK, and the first four of these peaks are
associated with the 110, 111, 200, and 211 planes,
respectively
[3]
. Similar diffraction peaks are present
in PEK, a result reinforced by the similarity in
principal X-ray reflections for PEEK and PEK
[2]
.In
addition, the relative intensity of the 110 and 200
peaks (19
and 23
) is reversed in PEK compared
with PEEK. In PEKK, the overall intensity of the
diffraction peaks is reduced, but an additional
diffraction peak appears around 2
h
ΒΌ
similarly reveal the effects of preferred chain
conformations associated with crystallinity
[21,22]
.
The calibration and use of the ratio of the
1305 cm
1
and 1280 cm
1
bands have been
described
[20]
and standardized in ASTM F2778, in
which specular reflectance FTIR is used as a basis for
determining the degree of crystallinity in the sample.
A similar technique for comparing the areas of the
966 cm
1
and 952 cm
1
peaks has also been used
[22]
. The specular reflectance method is suitable for
any smooth surface and allows for relatively conve-
nient evaluation of a variety of geometries. Because
of the penetration depth of the IR radiation, however,
this method provides information about the surface
of the sample and does not provide information about
the bulk. Considering that the skin of an injection-
molded part can be 200
e
300
m
m
[23]
, hundreds of
times more than the typical 1- to 2-
m
m penetration
depth in IR testing, this distinction can be important.
If the processing conditions create a skin
d
core
morphology with, for example, notably more crys-
tallinity in the bulk than at the surface
d
a surface
measurement will not capture the crystallinity that
dominates mechanical properties.
The FTIR method of ASTM 2778 was applied to
the three samples evaluated by WAXS, and an overlay
of the spectra is presented in
Fig. 4.5
. Although the
spectra are similar, they provide insight into the
polarity and interaction of the chains in the three
polymers. For example, in the fingerprint region, both
PEK and PEKK exhibit the same major absorbances
as PEEK, though some are shifted and the absorbance
at 1190 cm
1
, which is associated with asymmetric
stretching of one of the diphenyl ether groups, is
absent
[21]
(
Fig. 4.5b
). The two bands associated with
crystallinity are shifted slightly to lower wavenumber
(higher energy), perhaps because of the relative
effects of chain torsional angle for the different PAEK
polymers. By FTIR, average calculated crystallinity
for each sample was 35%, 30%, and 10%, respec-
tively. Thus, the agreement with WAXS data for
PEEK was excellent and within experimental error
for PEK. The lower crystallinity of the PEKK sample
measured by FTIR, compared with the value deter-
mined from WAXS, may be a skin effect.
16
. The
calculated degree of crystallinity is 35% for PEEK,
32% for PEK, and 17% for PEKK.
4.4.2.2 Small-Angle X-Ray
SAXS provides direct evidence of the physical
structure on a larger size scale, about 10
e
1000 times
greater than that of WAXS. It is used to analyze the
superstructures of polymers, providing information
about the periodicity of the crystalline
e
amorphous
stacks in lamellar crystals (i.e., the long period of the
semicrystalline structure)
[17]
. SAXS has been used
to determine crystal thickness in PEEK as a function
of crystallization temperature and to demonstrate the
lamellar thickening that occurs upon annealing
[4,5,9,19]
. SAXS has also identified precrystalliza-
tion density fluctuations in PEEK
[8]
.
4.4.3 FTIR Spectroscopy
FTIR spectroscopy provides information related to
the structure and mobility of dipoles and is
commonly used to characterize polymeric materials.
IR absorption peaks correspond to specific chemical
groups. In the case of PEEK, the ratio of the
absorption bands at 1305 cm
1
and 1280 cm
1
is
known to be sensitive to crystallinity
[20]
. These
bands have been associated with the carbonyl group
of the ketone linkage and the resonance of the
diphenyl ether group, respectively
[21]
. Other bands
Figure 4.5
FTIR spectra for the three PAEK polymers evaluated by WAXS as shown in
Fig. 4.4
. IR absorption bands
are different for the three materials, but the reference bands near 1305 cm
1
and 1280 cm
1
are present. (a) Overlay
plot of region identified in ASTM F2778. Using this method for all polymers, degree of crystallinity is calculated as 35%
for PEEK, 30% for PEK, and 10% for PEKK. (b) Stacked plot of fingerprint region identifying absorbance bands for the
three polymers. Image courtesy of Paul Ledwith, Exponent, Inc.
