Biomedical Engineering Reference
In-Depth Information
Acknowledgment
analysis often provides the most comprehensive
understanding of the unit cell, crystalline texture,
crystallite size, and degree of crystallinity. PEEK
exhibits an orthorhombic unit cell with a
c
-axis
dimension of about 10
˚
, reflecting a repeating
segment of the chain. Very similar dimensions
characterize the
c
-axis of other PAEKs, including
PEK, despite the differences in polymer repeat unit
length. The
b
-axis is radially oriented in the spheru-
litic structure of PEEK, and the (110) plane is the
plane of fastest growth. Both positive and negative
birefringences have been reported for PEEK spher-
ulites, but more investigation is required to under-
stand the observations.
Crystal density of 1.400 g/cm
3
is often used in
conjunction with a value of 1.265 g/cm
3
for amor-
phous density to calculate the degree of crystallinity
of PEEK. The results generally agree with those
calculated from WAXS, which has also been the
basis for validating the FTIR peak-ratio method of
ASTM F2778. DSC data should be reviewed with
caution in light of the morphological changes that
occur during the test.
Nucleation is typically heterogeneous and can be
affected by the presence of fillers, such as carbon
fibers. Crystal growth at temperatures near the melt
temperature exhibits regime I kinetics and is
adequately described by the Avrami equation with an
Avrami exponent,
n
, approximately equal to 3, indi-
cating predetermined nucleation of spherulitic crys-
tals. However, some evidence of prenucleation
ordering exists in materials crystallized at tempera-
tures approaching the melting point, and this has
been modeled using the Cahn
e
Hilliard approach.
Regime III kinetics occurs at larger undercooling,
where the Hoffman
e
Lauritzen approach has been
applied to describe crystal growth rates.
Carbon fibers have been shown to generate
epitaxially growing rodlike crystals in addition to the
spherulites that occur in neat PEEK. Spherulites are
typically too small to see by optical methods because
of nucleation density in commercial PEEK but can be
observed directly by electron microscopy when an
appropriate etchant is used.
As with any polymer, the reinforcing effects of
crystallites or fillers will affect mechanical properties
under static, fatigue, and impact conditions. Higher
crystallinity will result in higher modulus and yield
stress. Toughness and elongation, however, are typi-
cally controlled by crystal size and the mobility of the
amorphous phase.
The authors would like to thank Chris Espinosa,
Paul Ledwith, Shujun Chen, Ph.D., and Tao Xu,
Ph.D., Exponent, Inc. for assistance with the figures
and data in this chapter and Invibio, Inc. Oxford
Chem, and Solvay Advanced Polymers for samples
of their materials.
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