Morphology and Crystalline Architecture of Polyaryletherketones - PEEK Biomaterials - page 57

PEEK [ 5, 27 ]. However, typical PEEK injection
molding settings include a warmmold (180 e 220 C).
This does not create a quenched, amorphous surface
but can create a surface with higher crystallinity [26] .
In either case, the skin effect can be addressed by
subsequent thermal treatments, by machining away
of any variant crystalline skin, or by molding test
specimens of sufficient thickness as to render the
presence of a thin surface skin negligible [26] .
The microstructure developed during processing
affects mechanical response. For example, larger,
more perfect crystals are associated with weaker, less
damage-tolerant materials. This is because the
boundaries between crystallites are weak and allow
easier crack initiation and propagation [22] . Gener-
ally, though, mechanical properties of PEEK, such as
yield strength, elastic modulus, or toughness, are
determined by its degree of crystallinity.
The mechanical properties of PEEK generally
decrease with increased temperatures up to 250 C,
with a pronounced drop-off in properties above
150 C (i.e., for temperatures exceeding the glass
transition temperature) [28 e 32] . However, within the
context of biomaterial applications, where the
expected operating thermal environment is around
37 C (body temperature), the elastic behavior of
PEEK is relatively insensitive to temperature.
Implant applications that can involve heat generation,
such as impact loading during installation, or fric-
tional contact in a joint replacement, may involve
more detailed consideration of thermal effects on
mechanical behavior. Generally, the amorphous
phase controls yielding, plastic flow, and fracture
behavior at test temperatures below I g .
At ambient conditions, varying the strain rate in
uniaxial compression from 10 4 s 1 , correspond-
ing to nearly quasi-static loading, to 10 3 s 1 ,
corresponding to impact loading, increases the
yield strength of PEEK by around 30% [32] .This
is related to the ability of the individual polymer
chains to respond to loading. In PAEKs as in other
polymers, increasing the strain rate is similar to
decreasing the temperature, and at higher rates
molecular response is too slow, creating a stiffer,
more elastic response, and lower elongation at
break. The microstructure also affects high strain
rate and large deformations of PEEK associated
with impact. Such loading can create a number of
interesting micro- and macrothermomechanical
phenomena, including changes in crystallinity, defor-
mation-induced heating, macroscopic discoloration,

and viscoelastic recovery-induced rupture [32] .The
relevance of rate sensitivity should be consideredwhen
performing mechanical test evaluations of devices that
may be implanted by impact loading, such as PEEK
hip stems and bone anchors. Rate effects and elasticity
are both relevant to the fatigue performance of poly-
mers and should similarly be considered in designs that
include PAEK polymers [33] .

4.5.3 Effects of Fillers on Structure
and Properties
Inorganic fillers are often used to modify the
properties of polymers, and this is also the case for
PAEK polymers. Implantable PEEK is commercially
available in neat form, as well as with carbon fibers or
barium sulfate. The addition of these fillers alters the
two-phase morphology of PEEK, introducing a third
phase and changing the nucleation and growth of
crystals through both physical templating and
changes in thermal conductivity [18] .
As composite PEEK materials cool from the melt,
the manner in which polymer chain crystallize is
influenced by the presence of the filler [34] . Carbon
fiber-reinforced PEEK, for example, can exhibit both
spherulitic and epitaxial transcrystalline growth from
the fiber surfaces [35] . Recent research has further
shown that, at the microscale, composite PEEK
materials contain another region of short-range
order, referred to by Godara et al. [36] as an
“interphase region,” where the crystallization of the
PEEK matrix, and its localized micromechanical
behavior,

is influenced by the presence of the

reinforcing filler.

4.6 Summary and Conclusions

PAEKs are high-performance polymers that share
a predominantly linear aromatic backbone that
readily packs into orthorhombic crystals and generate
a two-phase morphology with up to about 40%
crystallinity. The unit cells of PAEK polymers are
quite similar because of the crystallographic equiv-
alence of the ether and ketone linkages. However,
glass transition temperatures increase with increasing
ketone content, as the ketone hinders mobility. The
constraining effect of crystallites has been observed
to increase T g as well.
Several techniques have been used to successfully
characterize PAEK structure, though combined