Morphology and Crystalline Architecture of Polyaryletherketones - PEEK Biomaterials - page 51

been reported [4] . Spherulites are orders of magni-
tude larger, about 25 e 40 m m in diameter [4] .For
example, PEEK spherulites formed by isothermal
crystallization at 320 C grew to more than 20 m m
before impinging [9] .
PEEK typically exhibits a high nucleation density.
As a result, polarized light microscopy, which is
commonly used to visualize crystalline detail in other
polymers such as PE, is not as effective for examining
the numerous spherulites that develop in PEEK,
especially when crystallized from the melt. Lamellar
detail of spherulites has been successfully visualized
using specialized etching techniques and examination
with a scanning electron microscope (SEM) [13] .
When fillers and reinforcements are introduced,
nucleation, growth, and crystal morphology can be
affected. Inorganic components can constrain chain
movement and can provide a template for crystal
growth [14] . For example, rodlike crystals have been
shown to nucleate at the surface of carbon fibers in
PEEK and grow in competition with the spherulites
nucleated in the neat resin away from the fiber [15,16] .
Furthermore, thermally conductive fillers can affect
heat transfer in PAEK polymers, and this has been
shown to affect the local crystallization conditions [11] .

approaching the melting temperature, chain folding is
rapid and crystallization is controlled by nucleation
density (i.e., regime I kinetics). At temperatures
approaching the glass transition temperature, crystal-
lization is controlled by diffusion of chains to the
crystal growth face (i.e., regime III kinetics) [6,8] .
Midway between the two temperatures, around
230 C for PEEK [4,18] , the crystallization half-life is
at a minimum as a result of competition between the
two factors [8] . PEEK samples crystallized at higher
temperatures have larger disorder parameters and
larger crystals [9] . In materials crystallized near the
melting point, some level of prenucleation ordering
s n d dm dwh e
Cahn e Hilliard approach [8] .
Nonisothermal crystallization when cooling
PEEK from the melt is more difficult to model. The
Ozawa equation, a modified form of the Avrami
equation that includes the effect of heating rate, is not
adequate, presumably because secondary processes
play a significant role in the crystallization kinetics.
A more generalized Avrami analysis applied at low
conversion, where the free spherulitic growth
approximation is valid, generates exponents that are
rate dependent and greater than those determined for
isothermal crystallization [5] .

4.3 Crystallization Behavior

4.4 Characterization Techniques

Both isothermal and nonisothermal crystallization
has been studied in PEEK. Isothermal crystal growth
can be modeled using the Avrami equation, which is
a general means of describing the empirical data of
materials crystallization. The Avrami exponent is
descriptive of either predetermined (nuclei begin
growing simultaneously) or homogeneous sporadic
(nuclei begin growing at a constant rate in space and
time) nucleation and one of several growth geome-
tries. For spherical crystal growth, the Avrami
exponents are 3 for predetermined nucleation, 4 for
sporadic nucleation, and 2.5 for sporadic nucleation
with diffusion-controlled growth [8,17] . Data from
PEEK isothermally crystallized from both the melt
and the rubbery amorphous phase correspond to an
Avrami exponent centered around 3, indicating that
the nucleation process is simultaneous (athermal) and
growth is spherulitic [5,6,8] .
High-temperature isothermal PEEK crystallization
has been successfully modeled with the Laur-
itzen e Hoffman equation [5] . This approach allows
three different regimes of nucleation. At temperatures

Numerous polymer characterization techniques
have been applied to better understand the structure of
PAEK polymers, especially the degree and nature of
crystallinity. These include the density gradient tech-
nique, wide-angle X-ray (WAXS) and small-angle
X-ray (SAXS) diffraction, infrared (IR) spectroscopy,
thermal analysis, and microscopy. X-ray provides
direct evidence of crystallinity in all the PAEK poly-
mers, whereas FTIR has been shown to provide a reli-
able, more broadly useful technique for determining
the crystallinity of at least PEEK. Density calculations
are effective, but rely on assumptions related to
uniformity and ideal crystal density, and are particu-
larly challenging for composite materials. Thermal
analysis provides insight into heat capacity andmelting
behavior, but standard testing conditions prevent direct
measurement of crystallinity at use conditions. Optical
and electron microscopy allow visualization of the
structure of PAEK polymers on various size scales,
providing insight into crystallite size, arrangement, and
birefringence.