Biomedical Engineering Reference
In-Depth Information
Although spherulites in PEEK are typically too
fine to allow desired visualization, crystallites in
PAEK polymers create birefringence that can be
observed via polarizing light microscopy in properly
prepared samples
[4]
. Birefringence in spherulites
occurs when the refractive index of a crystal in the
radial direction is different than that in the tangential
direction; the birefringence is positive when the radial
value is greater and negative when the tangential
value is greater
[17]
. Both positive
[4]
and negative
[9]
birefringence have been reported for PEEK.
adjacent amorphous material has been reported for
PEEK
[7,23]
. This is often recognized as a slight
increase in the glass transition temperature or an
increase in the breadth of the transition.
At temperatures close to the glass transition
temperature, molecular motion is sufficient to result
in slow densification of the amorphous phase. The
effect, described as physical aging, generally
appears as an enthalpic relaxation near
T
g
in DSC.
Aging does not affect the heat capacity of PEEK,
except in the glass transition region
[6]
. Physically
aged samples have increased density and a reduc-
tion in the toughness of the material
[7]
. Physically
aged PEEK exhibits increased tensile yield stress,
more localized yielding, and a decrease in impact
strength
[6]
.
Above
T
g
but below
T
m
, PAEK polymers exhibit
elasticity due to crystallinity. In this temperature
range, though, chain segments in the amorphous
region are sufficiently mobile to allow recrystalliza-
tion, lamellar thickening, and an overall increase in
the perfection of the crystallites
[19,23]
. Slow cool-
ing from the melt will also produce larger, more
perfect crystals
[22]
. This change in crystal charac-
teristics can have a significant effect on mechanical
properties, including stiffness
[16]
.
4.5 Structure ProcessingeProperty
Relationships
4.5.1 Thermal Transitions
Polymers exhibit characteristic thermal transitions
that are associated with molecular mobility of the
chains. For semicrystalline materials, this includes
a primary transition associated with the melting of
crystallites and described by a melting temperature
(
T
m
), as well as a secondary transition, the glass
transition (
T
g
), associated with the large-scale co-
operative motion of the amorphous phase. Polymers
may exhibit other secondary transitions associated
with small-scale molecular motion, and such a tran-
sition has been described for PEEK
[24,25]
. Dy-
namic mechanical analysis reveals three thermal
transitions in PEEK, melting (
a
transition), the glass
transition (
b
transition), and a third transition asso-
ciated with hydrogen bonding and the effects of
adsorbed water, detectable at very low temperatures
(
g
transition)
[24]
.
As would be expected for these molecular struc-
tures, limited chain flexibility combines with the
stereoregularity of the polymer backbone to create
polymers that exhibit high glass transition tempera-
tures and crystalline melt temperatures. At tempera-
tures relevant to the performance of medical devices,
PAEK polymers are below both their melting points
and glass transition temperatures, limiting recrystal-
lization or physical aging. This is significant for the
medical device designer.
The rigid polymer backbones of PAEK polymers
require more thermal energy to move than a more
flexible backbone, like that of PE, and exhibit corre-
spondingly higher temperatures to activate large-
scale cooperative motion. Crystallinity also hinders
molecular motion, and the effect of crystallinity on
4.5.2 Effects of Temperature on
Structure and Properties
Under clinically relevant temperatures and use
conditions, the morphology of PAEK polymers is
essentially stable. Thus, thermal history during
manufacturing is a controlling factor for the structure
and properties of PAEK-based devices.
PAEK polymers are readily formed by traditional
melt processes, including injection molding. In some
polymers, a high-crystallinity surface “skin,” in rela-
tion to the core of the specimen, can result from flow-
induced orientation of the polymer molecules
[26]
.In
PAEK polymers, the opposite can also be true, with
a less ordered, even amorphous surface developing in
response to the quenching effect of the mold and the
relatively low thermal conductivity allowing
continued molecular organization until the core also
cools. Because the polymer cooling rate varies with
thickness in injection molding conditions, a part may
exhibit spatially varying crystallinity and will be
susceptible to heterogeneous material properties.
Surface quenching has been described in
controlled studies investigating the crystallization of
