Biomedical Engineering Reference
In-Depth Information
Chapter 4
Morphology and Crystalline Architecture
of Polyaryletherketones
Maureen Reitman Sc.D., David Jaekel, Ryan Siskey, and Steven M. Kurtz Ph.D.
4.1 Introduction
conditions near the glass transition temperature, is
also presented.
Finally, this chapter also includes a brief review of
the connection between composition, morphology,
and basic physical properties such as stiffness,
strength, elongation, and fatigue behavior of neat
resins as well as PEEK composites.
Polyaryletherketones (PAEKs) are linear aromatic
polymers exhibiting desirable physical properties and
durability against environmental challenge. These
properties are the direct result of the chemistry and
architecture of the polymer backbone, the manner
and extent to which the molecules organize when the
polymer is solidified, and any morphological changes
that occur during subsequent thermal exposure. In the
sections that follow, commercial PAEKs are
described in terms of polymer backbone, crystal
structure, and basic morphological characteristics.
Because the literature is more fully developed for
polyetheretherketone (PEEK), the emphasis will be
on this material, with comparison with poly-
etherketone
4.2 Chain Architecture and
Packing
4.2.1 Polymer Backbone
PAEK polymers consist of aromatic rings joined by
ether or ketone linkages, the specific polymer
acronym (e.g., PEEK, PEK, PEKK, and poly-
etherketoneetherketoneketone) reflecting the number
and order of these linkages (
Fig. 4.1
). These highly
aromatic polymers are linear, with little or no
branching
[1]
. The linear, aromatic, ordered nature of
the PAEK polymers contributes to notable chemical,
radiation, and thermal resistance for these materials,
as well as desirable physical properties typically
associated with high-performance polymers.
Chain conformations are controlled by the steric
hindrance resulting from the aromatic groups and the
bond angles of the ether and ketone linkages. The
average inter-ring linkage angle has been reported as
124
[1]
, 125
[2]
, or 126.5
[3]
for PEEK and 123
[2]
or 124.6
[3]
for PEK. All are greater than the
expected bond angle of 120
[3]
. Although the chains
essentially adopt planar zigzag configurations in
crystals, the faces of the aromatic rings are slightly
twisted, with torsional angles of approximately 40
(PEK)
and
polyetherketoneketone
(PEKK) as appropriate.
This chapter includes a discussion of the semi-
crystalline nature of the PAEKs and the techniques
traditionally used to characterize their structures,
including density, X-ray diffraction, Fourier transform
infrared (FTIR) spectroscopy, differential scanning
calorimetry (DSC), and microscopy. Importantly, the
crystallization behavior of the PAEKs does not allow
for the degree of crystallinity at room temperature to
be obtained readily from DSC.
This chapter also contains a description of the
effects of thermal history, including annealing and
quenching, on structure and physical properties.
Typical nucleation and crystal growth is described for
both filled and unfilled systems, and generally
accepted models of crystallization kinetics are pre-
sented. A description of physical aging, which is
a relevant phenomenon for PAEKs encountering
