Biomedical Engineering Reference
In-Depth Information
4.4.4 Differential Scanning
Calorimetry
DSC provides a relative measure of heat flow,
which provides insight into molecular motion. DSC
is readily used to determine the heat capacity of
a polymer, the glass transition temperature, and the
melting point. Other information related to melting
and crystallization behavior can also be obtained. By
using two heating cycles, the effects of prior thermal
history generally can be erased, providing data
representative of the fundamental polymer behavior.
Data from DSC will reflect the effect of heating rate,
as well as any thermal processes occurring during
the test.
In PAEK polymers, DSC is used to determine
glass transition temperature and to assess melting and
recrystallization behavior. The data are generally not
used to determine degree of crystallinity under use
conditions because reordering and recrystallization
during the test negate the relevance of the calculation
from measured enthalpy. However, data from second
heating cycles are used to characterize the material.
For example, ASTM F2026,
Standard Specification
for PEEK Materials for Surgical Implant Applica-
tions
, includes a table of required thermal properties
for PEEK. These are listed in
Table 4.1
.
Heat of fusion for PEEK has been reported as
130 J/g, with a maximum rate of crystallization
Figure 4.6
DSC second heat thermogram for the three
PAEK polymers evaluated by WAXS and FTIR. Glass
transition and melting temperatures are indicated.
Characteristic values increase for PEKK compared
with PEEK and for PEK compared with both. Image
courtesy of Shujun Chen, Ph.D., and Tao Xu, Ph.D.,
Exponent, Inc.
occurring around 230
C
[4]
. The change in heat
capacity between liquid and glass phases of amor-
phous PEEK is reported as 0.28 J/g
[7]
, whereas the
thermodynamic melting point has been calculated as
395
C
[5]
.
The three samples evaluated by WAXS and FTIR
were also subjected to DSC. Second heat thermo-
grams are presented in
Fig. 4.6
. These illustrate the
effects of the greater polarity and barrier to rotation
associated with the ketone linkage compared with the
ether linkage and the effect of greater backbone
regularity on intermolecular interaction. Thus, PEKK
has a slightly higher
T
g
and
T
m
compared with PEEK
(162
C and 363
C compared with 151
Cand
340
C), and PEK has a slightly higher
T
g
(163
C)
and
T
m
(372
C) than both PEEK and PEKK. Similar
effects have been noted in model studies of PAEK
polymers
[1]
and a direct comparison of PEEK and
PEK
[2]
.
Table 4.1
Required Properties of Virgin PEEK
Resin per ASTM F2026
Parameter Method
Requirement
T
g
,
C
DSC, 20
C/min,
sealed sample,
taken on second
reheat
125e165
T
m
,
C
DSC, 20
C/min,
sealed sample,
taken as maximum
point on reheat
exotherm
320e360
4.4.5 Microscopy
Depending on the desired magnification, optical
and electron microscopy may be useful to examine
PAEK polymers. Both techniques can be used in
reflection or transmission to provide insight into
polymer structure. Etched surfaces can be examined
with SEM and transmission electron microscopy
(TEM) to reveal lamellae, and metallic shadowing can
be used to enhance contrast. To eliminate the effects of
electron beam damage, replicas are often used.
T
c
,
C
DSC, 20
C/min,
sealed sample,
taken as maximum
point on cooling
endotherm
260e320
