Anisotropic linear thermal expansivity
of poly(ether-ether-ketone)
R.K. Goyal
a,*
, A.N. Tiwari b, U.P. Mulik a, Y.S. Negi
c,*
a
c
Centre for Materials for Electronics Technology (C-MET), Department of Information Technology,
Government of India Panchwati, Off Pashan Road, Pune 411 008, MS, India
b
Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology, Bombay, Powai,
Mumbai 400 076, MS, India
Polymer Science and Technology Laboratory, Department of Paper Technology, Indian Institute of Technology, Roorkee,
Saharanpur Campus, Saharanpur 247 001, UP, India
Abstract
This paper deals with anisotropic behavior of thermal expansivity (a) of poly(ether-ether-ketone) (PEEK) sample prepared by compression molding above the melting point of polymer. The a was determined using thermo mechanical analyzer (TMA) for the temperature interval 50–250 C in expansion mode. It is found that out-of-plane thermal expansivity
(az) is 3.8-fold of the in-plane thermal expansivity (axy) below the glass transition temperature (Tg) of PEEK while az is
decreased to 1.8-fold of the axy above the Tg. Moreover, the average az over the range studied is about 2.2-fold of the
axy. This anisotropic behavior may be attributed to the alignment of polymer spherulites and chains along in-plane direction due to compressive forces under hot compression molding.
Keywords: PEEK; Thermo mechanical analyzer; Anisotropic; Linear thermal expansivity
1. Introduction
High performance thermoplastic semi-crystalline
poly(ether-ether-ketone) (PEEK) is an engineering
material having excellent resistance to moisture,
chemicals, wear and exceptional high temperature
thermal stability. It possesses high melting point
of 335 C, high glass transition temperature of
143 C and high continuous service temperature of
250 C. It exhibits excellent resistance to radiations,
and a V-0 flammability rating down to 1.45 mm
thickness with low smoke and toxic gas emission.
Therefore, it is used as a matrix for the preparation
of high performance polymer composites using various reinforcing fillers [1–8].
Although the morphological, mechanical and
thermal properties of PEEK have been reported in
details [9–11], there have been few reports on linear
thermal expansivity (a) of PEEK [12,13]. The a is a
significant thermal property and depends on the
backbone chemistry, morphology, crystallinity and
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molecular orientation of the polymers [14]. Moreover, it depends on the processing history of the
polymers. From X-ray diffraction technique it has
been studied that unit cell of semi-crystalline polymers such as PEEK and thermoplastic polyimides
exhibit different a along different lattice parameters
[12,15]. However, there are no reports for anisotropic a so far for the compression molded polymers, despite the fact that both the in-plane and
out-of-plane a are relevant to microelectronics
devices and printed wiring boards where stresses
arise during temperature fluctuation due to the mismatch between the a of the polymer and surrounding materials. Thus resultant stresses may cause
thermal fatigue failure of the devices.
Therefore in present communication, the anisotropic a of compression molded PEEK is reported
for the first time over the temperature range from
50 C to its maximum continuous service temperature (250 C). The out-of-plane (z-axis) and in-plane
(x-axis and y-axis) a was determined using thermo
mechanical analyzer (TMA). TMA is a tool used
for monitoring a of a material under negligible load
as a function of temperature under a controlled
atmosphere.
2. Experimental
The commercial PEEK (GATONETM 5300PF)
with mean particle size of 25 lm as determined by
laser particle size analyzer was dried over night at
150 C. The dried PEEK powder was filled in a tool
steel die and powder was heated under pressure to a
maximum temperature of 350 C. The pressure of
15 MPa was applied for 10 min at 350 C and then,
cooled naturally to a temperature below the glass
transition temperature (Tg) of PEEK in a mold.
The diameter and thickness of the molded pellet
was 25 mm and 2 mm, respectively. The experimental density of the pellet determined by the
displacement method was 1.3036 g/cc [3]. For a
measurement a rectangular specimen of 18 mm in
length, 13 mm in width and 2 mm in thickness was
cut from the compression molded pellet and polished by using emery paper to get smooth and parallel surfaces.
3. Characterization
The morphology of PEEK powder and molded
PEEK sample was studied using scanning electron
microscope (Philips XL-30) with an accelerating
voltage of 10–15 KV. Both the samples were coated
with gold by using a gold sputter coater [Polaron SC
7610]. The Perkin–Elmer DMA 7 e was used in
thermo mechanical analyzer (TMA) mode to determine the a along the in-plane (ax and ay) and outof-plane (az) direction of the PEEK sample under
expansion mode. To determine the ax and ay a sample holder was used to avoid any bending and to
hold the sample in perfect alignment with the LVDT.
Since the a of polymers varies with temperature, an
average value of a over a specific temperature range
such as: 70–100 (<Tg), 200–230 (>Tg), and 50–
250 C was determined. The instrument measures
elongation change between the probe and stage as
a function of temperature of the sample. A 50 mN
force was applied to make the probe in good contact
with sample. Before measurement, the specimen was
annealed in vacuum oven at 260 C. The sample was
held under pressure for 5 min and heated to 250 C
at a heating rate of 5 C/min in argon atmosphere.
The sample was then cooled to 50 C and reheated
at 5 C/min to 250 C. Thus, the instrument was
run for first, second and third cycles. A significant
difference was observed between the values of the
first and second run, while negligible difference was
observed between the values of the second and third
run. Therefore in present study, results were
reported for the second run only. The Tg of PEEK
determined by inflection in the curve between dimension change and temperature, is 153 C.
4. Results and discussion
Fig. 1a shows micrograph of pure PEEK powder.
The PEEK powder has irregular particles of rod like
shape. Fig. 1b shows micrograph of compression
molded PEEK sample at a magnification of 500
times. It can be seen that there is no porosity in
the sample. Therefore, the effect of porosity on the
polymer a can be neglected, although, it is reported
that presence of porosity decreases the a of materials [16].
Fig. 2 shows the dimension change relative to original PEEK sample as a function of temperature
from 50 to 250 C for the out-of-plane (z-axis) and
in-plane (x-axis and y-axis) direction. The in-plane
a was determined along the length (x-axis) and
width (y-axis) of the sample, which were perpendicular to each other and in-plane. It can be seen that
there is not significant difference in both the in-plane
a. Therefore an average value of in-plane a(axy) was
determined by the equation
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Fig. 1. SEM micrographs of: (a) PEEK powder and (b) compacted PEEK after polishing.
axy ¼ ðax þ ay Þ=2
ð1Þ
The values of the a determined below and above Tg
of PEEK during the second run are summarized in
Table 1. It can be seen from Fig. 2 that there is not
significant change in the dimensions in the early
stage of run. This may be due to temperature lag between the furnace and the sample. Moreover, the
slope between the dimension change and temperature is changed with temperature. Therefore an
average value of a for the temperature interval 70–
100 C (<Tg), 200–230 C (>Tg) and 50–250 C
was summarized in Table 1.
It is interesting to note that the rate of change in
slope is higher along the out-of-plane direction than
that of in-plane direction. The value of az determined over the temperature range from 70 to
100 C is 3.8-fold to that of the average in-plane
axy. Fig. 2 shows that the glassy to rubbery transition region is more diffuse, which may be attributed
to the restriction of chain motions imposed by the
PEEK crystallites. There is a rapid dimension
change along both planes above the Tg of PEEK.
Table 1 shows that above Tg the az is 3-fold to the
value of az below Tg while the average in-plane axy
is about 6-fold to the value of axy below Tg. The
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2.5
along z-axis
along x-axis
Dimensional change (%)
2
along y-axis
1.5
1
0.5
0
-0.5
50
100
150
Temperature (ºC)
200
250
Fig. 2. Dimension change (%) versus temperature of PEEK sample: along z-axis i.e. out-of-plane, along x-axis and in-plane, and along
y-axis and in-plane.
Table 1
Linear thermal expansivity (·106/C) of compression molded
PEEK during second run
Temperature
range (C)
70–100 (<Tg)
200–230 (>Tg)
50–250
Out-of-plane
thermal
expansivity (az)
60
181
124
In-plane thermal
expansivity
ax
ay
axy
15
93
54
17
107
58
16
100
56
az/xy
3.8
1.8
2.2
Maximum error is ±5%.
az: a along out-of-plane (z-axis).
ax: a along x-axis and in-plane.
ay: a along y-axis and transverse to x-axis, and in-plane.
axy: Average of ax and ay.
az/xy: The ratio of the a along out-of-plane to the average a along
in-plane.
values of az determined below and above the Tg of
PEEK, agree well with the values reported in the literature [12]. Table 1 shows that the ratio of the az–
axy varies from 3.8 for the a determined below Tg to
1.8 for the a determined above Tg. The maximum
dimensional change in PEEK over the temperature
range from 50 to 250 C is 2.47% (124 · 106/C)
increase along out-of-plane direction and 1.12%
(56 · 106/C) increase along in-plane direction.
This is much lesser than the most commercially used
epoxy, which expands more than 5% in the same
temperature range.
The lower a below Tg is due to the fact that below
Tg the free volume is too low to move molecules or
segments. However, this is sufficient to permit bond
vibrations which results in lower a as compared to a
above Tg. In order to movement of molecules or
segments of molecules from place to place, there
should be free volume into which these molecules
or segments may move. Above the Tg, there is sufficient energy for molecular movement and the free
volume increases sharply with an increase in temperature. In addition there is torsional and bending
motions of the PEEK chains, which results in higher
a above the Tg.
The difference between the out-of-plane and inplane a can be demonstrated with the help of model
shown in Fig. 3a–d. The compression molded
PEEK pellet is shown in Fig. 3a. It is well known
that crystalline polymers are heterogeneous materials consisting of two distinct phases [17,18]. Being
semi-crystalline, PEEK crystallizes from molten
state to a molecular composite, in which spherulites
are embedded in the amorphous matrix as shown in
Fig. 3b. The spherulite is an aggregate of lamellae
(crystallites), which grow radially from the same
nucleus within the bulk polymer as shown in
Fig. 3c. In fact spherulites are three dimensional
but not perfect spheres because they are truncated
by impingement with other spherulites [19]. Moreover, its morphology is very sensitive to the melting
and crystallization temperature of the polymer. The
spherulites crystallized at 310 C for PEEK containing 5 wt% carbon fibers are not fully developed and
have size 0.5–5 lm, whereas those crystallized at
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Fig. 3. Model for molecular composite: (a) compression molded pellet, (b) rectangular specimen taken from the pellet center, (c) spherulite
and (d) lamellae; (where t = 2–6 nm, depending on crystallinity, w = 10–100 nm, depending on the polymer chain length, l = 2–5 lm,
depending on process history and nucleation density).
330 C are well developed and have size 4–10 lm
[20]. These spherulites were grown at atmospheric
pressure (0.1 MPa). However, in present case PEEK
has been processed at 350 C under compression of
15 MPa, which is 150 times higher than that of
atmospheric pressure. Hence, it can be safely
assumed that spherulites may be preferably oriented
and developed along the in-plane i.e. in the x–y
plane of the specimen due to the constraints on its
growth along out-of-plane (z-axis) direction. In
addition the linear thermal expansivity also depends
upon the aspect ratio of the lamellae [18]. As shown
in Fig. 3d, in lamellae polymer chains are folded
with a periodicity of about 10 nm, with thickness
of 2–6 nm depending on the polymer crystallinity
level. The polymer chains are held together by
inter-chain Van der Walls forces. The lamellae
thickness increases from 2 to 6 nm as the crystallinity [6,9] level increases from 20% to 40%. The crystallinity of the studied PEEK [1] is 20%. The width
(w) of the lamellae depends on the length of the
polymer molecules. The lamellae length (l) is about
half of the spherulite size (2 l).
Thus transverse and longitudinal aspect ratio of
the lamellae is very high varying from 10 to 100 or
more. This results in higher aspect ratio, which
may act as plate like reinforcing filler in PEEK
matrix. Thus these plates like lamellae constrain the
expansion of polymer along radial direction. In addi-
tion there may be partial alignment of amorphous
PEEK chain along in-plane direction. Moreover,
during heating effective shortening along the chain
axis caused by the torsional and bending motions
[13] of the chain compensates the increase in thermal
expansion along the in-plane, which results in lesser
in-plane a as compared to out-of-plane a. Therefore
this anisotropic morphology results in different values of the a along the in-plane and out-of-plane.
The anisotropic linear thermal expansivity gives
the opportunity to tailor a by selecting the filler orientation to fulfill the demand of manufacturer for
electronic substrates where substrate’s in-plane a
must match with that of ceramic chip carrier and
the out-of-plane a with that of copper. The required
a can be tailored by incorporating appropriate
quantity and type of filler in polymer matrix. The
present study suggests an ample field for future
research to obtain a full understanding for the difference in out-of-plane a and in-plane a of the sample prepared under compression molding technique.
Acknowledgements
The authors are grateful to Dr. T.L. Prakash,
Executive Director of C-MET for his constant
encouragement and support. We would like to
thank to Dr. P.D. Trivedi, Polymer Division, M/s
Gharda Chemicals Ltd. India for providing PEEK.
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