CRYSTALLIZATION KINETICS AND THERMAL BEHAVIOR OF
POLY(ε-CAPROLACTONE)/MULTI-WALLED CARBON NANOTUBE
COMPOSITES
Erh-Chiang Chen, Tzong-Ming Wu
Department of Materials Science and Engineering, National Chung Hsing University
250 Kuo Kuang Road, Taichung 402, Taiwan
E-mail: d9266001@mail.nchu.edu.tw
ABSTRACT
In this work, Poly(ε-caprolactone) (PCL)/multi-walled carbon nanotube (MWCNT) composites were prepared via the
ultrasonically mixing of PCL and as-prepared MWCNT in a tetrahydrofuran solution. From TEM images, the fabricated
PCL/MWCNT composites contain exfoliated characteristic with well dispersion of MWCNT into PCL matrix. Effect of
MWCNT on the crystallization kinetics, thermal behavior and crystalline structure of PCL and PCL/MWCNT composites
were conducted by the differential scanning calorimetry (DSC), thermogravimetric analyzer (TGA), X-ray diffraction (XRD)
and polarized optical microscopy (POM) analysis. DSC isothermal crystallization results have revealed that the activation
energy of PCL significantly decreases with increasing MWCNT contents, indicating that the loading of MWCNT into PCL
matrix probably induced the more heterogeneous nucleation during crystallization processes. From TGA results, the
addition of small amount MWCNT into PCL matrix can enhance the thermal stability of PCL matrix. Isothermal degradation
data show that the activation energy Ed of composites is smaller than that of PCL. This phenomenon is probably due to the
incorporation of more MWCNT loading to PCL caused a decrease in the degradation rate and increased in the residual
weight for PCL/MWCNT composites.
Introduction
Multi-walled carbon nanotubes (MWCNT) first reported by Iijima in 1991 [1] contain unique mechanical properties, electrical
and thermal conductivity, and chemical stability [2]. Due to the excellent properties of MWCNTs, many investigators have
focused on exploiting these extraordinary characteristics for engineering applications, such as polymer composites,
chemical sensors, hydrogen storage, and Nanoelectronic devices [3-4]. For polymer/MWCNT composites, the addition of
MWCNT could significantly change the electronic, mechanical, and thermal properties of polymer matrix, which would
notably extend the application regions of polymer/MWCNT composites. Recently, some researches have been
demonstrated that MWCNT can be served as scaffolds for biotechnology applications [5], even though the MWCNTs are
not biodegradable.
Poly(ε-caprolactone) (PCL) is an aliphatic polyester with biodegradable and semi-crystalline characteristic. PCL can be
degraded by hydrolysis of its ester linkages in physiological conditions and has therefore received a wide range of possible
biomedical applications, including implantable biomaterial, microparticles for drug delivery and biodegradable packaging
materials. In particular, it is especially interesting for the preparation of long term implantable devices due to their lower
degradation as compared to that of polylactide [6-8]. Recently, Zeng et al. [9] using functionallized MWCNT as a reinforced
filler to prepared PCL/MWCNT composites with core/shell structures. They have been demonstrated that the possible
applications of these composites in bionanomaterials, biomedicine, and artificial bones. However, the techanical
development of PCL/MWCNT composites in these applications are limited by its high crystallinity and short useful lifespan.
However, there is little information about the crystallization behavior and thermal degradation of PCL/MWCNT composites.
In this study, the PCL/MWCNT composites have been prepared by mixing the PCL and as-prepared MWCNT in
tetrahydrofuran (THF) solution. The MWCNT with a lot of Van der Waals forces among themselves is very difficult to be
dispersed in a polymer matrix. Therefore the MWCNT was first added in tetrahydrofuran (THF) solution under ultrasonic
treatment for 24 h in order to well distribute the MWCNT in solution. The PCL/MWCNT composites were then prepared by
mechanically mixing the MWCNT and PCL in THF solution with magnetic stirring for 24 h. Effect of MWCNT on the
crystallization kinetics, thermal behavior and crystalline structure of PCL and PCL/MWCNT composites were conducted by
the diamond differential scanning calorimetry (DSC), thermogravimetric analyzer (TGA), X-ray diffraction (XRD) and
polarized optical microscopy (POM) analysis. The parameters of the isothermal crystallization kinetics, such as the Avrami
exponent (n) and crystallization rate constant (k), as well as the activation energy were studied. On the other hand, the
isothermal degradation of PCL/MWCNT composites is also discussed in this study.
Experimental
Specimens
Poly(ε-caprolactone) (PCL) pellets with Mn = 42,500 used in this study were obtained from the Aldrich chemical Company.
The multi-walled carbon nanotubes (MWCNT) were fabricated by ethylene chemical vapor deposition process with the
diameters in the range of 20 ~ 40 nm. In order to randomly distribute the MWCNT in polymer matrix, the MWCNT was
added in tetrahydrofuran (THF) solution under ultrasonic treatment for 24 h. The PCL/MWCNT composites were then
prepared by mechanically mixing the MWCNT and PCL in THF solution with magnetic stirring for 24 h. The solution were
then cast on the glass and dried in a vacuum oven at 40 oC for 24 h.
Thermal Analysis
The thermal analysis of PCL/MWCNT composites was preformed using a Perkin-Elmer PYRIS Diamond differential
scanning calorimetry (DSC). The temperature scale of the DSC was calibrated using the melting point of high purity indium
metal. All specimens were weighted in the range of 5 to 6 mg and were performed under a nitrogen atmosphere in each
experiment. The isothermal crystallization and melting procedures of PCL and PCL/MWCNT composites were obtained by
the follows: these specimens were heated to 90 °C at a rate of 100 °C/min and held for 10 min to eliminate any previous
o
thermal history; then they were quickly cooled to the proposed crystallization temperatures (Tcs) in the range of 44-50 C in
o
steps of 2 C, and maintained at Tc for various times. After the isothermal crystallization process, the exothermal curves of
heat flow as a function of time were recorded.
The thermal stability and isothermal degradation kinetics of PCL and PCL/MWCNT composites were performed on a Perkin
Elmer Thermogravimetric/Differential Thermal Analyzer (TG/DTA). All experiments are weighted about 8 mg and carried out
under a nitrogen atmosphere at a purge rate of 100 ml/min. For the thermal stability analysis, the specimens were heated
from 30 oC to 600 oC at a rate of 10 oC/min. For isothermal degradation, each sample was heated from 30 oC to 100 oC for 3
o
min to remove the residual water and then was used a heating rate of 10 C/min to reach the predetermined degradation
o
temperatures (Tds) at 320, 340, 350, 360, and 380 C for 40 min.
Structural Analysis
The PCL spherulite was observed using a polarized optical microscopy (POM) equipped with a Mettler FP82HT hot stage
and a digital camera. Specimens were prepared by melting the PCL and its composites on a slide glass on a hot stage at 90
o
C for 15 min to remove the previous thermal history and then quickly transferred onto another hot stage controlled to the
proposed crystallization temperatures. The spherulitic morphology was monitored between crossed polarizers, and
recorded at an appropriate time interval by a camera recorder mounted on the microscope. Transmission electron
microscopy (TEM) observations were made with a JEOL JEM-1200 using an acceleration voltage of 120keV. The specimen
fixed on the carbon-coated copper grid with a thickness about 100 nm, which was prepared by an ultra-microtome equipped
a diamond knife.
Results and discussion
Figure 1 shows the TEM micrograph of 1wt% PCL/MWCNT composites. From this image, it can be seen that the high
aspect ratio of MWCNT is well dispersed in PCL matrix. This result indicates that MWCNT was successfully separated in
PCL/MWCNT composite via solution mixing method under ultrasonic treatment. As expected from the high aspect ratio,
experimentally introducing MWCNT into PCL matrix might improves the crystallization and thermal properties of the pure
PCL. Therefore the crystallization kinetics of PCL and PCL/MWCNT composites under isothermal conditions can be
analyzed using the well known Avrami equation [10-11] as given in Equation (1):
Figure 1. TEM micrograph of 1 wt% PCL/MWCNT composites
1 − X t = exp[ − k (t ) n ]
(1)
-1
where the Xt is the relative crystallinity at different crystallization time, k is the crystallization rate constant (min ) related to
nucleation and growth parameters, and n is the Avrami exponent constant depending on the nucleation and growth
mechanism, t is the crystallization time, and the t1/2 can be defined as the time at which Xt is 50%. The equation (1) can be
rewritten as follows:
ln[ − ln( 1 − X t )] = n ln t + ln k
(2)
Figure 2 shows the plots of ln [-ln(1-Xt)] versus ln t for 0.25 wt% PCL/MWCNT composites at different crystallization
temperatures. The Avrami exponent (n) and crystallization rate constant (k) were obtained from the slope and intercept of
the straight line, respectively. Similar results are also observed for PCL and 0.5 wt% PCL/MWCNT composites and 1 wt%
PCL/MWCNT composites. Several crystallization parameters such as the t1/2, n, and k for PCL and PCL/MWCNT
composites are listed in Table 1. In general, t1/2 can be used to characterize the crystallization rate. The longer the t1/2, the
slower the crystallization rate. The values of t1/2 of the PCL/MWCNT composites are much smaller than that of PCL. These
results show that the isothermal crystallization rates of the PCL/MWCNT composites were faster than that of PCL at the
same Tc, indicating that the addition of small amount MWCNT can act as effective nucleating agents and accelerate the
crystallization rate of PCL. The values of k decreased with increasing Tc in all conditions. In addition, it is of great interest to
compare the Avrami exponent n of PCL/MWCNT composites with that of the pure PCL. The Avrami exponent n for the pure
PCL samples corresponding to the primary crystallization stage are close to 3 (range around 2.6~2.9), which imply that the
PCL polymer chains tend to take the three-dimensional crystal growth with heterogeneous athermal nucleation. However,
with the addition of the MWCNT in PCL, the mechanism of nucleation and the growth of PCL crystallite are slightly
influenced, leading to the slightly decrease of the Avrami exponent n in the PCL/MWCNT composites. The n values of
PCL/MWCNT composites are in the range of 2.4 to 2.6, which are explained by simultaneous occurrence of two- and
three-dimensional crystal growth. Therefore, these results illustrate that the incorporation of MWCNT into PCL might
causes heterogeneous nucleation induced by a change in the crystal growth process from three-dimensional crystal growth
to mixed two-dimensional and three-dimensional crystal growth [12-13].
Figure 2.
Avrami plots of ln[-ln(1-Xt)] versus ln t for 0.25 wt% PCL/MWCNT composites.
The isothermal crystallization for PCL is assumed to be thermal activated, thus the Avrami parameter k can be used to
determine the energy for crystallization. The crystallization rate parameter k can be approximately described by the
Arrhenius equation as follows:
ΔE a
1
(ln k ) = ln k 0 −
n
RT c
(3)
where the k0 is a temperature independent pre-exponential factor; ΔEa is an activation energy, and R is the universal gas
constant, and Tc is the crystallization temperature. Therefore ΔEa can be determined from the slope by plotting the
experimental data of 1/n (ln k) versus 1/Tc using curve fitting method. The values of ΔEa also listed in Table 1 are 338.8,
312.6, 301.8 and 293.5 kJ/mol for the PCL, 0.25 wt%, 0.5wt% and 1 wt% PCL/MWCNT composites, respectively. The
values of ΔEa were slightly decreases with the presence of MWCNT in PCL/MWCNT composites, and then continuously
decrease as MWCNT content increases. This result indicates that the addition of MWCNT into PCL probably induces the
heterogeneous nucleation (a lower ΔEa). The activation energy decreases with increasing MWCNT content, indicating that
the more MWCNT content in PCL matrix probably increase the more heterogeneous nucleation during crystallization
processes.
Table 1. Values of t1/2, n, k, and the activation energy ΔEa at various Tc for PCL and PCL/MWCNT composites.
PCL
0.25 wt%
PCL/MWCNT
0.5 wt%
PCL/MWCNT
1.0 wt%
PCL/MWCNT
o
Tc ( C)
44
46
48
50
t1/2 (min)
10.13
17.97
46.74
125.23
n
2.72
2.76
2.86
2.58
k
1.30E-03
2.30E-04
1.20E-05
4.60E-06
t1/2 (min)
2.49
4.74
10.07
22.72
n
2.52
2.63
2.55
2.51
k
7.00E-02
1.20E-02
2.00E-03
3.10E-04
t1/2 (min)
1.45
2.73
6.09
13.26
n
2.50
2.54
2.52
2.44
k
2.90E-01
5.90E-02
7.30E-03
2.10E-03
t1/2 (min)
1.39
2.54
5.33
10.24
n
2.44
2.49
2.42
2.39
k
3.30E-01
7.30E-02
1.30E-02
4.70E-03
ΔEa (kJ/mol)
338.8
312.6
301.8
293.5
POM micrographs of PCL and PCL/MWCNT composites after melting at 90 oC and then quenching to Tc are shown in the
Figure 3. The POM image of PCL exhibits a typical Maltese-Cross spherulite. By the addition of MWCNT to PCL, it can be
observed that the amount of spherulites of 1 wt% PCL/MWCNT composites is much higher than that of PCL. Base on these
observed POM images, the nucleation rate of 1 wt% PCL/MWCNT composites is higher than that of PCL, which is
consistent with the DSC data. These results indicated that well dispersion of MWCNT can effectively affect the
crystallization development and change the crystallization kinetics of PCL.
(a)
(b)
(c)
(d)
Figure 3. POM micrographs of (a) PCL and (b) 0.25 wt% PCL/MWCNT, (c) 0.5 wt% PCL/MWCNT, and (d) 1 wt%
o
PCL/MWCNT composites isothermally crystallized at 44 C.
In order to know the effect of MWCNT on the thermal behaviors of PCL matrix, DTA and TGA analysis were used to study
the thermal degradation of PCL and PCL/MWCNT composites. Figure 4 shows that DTA and TGA curve of PCL and
PCL/MWCNT composites at a heating rate of 10 oC/min. From the DTA data, the melting temperature (Tm) of PCL is about
o
66.5 C. By addition of small amount MWCNT into PCL, the Tm was slightly shifted to high temperature and the sharp of the
melting peak become broader. From the TGA data, the thermal stability of PCL shows similar tendency with that of
PCL/MWCNT composites. The onset temperature of degradation (Tonset) can be determined from the TGA curves by
o
extrapolating from the curve at the peak of degradation back to the initial weight of the polymer. The Tonset of PCL is 142 C
and increases as MWCNT content increases. From these experimental results, it can be seen that the presence of MWCNT
in PCL induced the better thermal stability which the degradation starting temperature shifted to higher temperatures.
DTA
TGA
Figure 4. The thermal behaviors of DTA curves and TGA curves of (a) pure PCL, (b) 0.25 wt% PCL/MWCNT, (c) 0.5 wt%
PCL/MWCNT and (d) 1.0wt% PCL/MWCNT composites.
The isothermal degradation behaviors of PCL and PCL/MWCNT composites were also investigated at predetermined
temperatures in a nitrogen flow environment. Figure 5 show the weight loss profiles of 0.25 wt% PCL/MWCNT composites
during isothermal heating in TGA at various temperatures. The residue weight decreased as the heating time increases and
the higher isothermal temperature was believed to cause the larger weight loss. It is clear that the residual weight of 1wt%
PCL/MWCNT composites is larger than that of PCL. This result shows that the addition of 1 wt% MWCNT can improve the
thermal stability of PCL.
Figure 5. Weight loss profiles under isothermal time of 0.25 wt% PCL/MWCNT composites at different isothermal
temperatures.
The degradation kinetics of PCL and PCL/MWCNT composites under isothermal conditions can be analyzed using the well
known method of Freeman and Carroll [14] to determine the order and activation energy of the degradation. The general
form of degradation is
− dW
= kdW n
dt
(4)
where kd is the degradation rate constant, W is the weight remaining and n is the order of the reaction. If the degradation
follows first order decomposition, then the equation (4) may be written as follows:
lnW0 − lnW = kd ⋅ t
(5)
where the W0 is initial weight. Plots of ln W versus t for 0.25 wt% PCL/MWCNT composites are shown in Figure 6. It is
o
found that a straight line is obtained for the isothermal degradation at 320 C which means that the degradation at this
temperature can be regarded as a first order decomposition with a steady rate constant. As the degradation temperature
o
continuously increases to 360 C, the plots of ln W versus t are close to linear. However, all the TGA curves for the
isothermal degradation at 380 oC have a concave shape with the reaction rate becoming smaller and smaller suggesting
that either the order of reaction is not one, or the rate constant is not a constant, or both.
Figure 6. The plot of logarithm of weight loss profiles versus isothermal time of 0.25 wt% PCL/MWCNT composites at
different isothermal temperatures.
Using Freeman and Carroll’s model, the kinetic parameters of the degradation were calculated assuming a first order
decomposition. The degradation rate constant, which is assumed to obey the Arrhenius equation similar to equation (3) as
follows:
ln k d = ln A −
Ed
RT
(6)
where A is the pre-exponential factor, Ed is the activation energy. From the Arrhenius plots of the ln (kd) versus 1/T for PCL
and PCL/MWCNT composites. The activation energies calculated using linear fitting of the data points are 222.8, 222.3,
218.4 and 213.9 kJ/mole for PCL, 0.25wt%, 0.5wt% and 1wt% PCL/MWCNT composites, respectively. The Ed value of
0.25wt% PCL/MWCNT composites is close to that of PCL. This result shows that the 0.25 MWCNT loading into PCL matrix
does not significantly changed the degradation behavior of PCL. But the addition of more MWCNT content into PCL, the Ed
value decreased. This is probably due to the incorporation of more MWCNT loading to PCL caused a decrease in the
degradation rate and increased in the residual weight for PCL/MWCNT composites. Because the Ed values of PCL and
PCL/MWCNT composites did not drastically changed, indicating that the addition of small amount MWCNT into PCL matrix
does not change the degradation mechanism of PCL [15].
Conclusions
From TEM images, the exfoliated PCL/MWCNT composites have successfully prepared using solution mixing under
ultrasonic treatment. DSC isothermal crystallization results have revealed that the activation energy of PCL significantly
decreases with increasing MWCNT contents, indicating that the loading of MWCNT into PCL matrix probably induced the
more heterogeneous nucleation during crystallization processes. From TGA results, the addition of small amount MWCNT
into PCL matrix can enhance the thermal stability of PCL matrix. Isothermal degradation data show that the activation
energy Ed of composites is smaller than that of PCL. This phenomenon is probably due to the incorporation of more
MWCNT loading to PCL caused a decrease in the degradation rate and increased in the residual weight for PCL/MWCNT
composites.
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