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Macromolecular Research, Vol. 15, No. 7, pp 662-670 (2007)
*Corresponding Author. E-mail: imss007@hanyang.ac.kr
Effect of A-Zeolite on the Crystallization Behavior of In-situ Polymerized
Poly(ethylene terephthalate) (PET) Nanocomposites
Young Hak Shin, Wan Duk Lee, and Seung Soon Im*
Department of Fiber & Polymer Engineering, College of Engineering, Hanyang University, Seoul 133-791, Korea
Received August 6, 2007; Revised August 29, 2007
Abstract: The crystallization behavior and fine structure of poly(ethylene terephthalate) (PET)/A-zeolite nanocom-
posites were assessed via differential scanning calorimetry (DSC) and time-resolved small-angle X-ray scattering
(TR-SAXS). The Avrami exponent increased from 3.5 to approximately 4.5 with increasing A-zeolite contents,
thereby indicating a change in crystal growth formation. The rate constant, k, evidenced an increasing trend with
increases in A-zeolite contents. The SAXS data revealed morphological changes occurring during isothermal crys-
tallization. As the zeolite content increased, the long period and amorphous region size also increased. It has been
suggested that, since PET molecules passed through the zeolite pores, some of them are rejected into the amorphous
region, thereby resulting in increased amorphous region size and increased long period, respectively. In addition, as
PET chains piercing into A-zeolite pores cannot precipitate perfect crystal folding, imperfect crystals begin to melt at
an earlier temperature, as was revealed by the SAXS profiles obtained during heating. However, the spherulite size was
reduced with increasing nanofiller content, because impingement between adjacent spherulites in the nanocompos-
ite occurs earlier than that of homo PET, due to the increase in nucleating sites.
Keywords: PET, A-zeolite, nanocomposite, crystallization behavior, fine structure.
Introduction
Poly(ethylene terephthalate) (PET) is a thermoplastic
polyester widely used in the manufacture of high-strength
fibers, photographic films, and soft-drink bottles owing to
its good mechanical properties with low cost.1 However,
PET has some drawbacks on the processability due to the
relatively slow crystallization rate. Accordingly, studies on
the PET nanocomposites filled with inorganic materials
have been investigated for the enhancement of their pro-
cessability and properties.2- 6
Zeolites are microporous crystalline aluminosilicates and
composed TO4 tetrahedra (T = Si, Al) with oxygen atoms
connecting neighboring tetrahedra.7 Zeolites have potential
applications such as catalysts, adsorption agents, and ion-
exchange resins on account of their regular pore structures
and high thermal stability. They were also employed as an
additive to impart antibacterial and flame retardant proper-
ties to polymers. LTA type A-zeolite having Si/Al ratio of 1,
sodium ions as a counter ion, a pore size larger than 4 Å,
and primary particle size of 10-30 nm were used as nano-
composite fillers in this study. Their general chemical for-
mula is Na96(Si96Al96O384) possessing SiO4
-, AlO4
- as basic
units. Recently, these zeolites were used to polymer com-
posite system as fillers, which play a role of reinforcement
filler and nucleating agent.8
In the polymer/zeolite nanocomposites, there are two con-
troversial reports about effects of zeolite on the composite
properties. One report9 is that the incorporated zeolites
decrease the mechanical properties of polymer due to poor
interfacial interactions between polymer and zeolite. The
other report is that zeolite bring on reinforcing effects on the
polymer, such as mechanical properties and crystallization
behavior, because polymer chains could pass through zeo-
lite pores in the nanocomposites. In particular, Frisch et
al.10-12 prepared interpenetrating polymer network (IPN) of
zeolite 13X and polystyrene or polyethylacrylate, which
prepared by using in-situ radical polymerization. In addi-
tion, Bein and Wu reported about preparation of molecular
wires and conducting polyaniline filaments from polymer
chains constrained into zeolite pores.13,14
In this paper, PET/A-zeolite nanocomposites were pre-
pared by using in-situ polymerization, which could be
applied as an effective method for improvement of zeolite
filler dispersion. And then the effects of A-zeolite loading
on isothermal crystallization kinetics, melting behavior,
and crystalline structure were investigated and discussed
briefly.
Effect of A-Zeolite on the Crystallization Behavior of In-situ Polymerized PET Nanocomposites
Macromol. Res., Vol. 15, No. 7, 2007 663
Experimental
Materials and Preparation of Nanocomposites. PET
nanocomposites containing A-zeolites (0-2.0 wt%) used in
this study were prepared by using in-situ polymerization.
In-situ polymerization method is advantageous because it
produces a homogeneous dispersion of A-zeolite particles
in the polymer matrix.15 Dimethyl terephthalate (DMT) and
Ethylene glycol (EG) (+ 99%) purchased from Sigma-Ald-
rich were used without further purification. A-zeolites were
kindly supplied from Korea Institute of Ceramic Eng. &
Tech. (KICET). PET/A-zeolite nanocomposites were pre-
pared by conventional two-step polycondensation. A-zeo-
lites and zinc-acetate catalyst were firstly dispersed in EG
solution by Ultra sonic vibration for 10 min before ester
interchange reaction. During this process, the EG monomer
and catalyst can effectively incorporate into A-zeolite pores.
EG slurry was then mixed with DMT, and temperature was
slowly increased to 210 oC with stirring. Finally, Ester
interchange reaction was carried out for 3 h with continu-
ously removing byproduct (methanol). Second polycon-
densation reaction was carried out at temperatures ranging
from 180 to 285 oC with typical antimony oxide catalyst
under vacuum below 0.1 torr for 2 h. The synthesized all of
the nanocomposite samples were dried in vacuum for 24 h
at 70 oC.
Characterizations of Nanocomposites.
Isothermal Crystallization: The isothermal crystalliza-
tions and melting behaviors of the PET/A-zeolite nanocom-
posites were measured using a Perkin-Elmer DSC 7
instrument in a nitrogen atmosphere. The nanocomposite
samples were heated to 300 oC, kept for 5 min to eliminate
thermal history, and then cooled rapidly (200 oC/min) to the
crystallization temperature within the range of 190-220 oC
and maintained for 30 min. During isothermal crystalliza-
tion, exothermic heat flow was recorded as a function of
time at a different crystallization temperature. After crystal-
lization, the fully crystallized samples were cooled down to
40 oC at a rate of 200 oC/min, and then second heating run
was carried out at a rate of 10oC/min.
Synchrotron X-Ray Scattering Measurements: Syn-
chrotron X-ray scattering experiments were carried out at
the 4C2 SAXS beam line of the Pohang Accelerater Labo-
ratory (Pohang, Korea). The X-ray wavelength (λ) used was
0.154 nm and the beam size at the focal point was less than
1mm
2, focused by platinum coated silicon premirror
through a double crystal monochromator. The scattering
intensity was detected with a two-dimensional CCD cam-
era. The scattering angle was calculated with a Bragg spac-
ing of 32.5 nm for a SEBS crystal at q= 0.1933 nm-1 as a
reference peak for SAXS. The samples were completely
melted at 300 oC to remove their any thermal history in the
melting chamber, and then they were quickly moved into
the crystallization chamber set to same isothermal crystalli-
zation temperatures mentioned above.
SAXS Data Analysis: All SAXS data were corrected for
the background, including a dark current and air scattering,
and were plotted as a Lorentz-corrected form, Iq2 against q,
where q is the scattering vector given by the following
form:
(1)
where λ is the wavelength and θ is the scattering angle. The
morphological parameters, such as the long period, average
lamellar thickness, and amorphous region size, were calcu-
lated from the one-dimensional correlation function G(r) by
assuming ideal two-phase model.
(2)
q = 4π
λ
------ s i n θ
Gr() = 1
2π
------
⎝⎠
⎛⎞
2
q2Iq()cos qr()qd
∫
Figure 1. DSC isothermograms of (a) homo PET and (b) nano-
composite filled with 2.0 wt% A-zeolite (AZ20).
Y. H. Shin et al.
664 Macromol. Res., Vol. 15, No. 7, 2007
where r is the correlation distance. The long period (L) was
determined from the first maximum of the correlation
curve. The average lamellar thickness (lc) was determined
from the x-axis values of an intersection point between the
tangent-line at G(r) = 0 and the tangent-line at the first min-
imum in the correlation curve. The size of amorphous
region (la) was obtained by L and lc (la=L-lc).
Results and Discussion
Avrami Theory Analysis. The isothermal crystallization
exotherms of the nanocomposite samples are shown in Fig-
ure 1. The relative crystallinity, Xt, was obtained from the
ratio of the area of the exotherm up to time t divided by the
total exotherm. The development of the relative crystallinity
with time of the nanocomposite samples is shown in Figure
2. All isotherms exhibited a sigmoidal dependence on time.
The crystallization rate of the nanocomposite samples
increased with increasing A-zeolite contents. It is indicated
that A-zeolite can be acted as a heterogeneous nucleation
agent in the PET matrix. In addition, the crystallization rate
of every sample showed a decreasing trend with increasing
the crystallization temperature.
The crystallization kinetics of the nanocomposite samples
were analyzed by using typical Avrami equation,16,17 which
can be expressed as the following form:
1-Xt= exp(- ktn)(3)
where k is the temperature-dependent rate constant, t is the
crystallization time, and n is the Avrami exponent, indicat-
ing the type of nucleation and dimension of crystal growth.
Taking logarithms, eq. (3) can be expressed as the follow-
ing form:
log[- ln(1 - Xt)] = nlogt+logk(4)
Here, n and k are obtained from the slope and intercept of
the linear plot of log(- ln(1 - Xt)) against log(t) in the pri-
mary crystallization portion, respectively. The plots of log
Figure 2. Relative crystallinity versus time of the nanocomposites; (a) homo PET, (b) AZ05, (c) AZ10, and (d) AZ20.
Effect of A-Zeolite on the Crystallization Behavior of In-situ Polymerized PET Nanocomposites
Macromol. Res., Vol. 15, No. 7, 2007 665
(- ln(1 - Xt)) against log(t) for the nanocomposite samples
are shown in Figure 3. The plots of all samples shifted
toward longer times with increasing the crystallization tem-
perature. It is indicated that the time to reach the maximum
crystallinity increased as the crystallization temperature
increases. The Avrami kinetic parameters are listed in Table
I. The n values for homo PET lies in the nearly 3.5. On the
other hand, the n values for the nanocomposite samples
have higher value than that of homo PET. In addition, when
the n values between nanocomposite samples were com-
pared with each other, the n value increased slightly with
increasing A-zeolite contents at the range of 4-5. It indicates
that crystal growth dimension of nanocomposites is more
complicate than that of homo PET. The rate constant, k,
shows a very temperature-sensitive decrease in all samples,
and k values increase with increasing A-zeolite contents.
The half-time of crystallization, t1/2, can be useful indica-
tor of the crystallization rate. Usually, the reciprocal of t1/2 is
used to describe the crystallization rate, G=(t1/2)-1. The
greater value of t1/2 means the slower crystallization rate.
The t1/2 for all samples is shown in Figure 4. The incorpora-
tion of A-zeolites decreased the t1/2 of the PET matrix, indi-
cating the nucleating effect of A-zeolites for PET. Further-
more, the value of t1/2 is less dependent on temperature with
increasing A-zeolite contents. Thus, A-zeolite acts as heter-
ogeneous nucleating agent. The half time of crystallization
is listed in Table I.
Crystallization Activation Energy. The crystallization
rate constant of Avrami kinetic parameter, k is assumed to
be thermally activated and can be used to determine crystal-
lization activation energy (ΔE). The k can be approximately
Figure 3. Avrami plots for (a) homo PET and (b) AZ20 nano-
composite crystallized at various temperatures.
Table I. The Avrami Parameters for the Various Crystalliza-
tion Temperatures
Sample nK (s )t (sec)
Homo-190 3.5 1.013 × 10 066.6
Homo-200 3.5 1.594 × 10 103.2
Homo-210 3.4 1.357 × 10 172.2
Homo-220 3.4 1.327 × 10 570.6
AZ05-190 3.9 2.373 × 10 025.2
AZ05-200 4.0 2.359 × 10 041.4
AZ05-210 4.2 1.400 × 10 067.8
AZ05-220 4.3 4.756 × 10 231.0
AZ10-190 4.0 7.561 × 10 017.4
AZ10-200 4.6 5.039 × 10 021.6
AZ10-210 4.8 1.382 × 10 040.2
AZ10-220 4.7 5.831 × 10 139.2
AZ20-190 4.3 7.242 × 10 014.4
AZ20-200 4.6 1.112 × 10 030.0
AZ20-210 4.6 2.893 × 10 040.2
AZ20-220 4.8 2.319 × 10 094.2
Figure 4. Half-time of crystallization versus crystallization tem-
perature.
Y. H. Shin et al.
666 Macromol. Res., Vol. 15, No. 7, 2007
described by the Arrhenius equation:18-20
(5)
Taking logarithms, eq. (5) can be expressed as the following
form:
(6)
where k0 is a pre-exponential constant, R is the gas constant,
ΔE is the crystallization activation energy, and Tc is the crys-
tallization temperature. Arrhenius plots of 1/n(lnk) against
1/Tc for all samples are shown in Figure 5 and are linearly
fitted. Here, ΔE can be obtained from the slope of the plots
(Table II). ΔE is strongly dependent on A-zeolite contents.
ΔE decreased with increasing A-zeolite contents, and drasti-
cally decreased over the 2 wt% of A-zeolite. It is indicated
that the incorporation of A-zeolite causes a heterogeneous
nucleation, which results in a lower ΔE.18-20 In other words,
the crystallization rate can be accelerated during the isother-
mal crystallization process owing to A-zeolite.
Melting and Equilibrium Melting Temperatures. Figure
6 represents the second heating run thermograms for the
completely crystallized samples. In the thermograms of all
nanocomposite samples, three melting peaks can be observed.
The lowest small melting peak appears at about 10-15 oC
above the crystallization temperature,21,22 which is called
annealing peak. The middle melting peak is corresponded to
the melting of original crystal lamellae grown during iso-
thermal crystallization, and the highest melting peak is cor-
responded to the melting of crystallites produced by melt-
recrystallization. Accordingly, in this work, the equilibrium
melting temperature was obtained by using the middle melt-
ing temperature (Tm).
The equilibrium melting temperature, Tm
o is the melting
temperature of infinitely stack crystals. It can be obtained as
a theory derived by Hoffman-Weeks.23
(7)
kt
1n⁄ = k0exp - ΔE
RTc
-----------
⎝⎠
⎛⎞
1
n
---
⎝⎠
⎛⎞
lnkt = lnk0 - ΔE
RTc
--------
Tm = Tc
γ
----- + 1 - 1
γ
---
⎝⎠
⎛⎞
Tm
o
Figure 5. Plots of (1/n)lnK versus 1/T for Avrami parameter K
deduced from isothermal crystallization.
Table II. Crystallization Activation Energy of the Nano-
composites
Sample Activation Energy (J/mol)
Homo -173
AZ05 -158
AZ10 -126
AZ20 -111
Figure 6. DSC second heating runs for the fully crystallized (a)
homo PET and (b) AZ20 nanocomposite at various crystalliza-
tion temperatures.
Effect of A-Zeolite on the Crystallization Behavior of In-situ Polymerized PET Nanocomposites
Macromol. Res., Vol. 15, No. 7, 2007 667
where Tm and Tm
o are the experimental melting temperature
and equilibrium melting temperature, respectively. Tc is the
crystallization temperature and γ is the lamellar thickening
ratio, which is supposed to always be greater than or equal
to 1.24,25
The fitting of Tm against Tc is approximately linear, which
is shown in Figure 7. The Tm
o is evaluated by the x-axis
value of an intersection of linear line obtained from experi-
mental data and Tm=Tc linear line. The Tm
o decreased with
increasing A-zeolite contents. It is suggested that the crystal
perfection of the nanocomposite fall off with increasing A-
zeolite contents.26 This result will be also confirmed from
SAXS results and will be discussed in detail at following
SAXS sections.
Secondary Nucleation Theory Analysis. The crystal
growth rates of the spherulite may be derived from the Lau-
ritzen-Hoffman secondary nucleation theory.24,27 In this
study, the reciprocal of half-crystallization time of isother-
mal crystallization in DSC measurement was used as the
crystal growth rate (G). The temperature dependence of G is
given as follows:
(8)
where G0 is the front factor, U* is the activation energy for
the segment diffusion to the site of crystallization, R is the
G = G0exp - U*
RT
c - T∞
()
------------------------
⎝⎠
⎛⎞
exp - Kg
TcΔT()f
------------------
⎝⎠
⎛⎞
Figure 7. Hoffman-Weeks plots of the nanocomposites.
Figure 8. Lauritzen-Hoffman plots of the nanocomposites.
Table III. The Kinetic Data for Crystallized Nanocomposites
Sample K (K )σ (J m )
Homo 2.75 × 10 10.12 × 10
AZ05 2.53 × 10 09.36 × 10
AZ10 1.86 × 10 06.99 × 10
AZ20 1.51 × 10 05.69 × 10
Figure 9. Change of morphological parameters during isother-
mal crystallization at (a) 190 C and (b) 220 C.
Y. H. Shin et al.
668 Macromol. Res., Vol. 15, No. 7, 2007
gas constant, T∞ is the hypothetical temperature below
which all viscous flow ceases, Kg is the nucleation parame-
ter, ΔT is the degree of supercooling defined as Tm
o-Tc and f
is a correction factor given as 2Tc/(Tm
o+T
c). Generally, tak-
ing logarithms, eq. (9) can be expressed as the following
form:
(9)
In this study, we used the universal values of U*= 6300
Jmol-1 and T∞=Tg- 30 K in all calculations.27 With this
assumption, Kg value is obtained from the slope of Plots of
lnG+U
*/R(Tc-T∞) against 1/Tc(ΔT)f as shown in Figure 8,
and can be expressed as the following form:24,28
(10)
where n is a constant equals to 2 for regime II, b0 is the mono-
molecular layer thickness, taken as the unit cell dimension of
b, σ is the lateral surface energy, σe is the fold surface free
energy, Δhf is the heat of fusion per unit volume (182.6 Jcm-3
= heat of fusion per unit mass (125.5 J/g15) * crystal density
(1.455 gcm-3))39 and κB is the Boltzmann constant.
On the other hand, the lateral surface energy, σ was often
estimated as:28
σ= α(Δhf)(a0b0)1/2 (11)
where α was derived empirically to be 0.11 by analogy with
the well-known behavior of hydrocarbons. The unit cell
dimensions a0 and b0 for PET used in this analysis are 4.57
and 5.95 Å, respectively.29 From eqs. (10) and (11), the fold
surface free energy, σe can be calculated and was shown in
Table III. The σe decreases with increasing A-zeolite con-
tents. It is indicated that the work required in folding the
molecules decreases with increasing A-zeolite contents.30
Consequently, it is concluded that the overall crystallization
rate was increased by the incorporation of A-zeolite in the
PET matrix.30
SAXS Studies During Isothermal Crystallization and
Melting Process. The changes of morphological parame-
lnG + U*
RT
c - T∞
()
------------------------
⎝⎠
⎛⎞
= lnG0 - Kg
TcΔT()f
------------------
Kg = nboσσeTm
o
ΔhfκB
------------------------
Figure 10. Microstructural parameters during heating after isothermal recrystallization at 190 C; (a) invariants, (b) long period, (c) aver-
age lamellar thickness, and (d) average amorphous size.
Effect of A-Zeolite on the Crystallization Behavior of In-situ Polymerized PET Nanocomposites
Macromol. Res., Vol. 15, No. 7, 2007 669
ters obtained from linear correlation function curve for
nanocomposite samples crystallized at 190 and 220 oC are
shown in Figure 9. As crystallization proceeded, L and la
drastically decreased, especially in the initial crystallization
period, at the each crystallization temperature. This could
be caused by the insertion of subsidiary lamellae between
two existing dominant lamellae.31-34 As the zeolite filler
content increased, lc for all nanocomposites are almost
same, because the lamellar thickness depends not on the
nucleation but on the molecular diffusion at a controlled
temperature. However, L and la increased slightly as the
zeolite content increased during isothermal crystallization.
This result is opposite to that of PET nanocomposites filled
with nano-sized layered materials.38,39 In the case of nano-
composites filled with layered materials, nanocomposites
could be crystallized more easily owing to the increase of
nanolayers acted as nucleation site, leading to the decrease
of the average amorphous region size. In the case of PET/
zeolite nanocomposite, however, it is believed that PET
molecules passed through zeolite pores. Therefore, PET
chains piercing zeolite couldn’t make appropriate folding
and some of them are rejected into amorphous region, lead-
ing to the increase of la and L, respectively.35
Figure 10 shows the change of morphological parameters
during heating after the isothermal crystallization at 190 oC
for 20 min. As shown in Figure 10(a) and 10(c), invariants
(overall scattering power) and lc for all nanocomposites
increased with increasing temperature owing to the crystal
thickening. And, invariants decreased abruptly after melting
over 275 oC. L and la also increased with temperature, indi-
cating the thermal expansion resulting from the increased
molecular mobility during heating, as shown in Figure 10(b)
and 10(d), respectively. In these figures, interestingly, the
temperature of the rapid increase in L and la, which resulted
from the onset of the crystal melting, was shifted to lower
temperature with increasing zeolite content. This result is
also opposite to that of nanocomposites filled with layered
materials.38 Based on the decrease of equilibrium melting
temperature with zeolite content as shown in Figure 7, it is
thought that PET molecules threading into zeolite pores
couldn’t make perfect crystal36,37 during isothermal crystal-
lization, leading to the decrease of those onset temperature
of crystal melting. Schematic diagram of lamellar structures
and spherulite for the isothermally crystallized PET and
PET/A-zeolite nanocomposites are presented in Figure 11.
It is believed that since PET molecules threading into A-
zeolite pores cannot make perfect crystal folding, imperfect
crystal start to melt at earlier temperature. In addition, L and
la increased with increasing zeolite content because some of
them are rejected into amorphous region during crystalliza-
tion, while lc was almost same. Accordingly, based on the
above results, the changes of morphological parameters for
nanocomposite samples were resulted from the molecular
threading. However, spherulite size decreased with increas-
ing nanofiller content, because impingement between adja-
cent spherulites in the nanocomposite occurs earlier than
that of homo PET owing to the increased nucleating site.
Conclusions
The PET/A-zeolite nanocomposites were prepared suc-
cessfully by in-situ polymerization. DSC analysis suggested
that crystallization rates of the nanocomposite samples were
increased because A-zeolites acted as nucleating agents.
The evolution of crystallization kinetics during isothermal
crystallization and subsequent melting was followed by
time-resolved SAXS and DSC measurements. It indicated
that the crystallization growth rates of the nanocomposite
samples increased with increasing A-zeolite contents. How-
ever, the crystals of nanocomposite formed less perfect
crystals owing to molecular threading of PET chains into A-
zeolite pores. It is confirmed from decreased equilibrium
melting temperatures and SAXS analysis. Since PET mole-
cules threading into A-zeolite pores couldn’t make perfect
crystal folding, and some of them were rejected into amor-
phous region, amorphous region size and long period were
increased.
Acknowledgements. This work was supported by the
Korea Foundation for International Cooperation of Science
& Technology (KICOS) through a grant provided by the
Korean Ministry of Science & Technology (MOST) in No.
Figure 11. Schematic diagrams of the nanocomposite crystal.
Y. H. Shin et al.
670 Macromol. Res., Vol. 15, No. 7, 2007
K20501000002-07-E0100-00210. This work was also par-
tially supported by Seoul R&D Program (10919).
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