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Journal of Polymer Research
ISSN 1022-9760
Volume 20
Number 2
J Polym Res (2013) 20:1-11
DOI 10.1007/s10965-012-0070-8
Dynamic crystallization and melting
behavior of β-nucleated isotactic
polypropylene polymerized with different
Ziegler-Natta catalysts
Jian Kang, Jinggang Gai, Jingping Li,
Shaohua Chen, Hongmei Peng, Bin
Wang, Ya Cao, Huilin Li, Jinyao Chen,
Feng Yang & Ming Xiang
1 23
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ORIGINAL PAPER
Dynamic crystallization and melting behavior of β-nucleated
isotactic polypropylene polymerized with different
Ziegler-Natta catalysts
Jian Kang &Jinggang Gai &Jingping Li &Shaohua Chen &
Hongmei Peng &Bin Wang &Ya Cao &Huilin Li &
Jinyao Chen &Feng Yang &Ming Xiang
Received: 6 November 2012 / Accepted: 28 December 2012 / Published online: 8 January 2013
#Springer Science+Business Media Dordrecht 2013
Abstract Large amount of work has be published on the
dynamic crystallization and melting behavior of β-nucleated
polypropylene (β-PP). However, the relationship between
molecular structure and dynamic crystallization behavior of
β-PP is still not clear. In this study, the dynamic crystalli-
zation and melting behavior of two β-nucleated isotactic
polypropylene (β-iPP) with nearly same average isotacticity
but different stereo-defect distribution, were studied by dif-
ferential scanning calorimetry (DSC), wide angel X-ray
diffraction (WAXD) and temperature modulated DSC
(TMDSC). The results indicated that stereo-defect distribu-
tion of iPP can significantly influence the dependence of the
β-crystal content and thermal stability on the cooling rate.
NPP-A with less uniform stereo-defect distribution favors
the crystallization at higher temperature region and the
formation of β-crystal with high thermal stability in all
cooling rates concerned, moreover, the β-crystal content is
influenced by cooling rate; for NPP-B with more uniform
distribution of stereo-defect, the crystallization temperature
and the regular insertion of molecular chains can be reduced
in a larger extent. NPP-B is more suitable for the formation
of high proportion of β-crystal in both low and high cooling
rates, meanwhile, the thermal stability of crystal is sensitive
to the cooling rate. This work provides a new insight into the
design of β-iPP in dynamic crystallization.
Keywords Isotactic polypropylene .β-crystal .Stereo-defect
distribution .Dynamic crystallization .β-αrecrystallization
Introduction
Polypropylene (PP) is one of the most widely used com-
modity polymers owing to its low manufacturing cost and
rather versatile properties [1–3]. Semicrystalline iPP exhib-
its a very interesting polymorphic behavior, depending on
the molecular structure [4], thermal history [5–9] and the
presence of extraneous materials [10,11]. Generally, four
crystalline structures of iPP are known, including the
monoclinic α-form, the trigonal β-form, the triclinic γ-form
[12,13], and the mesomorphic (smectic) form [14,15].
Among the crystalline structures, the β-modification has
received considerable interests because of several advanta-
geous properties, for instance, high deformation tempera-
ture, high impact and tear strength, in comparison with that
of α-iPP [16]. However, β-crystal has lower stability com-
pared with the α-modification, and can only be formed
under some critical conditions, e.g. quenching the melt to
a certain temperature range [5], crystallization in a thermal
gradient field [17], shearing or elongation of the melt during
crystallization [18], vibration-induced crystallization [10],
or the presence of β-nucleating agents [19–22]. Because
of the lower stability and lower melting temperature of β-
phase compared with α-modification, the crystalline phase
transformation (β-αrecrystallizsatoin) is a characteristic
feature of β-iPP [23]. Researchers studied the β-αtransfor-
mation of β-iPP during heating by combination of calorim-
etry measurement and simultaneous X-ray measurements
[24,25], and claimed that β-αrecrystallization includes
the temporary melting of the initial β-phase, the
J. Kang :J. Gai :J. Li :S. Chen :H. Peng :B. Wang :Y. Cao :
H. Li :J. Chen :F. Yang (*):M. Xiang (*)
State Key Laboratory of Polymer Materials Engineering,
Polymer Research Institute of Sichuan University, Chengdu,
Sichuan 610065, People’s Republic of China
e-mail: yangfengscu@126.com
e-mail: xiangming45@hotmail.com
J Polym Res (2013) 20:70
DOI 10.1007/s10965-012-0070-8
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synchronized recrystallization into α-form, and the final
melting of the α-form.
As one of the most important factor of the external
crystallization conditions, cooling rate plays an important
role in determining the crystallization of β-nucleated PP and
has been extensively studied, great progress has been
achieved. However, the influence of cooling rate on the
crystallization and melting behavior of β-nucleated iPP is
still under debate. Some researchers claimed that with lower
cooling rate during dynamic crystallization, the propor-
tion and thermal stability of the formed β-crystals were
enhanced [26–31].
Meanwhile, some other authors held different point of
views. Yu J et al. [32] studied the influence of cooling rate
on the microstructure and thermal stability of iPP nucleated of
a non-selective β-nucleating agent, and claimed that fast cool-
ing rate is favorable for β-form iPP formation; with slower
cooling rate, the stability of crystals was enhanced. Feng JC et
al. [33] claimed that a higher proportion of β-phase in PPR
containing βnucleating agent could be achieved under faster
cooling rates. Ernesto et al. [4] suggested that, the sensitivities
of Ziegler-Natta iPP and Metallocenic iPP to the cooling rate
is quite different.
Moreover, it seems that iPP nucleated by compound α/β-
nucleating agent systems has extraordinary response to the
cooling rates. Researchers [34,35] believed that in the dual
additive system of iPP, the results not only depend on the
nucleation efficiency and the relative content of the individ-
ual αand βnucleating agents, but also on the cooling rates
employed. The nucleating behavior of the additives can be
explained by the competitive nucleation.
The different results mentioned above might be attributed
to the following reasons: Firstly, the nucleating mechanism,
nucleating efficiency and selectivity of the nucleating agents
used might be quite different from each other; Secondly, the
molecular structure of the iPP applied in the studies, for
instance, the molecular weight and its distribution, the
defect concentration and its distribution, might also be
different. Thirdly, the complicated DSC melting curves
of β-iPP might confuse the obtained results [24]. There-
fore, further investigations concerning the influence of
the reasons mentioned above on the dynamic crystalli-
zation and melting behavior of β-nucleated iPP are still
needed.
Due to the complexity in ZN-iPP polymerization, it is not
easy to adjust the polymerization conditions and to obtain
ZN-iPP with same average isotacticities, but different defect
distributions. Currently, the effect of stereo-defect distribu-
tion on the dynamic crystallization and melting behavior of
β-iPP is still not clear. The aim of this study is to
investigate the influence of the stereo-defect distribution
on the crystallization and melting behavior of β-iPP
under dynamic crystallization conditions, in order to
provide a new insight for the structure–property rela-
tionship of β-iPP.
Experimental section
Materials and sample preparation
The preparation and microstructure characterization of the
iPP used in this study was reported in the previous work
[36]. The results of
13
C NMR, successive self-nucleation
and annealing, and FT-IR had been discussed there in detail.
A brief summary is given here.
The iPP samples studied (PP-A and PP-B) are iPP resins
for biaxially oriented polypropylene (BOPP) film, which are
polymerized by two different highly activity supported
fourth generation Ziegler-Natta (TiCl
4
= MgCl
2
) catalysts,
ZN-A and ZN-B, respectively. The catalytic activity of ZN-
A is higher than that of ZN-B. Other polymerization condi-
tions, for instance, the hydrogen concentration, Al/Si ratio,
temperature, pressure, of the two samples are the same. The
average isotacticities of the samples are nearly same, but the
stereo-defect distribution of PP-B is more uniform than
PP-A. Other molecular parameters of the samples can be seen
in the previous study [36].
The β-nucleating agent was a powder of metal salts
(tradename NAB83) purchased from Guangzhou Chenghe
(GCH) technology company and was a high efficiency
selective β-crystal nucleating agent, and was used as re-
ceived. However, we are not aware of the corresponding
work about chemical structure and crystal information of
NAB83 on the basis of the relevant patents.
The iPP pellets and NAB83 were mixed in the weight
ratio of 100:2 and then extruded by a twin-screw extruder
(SHJ-20, Nanjing Giant Machinery Co., Ltd, the screw
speed is 30 rpm and the temperature of each part is 185,
195, 200, 200 °C, respectively) and pelletized to obtain a
master batch. The master batch and iPP were mixed and
extruded by twin-screw again to obtain β-iPP pellets. The
concentration of nucleating agent was 0.05 wt.%. The
obtained β-iPP samples were named as NPP-A and NPP-B.
For the WAXD, DSC and TMDSC measurements, the
thin-sheet samples were prepared. The virgin polymers were
firstly molded at 190 °C, 10 MPa for 5 min into thin sheets
of 500 μm thickness, and then they were covered by glass
slides and heated in the heating oven under 200 °C for
20 min to erase any previous thermal history. After that,
two different thermal treatments were applied. One series of
samples labeled “air cooling”were taken out of the heating
oven and fast quenched down to room temperature. The
second one named “oven cooling”were kept in the drying
oven and slowly cooling down under the rate of about
2°C/min.
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Characterization
Differential scanning calorimetry (DSC)
All the calorimetric experiments were performed with a
Mettler Toledo DSC1 differential scanning calorimeter
(DSC), under nitrogen atmosphere (50 mL/min). The tem-
perature scale calibration was performed using indium as a
standard to ensure reliability of the data obtained. In order to
ensure the homogeneity of samples and the good contact
between sample and pan, the virgin polymer was molded at
190 °C, 10 MPa for 5 min into sheets of uniform thickness
about 500 μm. Then 5 mg round samples were punched out
of the sheets.
All the thermograms were fitted using Peakfit 4.12 soft-
ware according to the manner in literatures [24], and the
relative percentage crystallinities of α-crystal (α
c
) and β-
crystal (β
c
) were estimated from DSC by the following
expressions [37]:
ac¼ð1lÞa½ð1lÞbþð1lÞa
.ð1Þ
bc¼ð1lÞb½ð1lÞbþð1lÞa
.ð2Þ
where the degree of crystallinities (1-1) associated with each
phase, (1-1)
α
and (1-1)
β
, were calculated from the ratio
ΔH/ΔHu.ΔHand ΔHu are the apparent and completely
crystalline heats of fusion, respectively, and the values used
for ΔHu for 100 % crystalline iPP, was 209 Jg
−1
[38,39].
The nonisothermal programmes involved melt crystalliza-
tion at various cooling rates: 2, 5, 10, 20, 30 and 40 °C/min.
Temperature Modulated DSC (TMDSC)
The TMDSC melting curves of the samples were recorded with
a Mettler Toledo DSC1 Instrument at V
h
=1 °C/min, ±0.1 °C
with 10 s modulation, ranging from 50 °C to 200 °C.
WAXD
Wide-angle X-ray diffraction (WAXD) patterns were
recorded with a DX-1000 diffractometer. The wavelength
of CuKαwas 1=0.154 nm and the spectra were recorded in
the 2θrange of 5–35°, a scanning rate of 2°/min, and a
scanning step of 0.02°. The crystallite size Lof each plane
of samples was determined from the XRD using the Debye-
Scherrer’s equation [40]:
L¼0:9lbcos θ
=ð3Þ
where 1is the X-ray wavelength of radiation used, θis the
Bragg angle and βis the full width of the diffraction line at
half maximum (FWHM) intensity measured in radians.
The content of the β-crystal was determined according to
standard procedures described in the literature [41], employ-
ing the following equation:
kb¼Hbð300Þ
Hbð300ÞþHað110ÞþHað040ÞþHað130Þð4Þ
k
β
denotes the relative content of β-crystal form
(WAXD), H
α
(110), H
α
(040) and H
α
(130) are the intensities
of the strongest peaks of α-form attributed to the (110),
(040) and (130) planes of monoclinic cell, respectively.
H
β
(300) is the intensity of the strongest (300) diffraction
peak of the trigonal β-form.
Results and discussions
Nonisothermal crystallization and melting behavior
In nonisothermal crystallization kinetics, samples were firstly
cooled from the melt to 30 °C at various cooling rate, 2, 5, 10,
20, 30 and 40 °C/min, then they were heated under the rate of
10 °C/min to 200 °C.
Crystallization behavior
The crystallization curves of the samples at different cooling
rates are shown in Fig. 1. The crystallization parameters
plotted as a function of the cooling rate are shown in
Fig. 2, where T
onset−
T
endset
denotes to the crystallization peak
width. The larger the T
onset−
T
endset
, the wider the crystalliza-
tion temperature range.
In the previous study [36], it was found that the crystal-
lization temperature of PP-A is obviously higher than that of
PP-B. Interestingly, Fig. 2shows that the T
c
,T
onset
and
T
endset
of NPP-A are still higher than that of NPP-B. On
the other hand, the crystallization peak width T
onset
-T
endset
of
NPP-A is larger than that of NPP-B at low cooling rates
(2, 5, 10 and 20 °C/min); At high cooling rates (30 and
40 °C/min), the T
onset−
T
endset
of the samples become quite
close.
Taking the stereo-defect distribution into consideration,
the stereo-defect distribution of NPP-A is less uniform com-
pared with NPP-B and contains more amount of high iso-
tactic fractions. During the cooling process, these high
isotactic fractions might begin to crystallize at relative high
temperature, leading to a higher onset temperature of crys-
tallization T
onset
and a higher crystallization peak tempera-
ture T
c
; On the other hand, NPP-A also contains fractions
with medium and low isotacticity, which can only crystallize
at relative lower temperature, resulting a relatively lower
endset crystallization temperature T
endset
. Finally, the crys-
tallization peak width of NPP-A is larger than that of NPP-
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B. As the cooling rates increases from 2 to 40 °C/min, the
crystallization temperature difference between high and low
isotactic fractions becomes less significant.
The accumulated curves of the relative degree of crystal-
linity as a function of crystallization time are calculated as
showninFig.3. The half crystallization time (t
1/2
)of
nonisothermal crystallization, defined as the half period
(i.e. 50 % crystallization), from the onset to endset of crystal-
lization, is a direct measure of crystallization rate and is
calculated as shown in Fig. 4.
Figure 4reveals that at low cooling rates (2, 5, 10 and
20 °C/min), the t
1/2
of NPP-A is larger than that of NPP-B,
Fig. 1 Cooling curves of aNPP-A and bNPP-B at different cooling rates
Fig. 2 Crystallization parameters of NPP-A and NPP-B at different cooling rates. aT
c
,bT
onset
,cT
endset
and dT
onset
-T
endset
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indicating that the time for NPP-A to finish the crystalliza-
tion is longer than NPP-B; however, at high cooling rates
(30 and 40 °C/min), the t
1/2
of the samples are quite close.
The difference in t
1/2
of the samples is in correspondence
with the crystallization peak width T
onset−
T
endset
results.
During dynamic crystallization at low cooling rates, it is
easier for NPP-A to crystallize at higher temperature com-
pared with NPP-B, meanwhile, it takes less time for NPP-B
to finish the crystallization.
Melting behavior
The melting curves at the heating rate of 10 °C/min are
shown in Fig. 5. According to Fig. 5, the relative percentages
of crystallinity of α-crystal α
c
and β-crystal β
c
, the degree of
crystallinity of α-crystal (1-λ)
α
and β-crystal (1-λ)
β
are cal-
culated as plotted in Fig. 6.
As can be seen from Fig. 6, when the cooling rate
increases from 2 to 40 °C/min, for NPP-A, the β
c
increases
from 6.5 to 28.9 % and the (1-1)
β
increases from 3.4 to
13.6 %; however, for NPP-B, the β
c
and (1-1)
β
decrease
from 93.3 % and 41.6 % to 68.9 % and 35.2 %, respectively.
Meanwhile, it can be observed that as the cooling rate
increases, the α
c
and (1-1)
α
of NPP-A decrease gradually,
and these parameters of NPP-B increase gradually.
The results above reveal that the β-crystal content of
NPP-B is significantly higher than that of NPP-A at the
same cooling rate, and NPP-B is more favorable for the
formation of high proportion of β-crystal. Moreover, the
cooling rate dependence for β-crystal content obtained from
DSC heating curves of NPP-A and NPP-B are quite
different.
During the melting of β-iPP, the β-αrecrystallization
usually takes place, which results in the overlap of the
endotherm of the melting β-form, the crystallization exo-
therm from molten amorphous into α-form and endotherm
of the final melting of α-form, consequently confuses the β-
crystal content calculated from the final DSC melting curve.
Since it is usually not possible for one kind of polymer to
have two contrary cooling rate dependences, the different
cooling rate dependences of β-crystal content between NPP-
A and NPP-B might be attributed to the different thermal
stability of the formed β-crystal.
DSC measurement after elimination of the β-α
recrystallization
Varga et al. [5,25,41–43] reported the complex melting and
recrystallization behavior of β-iPP, and claimed that the
melting behavior strongly depends on the thermal post history
of the samples. During the partial melting of the β-crystal, β-
iPP cooled below the critical temperature ðT*
R¼100CÞ
recrystallizes into the α-phase, leading to an enhanced
Fig. 3 Profiles of relative crystallinity as a function of crystallization time of aNPP-A and bNPP-B at different cooling rates
Fig. 4 The half crystallization time t
1/2
of NPP-A and NPP-B at
different cooling rates
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apparent α-content determined from the DSC melting curves.
On the contrary, if β-iPP is not cooled down below T*
Rafter
crystallization, no βα-recrystallization occurs during heating,
and a separate β-melting peak appears on the DSC trace. In
this way, the polymorphic composition of β-nucleated iPP can
be determined from the DSC melting curves [5,41,43,44].
In this study, the end temperature of recooling (T*
R) was
set to T*
R¼120C in the subsequent series of experiment
(second run), in order to eliminate the disturbing effect of β-
αrecrystallization. The consequent melting curves after
cooling at different cooling rates (2, 5, 10, 20, 40 °C/min)
are recorded as shown in Fig. 7.
According to Fig. 7, the relative percentages of crystallinity
of α-crystal α
c
and β-crystal β
c
, the degree of crystallinity of
α-crystal (1-λ)
α
and β-crystal (1-λ)
β
are calculated and
plotted in Fig. 8, as a function of cooling rate.
After elimination of β-αrecrystallization, the β-peak
profile of NPP-B gradually shifts to lower temperature
region as the cooling rate increases, suggesting a decrease
of the structural stability of the crystal. However, this phe-
nomenon is hardly observed for NPP-A, indicating that the
crystal structural stability of NPP-B has less dependence on
the cooling rate, and is higher than NPP-A.
On the other hand, the β-crystal content of NPP-A
increases gradually as the cooling rate increases, which is in
accord with the results from the samples recooled to room
temperature, revealing that for NPP-A, the thermal stability of
the β-crystal is high, and the β-αrecrystallization is relatively
difficult to occur. However, after the exclusion of β-αrecrys-
tallization, the β-crystal content is still low, indicating that
high content of β-crystal can hardly be formed in NPP-A.
Meanwhile, high content of β-crystal is obtained from
NPP-B in all cooling rates used, and the β-content of
remains nearly unchanged as the cooling rate increases, sug-
gesting that the cooling rate has little influence on the exact β-
crystal content of NPP-B; however, it can significantly affect
Fig. 5 Melting curves of aNPP-A and bNPP-B at different cooling rates
Fig. 6 a Relative percentages of crystallinity of α-phase α
c
and β-phase β
c
and bthe degrees of crystallinity of α-phase (1-1)
α
and β-phase (1-1)
β
of NPP-A and NPP-B as a function of cooling rate. All the values were evaluated from heating curves in Fig. 5
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the thermal stability of the β-crystal. The higher the cooling
rate, the lower the thermal stability of the β-crystal, and the
larger extent of β-αrecrystallization during heating.
Characterization of the air-cooling and oven-cooling
samples
In order to understand the interesting crystallization behaivor
of NPP-A and NPP-B fully, a combination analysis of
WAXD, DSC and TMDSC is applied. Two different thermal
treatments were applied to the samples. One series of samples
labeled “air cooling”were taken out of the drying oven and
fast quenched down to room temperature. The second one
named “oven cooling”were kept in the drying oven and
slowly cooling down under the rate of about 2 °C/min.
DSC analysis
The melting curves of the samples at the heating rate of 10 °
C/min are shown in Fig. 9, and β
c
,(1-1)
β
,α
c
and (1-1)
α
are
calculated using Peakfit 4.12 as shown in Table 1. It should
be noted that in Fig. 9(a), the melting curves of NPP-A and
NPP-B exhibit multiple overlapped peaks, which is attrib-
uted to the presence of β-crystal with different thicknesses.
As can be seen from Fig. 9and Table 1,foraircooling
samples (rapid cooling), a β-crystal content of less than 40 %
has formed in both NPP-A and NPP-B; for oven cooling
samples however, the β-crystal peak of NPP-A is very small,
but a high β-crystal content of 81.0 % has formed in NPP-B.
These phenomenon are in accord with the findings in the DSC
nonisothermal crystallization analysis above.
WAXD analysis
The WAXD profiles of the samples are shown in Fig. 10.
The crystalline parameters calculated from Fig. 10 are
shown in Table 2.
Since the WAXD is performed under room temperature,
the confusion of β-αphase transformation can be excluded.
As can be seen from Fig. 10 and Table 2, the crystallite sizes
Fig. 7 Subsequent melting curves of aNPP-A and bNPP-B after recooled under different coolnig rate to the end temperature of T*
R¼120C
Fig. 8 a Relative percentages of crystallinity of α-phase α
c
and β-phase β
c
and bthe degrees of crystallinity of α-phase (1-1)
α
and β-phase (1-1)
β
of NPP-A and NPP-B as a function of cooling rate. All the values were evaluated from heating curves in Fig. 7
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of each plane of NPP-A is larger than NPP-B under the same
thermal condition;
On the other hand, for air-cooling samples, a sharp peak
at θ=16°, characteristic of β(300) plane, can be found from
both NPP-A and NPP-B, and the k
β
of NPP-B (83.4 %) is
obviously higher than that of NPP-A (44.3 %). Compared
with the results obtained from DSC (Fig. 9and Table 1), for
NPP-A, the β
c
(34.9 %) is close to k
β
(44.3 %), suggesting
that the β-crystal thermal stability of NPP-A is high; how-
ever, for NPP-B, the β
c
(39.8 %) is much smaller from k
β
(83.4 %), suggesting that the thermal stability of NPP-B is
much lower than that of NPP-A, which might be attributed to
the molecular structure differences between PP-A and PP-B.
On the other hand, as the cooling rate increases, the β-
crystal content k
β
of NPP-A increases significantly from
11.5 % to 44.3 %, meanwhile, the k
β
of NPP-B remains
more than 80.0 % and is almost unchanged, which is in
accord with the DSC analysis of T*
R¼120C recooled
samples.
Temperature modulated DSC (TMDSC) analysis
In order to gain further insight to the nature of the crystal-
lization behavior of NPP-A and NPP-B, TMDSC is
performed on the samples as shown in Fig. 11. Compared
with conventional DSC, TMDSC has been shown to be a
useful technique to clarify the multiple transitions of poly-
mers [45–50]. In the measurement, the obtained total heat
flow, approximately equivalent to that from a conventional
DSC, can be divided into a (capacity-related) reversible heat
flow and a (kinetic) nonreversible heat flow [51].
Air cooling
Oven cooling
Generally, crystallization and enthalpy relaxation (or recovery)
appear only in the nonreversible signal, whereas melting
occurs in both reversible and nonreversible signals [48,
Fig. 9 DSC heating curves of the β-iPP samples under different cooling rates. aAir cooling (about 80 °C/min), bOven cooling (about 2 °C/min)
Table 1 Melting parameters of β-iPP samples under different cooling
rates
Thermal
condition
Sample β
c
(%) α
c
(%) X
β
(%) X
α
(%)
Air cooling PP-A + NAB83 34.9 65.1 14.8 27.5
PP-B + NAB83 39.8 59.9 16.8 25.2
Oven
cooling
PP-A + NAB83 8.9 91.1 4.4 45.2
PP-B + NAB83 81.0 19.0 39.3 9.2 Fig. 10 WAXD profiles of theβ-iPP samples under different cooling
rates
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51–54]. Namely, only melting appears on the reversing signal.
Therefore, the reversing signal can be used to analysis the
separated melting behavior of the sample, including the melt-
ing of β-phase and the final melting of the α-phase. For air
cooling samples (Fig. 11(a and b)), only a small β-crystal
melting peak emerges in the reversing and total signal of
NPP-A, suggesting that only a small amount of β-crystal has
formed; meanwhile, a more obvious β-crystal melting peak
appears in the reversing signal of NPP-B. However, due to the
low thermal stability of NPP-B, the β-αrecrystallization exo-
thermic peaks is overlapped with the endothermic β-crystal
melting peak, and finally decreases the intensity of the β-
crystal melting peak in the total signal.
Figure 11(c and d) reveal that for oven cooling samples,
the total signal and the reversing signal of NPP-A are quite
similar, which might indicate that the crystal structure
formed is quite stable. However, the β-melting peak is quite
ambiguous, which can hardly be observed; For NPP-B, a
Table 2 WAXD parameters of
β-iPP samples under slow
cooling rate (2 °C/min)
Parameters Sample α(110) β(300) α(040) α(130) α(111)
L(nm) Air cooling NPP-A 118 294 196 157 161
NPP-B 66 287 192 –159
Oven cooling NPP-A 144 340 215 208 214
NPP-B 118 296 213 191 261
k
β
(%) Air cooling NPP-A 44.3
NPP-B 83.4
Oven cooling NPP-A 11.5
NPP-B 81.6
Fig. 11 TMDSC melting curves of NPP-A and NPP-B at 1 °C/min. Air cooling samples of aNPP-A and bNPP-B, and Oven cooling samples of c
NPP-A and dNPP-B
J Polym Res (2013) 20:70 Page 9 of 11, 70
Author's personal copy
sharp β-crystal melting peaks can be found on the reversing
signal, indicating the formation of a large amount of β-
crystal. A large β-crystal melting peak can also be found
in the total signal, which might be due to the enhancement
of the β-crystal thermal stability.
The results above demonstrate that, the stereo-defect
distribution significantly influences the dynamic crystalliza-
tion behavior of β-nucleated iPP.
For NPP-A, the less uniform distribution of stereo-defect
results in the high crystallization temperature, which might be
too high for the formation of β-crystal; On the other hand,
chains movement and regular insertion of the molecular
chains during crystallization is quite strong, resulting in a
strong tendency for the formation of α-phase and high thermal
stability of the crystal. High cooling rate can lower the crys-
tallization temperature and reduce the tendency of α-phase
formation, and therefore enhance β-crystal content. Anyway,
it is hard for NPP-A to produce high proportion of β-crystal.
For NPP-B, the more uniform distribution of stereo-
defects decreases both the crystallization temperature and
the regular insertion of molecular chains during the crystal-
lization, which might be more suitable for the formation of
β-crystal. Therefore, under all cooling rate studied, a high
proportion of β-crystal can be formed, and it seems that the
cooling rate has little influence on the β-crystal content; On
the other hand, the thermal stability of β-crystal is greatly
influenced by the cooling rate. The lower the cooling rate,
the higher the thermal stability of the β-crystal.
Conclusions
In this study, the dynamic crystallization and melting be-
havior of two β-nucleated ZN-iPP with nearly same average
isotacticity but different stereo-defect distribution, were in-
vestigated by means of DSC, WAXD and TMDSC. This
study takes the stereo-defect distribution into consideration,
and provides novel insight in designing the β-nucleated
ZN-iPP. The following conclusions can be drawn:
The stereo-defect distribution of iPP can significantly
influence the crystallization and melting behavior during
dynamic crystallization. ZN-iPP with different stereo-
defect distributions can exhibit different dependence on the
cooling rate.
For the ZN-iPP in this study, a more uniform distribution of
stereo-defect can decrease the crystallization temperature and
the regular insertion of molecular chains in a larger extent,
which is more suitable for the formation of high proportion of
β-crystal in both low and high cooling rates; meanwhile, the
thermal stability of crystal is sensitive to the cooling rate: the
higher the cooling rate, the lower the thermal stability of the
crystal. On the contrary, a less uniform stereo-defect distribu-
tion favors the formation of crystal with high thermal stability
in all cooling rates concerned; the β-crystal content greatly
depends on the cooling rate applied: the fasterthe cooling rate,
the higher the β-crystal content.
Generally, both the stereo-defect distribution and cooling
rate will influence the formation of β-crystal, it should be
carefully to choose proper ZN-iPP resins and crystallization
conditions during the producing to β-iPP.
Acknowledgment We express our sincerely thanks to the Program
for New Century Excellent Talents in University (NCET-10-0562).
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