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Polym Int 48 :53–56 (1999)
Polymer International
Influence of syndiotacticity and annealing
temperature on the double melting peak
behaviour of syndiotactic polypropylene
Junting Xu,*,1 Linxian Feng,1 Zhengxi Liu,1 Yi Deng,2 Chunming Cui2
and Wei Chen2
1 Department of Polymer Science and Engineering , Zhejiang Univers ity , Hangzhou 310027 , People’s Republic of China
2 Res earch Ins titute of Petroleum Proces s ing , SINOPEC , Beijing 100083 , People’s Republic of China
Abstract : Fractionated syndiotactic polypropylene (sPP) samples with homogeneous tacticity were
annealed at diþ erent temperatures. The inýuence of syndiotacticity and annealing temperature on
the double melting peak phenomena were investigated. It is found that all fractions show double
melting peaks at 75ÄC annealing temperature, while the low peak disappears when the fractions with
higher syndiotacticity are annealed at 85ÄC and above. The fraction with the lowest syndiotacticity
remains the same at any annealing temperature. In combination with wide-angle X-ray diþ raction
experiments, the double peaks are believed to correspond to the melting of cell II and III. The results
indicate that higher temperature and syndiotacticity are the external conditions and internal structural factor that permit cell II to transform into cell III.
( 1999 Society of Chemical Industry
Keywords : syndiotactic polypropylene ; syndiotacticity ; annealing ; double melting peak
INTRODUCTION
It is widely accepted that polymorphism occurs in
syndiotactic polypropylene (sPP) during crystallization. Corradini and co-workers1,2 proposed a
crystal structure with c-centered isochiral packing.
Lotz and co-workers3,4 suggested a structure of cell
II with antichiral packing of chains only along the a
axis and an orthorhombic cell III structure in sPP
isothermal crystallized at higher temperature, in
which right-handed and left-handed helices alternate
along both the a and b axes. (It is noted that cells I,
II and III are diþerent from forms I, II and III. In
ref. 5 both cells II and III correspond to form I, and
cell I corresponds to form I.) Moreover, the planar
zigzag structure can exist in the drawn sPP.6
Multiple melting peak phenomena have been
observed during heating for several polymers,
including sPP.7h12 Transformation of diþerent
crystal structures is considered as one of the origins
of double melting peaks. Boor and Youngman7
attributed these two peaks to the helical form and the
planar zigzag form, respectively. Cheng and coworkers11 observed that the crystal structure cell III
was formed during the transformation of double
peaks into a single peak, in an isothermal crys-
tallization temperature region between 70¡C and
120¡C. In addition, the molecular structure of the
polymer chain, such as tacticity and molecular
weight, may also inýuence the double melting peaks
of sPP. Marchetti and Martuscelli8 believed that the
two peaks correspond to the fusion of crystals
incorporated with stereodefects, and relatively perfect
crystals. Lovinger et al9 suggested that the lower
melting peak may be due to lower molecular weight
materials rejected during crystallization. Balbontin et
al13 held a similar viewpoint. They observed only a
single melting peak after isothermal crystallization
for samples with relatively narrow molecular weight
distributions. Previous studies showed that the
double peak of sPP was inýuenced by both structural
factors and crystallization conditions. These inýuences, however, have not yet been fully understood.
In our previous work, sPP was fractionated with a
preparative temperature rising elution fractionation
(TREF) technique, and sPP fractions with diþerent
syndiotacticity were obtained.14 In this publication,
these sPP fractions with homogeneous syndiotacticity were crystallized at diþerent temperatures. The eþects of syndiotacticity and
annealing temperature were investigated.
* Corres pondence to : Junting Xu, Department of Polymer Science
and Engineering, Zhejiang Univers ity, Hangzhou 310027, People’s
Republic of China
Contact/grant s pons or : National Natural Science Foundation of
China.
Contract/grant number : Project 59703002.
(Received 5 May 1998 ; revis ed vers ion received 25 Augus t 1998 ;
accepted 15 September 1998 )
( 1999 Society of Chemical Industry. Polym Int 0959–8103/99/$17.50
53
JT Xu et al
EXPERIMENTAL
Materials
Syndiotactic polypropylene samples prepared with a
SiO -supported metallocene catalyst in a slurry
2
process were supplied by SINOPEC. The weightaverage molecular weight (M ) and molecular weight
w
distribution (MWD) were 32.5 ] 104 and 2.2,
respectively. The polymer obtained was fractionated
using the TREF technique. Some fractions were
selected for crystallization experiments. The tacticity
distributions and viscosity average molecular weights
are listed in Table 1. The pentad distributions of
fractions were obtained using a Bruker AMX-400
spectrometer operating at 100.7 MHz in pulse
Fourier transformation mode at 100¡C. The intrinsic
viscosity of the samples was measured with a modiüed Ubbelohde type viscometer at 135¡C employing
decalin as solvent. The concentration of solution was
0.15 g ml~1 and a small amount of antioxidant was
added to the solution.
Crystallization and annealing
About 3 mg of the as-fractionated sPP sample was
placed in an aluminium pan and put on a hot plate
(180¡C, N ) where it was maintained for 5 min to
2
erase previous thermal history ; it was then cooled to
room temperature at a fast rate. Subsequently, the
samples were moved into a Perkin-Elmer DSC-7
instrument and annealed at 75¡C, 85¡C, 95¡C and
105¡C, respectively, for 6 h. The DSC melting
curves were recorded directly from the corresponding annealing temperature to 180¡C at a heating rate
of 10¡C min~1.
Wide-angle X-ray diffraction experiments
Wide-angle X-ray diþraction (WAXD) experiments
were carried out using a Rigaku 12 kW rotating
anode generator and diþractometer operated in the
reýection mode. The monochromatized X-ray beams
were Ni-ültered CuKa radiation with a wavelength
of 0.154 38 nm. The samples for WAXD experiment
were prepared by thermal pressing of sPP fractions
and the thin ülms obtained were subjected to the
same thermal treatment as that described in Crystallization and annealing.
RESULTS
The as-fractionated sPP samples all showed double
melting peaks when they were heated at a rate of
10¡C min~1. Similar phenomena were observed after
Fraction
Table 1. Pentad dis tributions and
molecular weight of s PP fractions
54
s
s
s
s
PP-2
PP-4
PP-5
PP-7
Figure 1. DSC thermograms of fractions annealed at (a) 75¡C and
(b) 85¡C.
these fractions were annealed at 75¡C (Fig 1a). The
enthalpies of fusion and melting temperatures of
both peaks were found to systematically increase
with increasing syndiotacticity (Tables 2 and 3). In
addition, the relative intensity of the hightemperature peak increases steadily as the syndiotacticity of fractions increases. In fraction sPP-2,
the high-temperature peak appears just as a weak
shoulder, but in fraction sPP-7, this peak becomes
larger than that at lower temperature. This agrees
with the results obtained by Boor and Youngman7
and in our work15 for non-isothermal crystallization
of these fractions. When fraction sPP-4, sPP-5 and
sPP-7 are annealed at 85¡C and above, the lowtemperature peak disappears, but the temperature of
the high-temperature peak remains basically
mmmm
mmmr
rmmr
mmrr
mrmm
] rmrr
mrmr
rrrr
rrrm
mrrm
M
v
(]10 É4)
0
0
0
0
1.9
0
0
0
3.1
2.6
2.3
2.0
6.7
5.5
4.5
4.0
5.0
3.6
2.4
1.6
0
0
0
0
74.9
81.2
85.2
87.4
8.5
7.1
5.5
5.0
0
0
0
0
12.8
24.7
29.3
40.5
Polym Int 48 :53–56 (1999)
Double melting peak behaviour of syndiotactic polypropylene
Fraction
s
s
s
s
Table 2. Effect of annealing
temperature on melting
temperature
Melting temperature (¡C )
PP-2
PP-4
PP-5
PP-7
75 ¡C a
85 ¡C a
95 ¡C a
105 ¡C a
112.4/117.8 (6.61 : 1)b
119.2/126.0 (2.99 : 1)
120.9/129.0 (2.09 : 1)
127.3 : 135.4 (1.34 : 1)
111.6/118.0
124.8
129.1
133.3
112.4/118.1
126.6
129.3
134.2
112.9/118.3
126.7
129.0
133.1
a Annealing temperature.
b Values in parenthes es are the area ratio of the two peaks .
unchanged (Fig 1b). In addition, annealing may lead
to a change in the apparent crystal size of the
polymer. Marchetti and Martsucelli8 found that the
apparent crystal size increased drastically after
the sPP samples were annealed around 85¡C. Cheng
and co-workers11 also found that when sPP fractions
were isothermally crystallized at 105¡C, the double
peaks disappeared, but the observed temperature in
their work is higher than that in our experiments.
This may arise from a diþerence in tacticity and
Table 3. Effect of annealing temperature on fus ion
enthalpy
Fus ion enthalpy (J g É1)
Fraction
s
s
s
s
PP-2
PP-4
PP-5
PP-7
75 ¡C a
85 ¡C a
95 ¡C a
105 ¡C a
13.10
22.80
24.96
29.00
14.94
27.05
29.13
33.88
12.93
29.13
31.86
35.43
7.00
22.58
27.04
30.83
a Annealing temperature.
Figure 2. WAXD patterns of s PP-5 annealed at (a) 75¡C and (b)
85¡C.
Polym Int 48 :53–56 (1999)
molecular weight of the samples used. As an exception, the DSC curves of fraction sPP-2, which possesses the lowest syndiotacticity, are nearly the same
at any annealing temperature.
Annealing causes changes not only in the DSC
traces but also in the crystal structure. Figure 2
shows WAXD patterns of sPP-5 annealed at 75¡C
and 85¡C. In Fig 2 (curve a) three strong peaks are
observed. In contrast, a new peak at 2h \ 18.8¡
appears in the pattern of sPP-5 annealed at 85¡C in
addition to the other three major reýection peaks
(Fig 2, curve b). This new reýection peak is characteristic of cell III (attributed to (211) plane), indicating antichiral packing of polymer chains.4,16 This
shows that a new crystal structure containing cell III
is formed when this fraction is annealed at higher
temperatures or is melted. The peaks at lower and
higher temperature may correspond to the fusion of
diþerent crystal structures, and the disappearance of
the low peak involves the transformation of crystal
structures.
DISCUSSION
Origin of double melting peaks
Because the cell I structure is found only in the übre
form and not in the bulk sPP, although cell I and cell
II have similar WAXD patterns,2 Fig 2, curve a
shows the WAXD pattern of cell II modiücation or
mixture of cell II and III modiücations. A similar
result was obtained by De Rosa et al17 in the fast
crystallization of sPP. Combining the annealing
experiment with the WAXD result, we can reach the
conclusion that the double melting peaks originate
from the diþerent crystal structures, ie cell II and
III modiücations. The formation of cell II structure
is kinetically favourable in non-isothermal crystallization at a fast cooling rate. Because the transformation of cell II into cell III involves the change
of handedness along the b axis,18 this process
requires the melting of crystals. When the samples
are heated to 85¡C, previously formed cell II crystals
may be partially molten, and some of them recrystallize with the cell III modiücation when the
samples are annealed at 85¡C.
In our experiments, the samples used have a
narrow syndiotacticity distribution.14 Therefore,
double melting peaks cannot be attributed to the
fusion of crystals incorporating diþerent stereodefects.17 Also, because the molecular weight of our
55
JT Xu et al
samples is relatively high with a narrow molecular
weight distribution characteristic of metallocenebased polymers, the amount of low molecular weight
polymer in our samples may be negligible, and the
lower melting peaks cannot be produced by low
molecular weight polymer rejected during crystallization.9,13 We believe the reason why Balbontin
et al13 observed only a single peak for fractions with
narrow molecular weight distributions is that too
high crystallization temperatures were used in their
isothermal crystallization experiments. The lowest
crystallization temperature employed by Balbontin et
al13 was 90¡C, while our work shows that double
melting peaks disappear above 85¡C.
Effect of annealing temperature and syndiotacticity
In polymer solids, suitable molecular mobility is
necessary for structural reorganization. The polymer
chains have a higher mobility at higher temperature.
The fact that two peaks are observed when a sample
is annealed at 75¡C and only a single peak is
observed when it is annealed at 85¡C and above, suggests that the transformation of cell II into cell III
structure cannot proceed at a temperature lower than
75¡C.
From the DSC results, it can be seen that the
DSC melting traces of sPP-2 basically remain the
same at all annealing temperatures. On the contrary,
only a single melting peak appears for other fractions
annealed at 85¡C and above. The major diþerence
between sPP-2 and other fractions is that sPP-2 has
the lowest syndiotacticity. This shows that syndiotacticity is an important factor that decides
whether cell II can be transformed into cell III or
not. Because the melting temperature of cell III is
higher than that of cell II, cell III is more regular
than cell II. When the syndiotacticity of a fraction is
too low, a highly regular structure cannot be formed
and the transformation of cell II structure into cell
III structure is then very difficult ; it may not even
be observed. In fraction sPP-2, the melting peak of
cell III appears as a weak shoulder. It can be predicted that this peak will completely disappear if a
fraction has syndiotacticity lower than that of fraction sPP-2. Syndiotacticity decides not only the
transformation of cell II into cell III, but also the
regularity of the formed cell III, because the temperature of the higher peak increases with the syndiotacticity of the fractions.
56
CONCLUSIONS
The double melting peaks of sPP originate from the
transformation of cell II into cell III during the
melting process. Both annealing temperature and
syndiotacticity of sPP have an important inýuence
on this transformation. Only at higher temperature is
the transformation permitted to take place. sPP with
lower syndiotacticity cannot form a highly regular
crystal structure, and the transformation of cell II
into cell III structure cannot happen.
ACKNOWLEDGEMENT
This Project 59703002 is supported by the National
Natural Science Foundation of China.
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Polym Int 48 :53–56 (1999)
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