Chemistry Reference
In-Depth Information
Table 3.
Physical properties of polyethylenes made in the presence of H
2
.
Catalyst
M
w
10
5
1
st
Heating
2
nd
Heating
P
ethyl/hydrogen
(g/mol)
M
w
/M
n
(-)
T
m
X
c
T
m
X
c
(
8
C)
(%)
(
8
C)
(%)
10: 1
5.23
10.2
141.2
72
139.0
67
TpTiCl
2
(OEt)
7: 1
2.24
6.3
141.7
68
140.1
65
5: 1
1.04
9.3
137.6
77
138.4
75
TpTiCl
2
(OiPr)
5: 1
0.72
4.8
140.2
80
138.7
73
TpTiCl
2
(OnBu)
5: 1
0.58
8.5
138.9
62
138.1
67
Conclusions
by hydrogen pressure (Table 3). Thus, when
the P
ethylene/hydrogen
ratio was increased,
the M
w
increased from 1.04
10
5
to 5.23
10
5
g/mol. This tendency was expected in
accordance with industrial practice, where
the decrease of the hydrogen pressure
reduces the probability of the transfer
reaction to occur as a consequence of the
drop of the hydrogen concentration in the
reaction medium around the active site. On
the other hand, the polyethylene polydis-
persities were lower than the copolymers
case. This might indicate that, for the
catalytic systems evaluated, hydrogen is
capable of inhibiting a fraction of active
species during the polymerization.
The polyethylenes showed melting tem-
peratures in the interval from 138 to 140
8
C
and crystallinities between 65 to 75%.
Additionally, no significant effects on
thermal properties were observed when
the type of alkoxyl group was changed. The
characterization results evidenced that all
the properties corresponded to a high
density polyethylene (HDPE) with molec-
ular weights in the range of commercials
grades.
As seen in Table 3, it was found that
the melting temperature and crystallinity
did not show significant variation between
the 1
st
and 2
nd
heatings. This behaviour
shows that molecular weight has an impor-
tant influence on the thermal properties
of the melt-crystallized sample: the differ-
ences in the thermal properties between the
nascent powders and the melt-crystallized
samples were higher when the molecular
weight increased.
The characterization by
13
CNMR, capillary
viscosimetry or GPC, and DSC, showed
that the catalytic system has an impor-
tant influence on the final properties of
nascent powders. Important differences
were observed between the thermal proper-
ties obtained from the first DSC heating
(nascent powders) and the second DSC
heating (melt-crystallized samples). This
behaviour was attributed to the crystal-
lization process during the polymer synthe-
sis which likely leads to less entangled
morphologies (better formed crystals) than
those obtained with the melt-crystallized
sample, due to the fact that the crystal-
lization kinetics of the nascent and molten
polyethylene are different. Furthermore,
an important influence of the molecular
weight on the melting temperature and
crystallinity of the melt-crystallized sam-
ples was corroborated.
We acknowledge the Che-
mistry Center of Venezuelan Institute for
Scientific Research (IVIC) for financial support
for this research and FONACIT for Grants
S1-2000000519 and F-2000001365.
Acknowledgements:
[1] H. Knuuttila, A. Lehtinen, A. Nummila-Pakarinen,
Adv. Polym. Sci. 2004, 169, 13.
[2] S. Ottani, R. S. Porter,
J. Polym. Sci.: Polym. Phys. B
1991,
, 1179.
[3] Y. M. T. Engelen, P. J. Lemstra,
29
Polym. Comm.
1991,
32, 3433.
