Two samples of LLDPE with different structural parameters studied in this
paper are listed in Table 1.
Specimens were prepared by cutting the granular resins into slices of mass ap-
proximately 5.5 mg. A Perkin-Elmer model DSC-2C differential scanning
calorimeter was used to measure the enthalpies of fusion and crystallization.
Three reference materials, indium, phenyl ether and o-terphenyl were used for in-
strument calibration. The temperature range of scanning was 213 K (-60~ to
443 K (170~ high purity nitrogen was used as purging gas. Unless indicated the
scanning rates (both heating and cooling) were 10 deg.min -1.
1. For the observation of structural differences between these two LLDPE
resins, identical thermal treatment were applied to all specimens in order to
eliminate the thermal history caused by processing and storage conditions. Each
specimen was heated up to 443 K and held at this temperature for 10 min, then
cooled down to 213 K and held isothermally for 10 minutes. The DSC measure-
ment was made during reheating to 443 K.
2. To observe the effect of heating rate on the melting behaviour, specimens
were isothermally conditioned at 443 K for 10 min cooled to 213 K, then reheated
up to 443 K at four separate heating rates (20, 10, 5 and 1 deg.min-t). A DSC
measurement was made during reheating.
3. To observe the effect of cooling rate on the crystallization behaviour, the
specimens were isothermally conditioned at 443 K for 10 min then cooled to
213 K at three cooling rates (20, 10 and 1 deg.min-~). Finally, the samples were
reheated up to 443 K and DSC measurement recorded during this reheating cycle.
4. To observe the annealing effect on the melting behaviour, the melt
specimens were cooled to four given annealing temperatures of 398 K, 393 K,
383 K and 373 K, and held at these temperatures for 10 and 120 minutes respec-
tively. The annealed specimens were cooled to 213 K and reheated up to 443 K
finally, DSC measurements being made for the final heating process.
The results of the effects of thermal history are shown in Fig. 1.
Comparing the peak shape of the two LLDPE samples as received in Fig. 1, it
can be seen that more comonomer content in LLDPE leads to a broader peak and
lower peak temperature. The amount of comonomer in Dowlex 2045 is less than
that in Stamylex 1048, i.e. the degree of linearity for Dowlex 2045 is higher than
that for Stamylex 1048. This is in accord with the structural parameters listed in
Table 1. The melting temperatures of both LLDPE samples measured after
eliminating thermal history were a little higher than those measured before. These
indicate that the crystalline integrity of both LLDPE increased during the new
thermal history. The changes of peak shape illustrate that the size distribution of
crystailites also changed. The changes of fusion enthalphy, AHf, for Stamylex
1048 were slightly higher than those for Dowlex 2045 before and after elimina-
tion of the thermal history effect. These reflected the small difference in crystal-
linity between these two samples. The effect of heating rate on melting behaviour
is shown in Fig. 2. It can be seen that the lower the heating rate, the better the
peak resolution, and consequently the clearer the shoulder peak. This may be in-
terpreted that the slower heating rates enable a semicrystalline polymer to have
more time for crystal growth prior to final melting. Curve 'a' in Fig. 2 illustrates
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