Saturday, December 18, 2010


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.

Results  and  discussion
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