M. Peeters et al., Morphology of homogeneous copolymers of ethylene and 1-octene. II. Structural changes on annealing, J POL SC PP, 37(1), 1999, pp. 83-100
Based on DSC evidence, annealing of ethylene-1-octene copolymers results in
a gradually increasing thermal stability of the original, metastable, crys
tals. SAXS and WAXD were used to monitor the structural changes involved af
ter isothermal annealing for a fixed time at step-wise higher temperatures.
A series of samples that differ in molar mass and comonomer content, rangi
ng from 0 to 11.8 mol % 1-octene, were cooled at two extreme rates from 150
degrees C, i.e., a quenching into liquid nitrogen and a controlled cooling
at 0.1 degrees C per minute to room temperature. The crystallinities of th
e quenched linear polyethylenes (LPEs), being included in this study as ref
erence materials, and of the quenched copolymer with a 1-octene content of
2.1 mol % are always found to be lower than the crystallinities of the slow
ly cooled samples. On the other hand, higher crystallinities can be found f
or the quenched copolymers with a higher comonomer content compared to the
slowly cooled specimens. A sequence of cocrystallization and recrystallizat
ion events is proposed to explain this contraintuitive, but reproducible ex
perimental fact. This reasoning can also account for the steeper increase o
f the amorphous layer thickness of the latter slowly cooled copolymers comp
ared to the quenched samples. All copolymers show a very moderate increase
of the lamellar thickness after each heating step. Besides additional cryst
allization and recrystallization, lateral growth of the crystals and an inc
rease of the crystallite density can account for the gradual increase of th
e thermal stability of copolymer crystals during prolonged annealing. The m
orphological effects observed for the LPEs confirm earlier findings. (C) 19
99 John Wiley & Sons, Inc.