Morphology of homogeneous copolymers of ethylene and 1-octene. II. Structural changes on annealing

Based on DSC evidence, annealing of ethylene-1-octene copolymers results in a gradually increasing thermal stability of the original, metastable, crystals. SAXS and WAXD were used to monitor the structural changes involved after isothermal annealing for a fixed time at step-wise higher temperatures. A series of samples that differ in molar mass and comonomer content, ranging 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 the quenched linear polyethylenes (LPEs), being included in this study as reference 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 slowly cooled samples. On the other hand, higher crystallinities can be found for the quenched copolymers with a higher comonomer content compared to the slowly cooled specimens. A sequence of cocrystallization and recrystallization events is proposed to explain this contraintuitive, but reproducible experimental fact. This reasoning can also account for the steeper increase of the amorphous layer thickness of the latter slowly cooled copolymers compared to the quenched samples. All copolymers show a very moderate increase of the lamellar thickness after each heating step. Besides additional crystallization and recrystallization, lateral growth of the crystals and an increase of the crystallite density can account for the gradual increase of the thermal stability of copolymer crystals during prolonged annealing. The morphological effects observed for the LPEs confirm earlier findings. (C) 1999 John Wiley & Sons, Inc.