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, 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.