Morphology of homogeneous copolymers of ethylene and 1-octene. III. Structural changes during heating as revealed by time-resolved SAXS and WAXD

The structural changes of two linear polyethylenes, LPEs, with different mo lar mass and of two homogeneous copolymers of ethylene and 1-octene with co mparable comonomer content but different molar mass were monitored during h eating at 10 degrees C per minute using synchrotron radiation SAXS. Two set s of samples, cooled at 0.1 degrees C per minute and quenched in liquid nit rogen, respectively, were studied. All LPEs display surface melting between room temperature and the end melting temperature, whereas complete melting , according to lamellar thickness, only occurs at the highest temperatures where DSC displays a pronounced melting peak. There is recrystallization fo llowed by isothermal lamellar thickening if annealing steps are inserted. T he lamellar crystals of slowly cooled homogeneous copolymers melt in the re verse order of their formation, that is, crystals melt according to their t hickness. Quenching creates unstable crystals through the cocrystallization of ethylene sequences with different length. These crystals repeatedly mel t and co-recrystallize during heating. The exothermic heat due to recrystal lization partially compensates the endothermic heat due to melting resultin g in a narrow overall DSC melting peak with its maximum at a higher tempera ture than the melting peak of slowly cooled copolymers. With increasing tem perature, the crystallinity of quenched copolymers overtakes the one of slo wly cooled samples due to co-recrystallization by which an overcrowding of leaving chains at the crystal surfaces is avoided. (C) 2000 John Wiley & So ns, Inc.