A USAXS STUDY OF MELT-PROCESSED PE WITH A SHISH-KEBAB STRUCTURE - THEINFLUENCE OF TEMPERATURE ON THE LONG PERIODS

The changes in the two axial long periods of injection moulded oriente d linear polyethylene (PE) with a shish-kebab structure have been inve stigated as a function of temperature by ultra-small angle X-ray scatt ering (USAXS) using synchrotron radiation. The aim of the present stud y is to investigate the origin of both periodicities and to establish whether both scattering maxima are mutually related. The gradual trans formation of the initial two long periods, L-1 = 36.4 and L-2 = 65.7 n m, measured at room temperature to the final single long period L-f = 120 nm at T = 130 degrees C has been demonstrated on high molecular we ight oriented PE samples. The detection of such high long periods (up to 150 nm for these samples) has been possible, for the first time, th anks to the use of the new high resolution USAXS beam line at the DORI S-bypass, DESY. On the other hand, low molecular weight PE shows, at r oom temperature, one long period corresponding to an oriented structur e with a wide lamellar distribution. The coherently diffracting domain s in the chain direction and the mean orientation of the lamellae deri ved from the azimuthal scanning of the SAXS profiles have also been ex amined as a function of temperature. The changes of the long period L- 1 with temperature are shown to be independent from the changes of the L-2 periodicity. From the results it is concluded that the oriented s hish-kebab structure, in the high molecular weight samples, consists o f two separate populations of lamellar stacks with the layer normals p arallel to the injection direction. During annealing only the thinner lamellae increase in thickness until they reach the size of the thicke r ones at about T = 130 degrees C. For higher temperatures than T = 13 7 degrees C the scattering maxima vanish suggesting that the lamellae melt. Finally, after cooling from the melt the lamellae crystallize ep itaxially on the preserved oriented shish fibrils and both stacking pe riodicities L-1 and L-2 appear again. (C) 1997 Elsevier Science Ltd.