COMPARATIVE-STUDY OF SIZE AND DISTRIBUTION OF LAMELLAR THICKNESSES AND LONG PERIODS IN POLYETHYLENE WITH A SHISH-KEBAB STRUCTURE

This study contains a combined application of three different techniqu es for the study of injection moulded polyethylene (PE), showing an or iented shish-kebab structure: small angle X-ray scattering (SAXS), low frequency Raman spectroscopy (LAM) and transmission electron microsco py (TEM). A series of linear PEs and molecular weights in the range 51 000-478 000 has been investigated and two injection temperatures have been used (T-m = 144 and 210 degrees C). SAXS patterns from the highl y oriented regions show the presence of either one axial long period ( L(1)) or two (L(1) and L(2)) depending on molecular weight ((M) over b ar(w)) and T-m. It is shown that L(1) and L(2) increase with (M) over bar(w) up to a given critical molecular weight (M) over bar(c). Above (M) over bar(c), L(1) and L(2) remain constant. Raman results qualitat ively confirm the existence of two separate distributions of straight- length chain segments for those samples having molecular weights above the critical value. Shorter segments are shown to be more abundant th an the longer ones. In the lowest molecular weight sample, results fro m SAXS, TEM and Raman spectroscopy seem to be consistent with each oth er, although in some cases a tilted molecular arrangement within the l amellae has to be invoked. On the other hand, in case of the highest m olecular weight sample, the length of the short straight-chain segment s derived from Raman spectroscopy agrees well with the double periodic ity obtained from SAXS. On the contrary, long periods measured from TE M only correspond to the shorter SAXS periodicity. This result is disc ussed by assuming the occurrence of crystalline bridges among adjacent lamellae.