Study of crystallization and isothermal thickening in polyethylene using SAXD, low frequency Raman spectroscopy and electron microscopy

Abstract

This study contains a combined application of three different techniques for the study of polyethy-lenes crystallized from the melt under different circumstances, small-angle X-ray diffraction (SAXD), low frequency Raman spectroscopy to examine the longitudinal acoustic ( LA) mode, and electron microscopy. In particular, the combination of SAXD and Raman methods enables the separation of the situation where there is only one lamellar structure which displays several orders in the SAXD pattern, from that where there is more than one type of lamellar thickness present. The superior power of the Raman method, which does not depend on the regularity in the lamellar stacking, becomes apparent. The multiplicity of the lamellar population could be associated with lamellae formed isothermally at the preselected crystallization temperature and with lamellae which originated from material which has remained uncrystallized at this temperature and formed subsequently with smaller lamellar thickness during cooling of the sample. The existence of the corresponding double lamellar population could be made directly visible using electron microscopy on freeze-cut and stained sections. The thinner lamellae in the double population could be extracted by solvents, removing the corresponding SAXD and Raman peaks, and leaving blank image areas in place of the thin lamellae in the electron-micrographs. These extracted thinner lamellae correspond to lower molecular weights as assessed by g.p.c. Thus molecular segregation during crystallization is involved. Furthermore the segregated texture units and their arrangement within the full morphology could now be identified. Pronounced changes in lamellar thickness with crystallization time were observed throughout and were associated in the early stages of crystallization with molecular fractionation and in the later stages with thickening of lamellae already present. An unexpected interrelation between nucleation density and the final lamellar thickness through the agency of isothermal lamellar thickening has been established. Examples are quoted which are contrary to the expected trend of lamellar thickness with crystallization temperature, but which are interpretable nevertheless in the light of the effect of isothermal lamellar thickening. The potential significance of all these findings and of this kind of approach for the characterization of crystalline bulk polymers is made throughout.