Evaluation of continuous and discrete melting endothermsfor determination of structural heterogeneities inZiegler-Natta catalyzed linear low density polyethylene

Ethylene / 1-butene copolymers at different comonomer levels were synthesized using a Ziegler-Natta catalyst to evaluate the applicability of thermal fractionation methods in predicting chemical composition distribution(CCD). The continuous melting endotherms obtained using DSC were converted to continuous CCD, and theaverage comonomer contents were compared with the NMR results. DSC underestimated the comonomer contentspecifcally at higher levels and demonstrated that it was more sensitive to the chosen baseline. The thermallyfractionated melting endotherms by using the SSA method were deconvoluted and transformed to discrete CCDs.The SSA method underestimated the average comonomer content even more than those obtained by DSC,nevertheless its results were more reproducible. The main shortcoming of the thermal methods was the inability ofshort ethylene sequences in forming discernible lamella thicknesses at high comonomer levels. Calibration curveswere created for converting the predicted comonomer contents into the absolute values and used for studyingindustrial LLDPEs with different comonomer levels. It was shown that SSA provided more reliable results andcorrelated more reasonably to the measured densities. Polyolefns J (2016) 3: 135-146

Keywords:

Chemical composition distribution , lamella thickness , short chain branching , thermal fractionation , linear low densitypolyethylene , Ziegler-Natta catalyst