Evaluation of continuous and discrete melting endotherms in determination of structural heterogeneities in Ziegler-Natta catalyzed linear low density polyethylene

Ethylene / 1-butene copolymers at different comonomer levels were synthesized using Ziegler-Natta catalyst to evaluate the applicability of thermal fractionation methods in predicting chemical composition distribution (CCD). The continuous melting endotherms by DSC were converted to continuous CCD, and the average comonomer contents were compared with NMR results. DSC underestimated comonomer content specifically at higher levels and was more sensitive to the drawn baseline. The thermally fractionated melting endotherms by SSA method were deconvoluted and transformed into discrete CCDs. SSA method underestimated average comonomer content even more, however results were more reproducible. The main shortcoming of the thermal methods was the inability of short ethylene sequences in forming discernible lamella thicknesses at high comonomer levels. Calibration curves were created for converting the predicted comonomer contents into absolute values and used for studying industrial LLDPEs with different comonomer levels. It was shown that SSA provides more reliable results and correlates more reasonably to the measured densities.