Evaluation of the comonomer distribution in ethylene copolymers using DSC fractionation

Abstract

The microstructures of ethylene random copolymers are evaluated using DSC fractionation by the degree of crystallinity. This method is based on the same principle of separation as temperature-rising elution fractionation (TREF) which uses a crystallization/dissolution procedure; however it does not physically separate the fractions. The resulting copolymers are sorted by crystallite sizes; the crystallite size and lamellae thickness are directly related to the ethylene block length. Copolymers including E/VA, E/nBA, E/nBA/CO, E/nBA/VA, E/MAA, E/AA and LLDPE have been analyzed by DSC fractionation to elucidate structural differences. We see that (a) specimens with high comonomer content are less crystalline; the ethylene segments are shorter, (b) the comonomer type makes a difference in fractionation if H-bonding is involved, (c) the narrow comonomer distribution, i.e. narrow ethylene-segment-length distribution, has fewer DSC fractions; the broad distribution has more fractions, (d) fractionation by crystallinity is affected if the molecular weight/viscosity is very high, (e) branch content reduces the crystallinity and shortens the ethylene blocks, and (f) there is no direct relation to molecular weight distribution or comonomer content.