Effects of chain defects on the thermal behaviour of polyethylene

Abstract

The annealing behaviour and thermodynamics of fusion of mats of solution-grown single crystals of ethylene-propylene and ethylene/1-butene crystallizable random copolymers are examined in comparison with linear polyethylene crystallized from the melt and from dilute solution. Correlation of quantities such as long spacing, annealing temperature, and apparent enthalpy of fusion, leads to the conclusion that the single crystal aggregates are involved in a morphological transformation during the process of annealing. The long spacing and the annealing temperature corresponding to this transformation decrease with the degree of constitutional defects present in the polyethylenic chain (percentage of propylene or 1-butene comonomeric units). Data in the plots of melting temperatures versus the reciprocal of the long spacing can be represented by straight lines. The thermodynamic equilibrium melting temperature T° m and the surface free energy σ e have been determined. Both T° m and σ e depend upon the number of constitutional defects in the chain. In the case of ethylene-propylene random copolymers σ e increases drastically with the percentage of CH 3 side groups. A completely opposite effect is observed in the case of ethylene/1-butene random copolymers; here decrease in σ e with the percentage of CH 3CH 2 side groups is observed. Both classes of polymers show a lowering in T° m with the number of side groups, though this effect is much more pronounced in the ethylene-propylene copolymers. The apparent enthalpy of fusion of annealed single crystal aggregates is not in linear relation with the reciprocal of the long spacing and with the corresponding melting temperature. Some consideration on the distribution of the defects in the crystals is also reported.