Splitting of DTA‐peaks in polyethylene and its relation to the crystalline texture

Linear polyethylene crystallized from the melt or heat‐treated at temperature above 110°C. shows in the DTA thermogram an additional endothermal maximum Tm1 at a temperature lower by 10 to 20°C. compared with the normal maximum Tm0. The appearanee of Tm1 depends on the crystallization temperature, time, and temperature of heat‐treatment. For a constant crystallization time, the difference between Tm0 and Tm1 decreases with increasing of the crystallization temperature; at a constant crystallization temperature it increases with the crystallization time. By treating those specimen with two endothermal maxima at a temperature near that of Tm1, Tm1 splits into two peaks. This effect is interpreted by the assumption, that those crystalline regions which first correspond to the Tm1 maximum, partly form more stable crystals of greater thickness and higher melting point, and partly melt with subsequent formation of little crystallites of lower melting point during cooling. Under suitable conditions, the three endothermic peaks also can be observed with branched polyethylene.