Equilibrium melting temperature of polyethylene with finite chain length

An attempt is made to determine the equilibrium melting temperature (T ) of polyethylenes with finite chain length, using the data on the observed melting temperature (Tm) and the heat of fusion (ΔH) of the samples annealed isothermally. Five fractions with the viscosity‐average molecular weight (M̄v) ranging from 2.8 · 104 to 3.80 · 106 were annealed in a Perkin‐Elmer differential scanning calorimeter at the different temperatures for various periods, and then heated up without cooling. For the fractions of low and moderate molecular weights, the partial melting was very rapid and the change in the DSC thermogram during annealing was mainly attributed to the partial melting followed by the subsequent recrystallization. Thus, T can be evaluated from the plot of Tm against ΔH by the following relation: , where t represents annealing time. Figures obtained for T are 142.0 ± 0.5°C and 139.5 ± 0.5°C for the fractions of M̄v = 2.10·105 and 2.8·104, respectively. These values are in good agreement with those calculated from the BROADHURST equation. On the other hand, the isothermal melting of the higher molecular weight fractions was slow and the recrystallization was not observable, implying that this method is inapplicable to these fractions.