Melting behaviors of polyethylene crystals: An application of fast-scan DSC

Melting behaviors of linear polyethylene (PE) have been experimentally examined by fast-scan and conventional differential scanning calorimetry (DSC). By fast-scan heating up to 104 K s−1, the melting peak temperature, Tpeak, is free from the influences of re-organization and re-crystallization and follows the shift only with superheating, which has been evaluated by a power-law behavior. The melting point, TM, of the chain-folded crystals has been determined as Tpeak extrapolated to zero heating rate without the influences of non-equilibrium effects. The Hoffman–Weeks plot of TM against crystallization temperature, Tc, suggests the equilibrium melting point T M 0 = 141.1 ° C of the linear PE having Mw = 52.0 × 103 and the doubling of lamellar thickness concluded from the linear relationship between TM and Tc with the slope close to 1/2. While the lamellar doubling is a well-known behavior of PE crystallized from the melt, the present results with Tc down to ∼100 °C suggest the doubling completed in a quite short time interval. The value of T M 0 = 140.4 ° C has also been obtained by the Gibbs–Thomson plot of TM against the inverse of lamellar thickness, (dc)−1, determined by small angle X-ray scattering. The agreement of the results within ±1 °C suggests the Hoffman–Weeks plot as an appropriate method for the melting point determination with fast-scan heating, only with which the determination procedure is free from re-organization and re-crystallization. The consistent behavior has also been confirmed in terms of the end-surface free energy, σe, determined by the Gibbs–Thomson plot and by the growth kinetics of crystallization.