Crystallization and melting of propylene-ethylene random copolymers. Homogeneous nucleation and β-nucleating agents

The different crystallization behavior of two propylene-ethylene random copolymers, (PP-R and PP-RCT), one of which (PP-RCT) contains a β-nucleating agent is investigated in the present paper. Isothermal and non-isothermal crystallization behavior was thoroughly studied. The subsequent melting behavior after isothermal crystallization was studied by means of standard and Step-Scan DSC. The equilibrium melting point of α crystals determined using the Hoffman-Weeks extrapolation was 11.8 °C higher than that for β crystals. The Lauritzen-Hoffman theory was applied and the nucleation constant K_g and the surface free energy σ_e was calculated. From the isothermal experiments it was concluded that the nucleating agent increased drastically the crystallization rate of the copolymer, also promoting the generation of β crystals. On cooling PP-RCT crystallized at about 8 °C higher temperatures than PP-R. The non-isothermal crystallization of the materials was examined and for the kinetics analysis the Avrami, Ozawa and Mo models were elaborated. Nucleation activity was calculated with the use of Dobreva's method. The effective activation energy was calculated for both polymers according to the iso-conversional method of Friedman and lower values resulted as expected in case of the PP copolymer with nucleating agent.