Nonisothermal crystallization kinetics of linear metallocene polyethylenes

The effect of weight-average molecular weight (M_w) on the nonisothermal crystallization kinetics of linear metallocene polyethylene (m-PE) was studied with modulated differential scanning calorimetry. Six linear m-PEs of molecular weights in the range 122-934 kg/mol were prepared by gas-phase polymerization. The cooling rate (R) was varied in the range 2-20°C/min, and it significantly affected the crystallization behavior. M_w had a weak influence on both the peak crystallization temperature and the crystallization onset temperature. All m-PEs showed primary and secondary crystallizations. At both low and high R's, the crystallinity showed a significant drop (∼ 30%) when M_w was increased from 122 to 934 kg/mol. At low R's (< 10°C/min), the rate parameters in the modified Avrami method [primary rate constant (k_R)] and Mo method [F(T)] of analyses agreed in suggesting that an increased M_w slowed the rate of crystallization. The M_w dependency of kR followed the Arrhenius type (k_R = k_Roe²⁸¹ / M_w, where k _Ro is a rate-dependent constant). However, at higher R's, k _R approached a constant value. The order parameters obtained by the different methods of analysis (n and a) were independent of M_w, which suggests that the crystal type remained the same. Hoffman-Lauritzen theory was used for data analysis, and activation energy per segment showed a significant decrease, from 225.0 to 11.8 kj/mol, when M_w was increased from 152 to 934 kg/mol. Finally, all methods of analysis suggested a significant effect of M_w on slowing the overall crystallization process.