Non-isothermal Crystallization Kinetics of Linear Metallocene Polyethylenes

The effect of Mw on the non-isothermal crystallization kinetics of metallocene linear polyethylene (m-PE) was studied using 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 was varied in the range 2–20C/min and it has significantly affected the crystallization behavior. The Mw had a weak influence on both the peak crystallization temperature, and the crystallization onset temperature. All m-PEs showed primary and secondary crystallizations. Both at low and high cooling rates, crystallinity showed significant drop (~30%) when Mw was increased from 122 to 934 kg/mol. At low cooling rates (<10C/min), rate parameters in modified Avrami (kR) and Mo [F(T)] methods of analyses agree in suggesting that increased Mw slows the rate of crystallization. The Mw dependency of the primary rate constant, kR, followed Arrhenius type (kR = kRo e , where kRo is rate dependent). However, at higher cooling rates kR approached a constant value. The order parameters obtained by the different methods of analyses (n and α) were independent of Mw suggesting that the crystal type remains the same. Hoffman-Lauritzen theory was used for data analysis and U* showed a significant decrease from 225.0 to 11.8 kJ/mol when the Mw was increased from 152 to 934 kg/mol. Finally, all methods of analysis suggest the significant effect of Mw on slowing the overall crystallization process.