Rheological evidence for high-temperature phase transitions in melts of high-density polyethylene

Melts of commercial high-density polyethylene (M̄_w = 36–85 × 10³) were subjected to the hydrodynamics of a Haake Rheocord blender at temperatures (T) spanning the normal processing range (190–260°C), far beyond the disappearance of solid crystals at about 140°C. Torque measurements revealed a peculiar dependence on T, including transitions at about 208°C and 227°C, and non-Arrhenius behavior above and below this range. Resemblances to liquid-crystal polymer phenomena are pointed out, and the impact on the melt processing industries is discussed.