Simultaneous measurement of normal force and electrical resistance during isothermal crystallization for carbon black filled high-density polyethylene

The kinetics of isothermal crystallization for high-density polyethylene (HDPE) containing different volume fraction (Φ) of carbon black (CB) have been evaluated by using differential scanning calorimetry (DSC) at 123, 124, and 125 °C where the rate of crystallization is moderate. Simultaneous measurement of normal force (F_N) and electrical resistance (R) has been performed to probe the process of isothermal crystallization at strain zero. Results reveal that, at the early stage of crystallization, F_N is almost independent of time (t) while relative resistance (R/R₀) changes slightly with increasing time t. However, a significant increment in F_N and a remarkable change in R/R₀ can be observed at the same critical time (t_c), and the value of t_c is dependent on the crystallization temperature and CB content, which is available for describing the isothermal crystallization as a characteristic parameter. It is found that t_c is greater than induction time of crystallization (t₀) due to the less sensitivity of mechanical and electrical responses than enthalpy to the structural changes in the composites. It is suggested that mechanical and electrical simultaneous measurement endows us a novel approach to probing the formation of percolation network involving in crystallization of polymer matrix.