Effect of Annealing Rate on Low-Temperature Impact Strength and Space Charge Characteristics of Isotactic Polypropylene

Isotactic polypropylene (iPP) is one of the most promising substitutes for XLPE material, which has attracted wide attention in recent years. Hot melting and annealing processes have great influence on the crystallization characteristics of iPP. To study the effect of annealing rate on iPP property, three kinds of non-isothermal crystallization samples with different annealing rates, namely iPP-Ice (ice water cooling), iPP-Air (room-temperature cooling), and iPP-Slow (slow cooling), were prepared, and their properties were studied by thermal analysis, low-temperature impact testing system, and pulsed electroacoustic method. The results show that with the increase of cooling rate, the crystallization temperature of iPP increased, while the melting point decreased. In contrast, the change of crystallinity increases first and then decreases. The 50% brittleness temperatures of iPP-Ice, iPP-Air, and iPP-Slow are 0.3
$^{\circ}\mathrm{C}, \boldsymbol{-5.6}^{\circ}\mathrm{C}$
, and
$-7.5^{\mathrm{Q}}\mathrm{C}$
, respectively. Among three groups of iPP samples, iPP-Ice has the most obvious space charge accumulation, the largest electric field distortion rate, and the lowest carrier mobility during depolarization. The accumulation degree of space charge in iPP material is positively correlated with the interface area of semi-crystalline polymer crystal region/amorphous region.