In this study, the effects of crystallization conditions (cooling rate and end temperature of cooling) on crystallization behavior and polymorphic composition of isotactic polypropylene/multi-walled carbon nanotubes (iPP/MWCNTs) composites nucleated with different concentrations of β-nucleating agent (tradename TMB-5) were investigated by differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD) and scanning electronic microscopy (SEM). The results of DSC, WAXD and SEM revealed that the addition of MWCNTs and TMB-5 evidently elevates crystallization temperatures and significantly decreases the crystal sizes of iPP. Because of the competition between α-nucleation (provided by MWCNTs) and β-nucleation (induced by TMB-5), the β-phase crystallization takes place only when 0.15 wt% and higher concentration of TMB-5 is added. Non-isothermal crystallization kinetics study showed that the crystallization activation energy ΔE of β-nucleated iPP/MWCNTs composites is obviously higher than that of pure iPP, which slightly increases with the increase of TMB-5 concentration, accompanying with the transition of its polymorphic crystallization behavior. The results of non-isothermal crystallization and melting behavior suggested that the cooling rate and end temperature of cooling (T end ) are important factors in determining the proportion and thermal stability of β-phase: Lower cooling rate favors the formation of less amount of β-phase with higher thermal stability, while higher cooling rate encourages the formation of higher proportion of β-phase with lower thermal stability. The T end = 100°C can eliminate the β-α recrystallization during the subsequent heating and therefore enhance the thermal stability of the β-phase. By properly selecting TMB-5 concentration, cooling rate and T end , high β-phase proportion of 88.9% of the sample was obtained.

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