Isothermal crystallization in polypropylene/ethylene–octene copolymer blends

Crystallization kinetics of polypropylene (PP)/ethylene–octene copolymer (EOC) blends was measured by differential scanning calorimetry (DSC) and by optical microscopy at various temperatures (123–140 °C). By DSC it was found that small amounts of EOC (10–30%) increase the bulk crystallization kinetics; in some cases even 80% increase was observed. Larger amounts of EOC (50–80%) have caused a decrease in crystallization kinetics. Optical microscopy has revealed that crystallization kinetics of a single spherulite is decreased by the presence of EOC, and at the same time there are more spherulites in the blend. EOC acts as a nucleation agent increasing the bulk crystallization rate and at the same time it acts as an obstacle to the growing front of a spherulite when the PP lamellae have to go around the EOC obstacle. This was confirmed by detailed analyses according to Avrami and Hoffman–Lauritzen. Structure was also observed by transmission electron microscopy (TEM). At low PP content (20%) PP forms very small isolated particles with size smaller than 1 micrometer. At 40% of PP a structure resembling a co-continuous one was found. When the PP is a majority phase (20–40% of EOC) rather large EOC particles were found. TEM also revealed presence of PP lamellae in EOC regions that together with Tg shift of PP and also EOC suggested partial miscibility. While at low temperatures the crystallization is very fast, at higher temperatures there is a competition of phase separation and crystallization.

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► Crystallization kinetics was tremendously decreased when the EOC content was very high (such as 70–80%).
► EOC in small amount acts as a nucleation agent for PP and increases the bulk crystallization rate.
► The analysis according to Avrami's equation revealed that the n exponent and the k rate constant initially increase and then decrease with increasing EOC content.
► The analysis according to Hoffman–Lauritzen theory revealed transitions in crystallization regimes at 130 and 136 °C and the increase in activation energy U*.
► Comparison of crystallization of the blend to pure PP revealed an existence of competition of crystallization with phase separation.