Continuous Cooling Curve Diagrams of Isotactic-Polypropylene/Polyethylene Blends: Mutual Nucleating Effects under Fast Cooling Conditions

Abstract

Crystallization kinetics under processing-relevant cooling conditions of polyethylene/isotactic-polypropylene (PE/PP) blends, with compositions similar to real recycled blends, was investigated through an in situ temperature monitoring technique. Continuous Cooling Curve (CCC) diagrams in a large crystallization temperature range were obtained for the two phases, thanks to a new apparatus allowing fast quenching of polymer samples. Using this technique, we found an "inversion point" in the crystallization order of the two polymers, which arises from the difference in crystallization rates between PP and PE with an increasing cooling rate. The order of crystallization of the two polymers at low cooling rates, that is, before the inversion point, can be tuned by employing non-nucleated or nucleated PP. In particular, non-nucleated PP crystallizes after PE above 0.5 °C/s, while nucleated PP solidifies before PE until 80 °C/s. Interestingly, mutual nucleating effects, previously shown only for ideal systems such as thin-layered films, were found at the interface between the phases, in correlation with the inversion point. During cooling, the phase which crystallizes first can enhance the onset of crystallization of the second lower crystallizing phase through nucleation at the interface. Ex situ morphological and wide-angle X-ray diffraction experiments confirmed the validity of the results shown by CCC diagrams. Our approach to characterize semicrystalline polymer blends with CCC diagrams facilitates tailoring their properties at cooling rates relevant for polymer processing. Moreover, the importance of knowing and controlling the type of components in blends obtained from recycling is demonstrated, given the variety of crystallization behaviors attainable.

Surface Nucleation of Dispersed Polyethylene Droplets in Immiscible Blends Revealed by Polypropylene Matrix Self-Nucleation

“…On the one hand, a decrease of T c was detected for blended PP with respect to neat PP. This phenomenon was already observed in our previous publications, , in which the reason for its occurrence was addressed as migration of some nucleating impurities from the PP phase toward the PE phase during melt-mixing. This explanation is reasonable also for the systems studied here.…”

“…On the other hand, a decrease of T m for both components after melt-blending was detected in all of the investigated compositions, as observed in our previous investigations for similar systems. 21,22 Finally, the shoulder to the main melting peak of neat and blended PP at about 148 °C indicates the presence of a certain amount of β-phase crystals, whose formation is favored at low cooling rates 24,25 and in the absence of impurities with nucleating activity. 21 In Figure S1 in the Supporting Information, representative AFM images of the prepared blends are reported.…”

“…21,22 Finally, the shoulder to the main melting peak of neat and blended PP at about 148 °C indicates the presence of a certain amount of β-phase crystals, whose formation is favored at low cooling rates 24,25 and in the absence of impurities with nucleating activity. 21 In Figure S1 in the Supporting Information, representative AFM images of the prepared blends are reported. PE is the minority phase and is well dispersed in the shape of droplets within the PP matrix for all three compositions.…”

“…This phenomenon was recently found by us on similar systems cooled from the melt at different rates, and it was investigated by morphological analysis. 21 In that report, we hypothesized that the presence or lack of a nucleating effect of the PP phase toward the PE phase depends on the difference in supercooling, i.e., in lamellar thickness, between the two phases in the blend. 28 In fact, a geometrical relationship between the lamellar thickness of PE and PP must be fulfilled for the occurrence of epitaxial growth of one phase on top of the other.…”

“…TCLs of PE formed at the interface with PP domains were already observed for PE/PP blends in previous works. 21,39,40 This peculiar crystalline morphology is formed because the density of nucleation sites is so high that impingements among the growing spherulites occur and, thus, further growth is possible only in the direction perpendicular to the interface. 41−44 Furthermore, the TCL can be identified because the nucleation at the interface is rather different than the nucleation in the bulk, which occurred as well (some lamellar stacks were nucleated in the center of the droplet and grew toward the droplet borders).…”