Pronouncedly elevated impact toughness of isotactic polypropylene upon annealing realized by introducing alkyl-terminated hyperbranched polyester

The promoted mobility of chain segments in the amorphous region of isotactic polypropylene (iPP) is beneficial to the improvement of its impact toughness. In the present work, two kinds of hyperbranched polyesters with different lengths of alkyl-terminated groups (octadecyl-terminated hyperbranched polyester HBP-C18, and hexyl-terminated hyperbranched polyester HBP-C6) were incorporated into iPP through melt-blending, respectively. Then the blends were annealed at different temperatures. Polarized optical microscopy and dynamic rheology results reveal that HBP-C18 only performs the function of lubricant on iPP. As a consequence, the incorporation of HBP-C18 weakens the impact strength of iPP, and further annealing treatment results in sharp depression of the impact toughness of iPP/HBP-C18 blend. On the contrary, dramatically enhanced impact toughness of iPP could be achieved by the incorporation of HBP-C6 and further annealing treatment. After being annealed at 120 °C for 3 h, the impact strength of iPP/HBP-C6 reaches 16.0 kJ m⁻², 119% higher than that of its unannealed counterpart, almost 186% higher than that of annealed iPP, and even elevated by 400% in contrast to that of unannealed iPP. Introducing a small amount of HBP-C6 with dual roles of plasticizer and pore-forming agent as well as further annealing treatment not only promote prominently the mobility of chain segments in the amorphous region of the iPP matrix, but also induce the formation of massive microvoids, giving rise to intense plastic deformation under the impact load condition, and therefore pronouncedly elevated impact toughness of iPP could be obtained. This work could provide a novel and effective strategy for iPP toughening.

Graphical abstract