Unique crystallization behaviors of isotactic polypropylene in the presence of MWCNT supported β nucleating agent: Lower temperature T(αβ)-T(βα) interval and fast cooling preferred formation of β crystals

- β nucleating agent for iPP is grafted onto the surface of MWCNTs through chemical bonding.
- The content of β crystals in iPP composite containing MWCNT-NA increase with increasing cooling rates.
- The α and β nucleation competition becomes more intensive due to the dual nucleating ability of MWCNT-NA.

Simultaneous improvement of stiffness and toughness of isotactic polypropylene (iPP) has caught tremendous effort for decades and previously we have shown that the modification of iPP with combined nanofillers and β-nucleating agent (β-NA) is an efficacious way to balance its stiffness and toughness. However, simultaneous introduction of reinforcing fillers and β-NA generates nucleation competition in the crystallization process and generally the formation of β crystals is depressed, while the nucleation competition induced by β-NA grafted onto the surface of reinforcing fillers was not clear yet. In this work, a kind of novel β-NA for iPP, N,N'-dicyclohexyl-1,5-diamino-2,6-naphthalenedi-carboxamide, was supported onto the surface of multi-walled carbon nanotubes (MWCNTs) through chemical bonds. The α and β nucleating competition among iPP nucleated by β-NA, physically mixed MWCNTs and β-NA (MWCNT + NA), and MWCNT supported β-NA (MWCNT-NA) was systematically investigated. Interestingly, in the presence of MWCNT-NA, iPP showed unique crystallization behaviors. The nucleation competition between α- and β-crystals became intensified and showed temperature dependence, and the relative content of β-crystals increased with increasing cooling rate, which is opposite to the general understanding of crystallization behaviors of semicrystalline polymers, but is highly desired in practical productions. A possible interpretation for this unusual crystallization behavior of iPP is provided.