High-pressure crystallization of iPP nanocomposites with montmorillonite and carbon nanotubes

High pressure facilitates crystallization of isotactic polypropylene (iPP) in the orthorhombic γ-form, differing in structure and properties from the α-form, in which iPP crystallizes under common processing conditions. In the study, the effect of high pressure on crystallization of iPP nanocomposites was examined. The iPP nanocomposites with 1-5 wt.% of well dispersed either o-MMT (PP/MT) or MWCNT (PP/CN) were prepared by mixing molten iPP with the nanofillers. The nanocomposites and neat iPP were crystallized during cooling under pressures up to 300 MPa. Regardless of crystallization pressure, the crystallization temperatures were similar whereas the sizes of the polycrystalline aggregates were only slightly smaller in PP/MT compared to those of neat iPP. On the contrary, the presence of MWCNT elevated the crystallization temperature of PP/CN by 8-13 K compared to that of neat iPP, and strongly decreased the grain size. This evidenced the nucleating activity of MWCNT during high-pressure crystallization of iPP in the γ-form. The higher crystallization temperature allowed a larger portion of the polymer to crystallize in the γ-domain before the α-domain was reached. As a result, PP/CN, especially crystallized under 50-100 MPa, contained more γ-form than neat iPP and PP/MT.