Structural Ensemble of Molecular Chains in Isotactic Polypropylene under Cylindrical Confinement

epitaxial Helical chain assembly has a great influence on the physical properties of polymeric materials. It has been known that the newly developed, anisotropic nanostructure of polymers in nanoconfinement is significantly different from that in the bulk. In this research, we demonstrate 3(1) helical chain assembly in isotactic polypropylene (i-PP) under 2D confinement in two main crystalline phases: alpha- and gamma-phase crystals. Samples synthesized with a metallocene catalyst yield the.-phases. We found the integrated assembly of the threefold helical chains in both alpha- and gamma-phase crystals under confinement. The helical chain packing at the four-methyl growth face prefers to be aligned with the rod long axis. Furthermore, we demonstrate two unit-cell orientations of the gamma-phase crystal in which the c(gamma)-axis is consistent with the radial or axial direction under cylindrical confinement. The formation mechanism for the radial gamma-phase crystals lies in that the rapid growth direction of the alpha-phase lamellar from which the gamma-phase crystals are epitaxially grown is kinetically favored to be consistent with the rod long axis. More interestingly, the new axial gamma-phase crystal reflects a complex, multi-fold structural ensemble of threefold helices in two phases via epitaxial relationships.

Automated summary

The research demonstrates the presence of three-dimensional helical chain assembly in isotactic polypropylene under 2D confinement, with integrated assembly observed in both alpha- and gamma-phase crystals. Additionally, two unit-cell orientations of the gamma-phase crystal are shown under cylindrical confinement. The formation mechanism for the radial gamma-phase crystals is explained by the kinetic favorability of alignment with the rod long axis.