Structural Ensemble of Molecular Chains in Isotactic Polypropylene under Cylindrical Confinement

Abstract

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: αand γ-phase crystals. Samples synthesized with a metallocene catalyst yield the γ-phases. We found the integrated assembly of the threefold helical chains in both αand γ-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 γphase crystal in which the c γ -axis is consistent with the radial or axial direction under cylindrical confinement. The formation mechanism for the radial γ-phase crystals lies in that the rapid growth direction of the α-phase lamellar from which the γ-phase crystals are epitaxially grown is kinetically favored to be consistent with the rod long axis. More interestingly, the new axial γ-phase crystal reflects a complex, multi-fold structural ensemble of threefold helices in two phases via epitaxial relationships.