Molecular ordering and molecular dynamics in isotactic-polypropylene characterized by solid state NMR

Toshikazu Miyoshi, The University of Akron

Abstract

The order−disorder phenomenon of local packing structures, space heterogeneity, and molecular dynamics and average lamellar thickness, , of the α form of isotactic polypropylene (iPP) crystallized at various supercooling temperatures, ΔT, are investigated by solid-state (SS) NMR and SAXS, respectively. increases with lowering ΔT, and extrapolations of −1 versus averaged melting point, , gives an equilibrium melting temperature, Tm0 = 457 ± 4 K. High-power TPPM decoupling with a field strength of 110 kHz extremely improves 13C high-resolution SS-NMR spectral resolution of the ordered crystalline signals at various ΔT. A high-resolution 13C SS-NMR spectrum combined with a conventional spin−lattice relaxation time in the rotating frame (T1ρH) filter easily accesses an order−disorder phenomenon for upward and downward orientations of stems and their packing in the crystalline region. It is found that ordered packing fraction, forder, increases with lowering ΔT and reaches a maximum value of 62% at ΔT = 34 K. The ordering phenomenon of stem packing indicates that chain-folding direction changes from random in the disordered packing to order in the ordered packing along the a sin θ axis under a hypothesis of adjacent re-entry structures. It is also found that forder significantly increases prior to enhancement of lamellar thickness. Additionally, annealing experiments indicate that is significantly enhanced after a simultaneous process of partial melting and recrystallization/reorganization into the ordered packing at annealing temperature ≥423 K. Furthermore, the center-bands only detection of exchange (CODEX) NMR method demonstrates that time−kinetic parameters of helical jump motions are highly influenced by ΔT. These dynamic constraints are interpreted in terms of increment of and packing ordering. Through these new results related to molecular structures and dynamics, roles of polymer chain trajectory and molecular dynamics for the lamellar thickening process are discussed.