Molecular Ordering and Molecular Dynamics in Isotactic-Polypropylene Characterized by Solid State NMR

Abstract

The order-disorder phenomenon of local packing structures, space heterogeneity, and molecular dynamics and average lamellar thickness, <l>, of the alpha form of isotactic polypropylene (iPP) crystallized at various supercooling temperatures, DeltaT, are investigated by solid-state (SS) NMR and SAXS, respectively. <l> increases with lowering DeltaT, and extrapolations of <l>(-1) versus averaged melting point, <T(m)>, gives an equilibrium melting temperature, T(m)(0) = 457 +/- 4 K. High-power TPPM decoupling with a field strength of 110 kHz extremely improves (13)C high-resolution SS-NMR spectral resolution of the ordered crystalline signals at various DeltaT. A high-resolution (13)C SS-NMR spectrum combined with a conventional spin-lattice relaxation time in the rotating frame (T(1rhoH)) 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, f(order), increases with lowering DeltaT and reaches a maximum value of 62% at DeltaT = 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 theta axis under a hypothesis of adjacent re-entry structures. It is also found that f(order) significantly increases prior to enhancement of lamellar thickness. Additionally, annealing experiments indicate that <l> 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 DeltaT. These dynamic constraints are interpreted in terms of increment of <l> 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.