CNDO CALCULATIONS FOR THE ROTATION OF AROMATIC RINGS IN MODEL ANALOGS OF PEEK

Rotational motions of the phenylene rings in PEEK were examined by means of semiempirical CNDO (complete neglect of differential overlap) molecular-orbital calculations for various model compounds. Results indicated that the ether-ether phenylene ring tends to stay coplanar with, whereas the ether-ketone ring prefers to lie vertical to, the plane of the backbone zigzag. This preferred arrangement results in low intramolecular hindrances for medium- to large-amplitude phenylene rotations or torsional motions of virtual bonds in amorphous PEEK. Upon crystallization, the phenylene rings give away their preferred angular positions and choose to incline by a low angle from the plane of the backbone zigzag in an alternating manner (as previous X-ray diffraction results dictate) to achieve efficient packing. The tight packing subsequently results in strong intermolecular interactions which, in combination with increased intramolecular hindrance (from stronger ring-ring repulsion at the low tilt angle), render motion of the phenylene rings restricted as observed in earlier solid-state NMR studies. Crankshaft types of chain motion are found consistent with the dielectrically determined activation energy for the gamma-relaxation of PEEK.