Semi-empirical calculations and dielectric spectrometry of molecular unitsin PEEK

Semi-empirical molecular orbital calculations and dielectric measurements h ave been conducted on DPE and DPK molecules and on PEEK films containing th ese molecular units. The calculations were performed using a modified-CNDO/ 2 method allowing us to take into account the van der Waals impenetrability zone of the electronic corona of the aromatic rings. From the analysis of the relative energy maps, it is shown, amongst other things, that the most probable route for a molecule to move is through gear rotations. The DPE mo lecule is also found to move more easily than the DPK molecule. Measurement s of the dielectric permittivity and of the refractive index were not possi ble for the DPE molecule in benzene solution due to its low dipole moment, this being confirmed by theoretical calculations. For the DPK molecule, an experimental mean value of 3.15 D is found for mu compared to a calculated value of 3.35 D. The analysis of the dissipation factor isochronal curves s hows that the activation energy for the DPK molecule is largely dependent o n its concentration in benzene. In the case of PEEK, two relaxations are se en at - 106 degrees C and - 73 degrees C for a frequency of 1 kHz. A parall el is drawn between isolated molecules of DPK in solution and DPK units tha t are distant from each other in a polymer chain, and between aggregated mo lecules of DPK in solution and DPK units that are close together in entangl ed chains. It is finally shown that the DPK molecular unit in a macromolecu lar chain is only able to reach the permitted conformations in low energy v alleys, corresponding to gear rotation motions. (C) 1999 Published by Elsev ier Science Ltd. All rights reserved.