Most aromatic polyimides and copolyimides show a β relaxation process below their α relaxation processes in dynamic mechanical and dielectric measurements. The origin of this β relaxation has been under active discussion. In order to gain a better understanding of the origin of the β relaxation process, the dynamic mechanical behavior of a series of segmented rigid‐rod polyimide and copolyimide films with different dianhydrides and diamines has been systematically studied. For some of the polymers, crystallinity and orientation effects on the β relaxation process were also investigated. It is found that the β relaxation process in the unoriented films can mainly be attributed to a relatively noncooperative motion in uncrystallized diamines. However, this noncooperative nature may be gradually lost by increasing the crystallinity and orientation. Molecular modeling and computation have led to a discussion of possible diamine motion in the β relaxation process.

No abstract

No abstract

A series of poly(enamine ketone)s (PEAKs) which exhibit liquid crystalline phases has been synthesized. The enamine ketone groups are able to exist in the cis and trans conformations. Molecular modelling results show that the cis conformation is energetically more favorable due to intramolecular hydrogen bonding. Model compound studies indicate that the formation of the mesogen requires the cis conformation in the enamine ketones to stabilize the liquid crystalline phase. This liquid crystalline phase is nematic for the model compound and the polymers. The heats of liquid crystal transitions in the model compound and in PEAKs are very close (4.7 kJ/mol in model compound and 4.0 kJ/mol of repeating units in polymers). This is not only an indication of a similar percentage of the cis conformation in both cases, but also an example that the flexible ethyleneoxy spacers in the polymers do not contribute to the orientational order of this liquid crystalline phase. PEAKs can crystallize through either quenching from the isotropic melt or from the liquid crystalline phase. Crystallization kinetics exhibit the effect of pre-ordering when these two different kinetics are compared.