Design, Preparation and Characterization of Thermotropic Liquid Crystal Polyamides

Abstract

There are few examples of thermotropic polyamides reported in the literature due to the strong tendency for hydrogen bonding between polymer chains. The vast majority of reported liquid crystalline (LC) polyamides are aromatic, highly crystalline and consequently infusible— and can only exhibit LC behaviour in solution (lyotropic behaviour). In this paper, the authors set out to synthesize novel amorphous wholly aromatic and semi-aromatic thermotropic polyamides. As crystalline aromatic polyamides are normally infusible, it was necessary to ensure that the target polyamides were amorphous with a definable softening point below their temperature of decomposition. The ‘Symmetry Index’ approach first developed by Dolden was used to ensure that the chosen monomer compositions were able to produce amorphous polyamides. Aharoni has reported that three amide linked aromatic rings is the minimum unit size needed to obtain mesogenic polyamides. Building on this principle, a new empirical predictive technique called the ‘Mesogenic Index’ is introduced and combined with the Symmetry Index to predict polyamides which are both amorphous and thermotropic. This approach was validated by the preparation of a whole new series of thermotropic amorphous polyamides which were based on 3,3′-dimethoxy benzidine, and the acid chlorides of terephthalic, isophthalic and adipic acids. Furthermore, this led on to the preparation of two more series of amorphous polyamides, believed by the authors to be the first wholly aromatic thermotropic polyamides to be reported, based on 3,3′-dimethoxy and 3,3′-dimethyl benzidine in conjunction with a variety of aromatic and cyclic diacids.