The investigations have led to a quantitative assessment of both the molecular rigidity and the macroscopic stiffness of a range of rigid-rod polymers. By measuring the persistence length in solution with several techniques, it was conclusively established that the molecular rigidity increases in the order polyester (PES1.1/R PES1.3/R) polyimide, poly(p-phenylene) (PPP3/R). The results with respect to the actually achieved experimental moduli showed main-chain values (corrected for the side-chains) around 150 GPa, without a specific relation with the anticipated molecular rigidity. An increasing discrepancy is observed between the actually achieved main-chain moduli and the theoretically predicted values in the order PES1.1/R PES1.3/R PES3.3/R PES3.5/R PPP3/R, which could mainly be ascribed to increasing processing difficulties. Consequently, a higher rigidity of the molecular chain does not unambiguously result in proportionally higher macroscopic main-chain moduli. On the other hand, the orientation process as observed in the drawing of films, turned out to be highly efficient for the polymers under investigation. Optimized rigid-rod polymers with flexible side chains might therefore present a unique combination of processing and performance characteristics.
The development of melt-processable liquid crystamers has received a great deal of attention in the past few years. Melt processability is usually attained by disrupting the regular structure of the rigid main chain by means of random copolymerization and/or the introduction of crankshafts or kinks. The copolymeric nature of the present LCPs inhibits a full understanding of the relation between the chain parameters and the macroscopic properties and as such hampers the further technical improvement of thpresent LCPs.
In this project optimization of macroscopic (mechanical) properties is pursued by retention and systematic variation of very high rigidity in the main chain. The polymers to be developed will all be based on the rigid phenylene group. These groups can either be polymerized directly, or with the aid of various linking groups (ester, amide, imide). With proper attachment of (small) side chains during the designing of the synthesis routes, a sequence of melt-processable polymers with distinct, very high cha rigidities results. These polymers will be characterized thoroughly, both with respect to the molecular (i.e. chain flexibility) and the macroscopic (mechanical thermal) properties. The results of this project may be applied to the improvement of present commercial LCPs, and to the development of new, innovative materials.