LATTICE-DYNAMIC PREDICTION OF THE LIMITING YOUNG MODULUS OF LIQUID-CRYSTALLINE ARYLATE POLYMERS - COMPARISON WITH TYPICAL RIGID-ROD POLYMERS

Based on the lattice dynamical equations, the theoretical Young's modulus E(l) has been calculated for a series of arylate polyesters: [GRAPHICS] The calculated results have been interpreted in connection with the mechanical deformation mechanism of the chains based on the concept of strain energy distribution to the internal displacement coordinates such as bond lengths, bond angles and internal rotations. Although the arylate polyesters are constructed by a series connection of rigid benzene and ester groups, the calculated moduli are only one third of the modulus predicted for a completely linear polymer chain such as poly-p-phenylene. This originates from the characteristic structure of the arylate polymers. The straight aromatic segments are tilted from the chain axis due to the existence of the ester group and, therefore, the strain energy concentrates mainly on the weak bond angle deformation of the ester groups and distributes only slightly to the benzene ring deformation. That is, the rigidity of the aromatic rings has been found not to be used as effectively as expected from the chemical structures. Comparison of the Young's modulus and molecular deformation mechanism has been made between the present liquid crystalline polymers and polymers with various types of chain conformation, including poly-p-phenylene terephthalamide (calc. modulus 182 GPa, X-ray obs. 200 GPa) and poly-p-phenylene benzobisthiazole (calc. 392 GPa, X-ray obs. 395 GPa).