Morphology and strength of high performance main-chain liquid crystal polymers

The impact of chain-length on formation and properties of thermotropic LC-phases is described for poly(didocecyl-p-phenylene). Isotropic, biphasic or anisotropic melts are observed. The zero-shear viscosity scales as η≈︁M_w^2.9 in the isotropic and as η=M_w^3.2 in the anisotropic melt. If quenched from the LC-melt, the material exhibits a smectic superstructure dominated by disclinations. The same feature is seen with the poly(diacetylene) P-4BCMU crystallized from lyotropic solution. The LC-phase consists of lamellae the thickness of which scales with the extended chain length. Segregation of the chains allows the lamellae to adapt to the strain field in the vicinity of the core of the disclinations. A banded texture observed when the LC-phase is subject to shear is resolved by TEM as composed of periodically distorted stacks of lamellae. Small modifications of the chain structure by comonomers has a substantial impact on the mechanical properties. Introducing only 5 mol percent of comonomers with sterically assuming substituents into lyotropic poly(p-phenylene terephthalamide) improves the compressional strength (Elastica test) of fibers by 25 percent.