Poly(diphenylacetylene) bearing long alkyl side chain via silylene linkage: Its lyotropic liquid crystallinity and optical anisotropy

The liquid crystalline properties and optical anisotropy of poly(diphenylacetylene) bearing an n-octadecylsilane moiety in the side chain, poly(1-phenyl-2-p-{dimethyl-n-octadecylsilylphenyl}acetylene) (C18-PDPA), were investigated in detail. C18-PDPA exhibited unexpected smectic phase liquid crystallinity in highly concentrated aromatic organic solvents such as toluene, despite its extremely high molecular weight of 4.18 × 10⁶ g/mol and a fairly wide polydispersity index of 2.5. This lyotropic liquid crystallinity was derived from: a high molecular stiffness (expected from the high viscosity index of a α ≈ 1.03), a high degree of intermolecular packing (the fractional free volume ≈ 0.16), and ultrahigh molecular weight. The two major absorption bands, located at 430 and 370 nm, are attributed to the π-π* transition parallel to the main chain and the localized π-π* transition with a charge-transfer characteristic among mesogenic repeating units perpendicular to the main chain axis, respectively. C18-PDPA exhibited highly polarized absorption and fluorescence bands in a sheared film. The main chain axis of the polymer was aligned parallel to the shearing direction, whereas the long axis of the stilbene-like side group was perpendicular to the shearing direction.