Molecular-Optical Anisotropy of Poly(Phenylquinoxaline) Thin Films with in-plane Orientation of Macromolecules

Abstract
For the uniaxial thin films of two poly(phenylquinoxaline) derivatives with in-plane orientation of macromolecules, the relationship between refractive indices n_j and polarizability parameters Γ {components γ_j, mean value γ= (γ_|| + 2γ_⊥)/3, anisotropy δγ = γ_|| - γ_⊥} of the biaxial monomer unit of the polymer chain for the light-wave polarization along (j = ||) and across (j = ⊥) the optical axis which is perpendicular to the film plane, was investigated. In the local-field tensor components f_j =1+L_j(n_j²–1), the Lorentz-tensor components values (L_j) were obtained using the experimental dependences of n_j(λ) on the light wavelength λ. The model tensors f_j, which are widely used for polymer films, lead to essentially understated values of δγ and distortion of the dispersion Γ(λ) for the investigated films with low birefringence Δn = n_||–n_⊥. It was observed for the first time, that for biaxial monomer units with two parameters of uniaxial orientational order (S, G) and two types of the polarizability anisotropy (Δγ, Δγ') the relation Delta;n ∝ δγ = SΔγ + GΔγ'/2 can lead to the equality Δn = 0 at S ≠ 0, G ≠ 0. Molecular and structural peculiarities required for realization of the film state with Δn = 0 were established.

Keywords: optical properties, thin films, poly(phenylquinoxaline) derivatives, molecular polarizability