The Photoinduced Birefringence in Thick Films of Chiral Azodye Copolymers

Abstract

The photoinduced birefringence, Δnind, in 19–35 μm thick films has been studied for two series of chiral photochromic liquid crystalline side chain copolymers. The birefringence measured is induced by excitation of the n‐π* band only. The Δnind value induced in 10 min is linear upon concentration of dye moieties but achieves saturation with increase of illumination intensity. Evolution of the birefringence can be described by double exponent growth; the faster exponent is the same for both copolymer series and should relate to the photoselection process, while the slower one should relate mainly to photo‐orientation and differs strongly dependent on chemical structure of photochromic groups. The different behaviour of two copolymer series during the growth governed by the second exponent is interpreted in terms of the “frictional model”. Due to elevated thickness of the films, rather moderate Δnind values result in huge phase retardation up to 270° and more. The materials are promising therefore for holography applications.

Effect of Molecular Weight on Photoinduced Birefringence in a Chiral Liquid Crystalline Azodye Polymer

“…On the other hand, the higher‐molecular‐weight fractions I and III seem to show a notably slower photoinduced reorientation, as compared with the oligomer fractions V, VII, and the raw polymer. In a previous paper,18 the evolution of the photoinduced birefringence for thin films prepared from the TGB‐like mesophase of several chiral photochromic copolymethacrylates have already been studied. The Δ n ind ( t ) curves obtained are successfully fitted with a biexponential growth function, Equation (1): …”

“…The two observed characteristic relaxation times, namely the fast time of τ 1 ≈ 60–150 s and the slow time of τ 2 ≈ 400–4 000 s, refer to the photoselection and photoreorientation processes, respectively 18…”

“…Transmission spectra of the P8*NN films were recorded with a self‐assembled set‐up based on a one‐beam monochromator MDR23 (LOMO, Russia) 18…”

“…For the measurements, a probe beam from a He/Ne laser at λ = 633 nm was used. The phase retardation was further recalculated to the Δ n ind ( t ) values, as described in detail in our previous publication 18…”

Two-photon-induced birefringence in azo-dye bearing polyimide; the birefringence changes versus the writing power

“…The biexponential kinetics of the writing process has been observed in the functionalized polymer systems containing the azobenzene molecules [5,6,21,22]. However, the authors gave a different attribution of the fast and slow contributions.…”