Light induced structures in liquid crystalline side-chain polymers with azobenzene functional groups

Abstract

We applied ellipsometry to study the distribution of azobenzene fragments in the films of two types of comblike polymers with azobenzene moieties in the side chains before and after ultraviolet (UV) light irradiation. The polymer with an alkyl chain at the end of the azobenzene fragment forms structures with preferred homeotropic alignment of the fragments. Irradiation of this polymer with nonpolarized UV light at normal incidence induces a homeotropic alignment of azobenzene fragments. Oblique irradiation induces tilted structures. Polarized UV light irradiation at normal beam incidence induces biaxial structures with a fanlike distribution of azobenzene fragments and preferably out-of-plane alignment. The polymer with polar nitro group at the end of the azobenzene moiety shows preferential in-plane orientation. The degenerate in-plane alignment is retained for normal irradiation with nonpolarized UV light. Excitation with polarized light provides highly ordered in-plane alignment of the azobenzene fragments perpendicular to the UV light polarization. Re-orientation of the fragments under a second UV exposure with orthogonal polarization involves out-of-plane rotation of the fragments.

Photoinduced three-dimensional orientational order in side chain liquid crystalline azopolymers

“…The stable POA was observed later on in polymers containing chemically linked azochromophores (azopolymers) [4]. It turned out that stable anisotropy can be induced in both amorphous and liquid crystalline (LC) azopolymers [4,5,6,7,8,9,10]. The efficiency of POA in LC azopolymers is generally much higher than it is for amorphous homologues.…”

“…[18] was found to be an experimental technique particularly suitable to characterize light induced anisotropy of orientational structures in azopolymer films. We have investigated these structures depending on the polarization state of the actinic light [10] and on the molecular constitution [15]. It was additionally found that the kinetics of POA involving both uniaxial and biaxial structures can be theoretically described by using the phenomenological approach suggested in Ref.…”

Kinetics of photoinduced anisotropy in azopolymers: models and mechanisms

“…We use the results of general analysis to formulate a simple model of the photoinduced ordering in LC azopolymers. In addition to the long term stability of POA, this model accounts for biaxiality of the photoinduced orientational structures [4,17,18,19,20,21] that was neglected in the models of Refs. [7,8,10,11].…”

Azobenzene Polymers for Photonic Applications

“…The director of a liquid crystal phase can be modified by orienting chromophores doped into the phase (Sun et al, 1992;Anderle et al, 1991) by using an azobenzene-modified ''command surface'' (Chen and Brady, 1993;Ichimura et al, 1993;Gibbons et al, 1991), using azo copolymers (Wiesner et al, 1991), and, of course, in pure azobenzene LC phases (Hvilsted et al, 1995;Stumpe et al, 1991). One can force the LC phase to adopt an in-plane order (director parallel to surface), homeotropic alignment (director perpendicular to surface), tilted or even biaxial orientation (Yaroschuk et al, 2001). These changes are fast and reversible.…”