Reversible photoinduced phase transition and image recording in polymer-dispersed liquid crystals

Abstract

The photoinduced phase transition of nematic liquid crystalline (LC) emulsions containing 4-n-butyl-4'-methoxyazobenzene (BMAZ) was studied in a polyvinylalcohol (PVA) film as a supporting medium. Phase transition was reversible and accompanied by a large change in transmittance as well as a change in birefringence. Photorecording by the method described provided a quick response and long-lasting images due to perfect fixation of LC emulsions into PVA film, unless reverse isomerization of BMAZ takes place. The 3 mu m thick films containing LC emulsions with diameter 2 mu m showed about 6 mu m of resolving power.

Photocontrollable Liquid‐Crystalline Actuators

“…Kawanishi et al prepared polymer/LMWLC composite fi lms with a thickness of 2-3 μ m using a mixture of a nematic LC and an AZ derivative dispersed in an aqueous solution of PVA by solventinduced phase separation. [ 68 ] Although the composite fi lms showed very low transmittance because of opacity, they became transparent upon irradiation at 366 nm, resulting from nematic reverse-mode switching were achieved in such AZ-containing polymer network systems by choosing network-forming materials and by tuning polymerization conditions. [ 69c ]…”

Photoregulation of Liquid-Crystalline Orientation by Anisotropic Photochromism of Surface Azobenzenes

“…PPT of polymer LCs (9, 10) affords long-term storage more than one month if kept below the glass transition temperature, while heating up may be required to write as fast as low-mass LCs. Long-term storage without sacrificing response properties has been demonstrated by applying the polymer dispersed LC film (PDLC) (11). PDLC is a polyvinylalcohol film in which microcapsules of a mixture of cyanobiphenyl LC analogues and 4-butyl-4'-methoxy-Az are embedded.…”

Photochromic Polymers

“…[88][89][90][91][92][93][94][95][96] Polymeric films demonstrating reversible birefringence alteration have been readily prepared by dispersing droplets of nematic liquid crystals with dissolved azobenzenes in an aqueous solution of poly(vinyl alcohol) to give cast films. 97 Extensive studies on photochromic liquid-crystalline polymers have been made by Krongauz et al 2 Liquid-crystalline phases caused marked colour changes of poly(acrylates) 98 and poly(siloxanes) substituted with spiropyran side chains upon UV irradiation owing to the aggregation of the photomerocyanines. 99 In contrast, spirooxazines attached to liquid-crystalline polymer backbones displayed no aggregation and hence exhibited normal photochromism similar to that in solution.…”

“…The essential difference between low-mass and polymeric systems was in the efficiency of the photoinduced phase change. Although UV exposure triggers the phase alteration of low-mass liquid crystals even at room temperature, [88][89][90][91][92][93][94][95][96][97] heating of the polymeric systems at a temperature close to the nematic-isotropic transition temperature was necessary to enhance the phase transformation. The photoinduced mesophase transformation of polymeric systems from a nematic into an isotropic phase was influenced by various factors including the molecular structure of the azo chromophores as well as that of the nonphotoactive mesogenic units, the molecular weight of the polymers, the spacer length between the mesogenic units and the polymer backbones, the content of azobenzene units, and, in particular, the temperature during photoirradiation.…”