Guest-host effect of dyes in polymer dispersed liquid crystals

Abstract

The guest-host [GH) effect of a tetracyanoquinodimethane (TCNQ) dye in nematic liquid crystals (LC) and polymer dispersed liquid crystals (PDLC) has been studied and compared with commercial azo and anthraquinone dyes. The adduct (MORPIP) is a conjugated molecule with a high dipole moment of 15 Debyes. E7 LC doped with MORPIP was found to give a dichroic ratio of 2.34 and an order parameter of 0.31; these values are lower than for the other dichroic dyes, typically 0.6 - 0.78. However, MORPIP showed a useful property When doped in LC and PDLC samples. The rise time of E7 was decreased by 21% when doped with 0.3 wt% MORPIP. It was suggested that MORPIP increased the dielectric anisotropy of E7 and hence reduced the rise time. Nevertheless, no change in dielectric anisotropy was observed. This observation favours a model in which the dipolar dopant reorients rapidly in the applied field and drives the reorientation of the surrounding LC The effects of the dyes incorporated in PDLGs were also studied, in order to explore their suitability for use in a colour reflective display. Dichroic PDLQ were made using the photo-initiated phase separation method. 0.37 wt7o MORPIP doped PDLCs showed a 45% faster response to an applied electric field than that of the undoped PDLCs. However, the contrast ratio of the dichroic PDLCs was not sufficiently high for display use due to insufficient scattering and dye trapping. Two new fabrication methods were developed to overcome this problem One method involved the preparation of a network type PDLC into which a doped LC was dispersed. Another method involved the preparation of voids by removing the LC from the PDLC In both methods, subsequent re-filling of the doped LC into the pre-made polymer matrix improved the contrast ratio and reduced the decay time of the dichroic PDLC