화학공학소재연구정보센터
Journal of Physical Chemistry B, Vol.107, No.51, 14211-14218, 2003
Exploring the photorefractive effect in polymer-dispersed liquid crystals using near-field scanning optical microscopy
Static and dynamic near-field scanning optical microscopy (NSOM) methods are used to study physical phenomena associated with photorefractivity in dye-doped polymer dispersed liquid crystals (PDLCs). Micrometer-sized liquid crystal droplets doped with perylene and NN-dioctyl-1,4,5,8-naphthalene diimide (NDI) dyes, which serve as the photoexcitable electron donor and acceptor, respectively, are studied. The droplets are encapsulated within a thin poly(vinyl alcohol) film. Simultaneously recorded topographic, fluorescence, and birefringence NSOM images show the films to be morphologically complex, containing ellipsoidal and collapsed ellipsoidal droplets. The presence of the collapsed droplets suggests the dissolved dyes may be surface active. Nevertheless, the NSOM images prove the vast majority of dye is contained within the well-defined liquid crystal droplets. Data on the local ion migration and liquid crystal reorientation dynamics is obtained by dynamic NSOM imaging methods. A sinusoidally modulated electric field is applied across the sample, between the aluminized NSOM probe and the electrically conductive film substrate. Lockin detection of the modulated optical signal resulting from field-induced liquid crystal reorientation is used to obtain amplitude and phase images of the sample. Comparison of the images recorded before, during, and after photogeneration of ions shows spatial variations in the relative importance of ion generation and migration on the liquid crystal orientation state. Computer simulations of the field-dependent ion migration dynamics and the liquid crystal response are used to better understand the data obtained.