Polymer-Dispersed Liquid Crystals: Boojums at Work
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more récent developments, materials incprporate polymer liquid crystals; both the polymer matrix and the droplets are biréfringent with the potential of direct-view liquid crystal flatpanel displays of unlimited view angle. 7 With the diversity and richness of physical properties in liquid crystal materials, the potential for new polymer-dispersed liquid crystal (PDLC) materials with unique optical features appears endless.
Nematic Droplet Structures Polymer dispersions introduce new and fascinating problems in materials science. One of thèse problems is the
finite size effect of the polymer cavity on the nematic liquid crystal. Under the confinement of a micron- or submicronsize cavity, large elastic distortion énergies imposed by the curvature of the cavity strongly compete with the molecular anchoring energy at the boundary wall to create unusual director configurations for the nematic droplet. 8 " 17 The configuration is further altered by an applied electric field.18 In a light shutter device, a director configuration governs the refractive index of the droplet and is important in the response time and voltage of the shutter. While many configurations are possible, two simple ones that nicely illustrate the principle are the radial and axial structures illustrated in Figure 1. Thèse structures can occur where the molécules are anchored perpendicular to the wall. If the anchoring strength is strong or the droplet is large enough, the radial structure is stable. The radial structure possesses a splay-type déformation with a point volume defect at the center topologically classified as a hedgehog . 1 U 8 If the anchoring strength is weak and the droplet sufficiently small or if there is an applied electric field, E, of suitable strength, the axial configuration is the most stable. 19 The axial structure may contain no defects unless the anchoring strength is large e n o u g h ,
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I I l
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1 ïï 1 I 1 1 ï I
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E (V/um) Figure 1. Régions of stability for the radial and axial structures. " The type of structure is governed by the droplet radius R, the surface extrapolation length d„ and the effects of an electric field E, as described by the value of the electric cohérence length f.
Figure 2. Observed radial ++axial transitions for différent size nematic droplets in a polyurethane polymer19 under différent applied field strengths demonstrating the expected knee of the curve in Figure 1. The black-and-white photos are of the polarizing microscopic textures of radial and axial droplets.
MRS BULLETIN/JANUARY1991
Polymer-Dispersed Liquid Crystals: Boojums at Work
when there may be an equatorial disclination not illustrated in Figure l. 20 The transformation from one configuration to the other is described in terms of surface extrapolation length de = K/W0 w h e r e W0 is the surface a n c h o r i n g strength in units of newtons/meter and K the a p p r o p r i a t e c o m b i n a t i o n of nematic elastic constants which hâve units of newtons. In the absence of an electric fi
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