LC microdroplets

FIGURE 7. Optical microscope picture with crossed polarizers of the microdroplets showing a liquid-crystal state filling the pores of a GDLC thin film. The orientations in microdroplets can be observed as the result of the specific sol-gel processing (parallel polarizers gave Maltese crosses of LC microdroplets and a black image of the silica-gel substrate)... 

The preparation of this type of materials was first reported by Fergason [3] consisting in a dispersion of LC microdroplets in solid polymer matrices [4-6]. Many efforts have been devoted since then to the development of these coatings, known as polymer dispersed liquid crystal (PDLC) in order to control specific electro-optical properties in a thin film [7-13]. 

Polymer-dispersed liquid crystals (PDLCs) are important to liquid-crystal technology. Nematic LC microdroplets are dispersed in a polymer matrix. These films are used similarly to liquid crystal displays such as displays, switchable windows, and light shutters. PDLC devices operate on the principle of electrical modulation of the LC refractive index to match or mismatch the refractive index of an optically transparent, isotropic solid. PDLC films are made between conductive, transparent substrates, and can be switched from being opaque to being transparent with the application of an electric field. Solid state NMR methods have been used to study (PCLCs) and to identify the LC/polymer interface [80,81]. Using NMR one can iden-... 

An atmospheric-pressure ion source for electrospray ionization consists of five parts (Figure 2) (1) the pneumatically assisted electrospray needle, used for the introduction of sample solution or LC mobile phase (2) the actual ion source region, where ions are generated from the microdroplets at atmospheric pressure (3) the ion-sampling aperture (4) the atmospheric-pressure to high-vacuum interface and (5) the ion-optical system, where the ions generated in the source are analyte-enriched and transported toward the high-vacuum mass analyzer. 

This extended series of developments culminated in the thermospray design (Blakley 1983) that was subsequently commercialized as an LC/MS interface. A solution of the analyte (e.g., HPLC eluant) and a volatile buffer (typically O.IM ammonium acetate added post-column) was evaporated from a heated capillary at a flow rate of up to LSml/min into a heated chamber (whence the name thermospray ), forming a mist of droplets containing relatively involatile analytes and solvent vapor as the solvent evaporated the analyte formed adducts with ions from the added salt. It is believed that the formation of free gaseous ions from the microdroplets then proceeds in a manner similar to that discussed for ESI in Section 5.3.6. Most of the neutrals are removed by a vacuum pump and the ions are extracted orthogonally by some electrostatic lenses and a repeUer through a pinhole (restricted to 25 (j,m to protect the vacuum in the mJz analyzer, typically a quadrupole because of its better tolerance to poor vacuum). Such an arrangement is found to be efficient... 

Field-induced reorientation of the director with attendant optical changes has recently been used in a novel application with the potential for large-area LCDs polymer dispersed LCs (PDLCs). A PDLC is a microemulsion of MLC dispersed in a conventional transparent polymer film. In the off state there is a mismatch between the refractive index of the MLC and that of the host polymer film. Hence the dispersion of MLC droplets scatters light very effectively, giving an optically opaque film (Fig. 5.14, left-hand side). On application of an external electric field (across a capacitor-like transparent coating of tin oxide on both sides of the polymer film), the director assumes the same orientation in all of the microdroplets. If the... 

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