Liquid crystals polarized light microscopy

Polarized light microscopy is the study of the microstructures of objects using their interactions with polarized light [1,2,23-27,31-33]. The method is widely applicable to polymers [34] and to liquid crystals [34-37]. The polarizing micro-... 

There are several techniques used to image the microstructure of fat crystal networks. (See Chapter 11 on Imaging. ) The most commonly used imaging method is polarized light microscopy (PLM) since fat crystals are birefringent and appear white, while the liquid oil is not and thus appears black. 

The characterization of liquid crystals by polarized light microscopy is the most straightforward method available and, whenever possible, it should be carried out in the initial stages of an investigation on new polymers. Thermal analyses alone can be misleading. In this procedure, a thin layer of the melt is kept at constant temperature on a hot-sta and obsawed between crossed polars. The appearance or texture of the melt is dependent on the structure of the mesophase, and, therefore, it is often possible to directly identify the type of mesophase present by this method. A good review of the microscopy of liquid crj b ajqjears in the books by Hartshome and by Demus and Richter... 

As briefly mentioned earlier, thermal studies have been used in conjunction with characterization by polarized light microscopy to determine the miscibility of polymeric and small molecule liquid crystals and low molecular weight mesogens, of the same or different types of liquid crystallinity, can also be used as plasticizers or diluents for polymers, as demonstrated in a study involving side chain liquid crystalline polymers... 

The formation of surfactant crystals (i.e. liquid crystals) at the oil-aqueous interface can be easily determined by the use of polarized light microscopy (35). 

Fig. 3.11 Polarized light Microscopy images of graphene oxide liquid crystals (a) and freeze fracture SEM image of nematic phase (b). Reproduced with permission from [115]. Copy right 2011 Wiley-VCH...

Thermal polarized light microscopy of liquid crystal systems still primarily involves the identification of phase types. Recently, however, a number of novel phases with complex structures have been discovered and detailed examinations of the configurations of their defects are required in order to provide a basis for future phase classification. Thermal microscopy is also used extensively in examination of the alignment processes of liquid crystals, and, in a related context, electric-field studies on meso-phases are carried out in aligned cells. Electric-field studies are now used as adjuncts to phase classification, e.g., antiferroelectric phases are sometimes identified in the microscope with the aid of electric-field studies. 

The heat of transitions, AHd h) in high melting peaks, is almost independent of tc- This implies that there is always a frozen liquid crystalline phase when quenching this polymer from 400°C to room temperature. In other words, the formation of liquid crystals from the isotropic phase can proceed very rapidly. Similar observations have been reported for a polyimide LCP quenched from 350°C in air and exhibiting a fine frozen liquid crystalline texture under polarized light microscopy [64,65],... 

Figure 2.11. Polarized light microscopy of polyacrylate-silica aerogel composites obtained by phoVipolymeriza-tion of octylcyanobiphenyl liquid crystal infiltrated in the porosity of the aerogel (in the cliche, liquid crystal and silica aerogel are, respectively, the brilliant and opaque phases). Courtesy ofPesce da Silveira N (UFRGS, Porto Alegre, Brazil) and Rigacci A. [99].

The polarizing light microscopy is the simplest method available to identify LC phases. This optical method has been used since the discovery of liquid crystals and has led to nematic, cholesteric and smectic classifications. The appearance of a specific texture of the melt is usually a function of the types of LC phase, and it is often possible to directly identify the type of LC phase present in a polymer melt by this optical method. The textures of various LC phases are caused by the existence of different types of defect present in the LC phases. It should be noted that microscopic observations are sometimes misleading because the development of specific textures in an LC phase can occur with great difficulty. This problem arises owing to their multiphase nature (the coexistence of polycrystalline and amorphous phases), polydispersity and/or higher viscosities of LCPs melts compared with those of LMLCs. In most cases, LCPs must be annealed for hours or days at suitable temperatures to develop specific textures. 

The lyotropic behavior of ethyl cellulose in various solvents (methanol, ethanol, diox-ane, acetic acid, acetic anhydride, m-cresol, phenol, etc.) was studied as a function of the critical concentration of the liquid crystal phenomenon produced by refractometry, polarized light microscopy, and optical transmission [118, 119]. It has been observed that critical concentration increases with decreasing the solubility of the solvent, and also that the critical concentrations for alcohols were much higher than those of other solvents with similar solubility parameters. 

Methyl cellulose is a derivative of cellulose soluble in water and widely used as a binder or thickener in pharmaceutical products, food products, in the field of ceramics, etc. Formation of the liquid crystal phase is dependent on molecular weight, concentration and temperature, as evidenced in different experimental studies employing differential scanning calorimetry, polarized light microscopy, optical rotatory dispersion [121]. This cellulose derivative has two stages of thermoreversible gelation in aqueous solution, as temperature rises, if concentration exceeds a certain critical value [117, 122]. Several studies [123] have revealed a crystal liquid phase in dilute solutions as well. 

The detection of liquid crystal is based primarily on anisotropic optical properties. This means that a sample of this phase looks radiant when viewed against a light source placed between crossed polarizers. An isotropic solution is black under such conditions (Fig. 12). Optical microscopy may also detect the liquid crystal in an emulsion. The liquid crystal is conspicuous from its radiance in polarized light (Fig. 13). The structure of the liquid crystalline phase is also most easily identified by optical microscopy. Lamellar liquid crystals have a pattern of oil streaks and Maltese crosses (Fig. 14a), whereas ones with hexagonal arrays of cylinders give a different optical pattern (Fig. 14b). 

Preliminary investigations of the liquid crystal phase behavior of these gold nanoparticles initially revealed an enantiotropic nematic phase (based on polarized light optical microscopy and thermal analysis) as well as some pattern formation of the gold nanoparticles in TEM experiments [540, 541],... 

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