Cross polarized microscopy

The compensation birefringence measurement is very easily coupled to optical microscopy in the transmission and reflection modes, thus allowing characterizing orientation with a spatial resolution of a few hundreds of nanometers [14]. Polarizing microscopes are widely available and are often used for birefringence studies even if spatial resolution is not required. Objectives specifically designed for cross-polarized microscopy are necessary to avoid artifacts. 

Therefore, local dissolution and recrystallization seem to play an important role in the gas uptake mechanism in these type of sensor materials. The coordination of SO2 to the platinum center (and the reverse reaction) is therefore likely to take place in temporarily and very locally formed solutes in the crystalline material, whereas the overall material remains crystalline. The full reversibility of the solid-state reaction was, furthermore, demonstrated with time-resolved solid-state infrared spectroscopy (observation at the metal-bound SO2 vibration, vs= 1072 cm-1), even after several repeated cycles. Exposure of crystalline samples of 26 alternat-ingly to an atmosphere of SO2 and air did show no loss in signal intensities, e.g. due to the formation of amorphous powder. The release of SO2 from a crystal of 27 was also observed using optical cross-polarization microscopy. A colourless zone (indicative of 26) is growing from the periphery of the crystal whereas the orange colour (indicative for 27) in the core of the crystal diminishes (see Figure 9). 

The detection of Hquid 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 Hquid crystal in an emulsion. The Hquid crystal is conspicuous from its radiance in polarized light (Fig. 13). The stmcture of the Hquid crystalline phase is also most easily identified by optical microscopy. Lamellar Hquid 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). 

Figure 8.1. (a) Spherulites growing in a thin film of isotactic polystyrene, seen by optical microscopy with crossed polars (from Bassett 1981, after Keith 196.3). (b) A common sequence of forms leading to sphertililic growth (after Bassett 1981). The fibres consist of zigzag polymer chains. 

By optical microscopy (OM), birefringent structures are observed in semicrystalline polymers, characterized by "Maltese-crosses" under crossed polars as seen in Figure 6. As these structures grow symmetrically in three dimensions... 

Figure 6 Spherulites of isotactic poly-l-butene (a, during growth) and of polyethylene (b, after completion) by optical microscopy (OM) under crossed polars. Reproduced from Ref. [3] with permission of John Wiley Sons, Inc.

Figure 3. Images of a cross-section of carbon fibers after propylene pyrolysis. 3a Scanning Electron Microscopy of a piece of the carbon cloth. 3b optical microscopy (crossed polarizers with a wave retarding plate).

Fig. 2 Optical microscopy image of a small section of a poly(ethylene oxide) (PEO) droplet dispersion sample, see text (1000-mm wide) obtained at Tc = - 2.6 °C. Amorphous droplets appear dark and semicrystalline droplets appear white under nearly crossed polarizers. The plot shows the fraction of crystallized droplets as a function of temperature upon cooling (0.4 °C min-1) for homogeneous nucleation. (Reprinted with permission from [84], Copyright 2004 by the American Physical Society)...

Video microscopy with crossed polarizers permits the direct and non-invasive observahon of the nucleahon and growth process for many substances, and thus the study of the hme evoluhon of the spherulite radius R t). When the growth is controlled by diffusion the radius of the spherulites increases as R t) a while when the growth is determined by a nucleation-controlled process (incorporahon of atoms or molecules to the surface of the crystalline part) the radius increases linearly with hme, R t) a t. 

Figure 5.17. Optical microscopy image (1.7 mm x 1.1 mm, crossed polarizers) of a partially transformed thin film of p-NPNN grown on a glass substrate a-phase (left) and /9-phase (right). Reprinted with permission from J. Fraxedas, J. Caro, J. Santiso, A. Figueras, P. Gorostiza and F. Sanz, Europhysics Letters 1999, Vol. 48, 461 67.

Fig. 2 (a) Transmission electron microscopy (TEM) image of a disc-like beidellite clay sample (sizes of the platelets range from 69 to 480 nm in diameter), and (b) lyotropic nematic phase of an aqueous suspension of the same beidellite sample at an ionic strength of 10 4 mol L-1 ( = 0.5%) observed between crossed polarizers [263], (Copyright 2009, American Chemical Society)... 

The use of a single polar is compatible with phase contrast microscopy. Crossed polars produce a dark field in which fine fibers will not be seen. If a compensator such as a first order red plate is also used, most of the fine fibers will be seen provided the light source (31) is sufficiently intense. 

In phase contrast microscopy when particles are viewed in brightfield, i.e., without the use of crossed polars, if particles have an n less than n of the medium, the particles appear very white unless the particles have a strong absorbance color. If n of the particles is close to n of the medium, the particles will appear faint blue. If n of the particles is greater than n of the medium, the particles will show sharp contrast. Edges and surface features will be easily seen. 

Figure 13. Coalescence of mesophase spherules to produce a new 2tt disclination. Hot-stage microscopy, crossed polarizers.

---