Electron microscopy of thermotropic

B. Donnio, D. N. Bruce, H. Delacroix, T. Gulik-Krzywicki. Freeze-fracture electron microscopy of thermotropic cubic and columnar mesophases. Liq... 

Fig. 3.3 Phase diagram of diacylphosphatidylcholines as suggested by X-ray, optical birefringence, and electron microscopy. Lj corresponds to the smectic A state and tp, Lf, Pp to the smectic B phase of thermotropic liquid crystals. The shaded area is a region of coexistence. (Reprinted from Fig. 2 of ref. 15 with permission from Wiley-VCH.)...

One of the most classic examples of chiral expression in thermotropic liquid crystals is that of the stereospecific formation of helical fibres by di-astereomers of tartaric acid derivatised either with uracil or 2,6-diacylamino pyridine (Fig. 9) [88]. Upon mixing the complementary components, which are not liquid crystals in their pure state, mesophases form which exist over very broad temperature ranges, whose magnitude depend on whether the tartaric acid core is either d, l or meso [89]. Electron microscopy studies of samples deposited from chloroform solutions showed that aggregates formed by combination of the meso compounds gave no discernable texture, while those formed by combinations of the d or l components produced fibres of a determined handedness [90]. The observation of these fibres and their dimensions makes it possible that the structural hypothesis drawn schematically in Fig. 9 is valid. This example shows elegantly the transfer of chirality from the molecular to the supramolecular level in the nanometer to micrometer regime. 

Freeze-fracture electron microscopy studies of the membranes of E. coli and A. vinelandii by Reusch et al.24 provide evidence of structural changes that support the fluorescence data (Figure 10). Freeze-fracture micrographs of log-phase cells show a typical mosaic of particles and pits on both concave and convex surfaces of the plasma membranes. However, as complexed PHB was increasingly incorporated into the membranes, as determined by analysis of the purified membranes and evidenced by the intensity of the thermotropic transition at - 56 °C, the micrographs revealed the formation of small semi-regular plaques in the plasma membranes (arrows) that possess shallow particles. The plaques grew in size and frequency as the concentration of membrane PHB and intensity of the PHB/polyP transition increased. 

Transmission electron microscopy (TEM) studies conducted by Thomas et al. [62,63] have permitted direct visualization of the molecular director distribution in flow-oriented thin films prepared from semiflex-ible thermotropic LCPs of the following structure ... 

Donald et al. [2] reported banded structures formed by several thermotropic polymers oriented by shear at temperatures above their softening points. Similar structures were also noted in fibers drawn from polymers with rigid backbones above the softening points. Viney et al. [3] point out that the banded structures observed in shear are due to the variation in the direction of the long molecular axis with respect to the direction of shear. Evidence obtained by both optical microscopy and electron diffraction measurements supports this view. Donald and Windle [4] studied the banded structure by electron microscopy and commented that The near sinusoidal variation in the direction of the principal axis of the refractive index ellipsoid is indeed reflecting the variations in the molecular orientation. Their transmission electron microscopy indicates that the transition from... 

Copper laurate can be processed into oriented fibers by melt spinning of its thermotropic columnar mesophase, X-ray diffraction and electron microscopy revealed a high degree of orientation of the spun fibers both in the crystalline and in the columnar states [45 a]. Neutron experiments have been performed on these fibers to study the motion of the hydrogen atoms of the chain [45 b]. 

Thermotropic aromatic copolyesters have a major advantage over the lyotropes, as the former can be melt processed. Temperature affects the orientation and the mechanical properties, and the copolyesters have been shown to be biphasic by SEM [694-696], optical, and TEM [697-699] techniques. The biphasic structure of X7G has been reported [699] for extruded fibers by optical and EM imaging and microdiffraction. Transmission electron microscopy micrographs of ultrathin longitudinal sections reveal a dense dispersed phase elongated along the fiber axis (Fig. 5.147). Microdiffraction from regions 20-100 nm across show the... 

The production of thermotropic APC cellulose derivatives micro fibers is reported (Canejo et al. 2008) for the electrospinning of APC. These were obtained from a lyotropic solution of APC at room temperature. Scanning Electron Microscopy (SEM) observations showed that the APC electrospun fibers exhibit a spontaneous twist along their axis. 

Transmission electron microscopy is able to identify the orientation of molecules within thin films of those thermotropic polymers which are sufficiently resistant to the necessary intense electron beam. The high resolution possible in TEM enables fine scale structures to be characterized which are invisible in a light microscope. Diffraction data can be recorded from the same area as bright and dark field images. However it must be recognised that the requirement for thin specimens can produce structures not necessarily typical of the bulk, particularly since surface interactions may be of importance. 

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