Liquid Crystalline Polymers by Vijay Kumar Thakur & Michael R. Kessler

Author:Vijay Kumar Thakur & Michael R. Kessler
Language: eng
Format: epub
Publisher: Springer International Publishing, Cham

9.3.2 Polarized Light Microscopy

PLM is a widely used technique to study and identify the phases exhibited by LCs. A thin layer of a LC is placed in between two cover glass plates and viewed through a PLM under temperature or proper conditions depending on the type of LC phase, various textures can be observed. Beautiful kaleidoscopic images called textures are seen due to the interference between light waves passing through the specimen, which is already heavily given throughout the chapter.

The formation and growth of LCs in nanocellulose was observed by means of laser diffraction. The laser beam produces a diffraction pattern and the central spot of the ring grows into a steady circular ring showing the growth of LCs and their organization into larger assemblies which is confirmed by the appearance of concentric rings after some time. Optical microscopy is an important tool for the study of the structure of lamellar LCs. Many microscopic techniques like SEM, TEM and AFM are commonly used. Chiral nematic phases have been observed for concentrated solutions and melts of many cellulose derivatives (Gray 1995). Both left and right-handed solutions, melts and films have been observed for cellulose derivatives, but to date only left-handed helicoids have been observed for CNC suspensions and films. The assignment of handedness was based on TEM images of oblique microtomed sections of films cast from aqueous suspensions of CNCs, following the interpretation of the arcing observed in natural composites by Bouligand (2008). Arcing observed in TEM images of thin oblique cross-sections through a planar chiral nematic film of wood CNCs cast from aqueous suspension on a Teflon substrate is shown in Fig. 9.20 (Giasson 1995).

Fig. 9.20Arcing observed in TEM images of thin oblique cross-sections through a planar chiral nematic film of wood CNCs cast from aqueous suspension on a Teflon substrate. The sections were cut at angles such that the top left-hand corner of the image was tilted (a) towards and (b) away from the observer (Giasson 1995) (Reproduced with permission from Dissertation, McGill University, Chap.​ 5, p 167)

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