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|Experimental Studies on Crystal Polymorphism and Self-Assembled Structures in Soft Matter
|Soft condensed matter
|Raman Research Institute
|Ph.D. Thesis, Jawaharlal Nehru University, New Delhi, 2023
|The molecular self-organisation and the emergent physical properties of materials under different external conditions is an active field of research in soft condensed matter science. The supramolecular order leading to various advanced functional materials has attracted much attention due to their technological applications as well as to the fundamental understanding of their properties. In this thesis, two different types of soft materials which exhibit self-assembled structures have been studied experimentally. The first system studied is a calamatic liquid crystal compound (8OCB) consisting of highly polar rod-like molecules. This compound is known to exhibit three different metastable crystal phases and a stable solid phase on crystallisation from its solution. In this thesis, we study the crystal polymorphs of 8OCB liquid crystal formed from the supercooled smectic phase. Two different solid phases of this compound are often found to form during cooling from its melt. One of them is identified as a metastable crystal state known as long parallelepiped (PP) phase and the other one is the stable commercial power (CP) phase. The CP phase is found to be heterogeneous and exhibits a coexistence of fibrillar nano-crystallites embedded in a solid amorphous state. It is found that the CP phase exhibits both banded and non-banded spherulitic growth on sufficient supercooling from its melt. In quasi-two-dimensional geometry, the banded spherulitic domain with a circular boundary shows concentric colour bands between crossed polarisers. We have found that the concentric bands are formed due to the periodic variation of the composition of fibrillar crystallites and amorphous phase along the radial direction of the domain. This variation changes the effective birefringence of the sample giving rise to the concentric colour bands between crossed polarisers. The second system studied in this thesis is the films formed on drying gelatinized potato starch solution droplets on a flat substrate. Starch apart from being the most consumable carbohydrate is also the source of the largest polymers found in nature. The circular starch films are studied using a variety of experimental techniques. Our experimental results show that the starch films are optically biaxial except at their center where these are optically uniaxial having an optic axis along their normal direction. In the biaxial region of the circular starch films, the three principal indices α, β and γ are found along the normal, radial and azimuthal directions respectively. The starch films are found to be optically negative in both their uniaxial and biaxial regions.
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