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RISS 인기검색어
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- 저자 : Gonzalez-Ronda, Lebzylisbeth (검색결과 1 건)
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Microstructure and defects of electrooptically active bithiazoles and bisoxazoles
Gonzalez-Ronda, Lebzylisbeth The University of Michigan 2000 해외박사(DDOD)
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The solid-state structure and defects of π-conjugated nonylbithiazoles (NBTs) and nonylbisoxazoles (NBOs) has been studied with the aim of understanding their electrical, optical, and mechanical properties. X-ray diffraction patterns of NBT and NBO molecular weight series were consistent with an oblique, π-stacked structure. The side-to-side and π-π stacking molecular spacings varied with increasing chain length so as to maintain a near constant density. Models were proposed for PNBT and PNBO in which the chain axes stagger with respect to one another. The electronic structure of PNBT was found to be sensitive to processing conditions. The effective conjugation length of the samples varied with their degree of crystallinity; yellow samples were found to be only weakly ordered, while red and green samples were semicrystalline. Red and green specimens had similar crystal structures, although green samples had a higher degree of order. PNBT solutions were found to exhibit liquid crystalline order. Examination of a solution concentration gradient showed that the isotropic and liquid crystalline regions appeared yellow, while the solidified regions appeared red. Bending of the (100) lattice fringes, corresponding to the side-to-side molecular spacing, was observed in both PNBT (d = 2.4 nm) and PNBO (d = 2.7 nm) using high-resolution transmission electron microscopy (HRTEM). A quantitative method for generating two-dimensional maps of the local lattice curvature at the nanometer level was developed. Despite similarities in their chemical structure, the two polymers accommodate the deformation very differently. PNBT undergoes continuous deformation mediated by [001] dislocations, while PNBO undergoes discontinuous deformation mediated by the presence of grain boundaries. The bending constants and persistence lengths of PNBT, PNBO, and other semi-rigid polymers were calculated using a molecular mechanics approach. The calculated bending constants (390 to 940 kJ*m/mol) and persistence lengths (16 to 38 nm) are consistent with experimental values for semi-rigid polymers (20 to 60 nm). The rigidity of the simulated oligomers was anisotropic; bending about the π-π stacking direction was found to be energetically unfavorable.
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