Crystallization, spectroscopic changes

Tashiro, K., Y. Minagawa, K. Kobayashi, S. Morita, T. Kawai, and K. Yoshino. 1994. Crystal structure change of poly(3-alkylthiophene) induced by iodine doping as revealed by X-ray diffraction and infrared/Raman spectroscopic measurements. Jpn J Appl Phys 33 L1023. 

Of spectroscopic teclmiques, nuclear magnetic resonance (NMR) has been most widely used to measure orientational ordering in liquid crystals [M, 57 and ]. Most commonly, changes of line splittings in the spectra of... 

As the density of a gas increases, free rotation of the molecules is gradually transformed into rotational diffusion of the molecular orientation. After unfreezing , rotational motion in molecular crystals also transforms into rotational diffusion. Although a phenomenological description of rotational diffusion with the Debye theory [1] is universal, the gas-like and solid-like mechanisms are different in essence. In a dense gas the change of molecular orientation results from a sequence of short free rotations interrupted by collisions [2], In contrast, reorientation in solids results from jumps between various directions defined by a crystal structure, and in these orientational sites libration occurs during intervals between jumps. We consider these mechanisms to be competing models of molecular rotation in liquids. The only way to discriminate between them is to compare the theory with experiment, which is mainly spectroscopic. 

IR and Raman spectroscopic studies on films and powders of PDHS indicate that the hexyl side chains are crystallizing into a hydrocarbon type matrix (40). This is indicated by the presence of a number of sharp characteristic alkane bands which become dramatically broadened above the transition temperature. Similar changes are observed for n-hexane below and above the melting point. CPMAS 29Si NMR studies on PDHS also show that the rotational freedom of the side chains increases markedly above the transition temperature (41,42). All of the spectral evidence... 

Among crystalline solids, typical second-order transitions are associated with abrupt intermolecular conformational, rotational, and vibrational changes and/or with abrupt changes in crystalline disorder and/or defects [7], These changes in crystalline solids are sometimes difficult to assign without the use of appropriate spectroscopic techniques such as solid-state NMR or a diffraction procedure such as single-crystal X-ray diffraction. 

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