X-ray scattering study of molecular order in a liquid crystal-side group polysiloxane

Abstract

The molecular structure of a polysiloxane with phenylbenzoate mesogenic side groups was investigated in an x-ray scattering study in the partially crystalline, smectic and nematic phase, and in the melt. In the crystalline phase polymer molecules have the form of straight ribbons with a "double-comb"-conformation. A bilayer structure is built up by regular stacking. Layers are the dominating structure element not only in the crystalline and smectic phase, but also in the nematic phase, and even in the isotropic melt. Layers are planar in the smectic phase and curved in the nematic phase, with an asymmetric distribution of the normal vectors about the director. In the isotropic melt there is evidence for the occurrence of clusters with layer-like short-range order.

Molecular Association in Nematic Phases of Cyclic Liquid Crystal Oligomers

“…Since then, a systematic experimental investigation of cyclic LCs with other backbones with the same pendant mesogen was undertaken . First attempts to simulate molecular associations of cyclic LCs indicate that a reasonable match to the experimental d -spacings can be obtained assuming an overlap of mesogenic groups within small clusters of limited sizes. , Similar but weaker low-angle X-ray scattering was observed for linear siloxane LC polymers several years ago (and in this recent work) which were explained by a modulation of electron density within areas of separately packed siloxane backbones and mesogenic side groups. , However, these speculations are not supported by a complete analysis of the X-ray scattering and no quantitative models of molecular arrangement have been proposed yet.…”

“…Usually, short-range positional molecular ordering is expanded over three to four coordinate spheres. Various more complicated versions of nematic ordering with local molecular associations have been observed in nematic phases of various LCs and polymeric LCs with stronger intermolecular interactions. − The most known case is cybotactic nematics, where association of molecules in small clusters or “cybotactic” groups occurs (Figure b) c.…”

“…Various more complicated versions of nematic ordering with local molecular associations have been observed in nematic phases of various LCs and polymeric LCs with stronger intermolecular interactions. [2][3][4][5][6][7][8][9][10][11][12][13][14] The most known case is cybotactic nematics, where association of molecules in small clusters or "cybotactic" groups occurs (Figure 1b). 2 Another type of association observed previously in a few systems is shown in Figure 1c.…”

Structural Characterization of Biphenyl Ester-Based LC Molecules: Peculiarities of Cyclic Siloxane-Based Materials

“…The “strength” of these reflections was examined thoroughly by interpreting their occurence as the presence of short-range positional order along the molecular axis. This type of molecular association, termed “strings”, has been observed previously in other material systems, namely, small molecules with hydrogen-bonding characteristics and small dimer molecules with nonsymmetric lateral attachment among twin mesogens. − Some conventional side-chain LC compounds have also exhibited similar diffuse structure, particulary those based on siloxane backbones. ,− …”

“…[15][16][17] Some conventional side-chain LC compounds have also exhibited similar diffuse structure, particulary those based on siloxane backbones. 7,[18][19][20] To examine the effect of small structural differences on this observed atypical scattering behavior, 13,14 compounds with slightly different structures are studied here. Also reported are initial molecular modeling simulations used to explore the molecular origins of this observed association between molecules.…”