Well-Defined Phase Sequence Including Cholesteric, Smectic A, and Columnar Phases Observed in a Thermotropic LC System of Simple Rigid-Rod Helical Polysilane

Abstract

Liquid crystal phases formed by rigid-rod polymers have been studied extensively in both theoretical and experimental ways. In the initial stage of theoretical approach, it has been shown that a system of long, hard molecules can exhibit an orientationally odered nematic phase if the density is efficiently high. 1 Recently, computer simulation has played an important role in understanding the liquid crystalline behavior of systems composed of hard rod molecules and has given the first indication that the smectic and columnar phases can also be formed by systems of molecules that interact through excluded-volume interaction alone. 2 Prior to this simulation, it was thought that smectic phases could only be exhibited by conventional molecules composed of aromatic mesogen groups and aliphatic tail groups which exhibit mainly attractive interactions. 3 This prompted many further theoretical studies of simple hard-rod systems. 4-6 On the experimental side, lyotropic phases have been extensively studied in biological molecular systems. The columnar and smectic liquid crystals have been reported in lyotropic liquid crystals of synthetic polypeptides, 7 DNA, 8 and the tobacco mosaic virus. 9 These liquid crystal phases observed can be easily differentiated from nematic and cholesteric phases which have so far been well studied for these kinds of rodlike polymers, but the detailed identification and nature of these new phases are still unclear because of the difficulty in managing the lyotropic system.In this study, we treat the polysilanes with chiral side chains that assume a helical rigid-rod conformation with the persistent length of around 85 nm. The rigidity results from severe restriction of main chain's internal rotation by steric hindrance between neighboring side chains. 10,11 Such polymer chain stiffness is closely correlated to the liquid crystallinity in the lyotropic system. Here, we find that these helical polysilanes can form thermotropic liquid crystals (LCs) as well as the lyotropic LCs if the long alkyl side chains are attached to the main chain as a second side group, as reported in a previous study. 12 The ability of thermotropic LC formation is due to the long flexible side chains that act as solvents in the lyotropic system. 13 A typical polysilane that forms a thermotropic liquid crystal is poly[n-decyl-(S)-2-methylbuthylsilane] (PD2MBS) with the following formula:This PD2MBS polymer, having length 1.96 × n Å (n: degree of polymerization) and diameter 15 Å, can be an ideal molecule to use to verify the theoretical predictions of the hard-rod model because the molecules interact mainly through excluded-volume interactions because no significant electrostatic intermolecular attractions are present due to its nonpolar nature. The nonpolar nature is also advantageous for the preparation of a sample with a narrow molecular weight distribution. A simple fractional precipitation method can be used due to the lack of aggregation by the molecules in solution.The results show that the thermotropic phase beha...

Synthesis and property of helical poly(phenylacetylene)s bearing chiral ruthenium complexes and real space imaging of meso‐ and nanoscopic structures by atomic force microscopy

“…Recently, a number of artificial helical polymers bearing enantiomerically pure chiral or achiral bulky functional side groups have been successfully prepared by the precisely controlled polymerization of functional monomers with well‐defined catalysts 27–35. These synthetic helical polymers exhibit many interesting characteristics, including electronic and photophysical properties,21, 36 the formation of thermotropic liquid crystals,37, 38 chiral and chirality recognition ability,39–51 and helix–helix transition 48, 52–61. Therefore, understanding the relationship between the functions and the global structures and local conformations is crucial in this field 27–35.…”

Rigid-Rod Polyesters with Flexible Side Chains IX. Phase Behavior Including Nematic, Layered, and Hexagonal Columnar Phases in Poly(p-biphenylene terephthalate) with Alkoxy Side Chains

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] The most interesting property of these materials is an ability to form layered structures in crystals and liquid crystals when the alkyl side chains reach a critical length. The layered mesophases are characterized by a closed packing of the rigid-rod main chains into layers with the molten side chains occupying a space between the layers.…”

Nano-Segregation of Squalane between Smectic Layers of Rigid-Rod Polysilane

“…During the initial stage of these studies, the numerical experiments and computer simulations revealed that the most common LC sequence of the nematic-smecticcolumnar phases could be reproduced in the hard-rod particle model systems [1,2]. These theoretical predictions agreed well with the thermotropic LC behavior of rigidrod helical polysilanes with narrow molecular weight distributions [3][4][5][6][7][8][9].…”