Microstructure and Morphology of Thermotropic Amphiphilic Liquid Crystalline Materials
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INTRODUCTION A columnar liquid crystalline phase has become a subject of considerable interest because the molecular recognition and self-organization provide the basis of spontaneous generation of functional supramolecular architectures via self-assembly from their components [1-5]. The selfassembling ability, phase symmetry and dimension are greatly influenced by molecular interactions, shapes and microphase separation between different, e.g., aromatic and aliphatic moieties [6-10]. The control of the orientation of these assembled materials is essential to their properties such as photoconductive materials [11] and functional membranes [12]. In this paper, we discuss the effect of chemical architecture on orientation of the cylindrical assemblies. We show direct visual evidence of the phase symmetry, dimension and stability of hexagonal columnar packing.
EXPERIMENTAL Three amphiphilic liquid crystalline materials have been investigated. Compound 1 (whose structure is sketched below) is taper shaped molecule having a crown ether at one end which forms the center of the column and 3 fluorinated tails at the other.
F(CF2)8(CH 2)O•••
H2 0
:(CF)8(CH2)O8H2
0
560 C
1359C
Crystal --------LC -------Isotropic
F(C do A
-H 20
1
189 Mat. Res. Soc. Symp. Proc. Vol. 559 ©1999 Materials Research Society
Two dendrimeric compounds, shown schematically below, were also investigated: 2, an asymmetric dendrimeric material, 1,2.3-tris[(3,4,5-tris[(4-(n-dodecan-1-yloxy) benzyl) oxy] benzyl) oxy] - benzene, and 3, a symmetric (disk-shaped) dendrimer, 1,3,5-tris [3,4,5-tris[4-.(ndodecan-l-yloxy) benzyloxy] benzyloxy] benzene [9].
where, R=CREH 25
LC
2
3
86°C -.... sotropic
LC
128 °C -...... Isotropic
Recent characterization and the synthetic efforts indicate that these materials form a hexagonal columnar mesophase, and that similar compounds form either or both cubic and columnar mesophases, depending on chemical structure and temperature [13]. Synthetic procedures of the materials were described in detail [14]. Thermal transitions were determined by polarized optical microscope (Olympus BX-60), equipped with hot stage (Mettler FP 82). Orientation of the cylindrical assemblies was controlled in thin films by surface anchoring conditions. For planar alignment of the columnar materials, the thin films suitable for electron microscopy examination were cast from solution (-0.5wt%) onto distilled water, held at temperature: at which the columnar phase is stable, and retrieved on copper grid. Homeotropically aligned columnar specimens were obtained by placing cast films onto carboncoated copper grids, heating to the isotropic phase and cooling (at a rate of 5°C/main to a particular temperature, before final quenching). Films of each preparation were examined at 100kV under bright field conditions using a JEOL 100CX TEM using low dose procedures [1516]. Samples were examined with and without staining with RuO4 vapors (using solutions of -0.5wt% aqueous RuO 4 ). RESULTS AND DISCUSSION Identification of the hexagonal column
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