Alkyl Tail Length Dependence of Structures in a Series of Symmetrically Tapered Bisamides Exhibiting Self‐Assembled Supramolecular Columnar Phases

Abstract

Previously, we synthesized a series of symmetrically tapered 1,4‐bis[3,4,5‐tris(alkan‐1‐yloxy)benzamido] benzene bisamides (CnPhBA, where n is the number of carbon atoms in the alkyl chains, n=10, 12, 14, and 16). These bisamides consist of a rigid aromatic core with three flexible alkyl chains at each end of the core. It was found that all of these CnPhBA bisamides formed a highly ordered oblique columnar (Φ_OK) phase and a low‐ordered oblique columnar (Φ_OB) phase.[1,2] To study the dependence of structure on alkyl chain length in this series of symmetrically tapered bisamides, we focus on the structural and phase transitions of CnPhBA (n=6 and 8) via differential scanning calorimetry, one‐dimensional (1D) and 2D wide angle X‐ray diffraction (WAXD) and Fourier transform infrared spectroscopy. Structural evolutions and phase identifications were also supported by texture changes observed by polarized optical microscopy. It was interesting that there was only one ordered structure in C6PhBA which contains the shortest alkyl tails in this series of bisamide. Its 2D lattice (a=1.94 nm, b=3.43 nm, and γ=63.5°) was different from the samples with longer alkyl chains (n=10, 12, 14, and 16, n≥10, even).[1,2] However, the alkyl tail length did not have a significant influence on the packing along the column (c=0.96 nm). No liquid crystalline (LC) columnar phase was observed. When the length of alkyl tails contains eight carbons (C8PhBA), there was a sudden jump in the isotropization temperature of its LC phase compared with that of molecules with longer alkyl tails (n≥10, even).[1,2] Dramatic structure and phase behavior differences between C8PhBA and other CnPhBA bisamides were observed. This finding indicated that the C8PhBA having eight carbons in its alkyl tails represents a transition from a weak nano‐phase separation effect of the alkyl chains with respect to the rigid cores in CnPhBA with n≤6 (even) to a strong nano‐phase separation effect in CnPhBA (n≥10, even).

On the occasion of Prof. Phil Geil's 75th birthday for his pioneering contributions in polymer crystal physics.

Keywords:

• tapered bisamide
• self‐assembly
• supramolecular structure
• hydrogen bonding
• nano‐phase separation
• transition length

Acknowledgment

This work was supported by NSF DMR‐0516602. We also acknowledge that the DSC experiments in this article were carried out on a Perkin‐Elmer PYRIS Diamond DSC that had been set up in our laboratory by Perkin‐Elmer Co.

Notes

On the occasion of Prof. Phil Geil's 75th birthday for his pioneering contributions in polymer crystal physics.