Self-assembly of taper-shaped monoesters of oligo(ethylene oxide) with 3,4,5-tris(n-dodecan-1-yloxy)benzoic acid and of their polymethacrylates into tubular supramolecular architectures displaying a columnar hexagonal mesophase

Abstract

The synthesis of the monoesters of oligo(ethylene oxide) and octane-1,8-diol with 3,4,5-tris(n-alkan-1-yloxy)benzoic acids and of their corresponding polymethacrylates is described. The LiCF₃SO₃ complexes of the monoesters of triethylene glycol with 3,4,5-tris(n-dodecan-1-yloxy)-benzoic acid (3-12-3), and with 3,4,5-tris(n-octadecan-1-yloxy)benzoic acid (3-18-3), as well as the polymethacrylates of the monoesters of mono-(5-12-1),di-(5-12-2), tri-(5-12-3) and tetra-(5-12-4) ethylene glycol with 3,4,5-tris(n-dodecan-1-yloxy)benzoic acid self-assemble into a cylindrical supramolecular architecture which displays a columnar hexagonal (Φ_h) mesophase. This cylindrical supramolecular architecture was characterized by a combination of techniques including differential scanning calorimetry, wide- and small-angle X-ray scattering, thermal optical polarized microscopy, and molecular modelling. The characterization results suggest a model that resembles the cylindrical architecture that forms by the self-assembly of low molar mass and polymerized inverse micelles. A cross-section of the cylindrical assembly is constituted of about five molecules of 3-12-3 in its LiCF₃SO₃ complex or about four to five repeats units of 5-12-n with their oligooxyethylenic segments melted and segregated in the inner core of the cylinder and their melted alkyl tails radiating towards the periphery of the cylinder. The driving force for the cylindrical self-assembly is provided by a fine balance of exo- and endo-recognition processes. Exo-recognition is the result of the tapered shape and hydrophobic character of the 3,4,5-tris(n-dodecan-1-yloxy)benzoate group. Endo-recognition is generated by either the combination of dipolar and ionic interactions of the oligo(ethylene oxide) receptor (LiCF₃SO₃ complex of 3-12-3) or the combination of dipolar interactions and covalent bonding (5-12-n). The necessity of the presence of endo-recognition for the self-assembly of the cylindrical supramolecular architecture is demonstrated by the replacement of the oligo(ethylene oxide) receptor by a non-polar aliphatic spacer. Comparison between the ‘molecular’ polymethacrylate backbone (in 5-12-3) and the ‘supramolecular polymer backbone’(formed via ionic interactions in the complex of 3-12-3) indicates that in this particular example, the ionic interactions generated by the dissolved ion-pairs stabilize the supramolecular assembly to a greater extent than does the covalent bonding.