Self-assembling properties of a series of homologous ester-diamides – from ribbons to nanotubes?

Soft Matter Pub Date: 2013-07-26 DOI: 10.1039/C3SM51369F

Abstract

We have studied the aggregation properties of a series of compounds BHPB-n bearing two amide groups and an ester alkyl chain. The number (n) of carbon atoms in this alkyl chain has been varied from 5 to 16. These compounds form gels at low concentration in alkanes. The gel concentrations and the heats of formation of the gels, measured by DSC, are divided into three different regimes: n ≤ 7, 8 ≤ n ≤ 12 and n ≥ 13. Freeze fracture TEM shows that all the compounds self-assemble into high aspect ratio objects, i.e. flat ribbons for n ≤ 7 and irregular helical ribbons for n ≥ 13. For 8 ≤ n ≤ 12, the compounds form tubes (except BHPB-11) and were studied by SANS. The outer and inner diameter distributions of the tubes were determined from the scattered intensities with a model of paired tubes. The diameters are controlled by the length of the ester chain (n) and vary from 21 for BHPB-8 to 32 nm for BHPB-12 (outer diameters). The SANS measurements also provide an estimate of the ratio of paired tubes and the proportion of non-tubular objects that eventually co-exist with these tubes. Variable temperature FTIR and DSC proved that H-bonds between the amide groups are involved in all types of self-assemblies. In addition, FTIR spectra of helical tapes and nanotubes exhibit a double ester νCO band whereas flat tapes display a single band, which confirms the involvement of the ester in the self-assemblies.

Graphical abstract: Self-assembling properties of a series of homologous ester-diamides – from ribbons to nanotubes
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