Supramolecular Arrangement and Rheological Properties of Bisamide Gels.

We report a systematic study of the gelation behavior of n BA gelators in xylene, with odd and even n -methylene spacers between the amide groups ( n = 5-10) and 17 carbons at each end. The melting temperatures ( T m 0 ) of n BA gels are obtained from fitting our DSC N (T) model to the experimental DSC data. The found T m 0 of n BA gels is about 35 °C lower than T m 0 of the pure n BA gelators. This is reasonably well explained by a simple model combining theories of Flory-Huggins and Gibbs free energy of melting (FHM model). We attribute this depression to an increase in entropy upon melting of the gel due to mixing with the solvent. The odd-even alternation in T m 0 of n BA gels, which was also found for the n BA gelators, indicates that the solid structures inside the gels are somewhat similar. This was studied using XRD: similar 00 l reflections were found in the XRD patterns of all n BA gels and their n BA gelators. For even n BA gels, the same reflections in the 19-25° (2θ) region confirm that the sheetlike supramolecular structure of the gels is analogous to the lamellar structure of the solid gelators. For odd n BA gels, a slight difference in the reflections around 20-25° (2θ) implies a somewhat different side-by-side packing of odd n BA gels compared to the solid state. This variation is found for all the odd gels, and indeed, they show distinctly different morphologies compared to the even n BA gels. The possible effect of this on the rheological properties is discussed using some inspiration from the Halpin-Tsai model for composites where n BA gels are considered to be analogous to composite materials. The change of the storage modulus ( G ') with the shape factor of woven fibers and sheets in n BA gels (20 wt %) indicates that a rheological odd-even effect might indeed be present.