Supramolecular Arrangement and Rheological Properties of Bisamide Gels

We report a systematic study of the gelation behaviorof nBA 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 nBA gels are obtained from fitting our DSCN(T) model tothe experimental DSC data. The found T (m) (0) of nBA gels is about 35 & DEG;C lowerthan T (m) (0) of the pure nBA gelators. This is reasonably well explained by a simplemodel combining theories of Flory-Huggins and Gibbs free energyof melting (FHM model). We attribute this depression to an increasein entropy upon melting of the gel due to mixing with the solvent.The odd-even alternation in T (m) (0) of nBA gels, which was also found for the nBA gelators, indicates that the solid structures insidethe gels are somewhat similar. This was studied using XRD: similar00l reflections were found in the XRD patterns ofall nBA gels and their nBA gelators.For even nBA gels, the same reflections in the 19-25 & DEG;(2 & theta;) region confirm that the sheetlike supramolecular structureof the gels is analogous to the lamellar structure of the solid gelators.For odd nBA gels, a slight difference in the reflectionsaround 20-25 & DEG; (2 & theta;) implies a somewhat differentside-by-side packing of odd nBA gels compared tothe solid state. This variation is found for all the odd gels, andindeed, they show distinctly different morphologies compared to theeven nBA gels. The possible effect of this on therheological properties is discussed using some inspiration from theHalpin-Tsai model for composites where nBAgels are considered to be analogous to composite materials. The changeof the storage modulus (G & PRIME;) with the shapefactor of woven fibers and sheets in nBA gels (20wt %) indicates that a rheological odd-even effect might indeedbe present.