We have developed an analytical method to quantitatively analyze differential scanning calorimetry (DSC) experimental data. This method provides accurate determination of thermal properties such as equilibrium melting temperature, latent heat, change of heat capacity which can be performed automatically without intervention of a DSC operator. DSC is one of the best techniques to determine the thermal properties of materials. However, the accuracy of the transition temperature and enthalpy change can be affected by artifacts caused by the instrumentation, sampling, and the DSC analysis methods which are based on graphical constructions. In the present study, an analytical function (DSC_N(T)) has been developed based on an assumed Arrhenius crystal size distribution together with instrumental and sample-related peak broadening. The DSC_N(T) function was successfully applied to fit the experimental data of a substantial number of calibration and new unknown samples, including samples with an obvious asymmetry of the melting peak, yielding the thermal characteristics such as melting and glass transition temperature, and enthalpy and heat capacity change. It also allows very accurate analysis of binary systems with two distinct but severely overlapping peaks and samples that include a cold crystallization before melting.

This study intends to develop design rules for binary mixture of gelators that govern their assembly behavior and subsequently explore the impact of their supramolecular assembly patterns on the gels’ rheological properties. To achieve these goals, nBA gelators with odd and even parities [n-methylene spacers between the amide groups (n = 5-10) and 17 carbons at each end] were blended at different ratios. Such bisamides with simple structures were selected to study because their different spacer lengths offer the possibility to have matching or non-matching hydrogen bonds. The results show that the assembly behavior of binary mixtures of bisamide gelators is the same in the solid and gel states. Binary mixtures of gelators, which only differ two methylene moieties in the spacer length, form compounds and co-assemble into fibers and sheets observed for (5BA)₁(7BA)₁ and (6BA)₁(8BA)₁ mixtures, respectively. Binary gelator mixtures of the same parity and a larger spacer length difference still lead to mixing for the odd parity couple (5BA)₁(9BA)₁₎, but to partial phase separation for the even parity mixture (6BA)₁(10BA)₁. Binary mixtures of gelators of different parities gave complete phase separation in the solid state, and self-sorted gels consisting of discrete fibers and sheets in the gels of (5BA)₃(6BA)₁ and (5BA)₃(10BA)₁. The even-even binary gels (20 wt %) consisting of co-assembled sheets show higher G′ than odd-odd binary gels (20 wt %) consisting of co-assembled fibers. In general, the self-sorting of odd and even molecules into the separate primary structures results in a dramatic decrease of G′ compared to the co-assembled gels (20 wt %), except for (5BA)₁(9BA)₁ gel (20 wt %). It might be due to larger woven spheres in (5BA)₁(9BA)₁ gel (20 wt %), which probably have a less entangled gel network.

We report a systematic study of the gelation behavior of 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 (Tm0) of nBA gels are obtained from fitting our DSCN(T) model to the experimental DSC data. The found Tm0 of nBA gels is about 35 °C lower than Tm0 of the pure nBA 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 Tm0 of nBA gels, which was also found for the nBA gelators, indicates that the solid structures inside the gels are somewhat similar. This was studied using XRD: similar 00l reflections were found in the XRD patterns of all nBA gels and their nBA gelators. For even nBA 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 nBA gels, a slight difference in the reflections around 20-25° (2θ) implies a somewhat different side-by-side packing of odd nBA 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 nBA gels. The possible effect of this on the rheological properties is discussed using some inspiration from the Halpin-Tsai model for composites where nBA 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 nBA gels (20 wt %) indicates that a rheological odd-even effect might indeed be present.

The crystal structure and phase behavior of bisamide gelators are investigated using differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy, X-ray diffraction (XRD), and molecular modeling, aiming at a better understanding of bisamide gel systems. A homologous series of bisamide model compounds (nBAs) was prepared with the (CH₂)_nspacer between the two amide groups, where n varies from 5 to 10, and with two symmetric C17 alkyl tails. With increasing spacer length, the thermal properties show a clear odd-even effect, which was characterized using our newly developed analytical model DSC_N(T). Using XRD, all studied nBA compounds turn out to have a layer-like structure. The XRD patterns of the odd BA series are very similar but show marked differences compared to the XRD patterns of the even series, which in turn are very similar. The odd-membered 5BA molecules are nearly perpendicular to the stacked layers, as described by a pseudo-orthorhombic unit cell, whereas the even-membered 6BA molecules are tilted at an angle with respect to the layer normal, as described by a triclinic unit cell. In both the odd and even series, the inter-layer interaction is the van der Waals interaction. The 6BA hydrogen bonding scheme is very similar to that of Nylon 6,10 α, unlike the 5BA H bonding scheme. The packing of the C17 alkyl tails in the 5BA layers is similar to polyethylene, and unlike 6BA. The slightly higher crystalline density of 6BA (1.038 g cm^-3) as compared to 5BA (1.018 g cm^-3) explains the higher melting point, higher enthalpy of fusion, and the observed shift of N-H stretch bands to higher wave numbers. The structural differences observed between the odd and even BA series reflect the different structure-directing effect of parallel versus antiparallel amide hydrogen bonding motifs. These differences underlie the observed odd-even effect in the thermal properties of nBA compounds.