Small Angle Neutron Scattering (SANS) Studies on the Structural Evolution of Pyromellitamide Self-Assembled Gels

Abstract

The kinetics of aggregation of two pyromellitamide gelators, tetrabutyl- (C4) and tetrahexyl-pyromellitamide (C6), in deuterated cyclohexane has been investigated by small angle neutron scattering (SANS) for up to 6 days. The purpose of this study was to improve our understanding of how self-assembled gels are formed. Short-term (< 3 h) time scales revealed multiple phases with the data for the tetrabutylpyromellitamide C4, indicating one-dimensional stacking and aggregation corresponding to a multifiber braided cluster arrangement that is about 35 Å in diameter. The corresponding tetrahexylpyromellitamide C6 data suggest that the C6 also forms one-dimensional stacks but that these aggregate to a thicker multifiber braided cluster that has a diameter of about 62 Å. Over a longer period of time, the radius, persistence length, and contour length all continue to increase in 6 days after cooling. These data suggest that structural changes in self-assembled gels occur over a period exceeding several days and that fairly subtle changes in the structure (e.g., tail-length) can influence the packing of molecules in self-assembled gels on the single-to-few fiber bundle stage.

Drying Affects the Fiber Network in Low Molecular Weight Hydrogels

“…Since the scattering is collected in the wet state, it is tempting to assume that the microscopy suffers from drying artifacts, especially since the structures imaged have a higher apparent radius than that suggested by the scattering data. 34 − 39 …”

Thermal annealing behaviour and gel to crystal transition of a low molecular weight hydrogelator

“…In other systems, gel aging involves Ostwald ripening where smaller fibres are lost in preference to larger and more stable structures over time, leading to a more robust gel. 31,32 Other gel systems may become more homogeneous over time after the initial formation of the network, again leading to a more robust gel. 33 A fascinating recent example of a gel-gel transition is a carbonate-triggered transition from an opaque to transparent dipeptide gel, accompanied by an increase in gel strength, explained by a transition from spherical to fibrous structures.…”

Controlling self-assembly of diphenylalanine peptides at high pH using heterocyclic capping groups

“…SANS measurements have been extensively used to probe the native structure of peptide-based hydrogels, most often on hydrogel samples which already exhibit a well-defined network151617181920212223242526. Less well studied is small angle scattering on transitions within gelators; however there are examples of SANS being used to examine the transition of a hexapeptide from ribbons to fibers27 and using SANS to look at the evolution of a pyromellitamide organogel over several days28. Curiously, there are limited examples of small angle scattering being used to probe the early stages of gelation in a supramolecular peptide hydrogel1529, perhaps owing to difficulties in obtaining usable time resolution.…”