Zooming in: Structural Investigations of Rheologically Characterized Hydrogen-Bonded Low-Methoxyl Pectin Networks

Self-assembled hydrogen-bonded networks of the polysaccharide pectin, a mechanically functional component of plant cell walls, have been of recent interest as biomimetic exemplars of physical gels, and the microrheological and strain-stiffening behaviors have been previously investigated. Despite this detailed rheological characterization of preformed gels, little is known about the fundamental arrangement of the polymers into cross-linking junction zones, the size of these bonded regions, and the resultant network architecture in these hydrogen-bonded materials, especially in contrast to the plethora of such information available for their well-known calcium-assembled counterparts. In this work, in concert with pertinent rheological measurements, an in-depth structural study of the hydrogen-bond-mediated gelation of pectins is provided. Gels were realized by using glucona-delta-lactone to decrease the pH of solutions of pectic polymers that had a (blockwise) low degree of methylesterification. Small-angle X-ray scattering and transmission electron microscopy were utilized to access structural information on length scales on the order of nanometers to hundreds of nanometers, while complementary mechanical properties were measured predominantly using small amplitude oscillatory shear rheology.