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

Order–Order Transition between Equilibrium Ordered Bicontinuous Nanostructures of Double Diamond and Double Gyroid in Stereoregular Block Copolymer

While ordered bicontinuous double diamond (OBDD) in block copolymers has always been considered as an unstable structure relative to ordered bicontinuous double gyroid (OBDG), here we report the existence of a thermodynamically stable OBDD structure in a diblock copolymer composed of a stereoregular block. A slightly asymmetric syndiotactic polypropylene-<i>block</i>-polystyrene (sPP-<i>b</i>-PS) as cast from xylene was found to display the OBDD morphology. When the OBDD-forming diblock was heated, this structure transformed to the OBDG phase at ca. 155 °C. Interestingly, OBDD was recovered upon cooling even in the temperature range above melting point of sPP, indicating that OBDD was a thermodynamically stable structure for sPP-<i>b</i>-PS melt, which was in contradiction to the conventional view. We propose that the larger free energy cost encountered in OBDD due to the larger packing frustration may be compensated sufficiently by the release of free energy due to local packing of the conformationally ordered segments of sPP blocks, which stabilizes the OBDD structure at the lower temperatures

Hierarchical Structure and Crystal Orientation in Poly(ethylene oxide)/Clay Nanocomposite Films

Water-cast nanocomposite films formed by poly­(ethylene oxide) (PEO) and Laponite clay were found to display three characteristic levels of structure with large-scale orientation. The first level with the length scale of ca. 30–50 nm was the clay lamellar bundles, which tended to stack perpendicularly to the film surface. The second level with the characteristic length of 1.8 nm was associated with the alternating stacking of the silicate layers and the PEO chains sandwiched between them. The preferred orientations of these two levels of structure were independent of clay content, solvent removal rate for the film preparation, and the crystallization temperature of the PEO chains situating outside the clay bundles. The third level of structure was characterized by the preferred orientation of the PEO crystalline stems with respect to the surface of the silicate layers. Perpendicular orientation always dominated in the nanocomposite films prepared by slow solvent removal irrespective of crystallization temperature. In the films prepared by fast solvent removal, however, parallel crystal orientation set in as the clay concentration exceeded ca. 33 wt %. The preferred crystal orientation was ascribed to the confinement effect imposed by the clay bundles to the crystallization of the PEO chains situating in the interbundle region. In the films cast by slow solvent removal, the weaker confinement associated with the larger interbundle distance led to perpendicular crystal orientation. When the interbundle distance was reduced to ca. 30 nm in the films prepared by rapid solvent evaporation, the strong confinement directed the crystals to form parallel orientation