Effect and Evolution of Nanostructural Complexity in Sensitive Polymer Gels

Abstract

Sensitive polymer gels consist of swollen networks of crosslinked macromolecules subject to a delicate interplay with their environment, allowing them to be swollen and deswollen or crosslinked and decrosslinked upon external stimulation. Such environmental sensitivity can be realized by the use of covalently crosslinked polymer networks that exhibit critical miscibility with their swelling medium or by the use of transient and reversible, supramolecular chain crosslinking. Both classes of sensitive gels are established and used in a variety of applications. However, to make this truly useful, systematic relations between the structure, dynamics, and properties of these gels must be derived. In addition to the specific trigger of gel responsiveness, a prime factor of influence in this interplay is the nanostructural polymer‐network topology. Whereas a simplistic view of a polymer network is that of an ideal, regular array of monodisperse meshes, real polymer networks often display polydisperse mesh sizes, along with structural imperfections such as loops and dangling chains, and along with further spatial inhomogeneities of their crosslinking density on a length scale of several tens of nanometers. This article summarizes recent effort to consistently probe the impact and temporal evolution of such nanostructural complexity in both gels with critical polymer–solvent miscibility and in gels with transient supramolecular crosslinking. For this purpose, different designed model systems are introduced to control and probe the polymer‐network nanostructural heterogeneity as an experimental variable. This is done using techniques such as rheology, fluorescence recovery after photobleaching, light scattering, and neutron scattering.

RETRACTED: Controlled Light Cross-Linking Technique to Prepare Healable Materials

“…Furthermore, with SAXS, it is not possible to distinguish between a mesh resulting from chemically cross-linked polymer chains or from physical entanglements of chains or a mixture of both. Thus, we must assume that a physical reorganization of the network structure is also included (for a detailed investigation of this nano structural complexity, see [34].…”

Influence of supramolecular forces on the linear viscoelasticity of gluten

“…The concept of reversibility of non-covalent transient bonds in network generation is well established in the area of supramolecular polymeric materials. In contrast to "conventional" polymeric materials, the strength of supramolecular systems is generated by the "stickiness" of the transient interactions and not only by the covalent bonds or chain entanglements (Herbst et al 2013;Seiffert 2015;Seiffert and Sprakel 2012). Sticky reptation dynamics (Bromberg 1998;Leibler et al 1991;Rubinstein and Dobrynin 1999) is frequently employed in rheology of biological materials, as for instance, in gelatin gels (van Vliet and Walstra 1995), alginate gels (Baumberger and Ronsin 2009), hybrid hydrogels (Sun et al 2012) or gluten composites (Kontogiorgos and Dahunsi 2014) where supramolecular forces prevail.…”

Proton conductivity in crosslinked hydrophilic ionic polymer system: Competitive hydration, crosslink heterogeneity, and ineffective domains

“…Generally, when mixture properties vary by several orders of magnitude, it is an indication of the formation of heterogeneous domains which grow and interconnect with each other . One of the sources of heterogeneity in crosslinked hydrophilic polymers is due to the stacking of crosslinking points termed as crosslink heterogeneity, also called static inhomogeneity . The stacking refers to higher density of crosslink nodes, leading to formation of domains.…”