Molecular anisotropy and rearrangement as mechanisms of toughness and extensibility in entangled physical gels

Abstract

Dynamic networks formed by physically crosslinked, entangled polymers have emerged as self-healing, stretchable, and functional materials. Entangled associative gels with remarkable toughness and extensibility have been produced by several distinct chemical approaches, suggesting that these enhanced mechanical properties result from molecular-scale topology. Previously, artificially engineered associative proteins were designed to provide an ideal model system to investigate the role of entanglement on gel mechanics via welldefined entangled or unentangled physical gels. Herein, uniaxial strain-induced structural changes in these model gels were observed using in situ small-angle x-ray scattering (SAXS) and in situ polarized optical microscopy (POM) up to 2000% engineering strain. Anisotropic optical responses to uniaxial strain at the nano-, micro-, and macroscales suggest that stress dissipation mechanisms enable high extensibility and toughness. Nano-and microscopic anisotropy observed by SAXS indicate stretching and alignment of flexible polymer strands along the straining axis, and nonmonotonic macroscopic anisotropy observed by POM suggests relaxation within the hydrogel due to rearrangement of associative network junctions. Unentangled hydrogels exhibit low toughness and a strain-rate-dependent transition from ductile to brittle tensile behavior, which is typical for associative polymer solutions. These findings indicate that topological entanglements and the freedom of individual chains to align at the nanoscale due to junction relaxation are both critical to achieving high toughness and elongation in entangled physical gels.

De novoevolution of macroscopic multicellularity

“…As these clusters are densely packed, moving one component would require moving many other components as well. Based on these observations, we hypothesized that branches are entangled, in a manner reminiscent of physical gels (30) and entangled granular materials (31). Entanglement would provide a mechanism for branches of cells to remain in the same, densely packed group even after cell-cell bonds break.…”

Extreme Extensibility in Physically Cross-Linked Nanocomposite Hydrogels Leveraging Dynamic Polymer–Nanoparticle Interactions

“…The Deborah number ( De ) is defined as , which represents a ratio of the time scales of interest (τ R or τ S ) and the time scale of the applied strain rate of our fiSER experiments. 57 When both the bulk relaxation and recovery time scales are nondimensionalized and plotted against the extensional strain to break, a clear trend emerges with the bulk relaxation time ( Figures 7 and S15 ), suggesting that the faster relaxation dynamics of PNP hydrogels comprising more hydrophobic NPs play a role in increasing the extensibility of these materials at these time scales. While the bulk relaxation time scale trends with extensibility, it is possible that structural changes arising from incorporation of the modified NPs may occur that also affect the extensibility.…”

“…It is important to note that these materials are formulated with 93% water content, which is significantly higher than previously published physically cross-linked hydrogel materials exhibiting high extensibility. 8 , 15 , 49 , 57 Stress data during extension demonstrated that the PNP hydrogels containing hydrophilic NPs reached higher stress values with deformation, approaching their yield stresses, but then dramatically failed ( Figure S9 ). In contrast, PNP hydrogels containing hydrophobic NPs gradually dissipate stress well above their yield stress values during elongation.…”

“… Ashby style plot of the materials space of dynamic yield stress and extensional strain to break. Three clear regions emerge in this space: (i) the green oval represents standard yield stress materials, 15 (ii) the purple oval represents designer materials 2 , 15 , 57 , 65 and consumer products 15 with high yield stress behavior and high extensibility, and (iii) the red oval represents a new design space enabled by this work with materials exhibiting low yet measurable yield stress behavior and high extensibility. Data shown with black symbols are reproduced from Nelson et al 15 …”

Intelligent Polymers, Fibers and Applications

“…From the level of macroscopic consideration, the adaptive performances of intelligent materials are achieved by the autonomous behaviors of molecules or atoms at a nanoscopic level [ 35 ]. When stimuli are introduced, aggregation, rearrangement and directional movement occur among the molecules and atoms [ 36 , 37 ].…”