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1. Structure and morphology of calcium-silicate-hydrates cross-linked with dipodal organosilanes

   https://doi.org/https://doi.org/10.1016/j.cemconres.2020.106076  

   Moshiri, A. (July 2020, Cement and concrete research)

   Coupling of organic and inorganic chemistry presents a new degree of freedom in nano-engineering of thermo-mechanical properties of cement-based materials. Despite these vast technological benefits, molecular scale cross-linking of calcium-silicate-hydrate (C-S-H) gel with organic molecules still presents a significant challenge. Herein, we report experimental results on sol-gel synthesis, structure and morphology of nanocrystalline C-S-H cross-linked with dipodal organosilanes. These novel organic-inorganic gels have layered turbostratic molecular structure with similarities to C-S-H precipitating in hydrating cement paste. The organic molecules' chain length controls the interlayer spacing, which shows little to no shrinkage upon dehydration up to 105 °C. However, the structure gets distorted in the basal crystallite plane, in which dimer and trimer Si-polyhedra structures condense on a 2D hexagonal Ca-polyhedra layer. Cross-linked C-S-H gels display plate-like morphology with tendency toward stacking into agglomerates at the larger scale. If successfully realized in cement environment, e.g. high concentration seed, such novel organic-inorganic C-S-H gels could potentially provide cement-based matrices with unique properties unmatched by classical inorganic systems. 

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2. Local deformation and dynamics of cross-linked hyaluronic acid gels at charged interfaces

   https://doi.org/10.1039/D5SM00616C  

   Dhakal, Sujata; Chandrasekar, Samyuktha; Adedeji, Adediwura Deborah; Morozova, Svetlana (October 2025, Soft Matter)

   Hydrogel adhesion is a complex process that involves chain dynamics, thermodynamics, chemistry, and topology. Using fluorescent confocal microscopy in combination with fluorescent differential dynamic microscopy (fDDM), we have determined surface deformation and dynamics of cross-linked hyaluronic acid (HA) gels, equilibrated against 1–1000 mM NaCl solutions, at positively and negatively ionized surfaces. Due to the negative ionization of HA, the gels are repelled from negatively ionized glass surfaces creating a fluid separation layer and repulsion remains unaffected by salt concentration. At these interfaces, the gel network motion is slowed, as determined with fDDM in 167 mM ionic strength. To create positively ionized surfaces, poly-L-lysine is deposited on the glass surface. At higher salt concentrations, surface ionization has little effect, while in lower salt concentrations, the softer gels are compressed 4–6 times by the surface forces. In lower salt concentrations, the surface interactions are less screened and the gels are softer, leading to greater deformation. These results reveal that gel deformation and interfacial dynamics are governed by a delicate interplay between gel modulus, surface ionization, and ionic strength, underscoring the need for new theoretical models to predict soft gel behavior at interfaces and enabling the rational design of gel-based adhesives, coatings, and biointerfaces. 

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3. Fracture, fatigue, and friction of polymers in which entanglements greatly outnumber cross-links

   https://doi.org/10.1126/science.abg6320  

   Kim, Junsoo; Zhang, Guogao; Shi, Meixuanzi; Suo, Zhigang (October 2021, Science)

   Longer and stronger; stiff but not brittle Hydrogels are highly water-swollen, cross-linked polymers. Although they can be highly deformed, they tend to be weak, and methods to strengthen or toughen them tend to reduce stretchability. Two papers now report strategies to create tough but deformable hydrogels (see the Perspective by Bosnjak and Silberstein). Wanget al. introduced a toughening mechanism by storing releasable extra chain length in the stiff part of a double-network hydrogel. A high applied force triggered the opening of cycling strands that were only activated at high chain extension. Kimet al. synthesized acrylamide gels in which dense entanglements could be achieved by using unusually low amounts of water, cross-linker, and initiator during the synthesis. This approach improves the mechanical strength in solid form while also improving the wear resistance once swollen as a hydrogel. —MSL 

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4. Chemorheological Monitoring of Cross-Linking in Slide-ring Gels Derived From α-cyclodextrin Polyrotaxanes

   https://doi.org/10.3389/fchem.2022.923775  

   Dikshit, Karan; Bruns, Carson J. (July 2022, Frontiers in Chemistry)

   Despite hundreds of studies involving slide-ring gels derived from cyclodextrin (CD)-based polyrotaxanes (PRs), their covalent cross-linking kinetics are not well characterized. We employ chemorheology as a tool to measure the gelation kinetics of a model slide-ring organogel derived from α -cyclodextrin/poly (ethylene glycol) PRs cross-linked with hexamethylenediisocyanate (HMDI) in DMSO. The viscoelastic properties of the gels were monitored in situ by small-amplitude oscillatory shear (SAOS) rheology, enabling us to estimate the activation barrier and rate law for cross-linking while mapping experimental parameters to kinetics and mechanical properties. Gelation time, gel point, and final gel elasticity depend on cross-linker concentration, but polyrotaxane concentration only affects gelation time and elasticity (not gel point), while temperature only affects gelation time and gel point (not final elasticity). These measurements facilitate the rational design of slide-ring networks by simple parameter selection (temperature, cross-linker concentration, PR concentration, reaction time). 

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5. Relaxation dynamics of supramolecular polymer networks with mixed cross-linkers

   https://doi.org/10.1122/8.0000421  

   Xu, Donghua; Olsen, Bradley D.; Craig, Stephen L. (November 2022, Journal of Rheology)

   The linear rheological properties of supramolecular polymer networks formed by mixtures of two different bis-Pd(II) cross-linkers with poly(4-vinylpyridine) in dimethyl sulfoxide are examined. The changes in storage and loss moduli of the networks with mixed cross-linkers are compared to those of samples with a single type of cross-linkers. While the plateau moduli, and presumably network topology, of the networks remain equal regardless of the cross-link distribution, the relaxation time contributed by the faster cross-linkers is increased (by a factor of about 1.5 for the specific samples used in this work) by the presence of the slower cross-linkers, while the reverse influences are not significant. This effect can be explained by the fact that a certain fraction of the elastically effective strands cross-linked with fast cross-linkers is pinned on one end by slow cross-linkers, reducing by half the rate of fast chain relaxation. This effect is anticipated to be general for gels with two well-separated relaxation times. 

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