Covalent adaptable networks using boronate linkages by incorporating TetraAzaADamantanes.

Boronic esters prepared by condensation of boronic acids and diols have been widely used as dynamic covalent bonds in the synthesis of both discrete assemblies and polymer networks. In this study we investigate the potential of a new dynamic-covalent motif, derived from TetraAzaADamantanes (TAADs), with their adamantane-like triol structure, in boronic ester-based covalent adaptable networks (CANs). The TetraAzaADamantane-boronic ester linkage has recently been reported as a more hydrolytically stable boronic ester variant, while still having a dynamic pH response: small-molecule studies found little exchange at neutral pH, while fast exchange occurred at pH 3.8. In this work, bi- and trifunctional TetraAzaADamantane linkers were synthesised and crosslinked with boronic acids to form rubber-like materials, with a Young's modulus of 1.75 MPa. The dynamic nature of the TetraAzaADamantane networks was confirmed by stress relaxation experiments, revealing Arrhenius-like behaviour, with a corresponding activation energy of 142 ± 10 kJ/mol. Increasing the crosslinking density of the material from 10% to 33% resulted in reduced relaxation times, as is consistent with a higher degree of crosslinking within the dynamic networks. In contrast to the reported accelerating effect of acid addition to small-molecule TetraAzaADamantane complexes, within the polymer network the addition of acid increased relaxation times, suggesting unanticipated interactions between the acid and the polymer that cannot occur in the corresponding small-molecules analogues. The obtained boronate-TetraAzaADamantane materials were thermally stable up to 150°C. This thermal stability, in combination with the intrinsically dynamic bonds inside the polymer network, allowed these materials to be reprocessed and healed after damage by hot-pressing.

In-situ and quantitative imaging of evaporation-induced stratification in binary suspensions.

The drying of a multi-component dispersion, such as water-based paint, ink and sunscreen to form a solid film, is a widespread process. Binary colloidal suspensions have proven capable of spontaneous layer formation through size segregation during drying. To design bespoke stratification patterns, a deeper understanding of how these emerge is crucial. Here, we visualize and quantify the spatiotemporally evolving concentration profiles in situ and with high resolution using confocal fluorescence microscopy of custom-designed binary dispersions in a well-defined geometry. Our results conclusively establish two distinct stratification routes, which give rise to three layered structures. A first thin layer develops directly underneath the evaporation front in which large particles are kinetically trapped. At later times, asymmetrical particle interactions lead to the formation of two subsequent layers enriched in small and large particles, respectively. The spatial extent and magnitude of demixing strongly depend on the initial volume fraction. We explain and reproduce the experimental concentration profiles using a theoretical model based on dynamic arrest and higher-order thermodynamic and hydrodynamic interactions. These insights unravel the key mechanisms underlying colloidal auto-stratification in multi-component suspensions, and allow preprogramming of stratification patterns in single-deposition formulations for future applications.