Phase Change Transformations with Dynamically Addressable Aminal Metallogels

Abstract

Dynamic polymers assembled through hemiaminal and aminal functionalities reversibly fragment upon binding to trivalent metals. Gels produced with these dynamic polymers are broken down to liquids after the addition of metal salts. Nuclear magnetic resonance spectroscopy studies and density functional theory calculations of intermediates reveal that the presence of these metals causes shifts in the energetic landscape of the intermediates in the condensation pathway to render stable nonequilibrium products. These species remain stable in the liquid phase at room temperature but convert to gels upon heating. With thermal activation, the fragmented ligands transform catalytically into closed-ring hexahydrotriazine products, which are macroscopically observable as new gels with distinct physical properties. The interplay between equilibrium and nonequilibrium gels and liquids and the ligands responsible for these transformations has been observed rheologically, giving controlled gel times dictated by the thermodynamics and kinetics of the system. This constitutionally dynamic macromolecular system offers the possibility of harnessing an equilibrium/nonequilibrium system in tandem with its inherent self-healing properties and triggered release functionality.

Coordination Dynamics and Thermal Stability with Aminal Metallogels and Liquids

“…These transformations occur over approximately 30 min at 0 °C. The depletion of these peaks indicates the conversion of hemiaminal in to aminal (hexahydrotriazine) [11]. Figure 3b shows the spectral region from 4.93 to 4.69 ppm where the R–NH– CH 2 –OH functionalities of the aluminium catalysed system deplete over 3 h at −25 °C.…”

“…This acyclic hemiaminal is believed to be responsible for the initial gel state observed at room temperature. The ring-closing reaction leading to products III and VI requires either heating under pressure (i.e., ≥3 MPa) or mild heating (i.e., ≥70 °C) in the presence of a catalyst [11].…”

“…The addition of more metal was found to slow the reaction as the metal stabilizes the starting materials and earlier, less networked reaction intermediates. The relative molar equivalents of aluminum chloride to the triamine increased from 1.1 equivalents to 2.3 equivalents giving sol-gel transitions in the range of 0.6 to 6.3 h, correspondingly [11].…”

“…We have previously demonstrated the possibility of using these materials for controlled triggered additive release [10], reversible gelling fluids, and self-healing polymer gels. In this article, we describe the dynamic chemistry of PHTs to highlight the effect of metals on the thermodynamics and kinetics of the condensations between amines and aldehydes to hemiaminal gels, and finally the PHT gel phase [11].…”

Conductive Recyclable Organogel Composites

“…PEG‐PHT networks are not thermally accessible since PEG decomposition occurs at temperatures lower than what is required for the ring closing step ( Figure ); therefore, accessing it through solvent choice is an attractive alternative synthetic approach and negates the addition of a catalyst . The resulting PHT network was stable in water indefinitely and had a higher modulus (≈6 kPa) than HDCNs due to the high crosslink density and lower solvent content (Figure S17, Supporting Information).…”