Supramolecular motifs in dynamic covalent PEG-hemiaminal organogels

Dynamic covalent materials are stable materials that possess reversible behaviour triggered by stimuli such as light, redox conditions or temperature; whereas supramolecular crosslinks depend on the equilibrium constant and relative concentrations of crosslinks as a function of temperature. The combination of these two reversible chemistries can allow access to materials with unique properties. Here, we show that this combination of dynamic covalent and supramolecular chemistry can be used to prepare organogels comprising distinct networks. Two materials containing hemiaminal crosslink junctions were synthesized; one material is comprised of dynamic covalent junctions and the other contains hydrogen-bonding bis-hemiaminal moieties. Under specific network synthesis conditions, these materials exhibited self-healing behaviour. This work reports on both the molecular-level detail of hemiaminal crosslink junction formation as well as the macroscopic behaviour of hemiaminal dynamic covalent network (HDCN) elastomeric organogels. These materials have potential applications as elastomeric components in printable materials, cargo carriers and adhesives.


Supplementary Tables
.  Table S3. However, the network formed by PEG diamine and paraformaldehyde completely reverts back to the starting material (PEG-diamine) in the presence of water, as indicated in Figure S4 and Figure S5. When the spectrum for a degraded HDCN was compared to a spectrum taken during the HDCN forming reaction at t < t gel (~30 min.), the NMR spectrum for the degraded HDCN indicates that we have reverted the network to primarily PEG diamine. The triplet at  = 3.13 ppm and singlet at  = 2.87 ppm, attributable to H-bonded NMP in the network, disappears after water treatment. Powdered PEG diamine can be recovered after the HDCN structure has been reverted in water.
The reaction mechanism proposed in Figure 2 relies upon the presence of water.
We argue that the concentration of water required to drive the formation of hemiaminal linkages is low (3 mg to have a stoichiometric equivalency). The spectra shown in Figure S7 indicate that dry conditions drastically slow the reaction. The kinetics are significantly slowed compared to "wet" conditions and mono-hemiaminal 2 is cleanly formed within the first 5 min at room temperature. After 1 hour, two more hemiaminal

H NMR Characterization of HDCN Networks and Model Compounds
We studied the time-dependent evolution of a PEG-HDCN network by sampling aliquots of HDCN precursors in a 50°C network-forming reaction. The signal for hexahydrotriazines as well as hemiaminals in the PEG-diamine HDCN forming reaction, if present, would be engulfed by the signal for PEG (~3.30 ppm). The signals for hexahydrotriazine (8) and hemiaminals (6 & 7) are not observed in Figure S2.
The model compound for PEG-diamine (2-methoxyethylamine, 1) was reacted with 2.2 equiv. paraformaldehyde at 110°C for 1.5 hours in d 6  recommendations for molecules optimized in implicit solvent. 16 Normal modes of all structures were examined to verify that equilibrium structures possess no imaginary frequencies and that one imaginary frequency corresponding to bond formation or bond breaking was obtained for transition state structures. Intrinsic reaction coordinate (IRC) calculations were also performed to verify that transition states are connected to reactant complexes and intermediates on the potential energy surfaces of reactions.
Partial atomic charges have been computed from an electrostatic potential fit that preserves total molecular charge, dipole and quadrupole moments.
The following settings were used in GAMESS-US.

Compressive Mechanical Characterization
The compressive modulus and hysteresis behavior of HDCN organogels were characterized using an Instron 5800 testing station equipped with a 10 kN load cell. The hysteresis behavior of gels was explored by cyclically loading and unloading HDCN gels within the elastomeric range of organogels. The stress-strain behavior of these gels was characterized using compressive mechanical analysis. Samples for compressive mechanical testing were formed in 25 mm diameter reaction vials. The compressive mechanical testing protocol was developed using this specimen size. Compressive mechanical testing experiments were conducted at ambient conditions; HDCNs were compressed at a rate of 0.1 (mm/mm)/min. Hysteretic compressive experiments were conducted within the range of 60% deformation at the same compressive load and unload rate. The energy dissipated by compressing the elastomeric gel networks was quantified by measuring the area between loading and unloading cyclic curves. The energy dissipated is expressed in terms of the Joules per mol PEG to normalize for the polymer content in each sample.