Phase separation in supramolecular and covalent adaptable networks.

Phase separation phenomena have been studied widely in the field of polymer science, and were recently also reported for dynamic polymer networks (DPNs). The mechanisms of phase separation in dynamic polymer networks are of particular interest as the reversible nature of the network can participate in the structuring of the micro- and macroscale domains. In this review, we highlight the underlying mechanisms of phase separation in dynamic polymer networks, distinguishing between supramolecular polymer networks and covalent adaptable networks (CANs). Also, we address the synergistic effects between phase separation and reversible bond exchange. We furthermore discuss the effects of phase separation on the material properties, and how this knowledge can be used to enhance and tune material properties.

Metal Coordination in Polyimine Covalent Adaptable Networks for Tunable Material Properties and Enhanced Creep Resistance.

Covalent adaptable networks (CANs) can replace classical thermosets, as their unique dynamic covalent bonds enable recyclable crosslinked polymers. Their creep susceptibility, however, hampers their application. Herein, an efficient strategy to enhance creep resistance of CANs via metal coordination to dynamic covalent imines is demonstrated. Crucially, the coordination bonds not only form additional crosslinks, but also affect the imine exchange. This dual effect results in enhanced glass transition temperature (Tg ), elasticmodulus (G') and creep resistance. The robustness of metal coordination is demonstrated by varying metal ion, counter anion, and coordinating imine ligand. All variations in metal or anion significantly enhance the material properties. The Tg and G' of the CANs are correlated to the coordination bond strength, offering a tunable handle by which choice of metal can steer material properties. Additionally, large differences in Tg and G' are observed for materials with different anions, which are mostly linked to the anion size. This serves as a reminder that for coordination chemistry in the bulk, not only the metal ion is to be considered, but also the accompanying anion. Finally, the reinforcing effect of metal coordination is proved insensitive to the metal-ligand ratio, emphasizing the robustness of the applied method.

Raman Spectroscopy Reveals Phase Separation in Imine-Based Covalent Adaptable Networks.

The introduction of dynamic covalent bonds into cross-linked polymer networks enables the development of strong and tough materials that can still be recycled or repurposed in a sustainable manner. To achieve the full potential of these covalent adaptable networks (CANs), it is essential to understand-and control-the underlying chemistry and physics of the dynamic covalent bonds that undergo bond exchange reactions in the network. In particular, understanding the structure of the network architecture that is assembled dynamically in a CAN is crucial, as exchange processes within this network will dictate the dynamic-mechanical material properties. In this context, the introduction of phase separation in different network hierarchies has been proposed as a useful handle to control or improve the material properties of CANs. Here we report-for the first time-how Raman confocal microscopy can be used to visualize phase separation in imine-based CANs on the scale of several micrometers. Independently, atomic force microscopy (AFM) confirmed the phase-separated domains inside the polymer. Remarkably, the materials were found to undergo phase separation despite being built up from miscible monomers, which arguably should yield homogeneous materials. We found that the phase separation not only affected the appearance of the material but-more notably-also had a noticeable effect on the thermal-mechanical properties of the material: CANs (of equal aliphatic/aromatic monomer composition) that displayed phase separation had both a higher crossover temperature (T cross, where tan(δ) = 1, and where the material transits from a rubbery to a viscous state) and an increased elastic modulus (G'). By modifying the CAN architecture, we were able to either suppress or enhance the phase separation, and we propose that the phase separation is driven by favorable π-π interactions between the aromatic components. Our work further shows the importance of phase separation in CANs, including in networks built from miscible components, and provides a handle to control the dynamic material properties. Moreover, our work underlines the suitability of Raman imaging as a method to visualize phase separation in CANs.

Molecular control over vitrimer-like mechanics - tuneable dynamic motifs based on the Hammett equation in polyimine materials.

In this work, we demonstrate that fine-grained, quantitative control over macroscopic dynamic material properties can be achieved using the Hammett equation in tuneable dynamic covalent polyimine materials. Via this established physical-organic principle, operating on the molecular level, one can fine-tune and control the dynamic material properties on the macroscopic level, by systematic variation of dynamic covalent bond dynamics through selection of the appropriate substituent of the aromatic imine building blocks. Five tuneable, crosslinked polyimine network materials, derived from dianiline monomers with varying Hammett parameter (σ) were studied by rheology, revealing a distinct correlation between the σ value and a range of corresponding dynamic material properties. Firstly, the linear correlation of the kinetic activation energy (E a) for the imine exchange to the σ value, enabled us to tune the E a from 16 to 85 kJ mol-1. Furthermore, the creep behaviour (γ), glass transition (T g) and the topology freezing transition temperature (T v), all showed a strong, often linear, dependence on the σ value of the dianiline monomer. These combined results demonstrate for the first time how dynamic material properties can be directly tuned and designed in a quantitative - and therefore predictable - manner through correlations based on the Hammett equation. Moreover, the polyimine materials were found to be strong elastic rubbers (G' > 1 MPa at room temperature) that were stable up to 300 °C, as confirmed by TGA. Lastly, the dynamic nature of the imine bond enabled not only recycling, but also intrinsic self-healing of the materials over multiple cycles without the need for solvent, catalysts or addition of external chemicals.