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

Abstract

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 dynamicmechanical 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 iminebased 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.

Phase separation in supramolecular and covalent adaptable networks

“…In our own studies on the phase separation in polyimine CANs, we also observed certain threshold concentrations for aromatically linked dianiline monomers compared to aliphatically linked imines. 66 Below this threshold concentration (generally between 10–20%, depending on the structure of the dianiline), no phase separation occurred, and the aromatic monomers blended in with the aliphatic matrix.…”

“…14A). 66 By making images using Raman confocal microscopy, we observed that polyimines synthesised from the non-sterically bulked 4,4'-methylenedianiline (MDA) showed phase separation into domains with sizes up till approximately 10 micrometer. However, when sterically demanding groups were introduced on the dianilines (either on the aromatic rings or on the methylene bridge), the phase separation was lost.…”

“…2). 7,66,67 It is also seen more often that analysis methods are combined, such as the combination of AFM with FT-IR (AFM-IR) 68,69 or AFM with tip-enhanced Raman spectroscopy (TERS). 70 These techniques are able to show phase separation on the scale of nanometers to micrometers.…”

“…We also observed in earlier work that phase separation in polyimine CANs was reversible, even after curing. 66 When paired with functionalities such as reduction of creep and shape memory, this becomes a viable method for creating smart materials.…”

Reprocessable Biobased Statistical and Block Copolymer Methacrylic-Based Vitrimers with a Shape Memory Effect

“…Another strategy for controlling creep in vitrimers involves utilizing phase separation. Although the influence of phase separation on viscoelastic properties of polymers is well known, only a handful of studies have explored it for vitrimers. − For example, Epps’ and Sumerlin’s groups investigated block copolymer vitrimers based on vinylogous urethane acrylates . They reported that compared to homogenous counterparts, the microphase-separated vitrimers exhibited lower creep due to lower network strand diffusion.…”