Correlation between Self-Assembled Nanostructures and Bond Exchange Properties for Polyacrylate-Based Vitrimer-like Materials with a Trans-N-Alkylation Bond Exchange Mechanism

Covalent Adaptable Networks Mediated by Redox-Responsive Neighboring-Group-Participating Transalkylation

Covalent adaptable networks (CANs) typically require external catalysts to facilitate efficient crosslinker exchange, which can limit the reprocessability of the network due to leaching and degradation of the catalyst. In this study, the use of catalysts was avoided by employing a bicyclo[3.3.1]nonane (BCN) bis-alkyl halide crosslinker with selenium-based neighboring-group-participation (NGP) to enhance the rate of bond exchange. This thermally mediated C─N alkyl exchange and the associated flow behavior enabled the intrinsically ionic network (which possesses antimicrobial properties) to be both chemically recycled and repaired and reprocessed under mild conditions. Furthermore, the dynamic behavior of the network can be regulated by the reversible redox responsiveness of selenium atoms within the network. This novel type of NGP-based CAN therefore has the potential to enrich designs for catalyst-free dynamic networks with high performance and modulated dynamicity.

Understanding the Topology Freezing Temperature of Vitrimer-Like Materials through Complementary Structural and Rheological Analyses for Phase-Separated Network

Vitrimers are sustainable cross-linked polymers characterized by an associative bond exchange mechanism within their network. A well-known feature of vitrimers is the Arrhenius dependence of the viscosity or relaxation time. Another important aspect is the existence of a topology-freezing temperature (Tv), which represents a transition between the viscoelastic solid state and the malleable viscoelastic liquid state. Various methods, including viscosity-temperature plots and temperature-ramp creep (or dilatometry), have been proposed for determining the Tv. In this study, we complementarily employ X-ray scattering-based structural analysis and rheological analysis to assign Tv in phase-separated vitrimer-like materials undergoing trans-N-alkylation bond exchange. Note that the trans-N-alkylation progresses via the dissociative bond exchange pathway, whereas our previous studies demonstrated that the temperature-dependence of relaxation time followed the Arrhenius dependence, which was the reason for the classification as a vitrimer-like material. Specifically, we identify Tv as the temperature at which an anomalous increase in domain distance occurs during the rubbery state in the structural analysis. In the rheological analysis, Tv corresponds to the transition temperature marking the shift from the Williams-Landel-Ferry dependence to the Arrhenius dependence in the shift factors used to create master curves for frequency sweep rheology data. Importantly, both methods yield nearly the same Tv, validating the accuracy of the proposed assignment and, thus, providing valuable insights into the specific properties of vitrimers.

Structure-property relationships to direct the dynamic properties of acylsemicarbazide-based materials

Secondary interactions, such as hydrogen bonding or phase separation, can enhance the stability of dynamic covalent materials without compromising on desired dynamic properties. Here, we investigate the combination of multiple secondary interactions in dynamic covalent materials based on acylsemicarbazides (ASCs), with the aim of achieving tunable material properties. The effects of different ASC substituents on the dynamic covalent and hydrogen bonding capabilities were investigated in a small molecule study using a combined experimental and theoretical approach, and revealed the presence of cooperative hydrogen-bonding interactions in 2 directions in one of the derivatives. The different motifs were subsequently incorporated into polymeric materials. Combining ASC motifs capable of strong, multiple hydrogen bonding with a polydimethylsiloxane backbone introduces structure-dependent, ordered nanophase separation in polymeric materials. The thermo-mechanical properties of the materials reveal a strong dependance on the hydrogen-bonding structure and exact nature of the ASC bond. The dynamic behavior in bulk shows that bond exchange depends on the dissociation rate obtained from ASC model compounds, as well as the strength of the secondary interactions in these materials. Differences in hydrogen-bonding structures of the ASC motifs also cause differences in creep resistance of the materials. Interestingly, the materials with strong, ordered and cooperative hydrogen-bonded clusters show the highest creep resistance. Our results demonstrate that tuning both the dissociation rate and the secondary interactions by molecular design in dynamic covalent materials is important for controlling their thermal stability and creep resistance.

Vitrimer-like elastomers with rapid stress-relaxation by high-speed carboxy exchange through conjugate substitution reaction

We report vitrimer-like elastomers that exhibit significantly fast stress relaxation using carboxy exchange via the conjugate substitution reaction of α-(acyloxymethyl) acrylate skeletons. This network design is inspired by a small-molecule model that shows the carboxy exchange reaction even at ambient temperature in the presence of 1,4-diazabicyclo[2.2.2]octane (DABCO). The acrylate and acrylic acid copolymers are cross-linked using bis[α-(bromomethyl)acrylates] and doped with 10 wt% DABCO, exhibiting processability to obtain a transparent film by hot pressing. The high-speed bond exchange in the network, validated by stress-relaxation tests, allows quick molding with household iron. In addition, the material is applied as an adhesion sheet for plastic and metal substrates. Because dynamic cross-linking with the proposed bond exchange mechanism can be implemented for any polymer bearing carboxyl pendants, our approach can be applied to versatile backbones, which must thus be meaningful in the practical sense.

Dynamics in polymers with phase separated dynamic bonds: the case of a peculiar temperature dependence

The topic of polymers with dynamic bonds (stickers) appears as an exciting and promising area of materials science, thanks to their attractive self-healable, recyclable, extremely tough, and super extensible properties. Polymers with phase separated dynamic bonds revealed several unique properties, but mechanisms controlling their viscoelastic properties remain poorly understood. In this work, we present a dynamic analysis of a model polymer system with phase separated hydrogen bonding functionalities. The results confirm that terminal relaxation in these systems is independent of polymer segmental dynamics and is instead controlled by structural relaxations in clusters of stickers. Detailed analysis revealed a surprising result: terminal relaxation time of these systems has weaker temperature dependence than that of structural relaxation in clusters, although the former is slower than the latter. Borrowing ideas from the field of block copolymers, we ascribed this unusual result to an LCST-like behavior for the miscibility of the stickers in the polymer matrix. The presented results and ideas deepen the understanding of the viscoelasticity for polymers with dynamic bonds, enabling intelligent design of functional materials with desired macroscopic properties.

Trapping bond exchange phenomenon revealed for off-stoichiometry cross-linking of phase-separated vitrimer-like materials

Vitrimer materials combined with nano-phase separated structures have attracted attention, expanding the tuning range of physical properties, such as flow and creep properties. We recently demonstrated a preparation of vitrimer-like materials with phase-separated nanodomains in which dissociative bond exchange via trans-N-alkylation of quaternized pyridine was operated. In this study, we demonstrate a new finding about the bond exchange mechanism: that is, the trapping bond exchange phenomenon. The component polymer is a poly(acrylate) containing pyridine side groups randomly along the chain, which is cross-linked by diiodo molecules via pyridine-iodo quaternization, where the quaternized pyridines are aggregated to form nano-size domains. When the cross-linking reaction is performed at an off-stoichiometric pyridine : iodo ratio (i.e., an excess of pyridine groups), free pyridine groups are located in the matrix phase. Since the bond exchange in the present system progresses in an inter-domain manner, the dissociated unit bearing pendant iodo is trapped by the free pyridine groups in the matrix, which generates other small aggregates. This trapping phenomenon greatly affects the relaxation and creep properties, which are very different from those found in conventional knowledge about vitrimer physics.

Microscopic Evidence for the Topological Transition in Model Vitrimers

In addition to the glass transition, vitrimers undergo a topological transition from viscoelastic liquid to viscoelastic solid behavior when the network rearrangements facilitated by dynamic bond exchange reactions freeze. The microscopic observation of this transition is elusive. Model polyisoprene vitrimers based on imine dynamic covalent bonds were synthesized by reaction of α,ω-dialdehyde-functionalized polyisoprenes and a tris(2-aminoethyl)amine. In these dynamic networks nanophase separation of polymer and reactive groups leads to the emergence of a relevant length scale characteristic for the network structure. We exploited the scattering sensitivity to structural features at different length scales to determine how dynamical and topological arrests affect correlations at segmental and network levels. Chains expand obeying the same expansion coefficient throughout the entire viscoelastic region, i.e., both in the elastomeric regime and in the liquid regime. The onset of liquid-like behavior is only apparent at the mesoscale, where the scattering reveals the reorganization of the network triggered by bond exchange events. The such determined "microscopic" topological transition temperature is compared with the outcome of "conventional" methods, namely viscosimetry and differential scanning calorimetry. We show that using proper thermal (aging-like) protocols, this transition is also nicely revealed by the latter.

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.

Unusual Stress Upturn in Elastomers Prepared Using Macro Cross-Linkers with Multiple Vinyl Side Groups

In this study, the unique tensile properties of acrylate elastomers prepared using macro cross-linker polymers with multiple vinyl side groups are analyzed. For the preparation of the macro cross-linker, poly(ethyl acrylate) copolymers bearing hydroxy functional groups are synthesized, followed by the hydroxy-isocyanate reaction with 2-isocyanatoethyl acrylate. Subsequently, the elastomers samples are prepared by UV polymerization of ethyl acrylate in the presence of the macro cross-linkers. The tensile properties of the elastomers in the small elongation region are similar to those of typical elastomers prepared using divinyl cross-linkers, whereas the stress upturn in the large elongation region is considerably different. The stress upturn varies based on the fraction of vinyl side groups in the macro cross-linkers, whereas stress in the small elongation region remains unchanged. These properties are analyzed using various theoretical models. The results reveal that there is artificial inhomogeneity in the cross-link density for samples prepared by the macro cross-linkers, where the short poly(ethyl acrylate) strands inside the macro cross-linker limit the overall chain stretchability. On the whole, this study demonstrates a new method for tuning elastomer properties, especially at large deformation.