Session B25: Dynamic Covalent Polymer Networks and Gels

Orthogonal Dynamic Bonds Generate Multiple Relaxation Processes in Soft Networks with Different Polymer Architecture

Polymer with tunable viscoelasticity is of paramount importance in damping applications. For example, polymers with hierarchical relaxation processes facilitate energy dissipation over a wide frequency range. However, it is still challenging for the rational design of polymer networks with such properties, as many competing factors affect dynamics. Here, mixed orthogonal dynamic bonds with 10,000 times different exchange kinetics were introduced into PDMS with both pendant and telechelic architecture to understand how the network connectivity and bond exchange mechanisms determine the overall relaxation spectra. A hydrogen-bonding group and a vitrimeric imine crosslinker are combined into the same network, and well-resolved multiple relaxation processes are observed with both pendant and telechelic PDMS architecture. Two modes are not observed when mixed bonds share an exchange mechanism. With both orthogonal dynamic bonds, excellent damping and improved mechanical properties are observed and strongly correlated with the complex viscoelasticity. In addition, hydrogen-bond exchange can result in two adjacent relaxation processes depending on the network architecture. This work provides molecular insights for the predictive design of hierarchical dynamics in soft materials.   

Crystallization in vitrimers probed by time-domain NMR and complementary methods

Partial crystallization in vitrimers can improve their mechanical properties via suppressing creep. However, the interplay with the degree of crosslinking requires consideration [1-3]. Here, we discuss results of NMR studies on two different kinds of semicrystalline vitrimers [2,3], providing the absolute level of crystallinity and addressing the interplay of crystallization and the bond exchange reaction.

In one case, the polymorphism of "ethylene vitrimers" made from (α,ω)-diamino alkanes and boric acid [2] can be probed via changes in dipolar NMR line width. We confirm crystallinities beyond 90% and observe the crystallization kinetics as well as crystal-crystal transformations in real time [4]. A long-time increase of T_m and the heat of melting upon long-time annealing is found to occur at constant high crystallinity, suggesting that both are governed by the crystal perfection, as confirmed by NMR. In the other case of thiol-ene crosslinked vitrimers featuring thiol-thioester exchange reactions along the network chains [3], we elucidate the interference of the main-chain random isomeric structures resulting from the bond exchange with the crystallization.

[1] Y. Golitsyn et al., Polymer 165, 72 (2019)

[2] B: Soman, C. Evans, Soft Matter 17, 3569 (2022)

[3] A.S. Kuenstler, C.N. Bowman, ACS Macro Lett. 12, 133 (2023)

[4] K. Saalwächter et al., Front. Soft Matter 3, 1208777 (2023)   

Influence of associative dynamic covalent cross-links on poly(ethylene oxide)-block-polystyrene vitrimers

Vitrimers are polymer networks cross-linked by associative dynamic covalent bonds, which achieve bond exchange through an addition-elimination mechanism. This allows the network to fluctuate its topology while still maintaining connectivity. Converting a block copolymer to a vitrimer potentially offers an additional pathway toward controlling self-assembly. In this work, we study how adding associative dynamic covalent bonds affects the block copolymer order-disorder transition temperature (T_ODT) and nanostructure. To prepare model block copolymer vitrimers, low molecular weight poly(ethylene oxide)-block-poly(styrene-stat-4-((4-vinylbenzyl)oxy)benzaldehyde) block copolymer precursors were synthesized using a two-part synthetic scheme. The styrenic block was then cross-linked with a diamine crosslinker to form a vitrimer that undergoes an imine metathesis cross-link exchange reaction. The combination of small angle X-ray scattering (SAXS) and oscillatory dynamic mechanical analysis (DMA) were used to measure the effect of cross-link density on the T_ODT and accessible morphologies of the block copolymers.   

Mechanical Insights into Hybrid Vitrimer Network Behavior

Vitrimers fall under the purview of associative covalent adaptive networks, enabling them to undergo topology-altering bond exchange reactions as a response to external stimuli such as temperature.  This ability allows these networks to exhibit desirable properties like reprocessability, extreme stretchability,  and self-healing while maintaining their solvent resistance and crosslinking density.  Vitrimers have also been observed to undergo bond exchanges on applying stress below the glass transition which significantly reduces their operational capabilities.  One method to improve their mechanical response at low temperatures is by creating hybrid vitrimer networks consisting of dynamic and permanent covalent linkages.  The primary objective is to determine a critical concentration of the dynamic bonds that would suppress the low-temperature creep while retaining the self-healing and reprocessability of vitrimer networks.  In this study, we use a hybrid Molecular Dynamics – Monte Carlo model to study the dynamics of glassy hybrid vitrimer networks with different reactive sites while subjecting them to triaxial stretching experiments and comparing their behavior to permanently crosslinked networks.  Our results show that vitrimer networks under triaxial stress successfully relax stress through bond exchange reaction even in the glassy regime.  The networks exhibit crazing through the bulk before undergoing ultimate failure.  Also, for these hybrid vitrimers, the strain at fracture is a direct function of the number of available reactive sites in the network.    

Dynamic Bonds Drive Broad Fluctuations of Chain Stretch in Elongated Associative Polymer Melts

Associative polymer networks (APNs) form through the self-assembly of thermoreversible associative bonds, enabling them to be reprocessed at elevated temperatures. However, these fluctuating chemical bonds also produce complex chain dynamics during flow that are not well understood. Here we apply coarse-grained molecular dynamics simulations to model the nonequilibrium dynamics of APNs during nonlinear elongational flows for varying association strength, coordination, and strain rate. We observe that the coupling between chain and network relaxation drives a strong heterogeneity in chain elongation during deformation, producing broad distributions of chain stretch at all strain rates. This produces a remarkable plateau in extensional stresses over a wide range of strain rates and strong rate-thinning of the extensional viscosity. We show that the broad fluctuations in chain stretch are caused by a new form of extensional tumbling where chains cyclically fluctuate between collapsed and highly extended states, due to dynamic fluctuations in the associative bonds. These fluctuation-mediated dynamics cannot be described by established mean-field models for APNs but may enable the design of polymer networks with novel nonlinear viscoelastic properties.   

Challenges & recent progress in the processing of vitrimers

Vitrimers aim at incorporating in polymer networks dynamic covalent crosslinks governed by associative, exchangeable reactions.[1] At high temperatures or in the presence of catalysts, chemical equilibria enable large scale reorganisation and stress relaxation of the network, and thus plastic deformation and reprocessing/welding of the sample.

This versatile concept has been applied to different polymer systems by taking advantage of a variety of equilibrated reactions.[2,3]

Current syntheses and processing of vitrimers are strongly focused on thermosetting formulations, i.e. liquid precursors cured into a dynamic crosslinked network, or modification and functionalization of thermoplastics in the melt.

We will discuss the challenges encountered during (re)processing of vitrimers, and in particular the difficult compromises between creep resistance at service temperature, and fast processing at high throughputs. We will illustrate recent promising strategies:

In a first part, we will discuss deactivation of exchange reactions, particularly useful in fast relaxing systems, that can be quickly processed but require further deactivation of exchanges to suppress creep issues. Advanced characterization of such catalytic deactivations will be carried out using ad hoc rheological models and illustrated by the recycling of conventional silicone elastomers by catalyst promoting siloxane bond exchanges.[4].

In a second step, we will present our recent success in the formation of phase separated blends between a vitrimer phase and a thermoplastic phase by reactive processing. A dynamically  cross-linked percolating vitrimer network can be obtained, that demonstrates yield stress properties and can be reversibly disrupted at high shear. This blending strategy enabled in particular to upscale the production of vitrimer/PP blends to conventional industrial production equipment and correspondingly to conventional processing tools (injection press, 3D printing).   

Using Catalytic Control of Dynamic Bond Exchange to Understand Flow and Self-Assembly in Model Networks

The introduction of dynamic covalent bonds (DCBs) into polymer networks facilitates plastic flow upon activation of molecular-level scale reactions in response to stimuli. Thus, by tailoring the onset and extent of bond exchange, these materials present  opportunities to access both thermoset and thermoplastic-like properties depending on the conditions of use. While this presents significant opportunities to access recyclable thermosets or to facilitate advanced manufacturing, there remains a lack of fundamental understanding of connecting the chemical details of molecular-scale exchange reactions to bulk thermomechanical properties. In this work, we will discuss recent efforts in our lab to link the rheological behavior at varying amplitudes and frequencies to underlying chemical mechanisms of exchange using model systems where the loading or species of catalyst can be used to tune bond exchange in chemically-identical networks. Additionally, we extend this understanding to control self-assembly in well-defined networks. Finally, we will present best practices for characterizing these materials to understand dynamics at various lengthscales.   

Mechanism of Shear Thickening in Dynamic Covalent Hydrogels

Hydrogels with dynamic covalent linkers have garnered intense interest as extracellular matrix (ECM) mimics and injectable delivery vehicles due to their tailorable viscoelasticity, stress relaxation, and self-healing behavior. While dynamic covalent hydrogels with a range of bond exchange timescales have been developed, their properties under flow are less well studied. In this context, we have developed synthetic multi-arm poly(ethylene glycol) (PEG) hydrogels with three different dynamic covalent linking chemistries: boronic ester, hydrazone, and thia-conjugate addition bonds. This suite of dynamic covalent linkages allows control over the bond exchange kinetics across three orders of magnitude, which dictates hydrogel viscoelasticity under small amplitude oscillatory shear. Interestingly, the hydrogels exhibit non-monotonic flow curves under steady shear, with shear thickening behavior that depends on the crosslinking bond exchange kinetics and polymer concentration. To probe the mechanism of shear thickening, reversible shear rate sweeps were performed, as well as absorbance measurements at different oscillatory strains. Our data point to non-Gaussian chain stretching as a primary driver of shear thickening behavior. Overall, these results provide insight to the molecular and structural characteristics that govern dynamic covalent PEG gelation, mechanics, and flow, while also expanding the types of scaffolds applicable to tissue engineering and therapeutic delivery.   

Exploring the Application of Electrochemical Stimulus to Dynamic Disulfide Based Polymers

Dynamic polymers incorporating disulfide bonds have been studied on account of the wide range of available stimuli that can be used to break the bond and induced dynamic bond exchange, such as heat, light, and base. However, the application of voltage as an electrochemical stimulus has been largely unexplored. Here, this talk will focus on how we are leveraging the electrochemical properties of disulfide-based polymer particles to demonstrate unique adaptive functionality. Specifically we have synthesized poly(glycidyl methacrylate) microparticles crosslinked with redox-responsive bis(5-amino-l,3,4-thiadiazol-2-yl) disulfide moieties (DS) to yield redox active particles (DS-RAPs). The resulting DS-RAPs show improved electrochemical reversibility compared to a small molecule disulfide analogue in solution, attributed to spatial confinement of the polymer-grafted disulfides in the particle. Moreover, by taking advantage of the electrochemical stimulus response, we have shown DS-RAPs particles can be cleaned from a fouled electrode surface under reductive potential and convective fluid flow, and thus introducing an innovative particle design strategy with intrinsic cleaning functionality. Lastly, the electrochemical stimulus-response and the resulting controlled electrolyte swelling has opened up a new pathway for responsive colloidal particles in solution. Overall, the novelty of this work focuses on the application electrochemical stimulus to drive the redox reactivity and dynamic nature of the DS-RAPs to enable unique functionality.     

Covalent Adaptable Networks from Ethylene/1-Octene Multi-block Copolymers: Effects of Melt Flow Index and Crystallinity on Thermomechanical Properties and Reprocessability

Olefin block copolymers (OBCs) such as ethylene/1-octene multi-block copolymers are widely produced for numerous industrial applications. The properties of OBCs stem from parameters such as average block length, number of blocks per chain, etc., which lead to complexity associated with establishing structure-property relationships between precursor OBCs and properties of both permanently and dynamically cross-linked OBCs. Here, we synthesized dynamically cross-linked OBCs, or OBC covalent adaptable networks (CANs), by melt-state reactive processing of neat OBCs of varying crystallinity and melt flow index (MFI) with a cross-linker capable of dialkylamino disulfide dynamic chemistry. Increasing crystallinity and decreasing MFI in precursor OBCs lead to higher cross-link densities in OBC CANs. Dynamically cross-linking OBCs into CANs also significantly improves their elevated-temperature creep resistance. Distinct from other CANs capable of dialkylamino disulfide chemistry for which the stress relaxation behaviors are largely governed by the dissociation of dialkylamino disulfide bonds, the stress relaxation behaviors of OBC CANs of higher cross-link density show evident dependence on their network viscoelasticity. Finally, the OBC CANs exhibit full cross-link density and thermomechanical property recovery after reprocessing, whereas permanently cross-linked OBCs cannot be reprocessed.   

Fully Reprocessable, Non-isocyanate Polyurethane Networks: Dual Thionourethane and Disulfide Cross-links in Non-Isocyanate Polythiourethane Networks Provide Advantages over Polyhydroxyurethane Network Analogues

Polyurethanes (PUs) rank sixth among all polymer species in annual worldwide production, with the vast majority in the form of permanent networks that have insufficient reprocessability to allow for high-value recycling or reprocessing. In addition, traditional PUs are synthesized from toxic isocyanates, leading to safety and human health hazards. Here, we report on the first study of the reprocessability and properties of non-isocyanate polythiourethane (NIPTU) networks with dual cross-links, thionourethane-based and disulfide-based, the latter obtained by auto-oxidation of pendant thiol groups. To provide a rational comparison, we have also produced structurally analogous polyhydroxyurethane (PHU) networks that contain only one cross-linker species, hydroxyurethane. Such PHU networks are commonly referred to as non-isocyanate polyurethane (NIPU) networks. Both NIPTU networks and PHU networks were made suing predominantly biobased or bio-derivable starting materials. With advantages in reactivity due to the thiol-based nature and in cross-link density and mechanical properties because of the dual cross-link nature, NIPTU networks can be favorable alternatives to PHU networks as replacements for PU networks. Additionally the NIPTU networks exhibit better water resistance with factor of ~3 reductions in water sorption relative to their PHU analogues. Both NIPTU and PHU networks exhibit excellent reprocessabilty with complete recovery of cross-link density after multiple reprocessing steps as well as potential as self-healing polymers. Notably, the NIPTU networks exhibit excellent creep resistance up to 80-100 degrees C as well as a high (> 90%) recovery of pure monomer, one of the best yields among reported studies of chemical recycling of polymers. Thus, the NIPTU networks also exhibit advantages in sustainability and circularity. Our NIPTU networks exemplify how renewable, non-isocyanate PU-like materials can be developed with both high-performance characteristics and the potential to contribute meaningfully to the circular economy.