Tuning the Viscosity Profiles of High-Tg Poly(1,2,3-triazolium) Covalent Adaptable Networks by the Chemical Structure of the N-Substituents

Polymerized and Colloidal Ionic Liquids─Syntheses and Applications

The breadth and importance of polymerized ionic liquids (PILs) are steadily expanding, and this review updates advances and trends in syntheses, properties, and applications over the past five to six years. We begin with an historical overview of the genesis and growth of the PIL field as a subset of materials science. The genesis of ionic liquids (ILs) over nano to meso length-scales exhibiting 0D, 1D, 2D, and 3D topologies defines colloidal ionic liquids, CILs, which compose a subclass of PILs and provide a synthetic bridge between IL monomers (ILMs) and micro to macro-scale PIL materials. The second focus of this review addresses design and syntheses of ILMs and their polymerization reactions to yield PILs and PIL-based materials. A burgeoning diversity of ILMs reflects increasing use of nonimidazolium nuclei and an expanding use of step-growth chemistries in synthesizing PIL materials. Radical chain polymerization remains a primary method of making PILs and reflects an increasing use of controlled polymerization methods. Step-growth chemistries used in creating some CILs utilize extensive cross-linking. This cross-linking is enabled by incorporating reactive functionalities in CILs and PILs, and some of these CILs and PILs may be viewed as exotic cross-linking agents. The third part of this update focuses upon some advances in key properties, including molecular weight, thermal properties, rheology, ion transport, self-healing, and stimuli-responsiveness. Glass transitions, critical solution temperatures, and liquidity are key thermal properties that tie to PIL rheology and viscoelasticity. These properties in turn modulate mechanical properties and ion transport, which are foundational in increasing applications of PILs. Cross-linking in gelation and ionogels and reversible step-growth chemistries are essential for self-healing PILs. Stimuli-responsiveness distinguishes PILs from many other classes of polymers, and it emphasizes the importance of segmentally controlling and tuning solvation in CILs and PILs. The fourth part of this review addresses development of applications, and the diverse scope of such applications supports the increasing importance of PILs in materials science. Adhesion applications are supported by ionogel properties, especially cross-linking and solvation tunable interactions with adjacent phases. Antimicrobial and antifouling applications are consequences of the cationic nature of PILs. Similarly, emulsion and dispersion applications rely on tunable solvation of functional groups and on how such groups interact with continuous phases and substrates. Catalysis is another significant application, and this is an historical tie between ILs and PILs. This component also provides a connection to diverse and porous carbon phases templated by PILs that are catalysts or serve as supports for catalysts. Devices, including sensors and actuators, also rely on solvation tuning and stimuli-responsiveness that include photo and electrochemical stimuli. We conclude our view of applications with 3D printing. The largest components of these applications are energy related and include developments for supercapacitors, batteries, fuel cells, and solar cells. We conclude with our vision of how PIL development will evolve over the next decade.

Analysis of Poly(thiourethane) Covalent Adaptable Network through Broadband Dielectric Spectroscopy

The dielectric spectra of the poly(thiourethane) network, HDI-S3, have been analyzed to know the nature and the cooperativity of each of the six dielectric processes observed. At low temperatures, γ₁, γ₂, and β dielectric relaxations were attributed to noncooperative local motions in the glassy state, in which apparent activation energies are 30, 36, and 60 kJ·mol^-1, respectively. At higher temperatures, three dielectric relaxations are observed (α_Tg, α*, ρ). The α_Tg relaxation is attributed to the glass transition, and it is overlapped with the α* relaxation. The molecular origin of α* relaxation is associated with the bond exchange reaction. Finally, the ρ relaxation is ascribed to the heterogeneity of the sample although its origin is uncertain. The DC conductivity (σ_DC) is found to be an appropriate variable to analyze the bond exchange reaction. Accordingly, the HDI-S3 has a molecular exchange mechanism of dissociative nature.

Characterising different molecular landscapes in dynamic covalent networks

Dynamic covalent networks present a unique opportunity to exert molecular-level control on macroscopic material properties, by linking their thermal behaviour to the thermodynamics and kinetics of the underlying chemistry. Yet, existing methods do not allow for the extraction and analysis of the influence of local differences in chemical reactivity caused by available reactants, catalysts, or additives. In this context, we present a rheological paradigm that allows us to correlate the composition of a reactive polymer segment to a faster or slower rate of network rearrangement. We discovered that a generalised Maxwell model could separate and quantify the dynamic behaviour of each type of reactive segment individually, which was crucial to fully comprehend the mechanics of the final material. More specifically, Eyring and Van 't Hoff analysis were used to relate possible bond catalysis and dissociation to structural changes by combining statistical modelling with rheology measurements. As a result, precise viscosity changes could be measured, allowing for accurate comparison of various dynamic covalent network materials, including vitrimers and dissociative networks. The herein reported method therefore facilitated the successful analysis of virtually any type of rate-enhancing effect and will allow for the design of functional and fast (re)processable materials, as well as improve our ability to predict and engineer their properties for future applications.

Suppressing Creep and Promoting Fast Reprocessing of Vitrimers with Reversibly Trapped Amines

We report a straightforward chemical strategy to tackle current challenges of irreversible deformation in low T_g vitrimers at operating temperature. In particular, vinylogous urethane (VU) vitrimers were prepared where reactive free amines, necessary for material flow, were temporarily shielded inside the network backbone, by adding a small amount of dibasic ester to the curing mixture. The amines could be released as reactive chain ends from the resulting dicarboxamide bonds via thermally reversible cyclisation to an imide moiety. Indeed, (re)generation of the required nucleophilic amines as network defects ensured reprocessing and rapid material flow at higher temperature, where exchange dynamics are (re)activated. As a result, VU vitrimers were obtained with limited creep at service temperature, yet with good reprocessability at elevated temperatures. Thus, by exerting strong control on the molecular level over the availability of exchangeable functional groups, a remarkable improvement of VU properties was obtained.

Combining vinylogous urethane and β-amino ester chemistry for dynamic material design

This study combines vinylogous urethane (VU) and beta-amino ester chemistry for the synthesis of covalent adaptable networks (CANs). The resulting CANs are synthesised using a range of diacetoacetates and commercially...

Taking advantage of β-hydroxy amine enhanced reactivity and functionality for the synthesis of dual covalent adaptable networks

This study highlights the potential of β-hydroxy amines as building blocks for aza-Michael CANs.

Reprocessing of Covalent Adaptable Polyamide Networks through Internal Catalysis and Ring-Size Effects

Here, we report the introduction of internally catalyzed amide bonds to obtain covalent adaptable polyamide networks that rely on the dissociation equilibrium between dicarboxamides and imides. While amide bonds are usually considered to be robust and thermally stable, the present study shows that their dynamic character can be activated by a smart choice of available building blocks without the addition of any external catalyst or other additives. Hence, a range of polyamide-based dynamic networks with variable mechanical and viscoelastic properties have been obtained in a systematic study, using a straightforward curing process of dibasic ester and amine compounds. Since the dissociation process involves a cyclic imide formation, the correlation between ring size and the thermomechanical viscosity profile was studied for five- to seven-membered ring intermediates, depending on the chosen dibasic ester monomer. This resulted in a marked temperature response with activation energies in the range of 116-197 kJ mol^-1, yielding a sharp transition between elastic and viscous behavior. Moreover, the ease and versatility of this chemistry platform were demonstrated by selecting a variety of amines, resulting in densely cross-linked dynamic networks with T_g values ranging from -20 to 110 °C. With this approach, it is possible to design amorphous polyamide networks with an acute temperature response, allowing for good reprocessability and, simultaneously, high resistance to irreversible deformation at elevated temperatures.

Covalent Adaptable Networks Using β-Amino Esters as Thermally Reversible Building Blocks

In this study, β-amino esters, prepared by the aza-Michael addition of an amine to an acrylate moiety, are investigated as building blocks for the formation of dynamic covalent networks. While such amino esters are usually considered as thermally nondynamic adducts, the kinetic model studies presented here show that dynamic covalent exchange occurs via both dynamic aza-Michael reaction and catalyst-free transesterification. This knowledge is transferred to create β-amino ester-based covalent adaptable networks (CANs) with coexisting dissociative and associative covalent dynamic exchange reactions. The ease, robustness, and versatility of this chemistry are demonstrated by using a variety of readily available multifunctional acrylates and amines. The presented CANs are reprocessed via either a dynamic aza-Michael reaction or a catalyst-free transesterification in the presence of hydroxyl moieties. This results in reprocessable, densely cross-linked materials with a glass transition temperature (T_g) ranging from -60 to 90 °C. Moreover, even for the low T_g materials, a high creep resistance was demonstrated at elevated temperatures up to 80 °C. When additional β-hydroxyl group-containing building blocks are applied during the network design, an enhanced neighboring group participation effect allows reprocessing of materials up to 10 times at 150 °C within 30 min while maintaining their material properties.

Reversible crosslinking and fast stress relaxation in dynamic polymer networks via transalkylation using 1,4-diazabicyclo[2.2.2] octane

A dynamic covalent network using transalkylation of benzyl-DABCO crosslinkers features fast relaxation with a very strong temperature dependence. The network is de-crosslinked by an excess of DABCO.