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-Y197 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 degrees 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

Filler reinforced polydimethylsiloxane-based vitrimers

Vitrimers are a type of covalent adaptable networks that use an associative mechanism resulting in the ability to be thermally processed without losing their network integrity, even at elevated temperatures. Here we describe the synthesis of vinylogous urethane-based thermally stable, reinforced polydimethylsiloxane vitrimers with a special emphasis on the influence of the filler's acidic or basic functionalization on relaxation times. The vitrimer design is based on a straightforward condensation reaction between amino terminated polydimethylsiloxane and a trifunctional acetoacetate. The addition of fillers improves the mechanical properties, reduces the soluble fraction, preserves processability and, depending on the amount of filler added, maintains the ability to be recycled, without loss of properties. Furthermore, the relaxation times can be slightly tuned by using acidic or basic fillers, whilst keeping the corresponding material properties. These results show that fillers can be used as both reinforcing agents and catalysts in vinylogous urethane vitrimer systems

Rigid polyurethanes, polyesters, and polycarbonates from renewable ketal monomers

Two novel biobased diols containing rigid cyclic ketal functionalities have been synthesized on a relatively large scale from glycerol and two diketones, i.e., 1,4-cyclohexanedione and 4,4'-bicyclohexanone. The diols have been used in several types of step growth reactions, resulting in polycarbonates, polyesters, and polyurethanes. While molecular weights (M-n) of up to 50 kg mol(-1) are obtained for the polyurethanes, they also exhibit high glass transition (T-g = 95-150 degrees C) and degradation temperatures (T-d = 260-320 degrees C), are transparent, and show a high mechanical strength (E = 1.10-1.35 GPa) but brittle behavior. For the polycarbonates, M-n values higher than 24 kg mol(-1) and T-g's in the range of 70-100 degrees C have been obtained. Moreover, the polycarbonates show good mechanical properties like ductility and are transparent. Finally, a new generation of fully renewable polyesters have been synthesized with dimethyl succinate and dimethyl furan dicarboxylate. While moderate molecular weight values are obtained, the polyesters show T-g's of up to 96 degrees C as well as high T-d's (+/- 315 degrees C)

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

Influence of the polymer matrix on the viscoelastic behaviour of vitrimers

International audienceVitrimer materials are a rapidly growing field of research, in which still many fundamental aspects of material design remain to be explored. In this study, we report a systematic study about the effect of the choice of the matrix on a dynamic covalent bond exchange reaction in a polymer network. In some way, this investigation follows the logic of a ‘solvent effect’ study that is typical for the development of an organic reaction or synthetic method in solution. Thus, this work constitutes a study of matrix effects on the viscoelastic properties of vitrimers, in particular with regard to their stress-relaxation behaviour. For that purpose, the dynamic transamination within vinylogous urethanes (VU) vitrimers, derived from a wide range of different commercially available diols, was chosen as vitrimer chemistry reaction platform. Additionally, the influence of the molecular weight, and thus the cross-link density, was also investigated using two oligomeric diols of different molecular weights. It was found that changing the molecular weight resulted in vitrimers with significantly different activation energies (from 68 to 149 kJ mol−1) and relaxation times (from 7 to 230 min at 140 °C). These remarkable results suggest that both the molecular weight and the nature of the polymer matrix have a large influence on the resulting viscoelastic properties. However, no straightforward prediction of relaxation times or activation energies is possible at this stage, and it becomes clear that many more structure–reactivity studies will be required to arrive at a more general understanding of the factors that underly vitrimer propertie

Dynamic Curing Agents for Amine-Hardened Epoxy Vitrimers with Short (Re)processing Times

International audienceThis work presents a straightforward strategy to introduce highly dynamic and adaptable cross-links into common epoxy resin formulations. For this, an oligomeric amine-based curing agent containing vinylogous urethane (VU) bonds was developed. This novel polyfunctional amine curing agent can be used as a drop-in solution for existing epoxy resin technologies, resulting in transparent, rigid, and, at the same time, highly reprocessable catalyst-free epoxy vitrimers. The oligomeric VU-curing agents are prepolymerized via a straightforward condensation reaction between acetoacetates extended with different classical amine monomers and epoxy hardeners. It is found that vitrimer properties can be readily introduced into these epoxy formulations by converting <50 mol % of the hardener’s amine functionalities into dynamic vinylogous urethane bonds. In this way, epoxy vitrimers can be obtained with material properties comparable to ones of the VU-free epoxy formulations. In addition, remarkably short processing times are observed in the absence of any catalyst, and the material displayed very short stress relaxation times and good recyclability, actually representing the most performant VU-based vitrimers so far. Furthermore, a proof of concept for its use in obtaining glass-fiber-reinforced epoxy composites is also presented

Polyaddition Synthesis Using Alkyne Esters for the Design of Vinylogous Urethane Vitrimers

International audienc