Vitrimers are organic glass-like, thermoreversible chemically crosslinked materials, which have attracted increasing interest since their introduction by Ludwik Leibler’s team a decade ago. Due to the reversibility of their crosslinks, materials can be produced in a certain shape and then be reshaped and therefore recycled many times, besides showing self-healing capability. In this property they are superior to thermosets, which have to be polymerized in the finally desired shape and can be downcycled only at their end of life. Depending on the glass transition temperature and the temperature above which occurs the thermoreversible crosslink dynamics, vitrimers display shape memory properties. Vitrimers can be designed from small molecules (organic compounds) up to large molecules (macromolecules, colloidal particles) if they carry suitable and accessible functional groups. They can be obtained from a large range of base components, including biobased materials and they can be used as matrix materials to produce nanocomposites with high filler loadings. Using microphase separated block copolymers offers an additional feature to tailor mechanical properties. Since their introduction many chemical functionalities have been identified as useful thermoreversible bonds to build vitrimers. Here different vitrimers based on low molecular weight compounds [1,2], block copolymers [3] and also nanocomposites with organic [4] or inorganic [5] fillers will be presented using imines or vinylogous urethanes as thermoreversible crosslinks.

Polybenzoxazines are conventional thermosets with high thermo-mechanical properties that, however, lack recyclability. We have developed a straightforward method to turn them into a new class of vitrimers that relies on a previously unexplored dynamic covalent exchange reaction.

Polymerization of benzoxazines with additional amines has been known to beneficially affect material strength and toughness.[1] Recently, we were able to show that such crosslinked materials based on commercially available bisbenzoxazines and polyetherdiamine also feature vitrimeric properties such as Arrhenius-like stress-relaxation and reprocessability, in the absence of catalysts or any functional groups commonly used in vitrimers, like esters/alcohols or disulfide bridges.[2] Additional amines can induce opening of the oxazine rings resulting in a combination of different aminoalkylated phenolic polymer structures. In dependence of stoichiometry and polymerization conditions, they comprise nucleophilic primary, secondary and tertiary amines as well as electrophilic methylene bridges which can interact at elevated temperatures in a nucleophilic substitution-like reaction with proton transfer. This leads to a covalent exchange of amines in an equilibrium reaction that can explain the dynamic behaviour of BZ/amine-based vitrimers.[3]

In the last years, the world of microporous materials has seen a major development, given their outstanding chemical and physical features. Among them, Covalent Organic Frameworks (COFs) are flourishing. COFs can be defined as 2 or 3D networks based on covalent bonds and fully organic structures. They are synthesized starting from monomers with multiple complementary functionalities that combine to form highly ordered and crystalline materials. Their properties depend on their structure and, given the great amount of different available motifs, COFs are characterized by a wide array of different chemical and physical properties. Despite the great interest in the field, a possible industrial applicability of COFs has been hindered by some drawbacks, amongst which an inherent lack of processability is one of the most significant. COFs are obtained as powders and it is almost impossible to process them in complex shapes with good control. This work aims at solving this shortcoming through a polymer-supporting strategy. COFs have been synthesized and then exploited as active components in the synthesis of different polymers. This approach allowed the direct growth of polymeric chains on the COF backbone. These polymeric chains are expected to arrange between the COFs’ layers, resulting in an improved processability, possibly close to the one of more classical thermoplastic materials. The covalent bond between the COFs and the polymeric chains has been assessed as well as the thermal and morphological properties of the resulting materials. Processability tests were also carried out, with the successful preparation of films through solvent casting.

Biomaterials with precise and fine-tunable micro and macro properties are essential for recapitulating the native tissue environment. Dynamic covalent hydrogels (DCH) are potential biomaterials providing tunable and time-dependent mechanical properties, self-healing ability, and processability. In addition to the viscoelasticity of DCH, which better mimics the native tissue microenvironment, the reversibility of the dynamic bonds can be exploited to deliver active agents. To enhance the control on the hydrogel chemistry and properties, we have introduced light-responsive moieties to hyaluronic acid which upon UV and NIR light irradiation, altered the crosslinking chemistry and enhanced the mechanical and physical properties of the hydrogels. Furthermore, we used pH as an internal stimulus to manipulate the equilibrium rate constant of the dynamic bonds, both to control the hydrogel properties and to tune the release rate of the active agent. To prepare the desired DCH, we have used hyaluronic acid and alginate as two platforms with Schiff base crosslinks of imine, hydrazone, and oxime with distinct properties to influence cellular response and deliver active agents. The moduli and stress relaxation behavior of the hydrogels were shown to be influenced by pH and light. The induced spatiotemporal changes in the composition and properties of the macro and microenvironments were characterized and proved to affect the cell-material interactions and cellular behavior. Moreover, the prepared hydrogels showed self-healing ability and injectability, further highlighting their potential as biomaterials.

Polybenzoxazine is a new class of high-performance phenolic thermosetting material prepared by the ring-opening polymerization of benzoxazine monomers without needing any external catalyst. They outshine conventional thermosets due to their unique properties, high thermal stability, low water absorption, no or less volatile release during curing, and low volume shrinkage with excellent dimensional stability. The permanent covalent crosslinks are responsible for the exciting properties of thermosets however, they hinder their reprocessability, generating huge waste which ends up segregated into landfill. Thus, a severe solution is the need of the hour to eradicate plastic pollution. The inclusion of dynamic bond for instance, sulfur-sulfur linkages into the polybenzoxazines networks, imparts reprocessability, repairability and recyclability to the material due to their dynamic nature at room temperature or under weak stimuli. In this work, a multifunctional benzoxazine monomer based on cardanol and 4-Aminophenyl disulfide was designed in order to accommodate polysulfide linkages into the network based on mechanism such as sulfur inclusion into disulfide linkages polysulfides incorporation via inverse vulcanization , Sulfur Radical Transfer and Coupling Reaction, and Catalytic Opening of the Lateral Benzoxazine Rings by Thiols. Furthermore, the chemical, rheological and mechanical properties of the synthesized new age material were studied to obtain a fast-relaxing thermosetting polymer suitable to use as reprocessable adhesive for metal plates.

Chemical crosslinking of polymeric membranes has been explored for organic solvent nanofiltration (OSN) and gas separation to induce chemical stability, prevent membrane plasticization and prolong performance. However, covalent crosslinking leads to formation of polymeric networks that hinder recycling or reprocessing at the end-of-life. In this work, dynamic covalent bonds (specifically furan-maleimide Diels-Alder (DA) cycloadducts) are used as a new approach to crosslinking of OSN and gas separation membranes to obtain membranes which are simultaneously highly stable at operating conditions and recyclable at their end-of-life. A novel furan-modified poly(amide-imide) (PAI-FU) was prepared by imide-ring opening of commercial polyimide commonly used in membrane technology (Matrimid® 5218) with furfurylamine. PAI-FU was subsequently crosslinked with 4,4’-bismaleimidodiphenylmethane (BMI) to create PAI-FU-BMI CANs. Nanoporous OSN membranes were fabricated through non-solvent induced phase separation, whereas solvent evaporation was used to prepare dense membranes to measure gas permeabilities. PAI-FU-BMI OSN membranes exhibited solvent permeances (IPA, ethanol, ethyl acetate, acetone) between 0.04 and 3.68 L m-2 h-1 bar-1, and Rose Bengal rejections in acetone >90%. PAI-FU-BMI gas separation membranes exhibited N2 permeabilities of 0.1-0.3 Barrer, CO2 of 2-4 Barrer, and He of 9-13 Barrer, depending on degree of crosslinking. Owing to the thermo-reversible nature of the DA reaction, PAI-FU-BMI membranes were reprocessed after use to recreate materials with similar chemical and mechanical properties, thus contributing to themes of ‘closing cycles’ and ‘value from waste’. This work shows the potential to produce recyclable high-performance membranes, and demonstrates how membrane technology benefits from advances in supramolecular polymer chemistry towards sustainable transition.