No abstract

A series of tunable interpenetrating polymer network (IPN) hydrogels are designed by the orthogonal incorporation of two distinct types of reversible bonds, i.e., Schiff base bonds and metal−ligand coordination bonds. Two copolymers based on poly(N,N-dimethyl acrylamide) (PDMA) are synthesized and used as building blocks for the IPN hydrogels. The first one bears ketone pendant groups and is cross-linked by dihydrazide or bishydroxylamine compounds to form, respectively, acylhydrazone-or oxime-based dynamic covalent bond (DCB) networks. The second one bears terpyridine side groups and is cross-linked by the addition of two different transition-metal cations to obtain supramolecular networks based on metal−terpyridine bis-complexes. Several IPN hydrogels are prepared by combining these different types of reversible bonds to investigate how the two subnetworks influence each other. To this end, the influence of the cross-linker nature and of the hydrogel preparation protocol on the rheological properties of these IPNs are also studied in detail. In particular, we show that the obtained IPN hydrogels exhibit a higher modulus compared to the simple addition of the moduli of the single networks, which we attribute to the entanglements between the two networks. We then study how these IPN hydrogels can disentangle and partially relax if one of the reversible subnetworks is composed of cross-links with a shorter lifetime. Finally, pH is used as a stimulus to affect the dynamics of only one of the subnetworks, and the impact of this change on the properties of the IPNs is investigated.

Less than 10% of plastics are recycled worldwide, among which polyolefins, which form two-thirds of all produced polymers, are the most discarded ones. This stems from the nonpolar nature of polyolefins, which limits their degradation and recycling. As such, any approach that may promote the recycling of polyolefins can significantly shift the statistics of polymer recycling toward a more sustainable future. To account for that, the vitrimer concept proposes a unique solution that not only increases the polarity of polyolefins, and as such promotes their degradation, but also expands the utility of polyolefin elastomers and compensates for the significant loss of mechanical properties upon sequential mechanical recycling in thermoplastic polyolefins. Vitrimers are polymer networks formed by reversible covalent cross-links that undergo percolation-conserving exchange reactions at high temperatures. Therefore, while they form a solid network at the service conditions, they can be reprocessed and recycled above a temperature threshold. Thanks to this, they have applications at the interface of thermoplastics and thermosets ranging from recyclable, self-healing, and adhesive materials to creep-resistant and shape-memory networks. This Review aims to classify cross-linking chemistries employed for the production of polyolefin-derived vitrimers and highlight subsequent changes in material properties and applications to promote further development of novel polyolefin vitrimers.

Redox hydrogels are obtained by introducing into a poly(oligoethyleneglycol)methacrylate network 2,2,6,6-tetramethyl-1-piperidinyloxy radicals, which can be oxidized into oxoammonium cations.