Toughening vitrimers based on dioxaborolane metathesis through introducing a reversible secondary interaction

This research is led by Shilong Wu and Quan Chen (State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China).

A trade-off exists between hardness and stretchability of almost all polymeric materials like plastics, elastomers, and fibers. The hardness in polymers reflects the resistance to deformation. Improving the hardness requires a high degree of crystallinity, strong intermolecular forces (such as ionic or hydrogen-bonding interaction), dense cross-linking and entanglements, or a higher degree of chain orientation. To fulfill these requirements usually requires restriction of the mobility of the polymer chains. On the other hand, stretchability reflects a polymer’s ability to deform without breaking. The frequently adopted strategies to improve stretchability include the promotion of the crystallographic glide and lamellar sliding for semicrystalline materials, or choosing highly flexible polymer chains as the basis and reducing the degree of cross-links and entanglements for noncrystalline materials.

The trade-off explained above limits the toughness of the polymeric materials determined by integrating the area of the stress−strain curve. The toughness reflects the absorbed energy before fracture and thus is related to both the hardness and stretchability that determine the amplitude and extension of the stress−strain curve, respectively. In the last two decades, intensive approaches have been attempted to improve the toughness of polymeric materials. In particular, for elastomeric materials, one practical approach is attempted by Gong and co-workers, i.e., introducing double networks with one network to

maintain the network integrity and the other interpenetrated network to sacrifice and dissipate the energy. One disadvantage of the double networks developed by Gong is that the sample cannot recover to its original state after breaking the covalent bonds of the sacrificial network. In addition to the double networks, another intensively investigated system is the dual-cross-linked polymer containing permanent or dynamic covalent cross-links and temporary cross-links, e.g., hydrogen bonds or ionic associations. Ideally, the covalent cross-links help maintain the sample’s integrity, while the temporary cross-links help dissipate the energy. The dissipation is efficient when the time scale of the formation and breakup of temporary cross-links match the deformation rate.

Development has also been achieved in the theoretical prediction of the stress−strain curve for dual-cross-linked networks. The model developed by Davidson and co-workers could predict the strain-softening at low strain due to the disentanglement and the strain-hardening at high strain due to the finite extensible nonlinear elasticity (FENE) effect. These softening and hardening features are also noted in the dual-cross-linked network polymers, although with different molecular origins. For example, it was proposed that the

strain softening at low strain is partly due to strain-induced dissociation of the temporary cross-links. More recently, theoretical development by Dobrynin and co-workers gave a reasonable prediction of the stress−strain curve of the dualcross-linked network. The model was later modified by Konkolewicz and co-workers upon including a strain ratedependent element.

The current study investigates the stretchability and toughness of a dual-cross-linked network. In related to the current study, we designed several poly(hexyl methacrylate) (PHMA)-based unentangled systems constituted of two physical networks, i.e., a strong network based on the ionic interaction and a weak network based on hydrogen bonds, including the double, triple, and quadruple hydrogen bonds. We found that, when there was no hydrogen bond, the flow-induced dissociation of the ionic groups usually led to the brittle behavior of the ionomers. When double and triple hydrogen bonds much weaker than the ionic interaction were introduced, the ductility of the ionomer samples was greatly

improved. This improvement was prominent when the elongational rate fell between the dissociation rate of the ionic groups and that of the hydrogen bonds, where the faster breakup and re-formation of the hydrogen bonds significantly dissipated the energy. In contrast, incorporating the quadruple hydrogen bonds, equally strong as the ionic interaction, led to even more brittle behavior than the pristine ionomer because the flow-induced dissociation occurred almost simultaneously for both the hydrogen bonds and ionic associations, thereby resulting in more severe flow instability, i.e., the formation of defects or small cracks that can grow into the macroscopic fracture. In addition, we examined the role of the number density of the double hydrogen bonds on the ductility of ionomers with dual-cross-linked network and found that there was an optimized number density for improving ductility. This study constructs a dual-cross-linked network, i.e., PHMA-based vitrimer network based on the chemically reversible cross-links as well as the hydrogen-bonding crosslinks. Namely, the authors copolymerized hexyl methacrylate monomer (HMA) with a vitrimeric cross-linker containing two chemically exchangeable units and n-isopropyl methacrylamide (NIPAM) to prepare well-entangled dual-cross-linked samples. The purpose of this study is twofold. First, test the applicability of this molecular design in preparing entangled vitrimer samples with high toughness. To this end, compare the toughness of the current sample and other vitrimer and elastomer samples reported in the literature. Second, test the applicability of the state-of-the-art model in predicting the stress−strain curves of the dual-cross-linked samples thus prepared. Since the stress−strain curve of prepared samples shows strong strain-rate dependence, they test the model developed by Konkolewicz that incorporated this dependence. They find that the stress−strain curves fit quantitatively when the hydrogen bonds are sparse, where they still expect a relatively independent relaxation of these bonds. Nevertheless, the prediction is not equally good for the stress−strain curves of samples with higher densities of hydrogen-bonding monomers, where they expect strong coupling between the hydrogen bonds, where the assumption of a single characteristic time for the strain-induced dissociation does not hold anymore.

See the article:

Toughening Vitrimers Based on Dioxaborolane Metathesis through Introducing a Reversible Secondary Interaction

https://doi.org/10.1021/polymscitech.4c00008