Reversible fusion and defusion of immiscible polymers starting from crosslinked networks

All-interface polymer blends have long been imagined as the most efficient measure to maximize the synergistic effect of different polymers. Current approaches including the closest version, interpenetrating polymer networks (IPNs), are still far from meeting the goal of being mutually soluble on the molecular level because of the inevitable phase separation resulting from the very small entropy of mixing of conventional high molecular weight materials, and the very frequent positive heat of mixing of different chemical structures.

Recently, the group of Zhang and Rong at the Sun Yat-sen University proposes a facile strategy to synthesize molecular-level reversibly interlocked polymer networks (RILNs) starting from two pre-formed immiscible polymer networks based on thermodynamic principle and reversible covalent chemistry. [You et al., Materials Today (2020), doi.org/10.1016/j.mattod.2019.09.005] The frequently opening and closing of the single networks due to the rapid exchange reactions of the embedded dynamic covalent bonds, the stronger inter-component interaction and the effect of solvation are jointly used to induce sufficient interdifussion and interweaving of the chains from different single networks. The processes reduce heat of mixing and increase entropy of mixing so that they are thermodynamically favored. When the excitation condition that triggers reversibility of the dynamic covalent bonds and the good common solvent are removed, a molecular interlocking network is obtained owing to the reconstruction of reversible bonds that lock the nested states before the phase separation. By taking advantage of topological rearrangement, the approach breaks through the traditional concept that IPNs cannot be prepared by physical blending and performs much better than the traditional methods to synthesis molecular mixture of crosslinked networks. Dynamic exchange reactions turn the network molecules into living objects, not only promoting chains mixing and interdiffusion among different networks, but also prohibiting phase separation of the molecularly interlocked chains.

The resultant RILNs exhibit an overall significant improvement in both static and dynamic mechanical properties in comparison to the two starting networks as a result of stress equalization. Moreover, the RILNs possess attractive adaptive functions like self-healing, reprocessing and recycling capabilities inherited from the dynamic bonds. Under certain circumstances, moreover, the process of interlocking is reversed producing the original single networks. Such unlocking/relocking processes can be repeated for multiple times like the magic “linking rings”, which is potentially meaningful for the circular economy.

The intimate assembly of different macromolecules at the molecular level plus subtle design may help to build up an extensible platform for bringing in new materials and new functionalities, while the inherent reversibility further provides new opportunities for recycling and tuning the degree of interfacial interaction. In a follow-up work, for example, the same group combines two reversible polymer networks with different pH-stimulus responsibility to provide the RILNs with underwater self-healing ability in a much wider pH range from acidic to basic. [Peng, et al., ACS Appl. Mater. Interfaces (2020), doi.org/10.1021/acsami.0c07040]