Self-Healing And Recyclable Polymeric Materials Based On Nitrogen-Coordinated Boroxines

Polymeric materials with recycling and self-healing properties have gained extensive attention in recent years because they can significantly avoid the energy and environment problems caused by excessive waste of raw materials.Polymer materials will inevitably be damaged during their usage,which will greatly reduce their mechanical properties.Endowing polymeric materials with self-healing ability can enable damaged materials to be healed spontaneously or under external stimulus,which can improve their service life,reduce the excessive consumption of raw materials,and save natural resources.When suffering from severe damage that exceeds the healing ability of the polymeric materials,it is difficult to achieve the healing process completely of their original mechanical properties.Therefore,it is very necessary to endow the polymer materials with recycling abilities which makes them to be further applied through reprocessing,thus reducing the harm of waste materials that cannot be disposed to the natural environment.The development of new dynamic covalent bonds or non-covalent interactions is an important driving force for the fabrication of self-healing and recycling polymeric materials.Nitrogen-coordinated boroxines,which can provide access to tripodal molecular architectures,are very suitable for the fabrication of highly cross-linked polymeric materials with high mechanical strength.Meanwhile,due to the presence of N-B dative bond,the reversible reaction of nitrogen coordination boroxines can be carried out under mild conditions,which is conducive to the preparation of polymer materials with high healing and recycling efficiencies.Here,we developed a new type of dynamic covalent bond called nitrogen coordination boroxines,and prepared a series of polymeric materials with excellent self-healing and recycling abilities by utilizing the unique molecular structure and high reversibility of nitrogen coordination boroxines.The research contents are as follows:1.The dynamic reversibility of nitrogen-coordinated boroxines was explored systematically,and room temperature self-healing polymeric materials were prepared by using nitrogen-coordinated boroxines as dynamic cross-linkers.The three-dimensional molecular structure of nitrogen-coordinated boroxines was calculated from the density functional theory.The nitrogen-coordinated boroxines exist in two configurations: 1:1 triphenylboroxine-amine adducts with a N-B and 1:2 triphenylboroxine-amine adducts with two N-B bonds.Meanwhile,DTF calculation indicates that the B-O bonds near the N-B dative bonds in 1:1 and 1:2 triphenylboroxine-amine adducts are longer than those in triphenylboroxines,thereby expediting the dissociation of boroxines in N-boroxine-PPG.The highly dynamic and reversible properties of the nitrogen-coordinated boroxines were also explored by the exchange reaction between two model compounds.Finally,nitrogen-coordinated boroxines cross-linked polymeric materials were fabricated.Due to the high reversibility of the nitrogen-coordinated boroxines and the mobility of the polymer chains,the fabricated material can be easily healed at room temperature.2.We prepared a series of room temperature self-healing and recyclable polymer complexes by using nitrogen-coordinated boroxines and hydrogen-binding interactions as the main driving forces.By controlling the mass fraction of PAA in the polymer complexes,the mechanical properties of the complexes can be systematically tailored.The high reversibility of nitrogen-coordinated boroxines and hydrogen-binding interactions enabled the polymer complexes to be healed at room temperature efficiently.Meanwhile,the polymer complexes also exhibit excellent recycling ability.When the polymer complexes were cut into small pieces,they can be easily reshaped for multiple times and regain their original mechanical properties with just a few drops of ethanol and pressing under very small pressure.3.We designed two kinds of phenylboric acid terminated polymers with different topologies and mixed them with different molar ratios to prepare a series of high-strength supramolecular thermosets that can be healed and recycled under mild conditions.The mechanical strength of the supramolecular thermosets can be systematically tailored by changing the molar ratio of the two phenyl boric acid terminated polymers within the supramolecular thermosets.Based on the high reversibility of nitrogen-coordinated boroxines,the supramolecular thermosets can be healed under mild conditions.Meanwhile,the supramolecular thermosets can be recycled by hot-press to regain their original mechanical strength.4.We fabricated a nitrogen-coordinated boroxines cross-linked rubber that can be healed and recycled at room temperature.The relationship between the storage/loss modulus and the frequency of this rubber material were measured by using rheology experiment at different temperatures,and the stress-relaxation times at different temperatures of the rubber were obtained.According to Arrhenius' equation,the fitting curves of stress relaxation times and testing temperatures were obtained,and then the activation energy of the rubber material can be calculated to be 69 k J/mol.Such high activation energy indicates that the rubber material has good stability at room temperature,and the dynamic behavior of the material can be affected by temperature.The fast stress relaxation at high temperature of the rubber material indicates that the material has excellent self-healing and recycling abilities at high temperature.When the material is damaged,they can be healed at room temperature only under the stimulation of water.Meanwhile,this material can be recycled for multiple times by means of hot-press without losing its mechanical properties.