Preparation And Study Of Self-healing Polyur Ethane Based On Cellulose Nanocrystals

In order to increase the use and service life of polymer materials,self-healing function has been introduced into polymer materials.There are generally two ways to introduce self-healing function.One is to edit reversible bonds into polymer networks,which can respond directly to damage of materials.The other is to add a carrier with repair agent inside the material,which is broken when the material is damaged,and the repair agent can be self-repaired by flowing into the crack.Among them,the former has been widely studied for its mild self-healing conditions,high efficiency and repeatability.However,the mechanical properties of self-healing materials with reversible bonds are often poor,so how to improve the mechanical properties of self-healing materials has become one of the recent research hotspots.Because of its good thermal stability and high crystallinity,cellulose nanocrystalline is often used as filler for composite materials,and its sources are wide and the way of obtaining is simple,which can realize the sustainable utilization of natural resources.In this paper,we introduce a self-healing system of imine bond type into thermoplastic polyurethane,which endows polyurethane with self-healing properties at room temperature.Then cellulose nanocrystalline was used as the dispersed phase,and the two were combined by solution blending method to obtain the self-healing polyurethane/cellulose nanocrystalline composite film with excellent mechanical strength,good thermal stability and high self-healing efficiency.It is hoped that it can play its role in architecture,medicine and other fields.Firstly,cellulose nanocrystals were extracted from microcrystalline cellulose by sulfuric acid hydrothermal method.Through the infrared spectrum,it can be seen that the basic chemical structure of microcrystalline cellulose does not change under the erosion of sulfuric acid.According to XRD test analysis,the crystallinity of raw material MCC is about67%,and that of product CNC is about 78%,and the crystallinity has increased by about16.4%.It can be seen by transmission electron microscopy that CNC is in short rod shape with a length of about 200-300nm and a width of about 20nm.The aspect ratio reaches 10-15.Cellulose nanocrystals with nanometer size can improve the contact area with polyurethane and better improve the properties of materials.According to thermogravimetric analysis,the initial degradation temperature of CNC is about 273?,which has excellent thermal stability.Secondly,this paper uses p-hydroxybenzaldehyde and 4,4'-diaminodiphenylmethane as raw materials to prepare an Imine aromatic diol(Imine).According to the absorption peak at 1604cm^-1 in the infrared spectrum and 8.44ppm in the nuclear magnetic spectrum,it can be concluded that Imine has been successfully synthesized.Subsequently,a series of self-healing polyurethanes with different Imine contents were prepared by using Imine as chain extender using a two-step method,called PUI-1?4.The infrared spectra showed that the imine bond was successfully bonded to polyurethane.The hydrogen bond in polyurethane was realized by C=O and N-H in polyurethane,and the imine bond could not form hydrogen bond with N-H in polyurethane.DSC curve showed that the glass transition temperatures of PUI-1-4 were all below room temperature,and they still maintained good elasticity at low temperature,and the glass transition temperatures increased gradually with the increase of imine content.TGA curve shows that PUI-1?4 has good thermal stability,and the initial degradation temperature can reach 348?.According to the stress-strain curve,PUI-4 with the highest imine content has the best mechanical properties,with the fracture strength reaching 8.03mpa and the elongation at break reaching 883%.With the increase of polyurethane imine content,the breaking strength of polyurethane increases,but the elongation at break decreases gradually,but it still keeps a high level.From the point of view of self-repair efficiency,the self-repair efficiency of self-repair polyurethane materials increases gradually with the increase of the amount of imine aromatic diol.The material with the highest self-healing efficiency is PUI-4,which reaches more than96%.Therefore,the imine bond self-repair system is superior.Finally,PUI-4 was used as the substrate of self-repairing polyurethane,and cellulose nanocrystals were added to the self-repairing polyurethane as dispersed phase.The influence of cellulose nanocrystals on the mechanical properties and self-healing properties of self-healing polyurethane can be studied by changing the dosage of cellulose nanocrystals.The experimental results show that with the increase of the amount of cellulose nanocrystalline,the properties of the composites increase first and then decrease.The mechanical properties of PUIC7 are the best,and the fracture strength reaches20.59mpa,which is about 1.56 times higher than that of PUI-4 matrix,and the elongation at break reaches 979%.With the increase of the dosage of cellulose nanocrystals,the self-healing properties of self-healing polyurethane gradually decreased,because the presence of cellulose nanocrystals limited the molecular chain movement.To sum up,PUIC7 has the best comprehensive performance and excellent mechanical properties,and the self-repair efficiency reaches 84.7%,which still remains at an excellent level.