Preparation And Repair Behavior Of Imine Bond-based Self-healing Polyurethane

Polyurethane materials are widely used because of their excellent elasticity,mechanical strength,aging resistance,wear resistance and corrosion resistance.However,damages as scratches,cracks and fissures may present during processing and use due to external stimuli,such as stress,heat,chemical media,irradiation,and so on.These damages can lead to a decline in the mechanical properties,shorten the service life,and even cause structural damage and serious safety problems.At the same time,waste polyurethane can result in environmental problems.Self-healing polyurethane materials can restore their own performance through self-repairing damages.Thus,the service life is expected to extend while safety and reliability of polyurethane can be improved.However,problems such as strength and self-healing ability are unable to balance at the same time.Meanwhile,high temperature and long time are needed for damage repair.Moreover,low mechanical strength exists generally in most of self-healing polyurethanes at present.Therefore,it is imperative to develop polyurethane materials with both lower repair temperature and higher strength as well as good self-healing property.Imine bond can undergo exchange reaction without catalyst at lower temperature.Disulfide bond is a room temperature dynamic covalent bond.Theses offer the possibility to solve the problem of high repair temperature and long repair time.Meanwhile,not only self-healing efficiency can be effectively improved,but also mechanical properties of polyurethane can be significantly enhanced through using the combination of two bonds as hard-segment structures.In view of this,imine bonds are introduced into hard-segment structure of polyurethane materials in order to prepare thermoreversible polyurethane with both good mechanical properties and damage self-healing ability.Subsequently,disulfide bond is introduced into the hard-segment structure of the thermoreversible polyurethane based on imine bond.Take advantage of the interaction of rigid imine bond and flexible disulfide bond structures,the development of the polyurethane with good mechanical properties,gentler repair conditions and better repair performance can be desireable.The main contents and conclusions of this study are as follows:(1)Preparation and properties of thermally reversible polyurethanes containing imine bonds: Firstly,a symmetric and rigid chain extender(ID)containing imine bond is synthesized by the reaction of p-benzaldehyde and ethanolamine.Subsequently,thermoreversible polyurethane containing imine bond is fabricated through4,4-diphenylmethane diisocyanate(MDI),polypropylene glycol-2000(PPG-2000)and ID.The hardness,tensile strength and elongation at break of the obtained polyurethane can reach73.0 HA,11.0 MPa and 603%,respectively.What’s more,the degree of microphase separation,thermal stability,hardness and tensile strength enhance with the increase of hard-segment and ID content.(2)Study of self-healing behavior of thermally reversible polyurethanes containing imine bonds: The sample is cut off and heat-treated at a certain temperature.The rebinding ability of the sample is observed with an optical microscope and the recovery rate of tensile strength is investigated as well.It is found that the sample can well rebond and the healing efficiency reaches 90.7% when the sample is treated at 80 ℃ for 180 min.Furthermore,the healing efficiency decreases gradually but it maintains above 90% with the increase of hard segment and ID content.The self-healing mechanism is explored by in situ infrared spectroscopy.Results show that the repair is realized through the synergistic effect of hard phase melting,imine exchange reaction and thermal motion of molecular chains.Moreover,the self-repair efficiency enhances with the increase of temperature.(3)Preparation and properties of thermally reversible polyurethanes containing imine and disulfide bonds: The thermoreversible polyurethane based on both imine bond and disulfide bond is synthesized using MDI,PPG-2000,ID and diaminodiphenyl disulfide.The crystalline and mechanical properties of the resultant polyurethane are characterized.Results show that stuctrue and properties of polyurethane have the following change with the increase of DS: the degree of microphase separation and tensile strength decrease.Meanwhile,the hardness is invariable and the elongation at break increases.The hardness,tensile strength and elongation at break of the optimal sample can reach 73.0 HA,9.6 MPa and 623%,respectively.When compared to those of polyurethane only containing imine bonds,the hardness remains constant,and the tensile strength is reduced by 1.41 MPa while the elongation at break is increased by 20%.in addition,the thermoreversible polyurethane contained both imine bonds and disulfide bonds possesses better fatigue resistance.(4)Study of self-healing behavior of thermally reversible polyurethanes containing both imine and disulfide bonds: Compared to the self-healing polyurethane only contained imine bonds,the healing efficiency of polyurethane with the introduction of both imine bonds and disulfide bonds can reach 92.5% when treated at 70 ℃ for 150 min.It indicates that more efficient healing can be realized at lower temperature and shorter time.Furthermore,the healing efficiency enhances with the increase of DS.Results of Raman spectroscopy manifests that disulfide bond exchange reaction occurs firstly in the material,and which improves the chain mobility and further promotes the exchange reaction of imine bond.As a result,the self-healing ability of polyurethane is improved greatly.