Preparation And Properties Of Self-healing Poly(Ether-urethane)s Based On Schiff-base Groups

Polyurethanes（PUs）have been widely used in various fields due to their good mechanical strength,excellent radiation resistance,friction resistance,fatigue resistance,well biocompatibility and thermal insulation properties,such as smart electronics,medical equipment,building materials,aerospace aviation,shape memory materials,etc.However,PUs are easily damaged under external force,which reduces the serviceable life of the products and produces potential safety hazard.On the other hand,the discarded cross-linked PU can cause a series of environmental problems and resource wastes.Therefore,it’s necessary to solve the problems by improving the servicable life and efficiency of PUs,which possesses not only practical economic benefits,but also important value of environmental protection.Obviously,it is an effective measure by endowing matetials with self-healing property,and self-healing materials have become one of the current research hotspots.In this thesis,we designed and prepared a series of new PU materials with excellent tensile properties and high room-temperature self-healing efficiency by introducing Schiff-base groups into PU structure.The research content is mainly divided into the following two parts:I.Self-healing poly（ether-urethane）s based on Schiff-base covalent bonds.A tetrahydroxy compounds containing Schiff-base groups（2,2’-（1E,1’E）-1,4-Phenylene bis（formyl subunit）bis（azasubunit）dipropane-1,3-diol,PBB）was firstly synthesized by condensation reaction of2-amino-1,3-propanediol with 1,4-phthalaldehyde,and then the isocyanate-terminated poly（ether-urethane）prepolymers were cross-linked by PBB to give the poly（ether-urethane）（PBPU）with different content of Schiff-base groups.The chemical structures of PBB and PBPU were characterized by NMR,FT-IR and MS,and the physicochemical properties of the PBPU films,especially the influence of the PBB content on the mechanical and self-healing properties,were studied.TGA test showed that the initial decomposition temperature of PBPU film was higher than 200 ^oC,indicateding high thermal stability,and the thermal stability increased with the increase of PBB content which produced high cross-linking degree;it was found from the DSC curves that only one low glass transition temperature（T_g）was observed,it meant that the molecular segments of the polymer possessed a certain freedom degree at room temperature and could provid the necessary conditions for their self-healing.The increase of cross-linking degree endows PBPU with excellent tensile properties（ultimate stress:2.5～10.7 MPa;strain at break:1500～1994%）.In addition,PBPU films exhibited low hydrophilicity and swelling ability（<7%）.The test of self-healing performances showed that PBPU owned well room-temperature self-healing capacity.With the increase of PBB content,the self-healing efficiency first increases and then decreases,and the maximum room-temperature self-healing efficiency reached up to94.6%when the PBB content was 3.61wt%;the increase of healing temperature and time was also contributed to promote the self-healing efficiency.After teen-time cyclic test,the self-healing efficiency remained above 75%.Furthermore,it was found that PBPU films possessed a reversible transition of transparency in water,which meant a potential application in optical field.II.Self-healing poly（ether-urethane）s based on coordinate bonds between Schiff-base groups and Co²⁺.A dihydroxy compounds containing Schiff-base groups（2-[[（2-hydroxyphenyl）methylene]amino]-1,3-propanediol,HPA）was firstly synthesized by condensation reaction of2-amino-1,3-propanediol with salicylaldehyde,and the isocyanate-terminated poly（ether-urethane）prepolymers were then chain-extented with HPA.After the Co²⁺was added to form dynamic M-L coordinate bonds,the cross-linked poly（ether-urethane）（HPPU-Co）was obtained.NMR,FT-IR and MS were used to confirmed the chemical structures of HPA and HPPU-Co.Meanwhile,the physicochemical properties of the HPPU-Co films were also studied,and the influence of the the ratio of Co²⁺to HPA in HPPU-Co films were studied detailly.Thermal propertie tests manifested that the HPPU-Co owned high thermal stability,low crystallinity and T_g at low temperature.The existence of crosslinked structure formed by coordination bonds provided excellent tensile with ultimate stress of 25～54 MPa.In addition,it was found that the HPPU-Co films presented a reversible color change in water,endowing the material with potential application in other fields.The self-healing performance tests showed that HPPU-Co possessed high self-healing ability at room temperature,and the maximum healing efficiency reached 99.6%when the ratio of Co²⁺to HPA was 1:2.The results indicated that the coordination geometry and the content of coordinate bonds were closely related with the self-healing capacity.Furthermore,the self-healing efficiency of HPPU-Co films remained above 80%after teen-time cyclic tests.The cross-linked PU materials based on Schiff-base covalent bonds and Schiff-base-Co²⁺coordinate bonds exhibited good thermal stability,excellent tensile properties and self-healing properties,which provided a method and theoretical basis for the design and preparation of PU materials with high self-healing capacity.