Preparation And Properties Of Self-healing Polyurethane Hydrogel

The internal microcracks appear inevitably during the use of hydrogel,which can reduce the material performance and shorten their serviceable life.The self-healing hydrogels based on dynamic covalent bond not only possess the stability and strength of traditional hydrogels,but also can realize the recombination of molecular chains via dynamic exchange under appropriate stimulation（pH value,temperature,light,etc.,）.Therefore,self-healing hydrogel materials attract the attention of many researchers and has become a hot research direction.In this thesis,two kinds of self-healing polyurethane hydrogels based on disulfide and hydrazone bonds were designed and prepared,and their self-healing performances were studied.The main contents are divided into the following two parts:Ⅰ.Self-healing polyurethane hydrogel based on reversible disulfide bonds（SPUG）:The tetrahydroxy compound of 3,3’-disulfide dipropane-1,2-diol（DSO）was synthesized by oxidization of 3-mercapto-1,2-propanediol（TG）using dimethylsulfoxide as oxidant,and the prepolymer of NCO-terminated polyurethane（PU-R）based on polyethylene glycol（PEG）and isophorone diisocyanate（IPDI）were then crosslinked by DSO to form gels.Finally,polyurethane hydrogels containing disulfide bonds（SPUGs）were obtained via solvent replacement technique,and the corresponding dry gels（DSPUG）were acquired by freeze-drying method.DSO and DSPUG were characterized by ¹H NMR,¹³C NMR,FT-IR,MS and SEM.The SEM observation showed that DSPUG had a honeycomb structure.The physicochemical properties,redox-responsiveness and self-healing properties of the materials were studies detailly.All the DSPUG showed excellent swelling rate,which increased with the increase of PEG content and with the decrease of crosslinking degree.While the equilibrium swelling rate（ESR）was mainly affected by solid content,and the maximum ESR could be up to 1738%（SPUG-IV）.Due to the denser network structure and hydrophilic PEG segments,the SPUG exhibited good water-retaining capacity（WRC）,and the water-retaining rate of SPUG-Ⅰwas still above 75%after 6 days at room temperature.The thermal transition curve of DSPUG displayed a glass transition temperature（T_g）lower than room temperature,and TGA analysis showed that the materials possessed excellent thermal stability with initial decomposition temperature（T_i）higher than 250 ℃.The mechanical properties of SPUG were measured by compression and extension tests,and the results showed that the hydrogel possessed well mechanical properties with compressive strength of 138.1～365.2kPa,breaking strength of 68.6～134.4 kPa and elongation at break of 255.8～759.6%,which increased with the increase of solid content or/and crosslinking degree.The redox-responsiveness of the hydrogels was founded in the presence of dithiothreitol and hydrogen peroxide,the SPUG produced three times of gel-sol-gel cycles.The effects of temperature,time and healing cycle on the self-healing efficiency（SHE）of SPUG were researched.At the optimum healing temperature of 60 ℃,the SHE could exceed 90%after self-healing for 12 h,and the SHE increased with the increase of disulfide bond content.The SHE of SPUG-I,which had the highest disulfide bond content,could reach 97.45%after 12 h of healing,and the SHE remained above 80%after 10cycles.Ⅱ.Self-healing polyurethane hydrogel based on reversible hydrazone bonds（NPUG）:The tetrahydroxy compound of 1,6-bis（（E）3,4-dihydroxybenzyl）hexadioyl hydrazine（DBAH）was firstly synthesized via condensation reaction of 3,4-dihydroxybenzaldehyde and diadioyl hydrazine,and PU-R was crosslinked by DBAH to give gels with different contents of hydrazone bonds.Finally,hydrogels（NPUGs）were obtained via solvent replacement technique,and the corresponding dry gels（DNPUGs）were acquired by freeze-drying method.The chemical structures of DBAH and NPUG were confirmed by ¹H NMR,¹³C NMR,FT-IR and MS.SEM observation.The physicochemical properties,pH responsiveness and self-healing properties of the hydrogels and dry gels were studied.The swelling rate increased with the increase of PEG content and/or the decrease of crosslinking degree,and the DNPUGs exhibited excellent swelling rate with the maximum ESR of more than 600%.NPUGs exhibited outstanding WRC,and the water-retaining rate of NPUG-Ⅰwas higher than 80%after 5 days at room temperature.The results of thermal properties indicated that the DNPUGs had a T_g at low temperature and good thermal stability with T_i higher than 270 ℃.The measurements of mechanical properties manifested that NPUGs owned well compression performance（102.0～392.3 kPa）and tensile performance（breaking strength:77.54～139.1 kPa;elongation at break:359.9～809.4%）.The pH-responsive gel-sol-gel cycle was founded by adjusting the environmental pH value,but the cyclic times should not exceed 4 times.The influence of time and healing cycle on the SHE of NPUGs were studied,and the results showed that the SHE was closely related with the content of hydrazone bonds.After healing for 12 h at room temperature,all the NPUGs exhibited high SHE of more than 90%,and the SHE of NPUG-Ⅰ,which had the highest content of hydrazone bonds,could reached 98.36%.In addition,the SHE remained above 80%after 10 cycles.Crosslinked polyurethane hydrogels based on disulfide or hydrazone bonds possess outstanding mechanical properties,unique stimulus-responsivity and excellent self-healing properties,which can provide a design concept for other self-healing materials based on disulfide or hydrazone bonds.