Preparation And Performance Research Of Polyurethane Hydrogels Containing Tannin

Tannic acid（TA）is a kind of plant macromolecule with a wide range of sources.It can play the role of complexing and crosslinking through various chemical bonds and intermolecular interactions such as hydrogen bonds,ionic bonds,coordinate bonds and hydrophobic interactions because of abundant phenolic hydroxyl groups.Dopamine（DA）has a similar structure to TA.Due to its highly reactive amino group and functional catechol structure,it is mainly used for surface modification and preparation of functional materials.Hower,DA is expensive and its preparation process is complex,can not be applied industrially.In this paper,TA was used to replace DA to synthesize a series of polyurethanes.Their structure and thermal properties were characterized by IR,NMR,and thermal analysis methods.The difference in the preparation method and properties of polyurethanes hydrogels with different ways to introduce TA into polyurethane was studied,which included physical blending,chemical crosslinking and chemical end-capping.Investigation focused on the effects of TA content,pH,n(Fe³⁺)/n（TA）and n（PDBA）/n（TA）on the properties of the gels.The main research contents and conclusions are as follows:1.The water-soluble linear polyurethane（LPU）was synthesized using polyethylene glycol 2000（PEG2000）and isophorone diisocyanate（IPDI）,and its structure was characterized by IR and NMR.The solubility of TA in LPU was measured.A kind of supramolecular hydrogel based on LPU/TA/Fe³⁺mixed solution was prepared.Besides,the effects of pH and n(Fe³⁺)/n（TA）on the gel properties were studied.The results show that the pH determines the coordination number of catechol with Fe³⁺and n(Fe³⁺)/n（TA）determines the ratio of hydrogen bonds and coordination bonds in LPU/TA/Fe³⁺gels.In other words,only when pH and n(Fe³⁺)/n（TA）are set at proper values,can TA and Fe³⁺form more two-coordination and three-coordinate,and can TA maintain sufficient phenolic hydroxyl groups for hydrogen formation with PEG segments.Thus LPU/TA/Fe³⁺mixture can form a supramolecular gel by self-assembling.2.The catalytic rate of dibutyltin dilaurate（DBTDL）and triethylamine（TEA）for reaction between TA and IPDI was compared.TA was directly used as a cross-linking agent so that a kind of chemical cross-linked elastomer（TAPU）was synthesized utilizing PEG2000 and IPDI.And TAPU can swell in water to form hydrogel.The structure and thermal properties of TAPU were analyzed while the influence of TA content on the mechanical properties,swelling properties and hydrolytic properties of TAPU gel was studied.As a result,the catalytic effect of TEA is better than that of DBTDL,and all phenolic hydroxyl groups of TA molecular are reactive with–NCO group.That is,the lower the content of TA in TAPU,the fewer the residual phenolic hydroxyl groups of TA molecules,the higher the chemical crosslinking degree and the fewer the physical crosslinking points of TAPU gel.The mechanical properties and swelling properties of TAPU hydrogel exhibit corresponding regularity.Since the reaction between the phenolic hydroxyl group of TA and-NCO has a certain reversibility,the carbamate group can be converted into urea groups by reaction with water,and the TAPU hydrogel can be gradually hydrolyzed at a certain temperature.TA-terminated polyurethane（PU-TA）was synthesized by Two-step method.The-NCO terminated polyurethane prepolymer was first synthesized using PEG2000and IPDI,and then the prepolymer solution was added into the TA solution drop by drop to synthesize PU-TA.FeCl₃ and 1,4-benzenediboronic acid（PDBA）were used as cross-linking agents to prepare PU-TA/Fe³⁺solution and PU-TA/PDBA solution respectively.The pH was adjusted to prepare composite gel.IR and NMR were used to characterize the structure of PU-TA.The rheological,pH-responsive and self-healing properties of gels are the focus of performance research.The results show that both PU-TA/Fe³⁺gels and PU-TA/PDBA gels have excellent pH reversibility and self-healing properties.