Study On Synthesis, Structure And Property Of Zwitterionic Sulfobetaine Polymer Nanocomposite Hydrogels

Hydrogels of zwitterionic betaine polymers are a potential new type of thermosensitive and/or polyelectrolyte hydrogel. They exhibit many potential applications because of their unique properties, such as drug release, wound dressing, and anti-protein adsorption and antithrombotic materials, and so on. They attract much interest, especially the sulfobetaine polymer with simple synthesis method and unique upper critical solution temperature (UCST). Currently, most zwitterionic sulfobetaine hydrogels reported are chemically crosslinked, showing weak compression mechanical property. There is almost no report on their tensile mechanical property. Meanwhile, it is difficult to control their UCST. Therefore, it is very important to improve the mechanical property of zwitterionic sulfobetaine hydrogel and control the UCST effectively for building the response relationship between the gel and external environment.In this thesis, for the purpose of improving the mechanical property and well-controlling the UCST type thermosensitivity, zwitterionic organic/inorganic nanocomposite hydrogels(NC gels) with high mechanical property, UCST type thermosensitivity and salt tolerance property were successfully synthesized by in situ polymerization, using zwitterionic sulfobetaine monomer with similar structure and nonionic acrylamide monomer in the presence of clay platelets which were used as a crosslinker. The synthesis, network structure, mechanical property, thermosensitivity and the swelling/deswelling behavior in different solvents were studied systematically. The main contents and conclusions on the synthesis and properties of zwitterionic thermosemsitive NC gels with high mechanical property and salt tolerance are as follows:1. Zwitterionic sulfobetaine acrylamide polymer (poly(N,N-dimethyl (acrylamidopropyl) ammonium propane sulfonate), polyA3) and their physically crosslinked and chemically crosslinked hydrogels were synthesized by free-radical polymerization in an aqueous redox system. The Mark-Houwink-Sakurada equation of zwitterionic sulfobetaine polymer (α=0.78) was firstly obtained by relating the intrinsic viscosity and molecule weight which were measured in O.1M NaCl solution. The formation mechanism of physically crosslinked hydrogel was described. The UCST variations and the factors affecting the UCST were also studied. It was found that the physical state and transparency were strongly dependent on the monomer concentration and temperature. When monomer concentration is150g/L, polyA3began to form the physically crosslinked hydrogels because of the interaction between the negative and positive charged groups and the entanglement of the polymer chain. PolyA3showed the UCST type thermosensitivity (The range of UCST is20-60℃). The variation tendency that UCST increased and then decreased with increasing monomer concentration result from the combine effect of molecular weight and polymer concentration. Both the introduction of chemically crosslinked network and the copolymerization with N-isopropylacrylamide (NIPA) decreased the UCST.2. Zwitterionic sulfobetaine nanocomposite gel with UCST type thermosensitive property was firstly successfully synthesized by in situ free-radical polymerization of zwitterionic lfobetaine monomers(A3, A4, M3) in the presence of exfoliated clay platelets in an aqueous redox system. Viscosity test showed that the viscosity increase of the clay dispersion was depressed owing to the interaction between zwitterionic monomers and clay. Mechanical property test indicated that the introduction of clay improved the tensile strength of polyA3gel and polyA4gel greatly. Especially, A4-NC gel showed the maximum tensile strength (71.3kPa) and2000%elongation. The UCST was also well controlled. The UCST of polyA3gel and polyA4gel decreased from23.9℃and100℃to16.5and85℃, respectively. However, for M3-NC gel, the tensile strength was still very weak and the UCST did not change at all. These can be attributed to the interaction difference between the different polymers and clay platetlets casused by the length of alkyl chain and steric hindrance effect.3. In order to further improve the mechanical property and control the UCST of the zwitterionic NC gels, copolymeric zwitterionic NC gel (A3D7-NC gel and A4Dy-NC gel) was prepared successfully usingN,N-dimethylacrylamide (DMAA) as comonomer. The formation of polymer/clay network structure was confirmed by XRD, TEM and FTIR. The tensile strength measurement indicated that the introduction of DMAA had a different effect on their properties, as a result of the difference of interaction between the A3and A4polymer chains and clay. The tensile strength and modulus of A3Dy-NC3gels increased with increasing DMAA content. As for A4Dy-NC3gels, the tensile strength decreased first and then increased, showing a minimum at y=50. The thermosensitivity test showed that UCST shifted to low temperature with increasing DMAA content. So the UCST phase transition can be well controlled by altering DMAA content. It is found that the A3D10-NC3gel with10mo1%DMAA exhibited highly stretchable and mechanically tough properties, and UCST phase transition, becoming an ideal hydrogel material. At this molar ratio, increasing the clay content further improved the tensile strength and controlled the UCST. The tensile strength was up to200kPa and UCST decreased to4℃.4. Zwitterionic copolymeric A3D10-NC-MZ gels with different polymer concentrations were synthesized by altering the total monomer concentration (0.1-1M). The effects of polymer concentration on the structure and property of hydrogel were discussed. Dynamic viscoelasticity analysis indicated that A3D10-NC were gel state, while A3D10-OR were gradually changed from sol to gel. Mechanical property of NC gel and OR gel showed that as Cp increased, the improvement of the tensile strength for NC gel is much larger than OR gel. For NC gel, UCST was not observed at low Cp; UCST appeared and shifted to low temperature with increasing Cp. For OR gel and copolymeric physical gel (polyA3D10), UCST always can be observed and increased first and then decreased with increasing Cp. Based on the results of TEM and XRD, the model of polymer-clay network structure at different polymer concentration in the copolymeric hydrogel was proposed: polymer chain attached to the surface of clay platelets at low Cp; aggregation of clay platelets occurred with increasing Cp; polymer/clay network structure formed at high Cp.5. The swelling/deswelling behaviour, transparency change and UCST variation of A3-OR gel and A3-NC gel in different solvents were investigated. It indicated that in water at25℃, A3-OR gel deswelled, while A3-NC gel first swelled and then deswelled. The UCST of them increased. The difference from the nonionic NC gel is that both the maximum degree of swelling and water content at equilibrium state of A3-NC gel increased with increasing n. Zwitterionic hydrogel showed unique swelling/deswelling in NaCl solution with different concentrations. At0-0.001M NaCl solution, OR gel deswelled while NC gel swelled first and then deswelled. At0.001-1M NaCl solution, both OR gel and NC gel swelled and the degree of swelling increased with increasing NaCl concentration, which is very different from the nonionic NC gel. They shows the strong salt tolerance propery.In conclusion, the new zwitterionic nanocomposite hydrogel with high tensile strength, UCST themosensitivity and salt tolerance has been synthesized successfully, and the structure and property of zwitterionic NC gels were well controlled by altering polymer concentration and clay content. All these laid a solid theoretical foundation for the development of such applications hydrogel. The results that the difference interaction between polymers with different structures and clay platelets affected the property of NC gels enriched the interaction theory of organic/inorganic hybrid systems, and also provided a useful reference for understanding and building other systems.