Study On Synthesis And Properties Of Metal Oxide Nanoparticles Cross-linked Nanocomposite Hydrogels With High Strength

Hydrogels is a kind of soft materials that consist of cross-linked polymer networks andlarge amounts of water. Due to their excellent hydrophilicity, biocompatibility and stimuli-responsibility, hydrogels have been widely utilized in various biomedical appliations suchas tissue engineering, controlled drug release, artificial muscle and biosensors. However,conventional hydrogels (OR gels) cross-linked by organic cross-linkers always displayweak mechanical properties and are readily broken down under small force or deformation,which greatly limit their practical applications, especially in load-bearing systems. In orderto improve the mechanical properties and broaden the applied fields of hydrogels,researchers in different countries have developed a series of novel hydrogel materials withhigh strength over the past decades. Among them, nanocomposite hydrogels (NC gels),which employ inorganic nanoparticles as cross-linking agents, have aroused wide concernbecause of their outstanding mechanical, optical, swelling/de-swelling properties, fastresponsive rate as well as other specific properties. For now, NC gels have become thehotspot of hydrogel research. However, for a long time, the nanoparticles used to prepareNC gels were confined to a kind of synthetic clay nanosheets-Laponite XLG. This kind ofnano-clays has negative charges on their surfaces but positive charges on their edges. A“house of card” structure would be formed when it is dissolved in water with highconcentration, which leads to the high viscosity of clay dispersion, making it difficult tomix with monomers as well as uniformly disperse in polymer matrix. This disadvantagehinders the further improvement of mechanical properties of NC gels. Although otherinorganic or organic nanomaterials, such as graphene oxide, cellulose naonocrystals andpolystyrene nanoparticels, have been utilized to prepare new type of NC gels, metal oxidenanoparticles, which are broadly used in various industrial areas, have been rarely used ascross-linking agents for hydrogel preparation before.In this thesis, titania, alumina and functionalized silica nanoparticles were successfullyutilized as inorganic cross-linking agents to prepare novel high strength nanocomositehydrogels using acrylic acid (AA), acrylamide (AM), N, N-dimethyl acrylamide (DMAA)and N-isopropylacrylamide (NIPAM) as monomers via in situ free-radical polymerization.The cross-linking mechanisms, mechanical properties, microstructures, swelling propertiesand other specific characteristics were systematically investigated. The main contents and conclusions are as follows:1. A series of high-strength P (AA-co-DMAA)/TiO2nanocompoite hydrogels (TADgels) with different monomer ratios were synthesized by in situ free-radicalcopolymerization of AA and DMAAusing titania nanoparticles (TiO2NPs) as cross-linkingagents. The cross-linking mechanism of TAD gels was contribute to the complexationsbetween carboxylic groups on polymer chains and hydroxy groups on the surface of TiO2NPs. When the TiO2content was fixed, as the increasing of AA ratio in the initial solution,the cross-linking densities of TAD gels increased, leading to the increasing of elasticmoduli and decrease in toughness and swelling ratios. When the AA ratio was higher than40%, the obtained TAD gels were brittle and stiff, lack of toughness. By simple heatingtreatment, TAD gels with higher mechanical strengths and lower swelling ratios could beobtained. These hydrogels were able to maintain high strengths after swelling and showwater-activated shape memory effet.2. A series of high-strength P(AM-co-DMAA)/TiO2nanocompoite hydrogels (TMDgels) with different monomer ratios and TiO2contents were synthesized by in situ free-radical copolymerization of AM and DMAA in TiO2NPs solutions. The transmissionelectron microscopy and infrared spectroscopy indicated that the TiO2NPs were uniformlydispersed in polymer matrix and the cross-linking mechanism of TMD gels werecontributed to the hydrogen bondings between the amino groups on polymer chains andhydroxy groups on the surface of TiO2NPs. The effects of AM ratio and TiO2NPs contenton the properties of TMD gels were the same as those of AAratio and TiO2content in TADgels. Compared with TAD gels, the TMD gels displayed larger stretchability and swellingratio due to the weaker interactions between polymer matrx and TiO2NPs. The hydrogelfilm (0.5mm in thickness) prepared from TMD gels with AM ratio lower than50%showedhigh transparency in visible light but could completely block the UV light with wavelengthlower than366nm, indicating that TMD gels could be utilized as UV-shieding materials.3. A series of high-strength P(AM-co-NIPAM)/TiO2nanocompoite hydrogels (TMNgels) with different monomer ratios and TiO2contents were synthesized by in situ free-radical copolymerization of AM and NIPAM in TiO2NPs solutions. Since the cross-linkingmechanism of TMN gels was also contributed to the hydrogen bondings between the aminogroups on polymer chains and hydroxy groups on the surface of TiO2NPs, the effects ofAM ratio and TiO2NPs content on the properties of TMD gels were the same as those inTAD gels. Due to the introduction of NIPAM, the TMN gels exhibited composition- dependent temperature sensitivity. Taking the advantage of the different swelling andshrinking behaviors of TMN gels in pure water and20%NaCl solution, hydrogel biayerwith bidirectional bending and reversible shape changing behaviors was prepared and itspotential appications as soft actuators was demonstrated.4. A series of high-strength P(AA-co-DMAA)/Al2O3high-strength nanocompositehydrogels (AAD gels) with different monomer ratios and Al2O3contents were prepared byphotoinitiated free-raical copolymerization of AA and DMAA using Al2O3NPs as cross-linking agents. Similar with TAD gels, the cross-linking mechanism of AAD gels was alsocontributed to the complexations between carboxyl groups on polymer chains and hydroxygroups on the surface of Al2O3NPs. The obtained AAD gels displayed ultrastrongmechanical properties, high transparency and homogeneous microstructures when the AAratios were lower than50%. The mechanical properties and swelling ratios coud also betuned by altering the monomer ratios and Al2O3contents in the initial solution. Apart fromthe outstanding mechanical properties, AAD gels displayed anti-fogging and shape memoryproperties.5. A new type of high-strength PNIPAM/SiO2nanocomposite hydrogels (FS gel) wereprepared by using3-methacryloxypropyltrimethoxylsilane (MPS) functionalized silicananoparticles (FS NPs) and NIPAM as cross-linking agents and monomer, respectively.Compared with OR gel and nanocomposite hydrogels (PS gel) cross-linked by pristinesilica nanoparticles (PS NPs), FS gel not only exhibited much improved mechanicalproperties but also maintained the fast responsive rate and temperature-sensitivity ofPNIPAM hydrogels. Furthermore, the FS gel also displayed fast water activatd shapememory effect due to its high mechanical strength and low swelling ratio.