Preparation And Properties Of Multiple-Sensitive PEG-based Hydrogel With Improved Mechanical Properties

At present, smart hydrogels mainly have two drawbacks:(1) Involving in double or multiple responses of smart hydrogels were very limited;(2) the mechanical properties of the smart hydrogels were generally poor. Therefore, it is of great scientific significance to research and develop the smart hydrogels with multiple responsive and excellent mechanical properties simultaneously from both theoretical and experimental research.Until2006, Lutz et al., who used two kinds of PEG derivatives of2-(2-methoxyethoxy)ethyl methacrylate (MEO2MA) and oligo(ethylene glycol) methacrylate (OEGMA) have synthesized adjustable temperature-sensitive copolymer (P(MEO2MA-co-OEGMA)), ending that only the amide type polymer has temperature-sensitive of the times. Therefore, PNIPAM as an example of a thermo-sensitive polymer was challenged by the discovery that the random copolymer of P (MEO2MA-co-OEGMA) exhibits a LCST in water, which can be tuned between26℃to90℃depending on the OEGMA content. Moreover, in contrast with PNIPAM, P (MEO2MA-co-OEGMA) hydrogels are expected to be more suitable for the application of biomedical materials because of their non-toxic and non-immunogenic.In this thesis, from the views of the functionality, mechanical properties and application of hydrogels, first, dual temperature-and pH-sensitive P (MEO2MA-co-OEGMA-co-AAc)(PMOA) hydrogels were prepared by free-radical polymerization of MEO2MA and OEGMA as thermo-sensitive monomers, and acrylic acid (AAc) as a pH-sensitive monomer. The stimuli-responsive properties of PMOA hydrogles were studied in detail by means of swelling and de-swelling experiments. Secondly, dual-and multiple-responsive organic/inorganic nanocomposite (NC) hydrogels (AT/PMOA and PMOA/AT-Fe3O4) with excellent mechanical properties were synthesized by in situ polymerization of MEO2MA, OEGMA and AAc, as the polymeric matrix (PMOA), and fibrillar attpulgite (AT) and magnetic attapulgite (AT-Fe3O4), as the reinforcer and magnetic functional enhancers, respectively. The mechanical properties, stimuli-responsive properties as well as the swelling/de-swelling behaviors were investigated in detail. Furthermore, we successfully fabricated PMOA/AT-Fe3O4nanocomposite microgel. The morphology, structure and responsive behaviors of the prepared microgels were systematically characterized by fourier transform infrared spectroscopy (FT-IR), field emission scanning electron microscopy (FESEM), atomic force microscope (AFM), nano-particle size analyzer and vibrating sample magnetometer (VSM). The main contributions are shown as following:(1) The PMOA hydrogels exhibit reversible LCST behaviors in the deionized water with changing temperatures and excellent pH-responsive behaviors in the alkaline buffer solutions. The results of the swelling and de-swelling tests showed that the thermo-and pH-responsive behaviors of the hydrogels can be tuned by three comonomers of MEO2MA, OEGMA and AAc. The equilibrium swelling ratio of hydrogels in the deionized water increased with increasing OEGMA content and decreasing AAc content. However, the equilibrium swelling ratio of hydrogels in the alkaline buffer solutions increased with increasing AAc content. In addition, the phase transition temperature determination by tracking the change of equilibrium swelling ratios with temperature was consistent with the DMA testing results. The LCST values corresponding to PM9O1A-1and PM8O2A-1are around31℃and37℃, respectively. Moreover, the de-swelling kinetics was studied by changing temperature and/or pH, and they could be well described with a first-order kinetics equation, In comparison of three given conditions, the PMOA hydrogels exhibited faster de-swelling rates by changing the temperature and pH values simultaneously from pH8/18℃to pH2/55℃. Especially PM8O2A-1and PMgO2A-2hydrogels, the amounts of release of water almost reach90%and100%at80min, respectively.(2) The results of the tensile tests and the fracture morphology analysis demonstrated that the incorporation of AT nanoparticles significantly enhanced the mechanical properties of AT/PMOA NC hydrogels. As the content of AT increased, the tensile strength, tensile modulus and effective cross-linked chain density increased. The fracture morphology analysis suggested the orientation effect of the nanorods during tension, the energy dissipation mechanism as well as the hydrogen bonding interactions between PMOA matrix and nanoparticles may be the main reasons for such mechanical properties improvement. In addition, the compatibility and interaction between the nanoparticles and polymer matrix were effectively characterized by dynamic mechanical analysis (DMA). The results showed that the storage modulus of AT/PMOA NC hydrogels were increased and the glass transition temperatures shifted to higher temperature compared to the pure PMOA hydrogel, which further indicated that the enhancement of mechanical property depended upon the presence and content of AT. According to the Arrenius equation, the physical crosslink density (Xc) and the activation energy for hydrogen bonding dissociation (Ea) have been evaluated from the elastic modulus-temperature relationship. The results showed that Xc increased with increasing the AT content, which was consistent with the results of static stretching. Furthermore, the content of AT has also an obviously effect on the swelling/de-swelling behaviors of the nanocomposite hydrogels. It was found that the faster swelling rates of the NC hydrogels were observed in comparison to the corresponding physically cross-linked PMOA hydrogel, except for1%AT/PMOA sample. However, the de-swelling kinetics of NC hydrogels was obviously retarded, and the de-swelling rates decreased with increasing the AT content.(3) The magnetic nanocomposite particle was successfully prepared via co-precipitation technique in aqueous suspension of purified attapulgite. The obtained AT-Fe3O4nanoparticle was characterized by FT-IR, XRD, FESEM and VSM. The results showed that Fe3O4nanoparticles were well deposited on the surface of AT and the AT-Fe3O4nanocomposite particle exhibited better the superparamagnetic behavior.(4) The effect of the magnetic AT-Fe3O4content on the morphology, various responsive behaviors, as well as tensile properties of the multi-functional AT-Fe3O4/PMOA nanocomposite hydrogels (mf-NC hydrogels) was systematically investigated. The results showed that the addition of the small amount of AT-Fe3O4nanoparticles which were well dispersed in the hydrogel matrix. With further addition of AT-Fe3O4nanoparticles (5wt%), more space of the polymer network was occupied which lead to a slight agglomeration. The magnetic properties of the mf-NC hydrogels were characterized by applying the static and dynamic magnetic fields, respectively. The magnetic hysteresis loops indicated that the mf-NC hydrogel exhibited superparamagnetic behavior, and the saturation magnetization of that increased with increasing the AT-Fe3O4content. The mf-NC hydrogel can continue to swell in an alternating magnetic field (AMF) after equilibrium swelling in deionized water. Due to the effect of the AMF, the magnetic nanoparticles have uninterrupted vibration in the hydrogel network, and thus the arrangement of the magnetic nanoparticles become more loose. It is reasonable to suggest that an increase of the distance between the molecular chains lead to a larger space in the hydrogel networks, which allows water to enter further. This phenomenon can affect the swelling behavior of the whole system. In addition, the mf-NC hydrogel still possesses temperature-and pH-sensitive simultaneously. The temperature and pH dependent swelling of the hydrogel is reversible, although the magnetic AT-Fe3O4nanoparticles were introduced into the PMOA system. Moreover, the mechanical properties of the mf-NC hydrogels were significantly improved by addition of the magnetic AT-Fe3O4nanoparticles into the PMOA matrix, and the tensile strength of that increases with increasing of AT-Fe3O4content.(5) The temperature-, pH-and magnetic-field-sensitive PMOA/AT-Fe3O4nanocomposite microgel was prepared by in situ free-radical polymerization with ethylene glycol dimethacrylate (EGDMA) as a chemical cross-linker. FT-IR spectrum has been used to characterize the hydrogen bonding interactions between AT-Fe3O4and PMOA copolymer. The degree of hydrogen bonding (XH) was obtained from the ’peak resolve processing’ on the corresponding H-bonded carbonyl peaks. The results illustrated that the XH of PMOA/AT-Fe3O4nanocomposite microgel was higher than that of the pure PMOA microgel. Simultaneously, the mechanism of the formation of the hydrogen bond between AT-Fe3O4and PMOA matrix was analyzed. According to the results of FESEM, AFM and VSM, PMOA/AT-Fe3O4nanocomposite microgel had not only the typical core-shell microstructure, but also had good superparamagnetic behavior. In addition, the pH-and (or) temperature-responsive properties of microgels were analyzed by the particle size variation in different pH media and temperatures. The results showed that PMOA/AT-Fe3O4nanocomposite microgel has a good pH-and temperature-sensitive. When the pH value of the buffer solution was below4.5, the micrlge was easily to form precipitate in the dispersing medium. However, when the pH value was above4.5, the microgel could be well dispersed in the buffer solution, and the particle sizes increased with increasing the pH value of the dispersing medium. The particle sizes of PMOA/AT-Fe3O4microgel was gradually decrease with increasing temperature, and the volume phase transition temperature (VPTT) was around36.5℃.Simultaneously, the temperature dependence of the particle size of the PMOA/AT-Fe3Q4microgel in different pH buffer solution was also investigated. The results showed that when the microgel was dispersed in weak acidic mediums, which remained temperature-sensitive, and the particle size of the microgel was decreased with increasing the temperature; however, when the microgel was dispersed in strong alkaline buffer solution, the particle size of that was less prone to shrinkage with increasing temperature. It may be attributed that the completely dissociated carboxy groups in the polymer network, leading to the temperature-sensitive of the microgel decreased or even disappeared.