Preparation And Functionalization Of Complex Microgels And Nanocomposite Hydrogels

Hydrogels were widely studied because of their applications in biomedical areas for their high water content and potential biocompatibility. The purpose of this thesis is to develop methods to synthesis complex microgels and nanocomposite hydrogels (NC gels) with multiresponse. First, we used precipitation polymerization to prepare total degradable microgels free of self-cross-linking by selecting special monomers and initiator, and then prepared multicompartment core/shell microgels and nanostrucutred hydrogel microcapsules with multiresponse. Later, amine-laden microgels were prepared by controlling the repulsive interactions between polymers and then used to fabricate all microgel films with anionic microgels. At last, we prepared multiresponsive and transparent NC gels with ultrahigh tensibility by in-situ copolymerization of ionic monomers and N-isopropylacrylamide (NIPAm) in the Laponite suspension. The main works and results are as following:1. Poly(N-isopropylacrylamide)(pNIPAm) microgels were synthesized by precipitation polymerization at temperatures ranging from 37 oC to 45 oC using the redox initiator system ammonium persulfate (APS)/N,N,N',N'-tetramethylethylenediamine (TEMED), or the photoinitiator 2,2'-azobis(amidinopropane) dihydrochloride (V50). Photon correlation spectroscopy (PCS) and atomic force microscopy (AFM) studies revealed that spherical microgels with narrow size dispersities can be obtained with these methods, and that the resultant microgels have similar volume phase transition temperature around 32 oC . Additionally, the low temperature, redox initiator strategy produces microgels devoid of self-cross-linking, thereby permitting the synthesis of completely degradable microgels when using N,N'-(1,2-dihydroxyethylene)bisacrylamide (DHEA) as a cleavable cross-linker. We also demonstrate the potential utility of the approach in bioconjugate syntheses; in this case avidin immobilization is demonstrated by one-pot copolymerization at 45 oC.2. Multiresponsive poly(N-isopropylacrylamide-co-acrylic acid) (pNIPAm-AAc) microgels containing mechanically and thermodynamically decoupled poly(N-isopropylmethacrylamide) (pNIPMAm) cores have been prepared. To achieve this structure, pNIPMAm microgels were used as templates in the synthesis of a DHEA cross-linked pNIPMAm inner shell. A pNIPAm-AAc outer shell was then added, resulting in“core/double-shell”(CDS) microgels. Erosion of the inner shell via periodate-mediated cleavage of the 1,2-diol bond in DHEA produced multiresponsive core/shell microgels with independent cores. AFM height and phase images of CDS-D microgels clearly showed that the pNIPAm-AAc shell became really flat after drying on the glass without the support of pNIPMAm shell. The temperature dependence of size and light scattering intensity of microgels in different pH buffers indicated the successful preparation of core-shell separated multiresponsive core/shell microgels.3. One-pot precipitation polymerization was used to prepare pNIPAm/pNIPMAm core/shell microgels. The temperature dependence of size and light scattering intensity showed that the core/shell microgels had double temperature response. In addition, temperature responsive pNIPMAm microgel capsules containing multiple pNIPAm nanoscopic inclusions were prepared. This structure was achieved through the addition of a BIS cross-linked pNIPMAm shell to stable, low polydispersity aggregates of pNIPAm chains that resulted from APS/TEMED initiated free-radical precipitation polymerization of NIPAm in the absence of any cross-linker. Thus, upon decreasing the temperature following synthesis, the majority of the encapsulated pNIPAm chains escaped from the thin, porous pNIPMAm shell, resulting in nearly hollow pNIPMAm microcapsules. However, we have observed that there are remnant pNIPAm segments unable to escape from the microcapsule, which form nanoparticulate inclusions upon raising the temperature to 40 oC. AFM height images clearly showed that multiple pNIPAm nanoparticles can be formed in a swollen pNIPMAm shell.4. Surfactant-free, radical precipitation co-polymerization of NIPMAm and N-(3-aminopropyl) methacrylamide hydrochloride (APMH) was carried out to prepare microgels functionalized with primary amines. The morphology and hydrodynamic diameter of the microgels were characterized by AFM and PCS, with the effect of NaCl concentration and initiator type on the microgel size and yield being investigated. When a V50-initiated reaction was carried out in pure water, relatively small microgels (~160 nm diameter) were obtained in low yield (~20%). However, both the yield and size increased if the reaction was carried out in saline or by using APS as initiator. Stable amine-laden microgels in the range from 160 nm to 950 nm in diameter with narrow size distributions were thus produced using reaction media with controlled salinity. Microgel swelling and electrophoretic mobility values as a function of pH, ionic strength and temperature were also studied, illustrating the presence of cationic sidechains and their influence on microgel properties. Finally, the availability of the primary amine groups for post-polymerization modification was confirmed via modification with fluorescein-NHS.5. All microgel films were fabricated by amine-laden pNIPMAm microgels and anionic pNIPAm-AAc microgels through centrifuge deposit layer-by layer (LBL) way. With the increase number of microgel layers in the film, AFM height images showed that the microgel packing density increased on the glass substrate and fluorescence microscopy images showed that the number of pNIPAm-AAc microgels increased. The fluorescence intensity of microgel films linearly increased with the number of pNIPAm-AAc microgel layer. Besides, AFM showed that the film thickness was also linearly increased with the number of microgel layer. At last, we comparatively studied the tensile and healing properties of microgel films from pNIPAm-AAc/PAH and pNIPAm-AAc/amine-laden pNIPMAm microgels. The results showed that film from pNIPAm-AAc and pNIPMAm microgels was easier to be broken and had worse healing ability.6. Ionic NC gels cross-linked by Laponite XLS with ultrahigh tensibility were successfully synthesized for the first time via in-situ copolymerization of NIPAm and sodium methacrylate (SMA). The pH and temperature response, transparency, and mechanical properties of the ionic hydrogels were investigated. The results showed that the addition of only 2 mol% of SMA endowed the NC gels with pH response, while the temperature response remained in the whole pH range. All the as-prepared hydrogels demonstrated transparency higher than 75%. The tensile strength evidently decreased from 60 kPa to 45 kPa when the SMA content reached 8 mol%. The elongation at break increased with increasing SMA content and 2900% was achieved for the sample containing 10 mol% of SMA. The effective network chain density estimated from the equilibrium storage modulus was about 0.28 mol/m3 for all the samples. The low chain density was the intrinsic origin of the ultrahigh tensibility for these ionic NC gels.7. pNIPAm/Laponite NC gels were synthesized via in-situ polymerization of NIPAm in the Laponite suspension containing polyethylene glycol (PEG). The adsorption of PEG on Laponite platelets was characterized by (?) potential, which decreased with the PEG adsorption. The tensile strength decreased and elongation at break increased with increasing PEG concentration. The effective network chain density of pNIPAm/Laponite NC gels determined from the equilibrium modulus Ge decreased upon adsorption of PEG on the Laponite. All of these results revealed the preferential adsorption of PEG on the Laponite platelets occupying the active sites for the pNIPAm chain anchoring, which hindered their cross-linking effect in the NC gels. However, the temperature sensitive swelling behavior still remained in the pNIPAm/Laponite NC gels containing PEG with higher swelling volume below the LCST due to the lower cross-linker density. By adjusting the amount of added PEG, we can easily control the properties of the pNIPAm/Laponite NC gels.