Study On Molecular Characteristics Of Poly (N-isopropylacrylamide) Prepared By Free-radical Redox Polymerization In Aqueous Solution And Polymer/Clay Network Structure In Nanocomposite Gels

Hydrogel, consisting of three-dimensional polymeric network with a significant amount of water or biological fluids, has attracted numerous interests not only in the theoretical study but also in potential application during the past two decades. Due to their stimulus-responsive and biomimetic properties, hydrogels are regarded as biomaterials applied in drug delivery system, tissue and cell cultivation engineering, sensing and diagnostics and so on. Especially for poly (N-isopropylacrylamide) gel, a typical thermal-sensitive hydrogel, great efforts have been promoted because its lower critical solution temperature (LCST) is quite close to human body temperature.However, traditional hydrogels (chemically-crosslinked hydrogels, OR gels) are not qualified for practical use due to their serious shortcomings, such as mechanical weakness and slow or delayed response time, which hinder their development in biotechnology. Recently, important breakthroughs have been made, i.e. nanocomposite gels (NC gels) are successfully synthesized by Dr. Haraguchi K.’s research group through in-situ, free radical polymerization at high yield under mild conditions and most of the traditional limitation of hydrogels has been overcome. NC gels, composed of poly (N-isopropylacrylamide) (PNIPA) and inorganic clay (synthetic hectorite), exhibit extraordinary mechanical properties, which can undergo large reversible extensions (1000%) and show ultrahigh mechanical toughness (～3000 times that of OR gels). Furthermore, a number of superior characteristics, such as controllable swelling/deswelling behavior, high transparency, optical anisotropy, biocompatibility and unique surface properties have never been realized in conventional hydrogels. All these advantages are closely related to its unique organic (polymer)/inorganic (clay) network structure:exfoliated clay platelets act as multi-functional crosslinking agents and neighboring clay sheets are connected by many flexible PNIPA chains. In order to identify the origin of the excellent properties of NC gels, it is important to get a thoroughly understanding of the PNIPA/clay network structure. In this thesis, the characteristics of PNIPA, the main constituent of gels, as well as the role of clay in in-situ polymerization have been reported. In addition, structure analysis of copolymer NC gel with improved mechanical properties is also discussed. They are briefly described as following:(1) The molecular characteristics of PNIPA, prepared by free-radical polymerization using an aqueous redox initiator and reaction conditions comparable to those used in the synthesis of nanocomposite gels, were investigated by altering the monomer concentration ([NIPA]) and the polymerization temperature (Tp) across the transition temperature (LCST). In the case of Tp below LCST, there is a critical [NIPA] (= n*) above which PNIPA partially forms gels in the absence of a chemical crosslinker, and the gel fraction increases with increasing [NIPA] and decreasing Tp. In contract, it was observed that the self-crosslinked gel disappeared regardless of [NIPA] if Tp above LCST. The structure and mechanism of formation of self-crosslinked PNIPA gels are discussed. In the range of n< n*, the relationship between molecular weight and [NIPA] were obtained. In addition, solution properties of PNIPA were discussed, including Mark-Houwink-Sakurada equation in organic solvents, the effects of fractionation and solvents onαand the relationship betweenαand [η]. Effect of molecular weight on glass transition temperature (Tg) and lower critical solution temperature (LCST) was also revealed.(2) As for the structure analysis of NC gels, PNIPA was successfully separated from an NC gel network by decomposing the clay (hectorite) using hydrofluoric acid (HF). A very low HF concentration (0.2 wt%) was adequate for the decomposition of the clay without causing any damage to PNIPA. The molecular weight of PNIPA separated from NC gels increased slightly with an increase in monomer concentration [NIPA] from 0.2M to 1.5M. In the case of [NIPA] equal to 1M, the obtained PNIPA was soluble and had a high Mw (= 5.5×106 g·mol-1) and Mw/Mn (=3.5). Also, these values are almost constant regardless of the clay concentration (Cclay=1-25×10-2 mol·1-1), which meant clay would not affect polymer chain growth, even though the mechanical properties and swelling behavior of the NC gel varied widely over this Cclay range. Comparisons of NC gels, PNIPA (without adding any crosslinker), and SiO2-NC gels with well dispersed nano-scale silica particles indicated that the clay platelets specifically play an important role in preventing the formation of self-crosslinked PNIPA networks and improving the mechanical properties of the NC gels.(3) A series of copolymer nanocomposite hydrogels were prepared by polymerization of different monomers, N-isopropylacrylamide(NIPA), N,N-dimethylacrylamide(DMAA) and N,N-diethylacrylamide(DEAA), which were combined with two components respectively. Different from other copolymer systems showing intermediate stress-strain behavior, the resulting copolymer nanocomposite hydrogels (NIPA/DMAA-NC gels) exhibited improved mechanical properties than homopolymer NC gels, NIPA-NC gel or DMAA-NC gel. The improvement was attributed to the unique interior structure of NIPA/DMAA-NC gels, in which two kinds of cross linkers, physical cross linkers and self cross linkers formed. This complex crosslinked structure also affected other properties, such as depressed swelling behaviors and increased glass transition temperature.