Synthesis, Characteristics And Modification Of Poly(N-alkylacrylamide)/Clay Nanocomposite Hydrogels

Polymer hydrogels are soft materials comprising three-dimensional polymer networks and a large amount of water filling the interstitial space of the network. Recently, wide attentions have been paid to a new type of polymer hydrogels, that is nanocomposite gels (NC gels), which made revolutionary improvement in mechanical properties of the gels. NC gels almost solved all the problems associated in the traditional chemically crosslinked hydrogel (OR gel), such as very poor mechanical properties, optical opacity at high concentration crosslink agent and low swelling degree and slow deswelling rate. Meanwhile, NC gels have been found to exhibit a number of novel functions related to the properties of their gel-air and gel-water interfaces, coil-to-globule transition, molecular orientation, interactions with cells, control of morphology, self-healing and multiple chemical affinities as well as outstanding mechanical, optical and swelling/deswelling properties. NC gels were usually made of nonionic poly(N-alkylacrylamide) and water-swellable clay minerals, which have a cation exchange capacity (CEC) of 104 mequiv./100 g-clay and sodium counter-ions. Gels are formed by the physical interaction (hydrogen bonding) between the two major components. NC gels should be classified into polyelectrolyte gels due to the existence of ionic clay platelets. But many research groups ignored the polyelectrolyte behaviors with the purpose for simplicity. And as a physically crosslinked nanocomposite hydrogel with a great number of novel characteristics and tunable functions, structural stability of the network is very important in application of the gels. In the present dissertation, importance is attached to the polyelectrolyte behaviors of the gel and stabilities of the network, which are studied in detailed as follows. Many conclusions were made providing important, interesting and new information for basic research of NC gels and their application.(1) Swelling behavior of nanocomposite hydrogels (NC gels) having organic polymer/inorganic clay network structures were systematically investigated, focusing on the role of exfoliated clay platelets with sodium counter-ions in the network and effects of various swelling conditions. NC gels in water exhibited characteristic swelling-deswelling behavior, i.e., initial large swelling, maximum swelling, and subsequent deswelling toward an equilibrium state, under conditions where the water was changed frequently. Effects of swelling conditions, such as the frequency of changing the water, amount of water per unit gram of gel, salt concentration and pH of the swelling solvent, gel composition (e.g., kind of polymer, clay concentration (Cclay), and polymer concentration (Cp)), swelling temperature and gel size, were clarified. NC gels with different polymers were all found to exhibit swelling-deswelling behavior except at very low Cclay and high Cp. Spontaneous deswelling of the gels was attributed to the combined effects of high swelling capability of the NC gel as a polyelectrolyte gel and continuous release of sodium ions from the network during swelling. Furthermore, the swelling-deswelling behavior could be reversed by re-introducing sodium ions into the network. These characteristic swelling behaviors of NC gels with polymer/clay networks are completely different from those of hydrogels with chemically crosslinked polymer network structures.(2) The effects of replacing the native Na+ counter ions associated with the clay platelets by various other cations on the swelling behavior of nanocomposite gels (NC gels) consisting of an organic (polymer)/inorganic (clay) network were investigated. The negative surface charge of the clay platelet conferred an ionic nature on the NC gels making them a type of polyelectrolyte gel; consequently, the swelling behavior of the NC gels was strongly influenced by the valence of the co-existing counter ions. NC gels containing monovalent cations such as Na+, K+ and Li+ exhibited large swellings and subsequent deswelling in water after attaining maximum degrees of swelling, the extent of which was dependent on the cation concentration. In contrast, introduction of multivalent cations such as Ca24, Mg2+, and Al3+ into NC gels depressed the swelling behavior markedly; the extent of swelling was dependent on the valency and concentration of the cation, and subsequent deswelling was suppressed by incorporating these multivalent cations. The decreased swelling and suppressed deswelling with multivalent ions result from the formation of additional crosslinks through ionic interactions between the clay platelets and the multivalent cations. It was also found that the degree of swelling in the initial gel state prior to introducing the multivalent cation strongly affected the subsequent swelling behavior, and that similar swelling behavior was observed on introducting multivalent ions, independent of the concentration of clay in the NC gel. Further, reversible absorption/desorption and selective absorption of multivalent cations was observed for the NC gels examined.(3) Nanocomposite gels (NC gels) have a unique organic (polymer)/inorganic (clay) network structure which causes them to exhibit excellent mechanical and swelling properties. It was found that dried and re-swollen NC gels showed higher tensile moduli and strengths and low elongations at break as well as lower swelling when compared to those of NC gels as-prepared. The modification in properties brought about by drying was observed in all NC gels, regardless of the type of polymer used and the clay concentration. These changes were attributed to the irreversible rearrangement of the polymer/clay network structure in which additional crosslinks are formed due to additional contacts between polymer chains and clay and the strengthening of pre-formed loose contacts in the concentrated state. The rearranged network structure remained almost unchanged on repeated drying and heating in the dried state. In contrast, conventional chemically-crosslinked hydrogels showed no changes in properties in response to drying treatment.(4) The effects of replacing water solvent of nanocomposite gels with alcoholic solvent are investigated on structure stability, swelling and mechanical properties of nanocomposite hydrogels made of poly(N,N-dimethylacrylamide)/clay network (D-NC gels). D-NC hydrogels maintained the three-dimensional structure without destroying when exchanging water solvents with alcoholic solvents. It was found that NC gel that originally containing water exhibit unique exchanging and swelling behaviors depending on the type of alcohol in which the gels are immersed. D-NC gel swells monotonously in methanol as in the case of water, but shows two-step swelling behavior (first shrinking and then swelling) in other alcohols such as ethanol, propanol, and butanol. The shrinking of gel was not caused in a proportional way with gels deforming to a concave shape. Even after reswelling, gel remained slight concave in shape in cases of alcohol with high carbon numbers (e.g.,1-propanol and 1-butanol). The degree of equilibrium swelling of gel in alcoholic solvents decreases greatly with the increase of the carbon number of the solvents. Gels remain high optical transmittance in methanol, but show great decrease in transmittance in other alcoholic solvents. Furthermore, mechanical properties of the gels are highly improved when exchanging water to alcohols. In the case of methanol, tensile strength of the gel increased by 67% (from 155 kPa to 259 kPa), companied by 49% increase in modulus (from 7.5 kPa to 11.2 kPa). The low swelling and improved mechanical properties by alcoholic solvents exchanged results from the reason that alcohols are not the better solvents for the polymer/clay network than water. Effects of experimental factors, i.e., species of alcohols, clay content, reversibility and stability of the network structures were clarified.