Preparation, Properties And Applications Of Thermo-sensitive Ionic Microgels

Ionic liquids (IL), which are organic salts having low melting temperature. The most notable feature of ILs is the strong influence of the nature of the counter anion on their physical or chemical properties. By ion exchange reaction, the solubility, hydrophilicity & hydrophobicity, glass transition temperature, or thermal stability of IL will be changed. Microgels are three-dimensional cross-linked polymeric colloidal particles. By properly designing and choosing suitable monomers and comonomers, the resultant microgels could respond chemically or physically to the changes in external environment, e.g. temperature, pH, ionic strength and solvency, etc. Because of the unique properties of microgels, numerous investigations have been focused on the exploration of their potential applications as controlled-release systems, catalysis, separation technology, nanoreators, enzyme immobilization biosensors, and surface coatings, etc. Various multi-functional monomers such as N,N’-methylene bis acrylamide (BIS), divinylbenzene (DVB), and ethylene glycol dimethacrylate (EGDMA) were used as cross-linkers for the syntheses of thermo-sensitive microgels. Among the responsive microgels reported in literature, the microgels, which have ionic liquid moieties in the cross-linking network, are scarcely explored. Such ionic microgels may possess the characteristics of PILs besides the general properties of microgels. The synergetic properties of PIL and microgels might render them new functionalities and hence broaden their application potentials. However, there were only a few reports about the preparation of gels or microgels involving ILs or PILs. Moreover, the microgels involving ILs reported in the literature had problems of large size and broad size distribution.In this dissertation, a new type of thermo-sensitive ionic microgels with a narrow size distribution and unique feature of PIL was obtained. The microgels could be functionalized by anion exchange reaction, designation of novel cross-linking structure, or further quaternization. Such microgels were suitable for the treatment of anionic dye, control release system, degradable templates, and metal ions detection.1. One-pot or two-step methods for the fabrication of thermo-sensitive ionic microgels(1) The thermo-sensitive ionic microgels were successfully prepared via the simultaneous quaternized cross-linking reaction during the surfactant free emulsion copolymerization of N-isopropylacrylamide (NIPAm) as main monomer and 1-vinylimidazole (VIM) or 4-vinylpyridine (4VP) as comonomer.1,4-dibromobutane and 1,6-dibromohexane were used as the halogenated compounds to quaternize the tertiary amine in the comonomer, leading to the formation of cross-linking network and thermo-sensitive ionic microgels. The obtained ionic microgels were spherical in shape with narrow size distribution. These ionic microgels exhibited thermo-sensitive behavior and unique feature of IL in aqueous solutions, of which the counter anions could be changed by anion exchange reaction with BF4K or lithium trifluoromethyl sulfonate (PFM-Li). After anion exchange reaction, the ionic microgels were stable in aqueous solution and could be well dispersed in the solvents with different polarities, depending on the type of counter anion. The sizes and thermo-sensitive behavior of the ionic microgels could be well tuned by controlling the quaternization extent, the type of comonomer, halogenated compounds and counter anions. Furthermore, these ionic microgels also showed capabilities to encapsulate and release the anionic dyes, like methyl orange, in aqueous solutions.(2) A post quaternized cross-linking strategy was reported to fabricate thermo-sensitive ionic microgels from thermo-sensitive linear copolymer poly(N-isopropylacrylamide-co-1-vinylimidazole) [P(NIPAm-co-VIM)] or poly(N-isopropylacrylamide-co-4-vinylpyridine) [P(NIPAm-co-4VP)] in aqueous solution above its lower critical solution temperature (LCST).1,4-dibromobutane,1,5-dibromopentane, or 1,6-dibromhexane were chosen as the quaternization cross-linker to quaternize the imidazole or pyridine moieties, leading to the formation of cross-linking and ionic network. The size, swelling property, and uniformity of microgels could be tuned by selecting proper quaternization cross-linking reaction temperature and cross-linkers. The obtained ionic microgels showed the capability to adsorb the heavy metal salts like K2Cr2O7 in aqueous solution via the anion exchange reactions. It was also demonstrated that other functional groups could be introduced into the microgel networks if the unquaternized moieties were available.2. Degradable thermo-sensitive ionic microgelsDegradable thermo-sensitive ionic microgels were synthesized via surfactant-free emulsion polymerization of NIPAm and VIM at 70℃ with degradable 1,4-phenylene bis (4-bromobutanoate) or 1,6-hexanediol bis (2-bromopropionate) as quaternized cross-linkers.1-vinylimidazole could be quaternized by 1,4-phenylene bis (4-bromobutanoate) or 1,6-hexanediol bis (2-bromopropionate), leading to the formation of degradable cross-linking network and ionic microgels. The obtained microgels were spherical in shape with narrow size distribution and exhibited thermo-sensitive behavior and controllable degradation. The disintegration of the microgels was confirmed to be resulted from the hydrolysis of ester bonds of the cross-linkers. The degradation rate of the obtained microgels could be regulated by tuning the pH value of microgel suspensions. The PNI-Ph series of microgels fabricated with 1,4-phenylene bis (4-bromobutanoate) as the cross-linker could gradually degrade even in neutral solution with lifetimes ranging from 44 to 53 h, depending on the quaternization ratio. The degradation of PNI-Ph series of microgels experienced two reaction processes, that is, the hydrolysis of ester bonds of the cross-linkers and the oxidation of generated hydroquinone to form benzoquinone. It was also demonstrated that different silica nanostructures could be fabricated by using such degradable thermo-sensitive ionic microgels as the template at various temperatures.3. Thermo-sensitive ionic microgels for detection of trace heavy metal ions in aqueous solution4-(2-Pyridylazo)-resorcinol (PAR) functionalized thermo-sensitive ionic microgels were synthesized by one-pot quaternization method. The microgels were spherical in shape with radius of ca.166.0 nm and narrow size distribution and exhibited thermo-sensitivity in aqueous solution. Such microgels could optically detect trace heavy metal ions, such as Cu2+, Mn2+, Pb2+, Zn2+, and Ni2+, in aqueous solutions with high selectivity and sensitivity. The microgel suspensions exhibited characteristic color with the presence of various trace heavy metal ions, which could be visually distinguished by naked eye. The limit of colorimetric detection (DL) was determined to be 38 nM for Cu2+ at pH= 3,12 nM for Cu2+at pH=7, and 14 nM,79 nM,20 nM, and 21 nM for Mn2+, Pb2+, Zn2+, and Ni2+ at pH=11, respectively, which was lower than (or close to) the U. S. EPA standard for the safety limit of these heavy metal ions in drinking water.