Synthesis, Characterization, And Application Of Environmentally Sensitive Polymeric-materials

Environmentally sensitive polymers, also called smart polymers, represent an important and novel family of functional polymeric materials. Such functional polymers can fast respond to the external stimuli such as changes in temperature, pH, and ion strength, etc., resulting in the dramatical changes in their structures, physical and chemical properties. Therefore, these polymers have potential applications in controlled drug release, tissue engineering, adsorbents, and chemical and biosensors, etc. However, there are still many unresolved issues for the preparation, characterization and applications of the environmentally sensitive polymeric materials, including complicated synthetic processes and requirement for special equipments, difficulty in the purification and cleaning, poor microstructures and physical properties, and limitations of characterization techniques, which may hinder and limit the potential applications of these functional polymeric materials. On the other hand, the chemical sensors based on environmentally sensitive polymer coatings and quartz crystal microbalance (QCM) technique are under the preliminary investigation stage at present time, and it is neceassry to develop novel environmentally sensitive polymeric materials as QCM sensor coatings in the future research. Herein, a series of environmentally sensitive polymeric materials including thermo-sensitive and heavy metal ion sensitive polymers were synthesized by free radical polymerization. The structures and properties, especially the response behavior to the external stimuli, of the obtained polymers were systematically characterized. Their relevant applications like the detection of trace heavy metal ion in water were also investigated and discussed.Poly(N-isopropylacrylamide) (PNIPAm) is an important thermo-sensitvie polymer. Raising the temperature above the lower critical solution temperature (LCST), PNIPAm chains in solution will shrink and form the collapsed globules. And reducing the temperature below the LCST, PNIPAm chains in solution will redissolve into the water. Starting with PNIPAm, we firstly developed a facile method for fabricating thermo-sensitive organic/inorganic hybrid hydrogel thin films from a cross-linkable organic/inorganic hydrid copolymer, poly [N-isopropylacrylamide-co-3-(trimethoxysilyl)propylmethacrylate] [P(NIPAm-co- TMSPMA)]. Fourier transform infrared (FT-IR) spectra confirmed the formation of hybrid hydrogel thin films after hydrolysis of the methoxysilyl groups (Si-O-CH3) and subsequent condensation of the silanol groups (Si-OH). Atomic force microscopy (AFM) images revealed that the surface morphology of the hydrogel thin films depended on the supporting substrates. Microdomains were observed for the hydrogel thin films on gold surface, which can be attributed to inhomogeneous network structures. The thermo-responsive swelling-deswelling behavior and the viscoelastic properties of the hydrogel thin films were investigated as a function of temperature (25-45℃) by using QCM operated in water. The experimental results also indicated that the properties of organic/inorganic hybrid hydrogel thin films were strongly dependent on the content of the comonomer TMSPMA.Similarly, thermo-sensitive organic/inorganic hybrid poly[N-isopropylacrylamide-co-3-(trimethoxysilyl)propylmethacrylate] [P(NIPAm-co-TMSPMA)] microgels were successfully prepared via two different methods without addition of any surfactant. In the first method, the microgels were obtained by a two-step method, i.e. the linear copolymer P(NIPAm-co-TMSPMA) was first synthesized by free-radical copolymerization and the aqueous solution of the copolymer was then heated above its LCST to give colloid particles, which were subsequently cross-linked via the hydrolysis and condensation of the methoxysilyl groups to form the microgels. In the second method, the microgels were prepared via conventional surfactant-free emulsion polymerization (SFEP) of the monomers NIPAm and TMSPMA. TMSPMA can act as the cross-linkable monomer. Since no surfactant was involved in the preparation of the hybrid microgels, more clean microgel dispersions were obtained. The prepared microgels were rather spherical and exhibited reversible thermo-sensitive behavior. The size, morphology, swellability, and phase transition behavior of the microgels were dependent on the initial copolymer or monomer concentration, preparation temperature, and the content of TMSPMA. The sizes of microgels obtained by SFEP were found to be more uniform than those by the two-step method. The hybrid microgels obtained by these two methods had more homogeneous microstructures than those prepared via conventional emulsion polymerization with N,N'-methylene-bisacrylamide as chemical cross-linker. Thermo-sensitive P(NIPAm-co-AAm-co-TMSPMA) microgels were successfully fabricated from cross-linkable linear terpolymer poly [N-isopropylacrylamide-co-acrylamide-co-3-(trimethoxysilyl) propyl methacrylate] by incorporating of acrylamide (AAm). Such microgels exhibited a wider size distribution as compared to P(NIPAm-co-TMSPMA) microgels from the linear copolymers. The presence of AAm shifts the phase transition temperature of terpolymer microgels to the higher temperature The hydrodynamic diameter and stability of the P(NIPAm-co-AAm-co-TMSPMA) microgels strongly depend on the pH value, salt concentration and temperature of the microgel aqueous solution.The thermo-sensitive polymer, PNIPAm, was further used to prepare hollow silica microspheres. By utilizing the thermo-sensitive character of PNIPAm, the collapsed globules of PNIPAm chains can behave as templates for the formation of core-shell silica microspheres at elevated temperature (>LCST) and will then diffuse out of the cores at lower temperature (< LCST), leading to the formation of hollow silica nanospheres. The TEM, SEM, XRD, and N2 adsorption-desorption results indicated that the shell of such hollow silica nanospheres also contain large amount of irregular mesopores. This new strategy was also tested with another thermo-sensitive polymer, poly(vinyl methyl ether) (PVME). However, only solid silica nanospheres with broad size distribution were obtained. We speculated that the formation of hollow silica nanosphere with PNIPAm templates was due to the possible interaction between the N atoms in PNIPAm and Si(OEt)4. The effects of initial concentration of PNIPAm, molecular weight of PNIPAm, and the prehydrolysis of silica precursor on the morphology and size of the resultant hollow silica nanospheres were also investigated. The PNIPAm soft templates were confirmed to be recyclable.Finally, a novel copolymer P(MBTVBC-co-VIM) sensitive to heavy metal ions was successfully synthesized by free radical copolymerization, and this novel copolymer can be used as the QCM sensor coating for detection of the heavy metal ions in aqueous solution. The copolymer P(MBTVBC-co-VIM) contains many imidazole rings, nitrogen (N) and sulfur (S) atoms in the side groups as electron donors, which can easily form complexes with heavy metal ions. The strong interaction between the S atom and Au electrode of quartz crystal further assures the stability of copolymer thin films on the quartz crystal surface in aqueous media. The QCM results indicated that the P(MBTVBC-co-VIM)-coated sensor exhibited high sensitivity, stability and selectivity for the detection of Cu2+ in aqueous solution. The lowest detection limit can be 10 ppm Cu2+ in aqueous solution, which resulted in the frequency shift of 3.0 Hz (ΔF3/3). The P(MBTVBC-co-VIM)-coated QCM sensors had porous surface morphologies as revealed by AFM investigation. Such porous structures enhanced the surface areas of the copolymer thin films, which increased the contacting probability of the imidazole groups, N and S atoms with heavy metal ions in solution and improved the detection sensitivity of the copolymer-coated QCM sensors.