Doctor of Philosophy
Department of Chemistry
Thipmonta, Nuchalee, Thermally sensitive conducting hydrogels, Doctor of Philosophy thesis, Department of Chemistry, University of Wollongong, 2013. https://ro.uow.edu.au/theses/3961
This thesis aims to synthesize thermally sensitive electrically conducting hydrogels. Poly(N-isopropylacrylamide) (PNIPAM) is the most common thermally responsive hydrogel and exhibits a lower critical solution temperature (LCST) which can be reversibly switched from a hydrophilic, swollen state to a hydrophobic, collapsed state. For constructing thermally sensitive electrically conducting hydrogels, this project is limited to the use of single-walled carbon nanotubes (SWNTs) as these materials are especially good candidates from a variety of carbon nanotubes because of they have relatively low bending stiffness. PNIPAM hydrogel composites containing SWNTs are expected to be promising materials because they can combine two or more dominant properties from both materials to produce novel structures with new properties.
The investigation of hydrogel composite materials began with the encapsulation a SWNT Buckypaper with the thermally sensitive PNIPAM hydrogel. SWNT Buckypaper was fabricated using a vacuum-filtration method. The fabrication of PNIPAM hydrogel coating on SWNT Buckypaper was achieved using electrochemically induced free radical polymerization the based on the electrochemical reduction of persulfate anion. In addition, the encapsulation of gold coated Mylar was also investigated as a background electrode for comparison with Buckypaper. The PNIPAM hydrogel was formed within and/or adjacent to the electrode surfaces. It could be readily observed by eye and in more detail using scanning electron microscopy (SEM). In addition, the formation of PNIPAM on gold coated Mylar, RVC and Buckypaper electrodes was confirmed by cyclic voltammetry which was performed before and after polymerisation, the FTIR spectra of PNIPAM exhibit all of the bands expected for PNIPAM such as amide II, C=O, -CH2-CH3 and CONH2 and cyclic voltammograms performed after polymerisation in the presence of potassium ferricyanide comparing with bare electrodes. A PNIPAM hydrogel coated on gold coated Mylar underwent a transition when varying the temperature of the electrolyte in an aqueous solution containing sulfate with the LCST transition occurring between 10°C and 20°C. The effect of salt on the LCST of PNIPAM coated on gold Mylar was studied by UV-vis determining the change in absorbance and shows a decrease in LCST of the gel as a function of increasing salt (SO4 2-) concentration in the hydrogel. Importantly, electrochemically induced free radical polymerisation can be used to deposit smart coatings on electrode which regulate permeability of reactants based on temperature.
The second investigation was carried out with the use of SWNTs for the purpose of electronic conducting fillers within a thermally sensitive hydrogel. PNIPAM/SWNT hydrogel composites as shrinkable electronic conductors studied here were successfully synthesised with various SWNT loadings: 0, 0.05, 0.1, 0.5 and 1.0 wt% through free radical polymerisation. The SWNTs were homogeneously dispersed in the aqueous solution of PNIPAM using SDBS as a surfactant under sonication prior to polymerisation as evidenced by optical microscopy. The ability of hydrogel composites to swell in water decreased with further increasing SWNT loading. Thus, the swelling ratio of the composite gels depends mainly on the crosslinking density. For the temperature-response studied, the onset of the LCST was measured to be 30°:C. All hydrogels except 0.5 and 1.0 wt%, reached the equilibrium thickness at 40°C, with 0.5 and 1.0 wt% reaching equilibrium at 45°C. The extreme volume change of hydrogels with high SWNT loadings (0.5 and 1.0 wt%) are related to low equilibrium swelling ratios and equilibrium water contents due to SWNTs decreasing the extent of chemical crosslinking. In addition, the level of compression strength and breaking strain and compressive modulus increased as function increasing SWNT concentrations into the hydrogel matrix. The changes of the mechanical properties of the gels are observed corresponding to the large change in swelling ratio. However, the hydrogel containing low concentration of SWNT demonstrated no considerable differences in the mechanical properties, while 0.5 and 1.0 wt% SWNT can be significantly improved the mechanical properties. The electronic properties of SWNT/NIPAM thermally sensitive composites were quantified by EIS plots below and above the LCST. The composite hydrogels has been thermally cycled between two temperatures through the heating-cooling cycles. The resistance of hydrogels decreased as a function of increasing SWNT concentration. The extreme resistance changes of hydrogels with high SWNT loadings (0.5 and 1.0 wt%) are related to volume changes and low equilibrium swelling ratios and equilibrium water contents.
The final investigation was on the preparation hydrogel composites using Laponite XLG ([Mg5.34Li0.66Si8O20(OH)4]Na0.66) clay as physical crosslinker without the need of a chemical crosslinker by free radical polymerisation at room temperature. The FTIR spectra of PNIPAM exhibit all of the bands expected for PNIPAM such as amide II, C=O, -CH2-CH3 and CONH2 and for synthetic Laponite clay. The swelling ratio decreased with increasing clay loading. Clay concentration is the most important parameter of PNIPAM/clay gel for going from solid-like to liquid-like behaviour with these chemical compositions in this study as PNIPAM/clay hydrogels were observed that the extrudable testing of the gels strongly depends on clay content. Moreover, the elastic modulus significantly improved as a function of increasing clay contents as resulted from rheological oscillatory frequency sweeps, creep-recovery and shear yield experiment applying shear strain at constant shear rate tests. In addition, the elastic modulus of the hydrogel composites containing SWNTs significantly improved with increasing SWNT concentrations. The LCST transition-dynamic mechanical behaviour of the PNIPAM/clay and SWNT/PNIPAM/clay composite hydrogels show that elastic moduli rapidly increased around 35°C to 40°C in both composite gels. These approaches can be used to form thermally sensitive injectable hydrogels which are highly desirable in clinical applications.