Thermophysical behaviour and applications of N- isopropylacrylamide-based ionogels
Gallagher, Simon (2014) Thermophysical behaviour and applications of N- isopropylacrylamide-based ionogels. PhD thesis, Dublin City University.
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This thesis is focused on ways to control the characteristics of a thermo-responsive polymer called poly(N-isopropylacrylamide) (pNIPAAM). The thermal behaviour of pNIPAAM displays an inverse solubility upon heating above what is known as its Lower Critical Solution Temperature (LCST). Experimentally, it converts from a hydrophilic to a hydrophobic structure when placed in aqueous media between the temperatures 30-35 °C.
This phenomenon places pNIPAAM as a potential candidate for use as a soft polymer actuator, and we are specifically interested in exploiting this behaviour for potential applications in microfluidic valves and drug delivery. However, there are some drawbacks associated with this thermo-responsive polymer. Generally, the polymerization medium used for the synthesis of pNIPAAM is an organic solvent. This is a limitation for pNIPAAM-based hydrogels used in open atmosphere and in a wide range of temperatures, as the liquid phase of the gel can be prone to freezing or evaporation due to its volatility. Its poor mechanical properties, especially in its swollen state, are detrimental to its role as a vessel for drug delivery. Because of its non-biodegradable nature, surgical removal after drug release may be required. If the device were too soft and easily broken during handling, it would be difficult to be completely removed through traditional surgical procedures. Also, weak mechanical properties are not desirable for microfluidic valves which can be exposed to harsh chemicals and high solvent pressures. The main focus of this thesis is to address these limitations and further optimize the pNIPAAM hydrogel for application.
The main theme of my work was the replacement of the common organic solvent with Ionic Liquids (ILs) as a polymerization medium for pNIPAAM, in order to create a series of ionogels. ILs are molten salts that consist of a cation and anion held together via weak electrostatic interactions, which reduce the lattice energy of the salt. This chemical environment contributes to factors such as negligible vapour pressure and general high thermal stability. The tailoring of the anion or cation can also lead to a variation of properties.
The first chapter of my work described the physicochemical properties of free- standing crosslinked pNIPAAM gels generated in the presence of varying ILs. The resulting ionogels were found to exhibit LCST values different to that of pNIPAAM and improved swelling/shrinking behaviour compared with that of the conventional pNIPAAM hydrogel. Most interestingly, the pNIPAAM shrinking behaviour was completely eliminated with use of dicyanamide as the IL anion.
The ionogel from this report with the most desirable shrinking/swelling behaviour was then studied in terms of drug release. This ionogel was found to have greater water uptake, and better mechanical stability that the conventional hydrogel. When pre-loaded with a dye acting as a drug mimic it was also found to have greater release capabilities.
Following this, varying concentrations of pNIPAAM were integrated into a cross- linked thermo-responsive Polymeric Ionic Liquid (PIL), producing an interpenetrating network (IPN). This novel material is characterized as function of the concentration of pNIPAAM within the gel. The IPN with the highest concentration of pNIPAAM was found to display the most favourable swelling/shrinking properties.
Lastly, progressions from this area also led to the synthesis of 1-vinylimidazolium alkyl sulphate polymeric ionic liquids (PILs). These PILs were synthesized using a halide-free method and were found to possess sufficient thermal stability of up to 340 °C and mechanical stability with a storage moduli value as high as 4.64 x 106 Pa.
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