Research Doctorate - Doctor of Philosophy (PhD) / The adsorption characteristics of pH responsive tertiary amine methacrylate-based diblock copolymers have been investigated. The main focus of this work is on poly(2-(dimethylamino)ethyl methacrylate-b-poly(2-(diethylamino)ethyl methacrylate (PDMA-b-PDEA) adsorption to the silica/aqueous solution interface at pH 9. Differing degrees of polymerisation and quaternisation were investigated with some attention given to variation of the block hydrophobicity utilising poly(2-(dimethylamino)ethyl methacrylate-b-poly(2-(diisopropylamino)ethyl methacrylate (PDMA-b-PDPA). Principally, optical reflectometry (OR) and atomic force microscopy (AFM) have been employed to monitor the adsorption in terms of adsorbed mass and layer morphology. A variety of other techniques have been utilised to provide ancillary information, including quartz crystal microbalance, zeta potential, dynamic light scattering and contact angle measurements. The combined results have provided a comprehensive understanding of the adsorption characteristics for the copolymers studied. Micelles of the tertiary amine methacrylate-based copolymers adsorbed readily to silica from aqueous solution at pH 9. The adsorption isotherms were determined, exhibiting a high affinity Langmuirian shape where the CMC did not appear to impact on the adsorbed mass. The adsorption was rationalised by the interaction between the cationic PDMA corona of the micelles with the negatively charged substrate. The more hydrophobic PDEA core block increased the level of adsorption above that observed for the PDMA homopolymer. It was shown that the adsorbed layers were robust to rinsing with electrolyte at high pH, although reduction of the pH to 4 yielded significant desorption. The adsorbed layer morphology observed by in situ AFM exhibited distinct micellar structures. The combined adsorbed mass and AFM images showed a significantly higher surface aggregation number than the measured solution aggregation number, indicating a more complex adsorption process than simple direct micelle adsorption. The adsorption kinetics were studied to elucidate the adsorption mechanism and revealed complex dynamic processes. Particular focus was given to the adsorption of 0q PDMA93-b-PDEA24 where the impact of concentration was evident and three mechanistic regimes could be defined; below the CMC, just above the CMC and far above the CMC. Interestingly, the adsorption process just above the CMC indicates a surface aggregation mechanism, while well above the CMC, the adsorption proceeds via a process that includes both direct micelle and unimer adsorption. On longer timescales, the adsorption at higher concentrations revealed an additional induction period of micelle relaxation on the surface that allowed for further adsorption. Increasing the PDMA quaternisation was found to reduce post adsorption rearrangement and as result equilibrium was reached more quickly for the highly quaternised analogues. The response of the adsorbed PDMA-b-PDEA copolymer films to multiple changes in solution pH (9 and 4) was monitored. After the initial desorption of copolymer with rinsing at pH 9 and then at pH 4, the adsorbed mass of copolymer was found to be constant with multiple cycles of pH. The remaining robust adsorbed layers, then exhibited reversible uptake and release of water with multiple pH cycles as measured by QCM. This observation was readily rationalised by the observed changes in copolymer charge (and hence hydrophobicity) affecting the interaction of the copolymer chains with the surrounding solution. While these characteristics were found to be reversible with pH cycling it was found that the initial micelle structure of the adsorbed film was lost upon the first rinse to pH 4. Finally, the first low Tg micelle-micelle multilayers of up to six layers were constructed using alternating layers of cationic and anionic tertiary amine methacrylate-based copolymers at pH 9. The existence of true micellar structures within each layer was proven using in situ AFM imaging where the alternating layer characteristics were supported by measured force curves. The construction of the individual micelle layers was also monitored by OR, where clear evidence of layer build-up was shown. In addition, each layer was robust to rinsing with electrolyte at the adsorbing pH, although, the stability of the formed multilayer was found to be limited to six layers. Upon reduction of the pH, almost all the adsorbed material was instantaneously removed from the surface. The stimulus-responsive nature of such multilayer films augurs well for potential controlled uptake/release applications. These findings should greatly encourage a larger research focus on micelle-micelle multilayers.
| Identifer | oai:union.ndltd.org:ADTP/222131 |
| Date | January 2007 |
| Creators | Smith, Emelyn |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | Copyright 2007 Emelyn Smith |
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