Stimulus-Responsive Polymeric Particles Supramolecularly Assembled from Poly(N-isopropylacrylamide) Graft Copolymers: From Micelles to Crosslinked Nanogels
| Autor: | Yuan Hung Hsu, 許源宏 |
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| Rok vydání: | 2006 |
| Druh dokumentu: | 學位論文 ; thesis |
| Popis: | 94 In this thesis, graft copolymers comprising poly(acrylic acid) (PAAc) as the backbone and poly(N-isopropylacrylamide) (PNIPAAm) and monomethoxy poly(ethylene glycol) (mPEG) as the grafts were prepared and characterized. These copolymers with the AAc residues in the negatively charged state underwent self-assembling into large polymeric aggregates in water in the range of 32~35 oC due to the formation of the multi-core structure (comprising several solid-like hydrophobic PNIPAAm cores surrounded by the liquid-like interfacial layers) and the inter-core PEG connections. These rather labile inter-core PEG connections were partially destroyed due to the continual dehydration and solidification of the liquid-like interfacial layers with increasing temperature, thereby leading to fragmentation of the original aggregates into smaller particles with the more distinct core/shell structure. The final particle size was virtually governed by the structure of the hydrophobic PNIPAAm microdomains in response to the effects of the PNIPAAm concentration and temperature on the inter-core PEG connections. The effect of SDS on the hydrodynamic size and structure of these thermally induced polymeric micelles and/or particles was also investigated. Fluorescence study of the polymer solutions in H2O at the ambient temperature using pyrene as a probe indicates that the SDS micelles were induced by the presence of the graft copolymer at its concentration significantly lower than its CMC. Further fluorescence study showed that the SDS micelles occurred by its extensive binding with PNIPAAm grafts. Following the temperature increases of the polymer solutions to 60 oC by the fast heating approach, the copolymer underwent phase transition and, in the presence of polymer-bound SDS micelles, the thermally induced polymeric micelles showed smaller in their size and more hydrophobic in the interior cores, as compared to those in the absence of polymer-induced SDS micelles. It was found that the particle size was reduced by reducing the interactions between PNIPAAm and PEG grafts due to the presence of polymer-bound SDS micelles at low temperatures. After formation of polymeric micelles with increasing temperature, the polymer-bound SDS micelles were destroyed and the dissociated SDS molecules migrated to the core/shell interface within micelles. The stabilization of SDS at the interface and reduction in interactions of PEG with PNIPAAm grafts within micelles rendered the core regions of micelles more hydrophobic. The proposed hypothesis in the effect of polymer-bound micelles on polymeric particles was further confirmed by showing the changes in the particle size in response to the change in the CACSDS. pH/Temperature responsive crosslinked nanogels were prepared based on vinyl group-bearing PNIPAAm graft copolymers, poly(AAc-co-MEA)/PNIPAAm/mPEG. The structure of the graft copolymers was established by 1H NMR spectroscopy. The graft copolymers underwent self-aggregation into micelle structure with increasing temperature above their critical micelle temperatures (CMTs) in aqueous phase. Strong dependence of the micelle size and structure on pH was observed. Evaluation of the micelle structure by the variable temperature 1H NMR measurements in D2O shows the presences of both the liquid-like interfacial layers and solid-like inner cores in the hydrophobic PNIPAAm microdomains. Nanogels were obtained by radical polymerization of the MEA moieties at 60 oC and pH 5.0 under stirring. The nanogels exhibited capability of undergoing volume changes in response to changes in environmental pH and temperature. |
| Databáze: | Networked Digital Library of Theses & Dissertations |
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