Studies On Preparation Of Multiple Environmental Stimuli Responsive Graft Copolymers Based On Polysaccharide And Their Self-assembly In Water

In recent years, self-assembly of polymers in aqueous solutions driven by environmental stimuli has attracted great attention. Such self-assembled polymeric micelles are responsive to external stimuli, such as pH, temperature, light, electronic and magnetic fileds. Therefore, they are expected to be applicable for numerous purposes. Previous work in this field, however, has mostly focused on the single stimulus-responsive polymer systems, and there are only a few of reports dealt with multiple-stimuli ones, although the later have more advantages than the former. Multiple responsive polymers can not only offer more possibilities to construct polymer micelles with desirable structures and morphologies depending on the external stimuli, but also present more complicated self-assembly behavior than single responsive systems. Therefore, the study on multi-stimuli responsive systems would be helpful to understand more complex self-assembly processes in nature.In this thesis, we have synthesized a series of graft copolymers (CNipAa) having modified natural polysaccharide, hydroxyethyl cellulose ( HEC), as the main chain and two hydrophilic synthetic polymers, PNiPAAm and PAA as stimuli-sensitive grafts. Depending on the sensitivities of the grafts to temperature, pH value, and additive metal ion, a variety of intelligent polymer micelles can be achieved just by altering the environmental conditions. The resultant micellar structures can be fixed through chemical cross-linking of the PAA components. It is particularly worthy noting that a temperature-dependent transition between micelle and hollow-sphere can be observed if the component PAA of thermo-caused micelles was crosslinked at acidic conditions. In addition, we also devoted to know how the combined stimuli, such as, temperature-pH and temperature-metal ion Ni²⁺, influence the self-assembly and the resulting micellar structures of CNipAa in water. As a result, we discovered that, for the first time, the compound micelles of CNipAa/ Ni²⁺ form or not depending on the solution temperature when Ni²⁺ ion was added into CNipAa copolymer solutions.Four sections of our research work were carried out as follows:(1) Etherification of biodegradable polymer, hydroxyethyl cellulose (HEC), was conducted to get modified HEC (MHEC) with a higher solubility in water. MHEC initiated the graft polymerization of N-isopropyl acrylamide (NiPAAm) and then acrylic acid (AA) successively. HEC, modified HEC (MHEC), copolymers of HEC-g-PNiPAAm (CNip) and HEC-g-PNiPAAm&PAA (CNipAa), were all characterized by various methods such as FT-IR, ¹H-NMR, element analysis. DLS and SLS were used to get Mw, , of the graft copolymers, CNip and CNipAa.(2) Both pH and temperature changes can bring on micellization of CNipAa in aqueous solution, which was investigated in details. For the case of pH-induced micellization, as pH of the solutions decreases to below 4.6, micellar structure having insoluble complexes of PAA/PNiPAAm as the core and solvated HEC chains as the shell can be obtained due to hydrogen-bonding complexation between the proton-acceptors in PNiPAAm and the proton-donors in PAA. In this process, some factors such as temperature, graft copolymer composition, were found to influence the resultant micellar structure significantly. Typically, the micelles at pH 1.3 and 30℃have been proved to have solid and sphere morphology via TEM and AFM observations. For the case of thermo-caused micellization, as temperature is lifted to above the LCST (about 35.2℃), micellar structures having hydrophobic aggregates of PNiPAAm grafts as the core and solvated HEC and PAA chains as the shell can be achieved because of the hydrophobic interaction of PNiPAAm grafts. Typically, thermo-induced micelles at pH 12 and 45℃were found to have solid morphology by DLS/SLS measurements, though they look like hollow spheres in TEM observations. In addition, thermo-induced micelles from CNipAa copolymer at pH 12 and 45℃were found capable of making a remarkable response to pH changes, and then the resultant micellar structures were investigated via TEM and AFM.(3) Chemical crosslinking the PAA grafts of both pH-induced micelles (pH micelle) and acidified thermo-induced micelles (T-pH micelle) resulted in stable nano-particles. For the case of pH micelles, the cross-linking reaction caused a PAA network in the core, which prevented micelles from dissociation over a broad pH range. And the crosslinked pH micelles can respond further to both pH and temperature changes, but only temperature causes an apparent variation of the micelles. For the case of T-pH micelles, the crosslinked PAA chains in the shell keep micellar integrity even under the non-micellization conditions, and the fixed micelles were found capable of undergoing a temperature-dependent transition between micelle and hollow sphere morphologies.(4) There exists electrostatic interaction between CNipAa copolymer and metal ion Ni²⁺ in neutral water because the PAA grafts are negatively charged due to ionization of the carboxylic acid groups. However, soluble complexes instead of micellar structures were created when Ni²⁺ ions were added into the neutral aqueous solutions of CNipAa at 30℃. This is because the interaction is too weak to make the chains aggregate. It is interesting to find that the micellar structures can be achieved if such CNipAa copolymer solution containing Ni²⁺ undergoes a heating-cooling cycle between 30℃and 45℃. Obviously, the thermo-sensitive PNiPAAm chains favor the formation of the micelles. Once the micelles form, they even can keep their integrity even when the temperature is decreased to be lower than the LCST. In addition, the temperature-responsive behavior and morphology of Ni²⁺/CNipAa micelles were thoroughly studied by DLS/SLS, SEM and AFM.