Synthesis And Characterization Of Environment-Responsive Polymeric Hollow Materials

Environment-responsive polymeric hollow materials with the special hollow structures and the unique responsive characters, provide them the remarkably controllable and responsive functional features, and have variety of potential applications. According to the preparation strategies, the environment-responsive polymeric hollow materials could be divided into two species. One is the polymeric vesicles prepared from the self-assembly of block or graft copolymers; the other is the polymeric hollow spheres prepared from the polymerization of monomer or the assembly of current polymers. Various techniques have been successfully developed and utilized to prepare environment-responsive hollow materials. However, these techniques have some drawbacks which could limit the synthetic efficiency and their applications.The polymeric vesicles are usually prepared through the self-assembly of block or graft copolymers which are usually synthesized by"living"polymerization techniques. However, the procedures of"living"polymerization techniques are technically complicated and with high cost. Herein, a new strategy based on supramolecular strategy was proposed. The functional polymers obtained from conventional radical polymerization were connected through supramolecular interactions of the functional groups (end groups or side groups) on these polymers to form supramolecular copolymer systems. Environment-responsive vesicles were prepared through the self-assembly of these supramolecular copolymer systems. Conventional template strategy is usually applied to prepare polymeric hollow spheres. However, there are a number of shortcomings associated with this synthetic strategy. Herein, a new template with reversible phase transition capability was used to prepare the environment-responsive hollow spheres. In this strategy, as the reversible phase transition capability, the template could be deformed spontaneously, and no etching agents were needed. This strategy provided a one-pot and green procedure for the preparation of environment-responsive hollow spheres.Based on the proposed strategies, the following results were obtained.1. Polystyrene with carboxylic end groups (PSt-COOH) and poly(N-isopropyl acrylamide) with amino end groups (PNIPAM-NH2) were prepared by the chain transfer reaction in free radical polymerization. These two polymer chains could be connected through the ionic interaction of the amino groups and carboxylic groups to form amphiphilic supramolecular block copolymers (SBP) which could further self-assemble to form various vesicles in the selected solvents systems. In the dioxane/H2O system, spherical vesicles were obtained, and the size of these vesicles could be controlled by modulating the SBP concentration. In the dioxane/ethanol system, the morphologies of the formed aggregates were dependent on the SBP concentration. As the SBP concentration increased from 0.01 to 0.075 g mL-1, the aggregates changed from tubelike to spherical vesicles. A temperature-induced morphological transition was observed in the dioxane/ethanol system. The tubelike vesicles could be transformed to form monodisperse spherical vesicles by increasing the temperature of the solution from 30 to 60°C. Based on the strategy, poly(styrene-co-9-vinylcarbazole) (PSVC-COOH) with carboxylic end groups and PNIPAM-NH2 were introduced to prepare fluorescence labeled vesicles. This new strategy provided a general procedure for the preparation of polymeric aggregates.2. Poly(N-vinylpyrrolidone) with carboxylic end groups (PNVP-COOH) and amino end groups (PNVP-NH2) were prepared by the chain transfer reaction in free radical polymerization. Through the ionic interactions between the carboxylic end groups of PNVP-COOH (or amino end groups of PNVP-NH2) and the pyridine groups of poly(4-vinylpyridine) (PVPy) (or benzoic groups of poly(4-acrylamidobenzoic acid) (PABA)), PNVP-COOH (or PNVP-NH2) could be grafted onto PVPy (or PABA) to form PVPy/PNVP-COOH (or PABA/PNVP-NH2) supramolecular graft copolymers (SGP). The two SGP systems could self-assemble to form two pH-responsive vesicles with different pH-responsive ranges. The size of the vesicles prepared from the two SGP systems could be readily controlled through modulation of the mass ratio of PVPy and PNVP-COOH (PABA and PNVP-NH2). The two vesicles could well respond to the change of pH values. Drug-release studies showed that release rates of the loaded drug (sunset yellow) in the two vesicles could be well controlled by pH of the releasing solution. This strategy provided a new route for the preparation of other responsive polymeric vesicles.3. Based on the precipitation polymerization of N-isopropyl acrylamide (NIPAM), PNIPAM hollow spheres were synthesized with the PNIPAM nuclei formed in the nucleation stage as non-crosslinked templates with reversible phase transition capability. TEM, SEM and freeze fractured TEM have confirmed the hollow structures of the obtained PNIPAM spheres. The size of the hollow spheres could be easily controlled ranging from the submicrometer-scale to the micometer-scale through modulating the surfactant content and the electrolyte concentration of the synthetic solution. The PNIPAM hollow spheres showed remarkable thermo-responsive behavior. This strategy provided a general procedure for the preparation of environment-responsive hollow spheres.4. Based on the synthetic strategy mentioned above, at the nucleus-growing stage, pH sensitive monomers, N-(2-(dimethylamino)ethyl)acrylamide (DAEA) and acrylic acid (AA), were added into the solution accompanied with the cross-linker to finally form P(NIPAM-co-DAEA) and P(NIPAM-co-AA) hollow spheres after the spontaneously deformation of PNIPAM nuclei via reverse phase transition, respectively. TEM analysis confirmed the hollow structures of the two polymeric spheres. The result showed that, the two hollow spheres could well respond to the change of pH and temperatures. This strategy provided a general route for the preparation of multi-responsive hollow spheres.