Preparation And Characterization Of Magnetic And Thermosensitive Polymer Microcontainers

Environmentally responsive materials can be defined as the materials that undergo relatively large and abrupt,physical or chemical changes in response to small external changes in the environmental conditions.Environmentally responsive materials can provide a variety of potential applications in separation,catalysis,drug release, sensors,etc.Temperature-responsive polymer and magnetic nanomaterials are widely studied environmentally responsive materials.The change of temperature is not only relatively easy to control,but also easily applicable both in vitro and in vivo. Magnetic materials can be conveniently manipulated by the application of an external magnetic field.Polymer microcontainers have attracted great interest owing to their stable structure,inner void where a great deal of molecules or big molecules can be encapsulated.Based on the research background,this thesis has focused on the preparation of polymer microcontainers with thermosensitive and magnetic properties and the results of each part are listed as follows.1) A reliable and efficient route for preparing thermoresponsive microcontainers based on cross-linked poly(N-isopropylacrylamide)(PNIPAM) was developed.Firstly, monodisperse thermoresponsive core/shell microspheres composed of a P(styrene (St)-co-NIPAM) core and a cross-linked PNIPAM shell were prepared by seeded emulsion polymerization using P(St-co-NIPAM) particles as seeds.The size of the P(St-co-NIPAM) core can be conveniently tuned by different dosages of sodium dodecyl sulfate(SDS).As the amount of SDS increased from 0 to 0.20 g,the size of the microsphere measured by dynamic light scattering(DLS) decreased from 246 nm to 43 nm.The thickness of the cross-linked PNIPAM shell can be controlled by varying the dosage of NIPAM in the preparation of PNIPAM shell.When the crosslinking degree(defined as molar percentage ratio of the cross-linker(N, N'-methylenebisacrylamide,MBA) to NIPAM) was kept constant as 10%and the amount of NIPAM are 0.5,1.0 and 2.0 g,the thickness of PNIPAM shell measured at 25℃are 31,67 and 89 nm,respectively.The thermosensitivity of the core/shell microspheres relates to the crosslinking degree of the shell.With increasing the crosslinking degree from 5%to 10%and 15%,the swelling ratio defined as(D_20℃/D_45℃)³ decreases from 16.1 to 11.8 and 7.9.Then,PNIPAM microcontainers were obtained by simply dissolving the P(St-co-NIPAM) core with tetrahydrofuran(THF). The core/shell microspheres and the PNIPAM microcontainers were characterized by transmission electron microscopy,dynamic light scattering,atomic force microscopy, and Fourier-transform infrared spectroscopy.By removing the core,the PNIPAM shell was released from the constraint of the core and the PNIPAM microcontainers can shrink and swell over a wider range of volume.2) P(NIPAM-co-AA) microcontainers surface-anchored with magnetic nanoparticles were prepared using PSt/P(NIPAM-co-AA) core/shell microspheres and amino-modified Fe₃O₄@SiO₂ particles as building blocks.At first,the PSt/P(NIPAM-co-AA) core/shell microspheres were prepared by seeded emulsion polymerization,and the amino-modified Fe₃O₄@SiO₂ particles were prepared by a modified St(o|¨)ber method followed by functionalized by 3-aminopropyltriethoxysilane (APS).Then,the smaller amino-modified Fe₃O₄@SiO₂ particles were assembled onto the surface of PSt/P(NIPAM-co-AA) core/shell microspheres by electrostatic interaction.Subsequently,the two particles were permanently combined through amidation reaction between carboxylic groups and amino groups under the catalysis of 1-ehtyl-3-(3-dimethylaminopropyl)carbodiimide(EDC) and N-hydroxysuccinimide (NHS).The density of the absorbed Fe₃O₄@SiO₂ particles on the surface of the core/shell microspheres can be tuned by the weight ratio of the added two particles. With an increase in the amount of the added amino-modified Fe₃O₄@SiO₂ particles, more surface of the PSt/P(NIPAM-co-AA) core/shell microspheres is covered by magnetic particles.By dissolving the PSt core with THF,novel triple-functional microcontainers with superparamagnetism and pH- and temperature-sensitivity were obtained.3) Wormlike one-dimensional(1D) iron oxide/silica nanostructures were prepared through chainlike self-assembly of the magnetic nanoparticles followed by a sol-gel process of TEOS at the surface of the assemblies.Furthermore,1D magnetic PNIPAM microcontainers were prepared.Iron oxide nanoparticles with narrow size distribution were prepared by a high temperature hydrolysis reaction.The size of the magnetic nanoparticles can be controlled by the volume of the added NaOH/DEG solution.The XRD pattern and Raman spectrum confirm that the product is a mixture of Fe₃O₄ andγ-Fe₂O₃.The magnetization curve measured at 300 K and the temperature dependence of zero-field-cooled(ZFC) and field-cooled(FC) magnetization confirm the weak ferromagnetism of the prepared iron oxide nanoparticles at room temperature.The magnetic nanoparticles can self-assemble into 1D nanoparticle chains on substrates as well as in colloid dispersion through magnetic dipolar interaction without the help of an applied magnetic field.The 1D assemblies were further harnessed to prepare 1D nanostructures with aligned magnetic nanoparticle inclusions and a continuous silica shell.Wormlike 1D iron oxide/silica nanostructures were prepared through chainlike self-assembly of the magnetic nanoparticles in a mixture of ethanol,ammonia,and water followed by a sol-gel process of TEOS at the surface of the assemblies.The length of the nanostructures can be controlled by the amplitude of ultrasonication,and the thickness of the silica coating can be tuned by the dosage of TEOS.The 1D nanostructures exhibit high magnetic sensitivity.In the presence of an applied magnetic field,the nanostructures can be oriented and align along the direction of the external magnetic field. P(NIPAM-co-MPS) was prepared by solution polymerization of NIPAM and 3-(trimethoxysily)propyl methacrylate(MPS).Then,P(NIPAM-co-MPS) was grafted onto the 1D iron oxide/silica nanostructures.The 1D iron oxide/silica nanostructures surface-grafted with P(NIPAM-co-MPS) were encapsulated by crosslinked PNIPAM through the precipitation polymerization of NIPAM.By dissolving the SiO₂ with NaOH,1D magnetic PNIPAM microcontainers were obtained.