|Title:||Novel nanostructure engineering via self-assembly of amphiphilic hollow particles|
Hong Kong Polytechnic University -- Dissertations
|Department:||Department of Applied Biology and Chemical Technology|
|Pages:||xxxi, 204 leaves : ill. ; 30 cm.|
|Abstract:||The research work described in this thesis aims to the preparation of novel nanostructured materials starting from an inexpensive building block, amphiphilic hollow particles. These particles are composed of polyethyleneimine-g-poly(methyl methacrylate) (PEI-g-PMMA) copolymer which is derived from its corresponding PMMA/PEI core-shell particle. A wide range of interesting nanostructured materials such as nanotubes, nanofibers, urchin-like, snowflake-like hierarchical structures have been constructed by simply manipulating assembling conditions such as co-solvent composition, temperature and solution pH. Assembling mechanism for the formation of nanotubes from hollow particles of PEI-g-PMMA was first investigated at either 15 or 17℃ with fluid shear in a mixture of dichloromethane (DCM) and water. Effects of stirring rate and DCM to water volume ratio on the hollow particle assembly were systematically examined. Surface properties and morphology of the hollow particles as well as the resulting assemblies in both DCM and water were characterized by X-ray photoelectron spectroscopy and transmission electron microscopy. Results from these studies suggest four key features of this assembly process: 1) Morphology of the amphiphilic hollow particle is inversable in organic solvent and water. 2) The assembly process can only occur with appropriate fluid shear and DCM/water ratio. 3) The hollow particles can undergo deformation to ellipsoidal shape with fluid shear at 350 rpm in an appropriate DCM/water (e.g. 3:7 v/v) mixture. 4) The elongated hollow particles are able to assemble into linear aggregates via tip-to-tip connection, followed by coalescence and fusion to generate hollow nanotubes with diameters less than 150 nm. The lengths of the nanotubes can be extended to micron-scale, and they can be easily aligned via a simple dip-coating method. This simple and inexpensive assembly process using amphiphilic hollow particle as a building block is dramatically different from the well-known self-assembly of block copolymers into different nanostructures under equilibrium conditions.|
Assembling temperature has shown profound effect on the morphology of the hollow particles and resulting assemblies. Varying solution temperatures from 15 to 45℃ lead to the formation of different sizes of hollow particles ranging from 28 to 225 nm in diameter, respectively. The resultant hollow particles could be further converted into snowflake-shape under a rapid temperature quenching process. In the case of preformed nanotubes, increasing solution temperature can enhance the hydrophobic interaction between PMMA graft chains inside the nanotube, resulting in the transformation of nanotube to nanofiber with diameter ranging from 17 to 137 nm. Various novel hierarchical microstructures can also be fabricated using preformed nanotubes as building block through pH tuning. The formation of three-dimensional hierarchical structures in aqueous solution over a pH range from 5 to 13 are captured by field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) and atomic force microscopy (AFM). The hierarchical structures display distinct assembly profiles across three pH regimes. At low pH (pH =3.0), nanotube building blocks appear in random orientation and distribution. There is almost no hierarchical assembly under this condition. At pH between 6 and 8, the nanotube building blocks transform into straw-sheaflike bundle and fractal-like structures. At pH 9 which is close to isoelectric point of the nanotubular charge density as determined with a zeta-potential measurement, nanotubes align into parallel bundles and pack into highly packed columnar structures. When solution pH is above 10, the bundles (in straw-sheaf morphology) with loosely agglomerated fantail-shaped morphology are further assembled into plate-like morphology with interwoven networks. The in-depth understanding gained through this study can guide the design and manipulation of self-assembly of various amphiphilic hollow particles, thus generating diverse and intriguing nanostructured materials.
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