PhD Thesis
Template-Directed Synthesis of One-Dimensional Hybrid Nanostructures from Cylindrical Polymer Brushes
Markus Müllner (05/2012-06/2012)
Support: Axel H. E. Müller
Summary
The thesis deals with the study of cylindrical polymer brushes (CPBs) and their use in template chemistry. Herein, we developed template-directed syntheses of one-dimensional (1D) hybrid nanostructures in which CPBs served as 1D soft templates. So called molecular core-shell or core-shell-corona CPBs, as well as CPBs obtained through microphase separation of diblock copolymers, were applied as templates for the preparation of various types of 1D organic-inorganic hybrid nanomaterials. Well-defined molecular core-shell and core-shell-corona CPBs with a narrow molecular weight distribution in both the backbone and the side chains have been synthesized via the combination of several polymerization techniques. Anionic polymerization has enabled the precise synthesis of polymer backbones, whereas atom transfer radical polymerization and ring-opening polymerization have allowed the sequential growth of side chains via the grafting-from approach. Depending upon the desired functional 1D hybrid nanostructures, different combinations of core, shell and corona have been chosen. Core-shell-corona CPBs with a poly(tert-butyl acrylate) (PtBA) core, a poly(3-acryloylpropyl trimethoxysilane) (PAPTS) shell and a poly[oligo (ethylene glycol) methyl ether methacrylate] (POEGMA) corona were hydrolyzed by aqueous ammonia to produce water-soluble organo-silica hybrid nanotubes. As the trimethoxysilyl group was directly incorporated into the structure of the CPBs, we avoided the addition of an external inorganic precursor. Amphiphilic core-shell-corona CPBs with a hydrophobic poly(ε-caprolactone) (PCL) core, a hydrophilic poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) shell and a hydrophilic POEGMA corona were used as 1D templates and nanoreactors for the fabrication of titania dioxide (TiO2) semiconductor nanotubes. The cationic PDMAEMA shell was loaded with a negatively charged titania precursor, namely titanium(IV) bis(ammonium lactate) dihydroxide (TALH). TALH underwent hydrolysis within the CPB shell upon heating above 70 °C. Accordingly, crystalline (that is, anatase) TiO2 hybrid nanotubes with a very well defined thickness were obtained. Subsequently, the diameter of the hybrid nanotubes was adjusted by modifying the side chain length of the PDMAEMA shell. Amphiphilic core-shell CPBs with a hydrophobic PCL core and a hydrophilic PDMAEMA shell were employed for the fabrication of silica nanotubes with different aspect ratios. Tetramethyl orthosilicate (TMOS) was used for the deposition of silica into the PDMAEMA shell. Several polymers with different dimensions, with respect to length as well as core and shell diameter, were synthesized and used as templates for the fabrication 1D silica nanostructures. Furthermore, silica nanotubes were obtained after acid treatment or calcination, and catalytically active porous nanomaterials were produced via the embedment of metal nanoparticles within the silica shell. All the hybrid nanotubes templated by molecular core-shell and core-shell-corona CPBs were uniform in length as well as diameter, due to the narrow molecular weight distribution of the CPB backbone and side chains. Moreover, the POEGMA corona in the core-shell-corona CPBs served as a shielding layer and protected the nanotubes from agglomeration and intermolecular crosslinking during the hybrid formation. Additionally, POEGMA rendered the hybrid nanomaterials soluble in various solvents, including water. Water-soluble core-shell CPBs were obtained from microphase separated cast films of polystyrene-b-poly(allyl methacrylate) (PS-PAMA) diblock copolymers. After UV-crosslinking of the cylindrical PAMA domains, CPBs with a PS shell and a crosslinked PAMA core were obtained. Subsequent sulfonation rendered the PS shell into poly(styrenesulfonic acid) and thereby made it water-soluble. The negatively charged brush templates were infiltrated with oppositely charged TiO2 nanocrystals to produce crystalline TiO2 hybrid nanowires. Further, it was possible to adjust the crystal structure of the nanostructures to either a rutile or anatase structure. The presented novel 1D hybrid nanomaterials based on CPBs have many potential applications due to their electronic, catalytic and semiconducting properties.