Conjugated polymers in mesoporous hosts (2009)

The subject of this thesis is the synthesis and characterization of composite materials based on electroluminescent conjugated polymers in mesoporous hosts. These materials were studied with the goal of producing a structure in which the electrical properties on encapsulated conjugated polymer chains could be measured. Towards this goal, both the creation of thin film hosts with oriented and ordered mesopores and new methods for the incorporation of polymers into mesoporous hosts are described, along with characterization techniques for showing the polymer distribution on the nanometre scale. The preparation of the conjugated polymer poly(1,4-phenylene vinylene) (PPV) inside the 3.1 nm channels of a hexagonally ordered mesoporous silica material, MCM-41, is described. The MCM-41 surface was first derivatized with an organic base. Subsequent introduction of monomer dissolved in ethanol resulted in base-initiated polymerization in the pores of MCM-41. A pore size reduction of 0.3 nm was seen in the composite material by nitrogen physisorption. Electron-energy loss spectroscopy (EELS) showed that the composite had a distinct loss signal related to the π-electron system on the polymer. Energy-filtered transmission electron microscopy (EFTEM) with 200 keV electrons showed that the polymer was evenly distributed throughout the composite material through mapping of the π-electron losses near 6 eV. A polymer mass content of ~8 % indicated the presence of approx. 6 polymer chains in each pore. The photophysical properties of PPV inside the composite were found to be similar to bulk PPV. For the preparation of mesoporous thin films with channels oriented normally to the surface, three literature approaches were investigated: the self-assembly of mesoporous silica films with the SBA-2 structure, the thermal oxidation of FeO/SiO₂ films, and the anodic oxidation of aluminum substrates. The latter approach, carried out at low temperature, is shown to yield alumina films with the desired pore structure. Films with a pore size of 4 ± 1 nm are created with an applied potential o f 15 V in 1.2 M sulfuric acid (in 1:1 water:methanol) at -39 °C. EELS and EFTEM were applied to the analysis of composite materials created by the adsorption of a thin layer of poly[2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV) on the surface of porous alumina membrane with 60 nm pore diameter. The measurements were carried out with a 200 keV electron beam travelling parallel to the pores of the host membrane. The π-electron losses of the polymer could not be discerned in this geometry. Strong surface losses were present at 8, 13 and 18 eV. A long-range loss mode not associated with bulk or surface losses appeared at 7.0 eV, up to 30 nm from the pore surface. Both these loss modes interfered with the detection o f the π-electron losses and the polymer distribution could not be confirmed. The origin of the long-range loss mode was identified as the Cherenkov effect. EELS with 120 keV electrons shifted the peak energy of this loss mode to 8.3 eV, which indicated a dependence on the electron speed. Samples with different pore diameters but a fixed interpore spacing also showed shifts in the peak position. Theoretical modelling of the loss spectrum of a cylindrical pore suggested that these observations arise from the interaction of the generated Cherenkov radiation with the nearby pores in the membrane. This introduces the possibility of studying photonic nanostructures by EELS. Different methods for introducing the conjugated polymer into an oriented porous host are explored. The idea of creating surface-grafted conjugated polymers on silicon substrates through step polymerization is investigated; as proof of concept, a surface-grafted dimer is synthesized through the Wadsworth-Horner-Emmons reaction. It is further shown that simple centrifugation of a polymer solution, while allowing solvent evaporation, provides a sufficient driving force for polymer insertion into the host. This composite is investigated by electron microscopy. EELS and EFTEM analysis were also applied to ultramicrotomed thin sections of this material, with the electron beam perpendicular to the pore axis. The results showed that relativistic effects may also be important in this geometry, effectively masking the distribution of the π-electron losses associated with the polymer. Possible routes to the preparation of a light-emitting device (LED) based on porous alumina films are described. The use of the underlying aluminum substrate as electron-injecting electrode was investigated but devices prepared in this manner did not show electroluminescence. The formation o f porous alumina films on conductive substrates such as silicon, indium tin oxide and gold was also investigated and the encountered experimental difficulties are reported.