PORTFOLIO

Semiconductive and light emitting polymers attract great attention owing to their tuneable physical properties, low cost and ease of manufacturing. They constitute a growing and important slice of semiconductive materials used in every day electronic devices such as computers, mobile phones, integrated circuits. Their use has been explored also in highly demanding applications, such as photovoltaic cells, polymer light emitting devices (PLED) and lasers, but their efficiency compared to inorganic materials has seriously limited their exploitation in practical applications at large scale. Impediments to high efficiency in both light and charge transport materials when using polymer-based systems are mostly to be attributed to the presence of defects. Efficiency can basically be controlled by affecting the chemical structure and the spatial and structural organization of polymers in the solid state. While efforts have been done in the first direction, little success has been obtained for the improvement of their physical properties. In the present approach we propose a novel and different route by which these polymers can be structured in well ordered long-range structures organized in 1, 2 or 3 dimensions of periods varying between tens and hundreds of nanometers. This should have positive consequences in the final physical properties of the devices. To do so we use copolymers, i.e. heterogeneous polymers where electronic properties are provided by one segment (block) of the polymer and structural properties are secured by the presence of another structural insulating block. This route offers the advantages of achieving high efficiency and controllable optoelectronic properties without synthetic restrictions, and thus, we anticipate novel possibilities for the design of fully organic optoelectronic devices for every day applications.

The present project, which aims at demonstrating the feasibility of use of new technologies to design optoelectronic devices with improved performances, is based on a synergistic approach combining fields as different as material science, polymer physics, photonics and electronics. In order to acchieve the goal to design new materials for advanced electronic technologies based on structured plastics, it necessitates remarkable advancements in both chemical and physical aspects. The aim is to bring a substantial impact to the technology presently used to design electronic sensors and digital devices.

The first system studied is a blend of two di-block copolymers, one is electron acceptor and the other is electron donor; they are respectively: Polystyrene-Polyvinylpyridine (PS-PVP), and Polystyrene-Polyphenylene vinylene (PS-PPV). To properly blend both copolymers without phase separating them is delicate, as TEM pictures show. A multilayer morphology […PS-PVP-PS-PPV-PS…], which is totally suitable for photovoltaic applications has been observed.

The second system studied is based on an original di-block copolymer where the first block is an electron donor: Polythienylenevinylene (PTV, 3’000 Da); the second block is made of polystyrene (PS, 40’000 Da). Fullerene (C60) can be grafted onto the polystyrene block, which turns the block into an electron acceptor. The result is an electron donor-acceptor di-block copolymer suitable for photovoltaic applications. SAXS & WAXS and TEM carried out on material made of the di-block copolymers before C60 grafting show a morphology based on a bilayer of PTV rods, to form 13 nm thick platelets, dispersed in a PS matrix; which is consistent with the fact that the proportion of PS in the di-block is extremely large. Grafting C60 onto the polystyrene block, where 25% of the styrene is grafted, leads to the destruction of the platelet structure. Which can be expected since the proportion of PTV rods is one order of magnitude smaller than the proportion of PS-gC60. To balance the proportion of PTV in the material, PTV homopolymer is added. For C60 grafted subject plus homopolymer, no clear structure is present; however, a beginning of PTV aggregation seems to occur on the TEM pictures.

The third investigated system is the bulk self-assembly of a series of hybrid triblock copolymers formed by a poly(9,9-dihexylfluorene-2,7-diyl) (PHF) internal block and two poly( -benzyl-L-glutamate) (PBLG) external blocks. Since the -helical secondary structure of the PBLG block may be either maintained or suppressed depending on the solvent casting history, the copolymers PBLG-PHF-PBLG exhibit two different conformations: a rod-rod-rod or coil-rod-coil configuration, respectively. In order to provide insight on molecular architecture influence on self-aggregation of these systems, three copolymers with different block ratio are investigated in both conformations using small and wide angle scattering techniques completed with transmission electron microscopy. Time resolve photoluminescence measurements have been performed and correlated with the morphology.

The final part of the study consisted in the study of home-synthesized sPS-PMMa block copolymers as possible candidates to replace Nafion(TM) in commercial fuel cells. A detailed study, involving proton conductivity, neutron scattering and transmission electron microscopy was carried out. Our results clearly show that by an accurate design of the sPS-PMMA molecular architecture, the proton conductivity of these materials can be finely tuned. This part of the study has made the object of a publication, recently accepted in Macromolecules.

J. Ruokolainen, R. Mezzenga, G.H. Fredrickson, E.J. Kramer, P.D. Hustad, G.W. Coates "Morphology and thermodynamic behaviour of syndiotactic polypropylene-poly(ethylene-co-polypropylene) random diblock block copolymers prepared by living olefin polymerization", Macromolecules, 38, 851 (2005)
R. Mezzenga, G.H. Fredrickson, E.J. Kramer "Tailoring morphologies in high internal phase emulsions by solvent casting", Macromolecules, 36, 4457 (2003)
R. Mezzenga, J. Ruokolainen, G.H. Fredrickson, E.J. Kramer "High internal phase polymeric emulsion by self-assembly of colloidal systems", Macromolecules, 36, 4466 (2003)
R. Mezzenga, J. Ruokolainen, G.H. Fredrickson, E.J. Kramer, D. Moses, A.J. Heeger, O. Ikkala "Templating organic semiconductors by self-assembly of polymer colloids", Science, 299, 1872 (2003)
L. Rubatat, et al. Macromolecules, 41, 1846 (2008)
N. Sary et al. Macromolecules, 40, 6990 (2007)
L. Rubatat, et al. Macromolecules, in press.

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