Growth control and structure in (macro)molecular systems
Opto-properties of polymer semi-conductors (PSCs) are strongly anisotropic and depend crucially on chain orientation and crystallinity in thin films. The group has developed new orientation/crystallization methods based on high-temperature rubbing and directional epitaxial crystallization. Using a home-made machine, the group showed that high-T rubbing allows to align/crystallize numerous PSCs that are widely used in plastic electronics. This control made it possible to uncover fundamental aspects of polymer crystallization and structure by TEM (HRTEM and electron diffraction). In P3HT, correlations between the length of planarized chain segments in the crystals and the optical properties (exciton bandwidth) and charge transport (hole mobility) was demonstrated for the first time.
Thin films with a controlled structure and orientation helped determine the structure of some representative PSCs by electron diffraction (form I and form II of P3HT, PDOPT, PCPDTBT, p(NDI2OD-T2) and F-PCPDTBT). Noteworthy, the two polymorphs of P3HT, the working horse among semiconducting polymers, were determined by the group.
This approach was successfully used to identify a new zipper-like packing of donor-acceptor co-oligomers designed for the elaboration of solar cells (ANR Picasso) and control domain orientation in thin films. The following Figure illustrates the original structures of such co-oligomers obtained from electron diffraction analysis and structural modeling.
Beside, the crystallization and structure of PLLA, a major biopolymer, has also been investigated in the group. The expertise of the group in cryo-TEM helped unravel the structure of micelles made of amphiphilic bottlebrush block copolymers containing polylactide (PLA) and poly(ethylene oxide) (PEO) side chains.