LIQUID CRYSTAL AZOPOLYMERS OBTAINED BY ATRP: NEW BEHAVIOURS, PROPERTIES AND APPLICATIONS

Side-chain liquid crystal polymers (LCP) with tailored azobenzene moieties are widely studied due to the easy photomodulation of their properties (birefringence, non-linear optical properties and chiroptical properties) making them suitable for several applications as materials for holographic data recording, non-linear optics, artificial muscles or molecular switches.[1-3] The possibility given by Atom Transfer Radical Polymerization (ATRP) of obtaining macromolecules with controlled molecular mass and low polydispersity is an important tool which has been adopted to produce polymethacrylic esters functionalized with various chemical moieties [4]. Here, we present the synthesis by ATRP and the characterization of monodisperse azopolymers of well-controlled structure (linear and three-arms star shaped) and different average molecular weights. The photomechanical effects (i.e. the dependence of volume and density) exhibited upon trans-to-cis and cis-to-trans photoisomerization of the azobenzene mesogenic groups have been investigated by ellipsometry and related to molecular weight, with particular attention to important parameters for potential applications such as the relative variation of total volume, response time, stability and reproducibility to achieving optomechanic material. In similar way, brush polymers containing photosensitive mesogenic side chains have been directly synthesized on glass substrates by a grafting method involving polymerization of a methacrylic azobenzenic monomer under ATRP conditions. Brush polymer surfaces, with respect to traditional films, are resistant to solvents and to high temperatures due to the covalent bonds between chain and substrate, which stabilizes the film morphology. The peculiar characteristics of polymeric brushes within the possibility of a fine control on the structure, make them very attractive for application as command surfaces. Furthermore, the presence of a chiral group of one absolute configuration in the side chain can lead to induction of surprising supramolecular chirality in liquid crystal state. The chiroptical properties of thin films of the azopolymers were investigated before and after irradiation with right (R)- and left (L)- circularly polarized light (CP) by circular dichroism (CD). All the irradiated polymeric films displayed amplified CD signals, with the largest level of supramolecular photoinduced chirality exhibited by linear polymers. The switching of the chiroptical proprieties make these materials suitable to use as chiroptical switches and of potential interest in nanoscale technologies for all-optical manipulation of informations.