A study of dynamics and relaxation at different time and length scales in copolymers with fluorinated azobenzene side groups

This work aims at studying relaxation processes over different time- and length-scale in new side chain azobenzene photosensitive liquid crystalline polymers with relation to the understanding of the interplay between molecular composition and heterogeneities at nanometer scale. Such new synthesized polymers belong to the wide class of smart or stimuli-responsive polymers able to mimic the capability of living system to modulate their response and structure to varying environmental conditions in a reversible manner. In such materials small physical or chemical changes in the environment can induce and modulate macroscopic large response as an effect of local structural or functional changes. Polymers containing azobenzene moieties as side chains are interesting not only from an applicative point of view. Their study can enlighten about the cooperative mechanisms driving the main chain relaxation and its interaction with lateral co-units or the creation of heterogeneitis in the local dynamics. More specifically, the subject of study is a class of fluorinated side chain homopolymer and its copolymers whose development and synthesis take as a basis the results of preceding studies in the homopolymer PMA4 1 a nematic poly(methacrylate) with azobenzene chromophore in side chain developed in a CIPE-INFM project for the selection of azobenzene polymeric matrices suitable for optical writing and data storage. Electron Spin Resonance (ESR) investigations showed, among other interesting properties, the presence of dynamics heterogeneities on the nanometric length- scale and the possibility of modulating different dynamics sites by suitable thermal treatments also in the isotropic phase. The quantification of the heterogeneities, their persistence on temperature and relaxation have been revealed themselves a key issue for the selection of the more suitable matrices for optical writing. It is worth remembering that the writing process both in birefringence and by topographic relief have been successful right following the indications provided by results from ESR. New polymers, studied by ESR and rheology, differentiate from PMA series only for the substitution of the terminal groups the main chain composition remaining the same. Fluorinated azobenzene liquid crystalline polymers seem to be a very promising materials for optical data storage substrate, especially in holographic recording for their capacity of suppressing the surface relief. Moreover they importance in applied physics arise from their excellent water repellent capacity and the possibility of modulating wettability by the change in the dipole moment of the cis- trans photochromic species. It has been demonstrated, in fact, that the azobenzene molecular conformational rearrangements allow photomodulation of a monolayer’s water contact angle and surface potential reaching a very good photo-control and photopatterning of wettability. Collective processes and the interplay between the dynamics of side groups and those of the main chain play a major role in the tuning of the bulk response of such materials. ESR spectroscopy, due to its sensitivity to different stochastic reorientational model has resulted a very useful technique in understanding reorientation processes, cooperative degree and the correlation with local environment in side chain polymers. On the other hand the use of spin probes as paramagnetic tracers in diamagnetic hosts is recognized as the technique of choice in polymers investigations since it does not modify local structures of the host matrix and allows to infer very local properties (on the nanometer scale) in the vicinity of the radicals in different sites mapped. Spin probes reveal suitable for characterizing the collective behavior of molecules or, in other words, the supramolecular structure exhibited by such smart materials. Nitroxide radicals, due to their chemical and thermal stability are widely used as spin probes. Their various structures and geometries allow to selectively study different sites and consequently different aspects of the same material. The analysis of the temperature dependence of rotational dynamics of cholestane nitroxide radical, carried out in this work, allows to characterize the nature of heterogeneities and their spatial or temporal nature. Different dynamics regimes and the crossover between them at characteristic temperatures signalling the onset of cooperative phenomena have been found and discussed. Asystematic comparison with the structural relaxation of the polymers, as revealed by rheological measurements, allows the estimation of the cooperativity degree in the dynamics. The work is intended as the natural development of previous ESR studies carried out in PMA4 hopolymer and MA4-MMA random copolymers by Prof. Marco Giordano and Prof. Laura Andreozzi’s group in collaboration with Prof. Giancarlo Galli’s research group at the Department of Chemistry and Industrial Chemistry of the University of Pisa. Investigation in such side chain liquid crystalline polymers has defined a standard approach in studying azobenzene containing polymers in particular as regards thermal treatment and its influence in creating/modulating dynamics heterogeneities. Heterogeneities which are manifested as two sites with different reorientational dynamics revealed crucial for the selection of polymeric subtrates for nanowriting.