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Tesi etd-11242016-191306

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