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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
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
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
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.