Molecular relaxation in a photosensitive liquid-crystalline polymeric glass former

Abstract

Abstract. We have measured the relaxation to equilibrium in the supercooled nematic phase upon approach to T g of a fragile liquid-crystalline photosensitive polymeric glass former. Perturbation is introduced in a non-conventional way by optical pumping (orientation) of the nematic polydomain phase. Relaxation back to equilibrium is followed both on the microscopic scale, by means of depolarized time-resolved Raman experiments, and on the macroscopic scale, by means of quartz crystal microgravimetry.

Equilibrium and out-of-equilibrium dynamics in a molecular layer of azopolymer floating on water studied by Interfacial Shear Rheology

“…This material has been already investigated by us both in bulk [19][20][21] and as molecular layers [5,22]. More recently we also performed XPCS e xperiments on this system measuring its microscopic dynamics [23] finding an overall picture consistent with what is found for other polymers and arrested states, e.g.…”

“…Figure 6 is an Arrhenius plot summarizing the temperature evolution of the 2D generalized viscosity | 2D | measured by ISR and by other rheological techniques, and plotted together with the XPCS relaxation times measured in a PA4 film both in dark and under UV illu mination. Experimental data are compared with the Vogel-Fulcher-Tammann (VFT) law describing the viscosity of bulk PA4 in dark which was obtained by EPR (T 0 =-33 °C, activation temperature T A =997 °C) [27] and was confirmed by depolarized micro Raman and Quartz Crystal Microbalance experiments [21]. Inspection of Fig 6 shows that the same VFT law is observed for both trans and cis PA4, thus suggesting that it is the main chain dynamics to govern the temperature evolution of the viscosity, while the azobenzene side chains provide an extra hindrance to the motion, which is dependent only on the shape of the isomer and not on the temperature.…”

Photoinduced Morphological Changes and Optical Writing in a Liquid Crystalline Polymer on the Micron and Sub-Micron Scale

“…We could then follow the photoinduced, as well as the thermally induced morphological changes and film dewetting processes. We note in passing that the cis-to-trans back-relaxation in dark takes place over a time scale of several hours at room temperature for this material [3,5].…”

“…This class of materials features a rich and complex phenomenology, which makes them very interesting also from a fundamental point of view. Their dynamics is the result of the complex interplay between several types of interactions and processes: the mesogenic potential, the conformational main chain transitions, the glass transition, the molecular trans-cis isomerization [3]. The relative weight of such processes, and the coupling between side chain and main chain dynamics, depend of course not only on temperature, but it can be also externally influenced by optical pumping with light of the appropriate wavelength, polarization and intensity [4].…”

Layering and relaxation in molecular films of a photosensitive polyacrylate from X-ray reflectivity and in elastic neutron scattering experiments

“…This system is indeed much more complex than other polymers, but we choose it because in it we can induce an isothermal perturbation by means of the photoisomerization of the azobenzene chromophore which is the main constituent of the side chain. We then can follow the back-relaxatio n process in pump±probe experiments by a variety of experimental techniques: birefringence (Arisi et al 2001), depolarized micro-Raman (Cristofolini et al 1999) and ellipsometry (Cristofolini et al 2000) methods.…”