Investigation of Azo Polymers Local Mobility by Oscillatory Spectra of Chromophores
Derivatives of azobenzene chromophores with donor and/or acceptor substituent are anisotropic molecules with absorption bands in visible and UF spectrum. Nowadays, many different applications for materials with the specified molecules are found, including their use in the devices of optical memory for recording and storage of information , for light frequency conversion , as optical switches .
One of perspective representatives of azo-chromophores is the molecule of 4-amino‑4’-nitroazobenzol (DO3). The paper  demonstrated that the dipolar moment of optical transition μmn in DO3 molecules is maximal in the direction of the molecule principal axis. The probability of absorption by the molecule of radiation is maximal when the direction of its principal axis matches the direction of polarization of the incident field.
When absorbing linear polarized laser radiation, chromophores are selectively excited and, generally (for example, owing to photoisomerization cycles), can change the orientation (fig. 1) . It is possible even in the glass-like polymeric matrix since local heating leading to increase in local mobility of molecular fragments of polymer chains near chromophore is observed during absorption . Light absorption and reorientation of chromophores will occur until their dipolar moment is not perpendicular to polarization vector of exciting radiation. Detecting of the received orientation is possible by various methods , including methods of oscillatory spectroscopy , .
2. EXPERIMENTAL PART
Synthesis of epoxamine oligomer with azo chromophore covalently attached to the main chain DO3 (CFAO) (Tc = 130 °C) is described in paper  (fig. 2a). Films have been received from 10% CFAO solution in cyclohexanone applied on substrates KBr and BaF2. Films have been annealed at a temperature of 140 °C during an hour. Thickness of films have been 0.5–2 μm. The absorption spectrum of CFAO thin film (l = 280 nm) is received using spectrophotometer Lamda 35 (PerkinElmer, USA) and is presented in fig. 2a.
The scheme of experimental assembly for research of mobility of polymer chain by IR-Fourier’s spectroscopy is given in fig. 2b. Laser radiation linearly polarized along axis x with excitation line in CFAO absorption band was used for orientation of chromophores (λ = 532 nm). Probing was carried out by the polarized IR irradiation by means of IR-Fourier spectrometer Vertex 70, Bruker in photosteady mode. Temperature researches in the range from 30 to 140 °C were carried out using cryostat Specac regulated by temperature controller West 6100+. Accuracy of temperature maintenance was 1°.
3. DISCUSSION OF THE RESULTS
For chromophore fragments orientation in the optical field, the intensity of exciting light varied within 5–20 MW/cm2. The chosen intensity range does not lead to emergence of irreversible changes in absorption IR spectra. IR spectra of absorption of polymer before (black) and after (red) illumination are given in fig. 3. Dxx corresponds to parallel and Dxy to perpendicular polarization of exciting (532 nm) and probing IR irradiation. Blue color designates DO3 chromophore spectrum. When illuminating with exciting radiation, Dxx of a number of lines relating to fluctuations of chromophore group decreases. It testifies to orientation of chromophores perpendicular to polarization of the incident field. Rather intensive, well resolved absorption band of 139 cm‑1 relating to stretching vibrations ν(C-NN)  has been chosen for the analysis.
The degree of orientation of chromophores is conveniently analyzed using anisotropy ratio : d = (Dxx – Dxy) / (Dxx + Dxy). The kinetic curve of anisotropy ratio during illuminating and after switching exciting radiation off is given in fig. 4 with temperature T = 30 °C. One can clearly see that anisotropy ratio significantly changes, reaching value –0.14. After switching exciting laser radiation off, partial disorientation of chromophores is observed. It is apparently connected with relaxation of mechanical stresses in polymer chain caused by photoinduced orientation of chromophores . Since orientation occurs in non-equilibrium matrix, the polymer chain aims to return chromophore to initial state after removal of external influence.
Plot of dependence of anisotropy ratio on temperature for 1139 cm–1 absorption band during illumination by exciting radiation with intensity of I = 9.4 MW/cm2 is given in fig. 5. Each experimental point is received after radiation of film within 10 minutes (the time during which anisotropy ratio reaches 85% of the maximum value). One can see that nature of dependence d(T) changes at a temperature of 60 °C.
In our opinion, the change of nature of dependence d(T) is connected with emergence of additional channel of mobility of chromophore groups causing their disorientation. Inflection of plots is close to 60 °C. The received temperature within the errors matches temperature β1 of relaxation transition found in the paper  by means of dielectric spectroscopy. The authors connect this relaxation transition with the emergence of biphenol fragments mobility with adjacent ether groups and OCCO fragments of the main chain. Our data allow to conclude, that defrosting of orientation mobility of chromophore groups at this temperature also occurs. Their reorientation requires the emergence of a cavity which is close to chromophore in volume. This volume evaluated by us using increment technique  makes about 200 Е3.
Therefore, studying of temperature dependence of anisotropy ratio of absorption bands of the chromophores in the polymer chain allows assessing the nature of their mobility, to determine temperature of relaxation transition and to receive additional information on the size of the elements of free volume arising upon this transition.
Orientation of chromophore fragments covalently attached to main chain of epoxamine oligomer in external optical field is investigated. The technique of determination of relaxation transition temperature in relation to temperature dependence of anisotropy ratio is suggested.
The studies are executed using the equipment of FCCU of the Kazan Federal University. The work was performed with assistance of the Russian Federal Property Fund (No. 15-42-02339).