Dielectric relaxation in copolymethacrylates containing side-chain nonlinear optical chromophores

Molecular mobility in the films of chromophore-containing copolymethacrylates was studied by dielectric spectroscopy. Near the glass transition temperature α- and δ-relaxation processes were observed, being related to segmental mobility and the reorientation of chromophore-containing side groups, respectively. It was shown that the temperature dependences of relaxation times in the range of the δ-process are linear or curvilinear for copolymers with 20 or 50% molar fraction of chromophore-containing units, indicating that the local or cooperative character of chromophore mobility was due to the absence (presence) of inter-chromophore interactions. These data are very important for optimizing the chromophore molar content in NLO polymers since the inter-chromophore dipole-dipole interactions usually lead to the formation of centrosymmetric anti-parallel aggregates which cannot be oriented by external electric fields. It was shown that the δ-process occurs at higher temperatures than the α-process. This fact is of great importance for optimizing the poling temperature at which macroscopic non-centrosymmetric orientation of chromophore occurs under the external electric field applied across a polymer film. The poling temperature should be at least not lower than the glass transition temperature for the δ-process.