Non-linear optical diagnostics of phase transitions in 1,4-cis-polybutadiene polymers

The photoinduced optical second harmonic generation (PISHG) in 1,4-cis-poly butadiene has been investigated as a function of temperature in the vicinit y of the glassing and crystallisation temperatures. Both the addition of ce ntrosymmetric tetraphene molecules as well as the trans-isomers lead to an enhancement of the local non-centrosymmetric charge density distribution de termining optical second harmonic generation. Using ab initio quantum chemi cal calculations with the inclusion of electron-vibration anharmonic intera ctions and combined with experimental PISHG data we show that increasing mo lecular localisation favours increasing non-centrosymmetry detected by PISH G. Two maxima of the PISHG signals have been observed at temperatures of co ngruent to 52 and 153 K for the specimens doped with p-terphenyl molecules and those doped with isomer trans-polybutadiene phase. Good correlation of the experimental data is obtained with theoretical calculations. The main m echanism of the observed temperature anomalies is connected with the enhanc ement of electron-vibration anharmonicity during glassing and crystallisati on structural rearrangements. (C) 2000 Elsevier Science Ltd. All rights res erved.