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

The photoinduced optical second harmonic generation (PISHG) in 1,4-cis-polybutadiene has been investigated as a function of temperature in the vicinity of the glassing and crystallisation temperatures. Both the addition of centrosymmetric tetraphene molecules as well as the trans-isomers lead to an enhancement of the local non-centrosymmetric charge density distribution determining optical second harmonic generation. Using ab initio quantum chemical calculations with the inclusion of electron–vibration anharmonic interactions and combined with experimental PISHG data we show that increasing molecular localisation favours increasing non-centrosymmetry detected by PISHG. Two maxima of the PISHG signals have been observed at temperatures of ≅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 mechanism of the observed temperature anomalies is connected with the enhancement of electron–vibration anharmonicity during glassing and crystallisation structural rearrangements.