Rotational dynamics of disperse red 1 in polystyrene: α-relaxation dynamics probed by second harmonic generation and dielectric relaxation

Considerable interest has been generated in the last decade in using polymers for second harmonic generations (SHG), the conversion of light from frequency ω to ω. The study described here demonstrates how SHG can be used as a new technique to examine quantitatively and in detail the α-relaxation dynamics of polymeric systems over a broad dynamic range. A novel experimental protocol has been developed which allows us to monitor rotational dynamics over almost twelve decades in time. The decay-mode dynamics can be represented by a KWW equation for the PS + DR1 system. In agreement with predictions of the WLF equation, the temperature dependence of 〈τ〉 is very strong near T_g; below T_g, it is weaker but nevertheless substantial. A good agreement was found in the values of 〈τ〉 obtained from SHG and dielectric relaxation measurements. By scaling 〈τ〉 with the reduced parameter Tr/T, a substantial similarity in PIBMA and PEMA was revealed. However, the PS data show a much stronger temperature dependence above T_g. Extensions of these studies to the effects of chromophore size and concentration and covalent attachments of the chromophore to the polymer are underway.