Temperature-domain analysis of primary and secondary dielectric relaxation phenomena in a nonlinear optical side-chain polymer

Abstract

The investigation of dipole relaxation processes in nonlinear optical ͑NLO͒ polymers containing chromophore dipoles with large hyperpolarizabilities is important for optimizing the poling process and for predicting the long-term stability of the poling-induced order. The primary or ␣ relaxation is difficult to assess by dielectric spectroscopy in polymers with high glass transition temperature due to thermally induced chromophore degradation. A fast experimental procedure is developed for the investigation of dielectric relaxation processes in NLO polymers, without severely inducing chromophore degradation. The procedure is based on the measurement of the dielectric function (T)ϭЈ(T)ϪiЉ(T) at a few frequencies from 30 Hz to 30 kHz, while heating the polymer at a constant rate. The complex plane representation of the temperature-dependent dielectric function is used to determine the distribution of relaxation times, while the temperature-dependent mean relaxation time ␣ (t) is numerically determined from the dielectric loss Љ(T). With only three decades in frequency, information on five decades in time or 90 K in temperature is gained above the glass transition temperature. The strong ␣ and the weak ␤ relaxation below the glass transition are separately investigated by thermally stimulated depolarization after a suitable two-step poling procedure. The method has been applied to a typical polyimidelike side-chain nonlinear optical polymer with modified Disperse Red 1 chromophores. Even below room temperature, a ␥-relaxation process is observed, demonstrating significant mobility of the chromophores much below the glass transition. From the results of thermally stimulated depolarization it is concluded that the initial fast decay of the electro-optical response to a temporally stable value is related to the partial depolarization caused by the ␤ relaxation.

Pressure-temperature study of dielectric relaxation of a polyurethane elastomer

“…It is found that the parameters describing the distribution of relaxation times for most amorphous polymers change very little with temperature. 4 Thus, the temperature dependence of the effective relaxation time is a key parameter to characterize the temperature dependence of the relaxation process. Therefore, the activation energy (effective relaxation time) and characteristic transition temperature (T g ) for each process of a polymer are widely investigated.…”

Nonlinear optical polyimide/montmorillonite nanocomposites consisting of azobenzene dyes

“…This might be due to the fact that some relaxation of local stresses accumulated by sample preparation would cause the randomization of aligned NLO dyes, leading to depression of NLO activity. Cheng et al have reported the primary and secondary dielectric relaxation phenomena in an NLO polymer [50]. It was found that dielectric measurements on a typical NLO sidechain polymer exhibited multiple relaxation processes even below room temperature, which are all related to the chemically attached chromophore dipoles.…”

Nonlinear dielectric response of poled amorphous polymer dipole glasses

“…With a dye loading of 90 mol%, the P(S-MA)-DR1 side-chain polymer showed a glass-transition temperature T g = 137°C, as measured with DSC at the same heating rate of 10°C/min [10]. After thermally poling the polymers for 10 min 20°C above their respective glass-transition temperature with a poling field of 80 V/lm, stable, non-resonant electro-optical coefficients r 33 = 1.8 pm/V @ 1.55 lm and r 33 = 8.5 pm/V @ 1.55 lm were achieved for the guest-host [12] and the side-chain polymer, respectively [13].…”