Increased thermal stability of a poled electro-optic polymer using high-molar-mass fractions

Polymer fractionation yields materials with higher average molar masses and thus substantially increased glass transition temperatures. We apply this technique to a photoaddressable polymer that exhibits high electro-optic coefficients, in order to enhance the thermal stability of the poled material. The degree of orientation of the dipolar chromophores is investigated with both pyroelectric and electro-optic measurements. The pyroelectric measurements at elevated temperatures show no significant contribution of $\beta$ relaxation as compared to $\alpha$ relaxation.

Figure 1

Chemical structure of the investigated material.

Figure 2

Molar mass distribution function $W$ (normalized) of the original polymer (P) and its fractions F1–F5. The curves have been obtained in an analytical gel-permeation-chromatography experiment in N,N-dimethylacetamide at $40°\mathrm{C}$ under consideration of the standard PMMA calibration.

Figure 3

Pockels coefficients $r_{13}$ measured at $685\mathrm{nm}$ wavelength for a sample of the nonfractionated material (solid symbols) and a sample of the fraction F2 (open symbols). Both samples are stored in an oven at a temperature of $90°\mathrm{C}$ and taken out in regular intervals for the interferometric measurements.

Figure 4

Pyroelectric signal for a heating rate of $(0.2\pm 0.02)°\mathrm{C}∕\min$ as a function of temperature. The curves are normalized to the signal strength at a temperature of $70°\mathrm{C}$ and correspond (from right to left) to the fractions F1–F5 and to the nonfractionated polymer.

Figure 5

Normalized pyroelectric signal at a heating rate of $(0.2\pm 0.02)°\mathrm{C}∕\min$ as a function of temperature. The solid line represents measurement data of a sample of the high-$T_{\mathrm{g}}$ fraction F1. The dashed line represents the curve fit.