Macromolecular chemistry and physics vol:197 issue:12 pages:4135-4153
Two side-chain polyurethanes, HMDI-PU (29) and TDI-PU (2b), have been synthesized from 2,2'-iminodiethanol (''diethanolamine'') N-substituted with a nitrostilbene group on a hexamethylene spacer with, respectively, hexamethylene diisocyanate and 1-methyl-2,4-phenylene diisocyanate (''2,4-toluenediisocyanate''). 2a showed a smectic liquid-crystalline phase; 2b proved to be amorphous. The phase behaviour was characterized by means of differential scanning calorimetry, polarization microscopy and wide-angle X-ray scattering. Thermally stimulated discharge current measurements (TSD) revealed at least two strong relaxation processes above the glass transition temperature T-g which are related to the dynamic glass transition involving the backbone part (alpha) and the mesogenic units (lambda). Non-linear optical analysis of 2a showed that the Pockels coefficient r(13) was low in the mesophase and high in the isotropic phase. Contrary to expectation, poling in the mesophase did not enhance the Pockels effect. The amorphous 2b gave both a high and a stable Pockels coefficient (r(33) around 6.0 pm/V). The stability could be improved by ageing and prolonged poling at the ageing temperature. However, the Pockels coefficient proved to be dependent on the temperature: high above 85 degrees C, low below 85 degrees C (provided that the sample had been heated once before). This was attributed to an antiparallel association of the chromophores. The role of hydrogen bonds influencing poling has been assessed by temperature-dependent infrared spectroscopy.