Nematic Main Chain Polymers with Head-to-Tail Structure: Synthesis and Enhanced NLO Response

NLO-active semiflexible main chain polymers with a head-to-tail structure were synthesized from 2-nitro-4-carboxy-4‘-[ N -(11-hydroxyundecyl)- N -methylamino]azobenzene (CHRI) and 4-carboxy-4‘-[ N -(11-hydroxyundecyl)- N -methylamino]azobenzene (CHRII). The CHRI polymers, with molecular weight approximately 40 000, were not liquid crystalline. The electrooptical coefficient difference r 33 − r 13 ranged between 0.75 and 1.36 pm/V depending on the particular sample. Theory suggests that nematic head-to-tail polymers can exhibit greatly enhanced electrooptical (EO) response. The 50/50 copolymers from CHRII and p -hydroxybenzoic acid (molecular weight 38 000) were liquid crystalline. r 33 − r 13 was enhanced by up to a factor of 20 to r 33 − r 13 ∼ 21 pm/V, with a comparable poling field around 8 × 10 6 V/m. The corresponding χ (2) are around 62 pm/V. The dynamics of the induction of NLO response by a poling field was clearly high polymer-like. Relaxation of the EO response below T g is imperceptible. Considering the smaller hyperpolarizability and lower number density of CHRII (compared to in the CHRI polymer), this enhancement is effectively ×40. Powerful chromophores, less modest poling fields, and longer chains should yield much larger χ (2) using such polymers.