Synthesis and properties of hyperbranched polyester with second-order optical nonlinearity

A nonlinear optical hyperbranched polyester (PE-Azo/Hyper) was synthesized from 4-[N,N-bis(hydroxyethyl)amino-4'-formyl]azobenzene and cyanoacetic acid by a Knoevenagel polycondensation using 4-(dimethylamino)pyridine as a base. The resulting polymer was soluble in polar aprotic solvents such as NN-dimethylformamide and 1-methyl-2-pyrrolidinone and could be processed into optical quality films by spin coating. The molecular weight was determined to be M-w=61,800 (M-w/M-n=1.86) by gel permeation chromatography using polystyrene as a standard. No melting point was detected by differential scanning calorimeter, indicating that this polymer presents an amorphous state. It shows a glass transition temperature of 121 degreesC. The second-order nonlinear optical coefficient d(33) of the poled polymer determined by the second harmonic generation at 1064 nm was 25.4 pm/V.