Photocrosslinkable second‐order nonlinear optical polymers synthesized by cationic copolymerization

Photocrosslinkable second‐order nonlinear optical (NLO) polymers were synthesized from cationic copolymerization of a vinyl ether monomer bearing 4′‐nitrobiphenyl‐4‐oxy group as the NLO chromophore with a vinyl ether monomer bearing cinnamoyl group as the photoreactive moiety. To obtain a suitable poling method involving photocrosslinking, which is capable of inducing a higher and more stable second‐order nonlinear coefficient, d₃₃, for NLO polymer films, some poling procedures were investigated. An optimized poling method was as follows. Ultraviolet (UV) irradiation is performed for 90 sec during poling at 50°C for 20 min, followed by poling at 150°C for 20 min. By using this poling method NLO polymer films exhibited a higher and considerably stable d₃₃ value at room temperature, even though they had rather lower glass transition temperatures before photocrosslinking. Some photocrosslinking mechanism for NLO polymers investigated here were considered.