Nonlinear Optical Polymers. 2. Novel NLO Linear Polyurethane with Dipole Moments Aligned Transverse to the Main Backbone

This paper presents a novel class of nonlinear optical (NLO) polymer for second harmonic generation (SHG). This new class of NLO polymer consists of the linear polyurethane, T-polymer, of tolylene 2,4-diisocyanate (TDI) with 3-[(2-hydroxyethyl)amino]-5-(hydroxymethyl)-4‘-nitroazobenzene (T-AZODIOL), whose dipole moment is aligned transverse to the main chain backbone. For comparison, the linear polyurethane L-polymer, whose NLO chromophore is incorporated into the main chain, is synthesized from TDI with 4-[ N -(2-hydroxyethyl)- N -methylamino]-3‘-(hydroxymethyl)azobenzene (AZODIOL). T-polymer poled at optimum conditions of a corona poling voltage 8.0 kV, a temperature of 95 °C, and a time of 60 min shows a large second-order nonlinearity of d 33 = 1.6 × 10 -7 esu (67 pm/V) with good thermal stability at room temperature. The oriented NLO dipole moments for T-polymer do not show significant relaxation at ambient conditions after 60 days except for small depression a few days after poling, whereas, the SHG activity of L-polymer is largely decayed at room temperature. The better thermal stability of this new class of T-polymer is related to the smaller free volume in T-polymer.