Second-harmonic response and relaxation behavior off high glass-transitiontemperature polyphosphazene films

The second-harmonic response of 4- [(4 ' -nitrophenyl)azo] phenoxy- doped p olyphosphazene (PPZ) films with high glass-transition temperatures (T-g sim ilar to 150 degreesC) has been investigated. The nonlinear chromophore has been incorporated into the PPZ as (i) a guest molecule (system I), (ii) a s ide-chain group covalently attached to the linear PPZ (system II), and (iii ) a substituted group attached to a triphosphazene molecule that is dissolv ed in the PPZ (system III). The three components chi ((2))(15), chi ((2))(3 1) and chi ((2))(33) of the second-order nonlinear optical susceptibility t ensor have been determined for the three systems. Values for chi ((2))(33) are in the range 5-15 x 10(-9) esu (similar to1.7-6.0 pm/V), which appears significant and comparable to that found, under nonresonant conditions, in other polyphosphazene-chromophore systems with a much lower T-g. For equal chromophore concentration the guest-host system I shows the highest respons e. This has been correlated with a higher degree of molecular ordering achi eved during poling. At variance with previous work, the SHG response of our three PPZ systems is very stable and has been comparatively investigated f or various temperatures in the range room temperature-120 degreesC, always below T-g. At low enough temperatures (< 100 degreesC) substantial differen ces are found between the three systems. The guest-host system I containing the free chromophore shows the fastest decay, whereas the one containing t he triphosphazene molecule (system III) is the slowest. At 120 degreesC all systems present essentially the same behavior. The kinetics of the relaxat ion has been satisfactorily fitted to both the biexponential and KWW laws, and the values of the parameters have been discussed in light of previous r esults reported on other NLO organic polymers.