In-situ study of the nonlinear optical responses and dynamics for a crosslinkable polymer-pair

The nonlinear optical responses of crosslinked and uncrosslinked polymer-pairs were characterized by using in-situ second-harmonic-generation measurements. When the molecular dipoles were thermally crosslinked to the polymer-pair main chain along the electric field, the rotational mobility of the molecular dipoles was found to be suppressed, resulting in improved rotational relaxation. This is understood to be from the increased moment of inertia of the molecular dipoles attached to the crosslinked polymer system, which also give an increased second-harmonic-generation signal. From the second harmonic generation measurement, the second-order nonlinear optical coefficients, d₃₁ = 1.7 pm/V and d₃₃ = 6.6 pm/V, were obtained. On the other hand, the transient dynamics of molecular alignment was investigated for both crosslinked and uncrosslinked polymer-pairs at various temperatures. At temperatures above T₉ - 50 °C, the double exponential and Dissado-Hill model fit the relaxation data well while at temperatures below T₉ - 50 °C, the double exponential and stretched exponentials provided a good fit. This finding suggests that two relaxation times can characterize the relaxation behavior of the polymer-pairs over the temperature range from room temperature to the glass-transition temperature.