Electric field effects and molecular motion in poled polymer thin films for nonlinear optical applications
Abstract
Rotational dynamics of nonlinear optical chromophores embedded in amorphous polymer films are investigated using second harmonic generation. Second harmonic generation is a second order nonlinear optical process that is sensitive to local molecular motion. The stability of the bulk nonlinearity, which is related to the rotational motion of the chromophores, is affected by the poling electric field and the local mobility of the polymer matrix. Corona poling, a technique involving applying a strong electric field across the polymer film, is used to orient the chromophores into the bulk noncentrosymmetric structure required to observe second order nonlinearity of the material. The field effects are examined by simultaneously measuring the second harmonic signal (during and following poling) and the surface voltage decay (following poling). It is found that for short times the residual field following corona poling retards chromophore reorientation. A rotational Brownian motion model including the surface voltage effect and polymer restrictions is developed to simulate the temporal dependence of the second harmonic signals and to interpret the rotational processes of chromophores at different temperatures. Applications of the model in predicting the microscopic polymer dynamics in various guest-host systems are studied. Five NLO chromophores with a similar geometric structure but of different sizes are used to probe local polymer dynamics. It is expected that the motion of the chromophores could be coupled to different polymer segmental motion because of the different sizes of the chromophores. Therefore, by monitoring the thermal and temporal dependencies of the second order nonlinearity in these polymer systems, different relaxation modes in the polymer can be examined. In order to detect different microscopic relaxation mechanisms of the polymers, chromophores are also incorporated into the polymer main chain in different directions. It is found that for a kink polymer, in which the chromophores are directed at an angle away from the major molecular axis of the polymer chain, the motion of the tilted chromophores may occur through local segmental motion. For a linear polymer, which has the same chromophore placed parallel to the chain direction, a large scale main-chain motion is involved in orientation. This study is important for understanding the microscopic polymer physics in these second order nonlinear optical materials and for developing an efficient process for obtaining high nonlinear optical performance of polymers.
Degree
Ph.D.
Advisors
Lackritz, Purdue University.
Subject Area
Chemical engineering|Materials science|Electrical engineering
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