Dynamic Mechanical Analysis and Dielectric Relaxation for Second Order Nonlinear Optical Applications
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ABSTRACT Relaxation dynamics of poly(methyl methacrylate) (PMMA) and doped PMMA systems are investigated as a function of processing temperature and frequency. Polymer relaxations and changes in local mobility are responsible for chromophore reorientation and lead to a loss in second-order nonlinear optical (NLO) properties. Studying polymer relaxations enables the temporal and thermal stability of the NLO polymer systems to be more accurately and readily predicted. The polymer dynamics are studied by correlating mechanical and dielectric relaxations to polymer and chromophore relaxations that occur during chromophore reorientation in second harmonic generation experiments. Dielectric and mechanical relaxation techniques enable polymer relaxation dynamics to be observed over a broad frequency range. The plasticization effect of the NLO chromophores on the polymer relaxation dynamics is also investigated.
INTRODUCTION Chromophore-doped polymers have been studied for a decade in an effort to produce materials suitable for second-order nonlinear optical (NLO) device applications. NLO polymer systems have several promising attributes such as ease in processability and fabrication, large electro-optical coefficients, and low dielectric constants that make them of interest in the optics, semiconductor, and telecommunications industries.' Second-order NLO properties are observed if the chromophores are noncentrosymmetric or exhibit an overall polar orientation. For amorphous doped polymers, this is achieved by applying an electric field across the polymer film which aligns the chromophores in regions of sufficient free volume and local mobility. Upon removal of the electric field, the chromophores reorient, resulting in a loss of NLO properties. The loss in stability of chromophore orientation caused by polymer relaxations after poling currently prevents application of polymer systems in second-order NLO devices.1 Understanding the polymer relaxation dynamics that occur during chromophore reorientation enables the temporal and thermal stability of the NLO polymer systems to be more accurately and readily predicted. By identifying molecular groups responsible for the relaxation dynamics, the variation of impact strength and other physical properties as a function of processing temperature and frequency can be investigated. Additionally, understanding relaxation dynamics aids in developing new polymers with improved thermal and temporal stabilities. Second-order NLO polymer relaxation dynamics have been studied using second harmonic generation (SHG) experiments. 2 In these experiments, chromophore orientation during and after poling is observed and is related to the local mobility and relaxation of the polymer system. The polymer relaxation dynamics that occur during chromophore reorientation are not well known, but may be examined using correlations with dielectric and mechanical relaxation dynamics. Polymer relaxation dynamics have been extensively studied using the established techniques of dynamic mechanical analysis (
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