Nanostructured Materials for Organic Electro-optics

IPrime Microstructured Polymers presents a seminar by Professor Padma Gopalan, University of Wisconsin, Dept of Materials Science and Engineering.

Held March 3, 2009 at 9:45 a.m., Smith Hall 331, University of Minnesota.

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Seminar Abstract:
The area of electro-optic polymers has experienced unprecedented growth in the last five years with the development of high performance electro-optic chromophores and prototype devices such as broad-band optical modulators and photonic phase shifters. One of the critical molecular design issues identified in electro-optic materials is that the intermolecular electrostatic interactions of high dipole moment chromophores in poled polymers significantly limit the maximum possible macroscopic electro-optic coefficient. One well-explored solution involves designing new chromophores with more sophisticated shapes that minimize electrostatic interactions. Our approach is to fully explore the role of chromophore domains in a polymer matrix and examine how the interface between the two impacts the macroscopic electro-optic activity by designing nanostructured materials. We have shown that once selective incorporation of nonlinear optical chromophores in a block copolymer domain is attained, poling within these domains is considerably more complex than in conventional homopolymer systems. Achieving poled order in confined domains depends on the polymer host architecture, domain size, nature of interaction between the chromophore and polymer host, and concentration of the chromophore within the domains. While well-ordered phase separated systems such as block copolymers show at least a two fold increase in the electro-optic coefficient of a given chromophore over the guest-host systems, a more effective approach is to explore polymer chains end functionalized with chromophores. The results of this study are applicable to a broader field of nanostructured photonic materials in which periodic modulation of spatial optical properties is defined via chemical processes. Perhaps the most far-reaching conclusion of our study has been that a synthetically straightforward chromophore encapsulated in a BCP system can exhibit a wide range of second order nonlinear optical behavior simply as a function of the nanomorphology. The ability to control the orientation of functional domains in block copolymer thin-films would enable direct correlation of the observed properties with the morphology. I will briefly talk about the chemistry of photo-crosslinkable brushes that we have developed to create a non-preferential surface on substrates of arbitrary compositions. These brushes offer a number of advantages such as ease of synthesis, fast kinetics, photopatternability and effectiveness in very thin layers which is desirable for pattern transfer.