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42836-G7
Hydrogen-Bonded Self-Assembled Polar Subunits as Nanostructured Electro-Optic Materials

Padma Gopalan, University of Wisconsin (Madison)

Our work is in the area of creating nanostructured materials for electro-optics (EO). Our approach has been to tailor the chromophore/polymer interface to maximize the EO activity achievable from a given chromophore. In the last year we have essentially focussed on: 1) selective encapsulation of EO chromophores in a block copolymer template, and 2) studying the poling dynamics of the dipoles by in situ means. Poly (styrene-b-4-vinylpyridine) [PS-b-P4VP] is an excellent BCP candidate for the host polymer due to the vast body of literature available on the phase behavior of encapsulated system. We have successfully demonstrated the confinement of EO chromophores in the spherical, cylindrical and lamellar P4VP domain of the BCP. These morphologies were characterized by a combination of transmission electron microscopy (TEM) and X-ray diffraction studies. For EO activity it is essential to have a non-centrosymmetric materials or a poled polymer. Hence, much of the last year we focused on understanding the poling dynamics of the chromophores within the well-defined block copolymer domains. These studies have led to some very interesting findings: 1) In addition to the size of the block copolymer domains the distribution of the chromophores within the domains is of utmost importance. If the ratio of the chromophore to the vinylpyridine units in the P4VP domain of block copolymer is lower than 1:25 the EO activity is significantly quenched. 2) Examination of the second harmonic generation signal while poling the BCP/chromophore blend revealed a direct correlation between the stability of H-bonding and the efficiency of poling, and 3) Annealing of the BCP/chromophore blend further changes the spacing between the chromophores within the domain leading to significant changes in poling dynamics. All these observations are the very first studies on understanding the poling dynamics in confined domains. It is also clear that a linear-dendron architecture of block copolymer where the branched architecture due to the dendron part keeps the chromophores apart and the small domain sizes achievable would be best morphology to encapsulate the EO chromophes. Studies are underway to examine a linear-dendron block copolymer with chromophore encapsulated in the dendron block as a next step. A manuscript on these observations is currently in preparation for submission to Macromolecules.

The PRF grant has enabled dedicating one graduate student to primarily elucidate the poling dynamics of chromophores in confined domains. It has also contributed to the training of three undergraduate students on the chemistry and physics of EO polymers

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